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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 29 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 29 of 30)
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In order to regulate the supply of steam in proportion to the
load on the steam turbine, there are several devices employed. (1)
The pressure of the steam before admission to the steam chest may
be controlled by means of a throttling governor. (2) In the case of
impulse turbines with a series of nozzles, each may, if desired, be con-
nected to an independent valve under the control of the governor,
so that the number of open nozzles and the extent of the steam belt
acting on the turbine may be varied to conform to the load, the steam
being admitted to the nozzles at full pressure. (3) Steam may be
admitted to the turbine at full pressure at intervals of long or short
duration. Any of these devices may be arranged to take care of an
ordinary amount of overload. For very large overloads, a by-pass



443




444



STEAM TURBINES



125



valve is generally provided, which may be controlled either by hand
or by the governor, admitting high-pressure steam into one of the
lower stages of the turbine.




Fig. 97. Sets of Blades Assembled. Allis-Chalmcrs Co.

Of these devices, the second evidently is not applicable to the
reaction turbine or to an impulse turbine taking steam around
the entire periphery. The other methods of governing might be
used for any form of turbine.



445



126 STEAM TURBINES



Throttling. The simplest governor is undoubtedly the throttle
type, but throttling is not an economical way of regulating power; for,
by the throttling process, the steam expands somewhat to a lower
pressure without doing useful work. The work done goes to super-
heat the steam and, although no heat units can be destroyed, less can
be recovered from a pound of the throttled steam than from a pound
at the initial pressure. The smaller the load, and consequently the
more the throttling, the greater this loss will be. Any form of tur-
bine, therefore, taking steam about its entire periphery and fitted
with a throttling governor would probably be relatively uneconomical
at light loads.

It is not economical to throttle the steam pressure before admis-
sion to a set of properly designed nozzles. Nozzles are designed for a
definite steam pressure and, if this is varied, the efficiency is bound to
fall off. There is, therefore, a two-fold loss due to throttling in a
turbine of this type. In turbines like the DeLaval, supplied with
several nozzles, any of which may be opened or closed by hand, it can
readily be seen that if there are, for instance, four nozzles, three of
which are open under ordinary loads, then a 883-% or even 66%
under-load may be taken care of by hand regulation. A throttling
governor on such a turbine would necessarily regulate only between
these limits and might act only upon one nozzle. On account of its
simplicity, the throttling type of governor has been largely used and
is practically always used on the smaller turbines where it has
given results that are satisfactory.

Varying Number of Open Nozzles. Turbines employing partial
admission through a group of nozzles like those of the Curtis turbine
may regulate the steam supply by providing an independent valve
for each nozzle. This valve, being under the control of the governor,
may be opened or closed as needed. In the Curtis turbine, steam is
admitted through a series of valves, the number of which depends
upon the capacity of the machine. The valves are arranged to open
successively, two-thirds of them being open at full load. The action
of the valves is so regulated that they are either fully open or fully
closed. Any increasing load is taken care of by the opening of an
additional valve, this valve closing when the load falls off. Very
wide variations in load can thus be carried with but little effect on
steam economy. The friction, of course, is a very much larger



446



STEAM TURBINES



127



per cent of small loads, so that the steam consumption at small loads
would appear very much larger, even though it worked with the same
absolute efficiency as at high loads.




Fig. 98. Sectional View of Governor for Varying Number of Nozzles Opened.

These valves are controlled from the governor either by means of
some electrical device, by a direct mechanical control, or by hy-
draulic pressure. The power necessary to operate one of these
valves is necessarily so great that it cannot be moved directly by the
governor, but the governor can operate a small pilot valve, which



447



128



STEAM TURBINES



in turn may set some mechanism in motion that is powerful enough
to do the work.

The governor itself is of the centrifugal type, its action depending
on the balance between the forces exerted by the springs and the
centrifugal forces of the revolving weights. Fig. 98 shows a sectional
view of this governor. A is the revolving weight, which, acting




Fig. 99. Turbine Governor for Varying Time of Admission.



through the knife edges B, may move the rod C against the action
of the spring D. At E is a ball-bearing gimbal joint which forms a
junction between the revolving mass of the governor and the stationary
lever of the governing arm. The governor is provided with the
auxiliary spring F which may be used to vary the speed of the turbine.
G is a small motor which may be operated from the switch-board to
regulate the tension of this spring F, thus varying the speed of the
turbine. The movement of the governor lever is transmitted through
a connecting rod and lever to the pilot valve of an hydraulic cylinder.
To provide for overloads greater than 50%, an auxiliary set of admis-
sion nozzles are provided in the second stage of the turbine, which
may take full-pressure steam if needed.



