Abstract
This paper describes thermal analysis procedures of heat exchangers for air conditioning systems which are used as outside air cooling systems or heat recovery systems. The performance of heat exchangers can be evaluated by frictional pressure drops in the two fluids utilized and the amount of heat exchanged between those fluids. Moreover, the pressure drops and the exchanged heat can be evaluated as exergy. Therefore, in this paper, the performance of heat exchangers is measured by exergy. Exergy analyses are becoming more common in power generation plants, but little work has been done to analyze the performance of heat exchangers utilized in air conditioning systems. Air supplied from heat exchangers in air conditioners does not produce any power; it can only heat or cool room air. The concept of exergy is believed to be useful for evaluating power plants, but not for evaluating non-power plants. It is important to expand this concept, though supply air does not produce any power. It should be possible to measure it by exergy, since there is a temperature difference between the return air and the supply air. Conventional exergy analysis is presented in thermal engineering texts in general forms. For example, the exergy of a heat exchanger is calculated by performing an exergy flux analysis on the two fluids at each inlet and outlet. In the exergy analysis in this paper, some of the inlets and outlets (i.e. exhaust air) are omitted if they are not used. Thermal design procedures for air-to-air cross-counter flow heat exchangers are developed as an application of the exergy analysis. The design procedures are applied to a type of exchanger used in outside air cooling systems suitable for rooms containing high heat density equipment. The specified heat exchanger is similar to those commonly used in air conditioning systems. Empirical evidence has suggested that the new design procedures can provide acceptable designs without accounting the cost of heat exchangers.
