How is a hospital medical air plant sized?

In HTM 02-01, the informed design process (IDP) determines whether a site needs medical air at all, and the aspects of flow the guidance lists for designing the pipeline distribution system include the typical flow required at each terminal unit, the likely number of terminal units in use at any time, the total flow to each ward or department as the sum of its diversified flows, the flow in the main branches and risers as the summation of those flows, and the flow required at the plant.

Step 1: design flows and diversity

Table 7 of HTM 02-01 gives medical air at 400 kPa a design flow for each terminal unit and a diversified flow formula Q for each department, where n is the number of beds, treatment spaces or rooms in use, not the number of terminal units. Examples from the table:

One of the major uses of medical air is for patients' ventilators. HTM 02-01 notes that pneumatically powered ventilators can use up to 80 L/min of free air continuously, that anaesthetic ventilators may need peak flows of up to 80 L/min and average flows of 20 L/min, and that CPAP, BiPAP and NIV in critical care typically run at up to 80 L/min. Anaesthetic gas mixers, humidifiers and nebulisers should not exceed 10 L/min. The medical equipment department should provide the specifics of the ventilators used on site.

Step 2: compressors and receivers

HTM 02-01 asks for at least two compressors whose total capacity provides 100% of the system design flow, and its Table 16 sizes each compressor by configuration, for example 100% of design flow each in a duplex medical air plant and 50% each in a triplex (see how many compressors a medical air plant needs). A combined medical and surgical air plant can be sized on the combined usage and demand of both systems under normal conditions, and its overall capacity is typically lower than for separately specified systems. The minimum water capacity of the receivers should be 50% of the compressor output in one minute, and systems with a design flow above 500 L/min should have two receivers.

The Portuguese specification

ACSS ET 03/2006 (revised in June 2024) bases the selection and sizing of the medical air source on the hospital's estimated consumption and the expected frequency of deliveries. For a compressor plant it asks for three automatically operating compressor units, each rated at 100% of the nominal flow, and receivers sized so that each compressor makes no more than 20 starts per hour, with each receiver meeting the nominal flow of the installation.

ULTRAAR capacities

Ultra Controlo datasheets give the capacity of each ULTRAAR model. Every model in the ULTRAAR ME table is a three-compressor set, starting with the 3.15/500D, which has three compressors of 14.4 m³/h at 10 bar. ULTRAAR ESW plants are designed with 1 to 8 compressors or in other configurations, and ULTRAAR SC receivers range from 0.2 to 6 m³.