Cleanroom temperature and humidity control: large airflow, small temperature difference, narrow humidity band
A cleanroom recirculates a lot of air to carry particles away, so the supply is only a few kelvin below the room. That changes how the coil works and how the temperature and humidity tolerance can be held. With only a little outdoor air, the coil, reheat and humidifier are all still needed across the seasons, and we see why in numbers.
Situation
Say a 300 m³ cleanroom has a process that requires 22 °C ±1 K and 45 % ±5 % RH. To control particles the room is run at 60 air changes per hour (an example value), so the supply is 18,000 CMH. With that much air, 30 kW of equipment heat is removed with the supply only 5.0 K below the room. In other words the supply and room points almost touch on the chart. Even 10 % outdoor air moves the mixed point a long way with the seasons, and if the coil leaving state is not held precisely the room goes out of tolerance. Three things to find: where the supply point is, how much coil, reheat and humidification each season needs, and how tightly the humidity ratio must be held to keep both ±1 K and ±5 % RH. The conditions are illustrative; the required conditions, air change rate and heat gains come from the process and the design specification.
Given conditions
| Item | Value |
|---|---|
| Room R and tolerance | 22 °C ±1 K, 45 % ±5 % RH (example requirement) |
| Volume, air changes | 300 m³, 60 per hour, so 18,000 CMH (example; the real value follows the cleanliness class and process) |
| Room heat gain | Sensible 30 kW, latent 1 kW (equipment and people, example) |
| Outdoor air | 10 % by dry air mass (for pressurization and make-up of exhaust); the other 90 % is recirculated |
| Coil leaving | 95 % RH when dehumidifying. Humidification by steam (enthalpy 2676 kJ/kg) |
| Three outdoor cases | Summer 32 °C / 60 %, spring or autumn 15 °C / 60 %, winter -5 °C / 50 % |
| Pressure | 101.325 kPa (sea level) |
Reading it on the chart
- Plot the room R (22 °C, 45 %) and draw the tolerance rectangle from its four corners (21 °C and 23 °C, 40 % and 50 % RH).
- The supply S lies 5.0 K below R along the room ratio line. With so much air, S sits right under R.
- Plot the mixed point M of outdoor air and R at 10 % outdoor air; it is only 10 % of the way from R toward OA.
- If M's humidity ratio is above S's (summer), the coil cools to the 95 % RH point C, which brings the humidity ratio down to S's, and C is reheated to S. If M is drier than S (winter), heat it and add the moisture with steam.
Step-by-step calculation
1) Airflow and supply temperature. Room 22 °C / 45 %: WR = 7.39 g/kg, specific volume 0.8461 m³/kg. Supply 300 × 60 = 18,000 CMH gives ṁ = 18,000 / 3600 / 0.8461 = 5.910 kg/s. Moist air specific heat cp = 1.006 + 1.86 × WR = 1.0197 kJ/kg·K, so ΔT = 30 / (5.910 × 1.0197) = 4.98 K and supply dry-bulb 17.02 °C.
2) Supply humidity ratio. The 1 kW latent gain is 1.42 kg/h of moisture (÷ (2501 + 1.86 × 22)). WS = WR − 0.000393 / 5.910 = 7.33 g/kg. The supply only needs to be 0.07 g/kg drier than the room, which is 60.8 % RH.
3) Summer (32 °C / 60 %). M is 23.02 °C, 8.46 g/kg, enthalpy 44.67 kJ/kg. WM is 1.13 g/kg above WS, so dehumidification is needed. The coil leaving state C is 95 % RH at W = 7.33 g/kg, which is 10.16 °C (enthalpy 28.69). Coil load Q = 5.910 × (44.67 − 28.69) = 94.4 kW. Then reheat from C to S is 41.3 kW. Cooling and then heating is a double burden, but holding the humidity ratio closely means controlling the coil leaving dew point.
4) Spring or autumn (15 °C / 60 %). M is 21.30 °C, 7.29 g/kg, which is 0.04 g/kg drier than WS, so a small amount of humidification (0.8 kg/h) is needed. The air before the humidifier, H, is 17.01 °C, lower than M, so sensible cooling of 25.8 kW is needed too. With large equipment heat, cooling remains even when the outdoor air is mild.
5) Winter (-5 °C / 50 %). M is 19.33 °C, 6.78 g/kg, which is 0.55 g/kg drier than WS. The air before the steam, H, must be 16.94 °C (enthalpy 34.21 kJ/kg), which is actually lower than M. Even with sub-zero outdoor air, 90 % of the air is recirculated, so the mixed dry-bulb stays at 19.3 °C and the 30 kW of equipment heat still has to be removed. Winter therefore needs sensible cooling of 14.4 kW together with humidification of ṁ × ΔW = 5.910 × 0.00055 × 3600 = 11.7 kg/h.
