1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. Why Reheating of steam is used?
a) to increase efficiency
b) to increase work output
c) to increase Turbine Inlet Temperature
d) to reduce amount of fuel used
Explanation: Work output = work done by turbine – work done by compressor.
7. Reheating of steam _____________
a) decreases steam rate
b) increases steam rate
c) no effect on steam rate
d) none of the mentioned
Explanation: Since work output increases the steam rate decreases.
8. Efficiency of cycle will increase due to Reheating if _____________
a) mean temperature of heat addition in boiler is higher than in reheat process
b) mean temperature of heat addition in boiler is lower than in reheat process
c) it will increase without any conditions
d) it will not increase
Explanation: Efficiency = work output / heat supplied, heat supplied is less in this condition only.
9. Why don’t we use more than two reheats?
a) not economical
b) increased mechanical stresses
c) net efficiency is decreased
d) all of the mentioned
Explanation: The mechanical stresses increases in much higher proportion than the pressure because of the prevailing high temperature.
10.The cycle efficiency in a single reheat plant is influenced by _____________
a) pressure at tubine inlet
b) pressure at which steam is reheated
c) temperature at which steam is reheated
d) temperature at tubine inlet
Explanation: The cycle efficiency is a function of the ratio of reheat to initial pressure.