448



STEAM TURBINES 129



Varying Time of Admission. The third method of governing
that by varying the time of admission, is almost invariably used on
turbines of the Parsons type. An admission of steam occurs about
once in every thirty revolutions at approximately full load. The
pilot valve is continually oscillating, thus preventing any liability of
sticking, but its period of oscillation is varied directly by the governor.
Steam, therefore, enters the turbine in puffs, the duration of which
depends upon the load; at slight overloads the valve would be con-
stantly open. One disadvantage of this method of governing is that
there may be an opportunity for the turbine to cool between successive
blasts of steam, thereby causing initial condensation at moderate
and light loads. A by-pass valve, Fig. 94, is provided on Parsons
turbines, and is under the direct control of the governor, and, for
considerable overload, will open, letting steam into the intermediate
stages.

The details of the mechanism for controlling steam turbine
governors are not essentially different from those used in reciprocating
engines, except as would be required by the greater speed of revolu-
tion. In turbines of large size, the valve cannot be operated directly by
the governor, but must always be moved by means of pressure ad-
mitted through a small pilot valve. Fig. 99 shows the diagrammatic
arrangement of the Westinghouse-Parsons governing gear. A is the
pilot valve under the direct influence of the governor which admits
steam to the larger piston, shown at the left. This piston has
pressure enough upon it to operate the admission valves.



449



RE VI KW (QUESTIONS



PRACTICAL TEST QUESTIONS.

In the foregoing sections of this Cyclopedia nu-
merous illustrative examples are worked out in
detail in order to show the application of the
various methods and principles. Accompanying
those are samples for practice which will aid the
reader in fixing the principles in mind.

In the following pages are given a large num-
ber of test questions and problems which afford a
valuable means of testing the reader's knowledge
of the subjects treated. They will be found excel-
lent practice for those preparing for Civil Service
Examinations. In some cases numerical answers
are given as a further aid in this work.



451



R K V I E W QUESTIONS



ON THE SUBJECT OF



EXERCISE I.

1. What is the difference between heat and temperature ?

2. How are the freezing and boiling points of a thermom-
eter determined ?

3. To what temperature F does 10 C correspond?

4. To what> temperature C does 20 F correspond ?

5. What is the temperature of steam when the barometer
stands at 30.05 inches ?

6. What is the absolute zero of temperature?

EXERCISE II.

1. Give an example of linear expansion from your own
observation.

2. A silver dollar is nicely fitted into an iron ring. If both
are heated, will the coin fit more or less tightly ?

3. A steel rail is 30 feet long at 20 C. How long is it at
0C?

4. A surveyor's steel tape 100 feet long is correct at dO F.
How much in error id it at 86 F ?

5. One end of every long iron bridge rests on rollers. Why
is this?

6. 700 cubic feet of air at 20 C are heated to 50 C. Find
the increase in volume.



453



HEAT



EXERCISE 111.

1. What is the difference between evaporation and ebulli-
tion?

2. For what reason is it not difficult to boil water on top of
a high mountain?

3. If a plug of tin were screwed into a steam boiler, would
it remain in its position if the steam pressure were raised to 100
Ibs. per square inch ?

4. Upon what does the process of distillation depend?
Why is the cooling water fed in at the bottom rather than at the
top?

5. What is a British thermal unit?

6. What is the maximum possible efficiency of a steam
engine taking steam at 185.3 pressure (gage) and exhausting at
15 pounds absolute pressure ?

EXERCISE IV.

1. What is meant by the latent heat of fusion? Does the
temperature of a substance change during solidification ?

2. How many pounds of water at 60 C will be required to
melt ten pounds of ice at 5 C ?

3. How many pounds of steam at 212 F will just melt 100
pounds of ice at 32 F ?

4. How many pounds of steam at atmospheric pressure will
raise 2,000 pounds of water from 40 F to 50 F ?

5. Define specific heat in your own words.

6. What is superheated steam ?

EXERCISE V.

1. Is there any thermal advantage in making a boiler of
copper rather than of iron ?

2. What is the difference between conduction and convec-
tion ?

3. Suppose you wished to heat a liquid that contracted con-
siderably on heating. Would you apply the heat at bottom or at
top?



454



REVIEW QUESTIONS



ON THK SOn.TKCT O V



THE STEAM ENGINE.



