Conference Year

2012

Keywords

Brayton, Desiccants, Refrigeration

Abstract

The phase-out of CFCs has shed a new light over natural refrigerants, which have null global warming potentials. Air would be a natural choice, and although the Brayton cycle usually exhibits a lower coefficient of performance when compared to vapor-compression systems of same capacity, it has been considered in applications other than aircraft cooling. These include gas separation, food processing and preservation, refrigerated containers and train air-conditioning. Price perspectives in the oil market also make the Brayton cycle an alternative to be considered as an option for automotive air conditioning. Even though the Brayton cycle is often employed in low temperature applications, the ambient humidity level is essential for the uninterrupted operation. For applications far below the ambient air dew point, the condensate is likely to cause icing at the turbine outlet, causing duct obstruction and system failure. The use of a solid desiccant would provide a thorough humidity control, allowing for increased pressure ratios (and thus lower expansion temperatures) even for significant ambient humidity levels. In the standard Brayton refrigeration cycle, the air is collected by the compressor at ambient conditions, and compressed through a specified compression ratio. The air is then cooled back to the ambient temperature at a regenerator, and sub-sequentially expanded through a turbine to the ambient pressure, at a low temperature. At the proposed cycle, the air is collected by a desiccant wheel and dehumidified, before it is admitted to the compressor. Accordingly, it can be compressed under a significant pressure ratio, without incurring in ice formation when later expanded. The desiccant wheel is dried using the hot air at the compressor outlet, by a heat exchanger which collects the heat that would be otherwise dumped by the regenerator. A mathematical model for the proposed cycle is developed, consisting of a system of non-linear equations which stems from mass and energy balances applied to each individual cycle component. The results show that the desiccant assisted cycle allows for frost-free operation even for temperatures below -60°C, which is required for fish preservation warehouses.

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