April 2003 Q&A

Energy Saving Features for Powder Cure Ovens

We have an old convection powder cure oven that we plan on replacing this year. It leaks heat into our plant and costs too much to operate, considering the cost of natural gas and our production requirements. What energy-saving features do you recommend on a new oven?

All of us are paying more for the natural gas we use to heat our homes and for our finishing processes. No one knows how long the spike in natural gas pricing will last or if it’s here to stay. Either way, it’s good common sense to make every effort to manage your energy costs.

The major heat load of a powder cure oven is the product, including hangers and conveyor. Since heating your product to a process cure temperature is the function of the oven, there is little you can do to reduce this heat loss.

However, there are several design considerations to keep in mind when planning for your new convection cure oven to reduce operating costs and conserve energy.

Besides the product heat losses, the remaining heat losses are incurred from the exhaust air, the product openings and the oven enclosure.

The exhaust system has several functions, including the required safety purge of the oven prior to igniting the burner and the removal of the products of combustion produced by the burner and the byproducts from the powder released during the curing process. NFPA bulletin 86 provides an outline that can be used to calculate the exhaust volume required for an oven.

To save energy with the exhaust system, include a two-speed motor on the exhaust fan or a modulating damper in the exhaust stack to reduce the volume of exhaust air during stops of the conveyor line or product gaps. This can be accomplished with relay logic or with the system’s PLC. This design feature could save thousands of dollars in operating costs per year.

The product openings should be as small as possible but large enough to safely allow product in and out of the oven. Bottom entry and exit with an elevated oven is the most desirable arrangement for product openings because hot air rises and therefore the oven will not leak heat.

If the oven is on the plant floor, side entry and exit is the only option. Utilize vestibules and powered air seals at the side openings when possible to reduce heat loss from the oven. To minimize the effect of drafts in the building, arrange the entry and exit next to each other [OK?] so that they are both exposed to the same surrounding pressure.

The oven enclosure should be appropriately insulated. A rule of thumb is one inch of insulation per 100?F operating temperature. Powder cure ovens are typically provided with four or six inches of insulated panels, preferably six inches. The oven panel construction should include slotted side channels to reduce the amount of conduction heat to the outside surfaces of the panel.

During installation of the oven, the areas where the panels join together and corner sections should be filled with insulation and sealed with high-temperature chalking and flashing. This will reduce the amount of air and heat that leaks from the oven.

Once your new oven is installed and operating, there are several maintenance steps you can take to keep it operating efficiently. Check the gas pressure and air/gas mixture monthly to ensure maximum combustion efficiency. Replace or clean the air filters on the combustion blower for proper operation. Clean the supply, recirculation and exhaust ductwork annually for best operating results.

Implementing the above features on your new powder cure oven along with a preventive maintenance program will provide significant savings in natural gas costs over the life of the oven.

Will the last one out of the building please turn down the thermostat?