1. Wh?t should be the rim weight of a oast iron fly- wheel
which is 15 feet in diameter, if the engine is of the Corliss type,
with an 18" X 3G" cylinder, and runs at 90 revolutions per
minute ?

2. In what way does the action of steam in the turbine
engine differ from the action of steam in the ordinary reciprocating
engine ?

3. What is the theoretical height of a simple pendulum
governor which makes 65 revolutions per minute?

4. In what way did Newcomen improve the steam engine ?

5. Explain the difference between the condensing and the
noncondensing engine, and show why the condenser increases the
power of the engine.

6. Describe two methods of overcoming the danger due to
high rotative speed in the steam turbine.

7. What two principles did James Watt follow in his
experiments on the steam engine?

8. What types of engines have no fly-wheel? Why have
they no fly-wheel?

9. What should be the thickness of rim for a cast iron fly-
wheel weighing 4,480 pounds ; the face is 12 inches wide, and the
diameter of the wheel 15 feet?

10. Name some of Watt's inventions and improvements in
the steam engine.



455



THE STEAM ENGINE.



11. What is meant by compound and triple expansion
engines?

12. What is a Woolf engine? What is a tandem engine?
A cross compound?

13. Explain with sketch the action of a sight-feed cylinder
lubricator.

14. What is the function of a governor?

15. Why is not the power given out by an engine constant
during a given time?

16. What are the relative advantages and disadvantages of
turbine engines as compared with reciprocating engines ?

17. What is the office of the fly-wheel?

18. How does the height of the governor affect its sensi-
tiveness ?

19. Why are lubricants used? Name the requisites of good
lubricants.

20. Name the advantages of the vertical engine.

21. Describe the two kinds of governors.

22. In what two ways do governors vary the work done on
the piston?

23. How are high-speed engines oiled?

24. Which class of engine needs the larger fly-wheel, and
why, high speed or low speed?

25. Describe the direct-acting steam pump.

28. How does the action of the Buckeye engine governor
in changing the amount of steam admitted per stroke differ from
the action of the straight-line engine governor?

2T. What are the advantages and the principal disadvan-
tages of the Corliss engine?

28. Name the requisites for high-speed reciprocating
engines.

29. Why do piunping engines usually have devices for
economy ?

30. Explain how the expansion of steam is obtained in each
of the two types of steam turbine.

31. If the height of a simple pendulum governor is 8 inches,
what is the equivalent height if a weight equal to 1| times the
weight of the balls is added ?



456



REVIEW QUESTIONS

ON THE StrnjKOX OK

THE STEAM ENOINE.

PART II.



1. Describe the method of finding the theoretical thermal
efficiency.

2. Explain how there is any gain by using a steam jacket.

3. Explain the formula expressing the relation between
pressure, volume and temperature of a perfect gas.

4. State Boyle's Law.

5. What effect does speed have on the economy of an
engine ?

6. Explain the advantages and disadvantages of super-
heating.

7. If steam exhausts into a jet condenser, how much water
will be required (theoretically) to condense 10 pounds of steam,
if the entering water is at 55 F, the discharge at 118 F and the
vacuum gage reads 13.7?

8. At what position of the crank is the maximum turning
moment?

9 . If the feed water is heated, is the economy of the en-
gine increased. ?

10. What is the absolute temperature of 150 F?

11. Other things being equal, is a large engine more eco-
nomical than a small one ? Why?

12. Explain initial condensation.

13. Describe the action of heat on a body of water under
atmospheric pressure.



457



THE STEAM ENGINE.



14. Define the terms : saturated steam, superheated steam,
wet steam and dry steam.

15. Name the properties found in the steam tables.

16. How does a condenser increase the power of an engine?
IT. Explain how re-evaporation is possible.

18. How is the amount of steam used per hour found ?

19. Explain with sketch how the crank effort is more
nearly constant if there is more than one crank on the shaft.

20. In what two ways does compounding effect a saving
of steam ?

21. Give the formula expressing approximately the relation
between pressure and volume of steam. What is it called?

22. Find the probable I. H. P. of a high-speed triple-expan-
sion engine for a battle-ship. The engine runs at 150 revolu-
tions; total ratio of expansion 12; initial pressure 145 pounds
(gage), condenser vacuum 14 inches of mercury; the low-pres-
sure cylinder is 72" X 36".

Ans. 2567.2 I. II. P.

NOTE. Use card factor of .60 and atmospheric pressure 14.7 pounds.

