Publication: HVAC & R Research
Publication Date: 01-MAY-07
Delivery: Immediate Online Access
Author: Chang, Young-Soo ; Kim, Min Seok
Article Excerpt
The outlet temperature of a gas cooler has a great effect on the efficiency of carbon dioxide heat pump systems. In order to obtain a small approach temperature difference at the gas cooler, a near-counterflow type heat exchanger has been proposed, and a larger heat transfer area is demanded. Therefore, the optimum design for gas coolers involving the analysis of trade-offs between heat transfer performance and cost is desirable. For this study, a simulation model has been developed for fin-and-tube type gas coolers, and an air-side heat transfer correlation is proposed. The developed model was confirmed by experimental results. The effects of geometric parameters, such as transverse tube spacing, longitudinal tube spacing, number of tube rows, and fin spacing, on the performance of heat exchangers were investigated using the developed model. This study suggests various simulation results for optimum design of gas coolers.
INTRODUCTION
Due largely to the recent outbreak of environmental problems caused by global warming, many ongoing studies are aiming to adopt heat pump systems using carbon dioxide (C[O.sub.2]), a natural refrigerant. As shown in Figure 1, the critical temperature of carbon dioxide is lower than ambient air temperature, which acts as a heat sink. Thus, this refrigeration system is different from others using conventional refrigerants; the condensing process does not exist and hot gas refrigerant is cooled in a single-phase gas cooling process. In this study, a performance analysis model is proposed that uses a fin-and-tube heat exchanger as the gas cooler of a C[O.sub.2] heat pump system.
Hot C[O.sub.2] is introduced through the tubes of a fin-and-tube heat exchanger, and an air-cooling process takes place. For fin-and-tube heat exchangers, air-side thermal resistance is a great portion of the total thermal resistance. Thus, in order to improve the performance of the heat exchanger, it is crucial to enhance the performance of the air-side heat transfer. Consequently, it is very important to understand air-side heat transfer characteristics for the design of a fin-and-tube gas cooler. The overall heat transfer coefficient is calculated from heat exchanger performance test data and [epsilon]-NTU relations considering the shape of the heat exchanger and operating condition, and the air-side heat transfer coefficient can be obtained from the overall heat transfer coefficient. However, Youn et al. (2000) verified that the discrepancy in air-side heat transfer coefficients could be as much as 20% according to the choice of [epsilon]-NTU relation for data reduction.
In this study, the tube-by-tube method was used as a gas cooler model because thermodynamic and transport properties of C[O.sub.2] show large variations during the gas cooling process. This method is relatively simple, and accurate results can be obtained considering the variations of heat transfer characteristics of each tube. Therefore, the tube-by-tube method is also used for obtaining air-side heat transfer coefficients instead of the [epsilon]-NTU relation method. Measured air-side heat transfer coefficients can be expressed by the correlation of Reynolds number and Prandtl number. The effects of geometric parameters, such as transverse tube spacing, longitudinal tube spacing, the number of tube rows, and fin spacing, on the performance of heat exchangers were investigated...
NOTE: All illustrations and photos have been removed from this article.
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