A supermarket refrigeration technician walking into a store will more than likely find a direct expansion (DX) system. Of all the different approaches to commercial refrigeration that have been tried, none is today more common than DX. Multicircuited, centralized, parallel compressor systems are in use in more stores than any other kind.
But as new stores are built and older systems in existing stores replaced, another approach called secondary refrigeration is gaining wider acceptance and is steadily increasing in numbers.