ANALYTIC RESEARCH INTO THROTTLING CONTROL METHODS OF LIQUID-COOLED ROTARY SCREW AIR COMPRESSORS, TO THE PURPORT OF MINIMIZING THE ENERGY COSTS

摘要

Capacity control by throttling of a liquid-cooled screw compressor, running at constant speed and producing compressed air, is an inferior method when the energy-economic point of view is taken into consideration. Manufacturers have tried to develop various kinds of control combinations tended to improve the energy economy. To a certain degree this has been successful, but incomplete knowledge about an optimum, theoretic control combination has been in the way of a really ideal control. A linear, mathematical model for an ideal screw compressor has been developed. The model shows the dependence of the power consumption of the compressor on the pressure and on the air output. By the aid of the model the energy consumption of the compressor is analyzed in two basic ways of control: by the modulating control and by the on/off line control, to which the reduction of counter-pressure by idling is combined. As a final conclusion from the analysis the criteria are obtained, on the basis of which an energy-economically correct control method can be chosen when the consumption of compressed air is known. It is found that between the modulating control and the on/off line control there is a certain consumption limit, and when the consumption not reaching this limit, the on/off line control, on certain presumptions, is more advantageous than the modulating control. A further presumption for the on/off line control to be advantageous is that the pressure difference ranges between suitable limits. These limits can be determined by the aid of the model. The limits can most substantially be influenced by varying the volume of the compressed air network. In the on/off line control the optimum pressure difference depends by the on the volume of the network and on the extent of consumption, and it can be definitely determined when the aforesaid parameters are known. The larger the volume of the compressed air network, the greater the savings reached by the on/off line control in comparison with the modulating control and the more freely the pressure difference can be chosen. The model is adaptable with great accuracy also to a real screw compressor. The necessary amendments can be made on the numeric values of the parameters of the model, when the inferences concerning the model remain valid. On the basis of the inferences a computer program can be made out, which identifies the correct control type for all performance situations of the compressor.