# Thermodynamics Questions and Answers – Mean Temperature of Heat Addition and Reheat Cycle

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This set of tough Thermodynamics Questions focuses on “Mean Temperature of Heat Addition and Reheat Cycle”.

1. In the Rankine cycle, heat is added reversibly at
a) constant pressure and constant temperature
b) constant pressure and infinite temperature
c) infinite pressure and constant temperature
d) infinite pressure and infinite temperature

Explanation: This is a basic fact about Rankine cycle.

2. The efficiency of Rankine cycle is given by
a) 1 – (Q1/Q2)
b) 1 – (Tmean/T2)
c) 1 – (T2/Tmean)
d) none of the mentioned

Explanation: Here T2 is the temperature of heat rejection and Tmean is the mean temperature of heat addition.

3. Which of the following statement is true?
a) for given Tmean, lower is the T2, higher will be the efficiency of Rankine cycle
b) the lowest possible temperature of heat rejection is the surroundings temperature
c) higher is the mean temperature of heat addition, higher will be the efficiency
d) all of the mentioned

Explanation: The efficiency of the Rankine cycle = 1 – (T2/Tmean).
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4. If we ____ the superheat at constant pressure then the cycle efficiency ____
a) decrease, increases
b) increase, decreases
c) increase, increases
d) decrease, decreases

Explanation: Increasing the superheat at constant pressure increases the mean temperature of heat addition and cycle efficiency also increases.

5. The maximum temperature of steam that can be used is not fixed.
a) true
b) false

Explanation: It is fixed from metallurgical considerations.

6. To prevent erosion of blades, quality should not fall below
a) 85%
b) 90%
c) 95%
d) 100%

Explanation: Thus the maximum moisture content which is allowed at the turbine exhaust is not to exceed 15%.

7. To fix the maximum steam pressure at the inlet of turbine we need to first fix
a) the maximum steam temperature at turbine inlet
b) minimum temperature of heat rejection
c) the minimum quality of steam at turbine exhaust
d) all of the mentioned

Explanation: These all are needed to be fixed to fix to maximum steam pressure at turbine inlet.

8. Which of the following is true about a reheat cycle?
a) used to limit the quality at turbine exhaust at 0.85 when steam pressure is higher than (p1)max
b) after partial expansion in turbine, steam is brought back to boiler
c) the steam is reheated by combustion gases
d) all of the mentioned

Explanation: This is the functioning of a reheat cycle.

9. The correct sequence of expansion in a reheat cycle is
a) HP turbine – LP turbine – constant pressure in boiler
b) HP turbine – constant pressure in boiler – LP turbine
c) LP turbine – constant pressure in boiler – HP turbine
d) LP turbine – HP turbine – constant pressure in boiler

Explanation: Here HP is the high pressure turbine and LP is the low pressure turbine.

10. Why is steam not allowed to to expand deep into two-phase region before being taken for reheating.
a) to protect the reheater tubes
b) to prevent solid deposits being left behind while evaporating
c) both of the mentioned
d) none of the mentioned

Explanation: These solid deposits are difficult to remove from the reheater tubes.

11. Why should the reheat pressure be optimized?
a) a low reheat pressure brings down the mean temperature of heat addition and hence the cycle efficiency
b) a high reheat pressure increases the moisture content at turbine exhaust
c) both of the mentioned
d) none of the mentioned

Explanation: These are the reasons why we need to optimize reheat pressure.

12. The optimum reheat pressure is ____ times that of the initial steam pressure.
a) 0.2
b) 0.23
c) 0.25
d) all of the mentioned

Explanation: It lies in the range 0.2-0.25 for most of the modern power plants.

13. With the use of reheat,
a) the net work output of the plant increases
b) there is only a marginal increase in cycle efficiency
c) the quality of steam at turbine exhaust is kept within a limit
d) all of the mentioned

Explanation: These can be considered the main advantages of using reheat.

14. More than three reheats have not been used so far.
a) true
b) false

Explanation: The maximum number of reheats used till now is two only.

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