Modeling, Hot Gas Bypass, Centrifugal Compressor, Load Stand
The increasingly competitive building equipment and control industry pushes manufacturers to devote more resources each year to research and development, continually improving the performance and efficiency of their products to develop and maintain a competitive edge. Compressor development is an expensive endeavor because of prototyping and testing costs, but the cost and time required for testing can be minimized by developing a model of the compressor test block to predict its behavior with a given prototype compressor at specified operating conditions. This paper presents a thermodynamic model of a hot gas bypass test block used to evaluate centrifugal compressor performance at a compressor development facility. The test block uses cooling towers to reject the heat of compression to outdoor air, and experience has shown that the range of achievable compressor test conditions can be limited by outdoor air temperature and humidity, which control the heat rejection rate. Therefore, one goal of the model development was to provide a means for evaluating the feasibility of tests at given ambient conditions. By incorporating models of the cooling towers into the test block model, test operators now are able to predict the range of compressor suction and discharge conditions that can be tested under the current outdoor air conditions. A second goal of the model was to assist in selecting the orifice plate used in the orifice flow meter that measures mass flow through the compressor. Operators previously had to make an educated guess as to the best orifice plate size in advance of running the tests, but the model now identifies the orifice diameters that result in pressure drops within the desired range, minimizing the trial and error involved in testing. The model assumes that the system operates at steady-state conditions and uses a compressor map to model expected prototype compressor performance. Therefore, this paper focuses on the condenser and cooling tower models, which are the most important elements for predicting the impact of outdoor conditions on cycle performance. It is shown that the resulting model achieves reasonable agreement with experimental data and provides a useful orifice selection routine.