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Smoky Oven

When I run a lot of powder coated metal through my straight shot oven, the building begins to fill with smoke. The building has a negative airflow. I know this because when I open the doors I feel a lot of air being pulled in. Do you know of a quick fix or maybe an airflow specialist who could help? Or could it simply be an exhaust problem?

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Q. I work for a sheet metal job shop. When I run a lot of powder coated metal through my straight shot oven, the building begins to fill with smoke. I run mainly TGIC and hybrid powders. I also run different metals, mainly galvanized and galvanneal. The building has a negative airflow. I know this because when I open the doors I feel a lot of air being pulled in. The problem is much worse in the winter than in the summer.

Do you know of a quick fix or maybe an airflow specialist who could help? Or could it simply be an exhaust problem?

 

A. You have some understanding of the causes of your problem already. Negative air within the plant creates back-pressure on the exhaust system and reduces the draw. It also makes it hard to maintain the negative pressure in the oven so it is easier for heated, expanded air to migrate from the openings. If the pressure on one side of the oven is greater than the pressure on the other side, the air can tunnel right through from one side to the other and carry gases into the plant, especially in a straight pass oven. It is also possible that your exhaust is too low relative to the volume of powder you are running through the oven. And it may be that the length of the vestibule is too short or you do not have any proper heat containment at the oven ends.
So, what to do? First, find out what your oven exhaust volume is. You may have the number on a print. If you do not have the air volume of exhaust in cubic feet per min (CFM), you will need to measure it. Start by measuring the area of the oven openings (height × width) to determine the square feet of opening. Next, use a velometer to determine the air speed in feet per min (FPM). The measurements across the openings will be different so you will need to take several to determine an average. Multiply the air speed times the area and you will have a rough idea of the air volume. The recommended volume is based on this formula:

  1. Powder in lb/hr × 9% × 360 ft3 × 4 × 60 min = CFM
  2. Multiply this number times 1.62 to correct for temperature of 400°F.

This tells you what your exhaust should be and you can then compare that with the actual exhaust and see if you need to speed up the fan and get more exhaust.

The negative air issue is harder. If you can install the correct volume of make-up air, that would be best. Air supply will provide the best function and most reliable containment of air in the oven and it is also the lowest-cost method of heating the building.

If the company is unwilling to invest in air make-up, you can do a better job of containment by extending the vestibules to create more cool air buffer between the heated area of the oven and the outside, add a heat-relief hood above the oven openings or add air curtains. Any or all of these features should fix the problem.

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