The role of optimum intermediate pressure in the design of two-stage vapor compression systems: A further investigation

The determination of the optimum intermediate pressure (pm,opt) is critical for the design of two-stage vapor compression (TSVC) systems, and there exists many studies on it. However, the inter-stage subcooling parameters (ε) and the respective efficiencies of the two compressors were not included in the existing estimation formulas for pm,opt. In this paper, six types of TSVC cycles are classified into two categories, and then based on a general expression of COP, the influences of ε and the compressor efficiencies on pm,opt are analyzed for the cycles using R22 and ammonia as the refrigerants. Results show that, for the ideal cycles, ε is a critical influence factor on pm,opt in addition to the condensing and evaporating conditions; whereas for the real cycles, both ε and the efficiencies affect pm,opt significantly. Two revised formulas are established to estimate pm,opt for the ideal and real cycles respectively with satisfactory results.