Performance analysis of screw compressors – numerical simulation and experimental verification

Abstract

This article describes a theoretical model and computer program for calculating the pressure–volume ( PV ) diagram and the efficiency of an oil-injected screw compressor. The proposed model considers the mass and energy conservation laws, the heat transfer between air and oil, the leakages through various paths, and the discharges of air and oil. The proposed program, which uses seven empirical constants to account for the difference between the flow and the heat-transfer rates in the screw compressor and those estimated by available correlations, solves for the efficiency and the pressure curve of the compressed air. A systematic methodology for the determination of the seven empirical constants is presented in this article. Optimization is carried out to determine the seven empirical constants. With the empirical constants, which are determined with four sets of experiments, the maximum difference between the calculated and measured results in the training process, the verification process and the total process is 2.0 per cent for the volumetric and isentropic efficiencies and 5 per cent for the pressure curve. In the discharge process, the pressure in the compression chamber is noted to be affected by the area of the outlet port and the pressure in the neighbouring chambers.

Calculation of temperature distributions in the rotors of oil-injected screw compressors

“…To provide experimental data for the determination of empirical constants, an experimental facility [19] is adopted in this study. Fig.…”

“…Equation (5) is the periodic boundary condition that describes the heat convection between the screw and the compressed air. In this study, the time-dependent properties of the compressed air and the performance of the oil-injected screw compressor are calculated by a lumped parameter analysis [19]. The errors of the calculated volumetric and isentropic efficiencies are both less than 2%.…”

“…(13), (15), and (17)] and power generated by the bearings [Eq. (19)] from the temperature of compressed air, the frictional power generated by the oil and the power generated by the bearing, under the five test conditions. In the forth step, the mathematical model is solved and the difference between the measured and calculated results under the five sets of operating conditions is minimized by the method of sequential quadratic programming (SQP) [29e32] to determine the six empirical constants.…”

“…To determine the periodic convective boundary condition, the convective heat-transfer coefficient is calculated as the product of an empirical constant and an ideal convective heat-transfer coefficient. The ideal convective heat-transfer coefficient is determined by the correlation proposed by Gnielinski [18] using the properties of the compressed air that are obtained with the lumped parameter analysis developed by the authors [19]. To determine the five steady boundary conditions, each of the heat fluxes at five boundaries is calculated as the product of an empirical constant and an ideal heat flux.…”

Experimental validation of a geometry model for twin screw machines

“…For example, Hsieh (10) presented a procedure to determine empirical constants in a 'training process' which optimises multiple constants using a range of test cases. Hsieh (10) and Wu (8) presented models which once calibrated, or trained, have good agreement between simulated and measured results. Caution should be exercised when using such models, which rely on empirical constants or coefficients, to predict the performance of compressors different than the machine used to derive the coefficients.…”