# Power Plant Questions and Answers – Brayton Cycle

This set of Power Plant Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Brayton Cycle”.

1. A gas turbine works on __________
a) Carnot cycle
b) Brayton cycle
c) Dual cycle
d) Rankine cycle

Explanation: A gas turbine works on 2 isentropic and 2 isobaric processes which is nothing else but Brayton cycle.

2. What is the range of Thermal efficiency in the Brayton cycle (ideal condition)?
a) 30-35%
b) 40-45%
c) 50-55%
d) 60-65%

Explanation: The thermal efficiency of an ideal Brayton cycle is in the range of 50-55%, however the efficiency of Brayton cycle in actual working condition is found to be between 30-45%.

3. What is the maximum possible TIT(Turbine Inlet Temperature)?
a) 1000 K
b) 1500 K
c) 950 K
d) 1300 K

Explanation: The maximum temperature is limited by metallurgical conditions.

4. A simple Brayton cycle uses Helium as the working fluid, has a maximum temperature of 1000 K, and a pressure ratio of 4. At the start of the compression, the Helium pressure and temperature are 50 kPa and 250 K. Based upon cold-air standard analysis assumptions, the thermal efficiency of the cycle is?
a) 0.13
b) 0.23
c) 0.43
d) 0.53

Explanation: Using the pressure ratio = 1 – rp(1 – k) / k.

5. A simple Brayton cycle has a pressure ratio of 5 and a maximum temperature of 900 K. Air enters the compressor at 100 kPa, 300 K. Based upon cold-air standard analysis assumptions, the back-work ratio of this cycle is?
a) 0.23
b) 0.53
c) 0.48
d) 0.36

Explanation: Using the pressure ratio and isentropic process relations, T2 = T1rp(k – 1) / k = 475 K and T4 = T3rp(1 – k) / k = 568 K. Application of the first law to the compressor and turbine processes and substituting the results into the back-work ratio definition produces rb = (T2 -T1) / (T3 -T4) = 0.53.

6. Which of the following methods can be used to increase efficiency of a Brayton cycle?
a) Regeneration
b) Increasing pressure ratio
c) Heat exchanger
d) All of the mentioned

Explanation: A heat exchanger that acts as a counter-flow energy recovery device positioned within the supply and exhaust air streams of an air handling system, in order to recover the waste heat.

7. For the same maximum pressure and heat input, the most efficient cycle is?
a) Brayton cycle
b) Carnot cycle
c) Rankine cycle
d) Dual cycle

Explanation: Efficiency of a Brayton cycle is directly proportional to pressure ratio.

8. The compression ratio is the ratio of ____________
a) Swept volume to total volume
b) Total volume to swept volume
c) Total volume to clearance volume
d) None of the mentioned

Explanation: According to definition, compression ratio is the ratio of swept volume to total volume.

9. A Sterling engine uses an energy source whose temperature is 727°C and an energy sink whose temperature is 27°C. How much heat must be added to the engine to produce 1 unit of work?
a) 1.10
b) 1.19
c) 1.43
d) 2.0

Explanation: Since the Sterling engine is totally reversible, its thermal efficiency is the same as that of a Carnot engine, η = 1 – Tl / Th = 0.7. Now, the definition of the thermal efficiency gives Qin / Wout = 1 / η = 1.43.

10. Which of the following is not a component of Brayton cycle?
a) Compressor
b) Combustion Chamber
c) Turbine
d) Pump

Explanation: Pump is used in Rankine cycle to circulate water from Condenser to Boiler.

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