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American Technical Society.

Cyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) online

. (page 24 of 30)
Online LibraryAmerican Technical SocietyCyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 24 of 30)
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With one-fifth cut-off and five expansions, the final volume of one
pound would be 5 X 2.75 = 13. 75 cubic feet, and this volume would be
reached at about 11.7 pounds gauge pressure. Fig. 23 will illustrate
this. The line b c d e is a curve representing the relation of pressures
and volumes of steam, as it expands adiabatically from 150 pounds
gauge pressure to the atmosphere and beyond the atmosphere into
partial vacuum. The total available work in the steam above at-
mospheric pressure, would be represented by the area of the diagram
a b e h. The greatest possible work that could be done in the cylin-
der, cutting off at one-third stroke and exhausting at atmospheric
pressure, would be the area ab c g h, which shows that a considerable
amount of the energy is lost. Even cutting off at one-fifth stroke, the
work represented by the area d e j is lost. To carry the expansion of
steam in a single-cylinder engine even to one pound above the
atmosphere, the boiler pressure must be greatly reduced, or the amount
of expansion increased materially. If this engine were made con-
densing, m k would represent the back-pressure line, and while the



349



34 STEAM TURBINES



total available energy would be increased by the area h c m k, the
work in the cylinder at one-fifth cut-off would be increased only by the
area hfj k, a very small part of the whole. In such case, the gain
would probably not pay for the cost of maintaining the vacuum.

In a compound two-cylinder engine taking steam at 150 pounds
gauge, the ratio of high- to low-pressure cylinder volumes would be
not over 1 to 5, and with cut-off on the high-pressure cylinder at one-
third stroke, there would be room for not over 15 expansions; that is,
the volume of steam at the end of the low-pressure stroke would be
not over 15 times the volume of the steam admitted. Now, if one
pound of steam at 150 pounds gauge pressure were expanded to 15
volumes, the result would be 15 X 2.75 = 41.25 cu. ft. One pound
of steam thus expanded from 150 pounds pressure will occupy 41.25



ISO Ibs. Gauge




Fig. 23. Relation of Pressure and Volume in Non-Condensing Reciprocating Engine.

cubic feet when the pressure has reached approximately 7.5 pounds
absolute, which would correspond to a vacuum of approximately 15
in. In other words, neglecting the condensation and other losses in
the cylinder, the ordinary compound engine with Corliss gear (an
engine in every way first class), cutting off at one-third stroke, cannot
expand steam at 150 pounds boiler pressure lower than to 15 in. vacu-
um. Any increase of vacuum beyond this point tends only to reduce
the back pressure on the piston, and the gain in work is slight, per-
haps not enough to pay for the additional work on the air pump,
increased size of condenser, and additional circulating water.

Fig. 24 shows, as before, the adiabatic expansion from 150
pounds gauge pressure. If a b represents one volume, h f would
represent fifteen, h f will be the back pressure line at 15 in. vacuum,
the maximum theoretical work done in the cylinder will be the area



350



STEAM TURBINES 35



a b d j h, and the work lost will be the area h f m k. Increasing the
vacuum below 15 in. gives only a little gain, represented by the area
hfgk, although more than in the previous case.

A triple-expansion engine will permit of about twenty expansions;
that is, the low-pressure cylinder will contain about twenty times
the volume displaced by the piston at cut-off in the high-pressure
cylinder. In such an engine, the final volume of one pound of steam
expanding from the previous pressure will be 55 cubic feet, and the
pressure corresponding to this volume would be 5.5 pounds absolute,
equal to about 19 in. vacuum. A condenser giving 24 in. vacuum
would allow just about difference enough to give a ready flow of steam




Fig. 24. Relation of Pressure and Volume in Condensing Reciprocating Engine.

from the engine to the condenser. If a greater vacuum is to be used
to advantage, the number of expansions must be increased. Even
here, increasing the vacuum beyond 19 in. gives relatively little gain
in the engine. To expand steam from 150 pounds gauge pressure
to 28.5 in. vacuum would require a final volume of 338 cubic feet for
each pound of steam admitted to the cylinder, and since one pound at
initial pressure occupies 2.75 cubic feet, the steam would have to ex-
pand 338 -v- 2.75 = 123 times, approximately. The utter impossibil-
ity of such expansion in the triple expansion engine will be evident
from the following consideration:

