Batch Indirect-Fired Furnaces

The principal purpose of indirect firing is to protect the furnace load from corrosion, oxidation, carbon and/or hydrogen absorption, or other reactions with the poc. The protection is accomplished by placing a solid barrier wall between the poc and the load, and by pumping an inert atmosphere into the chamber on the side of the wall where the load is located. The barrier wall may be refractory or metal, but it must 

There always will be a considerable temperature drop across a muffle wall or a radiant tube wall. Forced circulation on the load side of the wall helps reduce the resistance of the stagnant film clinging to the wall surface and minimize temperature nonuniformities within complex loads. 

The heating capacity of furnaces that are equipped with flame-in-tube muffles (radiant tubes) is limited by the heat that can be radiated from the tubes. The heating capacity of an indirect-fired furnace is less than that of a direct-fired furnace having 

The input to muffles or radiant tubes is limited by the strength, durability, and conductivity of their wall materials. The great temperature difference across a muffle or tube wall not only reduces its useful life but also causes the products of combustion to exit at a very high temperature, raising the fuel bill. For both reasons, muffle and tube walls are made as thin as practical, using a material that has both high thermal conductivity and resistance to heat. Alloy steels and silicon carbide are the most suitable materials for muffles and radiant tubes. Silicon carbide radiant tubes can withstand higher temperatures and are more resistant to oxidation than nickel-chrome alloy steel tubes, but the latter are less brittle and cheaper. 

Muffles are prone to leak, especially in furnaces above 1800 C (982 C), where most have been replaced by radiant tubes. For lower temperatures,electrically heated furnaces or furnaces with radiant tubes and forced circulation have largely replaced muffle furnaces, except for cover annealing furnaces. 

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H. Ramamurthy, S. Ramadhyani, and R. Viskanta, Modeling of Heat Transfer in Indirectly Fired Batch Reheating Furnace, in J. R. Lloyd and Y. Kurosaki (eds.), Proceedings of the ASME/JSME Thermal Engineering Joint Conference 1991, 5, pp. 205-215, ASME/JSME, New York/Tokyo, 1991. 

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