23. What is the temperature of saturated steam correspond-
ing to a pressure of 120 pounds (gage) ?

24. What is the heat equivalent of work ? How many foot-
pounds of work equal one B. T. U. ?

25. How many B. T. U. are required to furnish energy suffi-
cient to raise 500 pounds 5,280 feet? How many pounds of coal
will furnish this heat, assuming 6,800 B. T. U. per pound are
Available ?

f 3,393 B.T.U.
1 I pound (about).

26. How many cubic feet do 3 pounds of steam occupy
when at 45 pounds absolute pressure ?

27. How many B. T. U. are necessary to change 8 pounds
of water at 320 F into steam at the corresponding temperature
and pressure ?

28. What is the actual thermal efficiency of an engine ?



458



REVIEW QUESTIONS



ON THK SrTB.TKOT OB-



STEAM ENOINE INDICATORS.



1. A Brumbo Pulley has the following dimensions : lever 80
inches long, stroke 32 inches and radius of sector 10 inches. What
will be the length of the diagram ?

2. Define Mean Effective Pressure.

3. When is a three-way cock used?

4. Describe Watt's Indicator.

5. A steam engine has a stroke of 30 inches, diameter of
piston 18 inches, and the initial pressure 110 pounds (gage). If
steam is admitted during the whole stroke and the exhaust is at
atmospheric pressure, what is the theoretical M. E. P.?

6. In the above engine, what is the theoretical indicated
horse-power if the engine is running at 75 revolutions per minute?

Ans. 318 I. H. P.

7. Why is a reducing motion necessary in taking indicator
cards ?

8. After taking an indicator card, how is the atmospheric
line drawn?

9. Give the formula for finding the Indicated Horse-power,
explain the meaning of the letters, and why is the denominator
33,000?

10. What is a good height and length .for an indicator card ?
What data should be placed on the diagram ?

11. Describe the process of taking a card.



459



INDICATORS.



12. Explain with sketch the method of finding the area of
an indicator card by means of ordinates.

13. If you were told to find the I. H. P. of an engine what
data would you require ?

14. What peculiarity has the Tabor indicator?

15. What will be the fault with the card if the spring used
in an indicator is too light ? If too heavy ?

16. If the I. H. P. is 85.6 and the efficiency 90 per cent,
what is the available horse-power?

17. If the initial pressure of steam is 85 pounds absolute
and the exhaust pressure 2 pounds above the vacuum, what is the
thermal efficiency?

18. An indicator card gives a steam consumption of 28.6
pounds per I. H. P. per hour. If the mechanical efficiency is 87
per cent, what is the steam consumption per B. H. P.?

Ans. 32.87 pounds.

19. A rope brake is attached to the fly wheel of an engine.
The pulley is 36 inches in diameter and the rope 1 inch. If the
spring balance registers 220 pounds and the weight is 20 pounds,
what is the B. H. P. while the engine is running 300 revolutions
per minute? Ans. 17.6 B. H. P.

20. Are the cards most reliable when taken by two indica-
tors or when both are taken on the same paper by the same
indicator?

21. What is the speed of an engine if the B. H. P. is 2.84,
the length of arm of Prony brake 3 feet, and the weight at the
end of the arm 35 pounds ? Ans. 142 revolutions.

22. If the admission line of an indicator diagram curves
backward, what does it indicate ? How would you remedy it?

23. What is wire-drawing ? Explain why it is undesirable.

24. The piston of an engine is 19 inches in diameter; the
piston speed is 480 feet per minute. A card was taken with a
40-pound spring; the mean ordinate being 1.16 inches. What
was the I. H. P. ? Ans. 191.4 I. H. P.

25. What causes wavy lines in an indicator diagram?

26. What is the indicated horse-power of an engine 12" X 18*
when running at 90 revolutions, the M. E. P. being 42 pounds ?



4*0



REVIEW QUESTIONS



ON THE STJBJKOT OB-



GEARS.



1. Why cannot the displacement of the piston be found in
the same manner as that of the valve ?

2. What is the throw of the eccentric if the valve travel is
3 inches and the rocker arm has the following dimensions: AB =
12 inches, AG = 15 inches. See Figs. 19 and 20.

3. Explain why the eccentric radius is not at right angles
to the crank. What is lead ?

4. Why are laps used ? What effect does an increase of
outside lap have upon steam distribution ?

5. Describe an eccentric.

6. Sketch a plain slide valve and indicate ports, bridges,
exhaust cavity, inside and outside lap.

7. Give the advantages of the Corliss valve gear.

8. With a connecting rod of ordinary length, explain why
there is unequal steam distribution, if the valve is set for one end
and if the laps are equal.

