6. By permitting the boiler pressure to be lowered ten to twenty pounds
without reducing the power or the economy of the engine.
The discovery of the advantages arising from the condensation of steam
by _direct contact with water_ was accidental.
[Illustration: FIG. 578.]
In the earliest construction of steam-engines the desired vacuum was
produced by the circulation of water through a jacket around the
cylinder. This was a slow and tedious process, the engine making only
seven or eight strokes per minute. “An accidental unusual circumstance
pointed out the remedy, and greatly increased the effect. As the
engine was at work, the attendants were one day surprised to see it
make several strokes much quicker than usual; and upon searching for
the cause, they found, says Desaguliers, ‘a hole through the piston
which let the cold water (kept upon the piston to prevent the entrance
of air at the packing) into the space underneath.’ The water falling
through the steam condensed it almost instantaneously, and produced
a vacuum with far less water than when applied to the exterior of
the cylinder. This led Newcomen to remove the outer cylinder, _and
to insert the lower end of the water pipe into the bottom of the
cylinder_, so that on opening a cock a jet of cold water was projected
through the vapor. This beautiful device is the origin of the injection
pipe with a spray nozzle still used in low-pressure engines.”
The apparatus described above is called the _jet-condenser_ and is
in use up to the present day in various forms. In the Fig. 577, page
298, the jet is shown at C. It will be understood that steam enters
through the cock D and comes in contact with a spray of cold water at
the bottom, where it is condensed and passes into the air pump through
which it is discharged.
By this diagram, Fig. 577, may be understood in a simple yet accurate
manner _the course of steam from the time it leaves the boiler until it
is discharged from the condenser_.
Referring to the upper section of the plate, a sectional view of a
steam cylinder, jet condenser, air pump and exhaust piping is shown.
The high pressure steam “aa” is represented by dark shading, and the
low pressure or expanded steam “bb” by lighter shading.
The steam enters the side “aa,” is cut off, and expansion takes place
moving the piston in the direction of the arrow to the end of the
stroke. The exhaust valve now opens and the piston starts to return.
The low pressure steam instead of passing direct to the atmosphere, as
is the case of a high pressure engine, flows into a chamber “C,” and is
brought in contact with a spray of cold water. The heat being absorbed
by the water, the steam is condensed and reduced in volume, thus
forming a vacuum. It is, however, necessary to remove the water formed
by the condensed steam together with the water admitted to condense the
steam, also a small amount of air and vapor. For this purpose, a pump
is required, which is called the air pump.
[Illustration: FIG. 579.]
[Illustration: FIG. 580.]
[Illustration: FIG. 581.]
Condensers are classified into _surface condensers and jet condensers_,
both again being divided into direct connected and indirect connected
condensers.
The surface condenser (see Fig. 579) is mainly used in marine practice
because it gives a better vacuum, and keeps the condensed steam
separate from the cooling water; it consists of a vessel, of varied
shapes, having a number of brass tubes passing from head to head. The
ends of this vessel are closed by double heads, the tubes are expanded
into the inner one on one end, while their other ends pass through
stuffing-boxes in the other inner head.
The “admiralty” or rectangular surface condenser is represented in Fig.
579. This form occupies less floor space than the round shell, and is
preferred upon steam yachts and small vessels.
Steam is condensed on its introduction at the top of the apparatus
where it comes in contact with the cool surfaces of the tubes. Through
these water is circulated by a centrifugal pump driven usually by a
separate engine.
[Illustration: FIG. 582.]
_The water of condensation_ leaves the condenser at the bottom and is
drawn off by the vacuum pump. _The water from the circulating pump_
enters at the bottom right-hand end; following the direction indicated
by the arrows, it flows through the lower half of the tubes towards
the left whence it returns through the upper half of the tubes towards
the right and escapes overboard through the water outlet pipe.
It will be observed that the coolest water encounters the lowest
temperature of steam at the bottom, hence the best results are reached.
There is also a baffle plate just above the upper row of tubes to
compel a uniform distribution of exhaust steam among the tubes, as
shown in the engraving.
These tubes are usually small—1/2″ outside diameter—of brass and coated
with tin inside and outside to prevent galvanic action which is liable
to attack the brass tubes and cause them to corrode.
Fig. 581 shows an end view of the right-hand head of the surface
condenser here described.
[Illustration: FIG. 583.]
A single tube is shown in detail in Fig. 580. One end of the tube is
drawn sufficiently thick to chase upon it deep screw threads, while a
slot facilitates its removal by a screw-driving tool. The other end is
packed and held in place by a screw gland, which is also provided with
a slot. In this way the tube is firmly held in one head, and, though
tightly fitted in the other, is free to move longitudinally under the
influence of expansion or contraction, due to the varying heat.
In some cases engineers prefer the ordinary arrangement of screw glands
at both ends of the tubes, with the usual wick packing.
