Screw compressors and vapour injection scroll compressors have an additional port that opens into the middle of the compression. In the economiser circuit a small part of the liquid leaving the condenser is passed through a separate expansion valve and evaporated in a plate heat exchanger; in doing so it cools the main liquid line by a further 15–30 K. The vapour produced is fed to the compressor’s intermediate pressure port. The compressor compresses this vapour not from the beginning but from halfway. That is why the extra compression work spent remains small compared with the capacity gained.
Because colder liquid goes to the evaporator, every kilogram of refrigerant removes more heat; a higher capacity is obtained from the same compressor. The gain grows as the evaporating temperature falls: between −30 and −40 °C capacity increases by 20–40% and efficiency (COP) by 10–20%. At medium temperature the gain is small; this is why the economiser is used mainly in frozen storage, blast freezing and ice cream applications. Being able to select a smaller compressor for the same capacity also lowers the initial investment; manufacturers’ selection programs give the capacity separately with and without an economiser.
Two arrangements are common. In the (closed) type with a plate heat exchanger the liquid keeps its pressure, and the risk of flashing on long lines is reduced. In the (open) type with a flash tank all the liquid is brought down to the intermediate pressure and the vapour is separated from the liquid in a vessel; its efficiency is slightly higher, but because the liquid remains at the intermediate pressure the line design calls for care. The cool vapour fed to the intermediate port also lowers the discharge temperature; this protects the compressor at low temperature. The arrangement common in scroll compressors is the plate type.
The point to watch on site is the insulation of the subcooled liquid line; if it is not insulated the subcooling gained is lost on the way and the pipe sweats. The superheat of the economiser expansion valve must be set correctly, and no liquid must go to the intermediate port. When the compressor drops to low load through capacity control, the economiser port is out of action in most models; the capacity calculation must be made for this condition as well. A solenoid valve fitted in the intermediate pressure line closes when the compressor stops and prevents liquid migration to the intermediate port. At commissioning the gain is verified by measuring the intermediate pressure and the liquid temperature.