What you should know
- There is no universal cleanroom temperature and relative-humidity setpoint.
- In tropical climates, outdoor-air moisture and door infiltration can dominate the latent load.
- The cooling coil must generally reduce supply-air dew point below the room dew point to remove moisture.
- Reheat may be needed after deep dehumidification, but airflow, controls and heat recovery should be optimised first.
Start with process and product requirements
Temperature and relative humidity should be selected to protect the product and process, manage static or microbial risks where relevant, maintain material stability and support correctly gowned personnel. A commonly quoted comfort range is not automatically a cleanroom requirement.
Write the room setpoint together with its allowable range, measurement location, operating state, recovery expectation and alarm response. Relative humidity changes when temperature changes, even if the air's moisture content is unchanged, so dew point or humidity ratio is often more useful for diagnosing moisture control.
Sensible and latent loads behave differently
| Load source | Main effect | Design response |
|---|---|---|
| People | Sensible heat plus moisture | Use realistic occupancy, gowning and activity schedules |
| Outdoor air | Often a major latent load in warm, humid weather | Condition all required outdoor air and check the peak moisture condition |
| Door infiltration | Moisture and heat enter during pressure and door events | Coordinate door operation, airlocks, pressure and recovery |
| Process | Heat, vapour or direct exhaust | Use measured or vendor duty and include operating diversity |
| Fans and lights | Mainly sensible heat | Include fan heat at the correct point in the air path |
| Envelope | Sensible heat and possible vapour ingress | Review insulation, vapour control and thermal bridges |
Control moisture through dew point
A cooling coil removes moisture when its surface and leaving-air condition are cold enough for water vapour to condense. For stable dehumidification, the supply air normally needs a lower humidity ratio and dew point than the room. It can then absorb moisture from people, outdoor air, infiltration and the process before returning to the unit.
Deep cooling can leave supply air too cold for the room sensible load. Reheat, heat recovery, supply-air reset or an appropriate air-system arrangement can bring the dry air to a suitable supply temperature without losing moisture control.
A psychrometric calculation should use humidity ratio and mass flow, not relative humidity percentages alone.
Tropical-climate design sequence
Define indoor limits
Set temperature, RH or dew-point limits from the actual product, process and operator need.
Select outdoor design conditions
Use an appropriate local weather dataset and check peak moisture, not only peak dry-bulb temperature.
Calculate every load
Separate sensible and latent contributions by room, operating mode and schedule.
Build the air balance
Include outdoor air, process exhaust, toilet or safety exhaust, transfer air and room pressure offsets.
Select coil leaving condition
Choose a leaving humidity ratio that can offset the room latent load with practical coil performance.
Coordinate supply temperature
Check sensible capacity, diffuser performance and whether reheat or supply reset is required.
Test part-load control
Humidity problems often occur at low sensible load, low occupancy or rainy conditions, not only at peak cooling.
Controls that remain stable at part load
- Place temperature and humidity sensors away from direct supply jets, heat sources and doors.
- Coordinate cooling-valve, supply-temperature and reheat loops so they do not fight each other.
- Maintain required outdoor air and pressure balance when fan speeds reset.
- Trend room temperature, RH, dew point, coil leaving condition, valve position and door events together.
- Provide a condensate system that drains reliably under all fan pressure conditions.
- Check low-load operation, startup after shutdown and recovery after prolonged door opening.
Why cleanroom humidity runs high
| Observation | Possible cause | First checks |
|---|---|---|
| High RH with normal temperature | Supply dew point too high or latent load underestimated | Check coil leaving dew point, outdoor-air moisture and actual airflow |
| RH spikes during door use | Humid infiltration or airlock sequence | Trend door contacts, pressure and adjacent-space dew point |
| RH rises at low cooling load | Coil unloads before dehumidification is satisfied | Review control priority, minimum coil duty and reheat strategy |
| Some rooms high, others normal | Airflow imbalance, local sources or sensor issue | Verify sensor, supply quantity, return path and room moisture source |
| Condensation on surfaces | Surface temperature below local dew point | Measure surface temperature, dew point, insulation and air leakage |
Frequently asked questions
What temperature and RH should a cleanroom use?
There is no universal cleanroom setpoint. Define limits from product, process, material, static or microbial risk, gowning comfort, climate and applicable requirements. Record the operating state and tolerance with the setpoint.
Why is humidity difficult to control in tropical climates?
Warm outdoor air can carry a large amount of water vapour. Outdoor air, exhaust replacement and door infiltration can create a latent load that remains high even when the room sensible cooling load is low.
Why is reheat used in a cleanroom AHU?
Air may need to be cooled deeply to condense enough moisture, leaving it colder than the room needs. Reheat raises the supply dry-bulb temperature while retaining the lower humidity ratio. The system should be optimised to avoid unnecessary simultaneous cooling and heating.
Technical sources
Confirm the current controlled edition of every standard and the requirements that apply to your facility.
- ISO 14644-4:2022: Design, construction and start-upInternational Organization for Standardization
- EU Guidelines for Good Manufacturing Practice, Annex 1European Commission
- Sterile Drug Products Produced by Aseptic ProcessingU.S. Food and Drug Administration
Prepared by 360S Projects as a practical learning reference. Final design, validation and compliance decisions should be made by competent professionals using current project requirements, standards and manufacturer data.