Multicompressor refrigeration systems working with direct expansion of CO2
Refrigerated merchandisers are fully open to shoppers' view on the sales floor – as is indeed their intention – but the actual refrigerating equipment such as multicompressor refrigeration systems stays behind the scenes. These systems, required to supply refrigeration to the line-ups of refrigerated and freezer display cases, are located in the store's machine room, known appropriately as the backroom, and are made up of a number of compressors connected in parallel, a liquid refrigerant receiver and various control and safety devices.
Carbon dioxide and ammonia are enjoying a renaissance in multicompressor systems. As "natural" refrigerants, they are an interesting alternative to chlorinated fluorocarbons. By ecological standards, carbon dioxide, CO2, is almost the perfect refrigerant, since it has no ozone depletion potential, nor is it toxic or flammable. It is a chemically inactive substance and moreover decidedly cheap. Increasing use is being made of R 744 (CO2) as an alternative refrigerant in the refrigeration systems built by Linde AG. The new VCM multicompressor system developed for application in supermarkets and self-service department stores is the first low-temperature direct expansion system in which Linde uses CO2 as the refrigerant.
Technical note: To date, CO2 has been used in the industry only as a secondary refrigerant in systems for indirect cooling, where the liquid CO2 is itself cooled by a primary refrigerant while circulating through a separate circuit to absorb heat from the product or space to be cooled. Direct expansion systems working with a single refrigerant and circuit previously relied on the refrigerants R 404 A or R 22. Linde's new system is a major advance in that it allows using environmentally friendly CO2 in conjunction with the more economical direct expansion method of refrigeration employing a single circuit and heat transfer cycle.