POWER STATIONS
The term _power station_ is usually applied to any building
containing an installation of machinery for the conversion of energy
from one form into another form. There are three general classes of
station:
1. Central stations;
2. Sub-stations;
3. Isolated plants.
These may also be classified with respect to their function as
1. Generating stations;
2. Distributing stations;
3. Converting stations.
and with respect to the form of power used in generating the electric
current, generating stations may be classed as
1. Steam electric;
2. Hydro-electric;
3. Gas electric, etc.
=Central Stations.=--It must be evident that the general type of
central station to be adapted to a given case, that is to say, the
general character of the machinery to be installed depends upon the
kind of natural energy available for conversion into electrical
energy, and the character of the electrical energy required by the
consumers.
This gives rise to a further classification, as
1. Alternating current stations;
2. Direct current stations;
3. Alternating and direct current stations.
The alternators or dynamos may be driven by steam or water turbines,
reciprocating engines, or gas engines, according to the character of
the natural energy available.
[Illustration: FIG. 2,705.--Elevation of small station with direct
drive, showing arrangement of the boiler and engine, piping, etc.]
=Ques. Why is the reciprocating engine being largely replaced by the
steam turbine, especially for large units?=
Ans. Because of its higher rotative speed, and absence of a
multiplicity of bearings which in the case of a high speed,
reciprocating engine must be maintained in close adjustment for the
proper operation of the engine.
The higher speed of rotation results in a more compact unit,
desirable for driving high frequency alternators.
=Ques. Is the steam turbine more economical than a high duty
reciprocating engine?=
Ans. No.
=Location of Central Stations.=--As a rule, central stations should
be so located that the average loss of voltage in overcoming the
resistance of the lines is a minimum, and this point is located
at the center of gravity of the system. In fig. 2,706 is shown a
graphical method of locating this important spot.
[Illustration: Fig. 2,706.--Diagram illustrating graphical method
of determining the _center of gravity_ of a system in locating the
central station.]
Suppose a rough canvass of prospective consumers in a district
to be supplied with electric light or power shows the principal
loads to be located at A, B, C, D, E, etc., and for simplicity
assume that these loads will be approximately equal, so that each
may be denoted by 1 for example:
The relative locations of A, B, C, D, E, etc., should be drawn to
scale (say 1 inch to the 1,000 feet) after which the problem resolves
itself into finding the location of the station with respect to this
scale.
[Illustration: FIG. 2,707.--Exterior of central station at Lewis,
Ia.; example of very small station located in the principal business
section of a town. It also illustrates the use of a direct connected
gasoline electric set. The central station is located on Main Street,
which is the principal thoroughfare, and is installed in a low one
story building for which a mere nominal rental charge is paid, the
company having the option to buy the property later at the value of
the land plus the cost of the improvements and simple interest on
the same. To the front of an old frame building about 16 feet by
28 feet has been built a neat, well lighted concrete block room,
about 16 feet by 16 feet, carrying the building to the lot line and
affording ample space for the generating set and switchboards, and
such desk room as is needed for the ordinary office business of
the company. In this room, which is finished in natural pine with
plastered walls, has been installed a standard General Electric 25
kw. gasoline electric generating set consisting of a four cylinder,
four cycle, vertical water cooled, 43-54 H.P. gasoline engine, direct
connected to a three phase, 2,300 volt, 600 R.P.M. alternator with
a 125 volt exciter mounted on the same shaft and in the same frame.
With the generating set is a slate switchboard panel equipped with
three ammeters, one voltmeter, an instrument plug switch for voltage
indication, one single pole carbon break switch, one automatic oil
circuit breaker line switch and rheostats. Instrument transformers
are mounted above and back of the board. For street lighting service
a 4 kw. constant current transformer has been installed, and with
it a gray marble switchboard panel mounted on iron frames and
carrying an ammeter and a four point plug switch. On a board near
the generator set are mounted in convenient reach suitable wrenches,
spanners, and repair parts and tools. To cool the engine cylinders
five 6 × 8 steel tanks have been installed in the old building, a
pump on engine giving forced circulation.]
The solution consists in first finding the center of gravity
of any two of the loads, such as those at A and B. Since each of
these is 1, they will together have the same effect on the system
as the resultant load of 1 and 1, or 2, located at their center of
gravity, this point being so chosen that the product of the loads
by their respective distances from this point will in both cases
be equal.
The loads being equal in this case the distances must be equal
in order that the products be the same, so that the center of
gravity of A + B is at G, which point is midway between A and B.
Considering, next, the resultant load of 2 at G and the load of
1 at C, the resultant load at the center or gravity of these will
be 3, and this must be situated at a distance of two units from C
and one unit from G so that the distance 2 times the load 1 at C
equals the distance 1 times the load 2 at G. Having thus located
the load 3 at H, the same method is followed in finding the load 4
at I. Then in like manner the resultant load 4 and the load 1 at E
gives a load 5 at S.
The point S being the last to be determined represents,
therefore, the position of the center of gravity of the entire
system, and consequently the proper position of the plant in order
to give the minimum loss of voltage on the lines.
=Ques. Is the center of gravity of the system, as obtained in fig.
2,706, the proper location for the central station?=
Ans. It is very rarely the best location.
=Ques. Why?=
Ans. Other conditions, such as the price of land, difficulty of
obtaining water, facilities for delivery of coal and removal of
ashes, etc., may more than offset the minimum line losses and copper
cost due to locating the station at the center of gravity of the
system.
[Illustration: FIG. 2,708.--Map of Cia Docas de Santos hydro-electric
system; an example of station location remote from the center of
distribution. In the figure A is the intake; B, flume; C, forebay;
D, penstocks; E, power house; F, narrow gauge railway; G, general
store; H, point of debarkation; I, transmission line; J, dead ends;
K, sub-station. Santos, in the republic of Brazil, is one of the
great coffee shipping ports of the world, and for the development of
its water front has required an elaborate system of quays. These have
been developed by the Santos Dock Company, which holds a concession
for the whole water front. The company, needing electric power for
its own use, has developed a system deriving its power from a point
about thirty miles from the city, where a small stream plunges
down the sea coast from the mountain range that runs along it. The
engineers have estimated that 100,000 horse power can be obtained
from this source.]
=Ques. How then should the station be located?=
Ans. The more practical experience the designer has had, and the
more common sense he possesses, the better is he equipped to handle
the problem, as the solution is generally such that it cannot be
worked out by any rule of thumb method.
[Illustration: FIG. 2,709.--Station location. The figure shows
two distribution centers as a town A and suburb B supplied with
electricity from one station. For minimum cost of copper the location
of the station would be at G, the center of gravity. However, it is
very rarely that this is the best location. For instance at C, land
is cheaper than at G, and there is room for future extension to the
station, as shown by the dotted lines, whereas at G, only enough
land is available for present requirements. Moreover C is near the
railroad where coal may be obtained without the expense of cartage,
and being located at the river, the plant may be run condensing thus
effecting considerable economy. The conditions may sometimes be such
that any one of the advantages to be secured by locating the station
at C may more than offset the additional cost of copper.]
=Ques. What are the general considerations with respect to the price
of land?=
Ans. The cost for the station site may be so high as to necessitate
building or renting room at a considerable distance from the district
to be supplied.
If the price of land selected for the station be high, the
running expenses will be similarly affected, inasmuch as more
interest must then be paid on the capital invested.
The price or rent of real estate might also in certain
instances alter the proposed interior arrangement of the station,
particularly so in the case of a company with small capital
operating in a city where high prices prevail. In general,
however, it may be stated that whatever effect the price of real
estate would have upon the arrangement, operation and location
of a central station it can quite readily and accurately be
determined in advance.
=Ques. With respect to the cost of the land what should be especially
considered?=
Ans. Room for the future extension of the plant.
Although such additional space need not be purchased at the time
of the original installation it is well, if possible, to make
provision whereby it can be obtained at a reasonable figure when
desired. The preliminary canvass of consumers will aid in deciding
the amount of space advisable to allow for future extensions; as a
rule, however, it is wise to count on the plant enlarging to not
less than twice its original size, as often the dimensions have to
be increased four and even six times those found sufficient at the
beginning.
[Illustration: FIG. 2,710.--Section of the central station or
"electricity works" at Derby, showing boiler and engine room and
arrangement of bunkers, conveyor, ash pit, grates, boilers (drum,
heating surface and superheater), economizer, flue, turbines,
condenser pumps, etc.; also location of switchboard gallery and
system of piping.]
=Ques. What trouble is likely to be encountered with an illy located
plant after it is in operation?=
Ans. It may be considered a nuisance by those residing in the
vicinity, occasioning many complaints.
[Illustration: FIG. 2,711.--View of old and new Waterside stations.
The new station at the right has an all turbine equipment of ten
units, some Curtis and some Parsons machines, two have a capacity of
14,000 kw., and the remaining eight are of 12,000 kw. each. The old
Riverside station, seen at the left is described on page 1940.]
Thus, if the plant be placed in a residential section of the
community the smoke, noise and vibration of the machines may
become a nuisance to the surrounding inhabitants, and eventually
end in suits for damage against the company responsible for the
same. For these and the other reasons just given a company is
sometimes forced to disregard entirely the location of a central
station near the center of gravity of the system, and build at a
considerable distance; such a proceeding would, if the distance be
great, necessitate the installation of a high pressure system.
There might, however, be certain local laws in force restricting
the use of high pressure currents on account of the danger
resulting to life, that would prevent this solution of the
problem. In such cases there could undoubtedly be found some site
where the objections previously noted would be tolerated; thus,
there would naturally be little objection to locating next to a
stable, a brewery, or a factory of any description.
=Ques. Why is the matter of water supply important for a central
station?=
Ans. Because, in a steam driven plant, water is used in the boilers
for the production of steam by boiling, and if the engines be of the
condensing type it is also used in them for creating a vacuum into
which the exhaust steam passes so as to increase the efficiency of
the engine above what it would be if the exhaust steam were obliged
to discharge into the comparatively high pressure of the atmosphere.
The force of this will be apparent by considering that the
water consumption of the engine ordinarily is from 15 to 25 lbs.
of "feed water" per horse power per hour, and the amount of
"circulating water" required to maintain the vacuum is about 25 to
30 times the feed water, and in the case of turbines with their 28
or 29 inch vacuum, much more. For instance, a 1,000 horse power
plant running on 15 lbs. of feed water and 30 to 1 circulating
water would require (1,000 × 15) × (30 + 1) = 465,000 lbs. or
55,822 gals. per hour at full capacity.
=Ques. Besides price what other considerations are important with
respect to water?=
Ans. Its quality and the possibility of a scarcity of supply.
It is quite necessary that the water used in the boilers should be
as free as possible from impurities, so as to prevent the deposition
within them of any scale or sediments. The quality of the water used
for condensing purposes, however, is not quite so important, although
the purer it is the better.
If the plant is to be located in a city, the matter of water supply
need not generally be considered, because, as a rule, it can be
obtained from the waterworks; there will then, of course, be a water
tax to consider and this, if large, may warrant an effort being
made to obtain the water in some other way. In any event, however,
the possibility of a scarcity in the supply should be reduced to a
minimum.
If the plant be located in the country, some natural source of water
would be utilized unless the place be supplied with waterworks, which
is not generally the case. It is usual, however, to find a stream,
lake or pond in the vicinity, but if none such be conveniently near,
an artesian or other form of well must be sunk.
If abundance of water exist in the vicinity of the proposed
installation, not only would the location of the plant be governed
thereby, but the kind of power to be used for its operation would
depend thereon. Thus, if the quantity of the water were sufficient
throughout the entire year to supply the necessary power, water
wheels might be installed and used in place of boilers and steam
engines for driving the generators. The station would then, of
course, be situated close to the waterfall, regardless of the center
of gravity of the system.
[Illustration: FIG. 2,712.--View illustrating the location of a
station as governed by the presence of a water falls. In such cases
the natural water power may be at a considerable distance from the
center of gravity of the distribution system because of the saving
in generation. In the case of long distance transmission very
high pressure may be used and a transformer step down sub-station
be located at or near the center of gravity of the system, thus
considerably reducing the cost of copper for the transmission line.]
=Ques. What should be noted with respect to the coal supply?=
Ans. The facility for transporting the coal from the supply point to
the boiler room.
In this connection, an admirable location, other conditions
permitting, is adjacent to a railway line or water front so that
coal delivered by car or boat may be unloaded directly into the
bins supplying the boilers.
If the coal be brought by train, a side or branch track will
usually be found convenient, and this will usually render any
carting of the fuel entirely unnecessary.
In whatever way the coal is to be supplied, the liability of a
shortage due to traffic or navigation being closed at any time of
the year should be well looked into, as should also the facility
for the removal of ashes, before deciding upon the final location
for the plant.
[Illustration: FIG. 2,713.--View of a station admirably located with
respect to transportation of the coal supply. As shown, the coal may
be obtained either by boat or rail, and with modern machinery for
conveying the coal to the interior of the station, the transportation
cost is reduced to a minimum.]
