This set of Thermal Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Classification of Steam Turbines”.
1. Steam turbine produces useful work in the form of rotation of turbine shaft, by extracting thermal energy from pressurized steam.
Explanation: Steam turbines are one of the most important prime movers for power generation. In a steam turbine the potential energy of the steam is transformed into kinetic energy and it is then it is transformed into the mechanical energy of rotation of the turbine shaft.
2. According to the number of pressure stages, steam turbines are classified into _____
a) single cylinder and multi-cylinder
b) single stage and multi-stage
c) mono stage and multi-stage
d) axial and radial
Explanation: On the basis of number of stages, steam turbines are classified into single stage and multi stage turbines. Single stage turbines have only one pressure stage but can have multiple velocity stages. They are used for small power capacities like driving centrifugal compressors or blowers etc. Multistage turbine is a combination of impulse and reaction turbines and they are made for a wide range of power capacities.
3. According to the direction of steam flow, steam turbines are classified into _____
a) axial and radial
b) uniaxial and multi-axial
c) upstream and downstream
d) forward and backward
Explanation: According to the direction of steam flow, turbines are classified into axial and radial turbines. In case of axial turbine, the direction of steam flow is parallel to the axis of the turbine. In radial turbines steam flow direction is perpendicular to the axis of turbine.
4. Turbines with separate rotor shafts for each cylinder placed parallel to each other are known as _____
a) multi-rotor turbines
b) multiaxial turbines
c) single-cylinder turbines
d) multi-utility turbine
Explanation: Multiaxial turbines are multi-cylinder turbines with separate rotor shafts for each cylinder placed parallel to each other. Multi-cylinder turbines in which the rotors are mounted on one shaft and coupled to a single generator are known as single-shaft turbines.
5. On the basis of method of governing, steam turbines are classified into turbines with _____
a) diffuser governing and nozzle governing
b) throttle governing and nozzle governing
c) impulse governing and reaction governing
d) throttle governing and diffuser governing
Explanation: In case of turbines with throttle governing, fresh steam enters through one or more simultaneously operated throttle valves. in turbines with nozzle governing, fresh steam enters through two or more consecutively opening regulators.
6. Which of the following is NOT a valid type of steam turbines, based on heat drop process?
a) Condensing turbines with generator
b) Back pressure turbines
c) Topping turbines
d) Supercritical turbines
Explanation: According to heat drop process, steam turbines are classified into mixed pressure turbines, low pressure turbines, back pressure turbines, topping turbines, back pressure turbines with steam extraction from intermediate stages at specific pressure, condensing turbines with generator, condensing turbines with one or two intermediate stage extractions at specific pressures.
7. Which of the following statement regarding condensing turbines with generators is TRUE?
a) Exhaust steam is supplied to intermediate stages
b) Steam is not extracted from intermediate stages
c) The latent heat of exhaust steam during condensation is utilized in feed water heating
d) Steam at a pressure less than atmospheric is directed to a condenser
Explanation: In condensing turbines with generators, steam at a pressure less than atmospheric pressure is directed to a condenser. Also steam is extracted from intermediate stages for feed water heating. The number of extractions lying from 2-3 is as high as 8-9. In these turbines the latent heat of exhaust steam during condensation is lost completely.
8. Which of the following statements regarding topping turbines is TRUE?
a) Exhaust steam is utilized in low and medium condensing turbines
b) Always operate at low initial conditions (steam pressure and temperature)
c) It has highest efficiency among all the turbines
d) Exhaust steam is lost completely
Explanation: Topping turbines are similar to backpressure turbines, the difference being – in case of topping turbines, the exhaust steam is utilized in low and medium condensing turbines. Usually topping turbines are operated at high initial conditions i.e. high steam pressure and temperature.
9. Which of the following turbines have inlet pressure of steam greater than 225 ata?
a) Medium pressure turbines
b) High pressure turbines
c) Low pressure turbines
d) Supercritical turbines
Explanation: Supercritical turbines handle steam having pressure greater than 225 ata. Low pressure use steam at a pressure between 1.2 to 2 ata, medium pressure turbines upto 40 ata and high pressure turbines above 40 ata.
10. Stationary turbines with variable speed cannot be used to drive _____
Explanation: Stationary turbines could be of constant speed or variable speed. The variable speed stationary turbines are used to drive turbo-blowers, air-circulators, pumps etc. A non-stationary turbine with variable speed is required to drive locomotives like ship and railways.
11. Which of the following statements about steam turbines is FALSE?
a) Thermal efficiency of steam turbine is higher than that of steam engine
b) Internal lubrication is not required
c) Flywheel installation is not necessary
d) It has a non-uniform rate of power generation
Explanation: The rate power generation is uniform and hence flywheel installation is not necessary. Since there are no rubbing parts, internal lubrication is not required. The thermal efficiency of steam engine is less than that of steam turbine.
12. In Simple impulse turbine the pressure of the steam falls gradually.
Explanation: In case of simple impulse turbine the expansion of steam takes place in the nozzle, the pressure is not affected as steam moves over the blades. The gradual fall in pressure is observed in case of reaction turbines.
13. Which of the following statements about reaction turbines is TRUE?
a) Steam pressure drops suddenly
b) The complete expansion of the steam takes place inside nozzle
c) Steam pressure is not altered as the steam moves over the blades the turbine
d) Steam pressure gradually drops as the steam moves over the blades of the turbine
Explanation: In reaction turbines, continuous expansion of the steam takes place as it passes over the blades of the turbine and therefore there is a gradual fall in the pressure of the steam during the expansion. Steam pressure is not altered when the steam expansion takes place in a simple impulse turbine.
Sanfoundry Global Education & Learning Series – Thermal Engineering
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