THE EFFECTS OF NON-CONDENSABLE GASES IN HOUSEHOLD REFRIGERATORS

摘要

It is well known that the presence of non-condensable gases inside a compression vapour refrigerating circuit introduces an additional thermal resistance at the condenser and can spoil the energy efficiency of the system. However this problem so far has been investigated mainly for shell ant tube condenser of large capacity and scarce information is available on small systems, as it is the case of household appliances where the internal volumes are extremely reduced and therefore a very small amount of non-condensable gas has large effect. Moreover non-condensable gas behaves differently when condensation takes place outside tubes (shell and tube condensers) or inside tubes (condensers of small appliances); in the first case all heat transfer area is wrapped by a gas layer, whereas in the second case non-condensable gas is collected at the end of the tube. The effect of non-condensable gas in this work is experimentally investigated by injecting controlled amounts of air inside a refrigerating circuit and by recording the thermal and electric variables during different modes of operation (steady state and cyclic running). The tested refrigerating circuit is part of an appliance (a household refrigerator for fresh foods) and was slightly modified to simplify the analysis: the modification consists in the removal of the suction line/capillary tube heat exchanger. The presence of non-condensable gas was found to spoil energy efficiency, since brings about an increase in condensing pressure and a concomitant decrease in evaporating temperature, although larger liquid subcooling partially compensate for the first negative effects. The temperature distribution along condensers and evaporator tubes points out flooding of condenser and starving of evaporator: this is why performance is worsening, the more so as the fraction of non-condensable gases increases. In its turn, this behaviour is caused by the clogging effect of bubbles of gaseous mixture (air and refrigerant vapour) that enter the capillary tube. The experiments show that the minimum allowable amount of non condensable gas is significantly lower than the one so far deemed to be safe by manufacturers.