# Power Plant Question and Answers – Effect of variation of Steam Condition on Thermal Efficiency

This set of Power Plant Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Effect of variation of Steam Condition on Thermal Efficiency”.

1. The cycle efficiency ________________ with the _____________ of condenser pressure.
a) decreases, decrease
b) increases, decrease
c) remains same, decrease
d) does not vary, decrease

Explanation: As the decrease in condenser pressure mainly depends on the available cooling water temperature & thus on the climatic conditions of the place. Hence, cycle efficiency being a function of condenser pressure has an inverse relation with the condenser pressure.

2. What is the relation between cooling water temperature (tc1) & condenser pressure (pc)?
a) tc1 is directly proportional to pc
b) tc1 is directly proportional to 2xpc
c) tc1 is directly proportional to 5xpc
d) tc1 is directly proportional to 6xpc

Explanation: Cooling water temperature is directly proportional to condenser pressure 1xpc.

3. An increase in inlet steam temperature ___________ the heat rate.
a) lowers
b) increases
c) does not have any effect on
d) temperature has no significance in heat rate

Explanation: An increase in inlet steam temperature, i.e. an increase in superheat at constant inlet steam pressure & condenser pressure gives a steady improvement in cycle efficiency & lowers the heat rate due to an increase in the mean temperature of heat addition.

4. What is the effect of increasing superheat at constant inlet steam pressure & condenser pressure on cycle efficiency?
a) Steady decrement in cycle efficiency
b) Steady improvement in cycle efficiency
c) No effect on cycle efficiency
d) None of the mentioned

Explanation: An increase in inlet steam temperature, i.e. an increase in superheat at constant inlet steam pressure & condenser pressure gives a steady improvement in cycle efficiency & lowers the heat rate due to an increase in the mean temperature of heat addition.

5. What is the effect of increasing superheat at constant inlet steam pressure & condenser pressure on mean temperature of heat addition?
a) increases
b) decreases
c) remains same
d) none of the mentioned

Explanation: An increase in inlet steam temperature, i.e. an increase in superheat at constant inlet steam pressure & condenser pressure gives a steady improvement in cycle efficiency & lowers the heat rate due to an increase in the mean temperature of heat addition.
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6. What is the effect of increasing steam inlet temperature on turbine internal efficiency?
a) it remains same
b) it increases
c) it decreases
d) none of the mentioned

Explanation: Increasing the inlet steam temperature decreases the wetness of the steam in the later stages of the turbine & improves the turbine efficiency.

7. What is the percentage drop in the ultimate strength of unalloyed steels when the steam temperature is raised from 400-500 degree Centigrade?
a) 20%
b) 30%
c) 40%
d) 60%

Explanation: The ultimate strength of unalloyed steels is 30% when the steam temperatures are in the range 400-500 degree Centigrade. Alloying can increase this strength.

8. Apart from alloying, name a material that is used instead of ferritic steels to increase the ultimate strength of unalloyed steels.
a) Chromium
b) Molybdenum
c) Austenitic steel
d) None of the mentioned

Explanation: The use of Austenitic steels increases the ultimate strength of unalloyed steels at high temperatures. The temperature limits are in the range 538-565 degree Centigrade.

9. Which of these cases will have a higher cycle efficiency?
a) Condenser Pressure = 25 mm of Hg; Cooling Water Temperature = 12 degree Centigrade
b) Condenser Pressure = 50 mm of Hg; Cooling Water Temperature = 27 degree Centigrade
c) Condenser Pressure = 75 mm of Hg; Cooling Water Temperature = 36 degree Centigrade
d) Condenser Pressure = 100 mm of Hg; Cooling Water Temperature = 45 degree Centigrade

Explanation: In identical steam conditions cycle efficiency is inversely proportional to cooling water temperature.

10. For the given values of efficiency, inlet steam pressure and quality of steam, pressure limits can be determined by?
a) Mollier Charts
b) Gantt Charts
c) Mollier & Gantt Charts
d) None of the mentioned

Explanation: The pressure limits can be readily determined by drawing the corresponding expansion line of the turbine om a Mollier Diagram.

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