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Oven Heat Containment

A radiant preheat zone at the entrance end of the cure oven can be anexcellent enhancement. Curing will begin immediately and can beaccomplished faster than convection heating alone. Radiant cure canbe energy efficient if properly designed because it does not wasteBtus to heat the entire substrate or the surrounding air.

In most cases where there is a lot of product variety, curing with radi-ant emitters alone is not practical. The variety of shapes and sizesrequire convection heating to ensure that all areas reach the cure win-dow temperature and no area is overheated.

High purity (quartz lamp emitter) IR ovens are very effective for thispurpose and the bring up, or response time, is instant. The emitterscan be arranged in zones for different part geometries. Turning ondifferent numbers of emitters within a zone can vary the energy levels.The zone control can be turned on by a remote controller or operatedmanually.

Gas fired radiant emitters work very well also. Like quartz lamps, theycan be purchased in modular sections, they can be zoned, and theyhave a fast response time. The Btus required to operate a gas-fired IRemitter will typically cost less than the kilowatt-hours required for elec-tric infrared emitters.

Infrared preheat zones for powder cure ovens are typically around 30to 60 seconds. For exact process requirements, testing must be per-formed.

4 Oven Heat Containment

It is recommended that all ovens have some method to reduce heatspillage (rollout). The best containment is by use of bottom entry/ exiton an elevated or roof mount oven.

On all ovens with horizontal entry/ exits there are two (2) options:

1. Powered Air Curtains.

2. Heat relief hoods with a gravity stack and manual adjustable damper.Powered air curtains will help to reduce the heat rollout from the ovenand make it easier to achieve a temperature balance in the oven. Theyalso help to reduce the energy losses that occur as air is heated andexpands.

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Heat relief hoods reduce heat build up in the general area and are therecommended minimum for any oven installation.

Powered Air Curtains

The conveyor entrance and exit should be furnished with high velocityair to minimize the amount of heated air in the oven escaping to thesurrounding atmosphere, and to ensure the optimum efficient opera-tion of the oven. Each air curtain should be equipped with a fan as-sembly, complete with shaft, bearings, sheaves, belts, and motor. Theair curtain utilizes oven air and does not induce an additional fresh airload on the surrounding area. The discharge velocity should be around2,000 to 3,000 FPM.

Heat Relief Hood

The conveyor entrance and exit can be furnished with a canopy-typeheat-relief hood to capture the majority of the heat spillage from the

product opening and disperse it to the outside atmosphere. The hoodshould be constructed of galvanized steel and be designed to be ovensupported, with a manually adjustable quadrant damper in a 24" jointof stack. The entrance of a powder cure oven should have a heat reliefhood rather than an air curtain so that the uncured powder is not

Figure 5-6 Powder Air Curtain

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disturbed. In order for the air curtain to be effective, the dischargevelocity should be very high. If the air velocity is reduced to a pointslow enough to avoid disturbing the powder on the part, it will not besufficient for heat containment.

There is some concern about the energy losses that occur if the ovendoes not have an air curtain. A careful review of the facts shows thatthis should not be a concern.

The amount of energy loss around a product opening is typically around

50,000 Btus per square foot. At a typical cost in North America of0.40/ therm, the losses around a product opening of 2 wide x 5 high(0.6 x 1.5 meters) would be $4.00/ hour. An air curtain will be around70% efficient at best. So the same product opening would lose around$1.20/ hour with an air curtain. The difference in gas consumption is$2.80/ hour. Consider that the air curtain will cost several thousanddollars more than the heat relief hood and that it requires a motor of10 or 15 horsepower. With the electricity needs of the air curtain andthe need to recover several thousands dollars of additional investmentthe gas losses with a heat relief hood do not appear to be a veryimportant issue.

Figure 5-7 Heat Relief Hood