It may be possible to salvage some of the energy in the exhaust by using it to preheat incoming parts.


The first three installments of this topic outlined the steps you should take in drawing the best possible energy efficiency from your process heating equipment. To recap, they are:

1. Get the best performance you can from your existing equipment.

2. Investigate upgrades or improvements to your existing equipment to increase its efficiency.

3. Investigate more radical ways to improve energy efficiency, including replacement of the existing equipment.

OK, suppose you’ve done or looked at all these things and have come to the conclusion they just don’t deliver the efficiency you need. This might not be due to obsolescence or any fundamental shortcomings in your ovens or furnaces -- perhaps your product’s cost structure simply needs a greater cut in its energy cost component. What now?

This is where you begin to look at your heating equipment, and the way you use it, in a different light. In other words, it’s time to start with a clean sheet of paper. Here are some areas to investigate.

Do You Have the Right Equipment for the Job? If you’re now using batch-type ovens or furnaces to process material, would a switch to a continuous line make sense? Batch equipment is great when you’re handling small lots of material with widely varying heat processing requirements, but if everything is about the same size and weight, is produced at a relatively steady pace and takes a standard heating-cooling cycle, a single continuous heating line may let you do the job with a lot less energy.

Conversely, if running a lot of cats-and-dogs jobs through a continuous line forces you to continually reset temperatures and heating schedules and then hold the oven empty till the new settings take effect, you might be better off with several batch ovens.

Don’t Let That Heat Get Away. In most process flow layouts, placement of ovens and furnaces is a secondary decision (“If the spray booth is here, the oven has to be there…”). With energy costs running amok, it may be time to revisit the layout with an eye toward making it as energy efficient as possible. Do you have a product that gets heated more than once in its passage through the plant? Does it do nothing but cool down between heating cycles, simply because of distance or transit time between the furnaces? A basket or skid full of cooling parts, sitting in a corner or on a conveyor, can be a substantial waste of energy. Is there any way you can relocate the process heating equipment so the product leaves one line hot and goes directly into the other? If there’s some intermediate processing step between the heating cycles, can it be done to a hot part, or can it be shifted to some other point in the process flow?

This may require relocating equipment and production lines, but given the way many plants expand over the years, the existing layout may be due more to a past reluctance to move equipment where it should have been in the first place.

Squeezing More out of the Exhaust. I’ve always admired the tunnel kilns used in the clay products industry. These continuous furnaces operate at maximum temperatures 2,000oF (1,093oC) and higher, but their exhaust gases aren’t tossed out the stack til they’re down in the 600oF (316oC) range. The result is high thermal efficiency in spite of high operating temperatures. How do they do it? By forcing the combustion gases from the high temperature areas to pass counterflow over cooler incoming product. As they do, they transfer heat to the product, bringing it partway up to full processing temperature. The beauty of this is that as the gases cool, they continue to encounter cooler and cooler product, so heat transfer continues. And all this is being done with so-called “waste gases.” If you have a furnace or oven that operates at moderate to high temperatures, investigate design modifications that would route some or all of the exhaust over the cold incoming product. You may be pleasantly surprised at the benefits it can bring.

Cascading Heat. Sometimes it’s impractical or impossible to use the exhaust gases to preheat product as described above. In those cases, it may still be possible to salvage some of the energy in the exhaust by cascading it to a lower temperature process. This requires a little more effort -- for one thing, the donor and recipient heating equipment must be situated fairly close to each other so large amounts of heat are not lost during transfer. Second, the two pieces of equipment have to operate on compatible cycles. You don’t want to be forced into running Furnace 1 just to keep Furnace 2 going, and Furnace 2 should be operating frequently enough to make good use of #1’s exhaust. If you can’t get a perfect matchup of operating cycles, all’s not lost -- perhaps an auxiliary heater on #2 can be used to fill the gaps.

Don’t have a home for the waste heat, you say? Don’t limit your thinking to another piece of processing equipment. Maybe you can use the waste heat for comfort or water heating.

The point of this column is to prompt you to use your imagination in extracting more out of your energy expenditures. Not all of the suggestions may work for you, but these aren’t the only possibilities. Don’t be constrained by what’s been done, or not done, in the past. Times like these require bold thinking.    PH

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