Theoretical basis and practical implementation of “bi-refrigerant” and “two-compressor” refrigeration machine schemes

Abstract

The operating conditions of a refrigeration machine determine its energy efficiency. Some industries, particularly the food industry, are characterized by seasonal (short-term) consumption of cold in the low-temperature range (−40℃ and below) when rapid freezing of the product is required to achieve high quality because of finely dispersed crystallization of moisture and a “shock” temperature for psychrophilic microorganisms. To solve this problem, the periodic use of a low-temperature refrigerant, which, from a rational technical point of view, is not always combined with the constant required temperature of refrigerated storage because of possible operation under vacuum conditions and other difficulties, is advised. The difference between the temperature regime of the required short-term (seasonal or periodic) processing and the constant (main) temperature regime (e.g., refrigerated storage of products) often reaches 20℃ or more, which imposes certain restrictions on the design or operation of the refrigeration machine. In this regard, small and medium-sized refrigeration machines with the capability to operate on two refrigerants (e.g., low and medium temperatures, with periodic automatic mutual replacement) are promising. Such a refrigeration machine is referred to as “bi-refrigerant” in this study. The aforementioned difficulties are also solved by the proposed scheme of a “two-compressor” refrigeration machine, in which the possibility of its operation is realized during periodic change of modes from one stage to two stages, and vice versa, according to the need for the corresponding differences in boiling and condensation pressures.
 BACKGROUND: Environmental conditions and boiling point impose certain requirements on the selection of refrigerant and the design of the refrigeration machine, which often necessitates the division of the design period of operation of the refrigeration machine into two seasons, i.e., summer and winter, or the main operating mode (e.g., refrigerated storage) and nonessential (e.g., freezing seasonal or periodically supplied products to the refrigerator). This need may be especially acute in seasonal refrigerators and refrigerators operating under the conditions of large differences in ambient temperatures that periodically occur in some geographic areas. The need for a two-season (“bi-refrigerant”) refrigeration machine arises not due to fluctuations in the conversion coefficient under different environmental conditions (although this does occur) or a change in operating mode from the refrigeration cycle to the heat cycle (heat pump) but due to the seasonality of the supply of products subject to primary processing (quick freezing).
 AIMS: This work aims to provide theoretical justification for the practical implementation of the “bi-refrigerant” and “two-compressor” refrigeration machine schemes.
 MATERIALS AND METHODS: A theoretical investigation method was used to propose schemes for “bi-refrigerant” and “two-compressor” refrigeration machines, was used. The objects of the study were the diagrams of vapor compression refrigeration machines.
 RESULTS: A schematic diagram of a “bi-refrigerant” refrigeration machine operating by periodically replacing “seasonal” refrigerants has been developed. A schematic diagram of a “two-compressor” refrigeration machine, which provides the possibility of periodically changing a one-stage to a two-stage cycle, has been presented. A diagram of the automatic control of the proposed solutions is presented using the example of a “bi-refrigerant” refrigeration machine. Mathematical support and justification of the developed schemes are also presented.
 CONCLUSIONS: Our investigation revealed the theoretically justified possibility of using “bi-refrigerant” and “two-compressor” refrigeration machine schemes, e.g., in industries with seasonal short-term supply of short-term products subject to rapid (low-temperature) freezing or in other industries that require periodic changes in the boiling point of the refrigerant. Delaying factors in the application of the proposed “bi-refrigerant” refrigeration machine design may be the selection of compressor oil capable of operating on two refrigerants and the need to carefully solve the problem of circulation and its return to the compressor. However, this problem can be solved.