| Season (outdoor) | Mixed M (°C / g/kg) | Treatment | Coil (kW) | Reheat (kW) | Humidifying (kg/h) |
|---|---|---|---|---|---|
| Summer 32 °C · 60 % | 23.02 °C · 8.46 | dehumidify | 94.4 | 41.3 | 0.0 |
| Spring/autumn 15 °C · 60 % | 21.30 °C · 7.29 | humidify | 25.8 | 0.0 | 0.8 |
| Winter -5 °C · 50 % | 19.33 °C · 6.78 | humidify | 14.4 | 0.0 | 11.7 |
Interpreting the result
The tolerance is narrower than it looks. Looking only at 40 to 50 % RH, the humidity ratio span 6.56 to 8.22 g/kg seems generous. But the temperature also floats between 21 and 23 °C, so at a constant humidity ratio the RH falls when it warms and rises when it cools. With the humidity ratio fixed at the room target (7.39 g/kg), RH is 42.3 % at 23 °C and 47.8 % at 21 °C. To keep RH within 40 to 50 % wherever the temperature is, the humidity ratio must stay within 6.98 to 7.73 g/kg, or in dew point terms 8.7 to 10.2 °C (about ±0.8 K). The ±5 % RH is ±0.83 g/kg by itself, but once the temperature band overlaps it shrinks to ±0.38 g/kg.
Only 10 % outdoor air, yet the seasons matter. In summer the outdoor air lifts the mixed humidity ratio by 1.06 g/kg, costing 94.4 kW of coil and 41.3 kW of reheat. In winter the -5 °C air pulls the mixed humidity ratio down by 0.62 g/kg, so 11.7 kg/h of humidification is needed, while the mixed dry-bulb stays at 19.3 °C and 14.4 kW of sensible cooling remains. Cooling and humidifying are needed at the same time. Any swing in the outdoor humidity ratio goes straight into the room's humidity error, which is why cleanrooms often have a separate outdoor air unit that delivers make-up air at a steady dew point.
The supply point hugs the room point. With a ΔT of only 5.0 K, a 1 K error in coil leaving temperature is 20 % of the supply temperature difference and goes straight into the room. It is more stable to let the coil control the dew point (moisture removal) and let reheat, or a final supply temperature control, control temperature.
What the calculation leaves out
This is a steady-state heat balance. In a real cleanroom the air change rate and airflow pattern follow the cleanliness class and the process (verified with tests under the ISO 14644 series), and the air change rate sets the fan power and the airflow through the coil. HEPA filters and fans heat the air, so their heat adds to the room's sensible load; here that heat is taken as included in the 30 kW.
Dehumidification is not one-size-fits-all either. This example cools below the dew point and reheats; where the humidity band is tighter or the dew point must be lower, a desiccant dehumidifier is used. The 10 % outdoor air and the air change rate are examples, and the outdoor air needed for pressurization comes from leakage and exhaust. The tolerance comes from the user requirement specification, and ±1 K and ±5 % are illustrative.
Common mistakes
- Setting the humidity band from relative humidity alone. A ±1 K temperature swing moves RH by about ±2.7 percentage points by itself. Set the band on humidity ratio (dew point).
- Assuming an office-like supply temperature difference. A cleanroom has so much airflow that ΔT is about 5.0 K. A coil leaving temperature chosen as for an office would give supply that is far too cold.
- Ignoring 10 % outdoor air as small. Summer outdoor air lifts the mixed humidity ratio by 1.06 g/kg and creates dehumidification and reheat.
- Letting coil and reheat fight each other. If the coil is not on dew point control and reheat on temperature control, the two loops hunt.
- Leaving out fan and filter heat. A high air change rate means high fan heat. Include it in the sensible load so the supply temperature comes out right.
Try this calculation on the site
Open the summer process on the chart
The chart link plots the summer outdoor 32 °C / 60 %, room 22 °C / 45 %, the mixed point, the coil leaving point and the supply point, with cooling-dehumidifying and reheat process cards that give the coil kW and the reheat kW. For winter humidification in detail see Winter office humidification load, for coil leaving state and reheat see Summer cooling coil selection and reheat, for outdoor-air decisions see Economizer changeover, and for plotting see How to read a psychrometric chart.
References
- ASHRAE Handbook Fundamentals 2017 (SI), chapter 1 Psychrometrics: humidity ratio, enthalpy, specific volume and saturation vapour pressure (Hyland-Wexler). The psychrometric numbers here were computed with this site's engine, which implements those equations at 101.325 kPa.
- ISO 14644 series (cleanrooms and associated controlled environments): classification of air cleanliness (part 1) and test methods (part 3). Temperature and humidity are set by the user requirement; the 22 °C ±1 K, 45 % ±5 % and 60 air changes per hour here are illustrative, not values from the standard.
- ASHRAE Handbook HVAC Applications, Clean Spaces chapter: recirculation arrangements, air change rates and temperature and humidity control for clean spaces. The steam humidification balance follows the Humidifiers chapter of ASHRAE Handbook HVAC Systems and Equipment, and the steam enthalpy 2676 kJ/kg is the 100 °C saturated steam value (IAPWS-IF97 2675.6).
- Volume, heat gains, outdoor-air fraction and the 95 % RH coil leaving state are example values.