Years back, I received a call from a frustrated process engineer who wasn’t able to get his combustion system to perform. It went something like this:

“When the original burner wouldn’t get the product hot enough, we replaced it with one twice as large, but it still can’t do the job. What’s wrong?”

“OK, what are you trying to heat?”

“Tungsten wire. We have to heat it so we can draw it down to a smaller diameter.”

“And what temperature do you need for that?”

“Oh, about 5,000oF.”

“And you’re using natural gas and air?”

“Yup.”

“I think I see your problem. Your flame temperature is less than 3,500oF. You’re in for a long wait.”

Maybe it’s the Fireplace Mentality. Not warm enough? Make the fire bigger. That’s fine for relatively low temperature processes, but when you get to extremely high temperatures, you need a hotter flame. Otherwise, you can build a fire big enough to burn the place down, and the product will still be standing, unchanged, amid the ashes.

Everything Else Must Be Bigger Too

The plant manager ticked off all the equipment he had ordered to add 20 percent to the capacity of his dryer -- bigger burner, bigger air and fuel valves, bigger exhaust fan. I looked at the specs of the new fan -- 20 percent more flow capacity, all right…at about the same static pressure as the old one. When I told him the fan pressure wasn't high enough to pull all that extra air through the dryer, he looked positively sick. The Square Root Law, I explained -- pressure drop through all components of the system increases with the square of the flow ratio. If you want 20 percent more flow, you need 44 percent more pressure from the fan. What followed was a day of clambering all over a multi-story dryer, collecting dimensions and pressure readings to figure how to get the added flow without tearing everything down and starting over. Fortunately, the basic dryer was very generously sized, and I were able to identify some choke points in the ductwork that could be enlarged without breaking the budget and the manager's career.

The lesson is simple. If you want to increase the flow through anything, be ready to deal with a pressure drop that will increase with the square of the flow ratio.

Ten Pounds In a Five Pound Bag

It seemed logical. Increase the amount of material loaded into the furnace in the morning, so there'd be less labor required and time consumed to keep it filled throughout the day.

Indeed, they were right. By taking a few additional minutes to stack up extra work in the furnace in the morning, they could run longer before having to stop and refill it. It was a lot more pleasant on the help, too, because the furnace was cool at the start of the shift.

Everyone realized it would take longer to get the furnace up to temperature -- after all, it carried a heavier load -- but they could compensate by starting earlier. What came as a shock, though, was the realization that they had to stop production several times during the day because the pieces coming out of the furnace weren't hot enough, and many of them were not uniformly heated. So, the crew would take periodic breaks to let the furnace catch up. When the total labor time was added up, the breaks took longer than the one-time loading practice had saved. If that didn't clinch the matter, the next fuel bill did.

It's not that hard to understand. Furnace and oven designers allow all that free space between workpieces for a reason: heat transfer. Close up the spaces, and you block the pathways convection and radiation need to do their work.

Another Overloaded Bag

Nobody knew exactly what it was, but the heating system was putting out an unpleasant smell. A few at a time, customers would drift out of the building for a little fresh air, instead of staying inside and spending their money. The building was new, so there were plenty of suspects -- fresh paint, pipe leaks of some sort -- but none of these panned out.

Eventually, we determined the problem was in the gas-fired makeup air system. On opening the ductwork, we found an air-conditioning coil just a couple of feet downstream of the burner, upsetting airflow through the system. Some of the air leaving the burner recirculated back into the flame, generating aldehydes. It turned out the equipment manufacturer had plenty of experience with steam- and electrically heated systems, but this was their first venture with gas. They didn't realize more space was needed downstream of the burner to allow proper combustion and blending with the heating air.

The purpose of this column isn't to poke fun at the people who stumbled into these problems. They're all intelligent, well-meaning individuals who goofed. We all do. The lesson here is that it's a good idea to bounce your ideas of someone else with some expertise in the area. They might catch a problem before the metal gets cut.

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