Select a cleanroom fan by fixing the required air change rate first, then adding the full dirty-filter resistance, and only then choosing a backward-curved centrifugal fan or plenum fan array that meets that duty. Most ISO 14644-1 Class 7 and Class 8 recirculation systems land between 800 Pa and 1,800 Pa total static pressure, so a double-inlet backward-curved centrifugal fan with the motor outside the airstream is the normal answer. For Class 5 and cleaner, ceiling fan filter units (FFUs) with EC motors are usually preferred because each unit can be trimmed independently to hold uniform downflow velocity.
Step 1: Air change rate sets the airflow, not the cooling load
Cleanroom airflow is driven by particle dilution, not by sensible heat. Typical design bands are 10–25 air changes per hour (ACH) for ISO Class 8, 30–60 ACH for Class 7, and 70–150 ACH for Class 6. Class 5 and cleaner are normally unidirectional: 40–100 % of the ceiling is filter coverage and the design target is a downflow face velocity of 0.36–0.54 m/s, which is equivalent to several hundred nominal air changes per hour.
Convert to volume flow with Q = room volume × ACH. A 200 m² Class 7 room with a 3 m clear ceiling holds 600 m³, so 45 ACH gives a recirculation duty of 27,000 m³/h. Then add 10–15 % for envelope leakage, door swings and the make-up air needed to hold a 10–15 Pa pressure cascade to the adjacent grade. The fan is therefore selected at roughly 30,000–31,000 m³/h, not 27,000 m³/h.
Step 2: Add up the resistance at change-out condition, not clean
The single most common cleanroom fan mistake is selecting on clean-filter pressure drop. Filters are replaced when they reach roughly twice their initial resistance, and if the fan has no margin the room loses its air change rate months before the filters are due. Build the resistance schedule at change-out values.
| Component | Clean ΔP (Pa) | Change-out ΔP (Pa) | Notes |
|---|---|---|---|
| G4 / ISO Coarse pre-filter | 40–60 | 120–150 | Cheapest stage to change; change it often |
| F8–F9 / ISO ePM1 secondary filter | 120–180 | 300–400 | Protects the HEPA and drives its service life |
| H13–H14 terminal HEPA | 200–280 | 500–600 | Replace on ΔP, never on a calendar only |
| Cooling coil (6–8 rows, wet) | 150–300 | 150–300 | Add 20–30 Pa if a drift eliminator is fitted |
| Ductwork, dampers, silencer, grilles | 150–400 | 150–400 | Depends on duty and layout length |
Summing the change-out column for a typical Class 7 recirculation loop gives roughly 1,300–1,800 Pa. A Class 8 loop with only pre-filter, secondary filter and terminal HEPA on a short duct run may sit at 800–1,100 Pa. Those two numbers point at two different fan families, which is why the resistance schedule must come before the fan catalogue.
Step 3: Match the fan type to the pressure band
| Fan type | Static pressure | Typical peak efficiency | Best cleanroom use |
|---|---|---|---|
| Backward-curved plenum fan (plug fan) | 300–1,600 Pa | 72–80 % | AHU and fan-array recirculation, Class 6–8 |
| Double-inlet centrifugal, 4-72 / 4-79 type | 400–2,500 Pa | 78–86 % | Central recirculation and make-up air units |
| High-pressure centrifugal, 9-19 / 9-26 type | 2,500–15,000 Pa | 70–80 % | Dust extraction and process exhaust, not room air |
| EC fan filter unit (FFU) | 150–400 Pa | 45–60 % (unit level) | Class 5 and cleaner unidirectional ceilings |
| Axial fan, T35-11 type | 50–500 Pa | 65–75 % | Gowning-room transfer and low-resistance exhaust |
Forward-curved fans are widely available in the 300–900 Pa band but should be avoided in cleanrooms: their power curve rises steeply toward free delivery, so a filter change can overload the motor, and their efficiency at 1,000 Pa is typically 8–15 points below a backward-curved wheel of the same diameter.
Step 4: Decide the motor and speed control
Speed control is not optional in a cleanroom, because the duty point moves continuously as filters load. Fan laws make this cheap to exploit: airflow scales with speed, static pressure with speed squared, and shaft power with speed cubed. Running a fan at 80 % speed draws about 51 % of full-speed power, so a system commissioned with 30 % pressure margin can recover most of that margin as energy savings for the first half of the filter life.
EC motors are typically 85–92 % efficient across a wide turndown and integrate the drive, which suits FFUs and small plenum fans up to about 5.5 kW. Above roughly 7.5 kW an IE3 or IE4 induction motor with an external VFD is usually more economical and easier to service. Keep the motor out of the airstream wherever the fan sits downstream of the last filter, and specify totally enclosed frames of at least IP55 so that bearing grease and winding varnish cannot become particle sources.
