For commercial kitchen exhaust, baffle filters are the correct choice and mesh filters are generally prohibited in grease-laden Type I hood applications. A listed baffle filter captures roughly 60–80% of grease particles larger than 10 µm at a pressure drop of 60–90 Pa, drains condensed grease into a trough, and resists flame propagation because it presents no combustible accumulation surface within the airstream. Mesh filters trap grease inside layered wire, which holds fuel directly in the flame path and blinds within days. The only situations where mesh remains acceptable are non-grease Type II hoods handling steam, heat or dishwasher vapour.
How each filter type works
A baffle filter forces the airstream through a series of offset vertical channels, reversing direction two or three times. Grease droplets, having roughly 800–900 times the density of air, cannot follow the turns and impact on the baffle surface, then run down into a collection trough. Because the flow path contains no fine media, resistance stays essentially constant across the service interval – typically rising only 10–20 Pa between cleanings – and there is no path for flame to travel through the filter into the duct.
A mesh filter relies on interception: layers of aluminium or galvanised wire, often four to six deep, catch droplets as air passes through. Initial capture of particles below 10 µm can be better than a baffle, which is why mesh persists in marketing claims. In service, however, grease saturates the wire matrix, resistance can double within a week of heavy frying, and the filter becomes a wick holding several hundred grams of flammable residue directly above the cooking surface. That single fact is why fire codes and NFPA 96 direct grease-laden exhaust toward listed baffle or grease-extractor designs.
Filter type comparison
| Attribute | Baffle filter | Mesh filter | High-efficiency grease cartridge |
|---|---|---|---|
| Capture above 10 µm | 60–80% | 65–85% when clean | 85–95% |
| Capture at 1–5 µm | 10–25% | 20–35% when clean | 40–70% |
| Initial pressure drop | 60–90 Pa | 40–75 Pa | 150–300 Pa |
| Resistance rise in service | 10–20 Pa | 75–150 Pa within a week | 50–100 Pa |
| Fire performance | Blocks flame propagation | Retains fuel in airstream | Listed, blocks propagation |
| Cleaning | Weekly in a dishwasher | Daily, labour intensive | Weekly, plus cartridge service |
| Typical service life | 5–10 years | 1–3 years | 7–10 years |
| Code acceptance for Type I hoods | Standard | Generally not permitted | Permitted where listed |
Selection guidance
- Use baffle filters when the hood serves any grease-producing appliance – ranges, fryers, griddles, charbroilers, woks – which covers virtually all Type I applications.
- Use baffle filters when maintenance labour is limited, because a weekly dishwasher cycle is realistic where daily hand-scrubbing of mesh is not.
- Use mesh filters only when the hood is Type II over dishwashers, steamers, kettles or ovens producing heat and moisture but no grease.
- Use high-efficiency grease cartridges when exhaust discharges near operable windows, air intakes or residential neighbours and odour or grease deposition complaints must be prevented.
- Use a water-wash hood when the duty is extra-heavy solid fuel or continuous charbroiling, accepting the plumbing, detergent and drainage complexity in exchange for automated cleaning.
- Add a downstream grease removal stage when a rooftop fan discharges onto a trafficked roof, since even 80% capture leaves a meaningful aerosol load in the duct.
- Never mix filter types in one bank when resistances differ, because air short-circuits through the lower-resistance filters and capture collapses in the rest.
Sizing the filter bank
- Establish the hood exhaust rate from appliance duty. Typical wall-canopy values are roughly 1,100 m³/h per linear metre for light duty, 1,700 m³/h per metre for medium duty, 2,200 m³/h per metre for heavy duty, and up to 3,000 m³/h per metre for solid fuel.
- Divide the airflow by the rated capacity of each filter. A standard 500 × 500 mm baffle filter is normally rated 340–680 m³/h, so a hood exhausting 5,100 m³/h needs roughly 10–12 filters.
- Check the velocity through the filter face, which should land near 1.5–2.5 m/s. Below that range grease drainage is poor; above it, captured grease is re-entrained and carried into the duct.
- Install filters at 45° to 60° from vertical, never horizontally, so gravity drains grease into the trough rather than pooling it on the baffle.
- Maintain duct velocity of 7.5–10 m/s so residual aerosol stays airborne to the fan instead of settling as a fuel layer inside the duct.
- Fit a differential-pressure gauge across the bank and record the clean reading at commissioning, so drift is measurable rather than a matter of opinion.
Keruite manufactures the centrifugal and upblast exhaust fans used in commercial kitchen ventilation at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; each rotor is balanced to G2.5 and performance-tested before shipment.
Cleaning and fire safety practice
Clean baffle filters weekly in normal service and two to three times per week under heavy frying loads. A commercial dishwasher at 60–70 °C with an alkaline degreaser handles them well; soaking overnight in a hot detergent tank works equally well for stainless baffles. Reinstall only when dry and always in the correct orientation, since a reversed baffle drains grease into the airstream instead of the trough. Keep the trough emptied, because an overflowing trough puts liquid grease back into the exhaust path.
Remember that filters are only the first defence. Duct cleaning intervals still apply – typically monthly for solid fuel and high-volume operations, quarterly for moderate use and semi-annually for light use – and the fan housing, hinged access panels and the rooftop discharge area all accumulate residue. A hood that runs hot with an undersized exhaust rate deposits more grease downstream regardless of filter quality, so verify capture at the hood face, generally 0.4–0.5 m/s, before blaming the filters for a greasy duct.
Make-up air balance
Filter performance depends on the kitchen being in balance. Every cubic metre exhausted through the hood has to be replaced, and where dedicated make-up air is undersized the shortfall arrives through doors, service openings and the dining room. A hood exhausting 5,100 m³/h with only 3,500 m³/h of tempered make-up air leaves 1,600 m³/h to be pulled from the surrounding building, which creates cross-draughts at the cooking line, breaks the capture pattern at the hood lip and pushes grease-laden air into the room rather than the filters. Aim to supply 80–90% of the exhaust volume as dedicated make-up air, leaving a small negative offset of 10–20% so cooking odours do not migrate into the dining area. Interlock the make-up air unit with the exhaust fan so the two can never run independently, and re-verify the balance whenever an appliance or hood is changed.
FAQ
Are mesh grease filters allowed in commercial kitchens?
Generally no for grease-laden Type I hoods, because layered wire mesh retains flammable residue directly in the airstream. Fire codes direct these applications to listed baffle filters or grease extractors. Mesh remains acceptable in Type II hoods over dishwashers, steamers and ovens.
How often should baffle filters be cleaned?
Weekly in typical service and two to three times weekly under heavy frying or charbroiling. Wash at 60–70 °C with an alkaline degreaser, dry fully, and reinstall in the correct orientation. Track the differential pressure against the commissioning baseline to confirm the interval suits the actual load.
What pressure drop should I allow for grease filters?
Allow 60–90 Pa for a clean listed baffle filter, plus a further 10–20 Pa of in-service rise. Include that in the fan static calculation alongside duct losses, and keep filter face velocity in the 1.5–2.5 m/s band so drainage and capture both work as intended.
Do grease filters remove smoke and odour?
Only partially. Baffle filters capture droplets above about 10 µm efficiently but remove little sub-micron smoke and no gaseous odour. Where discharge is close to occupied areas, add a high-efficiency cartridge stage plus activated carbon or an electrostatic precipitator downstream.