How many compressors does a medical air plant need?

ISO 7396-1 deals with the rule that a medical gas supply system has at least three sources of supply; ISO 7396-3 refers to it for that rule. HTM 02-01 (2026), following BS EN ISO 7396-1, says all medical gas supplies should have three sources, called primary, secondary and tertiary. In the HTM examples for medical air compressor plants, the compressors provide the primary and secondary supply.

Compressor sizing in HTM 02-01

The HTM asks for at least two compressors, and more may be added as long as the total capacity provides 100% of the system design flow. Its Table 16 sizes each compressor by configuration:

Receivers should have a minimum water capacity of 50% of the compressor output in one minute. VSD compressors are sized individually for the configuration, and where a VSD compressor provides the primary control of demand it should be sized to no more than 80% of the required flow rate.

Primary, secondary and tertiary sources

HTM 02-01 Table 17 gives two examples. In a triplex plant, two compressors are the primary supply, the third compressor is the secondary supply, and automatic manifolds with locally based ready-to-use cylinders are the tertiary supply. In a quadruplex plant, three compressors are primary, one is secondary, and manifolds and cylinders are tertiary. National rules can differ from the NHS England guidance, so the specification that applies to the project should be checked.

ULTRAAR plants

The ULTRAAR SC, ESW and ME datasheets give a basic configuration of one, two, three or more compressors. ULTRAAR ESW plants are designed with 1 to 8 compressors, and every model in the ULTRAAR ME table is a three-compressor set, such as the 3.15/500D with three compressors of 14.4 m³/h at 10 bar. ULTRAAR SC and ESW receivers are prepared to receive an additional emergency compressor.