How paint baking ovens work
Wet paint, solvent-borne or waterborne, is baked on car bodies, two-wheeler parts, appliances and general fabrications in convection ovens at about 130 to 180°C for roughly 20 to 40 minutes. The oven has heat-up and holding zones, each with a heater box containing a gas burner and a recirculation fan. As the film heats, solvent and low molecular weight resin fractions evaporate into the oven air. To keep solvent concentration well below the lower explosive limit, a fixed exhaust is drawn continuously and replaced by filtered fresh air, much of it through air seals at the entry and exit. In automotive plants the exhaust goes to a thermal oxidiser, whose heat is often returned to the oven.
Where the FD fan sits
- Outdoor air intake
- Filters
- FD (supply) fan
- Air seal heater
- Air seals and heater boxes
- Oven chamber
The fresh air fan sits in an air handling unit on the oven roof or in a plant room, drawing outdoor air through filters and supplying the air seals at the oven entry and exit and the heater boxes in the zones. It handles cool, filtered air. It works against filters, heaters and ducts, typically 100 to 250 mmWC, and its flow is matched with the exhaust so the oven stays slightly negative.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the heater | Up to oven temperature, downstream of the fan |
| Dust | Outdoor dust, filtered to paint shop standards |
| Pressure at the fan outlet | Typically 100 to 250 mmWC |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
FD Fan selection for paint baking ovens
| Blade form | Backward curved or aerofoil, for efficiency on clean air |
| Material of construction | Galvanised or coated steel, with no loose coatings inside |
| Drive and arrangement | Direct drive with a variable speed drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive tracking exhaust flow |
| Also specified | Filters with pressure gauges, and silencers |
What this FD fan has to cope with
Dirt in the paint finish
Particles in the oven air settle on the wet film in the first zone and show as bits in the finish. The supply fan must not shed anything: no flaking paint inside the casing, no rubbing seals, and final filters downstream of the fan where the paint shop specifies them.
Air balance with the exhaust
If supply exceeds exhaust, the oven goes positive and solvent vapour leaks into the paint shop; if supply is too low, unfiltered shop air is drawn in at the openings and carries dirt onto the work. Tracking supply to exhaust flow with speed control keeps the balance.
Filter loading
Filters load steadily, and with a fixed-speed fan the supply flow falls week by week, upsetting the air seals and oven balance. A speed-controlled fan holding constant flow against filter pressure drop compensates, and the speed trend tells maintenance when to change filters before the fan runs out of speed.
Energy and control
Fresh air must be heated to oven temperature, so supply flow drives heating cost directly. Reducing supply to match an exhaust that has itself been cut to the safe minimum saves heat on both sides. A variable speed drive holding constant flow avoids oversupplying when filters are new, and turning the air seals down during breaks saves more.
ID fan and FD fan in paint baking ovens
| ID fan | FD fan | |
|---|---|---|
| Position | Oven roof, on the zone exhaust | Air handling unit on the roof or in a plant room |
| What it handles | Solvent-laden hot air | Filtered fresh air |
| Temperature at the fan | 130 to 180°C | Ambient, 20 to 45°C |
| Main design concern | LEL safety and resin deposits | Clean air and oven balance |
| Control | Minimum safe exhaust | Tracks exhaust flow |
See also the Induced Draft (ID) Fan for paint baking ovens.
Questions buyers ask
What does the fresh air fan do on a paint baking oven?
It supplies filtered outdoor air to the air seals at the oven openings and to the heater boxes, replacing the solvent-laden air removed by the exhaust fan. It handles clean, cool air, so it is a backward curved or aerofoil forced draft fan built to shed no particles into the airstream.
Why is there dirt in the paint after baking?
Common causes are dirty supply air from loaded or bypassed filters, particles shed from inside ducts or the fan, or an oven that is under too much suction and draws shop air in at the openings. Checking filter condition, supply and exhaust flows and oven pressure usually shows the source.
Should a paint oven be under suction?
Yes, slightly. The exhaust fan removes a little more than the supply fan delivers so that solvent vapour cannot leak into the paint shop. Too much suction draws unfiltered shop air in at the openings, which brings dirt onto the work and cools the entry zone.
Information needed to quote an FD fan for paint baking ovens
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Air seals. Number and size of the air seals, and their flow requirement.
- Filtration. Filter grades required by the paint shop, and whether final filters follow the fan.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.