Centralised Direct Expansion
Centralised Direct Expansion

This is a direct expansion (DX) system using HFC for both the low temperature (LT) and medium temperature (MT) loads (i.e. frozen and chilled display cabinets respectively).

Comprising two completely separate systems (LT and MT) it is typical of systems being installed in supermarkets throughout Europe today. Each system uses a central “multi-compressor pack” consisting of between three to eight semi-hermetic reciprocating or scroll compressors. The pack is located in a plant room and is connected to an external air-cooled condenser. High pressure refrigerant liquid is fed to the cabinets in the store, usually via a liquid receiver (not shown). The refrigerant vapour then returns to the compressor pack via the suction line. The condenser pressure is typically controlled by varying the air flow. This ensures that the condensing pressure is kept as low as possible, consistent with satisfactory expansion valve control when outdoor air temperatures are low. The minimum condensing condition is assumed to be 20°C for air-cooled vapour compression circuits.