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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 23 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 23 of 30)
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steam upon the vanes in the next wheel, called velocity compounding;
or by a combination of these two methods.

Suppose we start with steam at 150 pounds gauge pressure and
expand it to 28 in. vacuum, not in one expanding nozzle but in several
stages, so that the expansion in each would be to only about 60% of
the next higher pressure, in which case, diverging or expanding
nozzles would not be needed. The velocity of flow of the steam
would be somewhat less than 1,450 feet per second at pressures above
the atmosphere, and w r ould decrease slightly as the pressure lowered;
the lowest velocity, when discharging into a vacuum of 28 in., would
be about 1,250 feet per second, but, by letting the drop in pressure be
somewhat less than 60% in the higher stages, the velocity of flow
could be made approximately 1,250 feet per second throughout.
This is, of course, neglecting all losses. We could then have a steam
speed of about 1,250 feet per second to deal with, instead of 4,000; the

peripheral speed of the buckets would be = 625 cos a,

or, when a, is small, about 600 feet per second. For a wheel 5 feet
in diameter, this would mean about 2,300 revolutions per minute,
and the conditions arising from such a speed are much more easily
taken care of. This reduction in speed could be accomplished in
about ten stages. To reduce the speed to half, or 300 feet per



337



STEAM TURBINES



second, would require four times as many stages because the num-
ber of stages would be equal to the square of the ratio of reduction
of the steam velocity. Thus, the reduction from 4,000 feet per

(4 000 \ 2 *
F250' = 3 ' 2 = 10 '

approximately. To reduce the speed of the buckets from 600 to 300
feet per second, would evidently require four times as many, or
40 stages.

Suppose, now, the compounding were all in velocity stages, and
that the expansion occurred in one nozzle. The velocity of steam
would be nearly 4,000 feet per second, but it would have to pass
through three sets of revolving wheels to bring the relative speed at
each wheel to approximately 1,300 feet per second, and thus get the
same speed of revolution as in the previous case.

This method of compounding gives a very compact form of
turbine and one that has many mechanical advantages ; but the wheels
have to revolve in a bath of steam which makes the friction excessive,
and the efficiency correspondingly lower. This was the idea of the origi-
nal Curtis patent, but was soon abandoned for the combined pressure
and velocity turbine which is to-day the principal feature of the Curtis
design. In this combined method, there are two or more pressure
stages, and in present practice, not over two velocity wheels and one
set of guide vanes to each stage.* The older Curtis had even three
or four revolving wheels per stage with a corresponding number of
sets of guide vanes.

Compounding has been tried by the use of counter-running
wheels, but with little success. If the guide vanes were on wheels
free to turn, they would run in a direction opposite to the others,
and if the relative peripheral velocity of the two were, say, 1,200 feet
per second, it would mean that fop each wheel the absolute velocity
would be half this, or GOO feet per second. The difficulties of build-
ding and operating such a machine are considerable.

Types of Turbines. There are two main groups into which
steam turbines are usually divided, one known as the impulse, &nd
the other as the reaction type. It has become the general practice

*This does not apply to marine practice, the peculiar conditions of which
warrant the use of a larger number of A^elocity wheels per stage. Small
Curtis turbines and some special machines have three velocity stages.



STEAM TURBINES 23



to classify turbines under one or the other of these two general heads ;
but, as a matter of fact, every commercial turbine of the present time
really develops its power under the combined influence of action
and reaction. Yet there is a distinct difference between the expansion
of steam in these two types, as, for instance, in the DeLaval and
in the Parsons turbine. The use of the terms impulse and
reaction in reference to turbines is undoubtedly unfortunate, but
since their use has become practically universal, it is necessary to
understand the significance of their application.

In the so-called impulse turbine, the steam, expanding in a
nozzle or other suitable passage, thus attains a high velocity, and
impinges upon the vanes of a rotating wheel. The steam, in passing
through the wheel, gives
up a part of its kinetic
energy to the revolving
vanes, and leaves the
wheel at a lower velocity,
but at the same pressure
at wkich it left the noz-
zle. In the so-called re-
action type, the steam
enters the turbine at
boiler pressure, passes Fig. 1 7 Two Arrangement of Jet and Vane, (a) Pure

Action. (6) Action and Reaction.

through guide passages

onto the vanes of a rotating wheel, and little by little expands as it
passes through these vanes to subsequent guide passages and other
vanes, the pressure gradually becoming lower; the velocity which is
gained by the expansion in the guide passages and revolving vanes is
practically all imparted to the rotating drum. The pressure is less
on the one side of the vane than on the other, while, in the impulse
type, the pressure is the same on both sides of the vane.

