Thermal Engineering Questions and Answers – Steam Turbines – Methods of Reducing Rotor Speed

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This set of Thermal Engineering Quiz focuses on “Steam Turbines – Methods of Reducing Rotor Speed”.

1. How many set of nozzles are there in a simple impulse turbine?
a) one
b) two
c) three
d) four
View Answer

Answer: a
Explanation: There is only one set of nozzles in a simple impulse turbine. The complete expansion of steam takes place at once. The pressure of the steam doesn’t alter when the steam moves over the blades of the turbine.
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2. In steam turbine terminology, the loss of energy due to higher exit velocity of steam is called _____
a) velocity loss
b) energy loss
c) kinetic loss
d) carry over loss
View Answer

Answer: d
Explanation: The loss of energy due to higher exit velocity of steam in steam turbines is called carry over loss or leaving loss. This type of energy loss is observed in simple impulse turbines. The expansion takes place completely in one set of nozzles and there is only one set of blades, therefore, the exit velocity of the steam is high.

3. Which of the following statements about De laval turbine is FALSE?
a) It has only one set of nozzles
b) Impulse blades are attached to the rotors
c) It has multiple rings of moving blades
d) Exit velocity of steam is very high
View Answer

Answer: c
Explanation: De laval turbine is a simple impulse turbine. It has only one set of turbines and one set of blades, impulse type attached to the rotor. The exit velocity of steam is high, about 3.3 per cent of the nozzle outlet velocity.
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4. The wheel or rotor speed in case of De laval turbine is low as only one ring of moving blade absorbs the kinetic energy of the steam.
a) True
b) False
View Answer

Answer: b
Explanation: Since the expansion of steam takes place completely at once in De laval turbine, and only one ring of blades is present, the wheel or rotor speed is too high. It varies from 25000 to 30000 r.p.m. Some energy is lost as the exit velocity of steam is high.

5. Which of the following statements about multi-stage reaction turbine is FALSE?
a) Pressure drop takes place gradually over the fixed blades
b) Fixed blades act as nozzles
c) Moving blades absorb kinetic energy of the steam
d) Pressure is not altered while the steam is moving over the moving blades
View Answer

Answer: d
Explanation: In case of a multi-stage reaction turbine, gradual pressure drop takes place in both moving and fixed blades. Fixed blades acting as nozzles increase the velocity of the steam. The moving blades absorb the kinetic energy of the steam.
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6. Which of the following statements is FALSE about velocity compounding?
a) Steam is expanded through only one set of nozzles
b) Low cost and high efficiency
c) The kinetic energy of the steam is absorbed in stages
d) The fixed blades re-direct the direction of velocity
View Answer

Answer: b
Explanation: Velocity compounding is done to reduce the rotor speed. The kinetic energy of the steam is absorbed in stages. Complete expansion of the steam takes place at once in the nozzles. The initial cost is low due to lesser number of stages, and its efficiency is also low.

7. Which of the following statements is FALSE about pressure compounding?
a) Steam pressure is reduced when steam moves over the moving blade
b) Multiple set of nozzles is used
c) Rotor speed is not very high
d) Steam expansion takes place in steps
View Answer

Answer: a
Explanation: Pressure compounding is one of the methods to reduce rotor speed. In this method steam is expanded in steps. After every set of nozzles there is a set of moving blades to absorb the kinetic energy gained by the steam after expansion. Steam while moving through moving blades does not experience change in pressure.
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8. Which of the following turbine uses pressure compounding method for reduction of rotor speed?
a) Curtis turbine
b) Rateau and Zoelly turbine
c) Curtis and Moore turbine
d) De laval turbine
View Answer

Answer: b
Explanation: Pressure compounding is used in Rateau and Zoelly turbine for reduction of rotor speed. In this method the steam is expanded in steps and after each expansion the kinetic energy of the steam is absorbed by a set of moving blades.

9. Which of the following turbines do not resort to any method of rotor speed?
a) De laval turbine
b) Curtis turbine
c) Rateau and Zoelly turbine
d) Curtis and Moore turbine
View Answer

Answer: a
Explanation: De laval turbine is a simple impulse turbine. It has only one set of nozzles and only one set of moving blades. The complete expansion of steam takes place in the single set of nozzles and the steam pressure remains unaltered while the steam passes through the set of moving blades.
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10. Pressure velocity compounding is used in _____
a) Rateau and Zoelly turbine
b) Curtis and Moore turbine
c) De laval turbine
d) Curtis turbine
View Answer

Answer: b
Explanation: Curtis and Moore turbine employs pressure velocity compounding. The pressure drop is divided into stages and the velocity is also compounded in these stages. A set of nozzles is fixed at the start of each stage.

11. Identify the incorrect statement regarding impulse turbine.
a) Blade manufacturing of reaction turbine is difficult than that of impulse turbine
b) Impulse turbine makes use of profile type blades whereas reaction turbine uses aerofoil type
c) Pressure drop occurs in only nozzles in impulse turbines but also occurs in moving blades reaction turbine
d) Impulse turbine can generate more power than impulse turbine
View Answer

Answer: d
Explanation: Not much power can be generated with impulse turbine, but with reaction turbine power generation is better. Impulse turbine has profile type blades and reaction turbines have aerofoil type. Aerofoil blade manufacturing is difficult than profile type blade.

12. Which of the following turbines has the least efficiency?
a) Curtis and Moore turbine
b) Rateau and Zoelly turbine
c) De laval turbine
d) Curtis turbine
View Answer

Answer: c
Explanation: De laval turbine is a single stage turbine. Curtis turbine, Rateau and Zoelly turbine and Curtis and Moore turbine are all multi-stage turbines. De laval turbine has the maximum carry over loss, therefore, minimum efficiency.

13. In a condensing steam turbine the pressure of the steam leaving the turbine should be equal to _____
a) inlet pressure
b) condenser pressure
c) atmospheric pressure
d) boiler pressure
View Answer

Answer: b
Explanation: In a turbine the pressure of the steam during expansion is reduced to condenser pressure. And if the turbine is no-condensing type then the pressure is reduced to atmospheric. The inlet pressure of the steam is the equal to the boiler pressure.

14. Identify the incorrect statement about blades.
a) Fixed profile type blade redirects the steam
b) Steam pressure is dropped in moving as well as fixed aerofoil type blades
c) Fixed aerofoil type blades act as nozzles
d) Steam pressure us dropped in moving profile type blades
View Answer

Answer: d
Explanation: Steam pressure is not affected when the steam is passed through the profile type blades irrespective of them being fixed or moving. In case of aerofoil type blades, steam is pressure is dropped in moving as well as fixed blades. Fixed aerofoil blades act as nozzles.

15. In pressure velocity compounding, the total drop in steam pressure is divided into stages and velocity obtained in each stage is also compounded.
a) True
b) False
View Answer

Answer: a
Explanation: Nozzles are fixed at the start of each stage and a set of fixed blades and moving blades is installed in each stage for velocity compounding. The pressure remains unaltered during each stage. The method of pressure velocity compounding is used in Curtis and Moore turbine.

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