Heat From Ovens

Q. I operate a powder coating shop. Our part opening is 6 × 9 ft and we have a combination drying/curing oven. The drying oven is a straight shot and the cure oven is a two-pass, meaning it goes in, turns 180 degrees and then comes out. We find that we lose anywhere from 5–8% of the oven’s air volume depending on plant conditions (shipping doors opening and closing, etc.). We have air return soft seal ducting from the part openings back to the burner box (roof-mounted) and have also tried a hard seal, but the hard seal made things worse. We are located about 55 miles north of Miami and at times the shop gets to be 110°F with parts running through, etc. The majority of our parts are aluminum.

I have spoken with a variety of industry people and have been told heat will leave the oven no matter how hard we try to contain it due to the expanded volume and plant conditions such as the shipping doors mentioned above. We installed a 48-inch tube axial fan in the roof above the cure oven section entrance/exit, but my concern is we will actually suck additional hot air out of the oven.

Can you please recommend something? The costs of this are staggering! Thanks in advance. J.T.

A. It is true that air expands when heated and some hot air will migrate out of the openings. The recirculation heat seal that you mention is not effective because it does nothing to relieve pressure in the oven. A typical powered air seal does not help much either, especially with a big part opening. In addition, an air seal uses a fairly large electric motor that costs money to run.

You have two concerns: the heat gain in the building and the loss of energy. The best option for heat containment is an extended vestibule. Heated air migrates out of the opening into the vestibule and mixes with cooler air. Cooler air is drawn back to the oven to make up for oven exhaust. An extension of the vestibule to around 10 ft can do a lot to keep the heat in the oven. This reduces the heat gain in the building and reduces energy losses. The only problem is space. Most systems do not have enough space to extend the vestibule to 10 ft. Extend it as much as you can as the first step in solving your problem. The vestibule does not have to be insulated. It can be built of galvanized sheet metal.

It is often not possible to create enough vestibule length. Therefore, you may need to add heat relief hoods to the oven openings. A heat relief hood is a canopy that extends around 4 ft from the oven entrance and includes a stack at the top that goes out of the roof to allow expanded air to exit the building. It should not have a fan. It relies on normal warm air updraft. You can see a picture of what it looks like in the Chemical Coaters Association manual on Systems Design.

Your fan in the roof is not a bad idea. It will get rid of heat and it does not have an impact on how much heat leaves the oven. It does need supply to be effective so if the dock doors are not open and the building has negative air pressure you will need an air supply system to provide the necessary air turn over. Negative air pressure will also limit the effectiveness of the heat relief hood, so the supply air really helps with air turn over.