The engines of Hero, Wolfgang De Kempelen, and Avery were all
purely reaction types, but the Parsons acts by impulse as well as
reaction, and the Curtis and DeLaval, by reaction as well as by im-
' pulse. To make this clear, consider Fig. 17 (a), which shows a vane
and jet. The vane is so shaped that the jet leaves it at right
angles to the direction of impact. Here is a case of pure action, so
far as any force tending to move the vane in a direction parallel to the




2!



STEAM TURBINES



direction of the jet is concerned. There is, to be sure, a reaction of
the jet, but this reacting force is along a line A B at right angles to
the desired line of motion, and if the vane shown in this figure were
attached to the periphery of a wheel free to revolve, this force of

reaction would cause only an end-
thrust on the shaft, in no way
augmenting the force of rotation.
As previously shown on Page
10, to obtain the best efficiency
. the jet must be deflected through
an angle of 180. If the jet leaves
the wheel at any less angle than
90, for instance, angle B AC in
Fig. 17 (b), there is a reactive
force along the line AC, which can
be resolved into two components
one, A B, tending to cause
rotation, the other, B C, causing
an end thrust. A turbine thus
constructed, although called an
impulse turbine, evidently derives
an impelling force from this reac-
tion. A pure impulse turbine,
permitting no reaction of the jet,
would have a theoretical maxi-
mum efficiency of only 50%.
When the jet is turned through
an angle of 180, the reaction
becomes equal to the impulse.
The reaction is equal to the im-
pulse in any case, when the angle
at which the jet impinges upon
the vane is equal to the angle of
deflection measured from a plane
through the center of the rotating wheel at right angles to the shaft.
In this type of turbine, all the expansion takes place in the nozzles
or guide passages, none at all in the revolving vanes.

In the so-called reaction turbines, the expansion takes place




Steampipe



Fig. 18. Typical Features of Single-Stage

Impulse Turbine, with Relations of

Steam Pressures and

Velocities.



340



STEAM TURBINES 25



in. the revolving vanes as well as in the guide passages, and the
vanes and guides are placed so as to give a constantly increasing area
of passage to allow for the increasing volume of the steam as it
expands. As the steam expands in the guide passages, it acquires
velocity and impinges upon the running vanes, thus giving a decided
impulse to them, and as it again expands in the running vanes, the
reaction produces a further impelling force.

The distinguishing feature, then, between these two distinct
types of turbine is not to be found in the impulse or reaction of the
steam at all, for both types, as we have seen, act by virtue of both
forces; but the distinction lies in whether the expansion of the steam
takes place fully in a set of nozzles or guide passages with no expan-
sion in the moving vanes, or whether the steam expansion takes place
partly in the nozzles and partly in the revolving vanes. A turbine
might be so arranged that the expansion would take place entirely in
the moving vanes, the guide passages acting merely to change the
direction of the steam, but as yet no commercial turbine has been
built on these lines.

There are several distinct subdivisions of the two main types
of turbine. The simplest form is undoubtedly of the DeLaval type,
which consists of several diverging nozzles, expanding the steam
from boiler pressure to exhaust pressure, and directing the steam jets
onto the vanes of a single wheel. We have seen that the enormous
velocity of 4,000 feet per second will be attained in expanding from
150 pounds boiler pressure to 28| inches vacuum. The speed of
revolution must be very high and, although the velocity is greatly
lowered as the steam passes through the wheel, it will leave the
wheel with a considerable residual velocity which represents, of
course, so much lost energy. Fig. 18 illustrates the typical features
of this style of machine, the curves showing the relation of its
steam velocities and pressures. It will be noticed that the steam
pressure is a maximum, and equal to boiler pressure, at the inlet to
the nozzle, and will reach the condenser or exhaust pressure at the
nozzle outlet, as it impinges upon the vanes of the wheel. Clear-
ance, in this type of turbine, is of small consequence, for the wheel
revolves in steam of a uniform pressure, and there can, therefore,
be no leakage of steam without work being done. As there is
but one wheel revolving in the bath of steam, the friction would



341



2t;



STEAM TURBINES



not be very great, were it not that the friction increases very rapidly
with the speed, and in this single-wheel type, the speed of the steam is
very high. The chief loss will be due to the relatively high velocity
of the exhaust steam, and to the friction of the bearings on account of
the high rotative speed. To reduce these speeds of rotation to man-







Shaft



Boiler Press
















Throat Press


\




^
















>








>








,-


|








.f








!