9. How do the following changes affect steam distribution
with a plain slide valve, a. Increase in angular advance, b.
Increase in eccentricity.

10. Describe the essential features of the Joy valve gear.

11. Referring to Fig. 16, will the line RO be perpendicular
to XY when the piston is in the middle of the stroke ?

12. What should be the diameter of the steam pipe for an
18" X 24" engine that makes 100 revolutions per minute 4

Ans. 5 inches.

13. Describe how the governor regulates the engine by
means of the shifting eccentric.



461



VALVE GEARS



14. "When setting the Meyer valve, is the main valve given
a late or early cut-off ? Why ?

15. What relation between cut-off and compression can be
obtained with the Meyer valve that is impossible with the ordi-
nary slide valve?

16. Describe, with sketch, the Stephenson link.

17. Explain why an engine runs in the opposite direction if
the valve is disconnected from one eccentric and connected to the
opposite one.

18. What should be the width of the exhaust port ?

19. What is the advantage of the piston valve ?

20. What are the advantages of the radial valve gears ?

21. What is one disadvantage of the Corliss gear?

22. How may a valve be most easily balanced to overcome
weight of valve and gear as well as steam pressure ?

23. How does the Gooch link differ from the Stephenson
link?

24. Given in a certain valve:
Valve travel = 3 inches.
Lead angle = 6.
Cut-off at | stroke.
Compression at stroke.

Ratio of connecting rod to crank = 4.
Find by means of Zeuner's .diagram :
Linear lead.
Outside lap.
Inside lap.
Angular advance.

25. What are "crossed rods"? How is steam distribu-
tion with crossed rods different from that with open rods?

26. How does the passage in the trick valve affect lead?
How does it affect admission?

27. For what two reasons are link motions used on locomo-
tives?

28. Why does bringing the block nearer the center of the
Stepbenson link cause cut-off to occur earlier ?

29. What advantage has the double-ported slide valve ?

30. What is meant by the mid-position of a valve ?



REVIEW QUESTIONS

ON XHK SUBJECT Or

STEAM TURBINES

PART I



1. How does the steam economy of a good steam turbine com-
pare with that of the best Corliss engine of the same power?

2. What is the most serious difficulty that confronts the steam
turbine designer, and how may it be overcome?

3. What is the difference between the impulse and reaction
types of turbine?

4. In an impulse turbine designed for a given pressure and
vacuum, explain fully what effect on its economy would be produced
by changes in boiler pressure.

5. How does a nozzle giving maximum velocity to a steam
jet differ from one giving maximum velocity to a water jet, and why?

6. How does the steam expansion in the DeLaval differ from
that in the Rateau turbine?

7. Why are comparisons of performance based on steam con-
sumption per H. P. per hour, unreliable?

8. Why does no commercial turbine act by virtue of pure
reaction only?

9. How should a nozzle expanding steam at maximum
efficiency from 200 Ibs. absolute to 120 Ibs. absolute differ from one
expanding it from 200 Ibs. absolute to atmospheric pressure?

10. Explain the function of a Rateau accumulator?

11. What advantage is to be gained by compounding a steam
turbine? Why?



463



STEAM TURBINES



12. What are the chief sources of loss in the steam turbine?

13. What are the essential requirements in the design of a
steam turbine?

14. WTiat is the most serious objection to the compound velocity
step turbine, if the blade angles are all equal?

15. How are large overloads provided for in steam turbines?

16. If a nozzle makes an angle of 20 with the plane of rotation,
what is the proper peripheral velocity of the vanes for maximum
efficiency if the jet velocity is 4,000 feet per second?

17. In any reaction turbine what chief factor limits the economy?

18. WTiat is the most reliable method of comparing the per-
formance of two steam engines or turbines?

19. What is a so-called low-pressure steam turbine?

20. If a cylindrical nozzle delivers steam from a pressure of 100
Ibs. absolute into a pressure of 58 Ibs. absolute, how would the
energy of the jet be affected by raising the initial pressure to 200 Ibs.?
By lowering the final pressure to atmosphere?

21. How does the cost of a complete turbine power plant
including buildings compare with that of a reciprocating engine plant
of equal power?

22. What is the relation between the number of pressure stages
in a multi-stage machine and the reduction in the number of revolu-



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 29 of 30)