Copyright
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 22 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 22 of 30)
Font size
QR-code for this ebook


Stumpf turbine and is illustrated
by Fig. 9. Steam enters this
turbine through the nozzle A,
passes through the wheel buckets
to the other side, and discharges



Fig.



Hartman's Compound Impulse
Turbine.



into pipe B, which brings the steam around again to the inlet side.
Here it discharges through the opening b, against another bucket of
the same wheel, whence it is picked up by the opening x, in the pipe
C, and so on, finally exhausting through the pipe D. Somewhat
earlier, a turbine was invented that returned the steam in a similar



325



10



STEAM TURBINES



manner, but to another set of blades on the same wheel. This idea
has also been perfected by Riedler & Stumpf.

Moorehouse patented, in 1877, an improvement on the type
suggested by Real & Pichon in 1827. His chief claim was an allow-
ance for the increased volume of expanding steam in this type of
turbine.

DeLaval took out a patent on a reaction turbine of the Hero
type in 1883. It differed from the Avery turbine in detail, but not
in principle. This turbine was extensively used for running cream
separators, and was commercially successful, but was later aban-
doned for the present type of DeLaval motor.




Fig. 9. Compound Turbine with One Wheel.

In 1885, Parsons took out his first turbine patent on a motor
along the lines previously suggested by Wilson, and is responsible
for the successful development of this type of motor. His first tur-
bine, shown in Fig. 10, took steam in the center A, and exhausted
at both ends through the exhaust passage E E, thus avoiding any
end-thrust on the shaft B. At the same time, he patented his famous
flexible bearing, now in general use. In 1888, he patented the present
arrangement of grouping several rows of blades together increasing
the drum diameters step by step to provide for proper expansion, at
the same time patenting his balancing pistons, at present employed
to relieve end-thrust.

The expanding nozzle had been patented in 1867 for use in steam
injectors, but it was not until 1894 that anyone patented its use in



326



STEAM TURBINES



11



connection with a turbine. In this year, DeLaval secured this patent
and used the nozzle in connection with his turbine, for the purpose
of expanding the steam and getting a high velocity of jet with in-
creased kinetic energy.

During 1894 and 1895 there were issued a large number of
patents, many of which have been successfully developed. Among
them were Parsons', Rateau's, and the first patents for the use of
buckets of the Pelton type.

In 1896, Curtis patented the use of an expanding nozzle in com-
bination with a compound wheel of the type suggested by the Hart-




Fig. 10. Early Parsons Turbine.



man patents in 1858. Others had used both the expanding nozzle
and the same type of wheel, and only two years earlier patents were
taken out for a converging nozzle with a similar wheel. From a
study of nozzles, it will appear that the converging nozzle could be
used economically by increasing the number of stages used in the
expansion, but the turbine would be larger than the Curtis, and
probably less efficient.

Patents were issued in 1898 to Riedlei & Stumpf, whose turbine
appears to be an improvement on the Perrigault & Farcot, patented
about 1870.

In 1900, the Zoelly patents were issued. This turbine in prin-
ciple is similar to the Rateau, but different in construction.



327



12 STEAM TURBINES



The steam turbine patents issued since 1900 are altogether too
numerous even to mention, but from them a number of commercial
machines have been developed, and are now on the market. The
principal commercial turbines will be described later.

It must not be thought that this summary is at all exhaustive,
or that even all the noteworthy turbine patents have been mentioned.
There are hundreds of them, and it is possible here only to men-
tion those that are the immediate forerunners of our present com-
mercial types. This brief summary will show that the commercial
success of the turbine has been due to a more complete knowledge
of the properties of steam, improved details, and the possibility of
better workmanship, rather than to the development of new principles,
for the distinctive fundamental ideas of all of our commercial tur-
bines had been suggested years ago. It should be borne in mind,
however, that no fundamental principle can be successfully worked
out unless the minutest detail is correct, and these details may, and
in the case of the turbine did, prevent the successful carrying out of
the early ideas.

Fundamental Principles. The underlying principles of steam
and water turbines are alike a moving fluid impinges upon curved
vanes or buckets attached to the periphery of a wheel-, thus causing
it to revolve. The vanes or buckets change the direction of the actua-
ting fluid and absorb part of its energy, the fluid leaving the turbine
with a comparatively low velocity. To insure reasonable economy,
the fluid must impinge upon the vanes in a direction tangential to
their surface at the point of impact, so as not to impart any shock and
to avoid spattering. Further, the residual velocity at outlet should
be as low as possible.