1




Throat vel. _




\


V 1






/


\1






/


\S






/


^






-^

'


\


Pr^ss


in


'




\




'




\








\








Vet in Seam





fl e


,






\


Presk


re






A








\


Lost Vet.


Inlet'


'




In




..


isa


lr


w


0.


:.f"'


: ir


to










Steam Pipe "N&zie Wanes* >!No'zz/e ^Vones* ^Nozzle Wanes* "Nozzle 'Wanes*' Exhaust Pipe
\Thraat [Throat Throat Throat



Fig. 19. Features of Multi-Stage Impulse Type, Showing Relations of
Steam Pressures and Velocities.

ageable rates, gearing must be used, causing a further frictional
loss, or the diameter of the wheel must be abnormally great.

The velocity of the steam at the entrance to the nozzle is that
due merely to its flow through the pipe. At the throat of the nozzle,
the velocity, as we have previously seen, will be something under
1,500 feet per second, and at the mouth of the nozzle, if it is properly



342



STEAM TURBINES 27



designed, the velocity will approximate 4,000 feet per second, assum-
ing a boiler pressure of 165 pounds absolute and 28 -J- inches of
vacuum. This high velocity will not be maintained, however, as
the steam passes through the revolving vanes, but, at the condenser,
will have dropped to a value depending upon the amount of energy
absorbed from the steam during its passage through the vanes of the
wheel.

If wheels were arranged in successive chambers, so that the
steam could be expanded in several steps instead of in one, we should
have the essential elements of the Rateau type of turbine. Fig. 19
shows diagramatically the essential features of this type of turbine,
and the relation of velocities and pressures, as before. Each wheel
rotates in an independent chamber separated from the next by a
diaphragm provided with suitable expanding passages, so that the
steam, in passing from the first chamber to the second, will be under
conditions similar to those obtaining when passing from the boiler
into the first chamber, and again may attain a maximum velocity.

In a four-stage turbine of this sort, the pressure, as shown in the
curve in Fig. 19, should be a maximum (boiler pressure) at the inlet
to the first nozzle. At the throat of the nozzle, it should be approx-
imately 58% of the initial pressure. During its passage through the first
chamber, the steam pressure would be constant, and it would again
drop in a similar manner, in passing through the nozzles between the
first and the second chamber, the velocity rising with each drop in
pressure. With a four-stage turbine, the drop in pressure would be
such that one-fourth of the total available heat units would be
available in each chamber. The drop in pressure from chamber to
chamber would therefore not be uniform, for a given pressure
change represents more heat units in the lower than in the higher
ranges of pressure. The velocity at the inlet to the first nozzle would
again be merely the velocity of flow through the steam pipe; at the
throat of the nozzle, approximately 1,500 feet per second, and at the
outlet to the nozzle, where the steam impinges upon the vanes of the
first wheel, approximately 2,000 feet per second. This velocity will
drop as the steam passes through the wheel, rising again on its pas-
sage through the next nozzle, dropping again in the next wheel, and
so on, the residual velocity as the steam leaves the last wheel being
probably less than in the previous case.



28



STEAM TURBINES



Turbines are built on this principle by a number of manufac-
turers, the Rateau being the best known of this type. This particular
turbine has usually a large number of chambers, frequently 30 to 40,
and the drop in pressure from chamber to chamber is consequently

very small, so small in
fact that expanding noz-
zles are not




necessarv.



This type of turbine is
subject to leakage at A B,
Fig. 19, where the shaft
passes through the sta-
tionary diaphragm and
requires special packing.
This packing becomes
evidently inaccessible in
a multi-stage turbine.