[Illustration: FIG. 2,714.--Floor plan of part of the turbine central
station erected by the Boston Edison Co., showing two 5,000 kw.
Curtis steam turbines in place. The complete installation contains
twelve 5,000 kw. Curtis steam turbines, a sectional elevation being
shown in fig. 2,758, page 1,971.]
=Choice of System.=--The chief considerations in the design of a
central station are economy and capacity. When the current has to be
transmitted long distances for either lighting or power purposes,
economy is attainable only by reducing the weight of the copper
conductors. This can be accomplished only by the use of the high
voltage currents obtainable from alternators.
Again, where the consumers are located within a radius of two miles
from the central station, thereby requiring a transmission voltage of
550 volts or less, dynamos may be employed with greater economy.
Alternating current possesses serious disadvantages for certain
important applications.
For instance, in operating electric railways and for lighting it is
often necessary to transmit direct current at 500 volts a distance
of five or ten miles. In such cases, the excessive drop cannot be
economically reduced by increasing the sizes of the line wire, while
a sufficient increase of the voltage would cause serious variations
under changes of load. Hence, it is common practice to employ some
form of auxiliary generator or booster, which when connected in
series with the feeder, automatically maintains the required pressure
in the most remote districts so long as the main generators continue
to furnish the normal or working voltage.
The advantage of a direct current installation in such cases over a
similar plant supplying alternating current line is the fact that a
storage battery may be used in connection with the former for taking
up the fluctuations of the current, thereby permitting the dynamo to
run with a less variable load, and consequently at higher efficiency.
=Ques. Name some services requiring direct current.=
Ans. Direct current is required for certain kinds of electrolytic
work, such as electro-plating, the electrical separation of
metals, etc., also the charging of storage batteries for electric
automobiles.
[Illustration: FIG. 2,715.--Example of central station located
remote from the distributing center and furnishing alternating
current at high pressure to a sub-station where the current is passed
through step down transformers and supplied at moderate pressure
to the distribution system. In some cases the sub-station contains
also converters supplying direct current for battery charging,
electro-plating, etc.]
=Ques. How is direct current supplied?=
Ans. Sometimes the central station is equipped with suitable
apparatus for supplying both direct and alternating current. This may
be accomplished in several different ways: By installing both direct
and alternating current generators in the central station; by the use
of double current generators or dynamotors, from which direct current
may be taken from one side and alternating current from the other
side; or by installing, in the sub-station of an alternating current
central station, in addition to the transformers usually placed
therein, a rotary converter for changing or converting alternating
current into direct current.
Thus, it is evident that the character of a central station
will be governed to a great extent by the class of services to be
supplied.
An exception to this is where the entire output has to be
transmitted a long distance to the point of utilization.
In such cases a copper economy demands the use of high tension
alternating current, and its distribution to consumers may be made
directly by means of step down transformers mounted near by or
within the consumers' premises, or it may be transformed into low
voltage alternating current by a conveniently located sub-station.
Where the current is to be used chiefly for lighting and there
are only a few or no motors to be supplied, the choice between
direct current and alternating current will depend greatly upon
the size of the installation, direct current being preferable
for small installations and alternating current for large
installations.
If the current is to be used primarily for operating machinery,
such as elevators, travelling cranes, machine tools and other
devices of a similar character, which have to be operated
intermittently and at varying speeds and loads, direct current is
the more suitable; but if the motors performing such work can be
operated continuously for many hours at a time under practically
constant loads, as, for instance in the general work of a pumping
station, alternating current may be employed with advantage.
[Illustration: FIG. 2,716.--Diagram illustrating diversity factor.
By definition _diversity factor = combined actual maximum demand of
a group of customers divided by the sum of their individual maximum
demands_. Example, a customer has fifty (50) watt lamps and, of
course, the sum of the individual maximum demands of the lamps is 2.5
kw. watts ("connected load"). The customer's maximum demand, however,
is 1.5 kw. Hence, the diversity factor[A] of the customer's group of
lamps is 1.5 ÷ 2.5 = .6. In the diagram the ordinates of the curves
show the ratio _maximum demand_ to _connected load_ for various kinds
of electric lighting service in Chicago.]
[A] NOTE.--The diversity factor of a customer's group of lamps,
namely, the ratio of maximum demand to connected load is usually
called the _demand factor_ of the customer.
=Size of Plant.=--Before any definite calculation can be made, or
plans drawn, the engineer must determine the probable load. This is
usually ascertained in terms of the number and distances of lamps
that will be required, by making a thorough canvass of the city or
town, or that portion for which electrical energy is to be supplied.
The probable load that the station is to carry when it begins
operation, the nature of this load, and the probable rate of increase
are matters upon which the design and construction chiefly depend.
[Illustration: FIG. 2,717.--Load curve for one day.]
=Ques. What is the nature of the load carried by a central station?=
Ans. It fluctuates with the time of day and also with the time of
year.
=Ques. How is a fluctuating load best represented?=
Ans. Graphically, that is to say by means of a curve plotted on
coordinate paper of which ordinates represent load values and the
corresponding abscissæ time values, as in the accompanying curves.
=What is the nature of a power load?=
Ans. Where electricity is supplied for power purposes to a number of
factories, the load is fairly steady, dropping, of course, during
meal hours. In the case of traction, the average value of the load is
fairly steady but there are momentarily violent fluctuations due to
starting cars or trains.
[Illustration: FIG. 2,718.--Load curve for one year.]
=Ques. What is the peak load?=
Ans. The maximum load which has to be carried by the station at any
time of day or night as shown by the highest point of the load curve.
=Ques. Define the load factor.=
Ans. The machinery of the station evidently must be large enough to
carry the peak load, and therefore considerably in excess of that
required for the average demand. The ratio of the average to the
maximum load is called the load factor.
There are two kinds of load factor: the annual, and the daily.
The annual load factor is obtained as a percentage by
multiplying the number of units sold (per year) by 100, and
dividing by the product of the maximum load and the number of
hours in the year. The daily load factor is obtained by taking the
figures for 24 hours instead of a year.
[Illustration: FIG. 2,719.--Load curve of plant supplying power for
the operation of motors in a manufacturing district. The horizontal
dotted lines show suitable power ratings. A properly designed steam
plant has a large overload capacity, a hydraulic plant has a small
overload capacity, and a gasoline engine plant has no overload
capacity. Accordingly, the peak of the load (maximum load) may be 25
or 30 per cent. in excess of the rated capacity of a steam plant,
not more than 5 or 10 per cent. in excess of the rated capacity of a
hydraulic plant, not at all in excess of the rated capacity of a gas
engine plant.]
=Ques. What must be provided in addition to the machinery required to
supply the peak load?=
Ans. Additional units must be installed for use in case of repairs or
break down of some of the other units.
EXAMPLE.--What would be the boiler horse power required to
generate 5,000 kw. under the following conditions: Efficiency of
generators 85%; efficiency of engines 90%; feed water of engines
and auxiliaries 15 lbs. per I. H. P.; boiler pressure 175 lbs.;
temperature of feed water 150° Fahr? With a rate of combustion of
15 lbs. of coal per sq. foot of grate per hour and an evaporation
(from and at 212°) of 8 lbs. of water per lb. of coal, what area
of grate would be required and how much heating surface?
5,000 kw. = 5,000 ÷ .746 = 6,702= electrical horse power
To obtain this electrical horse power with alternators whose
efficiency is 85% requires
6,702 ÷ .85 = 7,885 brake horse power at the engine
This, with mechanical efficiency of 90% is equivalent to
7,885 ÷ .9 = 8,761 indicated horse power
Since 15 lbs. of feed water are required for the engines and
auxiliaries per indicated horse power per hour, the total feed
water or evaporation required to generate 5,000 kw. is
15 × 8,761 = 131,415 lbs. per hour.
that is to say, the boilers must be of sufficient capacity
to generate 131,415 lbs. of steam per hour from water at a
temperature of 150° Fahr. This must be multiplied by the _factor
of evaporation_ for steam at 175 lbs. pressure from feed water at
a temperature of 150°, in order to get the equivalent evaporation
"_from and at 212_°."
The formula for the factor of evaporation is
H - _h_
factor of evaporation = ------- (1)
965.7
in which
H = total heat of steam at the observed pressure;
_h_ = total heat of feed water of the observed temperature;
965.7 = latent heat, of steam at atmospheric pressure.
Substituting in (1) values for the observed pressure and
temperature as obtained from the steam table
1,197 - 118
factor of evaporation = ------------ = 1.117
965.7
for which the equivalent evaporation "_from and at 212_°" is
131,415 × 1.117 = 146,791 lbs.= per hour
=FACTORS OF EVAPORATION=
-------------+-----------------------------------------------------+
Temp of | STEAM PRESSURE BY GAUGE |
feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+
Deg. Fahr. | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 |
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
32 |1.214|1.216|1.220|1.222|1.225|1.227|1.229|1.231|1.232|
40 |1.206|1.209|1.212|1.214|1.216|1.219|1.220|1.222|1.224|
50 |1.195|1.197|1.201|1.204|1.206|1.208|1.210|1.212|1.214|
60 |1.185|1.188|1.191|1.193|1.196|1.198|1.200|1.202|1.203|
70 |1.175|1.178|1.180|1.183|1.185|1.187|1.189|1.191|1.193|
80 |1.164|1.167|1.170|1.173|1.175|1.177|1.179|1.181|1.183|
90 |1.154|1.157|1.160|1.162|1.165|1.167|1.169|1.170|1.172|
100 |1.144|1.147|1.150|1.152|1.154|1.156|1.158|1.160|1.162|
110 |1.133|1.136|1.139|1.142|1.144|1.146|1.148|1.150|1.152|
120 |1.123|1.126|1.129|1.131|1.133|1.136|1.138|1.140|1.141|
130 |1.113|1.116|1.118|1.121|1.123|1.125|1.127|1.129|1.130|
140 |1.102|1.105|1.108|1.110|1.113|1.115|1.117|1.119|1.120|
150 |1.091|1.095|1.098|1.100|1.102|1.104|1.106|1.108|1.110|
160 |1.081|1.084|1.087|1.090|1.092|1.094|1.096|1.098|1.100|
170 |1.070|1.074|1.077|1.079|1.081|1.083|1.085|1.087|1.089|
180 |1.060|1.063|1.066|1.069|1.071|1.073|1.075|1.077|1.079|
190 |1.050|1.053|1.056|1.058|1.060|1.063|1.065|1.066|1.068|
200 |1.039|1.043|1.045|1.048|1.050|1.052|1.054|1.056|1.058|
210 |1.029|1.032|1.035|1.037|1.040|1.042|1.044|1.046|1.047|
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
Temp of | STEAM PRESSURE BY GAUGE |
feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+
Deg. Fahr. | 140 | 150 | 160 | 170 | 180 | 190 | 200 | 210 | 220 |
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
32 |1.234|1.236|1.237|1.239|1.240|1.241|1.243|1.244|1.245|
40 |1.226|1.227|1.229|1.230|1.232|1.233|1.234|1.236|1.237|
50 |1.215|1.217|1.218|1.220|1.221|1.223|1.224|1.225|1.226|
60 |1.205|1.207|1.208|1.210|1.211|1.212|1.214|1.215|1.216|
70 |1.194|1.196|1.197|1.199|1.200|1.202|1.203|1.205|1.206|
80 |1.184|1.186|1.187|1.189|1.190|1.192|1.193|1.194|1.195|
90 |1.174|1.176|1.177|1.179|1.180|1.181|1.183|1.184|1.185|
100 |1.164|1.165|1.167|1.168|1.170|1.171|1.172|1.174|1.175|
110 |1.153|1.155|1.156|1.158|1.159|1.160|1.162|1.163|1.164|
120 |1.143|1.145|1.146|1.147|1.149|1.150|1.151|1.153|1.154|
130 |1.132|1.134|1.136|1.137|1.138|1.140|1.141|1.142|1.144|
140 |1.122|1.124|1.125|1.127|1.128|1.129|1.131|1.132|1.133|
150 |1.111|1.113|1.115|1.116|1.118|1.119|1.120|1.121|1.123|
160 |1.101|1.103|1.104|1.106|1.107|1.108|1.110|1.111|1.112|
170 |1.091|1.092|1.094|1.095|1.097|1.098|1.099|1.101|1.102|
180 |1.080|1.082|1.083|1.085|1.086|1.088|1.089|1.090|1.091|
190 |1.070|1.071|1.073|1.074|1.076|1.077|1.078|1.080|1.081|
200 |1.059|1.061|1.063|1.064|1.065|1.067|1.068|1.069|1.071|
210 |1.049|1.051|1.052|1.053|1.055|1.056|1.057|1.059|1.060|
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
Temp. of | STEAM PRESSURE BY GAUGE |
feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+
Deg. Fahr. | 230 | 240 | 250 | 260 | 270 | 280 | 290 | 300 | |
-------------+-----+-----+-----+-----+-----+-----+-----+----+------+
32 |1.246|1.247|1.248|1.250|1.251|1.252|1.253|1.254|
40 |1.238|1.239|1.240|1.241|1.242|1.243|1.244|1.245|
50 |1.228|1.229|1.230|1.231|1.232|1.233|1.234|1.235|
60 |1.217|1.218|1.219|1.220|1.221|1.222|1.223|1.224|
70 |1.207|1.208|1.209|1.210|1.211|1.212|1.213|1.214|
80 |1.196|1.198|1.199|1.200|1.201|1.202|1.203|1.204|
90 |1.186|1.187|1.188|1.189|1.190|1.191|1.192|1.193|
100 |1.176|1.177|1.178|1.179|1.180|1.181|1.182|1.183|
110 |1.166|1.167|1.168|1.169|1.170|1.171|1.172|1.173|
120 |1.155|1.156|1.157|1.158|1.159|1.160|1.161|1.162|
130 |1.145|1.146|1.147|1.148|1.149|1.150|1.151|1.152|
140 |1.134|1.135|1.136|1.137|1.138|1.139|1.140|1.141|
150 |1.124|1.125|1.126|1.127|1.128|1.129|1.130|1.131|
160 |1.113|1.115|1.116|1.117|1.118|1.119|1.120|1.121|
170 |1.103|1.104|1.105|1.106|1.107|1.108|1.109|1.110|
180 |1.093|1.094|1.095|1.096|1.097|1.098|1.099|1.100|
190 |1.082|1.083|1.084|1.085|1.086|1.087|1.088|1.089|
200 |1.072|1.073|1.074|1.075|1.076|1.077|1.078|1.079|
210 |1.061|1.062|1.063|1.064|1.065|1.066|1.067|1.068|
-------------+-----+-----+-----+-----+-----+-----+-----+-----+
One boiler horse power being equal to _an evaporation of_ 34½
_lbs. of water from a feed water temperature of 212° Fahr., into
steam at the same temperature_, the boiler capacity is accordingly
148,105 ÷ 34.5 = 4,293 boiler horse power.
