To size a fan for a paint booth, first set the required airflow from booth volume times air changes, typically 100 to 200 air changes per hour during spray and 10 to 20 for prep, then add the static pressure from filters, usually 250 to 500 Pa, plus any duct loss. The fan must also hold a slight negative pressure, around 5 to 15 Pa, so overspray never leaks into the shop. Get the air change rate right and the booth clears solvent fumes fast; get the pressure wrong and the filters choke the fan or the booth goes positive and smells up the building.
Step 1: Calculate Airflow from Air Changes
Take the booth clear-air volume, say 4 m by 3 m by 3 m equals 36 m³, and multiply by the duty air changes. A spray booth at 150 air changes per hour needs 36 times 150, or 5,400 m³/h, while a prep bay at 15 ACH needs only 540 m³/h. Cross-draft booths often target face velocity of 0.4 to 0.5 m/s across the opening, which for a 3 m by 2.5 m opening is about 10,800 to 13,500 m³/h. Always size to the more demanding of air-changes or face-velocity, because both protect the painter.
Step 2: Add the Static Pressure
Paint booths stack several resistances: a pre-filter at 50 to 100 Pa, a ceiling diffusion media at 30 to 80 Pa, an exhaust filter or paint arrestor at 80 to 200 Pa, and perhaps 30 to 100 Pa of duct. Clean-state total is often 250 to 500 Pa, but a loaded paint arrestor can climb to 600 or 800 Pa, so size the fan for the dirty condition or fit a pressure switch that alarms at 500 Pa. Undersizing here starves the booth exactly when the filter is fullest and the painter needs flow most.
Comparison of Booth Types
| Booth type | Air changes per hour | Face velocity (m/s) | Typical static (Pa) |
|---|---|---|---|
| Prep bay | 10 to 20 | 0.1 to 0.2 | 150 to 300 |
| Cross-draft spray | 100 to 150 | 0.4 to 0.5 | 300 to 500 |
| Downdraft spray | 150 to 200 | 0.2 to 0.3 | 400 to 600 |
| Side-draft | 120 to 180 | 0.3 to 0.4 | 350 to 550 |
Negative Pressure and Make-Up Air
A booth should run 5 to 15 Pa negative to the surrounding shop so solvent and overspray are pulled inward, never out. That negative pressure must be supplied by make-up air roughly equal to exhaust, minus the small leak, or the shop goes positive elsewhere. A 5,400 m³/h exhaust therefore needs about 5,000 to 5,400 m³/h of conditioned make-up, heated in winter to keep the painter comfortable and the paint flowing. Skipping make-up air starves the fan and pulls cold air through every crack.
Selecting the Fan
- Compute the larger of air-changes or face-velocity airflow, then add 10% margin.
- Size static pressure at the dirty-filter condition, often 500 to 800 Pa.
- Pick a centrifugal fan, such as a backward-curved or 4-72 style, for 300 to 800 Pa duty.
- Hold booth negative 5 to 15 Pa with balanced make-up air.
- Fit a pressure switch alarming near 500 Pa to catch a clogged arrestor early.
Explosion and Material Notes
Spray booths handle flammable solvent vapour, so the exhaust fan must be spark-resistant, with non-ferrous blades or a brass contact strip, and preferably an air-flow-over-motor design that keeps the motor out of the vapour path. In many markets this aligns with ATEX or local fire code, not with IP alone. A fan at 5,400 m³/h and 500 Pa on a 4 kW motor is typical, but the motor must be sized for the dirty-pressure rise, not just the clean point.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
Duct Layout and Air Velocity
Keep exhaust duct velocity at 12 to 18 m/s so overspray travels to the arrestor instead of dropping out and clogging the pipe, but avoid over 20 m/s where friction adds 100 to 200 Pa of needless pressure. A 5,400 m³/h flow in a 350 mm duct runs about 15 m/s, a good compromise, and a long 20 m run then costs roughly 150 to 250 Pa on top of the filters. Oversize the duct one size when the run is long, because the few percent extra metal beats a bigger motor and a noisier fan every time.
Filter Stage Selection
A two-stage bank, a pre-filter at 50 to 100 Pa plus a paint arrestor at 80 to 200 Pa, protects the wheel and extends the costly stage life by catching coarse overspray first. Changing the pre-filter at 150 Pa instead of waiting for 400 Pa keeps the fan near its design point and avoids the negative-pressure loss that lets solvent creep into the shop. Track pressure drop across each stage with a magnehelic gauge and you can time changes to the real load rather than a calendar.
Commissioning the Booth
At start-up, measure face velocity with a hot-wire or vane anemometer at nine points across the opening; aim for 0.4 to 0.5 m/s within plus or minus 0.1 m/s, because uneven capture lets plumes escape at the edges. Confirm booth negative 5 to 15 Pa with a manometer and verify make-up air matches exhaust within 10%. A booth that passes these three checks on day one stays compliant through its first inspection and keeps the painter's air clean during the longest shifts.
Filtration and Air Quality
Beyond capture, the exhaust train should protect the neighbourhood and the fan wheel, so add a final HEPA or activated-carbon stage when solvents or odours matter, which adds 50 to 150 Pa the fan must also cover. A spray booth pushing 5,400 m³/h through a 200 Pa arrestor plus 100 Pa carbon runs near 500 Pa total, squarely in centrifugal territory. Skipping the carbon may pass the booth but fail local odour rules, so size for the full stack, not just the capture face, and confirm the motor has the 10% to 20% margin for the loaded condition.
FAQ
How many air changes does a paint booth need?
Spray booths need about 100 to 200 air changes per hour to clear solvent fast, while prep bays need only 10 to 20. For a 36 m³ booth that is 3,600 to 7,200 m³/h for spray. Also check face velocity, often 0.4 to 0.5 m/s, and size for the larger of the two, because both protect the painter
What static pressure should I size for?
Size for the dirty-filter condition, typically 500 to 800 Pa once the paint arrestor loads, even though the clean booth may be only 250 to 500 Pa. A clogged arrestor can lift static from 250 to over 600 Pa and starve the booth. Fit a pressure switch alarming near 500 Pa so you change media before airflow drops.
Why must a paint booth be negative pressure?
A booth held 5 to 15 Pa negative to the shop pulls solvent and overspray inward, never out into the workspace, protecting other staff and meeting fire code. That negative is supplied by make-up air nearly equal to exhaust, about 5,000 to 5,400 m³/h for a 5,400 m³/h exhaust.
What fan type suits a paint booth?
A centrifugal fan, such as a backward-curved or 4-72 style, suits the 300 to 800 Pa duty and handles dirty air well. Size the motor for the dirty-pressure rise, often 4 kW at 5,400 m³/h and 500 Pa, and use spark-resistant construction with the motor out of the vapour path to meet ATEX or local fire rules for flammable solvent.