OK, I know what you're thinking -- the old boy's been standing over the exhaust stack too long. Fun with available heat? I've got more exciting things to do -- like watching paint dry.
But, wait a minute. The old available heat chart can tell you a lot more than just the efficiency of the combustion process. You just gotta know how to look at it.
Fun Thing #1. Predicting Immersion Tube Exhaust Temperatures. Say you're about to install a parts washer, and you want to know how hot the tube stack will be at the point where it will pass out through the roof. To find out, all you need to know is the tube's thermal efficiency and the percent excess air at which the burner will operate. (If in doubt, use 10%.) Say the tube efficiency is 70% -- that's a common rating. Start at 70% on the left-hand side of figure 1, read across to the 10% excess air line, and down to the exhaust temperature, which, in this case, is about 870oF (466oC). This number is the highest it can be -- at the roof line, it may actually be a little cooler because the stack will lose some heat along its length.
By the way, this only works on immersion tubes -- the efficiency of an immersion tube ignores tank wall, evaporation and other losses. It's also okay for a rough estimate of boiler stack temperatures (you'll be a little low on the temperature). But, don't use it on ovens and furnaces -- it just doesn't work very well for them. Can't have it all.
Fun Thing #2. Estimating Burner Flame Temperature. Actually, you can measure the temperature of the mixture of hot combustion products just past the flame tip. Start with the percentage excess air at which the burner will operate -- let's use 250%. Follow that excess air line down to the bottom of figure 1, where available heat equals zero. That's the combustion gas temperature: 1,300oF (704oC) in this case.
Fun Thing #3. Predicting Fuel Savings. That old clunk of a burner on your boiler needs replacement -- unless it's run with at least 50% excess air, it soots up and generates lots of carbon monoxide. The burner rep says his modern replacement can operate cleanly at only 10% excess air. In addition to all the other benefits, how much fuel can you expect to save, assuming the boiler stack temperature remains at 700oF (371oC)?
Go to 700oF on the bottom of figure 1, read up to the 50% excess air line and then across to the available heat, which is 68%. Do it again, but this time, use the 10% excess air line. Available heat will be 74% with the new burner. Figure your fuel savings with this equation:
% Savings = 100 x [1 - (AHb / AHa)]
where AHb is the available heat before the change was made, and AHa is the available heat after the change.
Plugging in the numbers from this scenario, we get:
% Savings = 100 x [1 - (68 /74)] = 8.1%
Remember, this saving applies only for the time the boiler is operating at high fire. At reduced firing rates, the stack temperature and percentage excess air are likely to be different.
Fun Thing #4. Comparing the Effect of Excess Air on Fuel Savings at Different Temperatures. Okay, I'm reaching for this one, but pretend you're curious about what effect the same change in excess air would have on fuel economy at a higher process exhaust temperature -- 2,000oF (1,093oC), for example. Go back to the chart, and you'll find that available heat is 25% at 50% excess air. That's AHb. At 10% excess air, AHa is 40%. Plug 'em into the equation, and it goes like this:
% Savings = 100 x [1 - (25 /40)] = 38%
Which just verifies something most people have known for a long time -- the higher the process temperature, the more important it is to control ratios closely.