The rate of evaporation is given at 8 lbs. of water (from and at
212° Fahr.), for which the fuel required is
148,105 ÷ 8 = 18,513 lbs. of coal per hour.
For a rate of combustion of 15 lbs. of coal per hour per square
foot of grate,
grate area = 18,513 ÷ 15 = 1,234 sq. ft.
For stationary boilers the usual ratio of heating surface to
grate area is 35:1, accordingly the heating surface corresponding
to this ratio is
1,234 × 35 = 43,190 sq.ft.
The above calculation is based on a rate of evaporation of 8
lbs. of water per lb. of coal and a rate of combustion of 15 lbs.
of coal per sq. ft. of grate. For other rates the required grate
area may be obtained from the following table:
----------------------------------------------------------------------
GRATE SURFACE PER HORSE POWER (KENT)
------------+------+-----+--------------------------------------------
|Pounds| |
| of | Lbs.| Pounds of coal burned per square foot of
|water | of | grate per hour
| from | coal+----+----+----+----+----+----+----+----+----
|and at| per | | | | | | | | |
| 212° | h.p.| 8 | 10 | 12 | 15 | 20 | 25 | 30 | 35 | 40
| per | per | | | | | | | | |
|pound | hour+----+----+----+----+----+----+----+----+----
| of | | Square feet grate per horse power
| coal | |
------------+------+-----+----+----+----+----+----+----+----+----+----
Good coal | }10 | 3.45| .43|.35 | .28| .23| .17| .14| .11| .10| .09
and boiler | } 9 | 3.83| .48| .38| .32| .25| .19| .15| .13| .11| .10
| | | | | | | | | | |
Fair coal |{8.61 | 4. | .50| .40| .33| .26| .20| .16| .13| .12| .10
or boiler |{8 | 4.31| .54| .43| .36| .29| .22| .17| .14| .13| .11
|{7 | 4.93| .62| .49| .41| .33| .24| .20| .17| .14| .12
| | | | | | | | | | |
Poor coal |{6.9 | 5. | .63| .50| .42| .34| .25| .20| .17| .15| .13
or boiler |{6 | 5.75| .72| .58| .48| .38| .29| .23| .19| .17| .14
|{5 | 6.9 | .86| .69| .58| .46| .35| .28| .23| .22| .17
| | | | | | | | | | |
Lignite and|}3.45 |10. |1.25|1.00| .83| .67| .50| .40| .33| .29| .25
poor boiler|} | | | | | | | | | |
------------+------+-----+----+----+----+----+----+----+----+----+----
=General Arrangement of Station.=--In designing an electrical
station, it is preferable that whatever rooms or divisions of the
interior space are desired should determine the total outside
dimensions of the plant in the original plans of the building than
that these latter dimensions be fixed and the rooms, etc., be fitted
in afterward.
=SAVING DUE TO HEATING THE FEED WATER=
Table showing the percentage of saving for each degree of increase in
temperature of feed water heated by waste steam.
----------------------------------------------------------------------
Init| Pressure of steam in boiler, lbs. per sq. inch above atmosphere
temp|-----------------------------------------------------------------
of |
feed| 0 | 20 | 40 | 60 | 80 | 100 | 120 | 140 | 160 | 180 | 200
----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
32°|.0872|.0861|.0855|.0851|.0847|.0844|.0841|.0839|.0837|.0835|.0833
40 |.0878|.0867|.0861|.0856|.0853|.0850|.0847|.0845|.0843|.0841|.0839
50 |.0886|.0875|.0868|.0864|.0860|.0857|.0854|.0852|.0850|.0848|.0846
60 |.0894|.0883|.0876|.0872|.0867|.0864|.0862|.0859|.0856|.0855|.0853
70 |.0902|.0890|.0884|.0879|.0875|.0872|.0869|.0867|.0864|.0862|.0860
80 |.0910|.0898|.0891|.0887|.0883|.0879|.0877|.0874|.0872|.0870|.0868
90 |.0919|.0907|.0900|.0895|.0888|.0887|.0884|.0883|.0879|.0877|.0875
100 |.0927|.0915|.0908|.0903|.0899|.0895|.0892|.0890|.0887|.0885|.0883
110 |.0936|.0923|.0916|.0911|.0907|.0903|.0900|.0898|.0895|.0893|.0891
120 |.0945|.0932|.0925|.0919|.0915|.0911|.0908|.0906|.0903|.0901|.0899
130 |.0954|.0941|.0934|.0928|.0924|.0920|.0917|.0914|.0912|.0909|.0907
140 |.0963|.0950|.0943|.0937|.0932|.0929|.0925|.0923|.0920|.0918|.0916
150 |.0973|.0959|.0951|.0946|.0941|.0937|.0934|.0931|.0929|.0926|.0924
160 |.0982|.0968|.0961|.0955|.0950|.0946|.0943|.0940|.0937|.0935|.0933
170 |.0992|.0978|.0970|.0964|.0959|.0955|.0952|.0949|.0946|.0944|.0941
180 |.1002|.0988|.0981|.0973|.0969|.0965|.0961|.0958|.0955|.0953|.0951
190 |.1012|.0998|.0989|.0983|.0978|.0974|.0971|.0968|.0964|.0062|.0960
200 |.1022|.1008|.0999|.0993|.0988|.0984|.0980|.0977|.0974|.0972|.0969
210 |.1033|.1018|.1010|.1003|.0998|.0994|.0990|.0987|.0984|.0981|.0979
220 | -- |.1029|.1019|.1013|.1008|.1004|.1000|.0997|.0994|.0991|.0989
230 | -- |.1039|.1031|.1024|.1018|.1012|.1010|.1007|.1003|.1001|.0999
240 | -- |.1050|.1041|.1034|.1029|.1024|.1020|.1017|.1014|.1011|.1009
250 | -- |.1062|.1052|.1045|.1040|.1035|.1031|.1027|.1025|.1022|.1019
----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
NOTE.--An approximate rule for the conditions of ordinary
practice is a saving of 1 per cent. made by each increase of
11° in the temperature of the feed water. This corresponds to
.0909 per cent. per degree. The calculation of saving is made as
follows: Boiler pressure, 100 lbs. gauge; total heat in steam
above 32° = 1,185 B.T.U. feed water, original temperature 60°,
final temperature 209°F. Increase in heat units, 150. Heat units
above 32° in feed water of original temperature = 28. Heat units
in steam above that in cold feed water, 1,185-28 = 1,157. Saving
by the feed water heater = 150 ÷ 1,157 = 12.96 per cent. The
same result is obtained by the use of the table. Increase in
temperature 150° × tabular figure .0864 = 12.96 per cent. Let
total heat of 1 lb. of steam at the boiler pressure = H; total
heat of 1 lb. of feed water before entering the heater = _h'_, and
after passing through the heater = _h''_; then the saving made by
the heater is (_h''_-_h'_) ÷ (H-_h'_).
Under usual conditions the plans of an electrical station are readily
drawn, as they are generally of a simple nature. The engines and
generators will occupy the majority of the space, and these are
usually placed in one large room; in some stations, however, they are
located respectively in two adjacent rooms. The boilers are generally
located in a room apart from the engines and dynamos, and in some
cases a separate building is provided for them; the pumps, etc., must
be installed not far from the boilers, and space must also be allowed
near the boilers for coal and ashes.
[Illustration: FIG. 2,720.--Floor plan of an electrical station
having a belted drive with counter shaft.]
Fig. 2,720 shows the floor plan of an electrical station, in
which a countershaft and belted connections are used between
the engines and generators. Referring first to the plan of the
building itself, A represents the engine and dynamo room, B
denotes the boiler room, C the office, D the store room, and E the
chimney connected with the boilers by means of the uptake _w_.
Referring next to the apparatus installed, S, S, S, S represents
a battery of four boilers; these are connected by steam piping
VV to the two steam engines, M and M, which are belted to the
countershaft O. Belted to the countershaft are the generators,
T, T, T, T, the circuits from which are controlled on the
switchboard, H.
=Ques. What are the objections to the arrangement shown in
fig. 2,720.=?
Ans. The large space required by the belt drive especially in
locations where land is expensive. Another objection is the
frictional loss due to the belt drive with its countershaft, etc.
[Illustration: FIG. 2,721.--Elevation of station having a belted
drive with countershaft, as shown in plan in fig. 2,720.]
=Ques. What are the desirable features of the belt drive?=
Ans. High speed generators may be used, thus reducing the first
cost, and the multiplicity of speeds and flexibility of the system
resulting from the use of a friction clutch.
Thus in fig. 2,720, each pulley may be mounted on the counter
shaft O with a friction clutch. A jaw clutch may also be provided
at Z, thus permitting the shaft O to be divided into two sections.
It is therefore possible by this arrangement to cause either of
the engines to drive any one of the generators, or all of them, or
both of the engines to drive all of the generators simultaneously.
=Ques. Under what condition is the counter shaft belt drive
particularly valuable?=
Ans. In case of a break down of any one of the engines or generators,
and also when it becomes necessary to clean them without interrupting
the service.
[Illustration: FIG. 2,722.--Plan of station arranged for extension.
The space required for a central station depends upon the number and
kind of lights to be supplied, and upon the character and arrangement
of the machinery. In calculating the size of building required, two
things must be carefully considered: first, the building must be
adapted to the plant to be installed in the beginning; and second, it
must be arranged so that enlargement can be made without disarranging
or interfering with the plant already in existence. This is usually
best secured by providing for expansion in one or two definite
directions, the building being made large enough to accommodate
additional units that will be necessary at some future time because
of the growth of the community and consequent increased demand for
electric current.]
=Ques. How may the design in fig. 2,720 be modified for the
installation of a storage battery?=
Ans. If a storage battery be necessary, a partition may be
constructed across the room A, as indicated by the dotted lines, and
the battery installed in the room thus formed.
[Illustration: FIG. 2,723.--Interior of old Riverside station
showing at the right, seven 6,000 horse power alternators driven by
reciprocating engines, and at the left, a number of turbine units
aggregating 90,000 horse power.]
=Ques. Mention a few details in the general arrangement of the
building fig. 2,720.=
Ans. Two doors to the room A may conveniently be provided at K and L,
the former connecting with the boiler room B, and the latter serving
as the main entrance to the station. There is little that need be
added to what has already been stated regarding the boiler room B.
The door at F provides for the entrance of coal and the removal of
ashes, while at P, the pump and heaters may conveniently be located.
In the office C, visitors may be received, the station reports made
out, bulletins issued from time to time, and whatever engineering
problems arise may here be solved on paper by the engineer in
charge of the plant. The store room D will be found convenient for
various supplies, tools and appliances needed in the operation of
the station. These may here be kept under lock and key and the daily
waste and loss resulting from carelessness avoided.
=Ques. What important point should be noted in locating the engines
and boilers?=
Ans. They should be so placed that the piping between them will be as
short and direct as possible.
