In a previous study similar to the present one, Chang and Kim (2006) simulated the effect of several parameters on the thermal hydraulic performance of a cross flow type of fin and tube gas cooler widely used in air conditioning systems。 Ba- sically, the effects of the frontal area, number of circuits, transverse and longitudinal tube spacing, number of tube rows and fin spacing were investigated。 Better thermal perfor- mance was observed when considering a larger frontal area (it reduces the approach temperature), higher number of cir- cuits (this reduces the CO2 pressure drop), smaller fin pitch and transverse tube pitch (this showed the negative aspect of a higher air side pressure drop) and longer longitudinal tube pitch。 These authors did not, however, address the effects of the air volumetric flow rate and tube diameter, or the adverse influ- ence of oil on performance。 Also, no comments were made concerning the impact of the significant changes in the in- tube CO2 thermophysical properties near of the pseudo- critical and critical points on the overall performance of the gas cooler, which can considerably influence the required size of the gas cooler and its working fluid  charge。

Ge and Cropper (2009) carried out performance simula- tions for finned tube CO2 gas coolers used in refrigeration systems, and for this they considered a distributed method as modeling strategy。 The authors highlighted that the simula- tion model was able to guarantee a high accuracy in the predicted results, since special attention was taken regard- ing the high variation of the CO2 thermophysical properties in the gas cooling process。 The main design parameter evalu- ated was the number of CO2 flow circuits, which showed a considerable influence on the approach temperature, i。e。 its re- duction, when increased from 1 to 3 circuits。 The reported reason was the increase in the CO2 heat transfer coefficient, which also implied in a higher heat transfer rate。 However, the results cannot be generalized, since they were obtained for a gas cooler structure with unchanged values of fin pitch and thickness, transversal and longitudinal tube pitch, and tube di- ameter。 Additionally, the heat conduction (spread) in the fins between parallel rows was not taken into account, and this may be the reason why a higher number of independent circuits showed better thermal performance (e。g。 the hot inlet flow in the first tube of the first row can considerably affect the tem- perature of the last tube of the second row – they are physically and thermally connected via fins – where reverse heat flux can happen if not well designed)。 Potentially, the same benefit can be obtained when considering only 1 circuit and a physical split of the fins between rows。 Finally, they also reported, as ex- pected, a decrease in approach temperature when the frontal air velocity is increased。

Jadhav et al。 (2012) numerically evaluated the perfor- mance of gas coolers for air conditioning applications。 A counter cross flow unmixed–unmixed plain fin and staggered tube con- figuration was adopted。 According to the authors, four rows gave the most compact design of gas coolers。 The correlations for heat transfer, air and CO2 sides, follow conventional ones found in the literature but nothing was mentioned about the pres- sure drops。 The effects of fin thickness and density, transverse and longitudinal tube spacing, gas cooler width and inlet air temperature on the gas cooler thermal performance were evalu- ated。 In summary, for the range evaluated, the most relevant parameters were transverse tube spacing, gas cooler width and air volumetric flow。 The simulations did not include the study of effects of: (i) the in-tube diameter, (ii) the changes in the working fluid properties around of the pseudo-critical and criti- cal points and (iii) lubricating oil。

Gupta and Dasgupta (2014) proposed a simulation     code

isenthalpic expansion。 For such an analysis, an optimum COP (coefficient of performance) was reported for a range of air ve- locities between 1 and 2 m s−1。 Finally, similar to the previous studies reported above, the influences of in-tube piping diam- eter, type of fins and lubricating oil, which can significantly affect the design and overall performance of the gas coolers, were not addressed。