Cleanroom airflow design:
what should the brief define?

Cleanroom airflow design determines how filtered air enters a controlled space, moves around people and equipment, collects contamination and leaves through returns or extract. A credible design connects that air path to the process risk, room classification, pressure regime, heat loads, occupancy and operating state.

Start with the protection duty, not a ventilation rate

The first question is what the airflow must achieve. Some rooms principally protect a product from particles released by people and processes. Others must contain a hazardous powder, biological agent or potent compound. Certain facilities need both product protection and containment, which can create competing pressure and air-movement requirements.

Map the critical work, contamination sources and vulnerable locations before selecting terminals or calculating fan duties. Record normal occupancy, gowning level, process equipment, door movements, heat gains and any local extract. Identify whether the room will operate continuously, use occupied and setback modes, or require recovery after a shutdown.

ISO 14644-4:2022 describes the process for creating a cleanroom from requirements through design, construction and start-up. It applies to new, refurbished and modified installations. The standard does not replace the project brief with one universal airflow solution; the user, specifier and design team still need to establish the relevant performance requirements.

Choose an airflow pattern suited to the critical activity

Cleanrooms are commonly described as using unidirectional or non-unidirectional airflow. In a unidirectional arrangement, filtered air moves in a broadly uniform direction across a critical zone. A non-unidirectional room relies on filtered supply air mixing with room air while contamination is diluted and carried towards returns. Hybrid arrangements may use local unidirectional protection within a wider non-unidirectional background.

The correct option follows the risk. A high-level ISO class does not automatically require the whole room to use one pattern, and installing more high-efficiency filters does not correct a poor relationship between supply points, equipment and returns. Large machinery, screens, operators and open doors can interrupt the intended route.

Draw the expected air path through each room and critical zone. Locate contamination-generating operations and ask where released particles will travel. For exposed product, consider whether clean air reaches the critical point before passing over a person, dirty surface or heat source. For containment, assess whether captured air can leave the hazard zone without carrying material into adjacent spaces.

ISO 14644-3:2019 provides cleanroom performance test methods for unidirectional and non-unidirectional airflow in as-built, at-rest and operational states. Naming the intended airflow type and test state in the brief helps prevent suppliers from proposing different concepts under apparently similar quotations.

Air changes per hour are an input, not proof of performance

Clean room air changes per hour, usually shortened to ACH, express the supplied air volume relative to the room volume. The basic calculation is supply airflow in cubic metres per hour divided by room volume in cubic metres. It is useful for sizing, comparison and recovery calculations, but it does not show where air travels or how effectively contamination is removed.

A single ACH value can produce very different results in two rooms. Terminal positions, return locations, ceiling height, equipment, occupancy and heat plumes all affect mixing and removal. Leakage and process extract can also mean that gross supply volume differs from the recirculated or replacement air duty.

Does an ISO class prescribe a fixed air-change rate?

ISO particle classification defines permitted airborne particle concentrations; it does not create one fixed air-change rate for every room of that class. The design rate should be justified against the cleanliness objective, contamination load, recovery requirement, airflow pattern and process. Sector-specific guidance, an approved quality system or a risk assessment may add further requirements.

Avoid copying a typical range into the User Requirement Specification without checking its source and purpose. Instead, ask the designer to state the proposed airflow duty, calculation basis, diversity assumptions, operating modes and predicted effect on recovery, pressure stability and energy use.

Set pressure relationships around product and containment risks

Pressure differences help establish the intended direction of air leakage through closed-room boundaries. A cleaner or more product-critical space is often kept positive to a lower-grade background. Where hazardous material must be contained, the required direction may reverse. The arrangement needs to consider the full suite, including airlocks, corridors, transfer hatches and process equipment.

For sterile medicinal manufacture, EU GMP Annex 1 gives 10 Pa as a guidance value between adjacent rooms of different grades. It also says the recommendations may need modification where hazardous material requires containment. That value belongs to the Annex 1 context and should not be presented as a universal rule for every UK cleanroom.

Specify which spaces are compared, the normal pressure range, alarm threshold, door state and response to loss of pressure. Check simultaneous door openings, process extract start-up, filter loading and equipment connections. A room can meet its set point with every door closed yet lose the intended protection during routine transfers.

Coordinate supply, returns, extract and equipment

Supply terminals should deliver the required volume without creating harmful turbulence at the process. Returns need clear paths and should not be blocked by benches, storage or future equipment. High- and low-level returns, local extraction or dedicated exhaust may be appropriate depending on the contaminant, heat source and process arrangement.

Build an equipment schedule showing dimensions, location, heat release, extract duty and operating sequence. A fume cabinet, biosafety cabinet or powder booth can materially change room balance. Confirm whether it recirculates or exhausts air, how replacement air is provided and what happens if either system fails.

Controls should coordinate supply, return and extract fans rather than treating them as isolated packages. Define start-up and shutdown sequences, occupied and setback modes, pressure control, damper response, sensor locations, alarm delays and links to building or environmental monitoring systems. Consider how the cleanroom reaches a stable condition before work is released.

Energy performance matters because cleanroom air systems can operate for long periods at substantial volume. Variable or setback operation may be possible, but the reduced mode needs a defined cleanliness, pressure and recovery strategy. Any change of duty should be commissioned and, where required, qualified before it becomes part of routine operation.

Prove the airflow under representative conditions

Design calculations and drawings show intent. Commissioning and qualification establish what the installed facility actually does. The test plan may include supply and extract volume, air velocity, pressure differences, recovery, installed-filter leakage, particle classification, controls challenges and airflow visualisation.

Airflow visualisation is especially useful because it reveals paths that a pressure reading or ACH figure cannot show. For sterile manufacture, Annex 1 expects airflow patterns to be visualised both at rest and in operation, including simulated operator interventions, and expects corrective action where movement presents a contamination risk. That requirement is specific to its regulatory scope, but the design principle is useful more broadly.

Agree the room state, door condition, equipment status, occupancy or simulation, acceptance criteria and recording method before testing. Retain results with current drawings and system settings so that later changes can be assessed against a known baseline. The cleanroom commissioning guide explains the engineering checks that should precede formal qualification, while the environmental-control guide covers routine monitoring and alarms.

What belongs in a cleanroom airflow design brief?

1. Product, process, personnel and containment risks.
2. Required classification or grade and each operating state.
3. Critical zones and intended unidirectional or non-unidirectional pattern.
4. Occupancy, equipment, heat loads and contamination sources.
5. Supply, return, recirculation, outside-air and extract duties.
6. Room pressure relationships, door assumptions and airlock strategy.
7. Proposed air-change rates and the calculation or risk basis.
8. Control sequences, setback modes, alarms and failure responses.
9. Access for filters, dampers, sensors, cleaning and maintenance.
10. Commissioning, visualisation, qualification and handover evidence.

Issue the same requirement to each bidder and ask them to identify design assumptions and exclusions. This makes airflow concepts, plant duties and lifecycle implications easier to compare. Our cleanroom design and build service explains how Cleanrooms Direct matches project briefs with suitable UK specialists.