=Ques. Why?=
Ans. The steam pipe should be short to reduce the loss of heat
between engine and boiler to a minimum, and both short and direct to
avoid undue friction and consequent drop in pressure of the steam in
passing through the pipe to the engine.
Entirely too little attention is given to this matter on the
part of designers and it cannot be too strongly emphasized that,
for economy, the steam pipe between an engine and boiler should
be as short and direct as possible, having regard of course, for
proper piping methods.
=Ques. What should be provided for the steam pipe?=
Ans. A heavy covering of approved material should be placed around
the pipe to reduce the loss of heat by radiation. For this purpose
hair felt, mineral wool and asbestos are used.
[Illustration: FIG. 2,724.--View of engine and condenser, showing how
to arrange the piping to secure good vacuum. _Locate the condenser as
near the engine as possible_; =use easy bends= _instead of elbows;
place the pump_ =below= _bottom of condenser so the water will
drain to pump_. At A is a relief valve, for protection in case the
condenser become flooded through failure of the pump, and at B is
a gate valve to shut off condenser in case atmospheric exhaust is
desired to permit repairs to be made to condenser during operation.
=A water seal= should be maintained on the relief valve and =special
attention= _should be given to the stuffing box_ of the gate valve
=to prevent air leakage=. _The discharge valve of the pump should be
water sealed._]
=Ques. How should the piping be arranged between the engine and
condenser, and why?=
Ans. It should be as short and direct as possible; especially should
elbows be avoided so that the back pressure on the engine piston will
be reduced as near as can be to that of the condenser.
That is to say, in order to get nearly the full effect of the
vacuum in the condenser the frictional resistance of the piping
should be reduced to a minimum.
Where 90° turns are necessary, easy bends should be used instead
of sharp elbows. The force of this argument must be apparent by
noting the practice of steam turbine builders of placing the
turbine right up against the condenser, and remembering that a
high vacuum is necessary to the economical working of a turbine.
See fig. 1,445, page 1,182.
=Ques. What are the considerations respecting the number and type of
engine to be used?=
Ans. In the illustration fig. 2,720, two engines M and M' are
employed, one belted to each end of the countershaft O. These engines
should be of similar or identical pattern; for a small output
they may be either simple or compound, as the conditions of fuel
expenditure may dictate, but if the output be large, triple expansion
engines or turbines are advisable.
[Illustration: FIG. 2,725.--"Dry pipe" for horizontal boiler: it is
connected to the main outlet and its upper surface is perforated with
small holes, the far end being closed. With this arrangement steam is
taken from the boiler over a large area, so that it will contain very
little moisture. _All horizontal boilers without a dome should be
fitted with a dry pipe;_ most engineers do not realize the importance
of obtaining dry steam for engine operation.]
Corliss or similar slow speed engines may advantageously be used
in either case. In all cases the engine should be run condensing
unless the cost for circulating water is prohibitive; even in such
cases cooling towers may be installed and effect a saving.
In operation, during the greater part of the day, one engine
running two or perhaps three of the generators, will carry the
load, but when the load is particularly heavy, as in the morning
and evening, both engines and all the generators may be required
to meet the demands.
[Illustration: FIG. 2,726.--Method of connecting a header to a
battery of boilers. Where two or more boilers are connected to a
single header, the use of a reliable non-return boiler stop valve is
necessary, and in some countries their installation is compulsory. A
non-return boiler stop valve will instantly close should the pressure
in the boiler to which it is attached suddenly decrease below that
in the header, and thereby prevent the entrance of steam from the
other boilers of the battery. This sudden decrease in pressure may be
caused by a ruptured fitting or the blowing out of a tube, in which
event an ordinary stop valve taking the place of a non-return boiler
stop valve would be inadequate, as the loss of steam from the other
boilers of the battery would be tremendous before an ordinary valve
could be reached and closed, assuming that it would be possible to do
so, which in the majority of cases it is not. Should it be desired
to cut out a boiler for cleaning or repairs, the non-return boiler
stop valve will not permit steam to enter the boiler from the header,
even should the handwheel be operated for this purpose, as it cannot
be opened by hand, but can, however, be closed. A non-return boiler
stop valve should be attached to each boiler and connected to an
angle valve on the header. A pipe bend should be used for connecting
the valves, as this will allow for expansion and contraction. The
pipe should slope a trifle downward toward the header and a suitable
drain provided. This drain should be opened and all water permitted
to escape before the angle valve is opened, thereby preventing any
damage due to water hammer.]
By exercising a little ingenuity in shifting the load on
different machines at different times, both engines and dynamos,
may readily be cleaned and repaired without interrupting the
service.
=Ques. For economy what kind of steam should be used?=
Ans. Super-heated steam.
The saving due to the use of superheated steam is about 1% for
every ten degrees Fahr. of super-heat. It should be used in all
cases.
=Ques. How should the machines be located?=
Ans. Sufficient space should be allowed between them that cleaning
and repairing may be done easily, quickly and effectually.
[Illustration: FIGS. 2,727 and 2,728.--Method of preventing vibration
and of supporting pipes. The figures show top and side views of
a main header carried in suitable frames fitted with adjustable
roller. While the pipe is illustrated as resting on the adjustable
rollers, nevertheless the rollers may also be placed at the sides
or on top of the pipe to prevent vibration, or in cases where the
thrust from a horizontal or vertical branch has to be provided for.
This arrangement will take care of the vibration without in any way
preventing the free expansion and contraction of the pipe.]
=Ques. How should the switchboard be located?=
Ans. In fig. 2,720, the switchboard H is mounted against the wall
dividing the room A from the room B, and is in line with the machines.
The advantages arising from a switchboard thus installed are,
that the switchboard attendant working thereon can obtain at any
time an unobstructed view of the performance of each individual
machine, and he has in consequence a much better control of them;
then, too, while he is engaged at the engines or generators he
can also see the measuring instruments on the switchboard, and
ascertain approximately the readings upon them.
In cases of emergency it is sometimes necessary for the
engineer in charge of a plant to be in several places at the same
time in order to prevent an accident, and that this seemingly
impossibility may be approximated as nearly as possible, it is
essential that the controlling devices be located as closely
together as is consistent, and that no moving belt or pulley
intervene between them.
These conditions are well satisfied in fig. 2,720, and owing to
the short distances between the generators and the switchboard the
drop of voltage in each of the conducting wires between them will
be low.
This latter advantage is worthy of notice in a station
generating large currents at a low pressure. To offset the
advantages just mentioned, the location of the switchboard in
line with the machines introduces an element of danger to the
switchboard, its apparatus, and the attendant, on account of the
possible bursting of a flywheel or other parts of the machines
from centrifugal force.
[Illustration: FIGS. 2,729 and 2,730.--Points on placing stop valves.
The first and most important feature is to ascertain whether the
valve will act as a water trap for condensed steam. Fig. 2,729
illustrates a common error in the placing of valves, as this
arrangement permits of an accumulation of condensed steam above the
valve when closed, and should the engineer be careless and open the
valve suddenly, serious results might follow owing to water-hammer.
Fig. 2,730 illustrates the correct method of placing the valve. It
sometimes occurs, however, that it is not convenient to place the
valve as shown in fig. 2,730 and that fig. 2,729 is the only manner
in which the valve can be placed. In such cases, the valve should
have a drain, and this drain should always be opened before the large
valve is opened.]
If the switchboard be placed in the dotted position at H', or,
in fact, at the opposite end of the room A, the damage to life
and property that might result from the effects of centrifugal
force would be eliminated, but in place thereof would be the
disadvantages of an obstructed view of the machines from the
switchboard, an obstructed view of the switchboard from the
machines, inaccessibility between these two, and a greater drop
of voltage in the majority of the conducting wires between the
generators and the switchboard.
=Ques. Describe a second arrangement of station with belt drive and
compare it with the design shown in fig. 2,720.=
[Illustration: FIG. 2,731.--Plan of electrical station with belt
drive without counter shaft. The installation here represented
consists of two boilers, S, etc., and three sets of engines and
generators, T, M, etc. Sufficient allowance has been made in the
plans, however, for future increase of business, as additional space
has been provided for an extra engine and generator set, as indicated
by the dotted lines.]
Ans. A floor plan somewhat different from that presented in fig.
2,720 is shown in fig. 2,731. Here a belt drive is employed, but no
countershaft is used. Each generator, therefore, is dependent upon
its respective engine, and in consequence the flexibility obtained by
the use of a countershaft is lost. On the other hand, there is less
loss of mechanical power between the engines and generators in the
driving of the latter, and less floor space is necessary in the room
A. If, however, the floor area of this room be made the same as in
the previous arrangement and the same number of machines are to be
installed, they may be spaced further apart, affording in consequence
considerably more room for cleaning and repairing them.
In operation, the normal conditions should be such that any two
of the engine and generator sets may readily carry the average
load, the third set to be used only as a reserve either to aid the
other two when the load is unusually heavy or to replace one of
the other sets when it becomes necessary to clean or repair the
latter.
The switchboard may perhaps be best located at H, as a similar
position on the opposite side of the room A would bring it beneath
one or more of the steam pipes and thus endanger it should a
possible leakage occur from these pipes. If located at H, however,
it will be in line with the machines, and therefore will be
subject to the disadvantages previously mentioned for such cases;
consequently it might be as well to place it at the further end
of the room, either against the partition (shown dotted) of the
storage battery room if this be built, or else (if no storage
battery is to be installed), against the end wall itself. The
nearer end of the room A would not be very desirable for the
switchboard installation on account of being so far removed from
the machines, and therefore more or less inaccessible from them.
Outside of what has now been mentioned, the division of the floor
plan and the arrangement therein is practically the same as in
fig. 2,720, accordingly what has already been stated regarding the
former installation applies, therefore, with equal force to the
present installation.
=Ques. Describe a plant with direct drive.=
Ans. This type of drive is shown in fig. 2,732. Each engine is
directly connected to a generator, that is, the main shafts of both
are joined together in line so that the generator is driven without
the aid of a belt.
=Ques. What is the advantage of direct drive?=
Ans. The great saving in floor space, which is plainly shown in fig.
2,732, the portion A' representing the saving which results over the
installations previously illustrated in figs. 2,720 and 2,731.
=Ques. How could the floor space be further reduced?=
Ans. By employing vertical instead of horizontal engines.
=Ques. What should be done before drawing the plans for the station?=
Ans. The types of the various machines and apparatus to be installed
should, as nearly as possible, be selected in advance so that their
approximate dimensions may serve as a guide in drawing up the plans
of the building.
[Illustration: FIG. 2,732.--Plan of electrical station containing
direct connected units. As shown, space is provided for an extra
boiler and engine and generator set, as indicated by the dotted
lines. Space also exists for a storage battery room if necessary, and
the partition dividing this room from the engine and dynamo room is
shown by a dotted line as in previous cases.]
Owing to the great difference in these dimensions for the
various types, and in fact for the same types as manufactured by
different concerns, no definite rules regarding the necessary
space required can here be given. In a general way, however, the
author has endeavoured to indicate by the drawings the relative
amounts of space that ordinarily would be considered sufficient.
=Ques. What is the disadvantage of direct drive?=
Ans. A more expensive generator is required because it must run at
the same speed as the engine, which is relatively low as compared
with that of a belted generator.
=Station Construction.=--The construction or rearrangement of the
building intended for the plant is a problem that under ordinary
conditions would be solved by an architect, or at least by an
architect with the assistance of an electrical or mechanical
engineer, still there are many installations where the electrical
engineer has been compelled to design the building.
In such instances he should be equipped with a general knowledge of
the construction of buildings.
=Foundations.=--The foundation may be either natural or artificial;
that is, it may be composed of rock or soil sufficiently solid
to serve the purpose unaided, or it may be such as to require
strengthening by means of wood or iron beams, etc. In either case any
tendency toward a considerable settling or shifting of the foundation
due to the action of water, frost, etc., after the station has been
completed must be well guarded against. To this end special attention
should be given to the matter of drainage.
=Ques. How should the foundation be constructed for the machines?=
Ans. The foundations constructed for the machines should be entirely
separate from that built for the walls of the building, so that the
vibrations of the former will not affect the latter.
If there be several engines and dynamos to be installed, it is
best to construct two foundations, one for the engines and one for
the dynamos. If, however, there be considerable distance between
the units, it may be advisable to build a separate foundation
for each engine and for each dynamo. The material of which these
foundations are composed should if the machines be of 20 horse
power or over, possess considerable strength and be impervious to
moisture. Brick, stone and concrete are desirable for the purpose,
and only the best quality of cement mortar should be employed.
Care must be taken that lime mortar is not used in place of
cement mortar, as the former is not well adapted to withstand the
vibrations of the machines without crumbling.
