Transcritical CO₂ Parallel Compression Refrigeration System Design And Performance Experimental Research

In order to host a "frugal,green,and smart" Winter Olympics,the National Speed Skating Oval "Ice Ribbon" adopted a carbon dioxide transcritical direct refrigeration system.To meet the demands of ice making and ice maintenance,a carbon dioxide transcritical parallel compression refrigeration system with adjustable operating modes was developed.System performance testing and ice surface temperature uniformity testing were conducted,and a system COP and ice rink evaporating temperature prediction model were constructed.The main conclusions are as follows:Based on the properties of carbon dioxide and the principle of parallel compression refrigeration system,a carbon dioxide transcritical parallel compression refrigeration system with two compressors in parallel operation and switchable parallel compression operation was developed according to the requirements of ice making and ice maintenance.Thermal parameter calculations were performed for the gas cooler,evaporator,and compressor of the test system,and their selection and verification were carried out.A system control program and human-machine interface were developed.The system performance under three operating modes,namely single machine operation,parallel operation,and parallel compression,was experimentally studied.During parallel operation,when the operating frequency of the auxiliary compressor is the same as that of the main compressor,the COP is higher.Both single machine operation and parallel operation have an optimal operating frequency for the compressor to maximize the system COP.During parallel compression,the maximum COP of the system is 2.19 when the main compressor frequency is 50 Hz and the auxiliary compressor frequency is 30 Hz,which is 11.59% higher than that of single machine operation with the same evaporating temperature and main compressor frequency.Ice surface temperature uniformity testing was conducted according to the requirements of GB/T 19995.3-2006 for sports ice rinks with ice thicknesses of 30,40 and 50 mm and ice surface temperatures of approximately-3,-6,and-9 ℃.It was found that the temperature difference on the ice surface was less than 0.4 ℃,which is much lower than the standard of the International Skating Union that the temperature difference on the ice surface should not exceed 1.5 ℃.This indicates that the ice surface temperature uniformity of the test system is good and can meet the requirements of ice sports.A system COP and ice rink evaporating temperature prediction model were constructed and compared with the test results.For COP prediction,the error under single machine operation is less than 2.8%,and under parallel operation,the error is less than 7.9%.To solve the problem of complex enthalpy value calculation in conventional theoretical prediction models,a parallel compression system COP prediction model based on pressure and dryness was constructed,and the prediction results of different main compressor operating frequencies were compared with the test results,with an error of less than 7.7%.A prediction model for evaporating temperature based on ice thickness and ice surface temperature was constructed,and the prediction results were compared with the test results,with an error of less than 2.9%.The above research results have been partially applied to the refrigeration system of the National Speed Skating Oval and can provide reference for the performance optimization of artificial ice rinks using carbon dioxide refrigeration systems.