How powder coating curing ovens work
Powder coated parts, hung on an overhead conveyor or loaded in batches, pass through a curing oven where the powder melts, flows and cross-links. A typical cure is 10 to 20 minutes at a metal temperature of 160 to 200°C, so oven air is held at about 180 to 220°C. Most ovens are convection ovens: a recirculation fan draws air from the oven, passes it over a heat exchanger or a burner, and blows it back through supply ducts. Heating is by a gas or diesel burner, firing directly into the air or indirectly through a heat exchanger, or by electric heaters. Powders release little volatile matter, but some cure by-products and, on direct-fired ovens, burner gases must be removed. An exhaust fan does this, and a combustion air fan feeds the burner.
Where the ID fan sits
- Burner or heat exchanger
- Recirculation fan
- Oven chamber
- Exhaust duct
- Exhaust (ID) fan
- Stack
The exhaust fan is mounted on the oven roof or beside it, drawing from the return side of the recirculation system or from hoods over the entry and exit openings. It works against modest resistance, typically 20 to 60 mmWC, and holds the oven slightly negative. On indirect-fired ovens the burner's own flue gas leaves separately through the heat exchanger flue, driven by the burner fan pressure or a small flue fan, and never mixes with the oven air.
Gas at the ID fan
| Air temperature, normal | 180 to 220°C at the exhaust |
| Maximum or upset | Up to about 250°C if the oven controller overshoots |
| Dust at the fan | Little dust; loose powder carried in on parts can reach the exhaust |
| Moisture and dew point | Dry; burner gases on direct-fired ovens add some water vapour |
| Fumes | Cure by-products, such as blocking agents from some polyurethane powders, which condense in cold ducts |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 62% 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. |
ID Fan selection for powder coating curing ovens
| Blade form | Backward curved, suited to a small hot flow |
| Material of construction | Mild steel, with external insulation |
| Drive and arrangement | Direct coupled or belt driven, motor away from the hot casing |
| Bearings and cooling | Shaft cooling disc and heat shield |
| Flow control | Exhaust set by damper at commissioning, or variable speed drive |
| Also specified | Insulated casing, flexible inlet connection, and a flow switch interlocked with the burner |
What this ID fan has to cope with
Fume condensation in the exhaust duct
Cure by-products condense on cold surfaces in the duct, fan and stack. Deposits build up as a yellowish, sticky film that can drip back into the oven or catch fire. Insulated ducts and casing keep the exhaust above the condensation point, and cleaning doors at bends and at the fan give access.
Hot bearings
The exhaust fan handles air near 200°C around the clock on multi-shift lines. Heat travelling along the shaft overheats bearings and breaks down the grease unless the fan has a cooling disc, a heat shield and bearings mounted clear of the casing on a pedestal.
Burner safety interlock
On direct-fired ovens the exhaust removes combustion products and is part of the safety system: the burner must not fire without proven exhaust. A flow or pressure switch on the exhaust duct, not just a motor running signal, interlocked with the burner, provides this proof.
Energy and control
Exhaust from a powder oven is heat thrown away: every cubic metre removed must be replaced and heated. Powders release little volatile matter, so the exhaust can often be reduced well below the figure set at commissioning, within the safety requirements of the burner system. A variable speed drive set to the needed flow, and reduced further when the conveyor runs empty, saves gas or electricity.
ID fan and FD fan in powder coating curing ovens
| ID fan | FD fan | |
|---|---|---|
| Position | Oven roof, on the exhaust duct | On the burner housing |
| What it handles | Hot oven air with cure fumes | Ambient air |
| Temperature at the fan | 180 to 220°C | 30 to 50°C |
| Main design concern | Heat and fume condensation | Air-fuel ratio |
| Control | Oven pressure | Burner firing |
See also the Forced Draft (FD) Fan for powder coating curing ovens.
Questions buyers ask
How much exhaust does a powder coating oven need?
Powder coatings release far less volatile matter than solvent paints, so exhaust needs are lower. There must be enough exhaust to remove cure by-products and, on direct-fired ovens, burner combustion products, and to hold the oven slightly negative. The exact figure depends on burner rating, powder load and the size of the oven openings.
What temperature does a powder coating oven exhaust fan handle?
Typically 180 to 220°C, matching the oven air temperature, with short peaks above that during heat-up or controller overshoot. The fan needs a shaft cooling disc, a heat shield and bearings outside the hot casing, and its rating is corrected for the lower density of hot air.
Why is smoke coming from my powder coating oven openings?
The oven is not under suction. Common causes are an undersized or fouled exhaust fan, a closed or stuck damper, or recirculation fans pushing harder than the exhaust can remove. Checking exhaust flow and measuring oven pressure at the openings usually shows which one it is.
Information needed to quote an ID fan for powder coating curing ovens
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Oven heating. Direct or indirect gas firing, diesel or electric heating, and burner rating.
- Oven openings. Size of the conveyor openings and any air curtains.
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.