[Illustration: FIG. 2,733.--Angle for foundation footing. In ordinary
practice the footing courses upon which the walls of the building
proper rest, consist of blocks or slabs of stone as large as are
available and convenient to handle. Footings of brick or concrete are
also used in very soft soils; footings consisting of timber grillage
are often employed. A grillage of iron or steel beams has also
been used successfully. The inclination of the angle φ, of footing
should be about as follows: for metal footings 75°; for stone, 60°;
for concrete, 45°; for brick, 30°. Damp proof courses of slate, or
layer of asphalt are laid in or on the foundations or lower walls to
prevent moisture arising or penetrating by capillary attraction.]
=Ques. Describe a method of constructing foundations.=
Ans. An excavation is made to the desired depth and a form inserted
corresponding to the desired dimensions for the foundation. A
template is placed on top locating all the centers, with iron
pipes suspended from these centers, two or three sizes larger than
the anchor bolts. At the lower end of the pipes are core boxes.
Concrete is poured into the mould thus formed, and when hard, the
forms are removed thus leaving the solid foundation. The anchor
bolts are inserted through the pipes and passed through iron plates
at the lower end as shown in fig. 2,734, being secured by nuts. By
using pipe of two or three bolt diameters a margin is provided for
adjustment so the bolts will pass through the holes in the frame of
the machine thus allowing for any slight errors in laying out the
centers on the template.
[Illustration: FIG. 2,734.--Concrete foundation showing method of
installing the anchor bolts.]
=Ques. What is the object of the openings in the bottom of the
foundation?=
Ans. In case of a defective bolt, it may be replaced by a new one
without injury to the foundation.
=Walls.=--Regarding the material for the walls of the station iron,
stone, brick and wood may be considered. Of these, iron in the form
of sheets or plates would be entirely fireproof, but being itself
a conductor would introduce difficulties in maintaining a high
insulation resistance of the current carrying circuits; it would
also make the building difficult to heat in winter and to keep cool
in summer. Stone in the form of limestone, granite or sandstone, as
a building material is desirable for solidity and attractiveness;
it is also fireproof and an insulator, but the high cost of such a
structure for an electrical station usually prohibits its use except
in private plants or in electrical stations located in large cities.
[Illustration: FIG. 2,735.--View showing part of template for
locating anchor bolt centers, pipes through which the bolts pass
and bolt boxes at lower end of bolts. The completed foundation is
shown in fig. 2,734, with template removed. The template is made of
plain boards upon which the center lines are drawn, and bolt center
located. Holes are bored at the bolt centers to permit insertion of
the pipes as shown.]
Brick is a good material and is readily obtained in nearly all parts
of the country; it is comparatively cheap, and is also an insulating
and fireproof material. The bricks selected for this purpose should
possess true sharp edges, and be hard burned.
=Ques. What are the features of wood?=
Ans. Wood forms the cheapest material that can be used for the walls
of electrical stations, and it usually affords satisfaction, but has
the disadvantage of high fire risk.
=Roofs=.--In fig. 2,736 is shown one form of construction for the
roof of an electrical station. The end view here presented shows
the upper portion of the walls at B and D; these support the iron
trusses C, and the roof proper MN. In many stations there is provided
throughout the length of the building, a monitor or raised structure
on the peak of the roof for ventilation and light. The end view of
the monitor is shown at S in the figure; its sides should be fitted
with windows adjustable from the floor.
[Illustration: FIG. 2,736.--One form of roof construction.]
=Floors.=--The floor of the station should be so designed that
it will be capable of supporting a reasonable weight, but as the
weights of the machines are borne entirely by their respective
foundations the normal weight upon the floor will not be great; for
short periods, however, it may be called upon to support one or two
machines while they are being placed in position or interchanged,
and due allowance must be made for such occurrences.
Station floors for engine and dynamo rooms are, as a rule,
constructed of wood. Where very high currents are generated, however,
insulated floors of special construction mounted on glass are
necessary as a protection from injurious shocks. Brick, concrete,
cement, and other substances of a similar nature are objectionable
as a floor material for engine and dynamo rooms on account of the
grit from them, caused by constant wear, being liable to get into the
bearings of the machines.
Where there are no moving parts, however, as in the boiler room, the
materials just mentioned possess no disadvantages and are preferable
to wood on account of being fireproof.
=THEORETICAL DRAFT PRESSURE IN INCHES OF WATER IN=
=A CHIMNEY 100 FEET HIGH=
(For other heights the draft varies directly as the height)
Temp. in TEMP. OF EXTERNAL AIR. (BAROMETER 30 INCHES)
Chimney, °F. 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100°
200° .453 .419 .384 .353 .321 .292 .263 .234 .209 .182 .157
220 .488 .453 .419 .388 .355 .326 .298 .269 .244 .217 .192
240 .520 .488 .451 .421 .388 .359 .330 .301 .276 .250 .225
260 .555 .528 .484 .453 .420 .392 .363 .334 .309 .282 .257
280 .584 .549 .515 .482 .451 .422 .394 .365 .340 .313 .288
300 .611 .576 .541 .511 .478 .449 .420 .392 .367 .340 .315
320 .637 .603 .568 .538 .505 .476 .447 .419 .394 .367 .342
340 .662 .638 .593 .563 .530 .501 .472 .443 .419 .392 .367
360 .687 .653 .618 .588 .555 .526 .497 .468 .444 .417 .392
380 .710 .676 .641 .611 .578 .549 .520 .492 .467 .440 .415
400 .732 .697 .662 .632 .598 .570 .541 .513 .488 .461 .436
420 .753 .718 .684 .653 .620 .591 .563 .534 .509 .482 .457
440 .774 .739 .705 .674 .641 .612 .584 .555 .530 .503 .478
460 .793 .758 .724 .694 .660 .632 .603 .574 .549 .522 .497
480 .810 .776 .741 .710 .678 .649 .620 .591 .566 .540 .515
500 .829 .791 .760 .730 .697 .669 .639 .610 .586 .559 .534
=Chimneys.=--These are generally constructed of brick and iron,
sometimes of concrete. Iron chimneys cost less than brick chimneys,
necessitate less substantial foundations, and are free from the
liability of cracking. They must be painted to prevent corrosion, are
less substantial, and lose considerably more heat by radiation than
do brick chimneys.
[Illustration: FIG. 2,737.--An example of direct connected unit with
gas engine power. The view shows a Westinghouse 200 kva., 4,000 volt,
three phase, 60 cycle alternator direct connected to a gas engine.]
[Illustration: FIG. 2,738.--Curves showing comparative costs of
chimney and mechanical draft. In certain of these, the cost of the
existing chimney is known, and that of the complete mechanical draft
plant is estimated, while in others, the cost of mechanical draft
installation is determined from the contract price, and the expense
of a chimney to produce equivalent results is calculated. Costs are
shown for both single, forced and induced engine driven fans and
for duplex engine driven plants, in which either fan may serve as a
relay. An apparatus of the latter type is the most expensive, and
finds its greatest use where economizers are employed.]
Both brick and iron chimneys, require an inner wall or lining of
brick, which forms the flue proper, and in order that this wall be
not cracked by sudden cooling an air space is left between it and
the outer wall. In a brick chimney the inner wall need not extend
much beyond half the height of the chimney, but when iron is used it
should reach to the top.
=Ques. Upon what does the force of natural draught in a chimney
depend?=
Ans. It depends upon the difference between the weight of the column
of hot gases inside the chimney and the weight of a like column of
the cold external air.
[Illustration: FIGS. 2,739 and 2,740.--Substituting mechanical
draught in place of chimney. The relative proportions of a brick
chimney, and of the smoke pipe required when mechanical draft is
introduced are forcibly shown in the illustrations, which show the
works of the B.F. Sturtevant Co., at Jamaica Plain, Mass. The removal
of the boilers to a position too far distant from the existing
chimney to permit of its longer fulfilling its office, led to the
substitution of an induced draft fan and the subsequent removal of
the chimney. The present stack or smoke pipe, barely visible in fig.
2,740, extends only 31 feet above the ground, and no trouble is
experienced from smoke.]
=Ques. How is the intensity of the draught expressed?=
Ans. In terms of the number of inches of a water column sustained by
the pressure produced.
=Ques. Are high chimneys necessary?=
Ans. No.
_Chimneys above 150 feet in height are very costly, and their
increased cost is not justified by increased efficiency._
[Illustrations: FIGS. 2,741 to 2,744.--Installation of forced draft
system to old boiler plant. The figures illustrate the simplest
method. The fan which is of steel plate with direct connected double
cylinder engine, is placed immediately over the end of a brick duct
into which the air is discharged. This duct is carried under ground
across the front of the boilers, to the ash pits of each of which
connection is made through branch ducts. Each branch duct opening
is provided with special ash pit damper, operated by notched handle
bar, as illustrated in the detail. This method of introduction
serves to distribute the air within the ash pit, and to secure even
flow through the fuel upon the grate above. Of course, the ash pit
doors must remain closed in order to bring about this result. A
chimney of sufficient height to merely discharge the gases above
objectionable level is all that is absolutely necessary with this
arrangement. Although the introduction of a fan in an old plant is
usually evidence of the insufficiency of the existing chimney to meet
the requirements, such a chimney, will, however, usually serve as a
discharge pipe for the gases when the fan is employed. The fan thus
becomes more than a mere auxiliary to the chimney; it practically
supplants it so far as the method of draught production is concerned.]
The latest chimney practice is to build two or more small
chimneys instead of one large one. A notable example is the
Spreckels Sugar Refinery in Philadelphia, where three separate
chimneys are used for one boiler plant of 7,500 horse power. The
three chimneys are said to have cost several thousand dollars less
than an equivalent single chimney.
=Very tall chimneys= have been characterized by one writer as
"_monuments to the folly of their builders._"
[Illustration: FIGS. 2,745 and 2,746.--Comparison of chimney draft
and mechanical draft. The illustrations show a plant of 2,400 H.P. of
modern water tube boilers, 12 in number, set in pairs and equipped
with economizers. Fig. 2,745 indicates the location of a chimney, 9
feet in internal diameter by 180 feet high, designed to furnish the
necessary draft; fig. 2,746 represents the same plant with a complete
duplex induced draught apparatus substituted for the chimney, and
placed above the economizer connections. Each of the two fans is
driven by a special engine, direct connected to the fan shaft, and
each is capable of producing draft for the entire plant. A short
steel plate stack unites the two fan outlets and discharges the gases
just above the boiler house roof. All of the room necessary for the
chimney is saved, and no valuable space is required for the fans.]
=Ques. How is mechanical draft secured?=
Ans. In two ways, known respectively as _induced draught_ and _forced
draught_.
=Ques. Describe the method of induced draft.=
Ans. A fan is located in the smoke flue, and which in operation
draws the gases through the furnace and discharges them into a
_short_ chimney.
=Ques. Describe the method of forced draft.=
Ans. In this method, air is forced into the furnace underneath the
grate bars by means of a fan or a steam jet blower.
[Illustration: FIG. 2,747.--Forced draft plant with hollow bridge
wall at the Crystal Water Co., Buffalo, N. Y. The air is delivered to
the ash pit via the hollow bridge wall, being supplied under pressure
by the blower seen at the side of the boiler setting. As shown, the
blower is operated by a small reciprocating engine; however, compact
blowing units with steam turbine drive can be had and which are
designed to be placed in the boiler setting.]
=Ques. What is the application of the two systems?=
Ans. Induced draft is installed mostly in new plants, while forced
draft is better adapted to old plants.
=Steam Turbines=.--It is not the author's intention to discuss at
length the steam end of the electric plant, because too much space
would be required, and also because the subject belongs properly
to the field of mechanical engineering rather than electrical
engineering. However, because of the recent introduction of the steam
turbine for the direct driving of large generators, and the fact
that it is now almost universally used in large central stations, a
detailed explanation of its principles and construction may not be
out of place.
[Illustration: FIG. 2,748.--Longitudinal section of elementary
Parsons type steam turbine. The turbine consists essentially of a
fixed casing, or cylinder, and a revolving spindle or drum. The ends
of the spindle are extended in the form of a shaft, carried in two
bearings A and B, and, excepting the small parts of the governing
mechanism and the oil pump, these bearings are the only rubbing
parts in the entire turbine. Steam enters from the steam pipe at C
and passes through the main throttle or regulating valve D, which,
as actually constructed, is a balanced valve. This valve is operated
by the governor through suitable controlling mechanism. The steam
enters the cylinder through the passage E and, turning to the left
passes through alternate stationary and revolving rows of blades,
finally emerging from them at F and flowing through the connection
G to the condenser or to the atmosphere, depending upon whether the
turbine is condensing or non-condensing. Each row of blades, both
stationary and revolving, extends completely around the turbine and
the steam flows through the full annulus between the spindle and
the cylinder. In an ideal turbine the lengths of the blades and the
diameter of the spindle which carries them would continuously and
gradually increase from the steam inlet to the exhaust. Practically,
however, the desired effect is produced by making the spindle in
steps, there being generally three such steps or stages, H, J and K.
The blades in each step are arranged in groups of increasing length.
