Industrial Process Heating Furnaces

In industrial processes heat energy is transferred by a variety of methods, including conduction in electric-resistance heaters conduction-convection in exchangers, boilers, and condensers radiation in furnaces and radiant-heat dryers and by special methods such as dielectric heating. Often the equipment operates under steady-state conditions, but in many processes it operates cyclically, as in regenerative furnaces and agitated process vessels. 

Electric furnaces for industrial process heating may use resistance or induction heating. Theoretically, if there is no gas or air exhaust, electric heating has no flue gas loss, but the user must recognize that the higher cost of electricity as a fuel is the result of the flue gas loss from the boiler furnace at the power plant that generated the electricity. 

There has not been a new text/reference book on industrial furnaces and industrial process heating in the past 30 years. Three retired engineers have given much time and effort to update a revered classic book, and to add many facets of their long experience with industrial heating processes—for the benefit of the industry s future and as a contribution to humanity. 

Since hydrogen burns cleanly and reacts completely with oxygen to produce water vapor, this makes it more desirable than fossil fuels for essentially all industrial processes. For example, the direct reduction of iron or copper ores could be done with hydrogen rather than smelting by coal or oil in a blast furnace. Hydrogen can be used with conventional vented burners as well as unvented burners. This would allow utilization of almost all of the 30 to 40% of the combustion energy of conventional burners that is lost as vented heat and combustion by-products. 

These two simple conversions will cover a large part of the stationary equipment. Gas fired home furnaces, water heaters, and stoves will all be handled in this simple manner at low cost. Coal and oil fired equipment will be less simple to convert. It will require the purchase of a low cost gas burner to replace the coal grates and oil injectors. Examples of industrial processes that can be converted in this simple manner include power plants, steel making, glass making, chemical processes, ore refining, and steam for heating. 

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... 

Figure 24.8 shows the layout for testing radiant tube performance in a pilot-scale industrial furnace (PSIF) of the Canmet Energy Technology Centre, Ottawa, Ontario, Canada [9]. The furnace is 4.5 m x 3.0 m x 1.0 m (inside dimensions) and it can be modified to simulate any industrial furnace geometry. With a firing rate of 1.2 MW, the temperature, heat transfer, and chemical environment found in most industrial processes can be emulated as well. The furnace is equipped with a calorimeter for total heat flux (34 cooled plates on the floor of furnace). 

Usually, a uniform heat flux is desired to produce better yields. Most industrial processes take advantage of flue gas to provide heat flux distribution inside the furnace. Note that in oxy-combustion, the flue gas volume is significantly reduced (by more than three times that of air-combustion). Hence, considerable efforts are needed to properly manage the flue gas recirculation pattern within the furnace and heat flux. Leroux et al. 

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