An operating theatre air handling unit must hold tight temperature and humidity while delivering high-grade filtration, because surgical site infection risk and equipment stability both depend on the air.
The Clinical Demands
An OR needs a controlled microclimate: a space temperature near 20โ24ยฐC and relative humidity around 50โ60% so the surgical team is comfortable in gowns and static control is maintained, while the ultra-clean zone over the table is supplied through HEPA-filtered laminar flow. Too dry and static discharge risks ignite anaesthetic vapours; too humid and microbial growth and condensation rise. The AHU must therefore cool, reheat, humidify, and filter in a coordinated loop, not a simple cooling coil, because any single stage alone cannot hold both temperature and humidity at once.
Pressure relationships matter too: the theatre is positively pressurized against corridors so contaminated air cannot drift in, and adjacent soiled areas are negative. The cascade is part of the infection-control design, and the AHU's supply-versus-extract balance is what enforces it hour by hour during a live list.
The heat load in a modern theatre is what makes the control problem hard. Surgical lights, a laparoscopic stack, warming devices, and a scrubbed team of eight or more can add several kilowatts of sensible load to a room of modest volume, and much of it appears suddenly when a list starts. Because humidity has to stay inside a band while temperature is trimmed to a surgeon's preference, the unit must cool to dehumidify and then reheat, which is why parallel coils rather than a single cooling stage are standard. The laminar canopy has a defined face velocity, commonly in the region of 0.2 to 0.3 m/s, and the supply-to-room temperature difference is kept small so the downward flow is not disturbed by buoyancy.
Requirements per ASHRAE 170 and HTM 03-01
ASHRAE 170 sets ventilation rates and temperature/humidity ranges for healthcare spaces; HTM 03-01 (UK) adds detailed guidance on air-change rates and filter grades for operating theatres. Both push for documented air changes per hour (often 20โ25 ACH in the theatre) and terminal HEPA (H14 class) at the supply diffuser, because the filter at the point of use, not just upstream, is what defines the clean zone over the patient.
| Item | Specification | Note |
|---|---|---|
| Temperature | 20โ24ยฐC | Surgeon comfort band |
| Humidity | 50โ60% RH | Static and microbial control |
| Air changes | 20โ25 ACH | Theatre ventilation rate |
| Terminal filter | H14 HEPA | Over the table |
| Pressure | +ve to corridor | Clean-to-less-clean cascade |
| Laminar face velocity | ≈ 0.2–0.3 m/s | Ultra-clean canopy over the table |
| Supply temperature difference | Kept small | Protects the downward flow pattern |
The AHU for such a space typically has parallel cooling and reheating coils, a steam or adiabatic humidifier, and a final HEPA bank, with redundancy and easy filter access built into the casing so a filter change does not expose the theatre to downtime. The control loop ties these stages together to hold the band despite varying patient and equipment load.
Filtration is staged rather than concentrated in one bank. A pre-filter protects the coils, a secondary filter protects the terminal stage, and the terminal HEPA does the clinical work, so the expensive final filter is not consumed by coarse dust. Each stage carries its own pressure-drop allowance, and the fan has to be sized for the dirty condition rather than the clean one, otherwise airflow falls below the required air-change rate long before a change-out is due. Terminal filters are leak-tested in place after installation and re-tested periodically, so the housing needs an upstream injection point and downstream scan access; a HEPA housing without test ports cannot be validated whatever the filter inside it.
Design Checklist
- Fix the cascade: Theatre positive, ancillary negative, measured by magnehelic at each door.
- Size ACH: Meet the cited standard's air-change rate for the theatre, not a generic office value.
- Select H14: Terminal HEPA at the laminar supply, with DOP test ports for the validation.
- Control humidity: Reheat plus humidification, not cooling alone, to hold the 50โ60% band.
- Plan redundancy: Size so a coil or filter outage does not close the theatre.
- Size the fan dirty: Use the fully loaded filter pressure drop so the air-change rate holds until change-out.
- Provide test ports: Terminal housings need injection and scan access or the filter cannot be leak-tested in place.
Why the Standard Exists
A defined theatre environment turns infection control from hope into a measurable specification that can be validated and re-validated. Keruite supplies air handling units with the cooling, reheat, humidification, and HEPA stages that healthcare AHU designs call for, built to the cited standard's intent so the validation team has the stages they need to pass.
Healthcare projects are validated rather than simply commissioned, so equipment has to arrive with data a validation team can use. Units built in an 80,000 m² plant by a manufacturer with more than 30 years of experience are supplied with coil, filter, and fan performance from an in-house performance laboratory, which is what allows a room to be signed off against the cited standard rather than against a nameplate. ISO 9001:2015 covers casing and filter-frame fabrication where leakage would undermine the whole filtration chain, CCC certification covers the electrical build, and deliveries into more than 50 countries mean documentation prepared for both ASHRAE and HTM conventions.
Our engineers respond to selection enquiries within 24 hours, including AHU staging and filter grades for operating theatre ventilation per ASHRAE 170 and HTM 03-01.