At the beginning of each of the larger steps, the blades are usually
shorter than at the end of the preceding smaller step, the change
being made in such a way that the correct relation of blade length
to spindle diameter is secured. The steam, acting as previously
described, produces a thrust tending to force the spindle toward
the left, as seen in the cut. This thrust, however, is counteracted
by the "balance pistons," L, M and N, which are of the necessary
diameter to neutralize the thrust on the spindle steps, H, J and K,
respectively. These elements are called "pistons" for convenience,
although they do not come in contact with the cylinder, but both
the pistons and the cylinder are provided with alternate rings
which form a labyrinth packing to retard the leakage of steam. In
order that each balance piston may have the proper pressure on both
sides, equalizing passages O, P and Q are provided connecting the
balance pistons with the corresponding stages of the blading. The end
thrust being thus practically neutralized by means of the balance
pistons, the spindle "floats" so that it can be easily moved in one
direction or the other. In order to definitely fix the position of
the spindle, a small adjustable collar bearing is provided at R,
inside the housing of the main bearing B. This collar bearing is
adjustable so as to locate and hold the spindle in such position so
that there will be such a clearance between the rings of the balance
piston and those of the cylinder, that the leakage of steam will be
reduced to a minimum and, at the same time, prevent actual contact
under varying conditions of temperature. Where the shaft passes out
of the cylinder, at S and T, it is necessary to provide against
in-leakage of air or out-leakage of steam by means of glands. These
glands are made tight by water packing without metallic contact. The
shaft of the turbine is extended at U and coupled to the shaft of
the alternator by means of a flexible coupling. The high pressure
turbines are so proportioned that, when using steam as previously
described, they have enough capacity to take care of the ordinary
fluctuations of load when controlled by the governor through the
valve D, thus insuring maximum economy of steam consumption at
approximately the rated load. To provide for overloads, the valve V
is supplied to admit steam to an intermediate stage of the turbine.
This valve shown diagrammatically in the illustration, is arranged
to be operated by the governor and is, according to circumstances,
located either as shown by the illustration, or at another stage of
the turbine.]
[Illustration: FIG. 2,749.--Arrangement of blading in Parsons type
turbine, consisting of alternate moving and stationary blades. The
path taken by the steam is indicated by the arrows.]
A turbine is a machine in which a rotary motion is obtained by
transference of the _momentum_ of a fluid or gas. In general the
fluid is guided by fixed blades, attached to a casing, and, impinging
on other blades mounted on a drum or shaft, causing the latter to
revolve.
Turbines are classed in various ways as: 1, _radial flow_, when the
steam enters near the center and escapes toward the circumference;
and 2, _parallel flow_, when the steam travels _axially_ or parallel
to the length of the turning body.
Turbines are commonly, yet erroneously classed as:
1. Impulse;
2. Reaction.
=Ques. What is the distinction between these two types?=
Ans. In the so called impulse type, _steam enters and leaves the
passages between the vanes at the same pressure_. In the so called
reaction type, _the pressure is less on the exit side of the vanes
than on the entrance side_.
Fig. 2,750 is a sectional view of the Parsons-Westinghouse
parallel flow turbine. Steam from the boiler enters first a
receiver in which are the governor controlled admission valves.
These valves are actuated by a centrifugal governor.
[Illustration: FIG. 2,750.--Sectional view of Parsons-Westinghouse
turbine, showing rotor and governor.]
_Steam does not enter the turbine in a continuous blast, but
intermittently, or in puffs._ The speed regulation is therefore
accomplished by proportioning the duration of these puffs to the
load of the engine, this being effected by the governor, fig.
2,752.
The governor of the turbine has only to move a small pilot
valve, or slide, E, which admits steam under the piston F, and
lifts the throttle valve proper off its seat.
As soon as the pilot valve closes, the spring shifts the main
throttle valve. Thus, at light loads, the main throttle or
admission valve is continually opening and shutting at uniform
intervals, the length of time during which it remains open
depending upon the load.
As the load increases, the duration of the valve opening also
increases, until at full load the valve does not reach its seat at
all and the steam flows steadily through the turbine. The steam
thus admitted flows into the annular passage A, fig. 2,750, by the
opening S, and then past the blades, revolving the rotor.
When the load increases above the normal rated amount a
secondary pilot valve is moved by the same means, this in turn
admitting steam to a piston, similar to F, which lifts another
throttle valve. This admits steam into the annular space I, so
that it acts upon the larger diameter of the drum or rotor, giving
largely increased power for the time being.
The levers or arms of the governor are mounted upon knife edges
instead of pins, making it extremely sensitive. The tension spring
may be adjusted by hand while the turbine is running.
[Illustration: FIG. 2,751.--Sectional view of a combination impulse
and reaction single flow turbine. This is a modification of the
single flow type, in which the smallest barrel of reaction blading is
replaced by an impulse wheel. Steam is admitted to the nozzle block
A, is expanded in the nozzles and discharged against a portion of the
periphery of the impulse wheel. The intermediate and low pressure
stages are identical with the corresponding stages in the single flow
type. The substitution of the impulse element for the high pressure
section of reaction blading has no influence one way or another on
the efficiency. That is to say the efficiency of an impulse wheel is
about the same at the least efficient section of reaction blading.
This design is attractive, however, in that it shortens the machine
materially, and gives a stiffer design of rotor. The entering steam
is confined in the nozzle chamber until its pressure and temperature
have been materially reduced by expanding through the nozzles.
As the nozzle chamber is cast separately from the main cylinder,
the temperature and pressure differences to which the cylinder is
subjected are correspondingly lessened. However, probably on account
of its small diameter at the high pressure section, the straight
Parsons type has always shown itself to be adequate for all of the
steam pressures and temperatures encountered in ordinary practice.]
The governor does not actually move the pilot valve, but shifts
the point L in fig. 2,752. A reciprocating motion is given to the
rod I by a small eccentric on the governor shaft; this is driven
by worm gearing shown near O in fig. 2,750, so that the eccentric
makes one revolution to about eight of the turbine. Thus, with a
turbine running 1,200 revolutions, the rod I would be moved up and
down 150 times per minute. As the points A and H are fixed, the
motion is conveyed to the small pilot valve E, thus giving 150
puffs a minute. The governor in shifting the point L brings the
edge of the pilot valve nearer the port and so cuts off the steam
earlier.
The annular diameter or space between the rotor and the stator
is gradually increased from inlet to exhaust, the blades being
made longer in each ring. When the mechanical limit is reached,
the diameter of the rotor is increased as at I and D so as to
keep the length of blade within bound.
Balance pistons as at B, C, F are attached to the rotor,
their office being to oppose end thrust upon those blades in
corresponding diameter of the rotor. Communication is established
through the passage V and pipe M between the eduction pipe and the
back of these pistons, thus increasing the efficiency of their
balancing and also taking care of any leakage past them.
A small thrust bearing T prevents end play of the rotor, and is
adjustable to maintain the proper clearance between the rings of
blades; this varies from ⅛ inch at the admission to 1 inch at the
exhaust. This bearing also takes up any extra unbalanced thrust. A
turbine should operate with a high vacuum, because without this it
does not compare favorably with an ordinary reciprocating engine
from the point of economy.
[Illustration: FIG. 2,752.--Sectional view of governor of the
Parsons-Westinghouse turbine.]
_Separate air pumps are provided to create the vacuum._
Where the ordinary type of vertical air pump is employed, a
booster or _vacuum increaser_ is added, as nothing below 26 inches
is advisable, 28 and 29 inches being always striven for. It is
also preferable to use a certain amount of _super-heat_ with steam
turbines.
To assist in producing the high vacuum, exhaust passages are
made large, the eduction passage E in fig. 2,750 being nearly
twenty-three times the area of the steam pipe.
Among other details, a noteworthy feature is a small oil pump K,
which circulates oil through bearings of the machinery, the oil
being drawn from the tank under the governor shaft and gravitating
there after use. No pressure of oil is employed. Stuffing rings
prevent leakage; these consist of alternate grooves and collars in
shaft and bearing, like the grooves in an indicator piston.
=Ques. Why is a high vacuum desirable?=
Ans. Because the turbine is capable of expanding the steam to a very
low terminal pressure, and this is necessary for economy.
=Ques. What may be said of the working pressures for turbines?=
Ans. To meet the varied conditions of service, turbines are designed
to operate with: 1, high pressure, 2, low pressure, or 3, mixed
pressure.
[Illustration: FIG. 2,753.--Sectional view of a double flow turbine.
The maximum economical capacity of a single flow turbine is limited
by the rotative speed. The economical velocity at which the steam may
pass through the blades of the turbine depends on the velocity of
the moving blades. The capacity of the turbine depends on the weight
of the steam passed per unit of time, which in turn depends on the
mean velocity and the height of the blades. For a given rotative
speed, the mean diameter of blade ring practicable is limited by
the allowable stresses due to centrifugal force, and there is a
practical limit for the height of the blades. Now if the rotative
speed be taken only half as great, the maximum diameter of the rotor
may be doubled and, without increasing the height of the blades, the
capacity of the turbine will be doubled. So with the single flow
steam turbine as well as with the single crank reciprocating engine,
there is a practical limiting economical capacity for any given
speed. If this limit be reached with a single crank reciprocating
engine, a unit of double the power may be produced at the same speed
by coupling two single crank engines to one shaft. Similar results
are secured making a double flow turbine which is in effect, as will
be seen from the figure, two single flow turbines made up in a single
rotor in a single casing with a common inlet and two exhausts. Steam
enters the nozzle block, acts on the impulse element, and then the
current divides, one-half of the steam going through the reaction
blading at the left of the impulse wheel; the remainder passes over
the top of the impulse wheel and through the impulse blading at the
right.]
High pressure turbines operate at about the same initial
pressure as triple expansion engines.
Low pressure, as here applied, means the exhaust pressure of the
reciprocating engine from which the exhaust steam passes through
the turbine before entering the condenser.
Mixed pressure implies that the exhaust steam is supplemented,
for heavy loads, by the admission of live steam.
=Ques. What determines the working pressure?=
Ans. When all the power is furnished by the turbine, it is designed
for high pressure; when operated in combination with a reciprocating
engine, low pressure is used for constant load, and mixed pressure
for variable load.
[Illustration: FIG. 2,754.--Sectional view of a semi-double flow
turbine. This is a modification in which the intermediate section
of reaction blading is single flow, and the low pressure section
only is double flow. This would be analogous to a four cylinder
triple expansion engine, that is, one with one high pressure, one
intermediate pressure and two low pressure cylinders--a design not at
all uncommon in very large engines in which the required dimensions
of a single low pressure cylinder would be prohibitive. Such turbines
are useful for capacities greater than is desirable for a single
flow turbine, and which are still below the maximum possibilities of
a double flow turbine of the same speed. In such machines the best
efficiency is secured by making the intermediate blading in a single
section large enough to pass the entire quantity of steam. A "dummy"
similar to those used on the single flow Parsons type, shown at the
right of the impulse wheel, compels all of the steam to pass through
the single intermediate section of the reaction blading, and balances
the end thrust due to this section. When the steam issues from the
intermediate section, the current is divided, one-half passing
directly to the adjacent low pressure section, while the other half
passes through the holes shown in the periphery of the hollow rotor
and through the rotor itself, beyond the dummy ring, into the other
low pressure section at the left hand end of the turbine.]
NOTE.--There are logical engineering reasons for the existence
of the several types of turbine, viz., single flow, double flow,
and semi-double flow. The double flow turbine is not inherently
superior to the single flow design, but is used under conditions
for which the single flow machine is unsuitable. Similarly, the
semi-double flow is recommended only for conditions which it can
meet more satisfactorily than either of the other types.
NOTE.--Low pressure turbines use exhaust steam from
non-condensing engines and are valuable as an adjunct to existing
plants for the purpose of increasing economy and capacity with a
minimum outlay for new equipment.
NOTE.--Bleeder turbines are for use in plants which are required
to furnish, not only power, but also considerable and varying
quantities of low pressure steam for heating purposes. In these
turbines a part of the steam after it has done work in the high
pressure stages may be diverted to the heating system, and the
remainder expanded through the low pressure blading and exhausted
into the condenser. In this way none of the energy of the heating
steam, due to the difference of pressure between the boiler and
the heating system is wasted. On the other hand if no steam is
required for heating purposes, the turbine operates just as
efficiently as though the bleeder feature were absent.
[Illustration: FIG. 2,755.--Westinghouse valve gear with steam relay.
In the smaller turbines, the governor acts directly on the steam
admission valves, opening first the primary valve, and then, if
necessary, the secondary valve, after the primary is fully open. In
turbines of the single flow Parsons type, the governor actuates two
small valves controlling ports leading to steam relay cylinders which
operate the admission valves. The little valve controlling the relay
cylinder for the secondary valve has more lap than the other and
consequently does not come into action until the primary valve has
attained its maximum effective opening. The figure shows the general
design of this type of valve gear.]
