In practice ARI Standard 500-2000 defines the compressor efficiency as the ratio of isentropic work to the actual measured input power. Therefore, the compressor efficiency becomes:
where m is the mass flow rate of refrigerant, kg/s; h2s is specific enthalpy of refrigerant vapor at discharge pressure at constant entropy (s1 = s2S), kJ/kg; h1 is specific enthalpy of refrigerant vapor entering the compressor, kJ/kg; P is the measured motor input power, kW; and nc is the compressor efficiency.
Note that the other compressor efficiency, the volumetric efficiency, can be approximately represented in terms of the ratio of the clearance volume to the displacement volume (R), and the refrigerant specific volumes at the compressor inlet (suction) and exit (discharge) (v1 and v2), as given below:
Also, note that the refrigeration capacity can be defined in terms of the compressor volumetric displacement rate (V,m3/s), compressor volumetric efficiency ( a v ) , density of the refrigerant at the compressor inlet (p1, kg/m3), and specific enthalpies of the refrigerant at compressor inlet and at expansion valve inlet. It is then written as
Further details on the practical performance evaluations and ratings of compressors, and definitions of compressor related items, are given extensively in ARI (2000) and Klein et al. (2000).
Although there are a number of issues that affect the compressor efficiency, the most significant one is the temperature lift (or compression ratio). To a lesser extent, the suction temperature, lubrication and cooling also play an important role. Therefore, the following solutions to increase the efficiency of the compressor become crucial.
• Minimization of temperature lift. The compressor is most efficient when the condensing pressure is low and the evaporating pressure is high, leading to the minimum temperature lift and compression ratio. The effect of operating conditions is illustrated by the compressor data example in Figure 3.16. In conjunction with this, a good system design should ensure that the condensing pressure is as low as possible and the evaporating temperature is as high as possible. Designing a system with a small condenser and evaporator to save capital cost is always a false economy. Using a larger evaporator and condenser often means a smaller compressor can be used and always reduces running costs. The additional benefit is that the compressor will be more reliable because it does not have to work as hard and operates with lower discharge temperatures.
• Lowering suction temperature. The lower the suction gas temperature the higher the capacity with no effect on power input. The discharge temperature will also be lower, thus increasing reliability. Suction line insulation is essential.
• Effective lubrication and cooling. The compressor must be lubricated and efficiently cooled. Insufficient lubrication increases bearing friction and temperature and reduces compressor efficiency, often resulting in failure.