To calculate airflow for an industrial ventilation system, multiply the room volume by the required air changes per hour, or sum the contaminant load and divide by the allowable concentration. The simplest formula is Q = (ACH × Volume) / 3,600 in m³/h, where ACH is typically 6 to 20 for industrial spaces. For contaminant control you instead use Q = G / Callow, ensuring dilution keeps workers below exposure limits. Both methods must then be checked against the available fan pressure.
Method 1: Air Changes Per Hour
Volume is length × width × height in cubic metres. A welding shop of 1,000 m² floor and 6 m height holds 6,000 m³; at 12 air changes per hour that requires 72,000 m³/h. Light assembly areas may need only 6 ACH, while paint booths and battery rooms often require 15 to 20 ACH. The ACH method is fast and conservative but ignores the specific contaminant, so it suits general comfort and mild hazard ventilation.
Method 2: Contaminant Dilution
When a known emission rate G (mg/h) must be held below an allowable concentration Callow (mg/m³), the needed flow is Q = G / Callow, again converted to m³/h. A process emitting 600 mg/h of solvent with an 8-hour limit of 50 mg/m³ needs at least 12 m³/h of fresh dilution air—clearly unrealistic at that number, so local capture hoods are added to cut the dilution burden. Always include a 1.2 to 1.5 safety factor on the result.
Method 3: Occupant and Heat Load
For occupied industrial spaces, add per-person ventilation of 20 to 60 m³/h depending on activity, plus make-up air to replace exhausted hoods. Heat removal uses Q = (Heat load in kW × 3,600) / (1.2 × ρ × ΔT), where ρ is the density and ΔT the allowed supply-return difference, often 5°C to 10°C. A 50 kW process with a 10°C rise needs roughly 15,000 m³/h of cooling air.
Face Velocity and Duct Sizing
Once total flow is known, size ducts for a face velocity of 4 to 10 m/s in mains and 2 to 5 m/s in branches to balance noise and pressure. A 10,000 m³/h main at 8 m/s needs about 0.35 m² of area, or a 670 mm round duct. Keeping velocity in range prevents both excessive friction loss (above 500 Pa per 10 m) and dust dropout (below 2 m/s in particulate lines).
Comparison of Calculation Methods
| Method | Input needed | Typical ACH / result | Best for |
|---|---|---|---|
| Air changes | Room volume | 6–20 ACH | General industrial ventilation |
| Dilution | Emission rate, limit | Q = G / C | Known contaminant sources |
| Heat load | kW, ΔT | 5–10°C rise | Process cooling air |
| Occupant | Headcount | 20–60 m³/h pp | Workshops with staff |
Calculation Checklist
- Use the air-changes method for general comfort ventilation at 6 to 12 ACH.
- Use the dilution method when a specific gas or vapour has a published exposure limit.
- Apply a 1.2 to 1.5 safety factor to any calculated flow before selecting the fan.
- Size ducts for 4 to 10 m/s mains to keep friction under control and noise acceptable.
- Confirm the chosen fan can deliver the flow at the system's dirty-filter static pressure.
Worked Calculation Walkthrough
Take a paint mix room of 15 m × 10 m × 4 m = 600 m³ needing 15 ACH for solvent control. Flow = 600 × 15 = 9,000 m³/h. Add a local booth capturing 3,000 m³/h at the source, so the total supply needed is about 12,000 m³/h. Apply a 1.3 safety factor for leaks and filter load, giving 15,600 m³/h. Size the duct at 8 m/s: area = 15,600 / 3,600 / 8 ≈ 0.54 m², or a 830 mm round main. This single example shows how ACH, capture, and safety factor combine into a real fan duty.
Balancing and Commissioning
After install, balance branches with dampers so each space gets its design fraction of total flow; unbalanced systems often show 20% to 40% deviation between rooms. Use a hot-wire or vane anemometer at grilles to confirm 0.15 to 0.5 m/s face velocity in occupied zones, and re-measure after filters load by one month. Document the as-tested flows so future maintenance can tell normal decline from a real fault, and tag dampers once set to prevent unauthorised adjustment that upsets the whole network balance and comfort.
Energy and Fan Power Check
Ventilation air costs energy. A 15,000 m³/h fan at 500 Pa needs about 2.6 kW shaft (Q×P/3,600/η with η = 0.8), so picking a high-efficiency wheel and right-sized motor saves 10% to 25% of running cost versus a cheap forward-curved unit. Where make-up air is heated or cooled, recovering energy with an ERV cuts the load by 60% to 85%. Always compare life-cycle cost, because a 10% efficiency gain on a 24/7 fan pays for a better unit within 2 to 4 years easily.
Avoiding Over- and Under-Ventilation
Too little air leaves hazards above limit; too much wastes fan power and conditioning energy and may cause uncomfortable draughts above 0.5 m/s. The fix is a proper calculation plus measurement, not a rule-of-thumb multiplier. For spaces with variable occupancy or process, add CO&sub2; or contaminant sensors that modulate the VFD between 30% and 100% of design flow, trimming annual energy use by 20% to 45% while still protecting air quality at peak demand periods reliably and compliantly.
Summary for Specifiers
Good ventilation starts with a real calculation, not a guess. Use ACH for comfort, dilution for hazards, and heat load for process cooling, then add a 1.2 to 1.5 safety factor and size ducts for 4 to 10 m/s. Measure after install and re-balance yearly. The small effort of calculating properly prevents both sick-building complaints and wasted fan power that can cost 20% to 40% of the energy bill over the building's whole service life.
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.
FAQ
What is a normal air changes per hour for a factory?
General industrial workshops usually need 6 to 12 air changes per hour, while spaces with fumes, paint, or batteries often require 15 to 20 ACH. The exact value depends on heat gain, contaminant load, and occupancy; when a specific hazard exists, the dilution method should override a simple ACH estimate for safety.
How do I convert ACH to m³/h?
Multiply the room volume in cubic metres by the desired air changes per hour, then divide by 3,600 to get cubic metres per second, or keep it in m³/h by using volume × ACH directly. For a 6,000 m³ room at 12 ACH the answer is 72,000 m³/h of total ventilation air.
Why add a safety factor to calculated airflow?
Real systems lose air through leaks, filter clogging, and duct roughness that estimates overlook, so a 1.2 to 1.5 factor protects against under-ventilation. Without it, the installed fan may deliver less than the design flow once the system is built, leaving workers or equipment above safe exposure or temperature limits.
What duct velocity should I design for?
Design main ducts for 4 to 10 m/s and branches for 2 to 5 m/s. Higher velocities raise friction loss and noise, while below 2 m/s in dust lines lets particles drop out and block the duct. Match velocity to the contaminant: gritty air needs higher speed, clean air can run slower and quieter.