In either class of turbine, rotation is caused, not by the statical
pressure of the actuating fluid, but by the velocity which it imparts
to the rotating turbine wheels. The kinetic energy of the fluid

WV 2
passing through the turbine is equal to , where W equals the

weight of the fluid per second and V is its velocity at entrance. Evi-
dently, the smaller W, the larger V mus't be to develop the same

WV 2
power. If the fluid leaves the turbine with the velocity V a , then ~ L



82R



STEAM TURBINES 13



represents the energy not absorbed by the turbine. If V a is small,
this wasted energy will be likewise small.

Since the fundamental principles of the turbines are the same,
it would seem at first sight as if steam and water turbines could be
built on similar lines. But this is not so, because the difference in
density and in elasticity of the two fluids requires different applications
of those principles. In the steam turbine, not only must proper steps
be taken to abstract the energy from the steam jet, but also to make
that energy a maximum by providing for the proper expansion of the
steam.

To make more clear the differences just mentioned, it should be
remembered that water is an inelastic fluid; that is, one having a
constant volume under all conditions of pressure. Therefore, in
flowing through a nozzle, if the velocity at the outlet is to be greater
than the velocity in the pipe, the area of the outlet must be smaller
than the cross-section of the pipe. Steam, on the other hand, is an
elastic fluid and expands rapidly as it flows through a nozzle. If
the increase in volume were in exact ratio to the increase in velocity,
then, for maximum efficiency, the nozzle would be parallel-sided.
This happens when the pressure at discharge is about 60% of the
initial pressure. But when discharging into a low pressure, the
volume of the steam increases more rapidly than the velocity, and
hence, if the mouth of the nozzle is to be capable of discharging the
same weight of steam per second as the throat ( the condition for
maximum efficiency), the cross-sectional area of the nozzle must con-
stantly increase toward the outlet.

Steam will expand as it passes through the turbine and if the
passages are correctly proportioned, so that this expansion can take
place only in one direction, that is, in the line of flow, the steam
particles will be forced forward in a nearly uniform jet; the steam,
by virtue of this expansion, will attain a very high velocity and the
jet will consequently have a high kinetic energy.

Water turbines use a relatively small head and a large quantity
of fluid; wrth the steam turbines, the quantity of fluid is small, but
the head is very large. To develop large powers with any form of
turbine, it is necessary that a number of wheels be used. With water
turbines, each wheel acts under full head, each using a relatively
small quantity of water. With steam turbines, however, it is the



329



14



STEAM TURBINES



Hollow Cube



head that must be divided into different steps; i. e., a single steam
turbine can use the full quantity of fluid but, if desired to run at
relatively low speeds, it can use but a portion of the total head.
To develop 1,000 H. P. on a turbine shaft, with a head of 150 feet,
would require approximately 4,800 pounds of fluid per second,
depending somewhat upon the design of the wheel. With a head
of 3,000 feet there would be required 242 pounds of fluid per
second, and with a head of 234,000 feet, comparable with that of
a steam turbine, the requirement would be about 3^ pounds of
fluid per second. It is thus clearly seen that the difficulty in
developing large powers with the water turbine is that of providing

for a sufficiently large quantity of
fluid through the turbines, and
with the steam turbine, that of
handling the great velocities re-
sulting from the enormous head.
In all steam turbines, the steam
is expanded in suitable nozzles
or passages. In some the expan-
sion is all in the nozzles; in others,
partly in the nozzles and partly in
the vanes or blades. In some,
the total expansion from boiler to
exhaust takes place in one nozzle,
and the energy is absorbed by a
single wheel; such a turbine is called a single-stage turbine. In
others, the expansion in one set of nozzles is only partial, and after
passing through one or more wheels, the steam again passes
through another set of nozzles and set of wheels, and so on,
until exhaust pressure is reached. This is a multi-stage turbine.
There may be all the way from one to forty stages, or even more,
in turbines of this type where all expansion is in the stationary vanes ;
and in turbines where part of the expansion is in the running
vanes, there may be 100 stages or more. Some turbines use nozzles
for expanding the steam, and some use stationary vanes for the
purpose, these vanes being so shaped as to provide suitable
passage areas to permit of steam expansion. The principle is the
same whether^ nozzles or blades are used, but blades are generally




_Q.