Cleanroom-specific construction requirements
- Casing leakage class L1 or L2 to EN 1886 for any section downstream of the final filter; a leaky casing short-circuits filtered air and defeats the pressure cascade.
- Smooth, cleanable, non-shedding internal surfaces: stainless steel or coated galvanised sheet, no exposed fibreglass insulation.
- Gasketed or gel-seal terminal filter frames, with a downstream test port for DOP or PAO scanning to ISO 14644-3.
- Flexible connections and spring or rubber isolators sized for 90–95 % vibration isolation efficiency at running speed.
- Differential pressure gauges or transmitters across every filter bank, so change-out is triggered by ΔP and not guesswork.
- Bearing L10 life of at least 40,000 hours, because cleanroom recirculation fans commonly run 8,760 hours per year.
When to choose which configuration
- Choose a single central centrifugal fan when the room is Class 7 or 8, the duty is below about 40,000 m³/h, and a short planned shutdown for maintenance is acceptable.
- Choose a plenum fan array (fan wall) when you need built-in redundancy, when the AHU has a tight installation height, or when the duty exceeds roughly 40,000 m³/h — six small fans in an array give an N+1 arrangement and a 5–10 point part-load efficiency advantage.
- Choose EC fan filter units when the grade is Class 5 or cleaner, when the ceiling layout will change with process equipment, or when velocity uniformity within ±20 % must be proved at each filter face.
- Choose an axial fan only for transfer, gowning-room extract or general exhaust below 500 Pa; do not use axial fans behind HEPA banks.
- Choose a separate high-pressure centrifugal fan when the room also has local dust or fume capture, because mixing a 3,000 Pa extraction duty into a 1,200 Pa recirculation loop wastes energy and destabilises room pressure.
Noise, vibration and balance targets
Cleanroom criteria are usually NC 50–65 for production areas and NC 40–45 where operators work an eight-hour shift, roughly 55–70 dB(A) at 1 m. Fan-generated noise is dominated by blade-pass frequency, so selecting the fan near its peak-efficiency point — ideally within 10 % of it — is the cheapest acoustic measure available. Residual unbalance drives both noise and bearing life; balance grade G2.5 or better is the practical requirement for wheels running above 900 rpm. Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory, and every wheel is balanced to G2.5 and performance-tested before shipment.
Five mistakes that cause cleanroom fans to fail acceptance
- Selecting on clean-filter resistance, then losing 20–30 % of design airflow within one filter cycle.
- Ignoring the pressurisation air, so the cascade collapses whenever an airlock door opens.
- Placing a belt drive downstream of the final filter, where belt dust becomes a particle source.
- Choosing a forward-curved wheel for a 1,200 Pa duty and tripping the motor overload after a filter change.
- Omitting filter ΔP instrumentation, which makes the fan the suspect when the real problem is a loaded secondary bank.
FAQ
What static pressure should a cleanroom recirculation fan be selected for?
Most ISO Class 7 and Class 8 recirculation loops need 800 to 1,800 Pa total static pressure once pre-filter, secondary filter, terminal HEPA, coil and ductwork are summed at change-out condition. Always build the schedule from dirty-filter values, then add roughly 10 percent installation margin.
How many air changes per hour does a cleanroom need?
Typical design bands are 10 to 25 air changes per hour for ISO Class 8, 30 to 60 for Class 7 and 70 to 150 for Class 6. Class 5 and cleaner use unidirectional flow instead, designed to a downflow face velocity of 0.36 to 0.54 m/s.
Are EC fan filter units better than a central air handling unit?
They serve different grades. Fan filter units suit Class 5 and cleaner ceilings because each unit is trimmed independently for velocity uniformity and layouts change easily. A central unit with a backward-curved centrifugal fan is more efficient and cheaper to maintain for Class 7 and Class 8 rooms.
Why avoid forward-curved fans in cleanroom air handling units?
Their power curve rises steeply toward free delivery, so a filter change can overload the motor, and peak efficiency is typically 8 to 15 points lower than a backward-curved wheel at 1,000 Pa. Backward-curved or aerofoil wheels are non-overloading and far more stable across filter loading.
How often should cleanroom filters be replaced?
Replace on differential pressure, not on the calendar. Coarse pre-filters usually change at roughly three times initial resistance, secondary filters at about twice, and terminal HEPA filters when they reach 500 to 600 Pa or fail an ISO 14644-3 scan test.