_The De Laval steam turbine_ is termed by its builders a high
speed rotary steam engine. It has but a single wheel, fitted with
vanes or buckets of such curvature as has been found to be best
adapted for receiving the impulse of the steam jet. There are no
stationary or guide blades, the angular position of the nozzles
giving direction to the jet. The nozzles are placed at an angle
of 20 degrees to the plane of motion of the buckets. The best
energy in the steam is practically devoted to the production of
velocity in the expanding or divergent nozzle, and the velocity
thus attained by the issuing jet of steam is about 4,000 feet per
second. To attain the maximum efficiency, the buckets attached to
the periphery of the wheel against which this jet impinges should
have a speed of about 1,900 feet per second, but, owing to the
difficulty of producing a material for the wheel strong enough to
withstand the strains induced by such a high speed, it has been
found necessary to limit the peripheral speed to 1,200 or 1,300
feet per second.
It is well known that in a correctly designed nozzle the
adiabatic expansion of the steam from maximum to minimum pressure
will convert the entire static energy of the steam into kinetic
energy. Theoretically this is what occurs in the De Laval nozzle.
The expanding steam acquires great velocity, and the energy
of the jet of steam issuing from the nozzle is equal to the
amount of energy that would be developed if an equal volume of
steam were allowed to adiabatically expand behind the piston
of a reciprocating engine, a condition, however, which for
obvious reasons has never yet been attained in practice with the
reciprocating engine. But with the divergent nozzle the conditions
are different.
_The Curtis turbine_ is built by the General Electric Company at
their works in Schenectady, N. Y., and Lynn, Mass. They are of the
horizontal and vertical types. In the vertical type the revolving
parts are set upon a vertical shaft, the diameter of the shaft
corresponding to the size of the machine.
The shaft is supported by and runs upon a step bearing at the
bottom. This step bearing consists of two cylindrical cast iron
plates bearing upon each other and having a central recess between
them into which lubricating oil is forced under pressure by a
steam or electrically driven pump, the oil passing up from beneath.
[Illustration: FIGS. 2,756 and 2,757.--Westinghouse valve gear with
oil relay. Governors for the larger turbines, particularly those of
the combination impulse and reaction double, or single double flow
type, employ an oil relay mechanism, as shown in the figure, for
operating the steam valves. In these turbines the lubricating oil
circulating pump, maintains a higher pressure than is required for
the lubricating system. The governor controls a small relay valve
A which admits pressure oil to, or exhausts it from the operating
cylinder. When oil is admitted to the operating cylinder raising the
piston, the lever C lifts the primary valve E. The lever D moves
simultaneously with C, but on account of the slotted connection
with the stem of the secondary valve F, the latter does not begin
to lift until the primary valve is raised to the point at which its
effective opening ceases to be increased by further upward travel.
In the Westinghouse designs, the operating valve, A is connected not
only to the governor, but also to a vibrator, which gives it a slight
but continuous reciprocating motion, while the governor controls its
mean position. The effect of this is manifested in a slight pulsation
throughout the entire relay system, which, so to speak, keeps it
"alive" and ready to respond instantly, to the smallest change in
the position of the governor. The oil relay can be made sufficiently
powerful to operate valves of any size, and it is also in effect
a safety device in that any failure of the lubricating oil supply
will automatically and immediately shut off the steam and stop the
turbine.]
A weighted accumulator is sometimes installed in connection with
the oil pipe as a convenient device for governing the step bearing
pumps, and also as a safety device in case the pumps should fail,
but it is seldom required for the latter purpose, as the step
bearing pumps have proven after a long service in a number of
cases, to be reliable. The vertical shaft is also held in place
and kept steady by three sleeve bearings one just above the step,
one between the turbine and generator, and the other near the top.
[Illustration: FIG. 2,758.--Elevation of new turbine central station
erected by the Boston Edison Co. The turbine room is 68 feet, 4
inches wide and 650 feet long from outside to outside of the walls.
The boiler room is 149 feet, 6 inches by 640 feet and equipped with
twelve groups of boiler, one group consisting of eight 512 H.P.
boilers for each turbine. The switching arrangements are located in
a separate building as shown in the elevation. The total floor space
covered by boiler room, turbine room and switchboard room is 2.64
square feet per kw. The boilers are all on the ground floor. See fig.
2,714 for plan.]
These guide bearings are lubricated by a standard gravity feed
system. It is apparent that the amount of friction in the machine
is very small, and as there is no end thrust caused by the action
of the steam, the relation between the revolving and stationary
blades may be maintained accurately. As a consequence, therefore,
the clearances are reduced to the minimum.
The Curtis turbine is divided into two or more stages, and each
stage has one, two or more sets of revolving blades bolted upon
the peripheries of wheels keyed to the shaft. There are also the
corresponding sets of stationary blades bolted to the inner walls
of the cylinder or casing.
The governing of speed is accomplished in the first set of
nozzles and the control of the admission valves here is effected
by means of a centrifugal governor attached to the top end of the
shaft. This governor, by a very slight movement, imparts motion
to levers, which in turn work the valve mechanism.
The admission of steam to the nozzles is controlled by piston
valves which are actuated by steam from small pilot valves which
are in turn under the control of the governor.
[Illustration: FIG. 2,759.--Illustration of a weir. To make a weir,
place a board across the stream at some point which will allow a
pond to form above. The board should have a notch cut in it with
both side edges and the bottom sharply beveled toward the intake,
as shown in the above cut. The bottom of the notch, which is called
the "crest" of the weir, should be perfectly level and the sides
vertical. In the pond back of the weir, at a distance not less than
the length of the notch, drive a stake near the bank, with its top
precisely level with the crest. By means of a rule, or a graduated
stake as shown, measure the depth of water over the top of stake,
making allowance for capillary attraction of the water against the
sides of the weir. For extreme accuracy this depth may be measured
to thousandths of a foot by means of a "hook gauge," familiar to all
engineers. Having ascertained the depth of water over the stake,
refer to the accompanying table, from which may be calculated the
amount of water flowing over the weir. There are certain proportions
which must be observed in the dimensions of this notch. Its length,
or width, should be between four and eight times the depth of water
flowing over the crest of the weir. The pond back of the weir should
be at least fifty per cent. wider than the notch and of sufficient
width and depth that the velocity of flow or approach be not over one
foot per second. In order to obtain these results it is advisable to
experiment to some extent.]
Speed regulation is effected by varying the number of nozzles
in flow, that is, for light loads fewer nozzles are open and a
smaller volume of steam is admitted to the turbine wheel, but the
steam that is admitted impinges against the moving blades with the
same velocity always, no matter whether the volume be large or
small. With a full load and all the nozzle sections in flow, the
steam passes to the wheel in a broad belt and steady flow.
WEIR TABLE
giving cubic feet of water per minute that will flow over a weir
one inch wide and from ⅛ to 20⅞ inches deep.
--------+------+------+------+------+------+------+------+------
Depth | | | | | | | |
inches | | ⅛ | ¼ | ⅜ | ½ | ⅝ | ¾ | ⅞
--------+------+------+------+------+------+------+------+------
=0= | .00 | .01 | .05 | .09 | .14 | .19 | .26 | .32
=1= | .40 | .47 | .55 | .64 | .73 | .82 | .92 | 1.02
=2= | 1.13 | 1.23 | 1.35 | 1.36 | 1.58 | 1.70 | 1.82 | 1.95
=3= | 2.07 | 2.21 | 2.34 | 2.48 | 2.61 | 2.76 | 2.90 | 3.05
=4= | 3.20 | 3.35 | 3.50 | 3.66 | 3.81 | 3.97 | 4.14 | 4.30
=5= | 4.47 | 4.64 | 4.81 | 4.98 | 5.15 | 5.33 | 5.51 | 5.69
=6= | 5.87 | 6.06 | 6.25 | 6.44 | 6.62 | 6.82 | 7.01 | 7.21
=7= | 7.40 | 7.60 | 7.80 | 8.01 | 8.21 | 8.42 | 8.63 | 8.83
=8= | 9.05 | 9.26 | 9.47 | 9.69 | 9.91 |10.13 |10.35 |10.57
=9= |10.80 |11.02 |11.25 |11.48 |11.71 |11.94 |12.17 |12.41
=10= |12.64 |12.88 |13.12 |13.36 |13.60 |13.85 |14.09 |14.34
=11= |14.59 |14.84 |15.09 |15.34 |15.59 |15.85 |16.11 |16.36
=12= |16.62 |16.88 |17.15 |17.41 |17.67 |17.94 |18.21 |18.47
=13= |18.74 |19.01 |19.29 |19.56 |19.84 |20.11 |20.39 |20.67
=14= |20.95 |21.23 |21.51 |21.80 |22.08 |22.37 |22.65 |22.94
=15= |23.23 |23.52 |23.82 |24.11 |24.40 |24.70 |25.00 |25.30
=16= |25.60 |25.90 |26.20 |26.50 |26.80 |27.11 |27.42 |27.72
=17= |28.03 |28.34 |28.65 |28.97 |29.28 |29.59 |29.91 |30.22
=18= |30.54 |30.86 |31.18 |31.50 |31.82 |32.15 |32.47 |32.80
=19= |33.12 |33.45 |33.78 |34.11 |34.44 |34.77 |35.10 |35.44
=20= |35.77 |36.11 |36.45 |36.78 |37.12 |37.46 |37.80 |38.15
--------+------+------+------+------+------+------+------+------
NOTE.--The weir table on this page contains
figures 1, 2, 3, etc., in the first vertical
column which indicates the inches depth of water
running over weir board notches. Frequently the
depths measured represent also fractional inches,
between 1 and 2, 2 and 3, etc. The horizontal
line of fraction at the top represents these
fractional parts, and can be applied between any
of the numbers of inches depth, from 1 to 25. The
body of the table shows the cubic feet, and the
fractional parts of a cubic foot, which will pass
each minute for each inch in depth, and for each
fractional part of an inch by eighths for all
depths from 1 to 25 inches. Each of these results
is for only one inch width of weir. To estimate
for any width of weir the result obtained for one
inch width must be multiplied by the number of
inches constituting the whole horizontal length
of weir.
[Illustration: FIGS. 2,760 and 2,761.--Samson vertical runner and
shaft, and complete Samson vertical turbine. The runner is composed
of two separate and distinct types of wheel, having thereby also
two diameters. Each wheel or set of buckets receives its separate
quantity of water from one and the same set of guides, but each set
acts only once and singly upon the water used, and the water does not
act twice upon the combined wheel, as some suppose. In construction=,
the lower or main set of buckets is made of flanged plate steel, and
cast solidly into a heavy ring surrounding the outer and lower edges,
and into a heavy diaphragm, separating the two sets of buckets.]
[Illustration: FIG. 2,762.--Water discharging from a needle nozzle
due to a pressure of 169 lbs. per sq. in.]
=Hydro-Electric Plants.=--The economy with which electricity can be
transmitted long distances by high tension alternating currents, has
led to the development of a large number of water powers in more or
less remote regions.
[Illustration: FIG. 2,763.--Photograph of an operating tangential
water wheel equipped with Pelton buckets.]
This economy is possible by the facility with which alternating
current can be transformed up and down. Thus at the hydro-electro
plant, the current generated by the water wheel driven alternator
is transformed to very high pressure and transmitted with economy
a long distance to the distributing point where it is transformed
down to the proper pressure for distribution.
A water wheel or turbine is a machine in which a rotary motion
is obtained by transference of the momentum of water; broadly
speaking, the fluid is guided by fixed blades, attached with a
casing, and impinging on other blades mounted on a drum or shaft,
causing the latter to revolve.
There are two general classes of turbine:
1. Impulse turbines;
2. Reaction turbines.
[Illustration: FIG. 2,764.--Sectional elevation of one of the 5,000
horse power vertical Pelton-Francis turbines directly connected to
generator, as installed for the Schenectady Power Co.]
=Ques. What is an impulse turbine?=
Ans. One in which the fluid is directed by means of a series of
nozzles against vanes which it drives.
=Ques. What is a reaction turbine?=
Ans. One in which the pressure or head of the water is employed
rather than its velocity. The current is deflected upon the wheel by
the action of suitably disposed guide blades, the passages being full
of water. Rotary motion is obtained by the change in the direction
and momentum of the fluid.
[Illustration: Figs. 2,765 to 2,768.--Cross sections of Lowel dam
power house, and wheel pits containing sixteen Samson turbines: The
section C-D gives an end view of the generator room showing the
locations of the generators below the head level water. They are
secure against flood water, or leakage, by well constructed stuffing
boxes in the iron bulkheads, through which the turbine wheel shafts
pass and connect to the generators. Section E-F gives an end view of
one of these wheel rooms or penstocks, and shows the extension of the
draft tube from wheel case into tail water. The section A-B shows the
sub-structure of gravel and macadam under the controlling gates, this
forming also a portion or extension of the dam proper. These gates
turn on an axis made of two 15 inch I beams securely riveted together
with plates and angle irons to which the wooden frame is attached.
