Question

A food storage chamber requires a refrigeration system of 12 tons capacity when evaporator temperature is $-8^{\circ }C$ and condenser temperature is ${30}^{\circ }C$. The system uses R-12 as refrigerant with properties given below. The refrigerant is subcooled by $5^{\circ }C$ before entering the throttle valve and vapour is superheated in the evaporator itself by $6^{\circ }C$ before entering the compressor. What is the COP of cycle?

• A. 5.8
• B. 3.1
• C. 6.1
• D. 7.1

Answer 1

The correct option is C 6.1


$h_4=h_3-c_{pl}\times \Delta T$

$h_4=h_3-c_{pl}\times 5$

$h_4=447.9-0.733\times 5$

$h_4=444.235kJ/kg[h_4=h_5,\text{due to throttling process}]$

$h_1=h_1^{\prime }+c_{pv}\times 6$

$h_1=569.8+1.235\times 6=577.21kJ/kg$

$s_1=s_2$ (as 1-2 is isentropic process)

$s_1^{\prime }+c_{pv}\ln \frac{T_1}{T_1^{\prime }}=s_2^{\prime }+c_{pv}\ln \frac{T_2}{T_2^{\prime }}$

$4.7564+1.235\ln \frac{271}{265}=4.7438+1.235\ln \frac{T_2}{(30+273)}$

$(\text{where}{T}_1=-8+6=-2^{\circ }C,T_1=-8^{\circ }C)$

$T_2=313.037K={40.037}^{\circ }C$

$h_2=h_2^{\prime }+c_{pv}(40.037-30)$

$h_2=598.896kJ/kg$

$COP=\frac{(h_1-h_5)}{h_2-h_1}=\frac{577.21-444.235}{598.896-577.21}$

$COP=6.131$