The four devices of a vapour compression refrigeration cycle are
Answer: A
The third option is the Rankine power cycle and the fourth belongs to absorption refrigeration. Naming the four devices correctly is the first step in analysing any point on the cycle. Each device changes one pair of properties.
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Match each device in the vapour compression cycle with what happens in it.
List I
A. Evaporator
B. Compressor
C. Condenser
D. Expansion valve
List II
1. heat is rejected and the refrigerant condenses
2. the refrigerant absorbs heat and boils
3. pressure falls at constant enthalpy
4. pressure and temperature are raised by work input
Answer: B
The cycle is a closed loop, so tracing what each device does in order is how any state point is located. The valve is the one place where the process is deliberately irreversible. The other three are treated as reversible in the ideal cycle.
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A turbine is not used in place of the expansion valve in a small refrigeration plant because
Answer: C
Correct answer: the work recoverable does not justify the cost
Expanding the liquid through a turbine would recover a little work and give a slightly greater refrigerating effect, and large plants do it. On a small machine the recoverable work is a few per cent of the compressor power, while the turbine adds cost, complexity and a maintenance liability. The throttle is chosen deliberately, not because the expansion must be isenthalpic - it is isenthalpic *because* a throttle is used.
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The coefficient of performance of a refrigerator is defined as
Answer: D
It measures what you want against what you pay for, which is why it exceeds one for a working machine. The second definition belongs to a heat pump, whose coefficient is larger by exactly one. Both describe the same machine viewed from different ends.
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A refrigerator removes 3 kilowatts from a cold space while consuming 1 kilowatt of compressor power. What is its coefficient of performance?
Answer: A
Correct answer: 3
\[ \mathrm{COP} = \frac{Q_C}{W} = \frac{3}{1} = 3. \] A value above one is normal and violates nothing: heat is being moved rather than generated, so three units are delivered from the cold space for one unit of work. Inverting the ratio gives the 0.33 offered, which would be a thermal efficiency and is the wrong quantity entirely.
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Assertion (A): A heat pump using the same equipment as a refrigerator has a coefficient of performance one greater.
Reason (R): The heat rejected at the condenser is the heat absorbed plus the work supplied.
Answer: B
Both are true and the reason gives the arithmetic: dividing that sum by the work gives the cooling coefficient plus one, which is why heat pumps look so favourable against direct electric heating. A coefficient of four delivers four units of heat per unit of work.
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A reversed Carnot cycle works between 260 and 300 kelvin. What is the highest coefficient of performance it could achieve as a refrigerator?
Answer: C
Correct answer: 6.5
\[ \mathrm{COP}_{max} = \frac{T_C}{T_H-T_C} = \frac{260}{300-260} = \frac{260}{40} = 6.5. \] The 7.5 offered uses the hot temperature in the numerator, which is the heat pump figure - and the two differ by exactly one, since a heat pump delivers the heat pumped plus the work put in.
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State whether the following is true or false.
The coefficient of performance falls as the temperature difference between the two reservoirs widens.
Answer: D
More work is needed per unit of heat moved as the lift increases, which is why a deep freezer is much less efficient than a chiller. The Carnot expression makes the dependence explicit. Freezer and chiller duties are often split for this reason.
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The main irreversibility in a simple vapour compression cycle is
Answer: A
Correct answer: the throttling of liquid through the expansion valve
Throttling is an entirely uncontrolled expansion: the pressure falls with no work extracted and entropy is generated, so it is the largest single source of irreversibility in the cycle. Boiling and condensation are close to reversible when the temperature differences are small, and pipe friction is minor by comparison. That is why the expansion turbine of the first item is worth considering at all.
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Consider the following statements about subcooling the liquid before the expansion valve.
1. It reduces the fraction that flashes to vapour at the valve.
2. It increases the refrigerating effect per kilogram circulated.
3. It requires additional compressor work.
Answer: B
The first two are correct and are why subcooling is worth arranging. The third is wrong, since the compressor sees the same suction condition and the gain is obtained without extra work. The extra cooling is usually free from the condenser.
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