A simple method of
compounding, but one
not likely to produce as
economical results, is
that shown diagramatic-
ally in Fig. 20, its vari-
ations of pressure and
volume being shown in
the curve. In this tur-
bine, steam is expanded
in a properly designed
diverging nozzle, from
boiler pressure to exhaust
pressure, and impinges
successively upon the
vanes of rotating wheels.
Between these wheels
are stationary guide vanes curved in the opposite direction, so that,
as the steam leaves the first set of vanes, it is redirected by these
guides upon the next set, and so on. The boiler pressure is exactly
similar to the boiler pressure shown in Fig. 18; the velocity of the
steam as it leaves the nozzle is also the same. This velocity drops



Boiler Press




Fig. 20.



Features of Impulse Turbine Compounded
by Velocity Steps Only.



344



STEAM TURBINES



29




lionary \-Nozzlea <.Stationaru
Vanes ,r*,r,~ Vanes



somewhat as the steam passes through the first set of running vanes,
remains constant as it passes through the first set of guide vanes,
again drops in the next set of running vanes, and again becomes con-
stant in the guide vanes, and so on; the velocity of the steam jet is
gradually lessened as it passes through wheel after wheel. The drop
in velocity in the steam in its passage through any one set of vanes
will, neglecting losses, be
approximately equal to
the total velocity divided
by the number of sets
of running wheels. In
this type of turbine, since
the velocity of the steam
is gradually decreased, it
is evident that if the same
quantity of steam is to
flow through successive
wheels in the same inter-
val of time, the passages
must gradually increase
in size. The velocity re-
maining constant in the
guide vanes,they may pro-
vide passages of uniform
section, as shown in Fig.
20, each set of passages,
however/ being larger
than the preceding set.
The principle just
described was the origi-
nal idea claimed in the
early Curtis patent, but was subsequently given up for the im-
proved arrangement shown in Fig. 21. This arrangement differs
from the other, in that, instead of fully expanding the steam in one
nozzle or set of nozzles from boiler to exhaust pressure, the expan-
sion is divided into two or more stages. This turbine contains cham-
bers, just as the Rateau type does, the difference being that in the
Curtis, each chamber contains two sets of running wheels and one set







Fig. 21. Features of Turbine Compounding by
Pressure Stages and Velocity Steps.



345



30



STEAM TURBINES



of guide vanes, while the Rateau chamber contains only one wheel
and no guide vanes. Turbines of the Curtis type have from two to
seven pressure stages, but at the present time, no more than two
sets of running vanes are used in each chamber*, although formerly,
more sets of running vanes were used. The relation of pressures to
velocities shown in Fig. 21 will be evident from the previous ex-
planations.

In the reaction tur-
bines, of which Parsons'
is the best known ex-
ample, the steam, as
already stated, gradually
expands in passing from
boiler to condenser pres-
sure. The velocity rises
in the first set of station-,
ary vanes, and drops as
the steam does work in
the first set of running
vanes. The velocity rises
again in the next set of
stationary vanes, drops
in the moving vanes, and
so on. Fig. 22 shows the
essential features of this
turbine and the relation
of pressures and volumes.
The stationary guide
vanes act just like small
nozzles, and allow the
steam to expand and acquire velocity. The moving vanes also allow
the steam to expand, arid the reaction of this expansion gives an
added impulse to the rotating wheel.

It is not intended that the foregoing shall be a description of
any turbine, but merely a description of the distinct and elementary
features of the action of steam in various types of turbine.




Fig. 22. Features of Reaction Type.



*See foot-note, Page 22.



STEAM TURBINES



In the one-stage, compound-velocity turbine, the steam leaves the
nozzle at exhaust pressure with a high velocity. If the expansion has
been complete, as intended,. the pressure remains constant as the steam
passes through the turbine, and there is no tendency to leakage.
The clearances between the blade tips and the casing can be made
as large as convenient, for it requires a difference of pressure to cause
steam leakage. If running on vacuum, there would be a tendency
for air to leak in around the shaft, and consequently this would need
to be well packed.

