The heat transfer process in a gas cooler can be divided into three parts

namely pre-

quasi-

and post-critical temperature intervals

which are based on the carbon dioxide (CO2) critical temperature at a maximum constant pressure specific heat capacity. Particularly for the quasi-critical interval

the proportion of heat release was reportedly 49.8% under exothermic process with CO2 critical temperature being lowered from 100 ℃ to 0 ℃. This finding was supported by a simulation based on the theory of entransy

which has been proven in subsequent experiments. On this basis

a simple algorithm for the optimal inlet pressure and temperature in the tested gas cooler was proposed according to the heat transfer characteristics of the quasi-critical section. The optimal inlet pressure was 11.34 MPa and temperature was 79.96 ℃ when the hot water (65 ℃) was obtained under the standard test condition through the algorithm. A test involving different inlet pressures with different compressor speeds was designed to validate the previous algorithm by calculating the system coefficient of performance (COP) and analyzing the heat transfer characteristics in a gas cooler byvarying water temperature with discharge tube distance. Consequently

when both CO2 and water flow rate was 15.0 g/s

the water temperature increased rapidly from 17 ℃ to 41 ℃ in the quasi-critical temperature intervals

which possessed huge entransy under 11.3 MPa inlet pressure with critical temperature of CO2 as 51.04 ℃. Finally

when the water outlet temperature was 64.1 ℃

heating amount was approximately 50.1%

and system COP was 3.32

which provided the best test condition among the three different sets.