If a triple-expansion engine were to expand the steam to this
pressure, with cut-off at one-third stroke, the low-pressure cylinder
would have a volume 123 -j- 3 = 41 times that of the high-pressure
cylinder, and its diameter would be to the diameter of the high,
as 1 is to the square root of 41, or about 6.5. This ratio is not
far from three times that found in general practice for such an engine,



351



36 STEAM TURBINES



and about four times that for a compound engine. Assuming that
the low-pressure cylinders are now as large as they can conveniently
be made, the complete expansion above outlined would require,
in the triple-expansion engine, three low-pressure cylinders of the
present size. Radiation loss and friction could easily overcome the
theoretical gain ; to say nothing of the prohibitive cost and weight of
the engine.

Consider the diagram in Fig. 25, which shows, as before, the
adiabatic expansion between 150 pounds gauge and 28.5 in. vacuum.
The black area represents the available work due to the complete
expansion of the steam, in excess of that available in the triple-ex-
pansion engine, running under 28.5 in. vacuum. This lost energy is



. er&stifufe




Atmospheric L ine



134 Volumes



Fig. 25. Energy in Steam not Available for Reciprocating Engine.

about 25% of the total energy available in the steam, or about 35%
of the energy available for use in the reciprocating engine with 28.5 in.
back pressure. Under the ordinary conditions of 25 in. back pressure,
the black area would be augmented by the crosshatched area, mak-
ing the lost energy about 40% instead of the 35% above. All of this
energy is lost by the triple-expansion engine, but can be utilized by
the turbine. Low vacuums cause large initial condensation in recipro-
cating engines, but do not have any disadvantageous effect on the
turbine.

The low-pressure turbine can be advantageously used in
connection with any reciprocating engine, and their combination
will always afford a considerable improvement in economy, and
increase the power without increasing the size of the boiler plant.
It often happens that engines are operated non-condensing because of



352



STEAM TURBINES 37



the expense of cooling water, and as we have already shown, the rel-
atively small gain would not pay for additional complications and
expense, especially if cooling towers have to be provided. The low-
pressure turbine, however, will provide enough additional power to
pay for the installation of proper equipment. There are already in
existence, plants where low-pressure turbines have been installed in
connection with engines previously used as non-condensing, and the
output has been practically doubled without increased cost for fuel.

It is readily seen from the previous discussion, that even in a
plant in which the engines are operated as condensing engines, a
considerable gain can be effected by installing a low-pressure tur-
bine, even though using the same condenser facilities as before.
In some ways, it is much easier to maintain a high vacuum in such
a combination, because the turbine will take the steam at -slightly
above the atmospheric pressure, and thus prevent a considerable
amount of air leakage, which always takes place through the the stuf-
fing-boxes of a low-pressure reciprocating engine.

If saturated steam expands adiabatically from 150 pounds
gauge to a pressure of 28.5 in. vacuum, practically half the
available energy is developed between the initial pressure and one
pound above the atmosphere, and the other half below the latter
pressure. It might be said, in explanation, that the work of expan-
sion can be considered as equal to the pressure times the volume; but
it is, perhaps, not often realized that the volume of steam will nearly
double in expanding from 26 in. vacuum to 28 in., and that,
therefore, the available energy is great, although the pressure is low.
In most condensing engines, the gain over non-condensing conditions,
as determined by actual experiment, does not exceed 30%, even under
favorable conditions of steady load. Under average conditions, the
gain drops to 25%, and under overload conditions, to a still lower
point. In general, a condensing reciprocating engine, if run non-
condensing, will carry about 70% of its maximum load, exhaust-
ing at, say, two pounds above atmospheric pressure, and, if the
steam from such an engine be exhausted into a low-pressure turbine
with proper condensing facilities, the latter will develop nearly as much
work as was developed by the engine itself, and there will result
from the two about 140% of the work which might be expected
from the reciprocating engine alone, if run condensing. It is inter-



35:*



38



STEAM TURBINES



esting to note that the discussion of Fig. 25 seems to show a
possible theoretical gain of about 40% over the engine condensing
at 25 in. vacuum, provided the turbine is run at 28.5 in. vacuum.