Here would seem to be a happy solution to the problem of steam
leakage, at the same time producing a most compact form of turbine;
but, unfortunately, a considerable loss is brought about by steam
flowing past the surfaces of both moving and guide vanes and by the
large amount of friction, due to the rotation of the many wheels
through the steam which fills the turbine. A further serious disad-
vantage is that an equal amount of work cannot be done in each set
of vanes if the entrance and exit angles of the vanes are made
equal, as is usually the case. For example, suppose a 4,000 foot
steam velocity to be reduced in four wheels, each wheel absorbing
1,000 ft. per sec. Then, if V v V v V 3 , and F 4 represent the respective
velocities at the entrance of each wheel, the available energy is

W V 2 W V 2 W
for the first wheel, ^- -^ = -^ X 7,000,000;

y[ y 2 WV * W
for the second, -^ ~- = X 5,000,000;

J *"*[/ J

for the third,] ^ X 3,000,000;

W
and for the fourth, X 1,000,000.

This difficulty will be remedied by increasing the number of
pressure stages, and decreasing the number of wheels in each stage
to a minimum. With a large number of velocity compound wheels,
the work done by the last wheel would be so small that the frictional
losses would be too large to make it at all economical. For example,
with six wheels, the last wheel would develop only 9% of the power
developed in the first. In turbines of this type, by a suitable



347



STEAM TURBINES



design of the nozzles and entrance and exit angles of the vanes, the
same amount of steam energy may be abstracted in each pressure stage.
The leakage in this type would be relatively small, only what
would pass from stage to stage. This would be comparatively small,
because the steam could escape only through the opening where the
shaft passes through the diaphragm (.4 B Fig. 19) that separates the
two chambers, and with small clearances this could not be large.
With a large number of stages, as in the Rateau turbine, leakage in
the high pressure end is not all lost, for it has an opportunity to
work in the lower stages.

In the reaction turbine, leakage of steam is a most important
factor. As the pressure on the two sides of the vane is different,
there is a tendency for the steam to escape between the tips of the
vanes and the outer casing A B, Fig. 22, also between the ends of the
guide blades and the rotor C D, Fig. 22. As the rotors are of large
diameter, a large area is offered for leakage, unless the clearances
are kept very small. Here, the steam leaking from the higher pres-
sures, will of course do work on the lower pressure, but at a less
efficiency, just as in the Rateau type. The successful turbine of this
type requires great nicety of workmanship in order that the clearances
may be adjusted to a minimum.

Low=Pressure Turbines. The greatest drawback to improve-
ment in any existing engine plant, or, in fact, in any mechanical in-
stallation, has always been the fact that the equipment already in-
stalled must be discarded, often thrown into the scrap pile, while still
in fairly good condition and capable of doing a considerable amount
of work. In the early installation of steam turbines, this was often
done, and in order to increase the capacity of the central station, good
reciprocating engines were often thrown out and turbines put in their
places. It was, however, soon discovered that this, in many cases,
was unnecessary, and that the desired increase in power could be had
by simply using low-pressure turbines in connection with the exist-
ing reciprocating engines. The low-pressure turbine takes the steam
exhausted by the engine, slightly above the atmospheric pressure,
and expands it to a lower vacuum than could be economically done
in the engine.

While the reciprocating engine is highly efficient for utilizing
the available energy of steam between boiler and atmospheric pres-



348



STEAM TURBINES 33



sure, it is relatively inefficient for utilizing the energy of steam in the
lower ranges of pressure, especially at pressures below 20 in. vacuum.
The steam turbine, on the other hand, utilizes the available
energy of steam in the lower more effectively than in the higher
ranges of pressure. Since there is about as much available energy in
steam below the atmospheric line as there is in steam above it, there
is every reason to believe that this combination of engine and turbine
will be a most efficient one. In order that the possibilities and limi-
tations may be fully stated, however, it will be necessary to investi-
gate some of the characteristics of steam expansion.

A single cylinder engine with cut-off at, say, one-third stroke, will
expand the steam to three times its initial volume, and if it takes steam
at 150 pounds gauge pressure, the volume of each pound of that steam
before expansion will be approximately 2.75 cubic feet. Now, if this
is expanded to three times its initial volume, every pound of steam
entering the cylinder will, at exhaust, occupy 3 X 2.75 = 8.25 cubic
feet. If the expansion has been adiabatic, that is, without the gain
or loss of heat, this pound of steam will occupy 8.25 cubic feet of
space when the pressure has reached 32 pounds by the gauge, and,
under the above conditions, an engine would release at this pressure
a manifest waste.



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 23 of 30)