Temperature and humidity control in cleanrooms

Particle control gets most of the attention in cleanroom design, but temperature and humidity control is often harder to achieve. Cleanrooms have large recirculation airflows, significant equipment heat, gowned operators who overheat easily, and products or processes that may react to small changes in moisture. In Canada, they also face very dry winters and humid summers.

This guide covers typical requirements, how to set realistic tolerances and the system design choices that make stable control possible.

Why temperature and humidity matter

Temperature

• Operator comfort. Cleanroom garments, hoods, masks and gloves trap heat. Operators who are too warm sweat and shed more particles and microorganisms. Rooms with full aseptic gowning are often held cooler than ordinary workplaces.
• Process stability. Precision measurement, photolithography and some chemical processes are sensitive to temperature changes, sometimes to fractions of a degree.
• Product stability. Many pharmaceutical products and materials have temperature limits.

Humidity

• Static electricity. Low humidity increases electrostatic charge, which attracts particles to surfaces and can damage electronic components or affect powder handling.
• Microbial growth. High humidity encourages microbial growth on surfaces and in building materials.
• Moisture-sensitive products. Hygroscopic powders, effervescent tablets and some medical devices require low humidity.
• Corrosion and material behaviour. Some processes are sensitive to moisture absorbed by materials.
• Operator comfort. Humidity combined with gowning affects comfort.

Typical design ranges

Requirements vary widely by industry and should be driven by product and process needs, not habit.

These are typical ranges only. GMP rules, including EU GMP Annex 1, do not set numerical temperature or humidity limits; they require conditions appropriate to the product and process, defined by the manufacturer.

Setting tolerances

Tolerances drive cost. Holding plus or minus 2 C and plus or minus 10 percent RH is relatively easy. Holding plus or minus 0.5 C and plus or minus 3 percent RH requires better sensors, more precise control, and careful design. Specify tight tolerances only where they are justified. Also define where they apply (room average, at the work zone or at a specific instrument) and over what time period.

System architecture

Separate latent and sensible loads

The most reliable cleanroom humidity control comes from separating the two jobs:

1. A makeup air unit conditions outdoor air to a controlled dew point. It handles almost all of the moisture load: outdoor air humidity in summer, humidification in winter, and moisture from people.
2. The recirculation system (air handler or fan filter units) removes sensible heat with cooling coils that operate above the room dew point. These "dry coils" do not condense water, so there are no wet surfaces in the recirculated air stream that could grow microorganisms or shed moisture.

With this arrangement, room humidity is controlled by controlling the dew point of the makeup air, and room temperature by the sensible coils. This decoupling is far more stable than trying to control both with one coil. Dew point concepts are explained in psychrometrics for HVAC engineers.

Dew point control in summer

To dehumidify, the makeup air unit cools outdoor air below the target dew point, then reheats it if needed. For typical cleanroom dew points this can be done with chilled water or DX coils. For low dew point requirements, desiccant dehumidifiers are used, often followed by cooling coils. See dehumidification strategies for commercial buildings.

Reheat should come from recovered heat where possible, such as condenser heat or a runaround loop, rather than new energy.

Recirculation coils

Chilled water supply temperature to dry sensible coils is set above the room dew point. Supplying warmer water also improves chiller efficiency and makes free cooling available more of the year.

Humidification in Canadian winters

In much of Canada, outdoor air in winter holds very little moisture. A cleanroom requiring 40 percent RH at 20 C needs significant humidification for months each year. Considerations:

• Clean steam. Steam from a building boiler often contains treatment chemicals such as amines. For pharmaceutical cleanrooms, clean steam generators fed with treated water are typically used, so no boiler additives reach the room air.
• Electric steam humidifiers suit smaller systems and allow clean steam without a central plant.
• Adiabatic humidifiers (high-pressure atomizing or wetted media) use less energy but require high water quality, such as reverse osmosis or deionized water, and careful hygiene to avoid microbial growth.
• Location. Humidify in the makeup air unit, upstream of final filters, with enough absorption distance that no water droplets reach filters or coils.
• Envelope. High indoor humidity in cold weather can cause condensation in walls and ceilings. The building envelope around the cleanroom must have an effective air and vapour barrier.

The broader issues are covered in humidification in cold climates.

Heat loads

Cleanroom cooling loads differ from offices:

• Fan heat from recirculation fans or fan filter units is substantial and continuous.
• Process equipment may dominate in production rooms.
• Lighting is often at higher levels.
• People contribute modest heat but significant moisture.
• Envelope gains are often small because cleanrooms are usually interior spaces.

Because recirculation airflow is often far more than needed for cooling, supply air temperature in a cleanroom may be only slightly below room temperature.

Controls and sensors

1. Place temperature and humidity sensors where they represent the controlled zone, usually in the return air path or at representative room locations, not near supply outlets or equipment exhaust.
2. Select humidity sensors with accuracy appropriate to the tolerance, and calibrate them regularly. Humidity sensors drift more than temperature sensors.
3. Use separate monitoring sensors in GMP areas where independent records are required.
4. Tune control loops to avoid hunting between humidification and dehumidification, often with a deadband.
5. Trend data and review it, especially during seasonal transitions.

Key takeaways

• Temperature and humidity requirements should come from product, process and gowned operator needs, with tolerances set only as tight as necessary.
• Separate latent control (makeup air dew point) from sensible control (dry recirculation coils) for stable, hygienic control.
• Summer dehumidification uses cooling coils or desiccants; winter humidification in Canada needs clean steam or high-purity adiabatic systems.
• Envelope vapour control is essential when humidifying cleanrooms in cold climates.
• Accurate, well-placed and regularly calibrated humidity sensors are critical to performance.