Fig. 11.



Apparatus for Showing Force
of a Jet.



330



STEAM TURBINES



15



used where many stages are employed and the drop in pressure is
small from stage to stage.

Before taking up the actual study of steam turbines, it will be
necessary to have a clear conception of a few elementary principles
of mechanics. Suppose a hollow cube to be filled with some fluid
(water or steam) at a given pressure, and to have an opening in one
side that can readily be closed. The arrangement is such that when
the outlet is opened, the internal pressure will remain the same. If
the outlet is opened, the fluid will rush out, as shown in Fig. 11, and,
if the jet is supposed to strike against a board free to move, the jet
will exert a force upon that board tending to swing it in the direction
of the jet. This force is called an impulse. At the same time there
will be a tendency on the part of the cube to move in the opposite
direction, and the force thus developed is called a reaction. It may
be explained in this way:

Suppose each side of the cube
to be one foot square, the area of
the opening, one square inch, and
the internal pressure, 100 pounds
per square inch. There will be
144 X 100 pounds pressure on each
side of the cube with the outlet
closed, but when the one-inch outlet
is opened, the total pressure on the

side containing the outlet will be reduced by the pressure of 100
pounds on the opening itself. This will leave an unbalanced force
of 100 pounds acting in the opposite direction, which is the origin
of the reactive force. This explanation is not strictly correct, but
serves to give an idea of these two forces, impulse and reaction.

Hero's turbine was a reaction turbine pure and simple, Branca's,
an impulse turbine; but what is called a reaction turbine at the present
time is not a simple reaction turbine in any sense, but one running
under the combined influence of reaction and impulse. Likewise,
the so-called impulse turbine is not a pure impulse turbine, but acts
under the combined influence of impulse and reaction. There is
no pure reaction turbine now on the market. The so-called impulse
turbine being rather simpler of explanation, for the present only
this type will be considered in the following explanations. How




Jet Deflected through 90.



331



10



STEAM TURBINES



these principles apply to the so-called reaction turbine will be
explained later.

Suppose a stream of water from a nozzle to impinge upon the
plate shown in Fig. 12, and so made that the jet is divided, and with-
out shock departs in a direction tangential to the plate and at 90 to
the line of impact. If the velocity of impact of the jet is V feet per
second, its velocity in the same direction after striking the plate will
be zero, and therefore, a definite force will be exerted on that plate,
equal to the force necessary to impart a velocity of V feet in one second
to the mass of water in the jet. The acceleration, therefore, will be
V feet per second, and since force is measured by mass times
acceleration, this force, acting on the plate, will be F = MV. If
the plate is allowed to move in the direction of the jet with a

velocity V v the relative velocity of
the plate with reference to the jet
will be V V v and the correspond-
ing force acting on the plate will
be F = M (V - VJ. Since work
is measured by the product of
force and distance, the force acting
through the space V l in one second,
will do the work W = FV l = M
(V- VJ V t foot pounds.

Now if the plate were shaped as
shown in Fig. 13, so that the direction of the jet were completely
reversed, that is, turned through 180, there would be an additional
pressure on the plate, due to the reaction of the jet leaving it. This,
neglecting friction, would be equal to the original impulse, thus
making the total force on the plate 2 F instead of F. It is quite
evident that if the force is twice as great, the work must also be
double, and the above expression for the work done becomes

W = 2FV l = M X 2 (V - l'\) l\

For this reason, turbine vanes are made so as to reverse the direction
of the jet as completely as possible. Complete reversal is not prac-
ticable because some clearance must be allowed for the deflected jet
to escape. This is especially true in the usual case in practice, where
the jet impinges upon the vanes from the side. Here, the angle has




Fig. 13. Jet Deflected through



332



STEAM TURBINES 17




to be such that the revolving wheel will clear both nozzle and
deflected jet.

