Midrange Heat Treat Furnaces

Bottom flueing is preferred, but in-the-wall vertical flues have been found too costly, and they pull a harmful negative pressure at the hearth level. With top firing, the best arrangement is hearth-level flues with automatic furnace pressure (damper) control. If fired with top and bottom burners, use of a roof flue with automatic furnace pressure control is suggested. The flue location should be determined to enhance the design circulation pattern. (See chap. 7.) 

If there is an operation bottleneck because of lack of heating capacity of a furnace in this temperature range, control techniques are available to increase capacity by raising the temperature of the furnace above the final product temperature. If bright metals such as stainless steel or titanium are to be heated, the rate of radiation will be low because of their lower emissivity (eq. 2.6) therefore, convection velocity should be increased. An excess of furnace or gas temperature over the desired final load temperature is permissible with steel provided the hottest location has a T-sensor to automatically control heat head. A fiue gas temperature somewhat higher than the final load temperature can be used with aluminum because of its lower absorptivity and higher thermal conductivity. 

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