STEAM ENGINE ECONOMY
Under the general heading of steam engine economy, such items as
cylinder condensation, steam consumption, efficiency, ratio of
expansion, under- and over-loading, condensing, etc., are treated.
The principal waste of steam in the operation of an engine is due to
condensation during the first part of the stroke. This condensation is
due to the fact that during expansion and exhaust the cylinder walls
and head and the piston are in contact with comparatively cool steam,
and, therefore, give up a considerable amount of heat. When fresh steam
is admitted at a high temperature, it immediately gives up sufficient
heat to raise the cylinder walls to a temperature approximating that of
the entering steam. This results in the condensation of a certain amount
of steam, the quantity depending upon the time allowed for the transfer
of heat, the area of exposed surface, and the temperature of the
cylinder walls. During the period of expansion the temperature falls
rapidly, and the steam being wet, absorbs a large amount of heat. After
the exhaust valve opens, the drop in pressure allows the moisture that
has collected on the cylinder walls to evaporate into steam, so that
during the exhaust period but little heat is transferred. With the
admission of fresh steam at boiler pressure, a mist is condensed on the
cylinder walls, which greatly increases the rapidity with which heat is
absorbed.
The amount of heat lost through cylinder condensation is best shown by a
practical illustration. One horsepower is equal to 33,000 foot-pounds of
work per minute, or 33,000 x 60 = 1,980,000 foot-pounds per hour. This
is equivalent to 1,980,000 / 778 = 2,550 heat units. The latent heat of
steam at 90 pounds gage pressure is 881 heat units. Hence, 2,250 / 881 =
2.9 pounds of steam at 90 pounds pressure is required per horsepower,
provided there is no loss of steam, and all of the contained heat is
changed into useful work. As a matter of fact, from 30 to 35 pounds of
steam are required in the average simple non-condensing high-speed
engine.
There are three remedies which are used to reduce the amount of cylinder
condensation. The first to be used was called steam jacketing, and
consisted in surrounding the cylinder with a layer of high-pressure
steam, the idea being to keep the inner walls up to a temperature nearly
equal to that of the incoming steam. This arrangement is but little used
at the present time, owing both to the expense of operation and to its
ineffectiveness as compared with other methods.
The second remedy is the use of superheated steam. It has been stated
that the transfer of heat takes place much more rapidly when the
interior surfaces are covered with a coating of moisture or mist.
Superheated steam has a temperature considerably above the point of
saturation at the given pressure; hence, it is possible to cool it a
certain amount before condensation begins. This has the effect of
reducing the transfer of heat for a short period following admission,
and this is the time that condensation takes place most rapidly under
ordinary conditions with saturated steam. This, in fact, is the
principal advantage derived from the use of superheated steam, although
it is also lighter for a given volume, and therefore, a less weight of
steam is required, to fill the cylinder up to the point of cut-off. The
economical degree of superheating is considered to be that which will
prevent the condensation of any steam on the walls of the cylinder up to
the point of cut-off, thus keeping them at all times free from moisture.
The objections to superheated steam are its cutting effect in the
passages through which it flows, and the difficulty experienced in
lubricating the valves and cylinder at such a high temperature. The
third and most effective remedy for condensation losses is that known as
compounding, which will be treated under a separate heading in the
following.
Multiple Expansion Engines
It has been explained that cylinder condensation is due principally to
the change in temperature of the interior surfaces of the cylinder,
caused by the variation in temperature of the steam at initial and
exhaust pressures. Therefore, if the temperature range be divided
between two cylinders which are operated in series, the steam condensed
in the first or high pressure cylinder will be re-evaporated and passed
into the low-pressure cylinder as steam, where it will again be
condensed and re-evaporated as it passes into the exhaust pipe.
Theoretically, this should reduce the condensation loss by one-half, and
if three cylinders are used, the loss should be only one-third of that
in a simple engine. In actual practice the saving is not as great as
this, but with the proper relation between the cylinders, these results
are approximated.