If the bucket shown in Fig. 13 were held stationary, the force
exerted by the jet would evidently be a maximum and equal to 2MV;
but the velocity of the bucket being zero, the work, equal to
the force multiplied by the space,
would also be zero. If, on the

other hand, the velocity of the ~^^|fe&, !

bucket were equal to the velocity
of the jet, the push would be zero,
and the work again zero. Some-
where between these limits, there
must evidently be a velocity which
will produce maximum results.

r/ '

c T7 Fig. 14. Jet Impinging upon Curved Vane

buppose now, that K, = : at an Angle with Plane of Rotation .

then

W = MX2(V- ] 2)~=M~,
but

= the kinetic energy of the jet, as it issues from the

nozzle. Therefore, if the speed of the bucket is one-half the velocity
of the jet, we have an efficiency of 100%, neglecting losses, and
this is, of course, the best obtainable. Therefore, the greatest effi-
ciency is obtained when the speed of the bucket is half the jet velocity,
provided the jet impinges upon the bucket in a direction parallel
to the line of movement of the bucket. For other angles, the speed
for maximum efficiency would be somewhat less.

If a jet with the velocity V strikes the bucket at an angle a,
as shown in Fig. 14, its velocity A B could be resolved into two com-
ponents one C B at right angles to the shaft, and one C A parallel
to the shaft. The one at right angles to the shaft, commonly known
as the velocity oj whirl, would produce a rotative impulse equal to
Vcos a, and V v the velocity of the vane necessary for maximum
efficiency, would be half this, or V l = $ Vcos a, provided the angle
with which the jet leaves the blade is equal to the angle of impinge-
ment. The component A C, parallel to the shaft, would have no



333



18 STEAM TURBINES




tendency to cause rotation, but would produce an end thrust on the
shaft. This component is called the velocity of flow.

Suppose a jet to impinge upon a curved vane at the angle
shown in a Fig. 15. If the jet strikes this vane tangentially, without
shock, the vane remaining stationary, the relative positions of the jet
before and after impact will be as shown. Now if the vane is

allowed to move with the velocity
Fj, the relative positions of the
vane and the nozzle will change,
and the jet will no longer glide
smoothly onto the vane, but will
strike the edges, and spatter.
To maintain the correct relative
positions, the nozzle must either

, be allowed to follow the vane, or its

(o) . . , , , ,

Fig. 15. Relative Positions of Jet position must be changed so that

and Vanc the direction and velocity of the

jet will be such that it may be resolved into two components,
one parallel with the direction of motion of the vane, and the
other tangent to the vane. The absolute direction of the jet
must be along the line AB,'(b, Fig. 15), but its direction rela-
tive to the moving vane will be along the line A C, and if A B
is drawn to a scale representing the actual velocity of the jet,
and C B laid off to the same scale to represent the velocity F x of
the vane, then A C will represent in magnitude and direction the rel-
ative velocity of the jet and the vane, which will be identical with
the absolute velocity in the first case where the vane is stationary.
Neglecting friction, the jet will leave the vane with the same relative
velocity. Draw E F = A C and E G = C B = the velocity V r Then
E PI, which we shall call V 3 , will represent in magnitude and direc-
tion the absolute velocity with which the jet leaves the vane.

W V 2

The energy in the jet before impact was - ,

IF V 2
after leaving the vane, -.

W F 2 WV 2 W
The energy absorbed was then Q 3 = (F 2 F 3 2 ).

For the best efficiency, F 3 should be small, but can never be zero



334



STEAM TURBINES 19



unless the jet angle a is zero, and the direction of the jet is reversed
through 180, an impracticable condition.

Nozzles. Steam does not cause rotation in the turbine because
of its statical pressure, but, as already stated, because of its velocity,
a difference in pressure acting indirectly, by imparting velocity to
the steam. It is evident, then, that in this class of motor, steam
velocities are all-important. The steam possesses energy by virtue
of the heat which it contains, but to make this energy available in
the turbine, it must be transformed into kinetic energy by the produc-
tion of a high jet velocity. The correct shaping of the nozzle is the
all-important factor in acquiring the requisite steam velocity, as will
appear from the following considerations:

