where W is the mass of the exhaust exhaust gases; C P is the specific heat of the exhaust gases; gases; TEXHAUST is the flue gas temperature entering the furnace exhaust system (stack); and TAMBIENT is the ambient temperature.
heat storage, wall, conveyor or radiation losses will be multiplied by the available heat factor
The highest priority is to minimize exhaust gas temperature and mass or volume of exhaust gases. The heat processing equipment exhaust gas temperature depends on many factors associated with the equipment operation and heat losses. It can be measured directly, or it can be assumed to be 100 to 200°F (55 to 111°C) above the control temperature for the heated zone where the flue gases are exhausted. The exhaust mass flow depends on the combustion airflow, fuel flow and the air leakage into the heat processing equipment. Measurement of fuel flow, together with the percentage of oxygen in the flue gases, can be used to estimate mass or volume of exhaust gases. The flue gas specific heat heat (CP) for most gaseous gaseous fuel-fired furnaces can be assumed assumed to be 0.25 BTU/lb/°F or 0.02 BTU/scf/°F for a reasonably accurate estimate of flue gas heat losses. Minimize Exhaust Gas Temperatures. Excessive Temperatures. Excessive gas temperatures can be the result of poor heat transfer in the heat processing equipment. If the combustion gases are unable to transfer the maximum possible heat to the oven, furnace or other heat processing equipment -- and its contents contents -- they will leave the equipment equipment at higher temperatures temperatures than necessary. necessary. Overloading Overloading heat processing processing equipment equipment also can lead to excessive excessive stack temperature temperatures. s. To get the proper rate of heat transfer, combustion gases must be held in the heating chamber for the right amount of time. The natural natural tendency of an overloaded overloaded oven or furnace is to run colder than optimal unless the temperature is set artificially high. This causes the burners to operate at higher than normal firing rates, which increases combustion gas volumes. The higher the gas flow rates and shorter time in the heat processing equipment cause poor heat transfer, resulting in higher temperature for the flue gases. Increased volumes of higher temperature flue gases lead to sharply increased heat losses.
Minimizing Exhaust Gas Volumes. Avoiding Volumes. Avoiding overloading and optimizing heat transfer are two ways to lower waste gas flows, but there are others. The most potent potent way is to closely control fuel-to -air ratios. Operating Operating the oven or furnace near the optimum fuel-to- air ratio for the process process also controls controls fuel consumption. consumption. The best part is that it usually can be done with A recuperator is a existing control equipment and a little maintenance attention. Some reduction in exhaust volumes will be the indirect result of efficiencies applied elsewhere. For instance, flue gas losses are a fixed percentage of the total heat input to the oven or furnace. Any reduction in heat storage, wall, conveyor or radiation radiation losses will be multiplied multiplied by the available heat factor. Use of Oxygen Enriched Combustion Air. Ambient Air. Ambient air contains approximately 21 percent oxygen, with nitrogen and other inert gases as the balance. The total volume of exhaust gases could be reduced by increasing the oxygen content of the combustion air, either by mixing extra oxygen into the ambient air or by using 100 percent oxygen. The reduced
gas-to-gas heat exchanger placed on the stack of the oven or furnace. It transfers heat from the outgoing exhaust gas to the incoming combustion air while keeping the two streams from mixing.
