When energy costs become high or when environmental concerns get a lot of press, government and industry renew their vows to energy conservation. Articles and seminars on saving energy regain fashion, and everybody seizes on some hot new technique for keeping fuel consumption to a minimum. Some of those techniques are superb; others are just plain goofy. Unfortunately, though they eventually get debunked, the goofy ones seem to resurface with each resurgence of energy awareness. Nothing seems to have the staying power of a really bad idea.
I may be mistaken, but one of those times may be approaching. Call it a gut feeling. So, to keep you from getting roped in by one of the perennially bad ideas in energy conservation, I'm going to take a look at excess air in ovens.
Mention excess air in the context of a gas-fired oven, and most people immediately think of burner air-to-gas ratios. There is an ideal ratio of combustion air to gas (about 10 to 1 for natural gas, 25 to 1 for propane). If you mix combustion air and gas at the ideal proportions, flame temperatures and combustion efficiency (also known as available heat) will be at their peak values. If you operate the combustion system with more than the ideal amount of air, flame temperatures and combustion efficiency decrease. And, the more excess air you put into the system, the lower they go.
Armed with this knowledge, people check their ovens' exhaust and go into shock when they find them operating with six to 20 times the air needed to burn the fuel. Their immediate reaction is to figure out how to get rid of some or all of that energy-wasting excess air.
Time OutExcess air has more purposes than controlling flame temperatures and combustion efficiency. If you ignore those other purposes, you risk messing up the productivity of the oven, perhaps damaging it or, worst of all, blowing it up.
So, before you reach for the handle on the air controller, understand all the things excess air does in an oven:
- First, it tempers the burner flame. The core of the burner flame is around 3,000oF (1,649oC) -- far too high for the oven and its contents to withstand. Try to use those hot combustion gases as is, and you'll reduce the oven and its contents to a smoking heap. Excess air is added for the same reason you add cold water to the hot in the bathtub.
- Second, excess air carries the heat from the flame to the work. In a convection oven, the burner is located remotely from the load, so there's no direct radiation. The heat has to be transported by something, and that something is the excess air.
- Third, it supports heat transfer to the load. Convection heat transfer, which is what you depend on in these ovens, is enhanced by having an ample supply of heated air impinging on and passing over the load. Excess air bulks out that supply, helping you achieve high heat transfer rates and high productivity.
- Fourth, excess air is a mass transfer medium. If you heat a product to drive moisture or solvents out of it but don't provide a steady stream of moving air, the moisture or vapors simply will collect in the oven enclosure, slowing down further evaporation and with it, production. The evaporated materials must be moved out as quickly as they form.
- Fifth, on all ovens evaporating flammable solvents, large amounts of excess air are needed to prevent an accidental explosion. If the solvent vapors build up to a concentration exceeding their lower flammable limit, the oven becomes a bomb. Add a source of ignition (a burner will do nicely), and an explosion definitely will occur. Regulations require that the solvent vapors be diluted to 25% of their minimum flammable concentration (50% if the oven has a continuous solvent monitor). That adds up to a lot of excess air.