Cleanroom HVAC design: an overview

A cleanroom is a room where the concentration of airborne particles is controlled, and often temperature, humidity and pressure as well. The HVAC system does most of that work. It supplies large volumes of filtered air, sweeps contamination away from the product, keeps cleaner rooms pressurized relative to less clean ones and holds environmental conditions steady.

Cleanroom HVAC is expensive to build and operate, so the most important design decisions happen early: what cleanliness level is truly needed, where, and in what operating state. This overview explains the main elements and links to more detailed guides on each topic.

Start with requirements, not equipment

A cleanroom design should begin with a clear statement of what the room must achieve. Key inputs include:

• Cleanliness class for each room or zone, normally expressed as an ISO 14644-1 class, and for pharmaceutical work also as a GMP grade.
• Occupancy state in which the class applies: as-built, at-rest or operational.
• Process requirements, such as heat loads, exhaust, fumes, vibration and sensitivity to temperature or humidity.
• Containment needs, where the product or process is hazardous and must be kept in rather than contamination kept out.
• Regulatory framework, for example Health Canada GMP requirements, USP 797 and 800 with NAPRA model standards for pharmacy compounding, or the Canadian Biosafety Standard for containment labs.
• Personnel flow and material flow, which drive the layout of airlocks and gowning rooms.

These requirements should be captured in a user requirements specification or owner's project requirements, which later become the basis for testing.

Classification

ISO 14644-1 classifies cleanrooms by the maximum allowed concentration of particles at specified sizes, from ISO Class 1 (cleanest) to ISO Class 9. Most pharmaceutical and hospital pharmacy cleanrooms fall between ISO 5 and ISO 8. Semiconductor fabs may require ISO 3 to ISO 5 in critical areas. See ISO 14644 cleanroom classification explained.

Not every room needs the same class. A common and cost-effective approach is to provide very clean conditions only where the product is exposed, for example inside a biological safety cabinet, isolator or unidirectional airflow hood, and to surround it with progressively less demanding background rooms.

Airflow patterns and air change rates

Two basic airflow patterns are used:

Unidirectional areas are designed around air velocity, commonly in the range of about 0.36 to 0.54 m/s (70 to 105 fpm) at the working position for GMP Grade A. Non-unidirectional rooms are designed around air change rate and supply diffuser layout. Air change rates are the largest driver of cleanroom fan energy, so they deserve careful justification. See air change rates and airflow in cleanrooms.

Returns

In non-unidirectional rooms, low-level returns near the floor help sweep particles downward and away from work surfaces. In unidirectional rooms, returns may be through raised perforated floors or low wall returns. Ceiling returns are generally avoided in cleaner rooms because they can short-circuit supply air.

Filtration

Final filters for cleanrooms are HEPA filters, and for the most demanding applications ULPA filters, usually installed in the ceiling at the point of supply. Upstream prefilters in the air handler protect the final filters and extend their life. Installed filters are leak-tested after installation and periodically thereafter. See HEPA and ULPA filter systems.

Two common arrangements deliver filtered air to the room:

• Ducted terminal HEPA filters fed by central air handlers.
• Fan filter units (FFUs), each with its own fan, drawing air from a ceiling plenum.

Each has trade-offs in cost, flexibility, noise and energy.

Air handling system architecture

Most cleanrooms recirculate much of their air because the required airflow is many times larger than the outdoor air needed for ventilation and pressurization. A typical arrangement separates three functions:

1. Makeup air unit: conditions outdoor air, controls humidity and provides pressurization air.
2. Recirculation air handler or FFUs: moves the large volume of air needed for cleanliness, with sensible cooling coils to remove heat.
3. Exhaust systems: remove process fumes and hazardous air, and set the pressure relationship.

Separating dehumidification in the makeup air unit from sensible cooling in the recirculation system makes humidity control more stable and allows the recirculation coils to run dry, which avoids wet surfaces in the cleanroom air stream.

Some cleanrooms, particularly those handling potent or hazardous materials, run on 100 percent outdoor air with no recirculation. This greatly increases energy use and should be justified by a risk assessment.

Pressurization

Cleaner rooms are typically held at a higher pressure than adjoining less clean rooms so that air flows outward through doors and gaps, carrying contamination away. Containment rooms reverse this, using negative pressure to keep hazards in. Airlocks between zones maintain the pressure relationship when doors open. EU GMP Annex 1 gives a guidance value of about 10 Pa between adjacent rooms of different grades. See cleanroom pressurization cascades.

Temperature and humidity

Cleanrooms often need tighter temperature and humidity control than offices, both for the process and for gowned operators, who are more comfortable in cooler rooms. Humidity affects static electricity, microbial growth, product stability and operator comfort. In Canadian winters, humidification can be a major energy and maintenance item.

Architecture and construction

HVAC performance depends on the room itself. Surfaces must be smooth, cleanable and non-shedding, penetrations sealed, and ceilings designed to hold filters with tight seals. Room leakage affects how much air is needed to maintain pressure.

Controls and monitoring

Cleanroom controls maintain airflow, pressure, temperature and humidity, and provide alarms when conditions drift. Regulated facilities often add an independent environmental monitoring system with validated data storage for particle counts and room pressures. Alarm delays are needed so that normal door openings do not trigger nuisance alarms.

Testing and certification

Cleanrooms are tested against ISO 14644-1 and ISO 14644-3 after construction and at regular intervals. Tests typically include airflow and air change rate, filter installation leak tests, room pressure differentials, particle counts, airflow visualization and recovery. Regulated facilities also complete qualification protocols. See cleanroom certification and testing.

Energy

Cleanrooms can use many times more energy per square metre than offices, mostly in fans and in conditioning outdoor air. Reducing air change rates where justified, using efficient fans and low pressure drop filters, and setting back airflow during unoccupied periods can cut energy use without compromising cleanliness.

Key takeaways

• Define cleanliness class, occupancy state and process needs room by room before selecting equipment.
• Concentrate the cleanest conditions where the product is exposed and use less demanding background rooms elsewhere.
• Separate makeup air dehumidification from recirculated sensible cooling for stable humidity and dry coils.
• Pressure cascades and airlocks keep contamination flowing from clean to less clean areas, or reverse it for containment.
• Testing and certification against ISO 14644 confirm that the design performs as intended.