HEAT TRANSFER IN INDUSTRIAL FURNACES

R. Viskanta, Impact of Heat Transfer in Industrial Furnaces on Productivity, in J. A. Rezies (ed.). Transport Phenomena in Heat and Mass Transfer, 2, pp. 815-838, Elsevier, Amsterdam, 1992. 

HEAT TRANSFER IN INDUSTRIAL FURNACES TABLE 2.8. Properties of air at one atmosphere... 

Aluminum reverberatory furnace. (From Baukal, C. E., Heat Transfer in Industrial Combustion, Boca Raton, FL CRC Press, 2000 courtesy of CRC Press.)... 

Temperature is a particularly important variable in industrial combustion applications because it directly or indirectly affects a number of other important variables. The product temperature is often a critical parameter in most processes. While there is usually a minimum temperature that must be reached for adequate processing, there may also be a maximum temperature above which product quality is reduced. Higher than necessary product temperatures not only increase fuel costs, but they may also increase cooling costs after the product exits the combustion process. Temperature affects the heat transfer in a furnace [1]. Thermal NOx emissions are exponentially dependent on flame temperatures [2]. Combustion chemistry is very complicated and dependent on temperature. High exhaust gas temperatures mean reduced thermal efficiency [3]. 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

Special problems arise in measuring local temperature within spray flames. Liquid and solid particles cause deposits and blockage of orifices in instruments. High-temperature conditions, with particles having high emissivity, result in complex radiative heat transfer which affects the accuracy of temperature measurement. In industrial furnaces and gas turbine combustion chambers, suction pyrometers have been used for... 

In the preliminary design of a furnace for industrial boiler, methane at 25 C is burned completely with 20% excess air, also at 25°C The feed rate of methane is 450 kmol/h. The hot combustion gases leave the furnace at 300°C and are discharged to the atmosphere. The heat transferred from the furnace (Q) used to convert boiler feedwater at 25°C into superheated steam at 17 bar and 250 C. 

H. A. J. Vercammen and G. F. Fromment, An Improved Zone Method Using Monte Carlo Techniques for the Simulation of Radiation in Industrial Furnaces, International Journal of Heat and Mass Transfer, vol. 23, pp. 329-337,1984. 

C. L. DeBellis, Evaluation of High Emittance Coatings in a Large Industrial Furnace, in B. Farouk et al. (eds.), Heat Transfer in Fire and Combustion Systems, 250, pp. 190-198, ASME, New York,... 

Another common way of classifying combustors is according to their geometry that includes their shape and orientation. The two most common shapes are rectangular and cylindrical. The two most common orientations are horizontal and vertical, although inclined furnaces are commonly used in certain applications such as rotary cement furnaces. An example of using the shape and orientation of the furnace as a means of classification would be a vertical cylindrical heater (sometimes referred to as a VC) used to heat fluids in the petrochemical industry. Both the furnace shape and orientation have important effects on the heat transfer in the system. They also determine the type of test that will be done. 

Because ample heat can usually be released at sufficiently high temperatures in industrial furnaces, the next problem to be studied in calculation of furnace capacity should be heat transfer to the furnace load and temperature equalization within the load. With adequate heat release at sufficiently high temperature assured, note the following factors that affect furnace capacity. 

Choudary, M. A Three-Dimensional Mathematical Model for Flow and Heat Transfer in Electrical Glass Furnaces, IEEE Transactions on Industry applications, Vol lA-22, No5, September/October 1986. 

For gas-fired systems the state-of-the-art is represented by the preheater described in Reference 69. A pebble bed instead of a cored brick matrix is used. The pebbles are made of alumina spheres, 20 mm in diameter. Heat-transfer coefficients 3—4 times greater than for checkerwork matrices are achieved. A prototype device 400 m in volume has been operated for three years at an industrial blast furnace, achieving preheat temperatures of 1670 to 1770 K. 

In the second example, that of an industrial pyrolysis reactor, simplified material and energy balances were used to analyze the performance of the process. In this example, linear and nonlinear reconciliation techniques were used. A strategy for joint parameter estimation and data reconciliation was implemented for the evaluation of the overall heat transfer coefficient. The usefulness of sequential processing of the information for identifying inconsistencies in the operation of the furnace was further demonstrated. 

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. 

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