Fig. 26 shows a study of the possibilities in connection with a
Rice-Sargent engine which has been operated for some years in the
plant of the General Electric Company at Schenectady, N. Y. This
unit operates a 250-v. direct-current generator, and ordinarily runs
with a load of 1,200 kw.

















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Results
connect!
Turbine
Points rr
equal s


th D.C.250Volt Generator
Compared with
obtainable from same ertyine in
on with low pressure A. C.

arhec/ O, X,9, correspond to
earn flows.


















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frtyinf and tow /are




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70O SOO IIOO 1300 /5OO I7OO /9OO 1OO 2300


2500



K.W. Output
Fifr. 26. Curves Showing Economy of Engine with Low-Pressure Turbine.

*The tests were made accurately by weighing condensed steam, the effect
of vacuum being determined by holding the steam flow constant, and chang-
ing the vacuum. The curves show performance under condensing and non-
condensing conditions, and also show what could be accomplished by this
engine in combination with a good low-pressure turbine. The rates of gain
here shown will seem extraordinary, but they are fairly representative of the
possibilities in the average condensing engine plant.

Referring to the curve-sheet, note that the upper curve represents the
engine operating non-condensing at 810 kilowatts, the steam consumption
being 30.6 pounds per kilowatt. With the load increased to 1,065 kilowatts,
the steam consumption is still 30.6 per kilowatt and with the load increased
to 1,265 kilowatts, the steam consumption is 33.6 pounds. Operating under
these conditions, 1,265 kilowatts is practically the maximum capacity of
the unit.

Now, operating condensing with a capacity of 1,140 kilowatts, the steam
consumption is 22 pounds per kilowatt; at 1,320 kilowatts, the steam consump-
tion is 24.6 pounds per kilowatt; and operating at 1,470 kilowatts, the steam
consumption is 28.8 pounds per kilowatt. Note, however, that the maximum
capacity of the unit has been increased from 1,265 to 1,470 kilowatts.

*Frora a paper by Chas. B. Burleigh, on the "Low-Pressure Steam Turbine."



354



STEAM TURBINES 39.



Now, by the assistance of the low-pressure turbine, vacuum conditions
remaining the same, the steam consumption at 1,550 kilowatts is 15.6 pounds
per kilowatt; at 2,020 kilowatts, the steam consumption is 15.4 pounds per kilo-
watt; and at 2,500 kilowatts, the steam consumption is 17 pounds per kilowatt.
By this combination, the maximum output of the unit has been increased from
1,265 kilowatts, non-condensing, to 2,500 kilowatts, or from 1,470 kilowatts
condensing, to 2,500 kilowatts.

It must not be thought that all this gain can be attained with
no compensating loss. In the first place, a surface condenser, to
maintain 28.5 in. vacuum, must be about twice the size of one to
maintain 26 in., and requires special apparatus that is not only costly,
but difficult to maintain. Again, the cost of maintaining a 28.5 in.
vacuum is very much more than that of maintaining a 20-in. vacuum,
leaving out of consideration the extra cost of condenser and cooling
water. After all, it is the dollars and cents that determine the best
efficiency, and it is poor economy to obtain the extra power at a
greater cost than the returns will warrant. A gain of .35% or more in
steam consumption may easily be effected by installing a low-pressure
turbine, but the gain in dollars and cents is seldom as great; just what
the gain may be, must of course depend upon the local conditions,
especially upon the conditions under which the reciprocating engine
is operating. In the majority of cases, such installations are worth
while, even though used with the usual vacuum.

