Instead of a separate unit for every room, two to eight compressors are connected to common suction and discharge headers, discharge into a common condenser, and every room or cabinet is connected to this line with its own evaporator, solenoid valve and expansion valve. This is called a central system or rack. Usually two separate racks are built, one for chilled (medium temperature) and one for frozen (low temperature) rooms.
Its advantages: since the compressors cut in and out with the load, part-load efficiency is high; when one compressor fails the others keep running; a single condenser and a single machine room make maintenance easier; heat recovery (hot water, space heating) is easy to apply. This is why supermarket chains and multi-room cold storage warehouses run on central systems.
In return it demands design and operating know-how: oil management (oil separator, reservoir, level regulators), long pipe runs, the size of the refrigerant charge and the leak risk, capacity control and automation. Because the F-gas regulation restricts large HFC charges, CO₂ (R744) transcritical racks and low-GWP refrigerants are coming to the fore in new central systems. Sonkar designs central systems on a project basis.