Low temperature vacuum sublimation refrigerators on carbon dioxide as working fluid

Abstract

The paper presents the results of theoretical and experimental analysis of a vacuum-freeze-drying refrigeration machine based on carbon dioxide. Such refrigeration machines can be equipped with refrigerators for the needs of medicine, where low temperatures are widely in demand in the range from minus 80 degrees Celsius to minus 130 degrees Celsius. The problem of developing alternative refrigeration machines is dictated by modern environmental requirements for working substances, which are gradually being tightened and soon the use of familiar freons will become impossible. For the air conditioning and commercial refrigeration industry, new substances are being actively developed, such as hydrofluoroolefins (HFOs), hydrocarbons can also serve as substitutes for hydrofluorocarbons and their mixtures, but it must be considered that they are combustible and explosive. A carbon dioxide vacuum sublimation unit may be an alternative to modern freon vapor compression refrigeration units for the indicated temperature range, but subject to comparable energy costs during its operation. The article examined the three-stage and four-stage cycles of a vacuum-freeze-drying refrigeration machine, conducted their theoretical comparison in terms of COP with a cascade cycle on hydrocarbon working substances. It can be noted that the COP of the considered cycles are close, but the environmental friendliness, safety and non-combustibility of carbon dioxide gives the advantages of a vacuum freeze-drying refrigeration machine.

Mathematical Modelling of Stingless Bee Honey Dewatering using Low-Temperature Vacuum Drying with Induced Nucleation Bubbling

“…However, LTVD alone is not sufficient to remove the water content within an acceptable time frame. Hence, LTVD was often coupled with other additional elements, such as the combination of microwave LTVD and vacuum freeze-drying [15][16] [17]. Unfortunately, exposure to microwave radiation has the potential to damage the sensitive nutritional content of SBH, whereas the freezing element deteriorates the enzymes in SBH.…”