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ISRO 2018 Refrigeration and Air-Conditioning Questions, Answers and Explanation

ISRO 2018 Refrigeration and Air-Conditioning Questions, Answers and Explanation

1. Carnot cycle consists of 
a) Two constant pressure and two isentropic processes
b) One constant isenthalpic, one constant volume and two constant pressure processes
c) Two isothermal and two isentropic processes
d) One constant pressure, one constant volume and two isothermal processes.

Answer
c) Two isothermal and two isentropic processes

Explanation [Thermal Engineering]
The most efficient heat engine cycle is the Carnot Engine cycle and it is represented as a rectangle in T-S diagram
Otto Cycle - 2 constant volume and 2 isentropic
Diesel Cycle - 1 constant pressure, 1 constant volume and 2 isentropic

2. One reversible heat engine operates between 1600K and T2 K and another reversible heat engine operates between T2 K and 400K. If both engines have the same heat input and output, then the temperature T2 is equal to
a) 800K
b) 1600K
c) 1200K
d) 6400K

Answer
a) 800K

Explanation [Thermal Engineering]
Reversible engine is a theoretical engine operating in according to Carnot cycle. If both engines have the same input and output their efficiencies must be the same as efficiency = work done/ heat input = (heat input - heat output)/heat input. The efficiency of the Carnot cycle operating between cold reservoir temperature Tc and hot reservoir temperature Th is given by 1 - Tc/Th.
1 - T2/1600 = 1 - 400/T2
T2/1600 = 400/T2
(T2)2 = 20x20x40x40
T2 = 800K

4. Reversed Joule cycle is known as
a) Rankine cycle
b) Carnot cycle
c) Bell Coleman cycle
d) Otto cycle

Answer
c) Bell Coleman cycle

Explanation [Thermal Engineering]
Joule cycle is also called as Brayton cycle. Brayton cycle consists of
- Isentropic compression
- Constant pressure heat addition (mostly combustion)
- Isentropic expansion
- Constant pressure heat rejection
Bell Coleman cycle consists of
- Isentropic compression
- Constant pressure heat rejection
- Isentropic expansion
- Constant pressure heat absorption (refrigeration effect)

8. A mixture of gases expand from 0.03m3 to 0.06m3 at constant pressure of 1MPa and absorb 84kJ of heat during the process. The change in internal energy of the mixture is
a) 54kJ
b) 30kJ
c) 84kJ
d) 110kJ

Answer
a) 54kJ

Explanation [Thermal Engineering]
Work done during a non-flow constant pressure process is PdV = 1000kPa x (0.06-0.03)m3
= 30kJ
From the first law of thermodynamics change in internal energy is the difference of heat absorbed by the system and work done by the system.
Change in internal energy = 84kJ - 30kJ = 54kJ

21. The purpose of thermostat in an engine cooling system is to
a) Indicate coolant temperature
b) Prevent coolant from boiling
c) Allow the engine to warm up quickly
d) Pressurize coolant for effective cooling

Answer
c) Allow the engine to warm up quickly

Explanation [Thermal Engineering]
The thermostat is like a valve that opens and closes as a function of its temperature. The thermostat isolates the engine from the radiator until it has reached a certain minimum temperature. Without a thermostat, the engine would always lose heat to the radiator and take longer to warm up. Once the engine has reached the desired operating temperature, the thermostat adjusts flow to the radiator to maintain a stable temperature.

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