Pressure cascades in operating theatre HVAC

The main purpose of operating theatre air conditioning is to keep the surgical site as clean as possible and to control the transport of particles and microorganisms. Besides filtration and air distribution, another equally important element is the pressure cascade between spaces. The pressure cascade ensures that air flows from cleaner areas towards less clean areas.

Why is a pressure cascade necessary?

Air flows from a higher-pressure space to a lower-pressure space. When the operating theatre is kept at positive pressure relative to the surrounding spaces, air flows outwards through door gaps and leakage points. This limits the entry of contaminated air into the theatre from the corridor or adjacent spaces.

In an operating suite, spaces are ranked according to their cleanliness level. In a typical cascade, the operating theatre is kept at the highest pressure, adjacent spaces such as preparation and scrub-up rooms at an intermediate level, and the corridor at a lower level. The positive pressure difference between the operating theatre and the adjacent space is generally in the order of +15 Pa. Exact values are determined according to the project and the standards in force. Standards such as DIN 1946-4 and national healthcare regulations are taken into account in this respect.

• If the pressure difference is too low, the airflow direction can easily be disrupted by door movements and wind effects.
• If the pressure difference is too high, doors become hard to open and whistling may occur at door gaps.
• Some spaces, such as infection isolation rooms, are kept at negative pressure; the location of these spaces within the suite is addressed separately.

The cascade is not limited to the operating theatre. Clean spaces such as sterile stores are also kept positive relative to their surroundings, while dirty utility and waste rooms are generally kept negative relative to adjacent spaces. This prevents air leaving dirty areas from spreading into clean corridors. The pressure relationship of the entire suite to the general areas of the hospital is also addressed in the design. We therefore begin the design by preparing a pressure diagram showing the target pressure and airflow directions for each space. This diagram becomes a common reference for the mechanical, architectural and automation teams.

How is pressure created?

The pressure in a space is created by the difference between the airflow supplied to it and the airflow extracted from it. More air is supplied to a space requiring positive pressure than its exhaust or return airflow. The excess air flows to adjacent spaces through door gaps and leakage points. There are therefore two key factors that determine the pressure difference:

• Airflow difference: The difference between supply and extract is calculated according to the target pressure and kept constant.
• Airtightness of the space: The more openings there are at doors, ceilings, lighting and service penetrations, the more air is needed for the same pressure.

This is why the pressure cascade is not purely a mechanical matter. Architectural details, door types and the airtightness of service penetrations also directly affect the result. At the design stage, we address these issues together with the architectural and electrical teams.

Control and monitoring

For pressure to remain stable throughout the day, airflows must be kept constant. As filters become loaded, system resistance increases and fan airflow tends to fall. This is compensated for with variable-speed fans and airflow control devices. Airflow control dampers used for each space keep supply and extract airflows at the set values.

• Differential pressure is measured between the operating theatre and adjacent spaces.
• The pressure value can be monitored on a display panel at the theatre entrance.
• When the value goes outside the set range, an alarm is raised with a suitable time delay to allow for door openings.
• Data is transferred to the building management system and recorded.

Airflows can be reduced to save energy when the operating theatre is not in use. Even then, the direction of pressure must be maintained. For this, a control scenario is set up so that the difference between supply and extract airflows is maintained in setback mode as well. The time to switch from setback mode to normal mode should also be defined in line with the operating plan.

Commissioning, TAB and periodic checks

It is not enough for the pressure cascade to be correct on paper; it must be verified by measurement on site. During commissioning, supply and extract airflows of the spaces are measured, airflow control devices are adjusted, and pressure differences are checked with the doors closed. This work is part of the testing, adjusting and balancing (TAB) process.

In hospitals, this verification is usually part of a more comprehensive validation. HEPA filter leak tests, airflow visualisation, particle counts, and temperature and humidity measurements are also assessed within this scope. After handover, filter changes, alterations or equipment failures can change the pressure balance. Measurements should therefore be repeated periodically. After alterations, especially if changes have been made to ceilings, doors or service penetrations, it is good practice to re-measure the pressure differences.

Correct pressure cascades in operating theatres are achieved when design, building details, the control system and on-site measurements are addressed together. At Ekura, we can provide support with the installation, TAB and commissioning of air conditioning systems in hospitals and healthcare facilities.