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or lean (excess air). Either way, it wastes fuel. Because there is not enough air for complete combustion, oper- ating the burners at rich combustion conditions wastes fuel by allowing it to be discarded with some of its energy unused. It also generates large amounts of carbon monoxide (CO) and unburned hydrocarbons (UHCs). At first glance, operating lean might seem to be a better proposition because all the fuel is consumed. Indeed, a lean operation produces no flammable, toxic by-products of rich combustion, but it does waste energy. Excess air has two effects on the combustion process. First, it lowers the flame temperature by diluting the combustion gases, in much the same way cold water added to hot produces warm water. This lowers the tem- perature differential between the hot combustion gases and the furnace and load, which makes heat transfer less efficient. More damaging, however, is the increased volume of gases that are exhausted from the process. The products of stoichiometric combustion and the excess are at the same temperature. The excess air becomes one more competitor for the energy demand in the process. Because this is part of the combustion process, excess air goes to the head of the line, taking its share of the heat before the furnace and its contents. The results can be dramatic. In a process operating at 2,000°F, available heat at stoichiometric ratio is about 45% (55% goes out the stack). Allowing just 20% excess air into the process (roughly a 12-to-1 ratio for natu- ral gas) reduces the available heat to 38%. Now, 62% of the total heat input goes out the stack, the difference being carried away by that relatively small amount of excess air. To maintain the same temperatures and pro- duction rates in the furnace, 18% more fuel must be burned. Air infiltration. Excess air does not necessarily enter the furnace as part of the combustion air supply. It can also infil- trate from the surrounding room if there is a negative pres- sure in the furnace. Because of the draft effect of hot furnace stacks, negative pressures are fairly common, and cold air slips past leaky door seals and other openings in the furnace. Figure 6 illustrates air infiltration from outside the furnace. Once in the furnace, air absorbs precious heat from the com- bustion system and carries it out the stack, lowering the fur- nace efficiency. A furnace pressure control system may be an effective way to deal with this. See the ITP tip sheet, "Reduce Air Infiltration in Furnaces,” for guidelines on esti- mating infiltration losses.1 The bottom line is that to get the best possible energy efficiency from furnaces and ovens, reduce the amount of energy carried out by the exhaust and lost to heat storage, wall conduction, conveying and cooling systems and radiation. Furnace scheduling and loading A commonly overlooked factor in energy efficiency is scheduling and loading of the furnace. “Loading” refers to the amount of material processed through the furnace or oven in a given period of time. It can have a significant effect on the furnace's energy consumption when measured as energy used per unit of production, for example, in British thermal units per pound (Btu/lb). Certain furnace losses (wall, storage, conveyor and radiation) are essentially constant regardless of production volume; therefore, at reduced throughputs, each unit of production has to carry a higher burden of these fixed losses. Flue gas losses, on the other hand, are variable and tend to increase gradually with production volume. If the furnace is pushed past its design rating, flue gas losses increase more rapidly, because the furnace must be operated at a higher temperature than normal to keep up with production. 1 The tip sheet on Air Infiltration was in development at the time of this publication. The tip sheet will be available online on the ITP BestPractices Web site at www.eere.energy.gov/industry/bestpractices. Figure 6. Air infiltration from furnace opening. Flue H Air Leak 4 BestPractices Technical Brief Waste Heat Reduction and Recovery for Improving Furnace Efficiency, Productivity, and Emissions PerformancePDF Image | Waste Heat Reduction and Recovery for Improving Furnace Efficiency, Productivity and Emissions Performance
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