An interesting application of the low-pressure turbine in con-
nection with rolling mill machinery and other intermittent work,
has been worked out by Professor Rateau, and has been made pos-
sible by the use of his steam accumulator, or regenerator. This ap-
paratus regulates the intermittent flow of steam exhausted from the
rolling mill engine, let us say, and intended to be used by a low-pres-
sure turbine. The accumulator may consist of a large tank in which
are numerous plates over which water can flow, or may contain simply
water rapidly circulated by artificial means. As the exhaust steam
from the engine enters this accumulator, it spreads out over the ex-
posed water surface, and some of it is condensed if there is an excess
of pressure due to more steam being supplied by the exhaust than is
being utilized by the turbine. On the other hand, if the turbine
utilizes more steam than is supplied by the exhaust, this causes a
lowering of the pressure in the accumulator, and a rapid vaporiza-
tion occurs from the exposed water surfaces, tending to equalize the



355



STEAM TURBINES



pressure. The accumulator thus bears the same relation to the
transfer of heat from the reciprocating engine to the turbine that a
fly-wheel bears to the transfer of work from the cylinder of the engine
to mill shafting. Fig. 27 shows one form of the Rateau accumulator.
It must be provided, of course, with a safety-valve, set at a pre-



Ejrhaust Steam from
Winding Engine





Bottom Basin
^ in one piece



Fig 27. Interior View of Rateau Accumulator, with Iron Trays.

determined pressure, and is usually provided with a reducing valve from
the boiler, so that in case the reciprocating engine should stop for a
considerable length of time, steam could still be supplied to the
turbine through the reducing valve.

The first apparatus of this kind was installed in 1902, and has
been very successful. The first to be installed in the United States was
at the Wisconsin Steel Company, in South Chicago. In this plant,



356



STEAM TURBINES 41



steam first goes to a receiver to take out the shock due to the puffs
of the exhaust. From here it passes to the regenerator. The
receiver is fitted with baffle plates and drains for water and oil, by
means of which they are thus separated from the steam. This
accumulator at South Chicago furnishes steam for a low-pressure
Rateau turbine which is used to furnish electric power for general
purposes.

Installation. The field of the steam turbine is unfortunately
limited in its usefulness by two very important factors; first, its
relatively high speed of revolution, even when compounded; and,
second, its non-reversibility. If, as in marine work, reversing is
absolutely 'necessary, then another turbine, which runs idle ordinarily,
with vanes set in the opposite way must be fitted on the shaft. To
make this reversing turbine as small as possible, efficiency is sacrificed,
but this is of small consequence, for it is used so little. It of course
adds materially to the first cost of the turbine and increases the
length of the necessary floor space.

The first and greatest field of turbine usefulness is undoubtedly
central station work for the generation of electricity by direct-con-
nected apparatus. It also has an important field in driving blowers,
centrifugal pumps, etc., where high speed of revolution is essential.
In such cases, it has a distinct advantage, for it may be direct-con-
nected, thus doing away with the belting necessary if reciprocating
engines were used. The turbine has been suggested to some extent
for driving mill shafting, in which case, of course, the speed is belted
down from a small pulley on the turbine to a large one on the counter-
shaft, but this appears to offer no particular disadvantage, for in
any case belting would be used, as the countershaft would never be
run at the same speed as the ordinary reciprocating engine.

In the field of electric generation the turbine to-day has prac-
tically superseded the reciprocator. The number of installations
is very great, and probably no new central station is now designed
for other than steam turbines. In 1906 the Committee of the Na-
tional Electric Light Association, after an extensive investigation
of turbines, reported a wide use of turbines for electric generation,
and their figures showed that about 75% of all the turbine units
of 500 kw. or over already installed in the United States, were for
electric purposes, and that practically only one new central station



357



42



STEAM TURBINES



abroad had been found installing reciprocating engines. The
distinct advantage of turbines for this work is the uniform turning
effort, the high speed of rotation permitting the use of a very, much
smaller generator, and the smaller floor space, requiring less capital
outlay in land and engine-house. These features place it in striking
contrast with the ponderous slow-moving Corliss engine.