The radius of the gates is 14 feet. They are designed to allow the
water to pass underneath the gate, thus controlling any height of
head water. They are intended to take care of an excess of water at
unusual stages of the river. The whole affair has been well designed
and executed. This plant furnishes a good example of a secure, and
level foundation, since the wheel houses and generator room are
immediately on the rock. It is necessary in all tandem plants to
provide a very secure, substantial super-structure so that the long
line of turbines and shaft will always remain straight and in proper
alignment with the generator and the turbine cases. Users cannot be
reminded of this too often.]
=Ques. Name three classes of reaction turbines.=
Ans. Parallel flow, inward flow, and outward flow.
Parallel flow turbines have an efficiency of about 70% and are
suited for low falls not over 30 feet. Inward and outward flow
turbines have an efficiency of about 85%. Impulse turbines are
suitable for high heads.
[Illustration: FIGS. 2,769 and 2,770.--Exterior and interior of
hydro-electric plant at Harrisburg, Va. It is located on the south
fork of the Shenandoah River, twelve and one-half miles distant. A
dam 720 feet long and 15 feet high was built on a limestone ledge
running across the river; which with a fall of 5 feet from the dam to
the power house, a quarter of a mile distant, secured an effective
head pressure of 20 feet. The power house, comprising the generator
room and the wheel room, also the machinery room, are here shown. The
wheel room, which is 20 × 40 feet, extends across the head race, and
rests upon solid concrete walls, forming the sides and ends of the
wheel pits. The end wall is 6 feet thick at the bottom, and 4½ feet
at the top. It has three arched openings, each 8 feet wide and 9 feet
high, through which the water escapes after leaving the turbines.
The intake is protected by a wrought iron rack 40 feet long. The
power is obtained by three 50 inch vertical shaft Samson turbines,
with a 20 inch Samson for an exciter. The three large turbines
have a rating of 1,350 horse power; and are connected to the main
horizontal line shaft by bevel mortise gears 7 feet diameter and 15
inches face. The couplings on the main shaft have 48 inch friction
clutch hubs, permitting either or each turbine being operated, or
shut down independently of the others. The main shaft is 85 feet long
and 6 inches diameter; making 280 revolutions. This shaft carries
two pulleys 70 inches diameter and 38 inches face for driving the
generators. The accompanying illustration shows the harness work,
gears, pulleys, etc., furnished with the turbines. The 20 inch
horizontal shaft Samson turbine of 72 horse power is direct connected
to an exciter generator of 20 kw., running 700 rev. per min. The
two large generators are driven 450 revolutions per minute by belts
producing a three phase current of 60 cycles of 11,500 volts for the
twelve and one-half miles transmission. The line consists of three
strands of No. 4 bare copper wire. This current is used for lighting
and power purposes, and the plant is of the latest improved design
and construction.]
=Isolated Plants.=--When electric power transmission from central
stations first came into commercial use, the distance from the
station at which current could be obtained at a reasonable cost was
exceedingly limited.
[Illustration: FIG. 2,769a.--Triumph direct current generator set with
upright slide valve engine.]
[Illustration: FIG. 2,770a.--Murray alternating current direct
connected unit with high speed Corliss engine and belt driven
exciter, 50, 75 and 100 kva. alternator and 150 R.P.M. engine.]
[Illustration: FIG. 2,771.--Direct connected direct current unit with
Ridgway high speed four valve engine.]
[Illustration: FIG. 2,772.--Buckeye mobile, or self contained unit
consisting of compound condensing engine, boiler, superheater,
reheater, feed and air pumps; it produces one horse power on 1½ lbs.
of coal, built in sizes from 75 to 600 horse power.]
[Illustration: FIG. 2,773.--Westinghouse three cylinder gas engine,
direct connected to dynamo, showing application of gas engine drive
for small direct connected units.]
Consequently, persons desiring electrical power were in the
majority of cases forced to install their own apparatus for
producing it, this being the origin of isolated plants.
From the nature of the case it is evident that an isolated
plant is as a rule smaller and more simple in construction than
a central station, and in consequence much more readily operated
and managed. It is generally owned by a private individual or a
corporation and operated in conjunction with other affairs of a
similar character. A basement or other portion of a building is
usually set aside in which the necessary apparatus is installed.
[Illustration: FIG. 2,774.--General Electric 25 kw., gasoline
electric generating set for lighting and power. The engine has
four cylinders 7¼ × 7½, and runs at a speed of 560 revolutions per
minute. The total candle power capacity in Mazda lamps is 20,000. The
ignition is by low tension magneto, coil and battery. Carburetter is
of the constant level type to which gasoline is delivered by a pump
driven by the engine. Forced lubrication; five crank shaft bearings
babbitted; valves in side; overall dimensions 96 × 34 × 60 high;
weight 5,000.]
Although electricity is now transmitted economically to great
distances from central stations, there is still a field for the
isolated plant.
The average type of isolated plant has enlarged from a small
dynamo driven by a little slide valve engine located in an out
of the way corner to direct connected generators and engines of
hundreds and even thousands of horse power assembled in a large
room specially adapted to the purpose.
In the more modern of these, the electrical outputs are each
frequently equal to that of a town central station of respectable
size, and the auxiliary equipments are similar in every
particular. As a matter of fact, in certain modern isolated plants
the only feature that distinguishes them from central stations
is that in the former case the owner of the plant represents the
sole consumer and conducts other business in connection with it,
whereas in the latter case there are a large number of consumers
uninterested financially in the enterprise, which is itself
generally owned and operated by a company conducting no other
business.
[Illustration: FIG. 2,775.--Plan of sub-station with air blast
transformers and motor operated oil switches and underground 11,000
or 13,200 volt high tension lines.]
=Sub-Stations.=--According to the usual meaning of the term, a
sub-station is a building provided with apparatus for changing high
pressure alternating current received from the central station into
direct current of the requisite pressure, which in the case of
railways is 550 to 600 volts.
Where traffic is heavy and the railway system of considerable
distance, sub-stations are provided at intervals along the line,
each receiving high pressure current from one large central station
and converting it into moderate pressure direct current for their
districts.
=Ques. Upon what does the arrangement of the sub-station depend?=
Ans. Upon the character of the work and the type of apparatus
employed for converting the high pressure alternating current into
direct current.
[Illustration: FIG. 2,776.--Plan of small sub-station with single
phase oil insulated self-cooling transformers and hand operated oil
switches 11,000 or 13,200 volts, overhead high tension lines.]
In general it should be substantial, convenient to install or
replace the heavy machines, and the layout arranged so that the
apparatus can be readily operated by those in attendance.
An overhead traveling crane is the most convenient method of
handling the heavy machinery, and is frequently used in large
sub-stations.
Fig. 2,776 shows a sectional view, and fig. 2,777, a plan for
a small sub-station containing two rotary converters and two
banks of three single phase static transformers operating on a
three phase system at 11,000 or 13,200 volts, together with the
auxiliary apparatus.
[Illustration: FIG. 2,777.--Elevation of small sub-station, as shown
in plan in Fig. 2,776.]
=Ques. For three phase installations, what are the merits of
separate and combined transformers?=
Ans. With separate transformer for each phase, repairs are more
readily made in case of accident or burnouts in the coils. The three
phase units have the advantage of low first cost.
Sub-station transformers produce considerable heat, due to the
hysteresis and eddy currents, and it is necessary to get rid of it.
Small transformers radiate the heat from the shell and the
medium sizes have corrugated shells which increase the surface and
provide more rapid radiation.
Large transformers are cooled by an air blast supplied by motor
driven blowers or by water pumped through a coil of pipe which is
immersed in the insulating oil of the transformer. The large size
oil insulated, water cooled transformers are used on circuits of
33,000 volts or more. In water turbine plants, the water may be
piped to the transformer under pressure and the pump omitted which
cuts down the cost of operating. Air blast transformers usually
have a damper or shutter for air control.
[Illustration: FIG. 2,778.--Marine portable transformer station on
Los Angeles Aqueduct. The view shows three 20 kva. Westinghouse out
door transformers installed on a float, 33,000 volts high pressure;
440 volts low pressure; 50 cycles.]
=Ques. Explain the use of reactance coils in sub-stations.=
Ans. In order that the direct current voltage of the ordinary rotary
may be regulated by a field rheostat, which calls for a corresponding
change in the alternating current voltage, a reactance coil is
provided between the low tension winding and the converter.
Without such a reactance the maintenance of the same voltage at
full load as at no load involves excessive leading and lagging
currents and consequently excessive heating in the armature
inductors, unless the resistance drop from the source of constant
pressure is small, or the natural reactance of the circuit high.
=Ques. What is the effect of weakening the converter field?=
Ans. A lagging current is set up which causes a drop in the reactance
coil.
[Illustration: FIG. 2,779.--Sectional elevation of portable outdoor
transformer type sub-station. The high voltage switching and
protective apparatus is mounted, out of the way, on the roof of the
car, but is operated from the switchboard with a standard remote
control handle. The transformer is carried directly over the truck at
the uncovered end of the car and the low-tension leads from it run in
conduit beneath the floor and up into the cab, (which contains the
converter and switchboard) to the converter. The positive lead runs
through a conduit and ends in a terminal on the roof. The energy thus
makes a complete circuit of the car leaving at a point close to that
at which it entered. The low pressure alternating current as well as
the direct current positive leads are carried below the car floor
in iron conduit supported from the channel frame. The field wires
are carried through this conduit to the rheostat. Wiring for the
lights is arranged to supply two, 5 light clusters. One is fed with
the 600 volt direct current and the other with 420 volt alternating
current. All lighting conductors are carried in metal moulding
carried between the flanges of the channel iron ribs. High wiring
is carried entirely on the roof of the car where it is entirely out
of the way and where the operator cannot come in contact with it.
The switchboard should be of the utmost simplicity. Usually the
negative and equalizer switches, and the field break-up switch are
mounted on the frame of the converter. The double throw switch for
starting and running the converter can be mounted under the floor
of the car and operated by handle at the switchboard. The rheostat
can be mounted back of the switchboard on brackets bolted to the car
super-structure. The switchboard need only carry the positive knife
switch and circuit breaker, and the alternating current ammeter,
voltmeter and power factor meter. Sometimes a watthour meter is
added. The positive lead is brought out through a conduit on the
roof of the car and is arranged for bolting to the positive feeder.
The negative and equalizer terminals are located at the cab end of
the car and are arranged so that connection can be easily made from
them to the ground and, if necessary, to an equalizer circuit. There
is usually a sliding door at each end of the cab and two windows on
each side. Above the doors, transoms, extending the width of the cab,
are arranged to drop so that a current of air will circulate through
the cab under the roof, carrying out the heated air. There are also
several ventilating holes beneath the converter in the floor of the
car. These provisions insure a constant circulation of air through
the car which carries away all heated air.]
=Ques. State the effect of strengthening converter field.=
Ans. A leading current is set up which gives a rise of voltage in the
reactance coil.
Hence when a heavy current passes through the series coil of a
compound wound converter and tends to produce a leading current,
the reactance coil will balance it, and improve the power factor
of the whole line.
[Illustration: FIG. 2,780.--Westinghouse 300 kw. converter in
portable sub-station.]
=Portable Sub-Stations.=--A portable sub-station constitutes a spare
equipment for practically any number of permanent sub-stations and
renders unnecessary the installation of spare equipment in each.
It can be used to increase the capacity of a permanent
sub-station when the load is unusually heavy, or to provide
service while a permanent sub-station is being overhauled or
rebuilt.
The transformer can be used for emergency lighting, the primary
being connected to a high pressure line and the secondary to the
load, if special provision be made at the time the transformer is
built to adapt it for these applications.
[Illustration: FIG. 2,781.--Switchboard end of Westinghouse portable
sub-station.]
When an electric railway has a portable sub-station, direct current
can be provided at any point on the system where there is track at
the high pressure line. The direct current can be made available
very quickly as its production involves only the transferring of the
sub-station, and its connection to the high pressure line.
Portable sub-stations range in capacity from 200 to 500 kw., and
for all alternating current voltages up to 66,000, and frequencies
of 25 and 60 cycles.
Although portable sub-stations usually must be of more or
less special design to adapt them to the conditions under which
they must operate, there are certain general features that are
common to all. All members are readily accessible and there are
no unnecessary parts. The weight and dimensions are a minimum
insuring ease of transportation. Live parts are so protected that
the danger of accidental contact with them is minimized.
[Illustration: FIGS. 2,782 and 2,783.--Views of levelling device for
Westinghouse converter.]
=Ques. What are the advantages of using outdoor transformers on
portable sub-stations?=
Ans. All high pressure wiring is kept out of the car. The transformer
is more effectively cooled and the heat dissipated by the transformer
does not warm the interior of the cab. The transformer is much more
accessible. The car can be run under a crane and the transformer
coils pulled out with a hoist.
Taps for different high and low pressure voltages can be readily
provided at the time the transformer is being built.