The coefficient of performance of a vapor compression refrigeration system:

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  1. Decreases with the decrease in evaporator temperature at a fixed condenser temperature.
  2. Increases with the increase in condenser temperature at a fixed evaporator temperature.
  3. Remains constant with the decrease in evaporator temperature at a fixed condenser temperature.
  4. Increases with the decrease in evaporator temperature at a fixed condenser temperature.

Answer (Detailed Solution Below)

Option 1 : Decreases with the decrease in evaporator temperature at a fixed condenser temperature.
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Concept:  

Capture 8

Capture 8 RRB JE ME 50 8Q TE CH 5 Hindi - Final 16

p - h and T-s diagram for ideal vapour compression refrigeration cycle are shown below,

F1 S.S Madhu 28.12.19 D1  F1 S.S Madhu 28.12.19 D2

The decrease in the evaporator pressure will not affect condensing temperature because the condensing temperature is a function of condensing pressure not of evaporator pressure. So, there is no change in the temperature of condenser. 

From the p-h diagram:

Let pE1 and p­E2 be the initial evaporator pressure and decreased evaporator pressure respectively.

At pE1,

W (Compressor work) = h2 – h­1

RE (Refrigeration Effect) = h1­ ­­– h4

At pE2,

W’ (Compressor work) = h2’ – h1

RE’ (Refrigeration Effect) = h1’ – h4’

From the p-h diagram,

W’ > W, Compressor work is increased

RE’ < RE, Refrigeration effect is decreased

There is an increase in compressor work and a decrease in the refrigeration effect.

The coefficient of performance of a vapor compression refrigeration system decreases with the decrease in evaporator temperature at a fixed condenser temperature.

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