The General Manager of the Metropolitan Street Railway Co.
of Kansas City is authority for the statement that in that station
six 5,000 kw. units of a well-known make of turbine could be installed
in space previously occupied by three 3,000 kw. engine-driven




1'lan and Elevation of 500-K. W. Westinghouse Turbo-Generator. This is
Same Scale as Fig 29; Notice Difference in Space Required.



units. Or in other words, 30,000 kw. of turbine power could be
put into a building where before only 9,000 kw. of engine power
had been possible. This probably is greater than would ordi-
narily be met with, but the difference in any case is large, the sav-
ing in space depending upon the type of turbine. The average
horizontal turbine and generator with auxiliary apparatus will occupy
about three-fifths of the space needed for a slow-speed, engine-driven
generator of the same power, and a vertical turbo-generator somewhat
less space than the horizontal.

A further distinct advantage of the turbine is in the fact that,
since there are no valves to adjust, the efficiency can be lowered only
by wear, and then only slightly; on the other hand, in reciprocating
engines, if the valves are not set exactly right, very poor economy



358



STEAM TURBINES



1:1



will result, and the opportunities for wear are far greater than in
turbine engines. Again, the turbine can use high degrees of super-
heat because there is no lubricant to burn; there is also little danger
of entrained moisture in the steam wrecking the turbine, and the





Fig. 29. Plan and Elevation of 500- K. W. Corliss Engine-Driven Generator Set.
Compare with Fig. 28.



absence of oil in the condensed steam greatly lessens trouble in the
boiler if the condensation is used for feed water. The economy of
space was graphically illustrated by Fig. 1, and Figs. 28 and 29 tell
the same story but with different types of engine and turbine.



359



44 STEAM TURBINES



Figures showing the relative space occupied by reciprocators
and turbines are of little value unless the size of condenser and
condensing auxiliaries are taken into consideration, for, as before
mentioned, they may easily be, in the case of the turbine, twice the
size of those used with a reciprocating engine of the same power.
The apparent saving of space, therefore, may be offset by these
auxiliaries. By placing the condensers underneath the turbine,
as is frequently done at the present time, not only may a consider-
able amount of floor space be saved, but the turbine can more readily
exhaust into the condenser. As we have already seen, at high vacuum
the volume of steam is very large, and the exhaust pipe from the
turbine will be proportionally large. It would thus appear that to
have the condenser any great distance from the low-pressure end of
the turbine would be not only a distinct disadvantage, but offer a
considerable practical difficulty.

Turbines, as we have seen, require very much smaller founda-
tions than reciprocating engines of the same power, and these founda-
tions will therefore cost very much less. It is hard to get a direct
comparison between turbines and reciprocating engines as a class,
because the foundations for high-speed reciprocating engines will
not be as massive as for the heavier, low-speed engines. The tur-
bine, occupying less floor space, will require smaller buildings and
less land, and this will in a number of cases be a substantial saving
in first cost and subsequent interest charges.

So far as the first cost of a generating plant goes, there is at
the present time very little difference between those using reciproca-
ting and those using turbine engines. The turbine itself costs more
than the reciprocating engine of the same power, but on the other
hand the generator for the turbine costs very much less. Again,
the condenser and pumps, if high vacuum is to be maintained, will
cost two or three times as much as for the reciprocating set, while
the cost of erection is decidedly in favor of the turbine. It is not
easy to get a direct line on the relative cost of turbine and engine
installations, for the figures available appear to vary about as much
between reciprocating engines and turbines as might be expected
to be found between various installations of reciprocating engines,
and undoubtedly turbine installations in some cases cost relatively
more than in others. It seems probable that the cost of the turbine is



360



STEAM TURBINES 45



regulated more by the cost of the reciprocating engine with which
it has to compete, than by the actual cost of manufacturing the
turbine. All in all, there is likely to be -a somewhat less cost of com-
plete installation in favor of the turbine, but the difference will not
be large- in any case, and in powers under about 100 kw., it is
probable that the engine installation is fully as cheap. This does not
take into account the value of land and buildings, which in all cases is
an important factor in favor of the turbine.

Performance. The losses occurring in the steam turbine con-
sist principally of loss of velocity of the steam itself due to friction
in contact with the vanes and guides; friction of the disks revolving
through a chamber filled with steam; eddying of the steam jet, due
to improper speed of the revolving disks; radiation; bearing friction.
The two latter items are not large, and under ordinary conditions



Online LibraryAmerican Technical SocietyCyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 24 of 30)