The mechanism illustrated in Figs. 582 and 583 shows _a combined
condenser and feed-water heater_. A compact and efficient method of
heating the feed-water from the hot well is of great importance; this
is the case in cold weather when the circulating water is at a low
temperature.
_The Volz apparatus_ is a combined condenser and feed-water heater; the
shell or exhaust steam chamber contains a set of tubes, through which
the feed-water passes, while the lower part contains the condensing
tubes, both parts being in proper communication with their respective
water chambers. The heater tubes being located immediately adjacent
to the exhaust inlet, are exposed to the hottest steam, and the
feed-water becomes nearly as high temperature as that of the vacuum.
Pages 304 and 305 show the sectional and outside views. The enclosing
shell containing the combined heater and condenser is a well ribbed
cylindrical iron casting; free and independent access is provided to
either set of tubes by removing corresponding heads.
The illustration, Fig. 584, is a longitudinal section of one side of
the condenser pump, and also a section of the condenser cone, spray
pipe, exhaust elbow and injection elbow. “A” is the exhaust to which is
connected the pipe that conducts to the apparatus the steam or vapor
that is to be condensed. The injection water is conveyed by a pipe
attached to the injection opening at “B.” “C” is the spray pipe, and
has, at its lower extremity, a number of vertical slits through which
the injection water passes and spreads out into thin sheets.
The spray cone “D” scatters the water passing over it, and thus ensures
a rapid intermixture with the steam. This spray cone is adjustable
by means of a stem passing through a stuffing-box at the top of the
condenser, and is operated by the handle “E.” The cone should be left
far enough down to pass the quantity of water needed for condensation.
_All regulation of the injection water must be done by an injection
valve placed in the injection pipe at a convenient point._
NOTE.—The surface condensers, Figs. 579-581, are made by the Wheeler
Condenser and Engineering Co., New York, as are also the Volz
combined surface condenser and feed water heater, shown in Figs. 582
and 583.
_The operation of this condensing apparatus is as follows_: steam being
admitted to the cylinders “K,” so as to set the pump in motion, a
vacuum is formed in the condenser, the engine cylinder, the connecting
exhaust pipe, and the injection pipe. This causes the injection water
to enter through the injection pipe attached at “B” and spray pipe “C”
into the condenser cone “F.” The main engine being started, the exhaust
steam enters through the exhaust pipe at “A,” and, coming in contact
with the cold water, is rapidly condensed. The velocity of the steam is
communicated to the water, and the whole passes through the cone “F”
into the pump “G” at a high velocity, carrying with it, in a comingled
condition the air or uncondensable vapor which enters the condenser
with the steam. The mingled air and water is discharged by the pump
through the valves and pipe at “J” before sufficient time or space has
been allowed for separation to occur.
[Illustration: FIG. 584.]
_The exhaust steam induction condenser_ is based upon the same
principle heretofore explained under the section relating to injectors.
See Fig. 585.
[Illustration: FIG. 585.]
The exhaust steam enters through the nozzle, A. The injection water
surrounds this nozzle and issues downward through the annular space
between the nozzle and the main casting. The steam meeting the water
is condensed, and by virtue of its weight and of the momentum which
it has acquired in flowing into the vacuum the resulting water
continues downward, its velocity being further increased, and the
column solidified by the contraction of the nozzle shown. The air is
in this way carried along with the water and it is impossible for it
to get back against the rapidly flowing steam in the contracted neck.
The condenser will lift its own water twenty feet or so. When water
can be had under sufficient head to thus feed itself into the system,
and the hot-well can at the same time be so situated as to drain
itself, it makes a remarkably simple and efficient arrangement. In
case the elevation is so great that a pump has to be used to force the
injection, the pump has to do less work than the ordinary air pump, and
its exhaust can be used to heat the feed water.
_The Bulkley “Injector” condenser_ is shown in Fig. 586, arranged so
that the condensing water is supplied by a pump. The condenser is
connected to a vertical exhaust pipe from the engine, at a height
of about 34 feet above the level of the “hot-well.” An air-tight
discharge pipe extends from the condenser nearly to the bottom of the
“hot-well,” as shown in the engraving.
The condenser is supplied by a pump as shown, or from a tank, or from
a natural “head” of water; the action is continuous, the water being
delivered into the “hot-well” below. The area of the contracted “neck”
of the condenser is greater than that of the annular water inlet
described above, and the height of the water column overcomes the
pressure of the atmosphere without.
[Illustration: FIG. 586.]
The supply pump delivers cool water only, and is therefore but
one-third of the size of the air-pump. The pressure of the atmosphere
elevates the water about 26 feet to the condenser.
The accompanying diagrams, Figs. 587 and 588, are worthy of study.
They represent _a condenser plant_ designed by the Schutte & Koerting
Co., Philadelphia, and placed on steam-vessels plying on fresh water.
