Heat Recovery

Q. Is there a way to take the waste heat that goes up the cure oven stack and reuse it in the dry-off oven? D.A.

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A. This is a question that is asked a lot and not well understood by many people. First, make sure that the exhaust volume is correct. You should be exhausting a certain amount of air in cubic feet per minute (CFM) or meters per minute (CMM) based on the pounds per hour of powder. In addition, you need some air for the by-products of combustion. An average cure oven may exhaust between 1,500 and 3,000 CFM (45 to 85 CMM). Of course, there are large ovens that could exhaust much more if the volume demands it but this is an average system in the 6–10 FPM range (2–3 MPM). At a stack gas temperature of 400°F (200°C) 3,000 CFM of air would contain a little over 1,000,000 Btu/h. If the cost of natural gas is $0.50 per hundred cu ft, the cost of the gas going up the stack is $5/hour.

You have to maintain some stack temperature to avoid condensation inside the stack, so you could potentially recover around 50% of the loss or around $2.50/hour. But that exhaust is laden with gas and you cannot dump it straight into the dry-off oven. It will have to go through a heat exchanger, and that will rob another 25–30% of your “waste” heat. Now you are down to saving around $2 per hour and you do not have enough Btu to dry your parts, so you still need additional heat in the dry-off oven.

The heat exchanger, fans, duct, dampers and controls cost you around $40,000, so you need a lot of payback. If the savings is $2 an hour, the payback is 20,000 hours, or around 10 years. You will also need to pay to run motors to drive the system. And that equipment will not last forever or be efficient without maintenance. Some clever manufacturer may have found a way around some of this, but I am not aware of anyone who has made it work. If you want to save money on drying, you should use more air and time and less gas. Also, be sure your insulation is thick enough. Many ovens bleed a lot of heat at the seams and around the burner box. Adding 2 inches of insulation will save a lot of money with a minor investment and no added operating cost. Look at heat loss at the oven entrance, too. An extended vestibule will limit heat roll out and improve efficiency.