Second law analysis of extra power requirements for a cascade of industrial compressors

Laboratory based research, development and testing of gas turbine and aircraft engine combustion processes requires a supply of compressed air at pressures, temperatures and flow rates comparable to the operating conditions from the compressors of the actual engine in which the compressor is powered by the turbine. However, in the laboratory, the compressed air is typically provided by a cascade of compressors with some intercooling and driven by large nominally fixed speed three phase induction motors. The control of the output pressure and flow has been performed with a combination of throttle and dump valves in the cascade while final temperature is attained by natural gas fueled heaters. An existing system has had an energy cost in excess of $2,000 per hour. While, it was recognized that some fundamentally wasteful techniques might be contributing to this substantial operating cost, it was not clear if there could be any practical and cost-effective method for reducing the power requirements: and costs. Thus, the authors undertook a 1st and 2nd law analysis for the purpose of identifying any such energy and cost saving options. It was expected that such an analysis might result in recommending modifications of the system in terms of additional intercooling, variable speed motors or installation of inlet guide vanes that would require substantial capital investment and installation to realize the benefits. Thus, the authors were surprised to find that the largest initial benefits could be achieved immediately with no capital investment, but by means of some minimal changes in the operating procedures. This paper describes the system, the author's analysis and the resulting changes in procedures that have resulted in a 15% reduction of required power and electric cost, along with describing additional computer based monitoring and equipment changes that are being considered or proposed.