Hygienic Ventilation and Clean Room Systems

What problem it solves

The purpose of hygienic ventilation is to keep the particle and microorganism load in a space under control. It does this through three mechanisms: it cleans the air by filtering it, it dilutes contamination by supplying enough clean air, and it prevents contaminated air from reaching clean areas by creating pressure differences between adjacent spaces.

Of these three, the pressure cascade is the one most often skipped. In a room fitted with HEPA filters but without pressure balance, corridor air floods in every time the door opens. The particles the filter stopped simply walk in through the door gap. That is why design in a hygienic area looks not at a single room but at the relationship between rooms.

The direction of pressure depends on the nature of the process. Where the product must be protected, the room is kept at positive pressure relative to its neighbours; air always flows from clean to less clean. If hazardous dust or vapour is produced, the direction is reversed: the room is kept negative and contamination does not escape. Some facilities have both requirements at once; in that case interlocked airlocks are used.

Applications

• Pharmaceutical and cosmetics production areas
• Food production, filling and packaging lines
• Hospital operating theatres, intensive care units and sterile stores
• Medical device production and assembly areas
• Analysis and quality control laboratories
• Sensitive assembly rooms in electronics production

System components

Filter stages

Filtration in hygienic systems is staged. The air handling unit contains coarse and fine filters; the final HEPA stage is placed as close to the room as possible, usually in ceiling terminal housings. The reason is simple: if the HEPA filter is in the AHU, the duct between filter and room becomes a source of re-contamination.

Terminal HEPA housings

Mounted in the ceiling in sealed, gasketed housings. Filter changes are designed to be done from the room side. After installation each housing is leak-tested individually; even if the filter itself is intact, a gasket leak invalidates the whole system.

Airflow pattern

In clean areas, where the air enters and leaves matters as much as the airflow rate. Supply is from the ceiling and extraction is usually at low level on the walls, so particles are swept from top to bottom. Putting the extraction in the ceiling leaves contaminated air circulating at working height.

Surfaces and sealing

Clean room panels, corner profiles and penetrations must be cleanable and airtight. Cable and pipe penetrations are sealed with gaskets. However well the ventilation is designed, a leaking suspended ceiling breaks the pressure cascade.

Testing and validation

A hygienic system is proven by measurement at handover. The main tests are:

1. Terminal filter airflows and total air change rate
2. Pressure differences between rooms
3. HEPA filter leak test
4. Airborne particle count
5. Temperature and relative humidity measurement
6. Recovery time: the time to return to the clean state after the room has been contaminated

The results are reported and repeated periodically. A system changes over time: filters load up, gaskets age, dampers drift. Validation is not a one-off event but a recurring routine.

A clean room that has not been measured is only a room assumed to be clean.

Points to watch

Filter life and differential pressure. Every HEPA stage should have a differential pressure gauge. Filters are changed according to measured differential pressure, not according to a calendar.

Door opening effect. Opening doors at the same time temporarily breaks the pressure cascade. Interlocked doors are used at critical transitions.

Quiet operation. A high air change rate increases noise. In spaces such as operating theatres and laboratories, sound level is a design criterion; duct velocities and silencers are chosen accordingly.

Cleanability. Grilles, ducts and terminal housings with cleanable surfaces are preferred. A profile that collects dust is a problem in a hygienic area.

What clean room classes mean

Clean room classification defines how many particles above a given size may be present in a cubic metre of air. The smaller the class number, the fewer particles are allowed, and so the higher the required air change rate and filter investment.

The critical point in practice is the state in which the class is measured. The same room gives different results when empty, with equipment installed but idle, and in full operation. When a clean room is designed, which class is to be achieved in which state must be agreed in writing from the start; otherwise expectation and measurement will not match at handover.

The second common misunderstanding is that a high class is needed everywhere. In reality facilities are usually built in stages: the most sensitive process is kept at a high class in a small area, while surrounding areas are left at a lower class. This approach significantly reduces both investment and operating cost.

Air change rate and recovery

In hygienic areas, the air change rate determines how quickly contamination generated in the room is diluted. A high air change rate sweeps particles away quickly but increases energy consumption linearly.

In practice the more meaningful criterion is recovery time: after the room is deliberately contaminated, the time to return to the target class is measured. This test shows that airflow and airflow pattern are working correctly together. In a room with high airflow but a poor airflow pattern, recovery takes longer than expected; the fix is not more airflow but better placement of supply and extraction.

Project process

1. Defining requirements. Which class in which area, in which state; what the pressure direction will be.
2. Room schedule and flow diagram. Relationships between rooms, airlocks and pressure cascades are mapped.
3. Calculation and selection. Airflow, filter stages, air handling unit and terminal housings are selected.
4. Fabrication and installation. Panels, ductwork, terminal housings and controls are installed.
5. Commissioning and validation. Airflow balancing, leak testing, particle counting and recovery time measurement.
6. Reporting and periodic validation. Results are reported and the retest interval is set.

Maintenance and filter management

In hygienic systems, unlike other ventilation systems, maintenance is a quality requirement. Filter changes are tied to measurement, not to the calendar: differential pressure is monitored at every stage, and the filter is changed as it approaches the final pressure given by the manufacturer.

The route in and out is planned in advance so that a dirty filter is not carried through the room during the change. After a HEPA stage is replaced, the leak test is repeated; a housing closed without testing may carry a leak that invalidates the entire system.

A maintenance log is kept: which filter was changed when, what the differential pressure was at that moment, and what the post-change measurements showed. This log is exactly the document auditors ask for.