Key takeaways
- Clean rooms are classified by airborne particle concentration under ISO 14644-1, from ISO 1 (cleanest) to ISO 9.
- HEPA filtration, airflow pattern and air change rate work together to reach and hold a class.
- Pressure cascades keep air flowing from cleaner to less clean areas, typically 10 to 15 Pa per step.
- Performance must be tested and documented, not assumed.
Pharmaceutical packaging, electronics assembly, medical device manufacturing, hospital operating theatres and food processing all depend on controlled environments. In each case the air-conditioning system does far more than keep people comfortable: it removes particles, holds rooms at defined pressures, and keeps temperature and humidity inside tight limits. This guide explains the principles behind clean room HVAC design.
What makes clean room HVAC different
A conventional comfort system recirculates air through a coarse filter and aims for a comfortable temperature. A clean room system is designed around four controlled variables:
- Particle concentration, controlled by high-efficiency filtration and dilution
- Airflow pattern, so contamination is carried away from the product
- Pressure, so air moves from cleaner to less clean spaces and never the other way
- Temperature and humidity, often to tighter tolerances than comfort requires
Clean room classes under ISO 14644-1
Clean rooms are classified by the maximum number of airborne particles allowed per cubic metre of air. The most frequently specified classes in industry are ISO 5 to ISO 8. Older projects often use the former US Federal Standard 209E class names, which map approximately as follows:
| ISO 14644-1 class | Max particles ≥0.5 µm per m³ | Approx. former FS 209E class | Typical use |
|---|---|---|---|
| ISO 5 | 3,520 | Class 100 | Aseptic filling zones, critical process areas |
| ISO 6 | 35,200 | Class 1,000 | Semiconductor and precision assembly |
| ISO 7 | 352,000 | Class 10,000 | Pharma background areas, device assembly |
| ISO 8 | 3,520,000 | Class 100,000 | Packaging, change rooms, support areas |
The required class comes from the product and the regulatory framework it is made under, not from the HVAC designer. Getting it confirmed at the start avoids expensive redesign later.
Filtration: pre-filters, fine filters and HEPA
Clean room air is filtered in stages. Pre-filters and fine filters in the air handling unit capture larger dust and protect the final stage. Final filtration is usually by HEPA filters in terminal housings at the ceiling, so air is filtered immediately before it enters the room. Under EN 1822, H13 filters are rated at 99.95% and H14 filters at 99.995% efficiency at the most penetrating particle size.
A filter is only as good as its seal. That is why installed HEPA filters are leak tested by scanning the filter face and frame with an aerosol challenge after installation.
Airflow patterns
Unidirectional (laminar) flow
For ISO 5 and cleaner zones, filtered air moves in parallel streams at a uniform velocity, usually downward from a full HEPA ceiling or a laminar flow unit, sweeping particles away from the critical work. Design velocities in the region of 0.36 to 0.54 m/s at the work area are commonly used.
Non-unidirectional (mixed) flow
ISO 6 to ISO 8 rooms typically use HEPA diffusers in the ceiling with returns at low level near the floor. Clean air dilutes the particles generated in the room and the low-level returns draw contaminated air down and out of the working zone.
Air change rates
The air change rate is the supply airflow divided by the room volume, expressed per hour. Guideline tables, such as those in IEST recommended practices, give broad ranges that rise steeply as the class gets cleaner, from tens of air changes per hour for ISO 8 rooms to hundreds for ISO 5 unidirectional zones.
A table is a starting point, not the answer. The right airflow depends on how many particles people and processes generate, how quickly the room must recover after a disturbance, and the heat load the air also has to remove. Over-specifying air changes increases fan energy for the life of the facility, so it is worth calculating rather than rounding up.
Pressure cascades, airlocks and containment
Rooms are held at different pressures so that air always moves from cleaner areas to less clean ones when doors open or through gaps. A pressure difference in the order of 10 to 15 pascals between adjacent rooms of different grades is widely used in practice.
- Positive pressure cascades protect the product: the cleanest room has the highest pressure.
- Negative pressure is used where the hazard must be contained, such as potent compounds or biological agents, so air flows into the room rather than out.
- Airlocks with interlocked doors maintain the pressure difference as people and materials move between grades.
Pressures are maintained by controlling supply, return and exhaust airflows and are displayed on differential pressure gauges or a monitoring system outside each room.
Temperature and humidity control
Many processes and garments-on operators need stable conditions. Achieving low humidity usually means over-cooling the air to condense moisture and then reheating it to the supply temperature, or using a desiccant dehumidifier where very low humidity is required. Chilled water systems are often preferred for this because they give finer control; our VRF vs chiller guide explains why.
Air handling and construction details
- Dedicated air handling units for clean areas, with variable-speed fans to hold airflow steady as filters load
- Sealed, cleanable ductwork and airtight penetrations
- Flush-mounted light fittings and HEPA housings in a sealed ceiling
- Low-level return risers positioned to support the airflow pattern
- Standby fans or units where downtime would stop production
Testing, qualification and monitoring
A clean room is not complete until its performance has been measured and documented. Typical tests include:
- Airflow volume and velocity measurement
- Installed HEPA filter leak testing
- Airborne particle count classification under ISO 14644-1
- Room pressure differential verification
- Recovery testing, showing how quickly the room returns to its class after contamination
- Temperature and humidity uniformity checks
ISO 14644 recognises three occupancy states for testing: as-built, at-rest and operational. Periodic re-testing intervals are set out in ISO 14644-2 and by your own quality system or regulator.
1 Worldcrafts designs and installs HVAC systems for clean room applications, including retrofits in operating facilities. Talk to our engineers about your process requirements.
Frequently asked questions
Not as the primary system for a classified clean room. Split units cannot provide HEPA filtration of the supply air, controlled airflow patterns or reliable room pressurisation. They may cool support areas, but classified rooms need a dedicated air handling system.
An ISO 7 room allows ten times fewer airborne particles of 0.5 micron and larger than an ISO 8 room: 352,000 per cubic metre compared with 3,520,000. Achieving ISO 7 generally needs more supply air, more HEPA coverage and tighter control of people and materials.
Positive pressure makes air flow outwards through doors and gaps, so unfiltered air and particles from surrounding areas cannot enter. Where a hazardous material must be kept in, the room is held at negative pressure instead.
Re-testing intervals for particle counts, airflow and pressure are set out in ISO 14644-2 and are often annual, but your quality system, customer or regulator may require more frequent monitoring and testing.
Planning a project like this?
Send us your drawings or scope. Our engineers will review the requirement and come back with an itemised proposal.
