Abstract
Towards the introduction of environmentally friendlier refrigerants, CO2 cycles have gained significant attention in cooling and air conditioning systems in recent years. In this context, a design procedure for an air finned-tube CO2 gas cooler is developed. The analysis aims to evaluate the gas cooler design incorporated into a CO2 air conditioning system for residential applications. Therefore, a simulation model of the gas cooler is developed and validated experimentally by comparing its overall heat transfer coefficient. Based on the model, the evaluation of different numbers of rows, lengths, and diameters of tubes, as well as different ambient temperatures, are conducted, identifying the most suitable design in terms of pressure losses and required heat exchange area for selected operational conditions. The comparison between the model and the experimental results showed a satisfactory convergence for fan frequencies from 50 to 80 Hz. The absolute average deviations of the overall heat transfer coefficient for fan frequencies from 60 to 80 Hz were approximately 10%. With respect to the gas cooler design, a compromise between the bundle area and the refrigerant pressure drop was necessary, resulting in a 2.11 m2 bundle area and 0.23 bar refrigerant pressure drop. In addition, the analysis of the gas cooler’s performance in different ambient temperatures showed that the defined heat exchanger operates properly, compared to other potential gas cooler designs.
Highlights
The use of air conditioning systems is expanding rapidly around the world
The results indicated that the performance of the gas coolers was enhanced by higher mass flow rates and lower tube diameters at the expense of higher pressure drops
An experimental campaign was carried out in order to validate the mathematical calculations for the performance of an air-finned CO2 gas cooler
Summary
The use of air conditioning systems is expanding rapidly around the world. An estimated amount of 700 million air conditioners will be operating in the world by 2030 [1]. Son and Park [12] carried out an experiment in order to investigate the gas cooling process of CO2 in terms of heat transfer coefficient and pressure drop of the refrigerant. Ge and Cropper [19] presented a detailed mathematical model for air-cooled finned-tube CO2 gas coolers They used a distributed method in order to obtain more accurate refrigerant thermophysical properties and local heat transfer coefficients during cooling processes. The heat transfer and pressure drop characteristics of the supercritical CO2 in tubes have been investigated extensively using experimental and theoretical methods, research on the air-cooled finned-tube CO2 gas coolers is still limited. The model was applied to identify a reliable and efficient finned-tube CO2 gas cooler design, as well as to evaluate its performance in different off-design conditions under varying ambient temperatures
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