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BestPractices Technical Brief high-temperature furnace can attest to the huge amount of thermal energy beamed into the room. Anywhere or anytime there is an opening in the furnace enclosure, heat is lost by radiation, often at a rapid rate. These openings include the furnace flues and stacks them- selves, as well as doors left partially open to accommodate oversized work in the furnace. Waste-gas losses. All the losses mentioned above – heat storage, wall transmission, conveyor and radiation – compete with the workload for the energy released by the burning fuel-air mixture. However, these losses could be dwarfed by the most significant source of all, which is waste-gas loss. Waste-gas loss, also known as flue gas or stack loss, is made up of the heat that cannot be removed from the combustion gases inside the furnace. The reason is heat flows from the higher temperature source to the lower temperature heat receiver. In effect, the heat stream has hit bottom. If, for example, a furnace heats products to 1,500°F, the combustion gases cannot be cooled below this tem- perature without using design or equipment that can recover heat from the combustion gases. Once the combustion products reach the same temperature as the furnace and load, they cannot give up any more energy to the load or furnace, so they have to be discarded. At 1,500°F temperature, the combustion products still contain about half the thermal energy put into them, so the waste-gas loss is close to 50% (Figure 4). The other 50%, which remains in the furnace, is called available heat. The load receives heat that is available after storage in furnace walls, and losses from furnace walls, load conveyors, cooling media and radiation have occurred. This makes it obvious that the temperature of a process, or more correctly, of its exhaust gases, is a major fac- tor in its energy efficiency. The higher that temperature, the lower the efficiency. Another factor that has a powerful effect is the fuel-air ratio of the burner system. Fuel-air ratios. For every fuel, there is a chemically correct, or stoichiometric, amount of air required to burn it. One cubic foot of natural gas, for example, requires about 10 cubic feet of combustion air. Stoichiometric, or on-ratio combustion will produce the highest flame temperatures and thermal efficiencies. However, combustion systems can be operated at other ratios. Sometimes, this is done deliberately to obtain certain operating benefits, but often, it happens simply because the burner system is out of adjustment. The ratio, as shown in Figure 5, can go either rich (excess fuel or insufficient air) Figure 3. Radiation loss from heated to colder surface. Figure 4. Exhaust gas heat losses vs. exhaust gas temperature and air-gas ratio. Fuel: Birmingham Natural Gas (1002 Btu/cu ft, 0.6 sp gr) 100 90 80 70 60 50 40 30 20 10 0 1200% 1000% 800 % 600% 400% 300% 250% 200% 150% 100% 50% 25% 10% 0 1000 2000 3000 Exhaust Gas Temperature, °F Figure 5. Effect of off-ratio operation on furnace efficiency. Correct Ratio Richer Leaner Air-Gas Ratio Waste Heat Reduction and Recovery for Improving Furnace Efficiency, Productivity, and Emissions Performance 3 0% Excess Air Process Efficiency Exhaust Gas Heat Loss, % of InputPDF Image | Waste Heat Reduction and Recovery for Improving Furnace Efficiency, Productivity and Emissions Performance
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