Indirect-fired furnaces

Indirectly fired furnaces find applications in metallurgy, paint enameling, the pharmaceutical industry, and other situations where it is necessary to control the chemistry of the furnace atmosphere [186]. An important aspect of the heat treatment of metals, for example, is the effect of the furnace atmosphere on the stock being heated. In most cases, the need is to minimize or eliminate completely the undesirable effects of furnace gases, such as oxidation or decarbonization. Both directly [187,188] and indirectly [189,190], natural-gas-fired furnaces have been analyzed, but the details cannot be included here. 

FIGURE 18.41a Multimode heating with an indirectly fired furnace. Shown is (a) predicted sheet temperature distributions. 

Indirect-fired furnaces are for heating materials and products for which the quality of the finished products may be inferior if they have come in contact with flame or products of combustion (poc). In such cases, the stock or charge may be (a) heated in an enclosing muffle (conducting container) that is heated from the outside by products of combustion from burners or (b) heated by radiant tubes that enclose the flame and poc. 

Radiant Tubes. For charges that require a special atmosphere for protection of the stock from oxidation, decarburization, or for other purposes, modern indirect-fired furnaces are built with a gas-tight outer casing surrounding the... 

Fig. 1.16. Indirect-fired furnace with muffles for both load and flame. Cover annealing furnaces for coils of strip or wire are built in similar fashion, but have a fan in the base to circulate a prepared atmosphere within the inner cover.

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... 

Fresh reducing gas is generated by reforming natural gas with steam. The natural gas is heated in a recuperator, desulfurized to less than 1 ppm sulfur, mixed with superheated steam, further preheated to 620°C in another recuperator, then reformed in alloy tubes filled with nickel-based catalyst at a temperature of 830°C. The reformed gas is quenched to remove water vapor, mixed with clean recycled top gas from the shaft furnace, reheated to 925°C in an indirect fired heater, and injected into the shaft furnace. For high (above 92%) metallization a CO2 removal unit is added in the top gas recycle line in order to upgrade the quaUty of the recycled top gas and reducing gas. 

Fired heaters differ from other indirect-fired processing equipment in that the process stream is heated by passage through a coil or tubebank enclosed in a furnace. Fired heaters are classified by function and by coil design. 

Indirect Heating If the process material cannot tolerate exposure to the combustion gas or if a vacuum or an atmosphere other than air is needed in the furnace chamber, indirect firing must be employed. This is accomplished in a muffle furnace or a radiant-tube furnace (tubes carrying the hot combustion gas run through the furnace). 

Direct-fired combustion equipment is that in which the flamt of combustion are used to achieve the desired result by r and convection. Common examples include rotary kilns and open-hearth furnaces. Indirect-fired combustion equipment is that in wh e... 

There are many other types of heat exchanger devices that can be used to heat the gas above the hydrate temperature. These could include shell and tube heat exchangers, electrical immersion heaters, furnaces, etc. However, the most common equipment type used to heat a well stream is the indirect fired water bath heater. 

Indirect-Fired Equipment (Fired Heaters) Indirect-fired combustion equipment (fired heaters) transfers heat across either a metallic or refractory wall separating the flame and products of combustion from the process stream. Examples are heat exchangers (discussed in Sec. 11), steam boilers, fired heaters, muffle furnaces, and melting pots. Steam boilers have been treated earlier in this section, and a subsequent subsection on industrial furnaces will include muffle furnaces. 

A furnace may be direct-fired or indirect-fired. The indirect-fired is known as a muffle furnace, and in such furnaces the combustion gases are separated from the stock being heated to prevent contamination. 

Modern olefins plants now under design and construction employ indirectly fired tubular pyrolysis reactors. A typical furnace housing the reactor is illustrated schematically in Figure 2. 

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. 

Radiant tubes are used in industry for heat treatment applications in which products are treated under a protective gas atmosphere within heat treatment furnaces. Therefore the heating of such furnaces are performed with indirect fuel-fired systems or electrical heating elements. For indirect fired applications, the flue gas of the combustion process can not enter into the furnace. The combustion takes place within radiant tubes and the heat is transferred—via radiation—from the outer surface of the tube to the process. There are different types of radiant tubes available. For all types the maximum transferred heat is one of the important features of such systems. That means that the maximum radiant tube temperature and the temperature uniformity are important characteristics of radiant tubes. Another issue is the efficiency of the radiant tubes. In... 

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