Question

A refrigeration system using refrigerant 22 is to have a refrigerating capacity of 80kw. The cycle is a standard vapor-compression cycle in which the evaporating temperature is -9 C and the condensing temperature is 42 C, sketch the cycle on pressure enthalpy coordinates.

Calculate

1. Determine the volume flow of refrigerant at the inlet to the compressor
2. Calculate the power required by the compressor
3. At the entrance to the evaporator what is the fraction of vapour in the mixture

Answer 1

As nothing is stated about superheating and subcooling, we will assume there is none. Using the information from the data provided, we can draw the process diagram as follows

Here 1-2 is the evaporator, 2-3 is a compressor, 3-4 is condenser and 4-1 is throttling valve

From a pH chart for R22, we get the enthalpy values. Please note that I am not allowed to post an actual pH chart because of legal reasons.

Point	Enthalpy
1	105
2	240
3	280
4	105

Thus refrigeration effect per unit mass flow rate of refrigerant is h2-h1 = 240 - 105 = 135 kJ/kg

It is given that the refrigeration effect is 80 kW. Thus the mass flow rate of refrigerant m is 80/135 = 0.592 kg/s

The specific volume at the compressor entrance (point 2) is obtained from the chart as 0.06 m3/kg

So volume flow rate at entrance of compressor is 0.06*0.592 = 0.03552 m3/kkg

The power input to compressor is the product of mass flow rate and change in enthalpy per unit mass refrigerant across the compressor. Hence compressor power is m*(h3-h2) = 0.592 (280-240) = 23.68 kW

From the pH chart, at point 1, we obtain the dryness fraction as close to 0.34. This denotes the amount of vapour in the flow of the refrigerant.

ANSWERS

• The volume flow rate of the refrigerant at the compressor inlet is 0.03552 m3/s
• Power required by the compressor is 23.68 kW
• The fraction of vapour in the mixture at the entrance of the evaporator is 34%

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