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