Thermodynamic model for reciprocating compressors

Refrigerators and heat pumps are thermodynamic cycles, which mostly work with hydrofluorocarbons. As opposed to hydrochlorofluorocarbons, hydrofluorocarbons do not have a potential for ozone depletion, but have a high global warming potential and therefore, are to be replaced in the near future. Finding individual replacement working fluids for current systems is also important in view of this context.

Fluid selection is a topic of concern and a number of methods have been developed in relation to the systemization of their selection for heat pumps and for Organic Rankine Cycles. In a good number of these cycles, for instance, refrigerators and heat pumps, compressors have been found to lead to large exergetic losses. Therefore, the compressor directly influences the coefficient of performance.

Several smaller thermodynamic cycles are normally designed with reciprocating compressors, therefore, the efficiency of the entire process is hinged on the compressor efficiency. For this reason, identifying appropriate as well as high efficient fluids will be facilitated, when the isentropic and volumetric efficiencies of the existing compressors as a function of operating conditions and the fluid, are easily predicted.

The current plants are operated in steady state, therefore, data at one point of operation with a single fluid can easily be obtained. This data point and a few compressor parameters will make it possible to realize an elementary model that will be accurate to be implemented in cycle modeling and would be helpful in predicting reliable coefficients of performance for various fluids in a cycle. This would be necessary for fluid recommendations for particular processes.

Dennis Roskosch, Valerius Venzik and Burak Atakan from The University of Duisburg-Essen in Germany introduced a differential compressor model for reciprocating compressors that predicted isentropic and volumetric efficiencies and could be easily fitted with measured data at one operation point of an existing compressor. Their research work is published in International Journal of Refrigeration.

The research team fitted two semi-physical correlations for the valve flows in a bid to characterize the influence of the various fluids. They validated the model on 63 measured points based on several fluids from one semi-hermetic reciprocating compressor.

The role of the valve flows was an important observation made by the authors. Referencing several measurements with various fluids under various operation conditions, they obtained two empirical equations, which correlated the effective flow areas. When the two equations were applied, the transferable parameters were reduced to relative clearance volume and friction pressure. Therefore, this model could be fitted to an existing compressor based on only one measured point with one fluid.

The findings for various fluids under varying operation conditions indicated good agreement with the measured values taken at constant rotation speed. The relative mean errors for volumetric and isentropic efficiencies were observed to be smaller than 3.0% for the compressor taken as the basis for the fitting procedure and lower than 4.4% for a different compressor.

The researchers found that the correlations for effective flow areas were inaccurate for rotation speeds. This could be however recovered by the fitted friction pressure, which should have been regarded as a fitting parameter. The proposed model is therefore a good basis for fluid replacement studies. However, additional verifications with varying piston compressors and other fluids would be necessary.