Combustion expert Dick Bennett shares some calculation shortcuts. If the need arises, you can scribble them on your palms.

At a meeting last week, I noticed one of the participants had written some numbers on the palm of his hand. I guess we've all done it at one time or another -- an important bit of process data, a phone number or meeting time to remember, and no paper to write it on. So, we use ourselves as memo pads.

While teaching seminars and training schools over the years, I've also noticed that many people are unfamiliar with a lot of the handy little shortcuts that heat processing pros take for granted. So it's time to share some of them with you. If the need arises, you can scribble them on your palms.

Handy Little Shortcut #1: One cubic foot of air can release about 100 BTU from most common hydrocarbon fuels. Knowing this lets you quickly figure the BTU potential of a burner system, if you know the capacity of the combustion air fan. For example, if the fan's rated at 10,000 scf/hr, BTU/hr capacity will be about 100 times that, or 1 million BTU/hr. Conversely, if you know the burner's BTU/hr rating, you can estimate the fan's capacity. The burner nameplate says 2,500,000 BTU/hr? As long as the fan is its only source of combustion air, you can figure it's good for at least 25,000 scf/hr.

Actually, with natural or LP gas, you can expect between 105 and 110 BTU per cubic foot of air, but since most burner systems are set up to run with around 10% excess air at high fire, using the 100 rule puts you pretty close to the mark.

Handy Little Shortcut #2a: For carrying ambient air (such as burner combustion air), the maximum scf/hr a pipe should handle equals 1,000 times the square of its nominal size in inches. For example, a 3" pipe should be limited to 3 x 3 x 1,000 scf/hr air, or 9,000 scf/hr max. This guideline does not apply to hot air and recirculating ductwork, so please don't try to use it there.

Handy Little Shortcut #2b: For natural gas, use Shortcut 2a, but multiply the square of the pipe size by 1,300 scf/hr.

Handy Little Shortcut #3: To convert oC to oF in your head, multiply the Celsius temperature by two, subtract 10% of what you get, and add 32. (My thanks to an old friend in the aluminum business for this one.) Try it with 350oC. First, double it to 700, then subtract 10% of that (70) to get 630. Finally, add 32 to come out with 662oF -- right on the money.

Going from oF to oC, unfortunately, isn't as simple. The mental gymnastics may be more trouble than they're worth, but here they are anyway. The result will be accurate within a degree. First, subtract 32 from the Fahrenheit temperature. Add 10% to the number you get and then add 1% to the new result. Last, divide by 2 to get oC. Try it, using 750oF as a start. 750 minus 32 equals 718. Add 10%, which is 72 in round numbers, to get 790. Add 1% to that number -- 8, in round figures, to arrive at 798. Now, divide by 2 to get 399oC, which compares pretty well with the actual figure of 398.9oC.

Handy Little Shortcut #4a: To convert percent excess air to air/fuel ratio, you first need to know what the stoichiometric (correct) air/fuel ratio should be. (I'll use 10:1, which is a good average for natural gas.) Add 100 to the percent excess air, then divide the result by 100. Multiply that number by the stoichiometric ratio. Let's run through it with an excess air level of 650%. Add 100 to get 750, divide by 100 to get 7.5, and then multiply this by 10. Voila! 75 to 1 air/gas ratio.

Handy Little Shortcut #4b: Want to go from air/gas ratio to percent excess air instead? OK -- start with the actual air/gas ratio, subtract the stoichiometric amount from it, divide by the stoichiometric ratio, and then multiply the result by 100. Try it using natural gas and a 45:1 air/gas ratio. 45 minus 10 is 35, divide by 10 to get 3.5. Multiply by 100, and you get 350% excess air.

Don't simply multiply by 10 to shorten the last two steps -- fuels with different stoichiometric proportions won't work out if you do. Take propane (25:1 stoichiometric ratio) for example, and the same 45:1 ratio. 45 minus 25 is 20; 20 divided by 25 equals 0.8, which we multiply by 100 to get 80% excess air. If you had just multiplied by 10, you would have gotten 200% excess air, which is incorrect.