Experimental Investigation of the Effect of a Recuperative Heat Exchanger and Throttles Opening on a CO2/Isobutane Autocascade Refrigeration System

Michał Sobieraj

doi:10.3390/en13205285

Abstract

An experimental evaluation of an autocascade refrigeration (ACR) system was carried out. A zeotropic mixture of isobutane and CO2 was employed as a working fluid in an autocascade refrigeration (ACR) system. An experimental system was designed and built to study the influence of the recuperative heat exchanger (RHX) and openings of the throttle valves on the system performance. The use of RHX facilitated the condensation process and improved the cycle characteristics. The working mass concentration of CO2 was higher, as it was closer to the nominal concentration and the discharge pressure was lower by 19% to even 39% when the RHX was employed in the system. An increase of up to 20% in the coefficient of performance (COP) was observed. Furthermore, the effects of the openings of the throttle valves on the system characteristics were studied. The change in the openings of the expansion valves affected the mass flows and the working mixture composition. The working CO2 mass fraction increased with higher openings of the evaporator throttle. The subcooling degree of liquid CO2-rich refrigerant increased with higher openings of the expansion valve under the phase separator. The results of the present work should be helpful for design and optimization of autocascade systems working with natural and synthetic refrigerants.

Highlights

Hydrofluorocarbons (HFCs) are being phased out because of their high GWP [1]
The experimental autocascade refrigeration (ACR) system was charged with 560.9 g of R744/R600a (51.6% of R744 by mass)
According to Bai et al [22] and former experimental results, the working mass concentration of the more volatile component, R744, is usually lower than the charging concentration. For this reason the system was charged with the mixture containing more R744 to attain the working concentration closer to the desired one

Summary

Introduction

Hydrofluorocarbons (HFCs) are being phased out because of their high GWP [1]. As none of the synthetic refrigerants can be considered a long-term solution, there is an increased interest in using natural substances as refrigerants in vapor compression system [2]. Hydrocarbon (HC) refrigerants, as well as CO2 (R744), possess all the properties required to make them suitable long-term replacements for fluorinated greenhouse gases [3]. It is the saturated vapor pressure curve that depicts the suitability of a refrigerant for given operating conditions. CO2 cascade systems are often used in supermarket applications in hot climates [4]. Another way to employ CO2 as a refrigerant is to use a high-pressure transcritical systems. Several research works have been carried out to make these systems suitable for high ambient temperature conditions, including ejectors [6] and mechanical subcooling [7]

Methods

Results

Conclusion