Engines in which expansion takes place in two stages are called compound
engines. When three stages are employed, they are called triple
expansion engines. Compounding adds to the first cost of an engine, and
also to the friction, so that in determining the most economical number
of cylinders to employ, the actual relation between the condensation
loss and the increased cost of the engine and the friction loss, must be
considered. In the case of power plant work, it is now the practice to
use compound engines for the large sizes, while triple expansion engines
are more commonly employed in pumping stations. Many designs of multiple
expansion engines are provided with chambers between the cylinders,
called receivers. In engines of this type the exhaust is frequently
reheated in the receivers by means of brass coils containing live steam.
In the case of a cross-compound engine, a receiver is always used. In
the tandem design it is often omitted, the piping between the two
cylinders being made to answer the purpose.
The ratio of cylinder volumes in compound engines varies with different
makers. The usual practice is to make the volume of the low-pressure
cylinder from 2.5 to 3 times that of the high-pressure. The total ratio
of expansion in a multiple expansion engine is the product of the ratios
in each cylinder. For example, if the ratio of expansion is 4 in each
cylinder in a compound engine, the total ratio will be 4 x 4 = 16. The
effect of a triple-expansion engine is sometimes obtained in a measure
by making the volume of the low-pressure cylinder of a compound engine 6
or 7 times that of the high-pressure. This arrangement produces a
considerable drop in pressure at the end of the high-pressure stroke,
with the result of throwing a considerable increase of work on the
high-pressure cylinder without increasing its ratio of expansion, and at
the same time securing a large total ratio of expansion in the engine.
In the case of vertical engines, the low-pressure cylinder is sometimes
divided into two parts in order to reduce the size of cylinder and
piston. In this arrangement a receiver of larger size than usual is
employed, and the low-pressure cranks are often set at an angle with
each other.
Another advantage gained by compounding is the possibility to expand the
steam to a greater extent than can be done in a single cylinder engine,
thus utilizing, as useful work, a greater proportion of the heat
contained in the steam. This also makes it possible to employ higher
initial pressures, in which there is a still further saving, because of
the comparatively small amount of fuel required to raise the pressure
from that of the common practice of 80 or 90 pounds for simple engines,
to 120 to 140 pounds, which is entirely practical in the case of
compound engines. With triple expansion, initial pressures of 180 pounds
or more may be used to advantage. The gain from compounding may amount
to about 15 per cent over simple condensing engines, taking steam at the
same initial pressure. When compound condensing engines are compared
with simple non-condensing engines, the gain in economy may run from 30
to 40 per cent.
TABLE IV. STEAM CONSUMPTION OF ENGINES
+-------------------------+--------------------------------+
| | Pounds of Steam per Indicated |
| | Horsepower per Hour |
| Kind of Engine +----------------+---------------+
| | Non-condensing | Condensing |
+-------------------------+----------------+---------------+
| { High-speed | 32 | 24 |
| Simple { Medium-speed | 30 | 23 |
| { Corliss | 28 | 22 |
| | | |
| { High-speed | 26 | 20 |
| Compound { Medium-speed | 25 | 19 |
| { Corliss | 24 | 18 |
+-------------------------+----------------+---------------+
Steam Consumption and Ratio of Expansion
The steam consumption is commonly called the _water rate_, and is
expressed in pounds of dry steam required per indicated horsepower per
hour. This quantity varies widely in different types of engines, and
also in engines of the same kind working under different conditions. The
water rate depends upon the "cylinder losses," which are due principally
to condensation, although the effects of clearance, radiation from
cylinder and steam chest, and leakage around valves and piston, form a
part of the total loss. Table IV gives the average water rate of
different types of engines working at full load.
The most economical ratio of expansion depends largely upon the type of
the engine. In the case of simple engines, the ratio is limited to 4 or
5 on account of excessive cylinder condensation in case of larger
ratios. This limits the initial pressure to an average of about 90
pounds for engines of this type. In the case of compound engines, a
ratio of from 8 to 10 is commonly employed to advantage, while with
triple-expansion engines, ratios of 12 to 15 are found to give good
results.
