Design Methodology and Impact of Cross-fow Heat Exchangers’ in Refrigeration and Air Conditioning Systems

摘要

This paper discusses the design of heat exchangers’ for refrigeration and air conditioning systems and also the use of extended surface heat transfer for these heat exchangers. Cross-fow heat exchangers are extensively used in refrigeration and air conditioning applications. The basic design includes a heat exchanger coil-block located in an airstream for cooling and heating purposes. Several construction options are available, including: (1) various spaced fn coils; (2) electric, hot gas or water defrosting; (3) fan(s). Also heat exchangers can be designed for low air velocity, standard velocity and as sloped unit evaporators. The size and cost of the heat exchanger is dependent upon the heat duty, the allowable pressure drop and the geometry of the heat transfer area. Often one heat transfer resistance within the overall heat transfer dominates and this determines the amount of surface area required for the duty. The dominance of this resistance is often broken by the use of extended surface (fns), which will result in a lower surface and hence smaller heat exchanger. The extended surface performance can be predicted by a number of indicators: fn effectiveness, fn effciency, enhancement and augmentation. This paper will also consider which, if any, of the above performance indicators are the best and the design methodology for the heat exchangers.