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The most potent way is to closely control fuel-air ratios. Operating the furnace near the optimum fuel-air ratio for the process also controls fuel consumption. The best part is that it can usually be done with the existing control equipment. All that is required is a little maintenance attention. The ITP tip sheet "Check Burner Air- Fuel Ratios" will provide a useful chart for figuring exhaust gas losses and shows how to figure the efficiency improvements that can come from controlling ratios more closely.3 Some reduction in exhaust volumes will be the indi- rect result of efficiencies applied elsewhere. As mentioned above, flue gas losses are a fixed per- centage of the total heat input to the furnace. As shown in Figure 8, any reduction in heat storage, wall, conveyor or radiation losses will be multiplied by the available heat factor. For example, on a fur- nace operating at 50% available heat (50% exhaust gas loss), lowering wall losses by 100,000 Btu per hour (Btu/hr) will permit a firing rate reduction of 200,000 Btu/hr. That is 100,000 Btu/hr for the wall loss and 100,000 Btu/hr for the accompanying exhaust gas loss. Use of oxygen enriched combustion air. Ambient air contains approximately 21% oxygen with nitrogen and other inert gases as balance. The total volume of exhaust gases could be reduced by increasing the oxygen content of combustion air, either by mixing in ambient air or by using 100% oxygen. Reducing exhaust gases would result in substantial fuel savings. The exact amount of energy savings depends on the percentage of oxygen in combustion air and the flue gas temperature. Higher values of oxygen and flue gas temperature offer higher fuel savings. Obviously, the fuel savings would have to be compared to the cost of oxygen to esti- mate actual economic benefits. Waste heat recovery. Reducing exhaust losses should always be the first step in a well-planned energy conser- vation program. Once that goal has been met, consider the next level – waste heat recovery. Waste heat recovery elevates furnace efficiency to higher levels, because it extracts energy from the exhaust gases and recycles it to the process. Significant efficiency improvements can be made even on furnaces that operate with properly tuned ratio and temperature controls. There are four widely used methods: 1. Direct heat recovery to the product. If exhaust gases leaving the high-temperature portion of the process can be brought into contact with a relatively cool incoming load, energy will be transferred to the load and preheats the load. This reduces the energy that finally escapes with the exhaust (Figure 9). This is the most efficient use of waste heat in the exhaust. Use of waste heat recovery to preheat combus- tion air is commonly used in medium- to high- temperature furnaces. Use of preheated air for the burners reduces the amount of purchased fuel required to meet the process heat requirements. Figure 10 shows the effect of preheating combus- tion air on exhaust gas heat losses. Preheating of combustion air requires the use of a recuperator or a regenerator. 2. Recuperators. A recuperator (Figure 11) is a gas-to-gas heat exchanger placed on the stack of the fur- nace. There are numerous designs, but all rely on tubes or plates to transfer heat from the outgoing exhaust 3 The tip sheet on Burner Fuel-Air Ratios 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 8. Multiplying effect of available heat on furnace losses. Flue Gas Losses Multiply Wall, Conveyor & Radiation Losses For every BTU lost through the walls, carried out by a conveyor or radiated out an opening... ...2 BTUS are required to replace it in a furnace operating at 1700°F & 10% excess air with ambient combustion air. Figure 9. Direct preheating of incoming work. 6 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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