Optimal interstage pressures of multistage compression with intercooling processes

Abstract

This study develops a method to determine the optimal interstage pressures for two-stage and three-stage compressions with intercoolers. The developed method can also be applied using multi-loop calculation for compression involving more than three stages. The developed relationship is expressed in terms of temperatures, densities and the derivative of entropy with respect to pressure. The properties used in this study are calculated from the multiparameter equations of state. Equations of optimal interstage pressures for the nonequal inlet temperature of compressors based on the ideal gas assumption, called an ideal gas model, are found and investigated. The study results show that for the high pressure and low temperature region, the ideal gas model cannot predict the interstage pressure that minimizes the total specific compression work. High pressure and low temperature are considered by comparing the critical pressure and temperature of the considered gas. The developed model can provide the optimal interstage pressure which causes minimum total compression work even if the compressions are conducted in a high pressure and low temperature region. For two-stage compression of CO2 from 6.25 MPa, 305 K to 20 MPa with isentropic efficiencies of 0.9, the developed method can save 16.63% of specific compression work, comparing to the ideal gas model. For the developed method, if the isentropic efficiency of the stage compressor increases, the calculated optimal interstage pressure raised by that stage also increases. The increase in outlet temperature of the intercooler causes increasing optimal value of interstage pressure raised by the previous compression stage.