by Marc Ward
Clay Times Feb.1997
In recent days I've received two questions about forced air burners. One deals with operation, while the other deals with construction of forced air burners. Here's the questions and answers;
Q. Do you carry a blower that puts out around 350 cfm? I'm building some burners that will be around 340,000 BTU each. I know that I need 10 cubic of air per thousand BTU.
A. This question illustrates a common mathematical error that most people make when trying to figure out what size blower they need for their homemade burners. Now, not being cute here, but most folks don't even go as far as this. Many times it's just guess work. Potter's don't guess about how much cobalt goes in that cool blue glaze. They measure with a gram scale. They don't guess about what temperature to fire to. They use cones. They don't guess about when the deadline is to get slides into the jury. It's circled in red on their calendar. Why then, when it comes to combustion equipment, do potters just guess and hope for the best? The person that asked this question wasn't just guessing and came up with the number of 10 cubic feet of air for every 1000 BTU's. They got the answer by reading Fred Olsen"s "The Kiln Book". It's a good book that should be in every potter's personal library. The mistake they made was a simple one.....BTU's are expressed as a per hour figure while air flow is based on CFM (cubic feet per MINUTE). They looked at the figure 340,000 and thought, "well, there are 340 thousands so I need 340 or so from my blower. Remember, it's 10 cubic feet of air PER 1000 BTU's, so you really need 10 times the 340 or 3400. But wait,.... that 3400 is per hour and blowers are rated in cubic feet per minute. Simple. Divide your number of 3400 by the minutes in an hour: 60. Bingo! Your answer is 56.6 CFM. Your basic squirrel cage blowers come in the following flavors; 60, 100, 140. If you're within 10% of using all your blowers capacity, step up to the next size. In our example, you would step up to the 100 CFM blower. The 60 CFM blower might not have enough "umph" if you have considerable back pressure, but with the 100 CFM, you now have a little more air than you need. This brings us to our next question;
Q. I'm able to get a good blue flame on my burners. But my temperature doesn't go up as fast as I want it to. How do I get more gas if my burners are already turned up full blast?
A. This is one of those things that takes practice. Most folks assume that a good blue flame is indicative of a good flame. Maybe yes, maybe no. With forced air, a loud blue flame might also be a sign of a lean flame (too much air in the mix). This kind of flame can be as, or more, wasteful than a reduction flame. In essence, you are cooling the flame down by spilling excess air. You could be wasting as much as 30% of your gas. That's a lot. 30% higher fuel bills. 30% longer firings. 30% better chance you'll miss dinner with your family. So if you have a blower that's oversized, which you might need, be careful. If you have a digital pyrometer, you can see what's happening (temperature wise) almost instantaneously when you adjust the burner. Here's what to look for...or rather what to listen for; There is a point in a forced air burner where the sound is loud and rumblely. When you turn up the gas or turn down the air, it all of a sudden gets quieter. That's the point when you start to enter reduction. If you keep your burners set at just the point before it gets quiet,....not as loud, but not so quiet, you'll be as close to a good ratio burn as you can get without an oxyprobe. I have yet to talk to a potter that doesn't say something like, "an oxyprobe is the best money I've ever spent". You may have spent years firing by the "seat of your pants" and think you have it down, but analytical tools might show you different.
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