Winning the Fan Shell Game
I was about eight years old when the carnival came to town and set up in the field behind our street. Well, OK, it was billed as a circus, but it was short on rings and long on games of chance, freak shows and other hustles.
It was my first exposure to the shell game, the one where the guy puts a pea under one of three walnut shells, shuffles them around and lets you guess (for a fee, naturally) which shell hides the pea. The game is fixed, of course, and after a couple of easy guesses, you're lured into putting down a bigger bet. Suddenly, the pea isn't where you expected. How could that be? I'm sure I kept my eye on the right shell.
I had the same feeling when I first got into oven and dryer heating systems years ago. Having learned the trade at a company dealing mostly with high temperature metallurgical and ceramic heating applications, I was conditioned to expect only one fan -- the combustion air blower -- on the furnace or kiln. Suddenly, I was staring at an oven with three fans and trying to figure out where the air was as it shuttled from one fan to the other.
Turns out I wasn't the only one -- there are a lot of others who struggle with all those fans, what they're doing and how they interact. Maybe its time to expose the "Fan Shell Game."
Still baffled? Take a Post-it Note, pretend it's the oven enclosure, and stick it over the recirculation fan and loop in the sketch. That's what you're dealing with -- as far as flow into and out of the oven is concerned, the recirculating fan doesn't exist.
Now we're down to two shells. Tracking the flow through the oven ought to be a little easier to understand now. To keep things simple, I'll just refer to flows in cfm, but I mean acfm (actual cubic feet per minute) -- air volume as measured at the conditions present, at that moment, in the fan, ductwork or oven chamber. I'll start with the exhaust fan because whatever that fan takes out of the oven, you have to replace from one source or another.
Say the exhaust fan removes 10,000 cfm of air, spent combustion products and water vapor from a drying oven. The burner contributes 7,500 cfm of this total. Water vapor from the load adds another 100, for a total of 7,600 cfm. If the oven has been designed properly, the other 2,400 enters through fresh air inlets, probably to the heater box, where it can mix thoroughly with the burner products.
What if the oven has undersized fresh air inlets that let in only 2,000 cfm? The shortfall, 400 cfm, will find its way in via some other route, like entrance and exit openings on a continuous oven or past door seals or other gaps on a batch oven. Cold spots inside the oven door or the gales blowing into both ends of the tunnel might just be the exhaust fan doing too good a job. Consider throttling it back a bit.
How about the opposite situation -- hot air rushing out of every opening in the oven? If the tunnel is open, some spillage probably is unavoidable, but if it seems excessive, the exhaust fan may be unable to remove all the cfm the burner system is putting in. It might be undersized, or maybe its damper simply isn't open far enough.
Then there's the split situation: Everything's fine with the burner at high fire, but cold air gets drawn in when the burner cycles to low -- or the opposite, all's well at low fire, but the oven gets a case of Hot Breath with the burner at high. Maybe the exhaust fan volume isn't synchronized with the burner input. That's inevitable if there are fixed exhaust dampers, but if the damper is motorized, it should be adjusted to track the burner input more closely.
That's about all there is to it. And see? I didn't mention the recirculating fan once.