[Illustration: Fig. 38. Action of Throttling Governor on Indicator
Diagram]
[Illustration: Fig. 39. Effect of Shaft Governor on Indicator Diagram]
[Illustration: Fig. 40. Increasing Power of Engine by Condensing]
[Illustration: Fig. 41. Decreasing Steam Consumption by Condensing]
The _thermal efficiency_ of an engine is the ratio of the heat
transformed into work to the total heat supplied to the engine. In order
to determine this, the _absolute_ temperature of the steam at admission
and exhaust pressures must be known. These pressures can be measured by
a gage, and the corresponding temperatures taken from a steam table, or
better, the temperatures can be measured direct by a thermometer. The
absolute temperature is obtained by adding 461 to the reading in degrees
Fahrenheit (F.). The formula for thermal efficiency is:
_T__{1} - _T__{2}
-----------------
_T__{1}
in which
_T__{1} = absolute temperature of steam at initial pressure.
_T__{2} = absolute temperature of steam at exhaust pressure.
_Example_:--The temperature of the steam admitted to the cylinder of an
engine is 340 degrees F., and that of the exhaust steam 220 degrees F.
What is the thermal efficiency of the engine?
(340 + 461) - (220 + 461)
Thermal efficiency = ------------------------- = 0.15
(340 + 461)
The _mechanical efficiency_ is the ratio of the delivered or brake
horsepower to the indicated horsepower, and is represented by the
equation:
B. H. P.
Mechanical efficiency = --------
I. H. P.
in which B. H. P. = brake horsepower,
I. H. P. = indicated horsepower.
All engines are designed to give the best economy at a certain developed
indicated horsepower called full load. There must, of course, be more or
less fluctuation in the load under practical working conditions,
especially in certain cases, such as electric railway and rolling mill
work. The losses, however, within a certain range on either side of the
normal load, are not great in a well designed engine. The effect of
increasing the load is to raise the initial pressure or lengthen the
cut-off, depending upon the type of governor. This, in turn, raises the
terminal pressure at the end of expansion, and allows the exhaust to
escape at a higher temperature than before, thus lowering the thermal
efficiency.
The effect of reducing the load is to lower the mean effective pressure.
(See Figs. 38 and 39.) This, in throttling engines, is due to a
reduction of initial pressure, and in the automatic engine to a
shortening of the cut-off. The result in each case is an increase in
cylinder condensation, and as the load becomes lighter, the friction of
the engine itself becomes a more important part of the total indicated
horsepower; that is, as the load becomes lighter, the mechanical
efficiency is reduced.
Effect of Condensing
So far as the design of the engine itself it concerned, there is no
difference between a condensing and a non-condensing engine. The only
difference is that in the first case the exhaust pipe from the engine is
connected with a condenser instead of discharging into the atmosphere.
A condenser is a device for condensing the exhaust steam as fast as it
comes from the engine, thus forming a partial vacuum and reducing the
back pressure. The attaching of a condenser to an engine may be made to
produce two results, as shown by the work diagrams illustrated in Figs.
40 and 41. In the first case the full line represents the diagram of the
engine when running non-condensing, and the area of the diagram gives a
measure of the work done. The effect of adding a condenser is to reduce
the back pressure on an average of 10 to 12 pounds per square inch,
which is equivalent to adding the same amount to the mean effective
pressure. The effect of this on the diagram, when the cut-off remains
the same, is shown by the dotted line in Fig. 40. The power of the
engine per stroke is increased by an amount represented by the area
enclosed by the dotted line and the bottom of the original diagram.
Assuming the reduction in back pressure to be 10 pounds, which is often
exceeded in the best practice, the gain in power by running condensing
will be proportional to the increase in mean effective pressure under
these conditions. For example, if the mean effective pressure is 40
pounds when running non-condensing, it will be increased to 40 + 10 = 50
pounds when running condensing, that is, it is 50/40 = 1.25 times as
great as before. Therefore, if the engine develops 100 I. H. P. under
the first condition, its final power will be increased to 100 x 1.25 =
125 I. H. P. under the second condition.
Fig. 41 shows the effect of adding a condenser and shortening the
cut-off to keep the area of the diagram the same as before. The result
in this case is a reduction in the quantity of steam required to develop
the same indicated horsepower. The theoretical gain in economy under
these conditions will run from about 28 to 30 per cent for simple, and
from 20 to 22 per cent for compound engines. The actual gain will depend
upon the cost and operation of the condenser which varies greatly in
different localities.