NON-CONDENSIBLE GASES

Other gases, mainly ambient air, may enter a refrigeration system as a result of incomplete evacuation before charging, opening of parts for maintenance or repair, or inward leaks on circuits operating below atmospheric pressure. These gases will be circulated with the refrigerant vapour until they are all in the con­denser and receiver. They cannot move further around the circuit because of the liquid seal at the outlet to the expansion valve.

Within the confines of the condenser and receiver, the gases will diffuse together and will exist in the same proportions throughout. The non-condensibles may therefore be removed through purge valves on either vessel, but such valves are commonly fitted on or near to the hot gas inlet to the condenser. The presence of non-condensible gas will be shown as an increase of condenser pressure (Law of Partial Pressures) and may be detected during normal operation if the run­ning log is accurate. The effect of this higher condenser pressure is to increase the compression ratio and so reduce the volumetric efficiency and increase the power. There will also be the effect of the gas blanketing the condenser surface, reducing heat flow.

Where the presence of such gas is suspected, a cross-check can be made, pro­viding the high-pressure gauge is of known accuracy. The method is to switch off the compressor after a short running period, and so stop the flow of thermal energy into the condenser, but continue to run the condenser until it has reached ambient conditions. The refrigerant vapour pressure can then be determined from the coolant temperature, and any increase indicates non-refrigerant gas in the system.

The bleeding of gas from the purge valve will release a mixture which can be estimated from the total pressure.

Example 11.2

A system containing R407C is cooled to an ambient temperature of 20°C and the condenser gauge then indicates 11.70 bar. What is the partial pressure of the non — condensible gas, and how much R407C must be lost to purge 10 g of this gas assuming that it is air?

Because R407C is a zeotropic mixture its vapour pressure is dependent on the proportion of vapour phase in the vessel. Assuming that this is small compared to the mass proportion of liquid in the condenser, the pressure will be the bubble point pressure at 20°C.

Vapour pressure of R407C at 20°C = 10.34 bar abs Observed pressure = 11.70 bar abs Partial pressure of non-condensible gas = 1.36 bar abs

R407C consists of R32/R125/R134a in proportions 23/25/52%. Therefore its molecular mass is:

0. 23 X 52 + 0.25 X 120 X 0.52 X 102 = 95

Note: The vapour composition in the condenser will differ slightly from the mass proportions given, but this will be ignored.

Gas

Proportion by pressure

Molecular

Mass

Proportion by weight

Weight ratio

Air

1.36

28.97

39.40

1

R407C

10.34

95

983.25

25.0

So 250 g of R407C must be wasted to purge 10 g of non-condensible gas. Purging must only be allowed if absolutely necessary, and must be carefully controlled.

Ammonia has a much lower molecular mass and the proportion by weight in this example would only have been approximately 40 g of ammonia lost. Also, ammonia is much cheaper than R407C!

Posted in Refrigeration and Air Conditioning