An assessment of heat transfer through fins in a fin-and-tube gas cooler for transcritical carbon dioxide cycles

In CO2 transcritical refrigeration cycles fin-and-tube coils are still considered as possible gas cooling devices, due to their lower cost when compared with recent aluminium minichannel heat exchangers. In spite of the very high working pressures, an “off-the-shelf” coil with four ranks of 3/8” copper tube and louvered fins was found suitable to work with high R744 pressures and has been studied as a gas-cooler in a test rig built for testing CO2 equipment operating with air as a secondary fluid. The test rig consists of two closed loop air circuits acting as heat sink and heat source for gas-cooler and evaporator respectively. The tested refrigerating circuit consists of two tube-and-fin heat exchangers as the gas-cooler and the evaporator, a back-pressure valve as the throttling device, a double-stage compound compressor equipped with an oil separator and an intercooler. A full set of thermocouples, pressure transducers and flow-meters allows measurement and recording of all the main parameters of the CO2 cycle, enabling to perform heat balance both air-side and refrigerant-side. Tests were focused on two different gas-coolers, with continuous and cut fins, and on two different circuit arrangements. Tests on each heat exchanger were run at three different inlet conditions, both for CO2 and air. A simulation model was developed for this type of heat exchanger and three models (Dang and Hihara 2004, Gnielinski 1976, Pitla et al. 2002) proposed for the carbon dioxide supercritical cooling heat transfer coefficients were implemented and compared in the code. The model results are compared with the experimental data for the finned coil: emphasis is given to the effect of heat conduction through fins between adjacent tube ranks on the system efficiency. In the paper, the experimental results for transcritical carbon dioxide entering the gas cooler at 87.0 °C (7.911 MPa), 97.6°C (8.599 MPa), 107.8°C (9.102 MPa) with air inlet temperature 20.3 °C, 21.5 °C and 23.0 °C respectively are presented. By using a coil with fins modified to reduce the heat conduction, a 3.7 % to 5.6 % heat flux improvement was gained. This improvement can be clearly translated in terms of Coefficient Of Performance, since a low value of the CO2 temperature at its outlet increases the cooling capacity. Considering a reference cycle with the same operating conditions, a 5.7 % to 6.6 % increase of COP can be obtained.