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Impact of Elevated Ambient Temperatures on Capacity and Energy Input to a Vapor Compression System: Literature Review. Letter Report.


pdf icon Impact of Elevated Ambient Temperatures on Capacity and Energy Input to a Vapor Compression System: Literature Review. Letter Report. (970 K)
Motta, S. F. Y.; Domanski, P. A.

Letter Report for ARTI 21-CR; Research Project 605-50010/605-50015; 9 p. 2000.

Keywords:

refrigerants; air conditioning; alternative refrigerants; coefficient of performance; refrigeration; coefficient of performance

Abstract:

Operation of a system at elevated ambient temperatures inherently results in a lower Coefficient of Performance (COP). This conclusion comes directly from examining the Carnot cycle. The COP relation, indicates that the COP decreases when the condenser temperature increases at a constant evaporation temperature. This theoretical indication derived from the reversible cycle is valid for all refrigerants. For refrigerants operating in the vapor compression cycle, the COP degradation is greater than that for the Carnot cycle and varies among fluids. The two most influential fundamental thermodynamic properties affecting refrigerant performance in the vapor compression cycle are refrigerant's critical temperature and molar heat capacity. For a given application, a fluid with a lower critical temperature will tend to have a higher volumetric capacity and a lower coefficient of performance (COP). The difference between COPs is related to different levels of irreversibility because of the superheated vapor horn and the throttling process. The levels of irreversibility vary with operating temperatures because the slopes of the saturated liquid and vapor lines change, particularly when approaching the critical point. Refrigerants with a low critical temperature have a high pressure, a low drop of saturation temperature for a given pressure drop, and a low condenser-to-evaporator pressure ratio. These properties offer some advantages, which can be exploited in a real system for the betterment of its performance. Some researchers reported that a low pressure ratio promotes an improved compressor isentropic efficiency. The low drop of refrigerant saturation temperature for a given pressure drop allows designing heat exchangers with a high refrigerant mass flux, which promotes an improved refrigerant-side heat transfer coefficient.