Freezing Technology

3.2.2. Thermodynamic properties

1. Boiling temperature

Should be low at atmospheric pressure. If it is high, compressor should be operated at high vaccum, reduces capacity and increase the cost of the system.

e.g.

Refrigerant	Boiling temperature (ºC)
R-12 (CCl2F2)	- 29
R-717 (NH4)	-33.3
R.744 (CO2)	-73.6
R-764 (SO2)	-10

2. Freezing temperature:

Should be below the operating temperature of evaporator. Freezing temperature of most of refrigerants are below -35^ºC

Refrigerant	Boiling temperature (ºC)
R-12	- 157.5
R-717	-35
R.744	- 56.7
R-764	- 75.6

3. Evaporator and condenser pressure

Both the evaporating and condensing pressure should be +ve (i.e. above atmospheric pressure). The + ve pressure are necessary to prevent leakage of air, moisture into the refrigeration systems. It also permits easier detection of leaks.

Critical temperature & pressure:

It is the highest temperature at which refrigerant  can be condensed to liquid, regardless of high pressure.

Refrigerant	Critical temperature (ºC)	Critical pressure (bar)
R-12	112	41.2
R-717	133	113.86
R.744	31	73.8
R-764	157	78.7

Coefficient of performance and power requirements

For an ideal refrigerant operating between -15ºC evaporator temperature and 30⁰C condenser temperature. The theoretical coefficient of performance for the reversed cycle is 5.74.

Refrigerant	Coefficient of performance	Power HP/TR
R-12	4.70	1.00
R-11	5.09	0.93
R-717	4.76	0.99
R.744	2.56	1.84
R-764	4.87	0.97