Lubricant Related Problems with Heat-Pumps

Abstract

Oil is required in refrigeration and heat pump systems for lubrication of the compressor, expansion valve and other moving p~rt7.Ho~ever, the oil is often highly m~sClble wlth the refrigerant, in proportlons which depend on pressure and temperature, as well as oil and refrigerant type.As some of the oil circulates around the system with the refrigerant, it follows that at all times a proportion of the refrigerant will be dissolved, as a liquid, in the oil.Not only does this oil-refri- gerant miscibility give rise to dilution of the oil, hence impairing its lubricating properties, but it also leads to a reduction in evaporator capacity.This is because, at the evaporator outlet, some of the refrigerant remains in the liquid state, dissolved in the oil, and is thus unavailable for evaporation to carry latent heat.This effect on its own would not be too serious, were it not that the coefficient of performance (COP) is also impaired due to oil circulation.This results from the fact that, while evaporator capacity is reduced, compressor power is largely unaffected by the oil circulation rate.This of course implies a reduced COP.In a heat pump design, COP is usually all-important, and this paper examines some of the effects of oil circulation on heat pump performance.Particular attention is drawn to designs using oil-flooded rotary sliding-vane compressors, where oil circulation is usually much 156