An improved approach to model compressors, condenser/gas cooler and their control system in refrigeration systems with R404A and R744 based on physical principles

Abstract

This study introduces an improved methodology to evaluate with accuracy the performance of a conventional system with R404A and a Booster system with R744. Reciprocating and scroll compressors are modeled with improved semi-empirical approaches, and their operation is ideally controlled to maintain the required refrigerant mass flow rate with an On/Off and variable speed control system. The R744 gas cooler is modeled with a four-zones model and the R404A condenser with a three-zones model. The R744 condenser is modeled with the superheated zone discretized into eight zones to avoid unrealistic results due to the variation of its thermophysical properties and the two-phase and subcooled zones with one zone respectively. In line with the design, the Booster condenser enters into the pseudo-critical zone at an ambient temperature of 23 °C, while the gas cooler is activated after 31 °C. The compressor models present errors lower than 10%, while the Booster model presents mean deviations lower than 2.3%. The Booster system presents better COPs than the conventional at ambient temperatures lower than 15ºC. At higher ambient temperatures, the COPs of both systems are similar: the conventional system overperforms the Booster up to 6%, being in disagree with the literature results where these differences increase up to 30%. The proposed methodology allows to reduce the overall model error by 3% with respect to a thermodynamic cycle-based model; allowing to improve the comparison between the Booster system with R744 and the conventional one with R404A, which leads to more transparent results than the reported common comparisons.