Excess Air: Is it Such a Big Deal?
Setting up combustion systems for too much excess air (or dilution air) wastes fuel. Here's how to know if you are wasting natural gas in your industrial oven.
% Fuel Savings = 100 x [1- (Available Heat, High XS Air/Available Heat, Low XS Air)]
If you know your oven or furnace exhaust or flue gas temperature and the amount of excess air you're operating at (you can determine this with an O2reading), you can calculate the savings resulting from judiciously trimming back excess air. The Available Heat Chart will give you the numbers you need.
For example, suppose your process has an exhaust temperature of 1,400 oF (760oC), and you measure 25 percent excess air in the stack. You know you're able to operate satisfactorily with only 10 percent excess air. How much can you save? From the chart, your present available heat is 52 percent, and lowering the excess air to 10 percent will kick it up to 57 percent.
Your savings? 100 x [1- (52/57)] = 8.8 percent. Not bad for a simple tweak of the ratio controls.
If you look at the Available Heat Chart, you'll notice the curves for various levels of excess air are widely spread at high exhaust temperatures but tend to converge on a point at lower exhaust temperatures. This means any deviation from the optimum excess air level extracts a heavier efficiency penalty as exhaust temperatures increase. The other obvious thing about this chart is that at low exhaust temperatures typical of many ovens and dryers, available heat levels are pretty high until excess air reaches about 400 percent. This raises the question whether excess air control is really worth the effort on equipment operating with relatively low temperatures. Is there a payoff? Obviously, it will vary from one installation to another, but here are some numbers from real-life installations to give you a feel for what's possible.
A drying/baking oven was found to be operating at an excess air level of 300 percent with a stack exhaust temperature of 450oF (232oC). On first flush, that didn't sound too bad, but research into operating records revealed the process was originally designed to run with about 100 percent excess air. Restoring excess air to its original level will raise available heat from 55 percent to 72 percent, lowering gas consumption by 24 percent. On another oven, the 350oF (177oC) exhaust contained 400 percent excess air, which could be lowered to 150 percent without any harm. Savings from this adjustment amounted to 25 percent of the original fuel consumption.
So even at low operating temperatures, getting excess or dilution air under control can pay handsomely.
Before you rush out to the floor, wrench and screwdriver in hand, find out what your excess air ought to be. The equipment manuals may have this information, though it's more likely they will simply contain air, gas and control valve setting data, on the assumption these settings will give you the right air-gas ratio. If you can't locate this information, either from the manuals or by contacting the equipment supplier, you're going to have to experiment.
There are a couple of very important things to deal with first. If your process evolves flammable vapors, such as solvents from finishes, the minimum allowable excess air will be determined by the dilution requirements of the vapor. Typically, you need to provide enough dilution air to reduce the vapor concentration to no more than 25 percent of its lower explosive limit (LEL), unless you have continuous solvent monitoring instruments, in which case, 50 percent of the LEL is permitted. If the rate of vapor evolution isn't constant, such as in a batch oven, the ventilation volume has to be sufficient for the worst possible conditions. Don't guess on this! An explosion or fire could be the payoff for getting too ambitious with your adjustments. Safety considerations always trump fuel savings.
If your product evolves water during drying or curing, be sure not to cut the dilution or excess air so low that the humidity in the dryer becomes excessive. Otherwise, you'll have to slow down the process to avoid having under-dried product.
Suppose you can't find any hard data on how low to take your excess air, but you're either operating well below 25 percent of the LEL (double check!) or don't have any volatiles to worry about. What now?
Begin by taking an O2reading in the stack and reduce the secondary airflow a bit. Read the O2again, let the process stabilize and monitor product characteristics like moisture content, weight loss or whatever is a key indicator of process quality. If everything looks OK, tweak the air again and repeat the process. Keep adjusting and checking until the product begins to show some signs of change. When you reach that point, back up to your previous setting to give yourself a cushion against the effects of atmospheric variations and between-maintenance deterioration.
Will this take some time? Absolutely. But isn't a 20 or 25 percent, or even 10 percent, reduction in fuel consumption worth it?