It has been well established by experiment that steam at high
pressure flowing into a space at lower pressure, through a nozzle with
parallel sides, cannot attain a velocity exceeding 1,450 to 1,500 feet
per second, no matter how high the initial pressure nor how low the
pressure into which the steam discharges. This limiting velocity is
due to the fact that at the throat of any nozzle there occurs a drop in
the pressure of the steam to about 58% of the initial pressure, and, if
the nozzle be a cylindrical one, this drop will remain practically con-
stant throughout the length of the nozzle. The velocity acquired
by virtue of this difference in pressure will therefore be about the same,
whether the absolute pressure into which the steam is discharged is
58% of the initial pressure or much less. In the latter case, the
throat pressure cannot change until the outlet is reached, when the
pressure drops suddenly to the pressure of the space into which the
steam is discharging, and the steam immediately expands in all
directions, thus dissipating its energy. The only case in which maxi-
mum efficiency is developed with orifices and short passages with
parallel sides is when the low pressure is greater than 5S% of the high
pressure.

This limiting value for orifices or parallel-sided nozzles, and the
consequent limit of steam velocity, makes it impossible to develop
the greatest energy of the steam when expanding to low pressures
except through a nozzle with flaring sides, in which the outlet is
greater than the inlet. In such a nozzle the steam expansion occurs
gradually in its flow, and is constrained to take place only in the
direction of the flow. In this, way, the velocity of the steam par-



335



20 STEAM TURBINES



tides is increased as it proceeds along its nozzle until a tremendous
speed has been developed, which will produce about 95% of the
available energy. Furthermore, steam may be expanded effectively
within the confines of such a nozzle from any high pressure to any
lower pressure, provided the increase of areas of cross-section of the
nozzle is proportional to the increase of specific volumes of the steam.
In other words, with a cylindrical nozzle, a limiting steam velocity
of 1,450 to 1,500 feet per second is possible, no matter whether the
initial pressure of the steam be 70 pounds or 200 pounds, or whether
the pressure into which the steam is finally expanded be 58% of the
initial or a 28" vacuum. Of course, as the weight of the steam per
cubic foot, varies with the pressure, a greater weight of steam will
be discharged per second at higher pressure, resulting in a somewhat
greater kinetic energy in the steam jet. On the other hand, if the
nozzle has flaring sides, the steam at, say, 150 pounds gauge pressure,

will have the same pressure at the
throat, that is, 58% of the initial
pressure, but will acquire a rapid-
ly increasing velocity from throat
f in.'jDer. to outlet, and, with a 28" vacuum

Fig. 16. Properly Designed Expanding ahead of it, will leave the IlOZzle

with a velocity of 4,000 feet per

second, assuming no friction in the nozzle. Fig. 16 shows a properly
designed nozzle for expanding steam from 150 pounds boiler pressure
to 28" vacuum.

Compounding. It has been explained that if steam is expanded in
a suitable diverging nozzle, nearly all the heat energy becomes available
as kinetic energy, and that this steam, when flowing from a boiler pres-
sure of 150 pounds to a vacuum of 28", may attain a velocity of approx-
imately 4,000 feet per second. If the linear velocity of the buckets
were to be approximately one-half the velocity of the jet, there
would be grave danger that the wheel would burst from centrif-
ugal force. A peripheral speed of 1,200 feet per second is the
limit in practice. 2,000 feet per second would mean about 12,750
revolutions per minute in a wheel 3 feet in diameter. This latter
velocity would mean great delicacy in balancing and difficulty in
providing suitable bearings, even if material could be found to
withstand the strain. A wheel 15 feet in diameter would have to




836



STEAM TURBINES 21



revolve 2,500 revolutions per minute if the above mentioned
peripheral velocity were to be obtained, and again the impossibility of
construction is evident. Some means must therefore be employed to
reduce the speed to manageable rates without unduly increasing the
size of wheel. This may be done in a single-stage turbine by means
of gearing, but here, if 1,200 feet is to be the maximum permissible
peripheral velocity, and 2,000 is the theoretical velocity, there will be
a loss of efficiency. As a matter of fact, the steam jet does not strike
the wheel in a line at right angles to the shaft; consequently the
velocity of whirl, as already seen, is Vcos a, and with friction allow-
ance, this is somewhat reduced, but, even with a bucket velocity of
1,200 feet per second, the revolutions will usually be too high.

Turbine speeds may be satisfactorily reduced without the use
of gearing, by what is called compounding; i. e., by dividing expansion
into separate stages, called pressure compounding; by passing the
steam over several wheels with guide vanes between to redirect the



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 22 of 30)