In these drawings the parts are designed by descriptive lettering
instead the ordinary way of reference figures; this adds to the
convenience of the student in considering this novel application of the
condenser-injector, the action of which is described in the following
paragraphs.
[Illustration: FIG. 587.]
For steamers plying on fresh water lakes, bays and rivers it is
unnecessary to go to the expense of installing surface condensers such
as are used in salt water; keel condensers, however, are used in both
cases.
[Illustration: FIG. 588.]
_The keel condenser_ consists of two copper or brass pipes running
parallel and close to the keel, one on each side united by a return
bend at the stern post. The forward ends are connected, one to the
exhaust pipe of the engine while the other end is attached to the
suction of the air pump.
In other cases both forward ends are attached to the exhaust pipe of
the steam engine while the water of condensation is drawn through a
smaller pipe connected with the return bend at the stern post which is
the lowest part of the keel condenser.
Fig. 587 is much used for vessels running in fresh water. The
illustration is a two-thirds midship section of a vessel with pipe
connections to the bilge—bottom injection—side injection into the
centrifugal pump, thence upward through suction pipe into the ejector
condenser where it meets and condenses the exhaust steam from the
engine and so on through the discharge pipe overboard. The plan of
piping with valves, drain pipes and heater are shown in Fig. 588.
In case of the failure of any of the details of this mechanism to
perform their respective functions a free exhaust valve and pipe is
provided which may be brought instantly into use. The discharge pipe
has a “kink” in it to form a water seal, as represented with a plug
underneath to drain in case of frost, or in laying up the vessel in
winter. A pipe leads from globe valve (under discharge elbow) to feed
pump for hot water.
_Condensing Surface Required._ In the early days of the surface
condenser it was thought necessary to provide a cooling surface in the
condenser equal to the heating surface in the boilers, the idea being
that it would take as much surface to transfer the heat from a pound
of steam to the cooling water and condense the steam as it would to
transfer the heat from the hot gases to the water in the boiler and
convert it into steam. The difference in temperature, too, between the
hot gases and the water in the boiler is considerably greater than that
between the steam in the condenser and the cooling water.
[Illustration: Surface Condenser.]
NOTE.—The following list gives the numbers with the corresponding
names of the parts of _the surface condenser_, shown in the above
outline sketch: 1, condenser shell; 2, outside heads; 3, exhaust
inlet; 4, exhaust outlet; 5, water inlet; 6, water outlet; 7,
peep holes; 8, tube heads; 9, partition; 10, rib; 11, tubes; 12,
stuffing-boxes.
[Illustration: Jet Condenser.]
NOTE.—The numbers and names of parts in the above figure,
representing in outline a _jet condenser_, are as follows: 1,
condenser body; 2, exhaust inlet; 3, discharge; 4, injection valve;
5, spray pipe; 6, spraying device.
_Steam, however, gives up its heat to a relatively cool surface much
more readily than do the hot furnace gases_, and the positively
circulated cooling water takes up that heat and keeps the temperature
of the surface down, while in a boiler the absorption depends in a
great measure upon the ability of the water by natural circulation
to get into contact with the surface and take up the heat by
evaporization. It has been found, therefore, that a much smaller
surface will suffice in a condenser than in the boilers which it serves.
The Wheeler Condenser and Engineering Company, who make a specialty
of surface condensers, say that one square foot of cooling surface is
usually allowed to each 10 pounds of steam to be condensed per hour,
with the condensing water at a normal temperature not exceeding 75°.
This figure seems to be generally used for average conditions. Special
cases require special treatment.
For service in the tropics the cooling surface should be at least ten
per cent. greater than this estimate. Where there is an abundance
of circulating water the surface may be much less, as with a keel
condenser, where 50 pounds of steam is sometimes condensed per hour per
square foot of surface; or a water works engine, where all the water
pumped is discharged through the condenser and not appreciably raised
in temperature, probably condensing 20 to 40 pounds of steam per hour
per square foot of surface.
Under the division of this volume devoted to “air and vacuum pumps,”
much information has been given relating to the principles of the
condensation of steam and also some illustrations of working machines.
Still it may be well to say this, in addition, that—
All questions in regard to a vacuum become plain when we consider
that the atmosphere itself exerts a pressure of nearly 15 pounds,
and measure everything from an absolute zero, 15 pounds below the
atmospheric pressure. We live at the bottom of an ocean of air. The
winds are its currents; we can heat it, cool it, breathe and handle it,
weigh it, and pump it as we would water. The depth of this atmospheric
ocean cannot be determined as positively as could one of liquid, for
the air is elastic and expands as the pressure decreases in the upper
layers. Its depth is variously estimated at from 20 to 212 miles. _We
can, however, determine very simply how much pressure it exerts per
square inch._
UTILITIES AND
ATTACHMENTS
[Illustration: WORKING SHIP PUMPS BY ROPES.]