This set of Power Plant Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Reheating of Steam”.
1. The product of steam rate & network when the steam rate is expressed in kg/kWh is equal to?
a) 36
b) 360
c) 3600
d) 60
View Answer
Explanation: The Steam Rate in terms of kJ/kWh is given by the expression,
Steam Rate = (3600/Wnet) kg/kWh.
2. The product of heat rate & efficiency when the heat rate is expressed in kJ/kWh is equal to?
a) 60
b) 36
c) 360
d) 3600
View Answer
Explanation: The heat rate in terms of kJ/kWh is given by the expression,
Heat Rate = (3600/n) kJ/kWh
where, n = efficiency.
3. What is the effect of reheat pressure on mean temperature of heat addition Tml?
a) Reheat pressure is directly proportional to Tml
b) Reheat pressure is inversely proportional to Tml
c) Reheat pressure is equal to Tml
d) Tml is independent of reheat pressure
View Answer
Explanation: As cycle efficiency is reduced with a decrease in reheat pressure. Hence, the mean temperature of heat addition also decreases.
4. Why is the steam not allowed to expand deep into the two phase region before it is taken to reheating?
a) to control flow rate
b) to control phase change
c) to protect reheat tubes
d) none of the mentioned
View Answer
Explanation: The steam is not allowed to expand deep into the two phase region before it is taken to reheating, because the moisture particles in the steam while evaporating would leave behind solid deposits in the form of scale which is difficult to remove. Hence, when the steam expands, the reheat tubes are damaged.
5. Net Work output of the plant __________ with reheat.
a) decreases
b) increases
c) remains same
d) none of the mentioned
View Answer
Explanation: With reheat, the area under the curve increases in comparison to what it was without reheat. Hence, Net Work output of the plant increases.
6. Which of the following problems are posed by increasing the number of reheats?
a) Cost & Fabrication problems arise
b) Heat transfer problems arise
c) Frictional losses arise
d) None of the mentioned
View Answer
Explanation: Higher the number of Reheats, still higher steam pressures could be used, but mechanical stresses increase at a higher proportion than the increase in pressure, because of the prevailing high temperature. The cost & fabrication difficulties will also increase.
7. What is the effect of decrease of reheat pressure on the quality of steam at turbine exhaust?
a) decreases
b) increases
c) remains same
d) none of the mentioned
View Answer
Explanation: For too low a reheat pressure, the exhaust steam may even be in the superheated state which isn’t good.
8. The optimum reheat pressure for most of the power plants is how many times of the initial steam pressure?
a) 0.1-0.15
b) 0.2-0.20
c) 0.2-0.25
d) 0.1-0.10
View Answer
Explanation: The efficiency increases as the reheat pressure is lowered & reaches a peak at a pressure ratio between 0.2-0.25.
9. What is the most preferable dryness fraction of the exhaust steam?
a) 0.99
b) 0.77
c) 0.66
d) 0.88
View Answer
Explanation: The dryness fraction of the exhaust steam is in the range of 0.86-0.88 for modern turbines.
10. For pressure ratio = 1, efficiency = 0, then?
a) reheat is used
b) reheat is not used
c) only reheat is used
d) none of the mentioned
View Answer
Explanation: There cannot be any reheat cycle employed when the cycle efficiency is zero and the pressure ratio is equal to 1.
Sanfoundry Global Education & Learning Series – Power Plant Engineering.
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