Lubricant Related Problems with Heat-Pumps

Roger Morgan, Lecturer, Brian L. Sutcliffe, Laboratory Technician, 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-refrigerant 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 School of Physical Sciences, The New University of Ulster, Coleraine, Northern Ireland. School of Physical Sciences, The New University of Ulster, Coleraine, Northern Ireland. School of Physical Sciences, The Ne~ University of Ulster, Coleralne, Northern Ireland. School of Physical Sciences, The New University of Ulster, Coleraine, Northern Ireland. School of Physical Sciences, The New University of Ulster, Coleraine, Northern Ireland. greater than with reciprocating compressors. BACKGROUND In this laboratory we have been concerned fo:t:· some time in the design and development of vapour compression heat pumps for a range of applications, and it has usually been our experience that performance is not quite as good as would be expected from the compressor manufacturer's performance charts. There are many reasons for this, but the short-fall in performance is not usually very great. However, recent tests using a rotary sliding-vane compressor showed up an abnormally wide and unacceptable discrepancy between observed performance and the manufacturer's data under certain conditions. Another factor was the consistent measurement of liquid line temperatures several degrees above saturation. These readings, indicating superheat, were somewhat puzzling as we could clearly see liquid refrigerant in the sight glass. After these obs~rvations, the accuracy of our measuring i~struments was thoroughly checked, and the tests repeated, still with the same results. At this point we realised that the effects of oil-refrigerant miscibility could be more important than we had thought previously, and a fairly trivial calculation using Raoult's Law showed that the presence of oil in small amounts could change the saturation temperature to the observed extent. From here our investigations followed in the footsteps of such as Bambach (1955) and Spauschus (1963). OIL-REFRIGERANT INTERACTION Oil is miscible with fluorocarbon refrigerants to a varying degree. In the case o~ liquid Rl~ it is totally miscible, the m~xture form~ng a single phase at all temperatures and pressures. The effects produced by the oil in a heat pump system can be summarised as follows: 1. It changes the working fluid from a pure refrigerant with well known properties to a poorly understood mixture, with properties which depend on the oil type and concentration. 2. It can affect the heat transfer processes in the evaporator and condenser. This effect can be either to improve or impair heat transfer, depending on oil concentration (Green, 1963) . 3. The boiling point of the mixture is elevated above that of the pure refrigerant, according to Raoult's Law. 4. The heat carrying capacity of the mixture is reduced during evaporation because the oil holds a proportion of the refrigerant in the liquid phase, thereby reducing the latent heat pick-up. As will be shown later, this effect implies a reduction in the coefficient of performance (COP). It follows from these points that the presence of oil in a heat pump circuit could have a significant effect on its performance. The following analysis is an attempt to predict and quantify the likely effects.