Direct-fired furnaces

Some gas processes use direct fired furnaces. Process fluid flows inside tubes that are exposed to a direct fire. In this case radiant energy is important. Furnaces are not as common as other devices used in production facilities because of the potential fire hazard they represent. Therefore, they are not discussed in this volume. 

The main use of residual fuel oil is for power generation. It is burned in direct-fired furnaces and as a process fuel in many petroleum and chemical companies. Due to the low market value of fuel oil, it is used as a feedstock to catalytic and thermal cracking units. 

Siennas and umbers are calcined in a directly fired furnace, and water is driven off. The hue of the products is determined by the calcination period, temperature, and raw material composition [3.4]. 

Direct-fired furnace. Fired heaters are designed to increase the process temperature of oil and gas streams. This increase of temperature in most every case does not change molecular structure. Thus, temperatures up to 500°F maximum with 400°F design are very common. Designs are usually cylindrical, with vertical radiant tube banks fired by oil/gas combination burners. 

Steam reforming is an important process to generate hydrogen for such uses as ammonia synthesis because of the high endothermic heat reaction and its rapidity. High heat fluxes with a direct-fired furnace are required. Although many steps of reactions are possible, the typical reaction steps are as follows ... 

Direct Fired Furnaces. For direct fired furnaces, radiative heat transfer from the flame and combustion products as well as from the walls to the load is usually the dominant heat transfer mode. Convection from the combustion gases makes a much smaller contribution. The radiative transfer within the furnace is complicated by the nongray behavior of the combus-... 

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 EDC pyrolysis unit operates at temperatures in the range of 500-550°C and pressure up to 25-30 atm. It is an endothermic reaction and is normally carried out as a homogeneous non-catalytic gas phase reaction in a direct-fired furnace. The pyrolysis is usually operated in the range of 50-60% conversion of EDC. 

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