Question

A Carnot vapor refrigeration cycle operates between thermal reservoirs at 4°C and 30°C. The working fluid is saturated vapor at the end of the compression process and saturated liquid at the beginning of the expansion process. Determine the operating pressures in the condenser and evaporator, in bar, and the coefficient of performance for each of the following as the working fluid. (a) Refrigerant 134a (b) Refrigerant R22 (c) water (d) ammonia

Answer 1

Answer:

COP=10.65

a)Evaporator pressure=337.7 KPa

Condenser pressure=770.2KPa  

b)Evaporator pressure=566.1 KPa  

Condenser pressure=1192 KPa

c)Evaporator pressure= 0.81 KPa

Condenser pressure=4.024 KPa

d)Evaporator pressure= 394.84KPa  

Condenser pressure=1167.2KPa

Explanation:

Given that cycle is working under Carnot cycle and we know that Carnot cycle is the ideal cycle for vapor compression.We also know that COP of Carnot cycle does not depends on the working fluids it depends only at the temperature limits.that is why COP of all fluids will be same.

COP of Carnot cycle given as follows

$COP=\dfrac{T_L}{T_H-T_L}$

Here given that  $T_L=277 K,T_H=303 K$

$COP=\dfrac{277}{303-277}$

COP=10.65

Now to find the condenser and evaporator pressure

We know that inside the wet region only one property needed to find other properties.Here given that temperature at condenser and at evaporator so the condenser and evaporator pressure will be the saturation pressure condenser and evaporator at given temperature for all fluids.

a)For refrigerant 134a

From properties table of  refrigerant 134a

Evaporator pressure=337.7 KPa   (pressure corresponding to  4°C)

Condenser pressure=770.2KPa  (pressure corresponding to  30°C)

b)For refrigerant R22

From properties table of  refrigerant R22

Evaporator pressure=566.1 KPa   (pressure corresponding to  4°C)

Condenser pressure=1192 KPa  (pressure corresponding to  30°C)

c)For refrigerant water

From properties table of  refrigerant water

Evaporator pressure= 0.81 KPa   (pressure corresponding to  4°C)

Condenser pressure=4.024 KPa (pressure corresponding to  30°C)

d)For refrigerant ammonia

From properties table of  refrigerant ammonia

Evaporator pressure= 394.84KPa   (pressure corresponding to  4°C)

Condenser pressure=1167.2KPa  (pressure corresponding to  30°C)