Furnace radiant tubes

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). 

Watson, J. Why Heat Recovery is a Natural for Radiant Tube Furnaces. Heat Treating, Feb. 1983. Also North American Handbook Supplement 204. ... 

Example 5 Radiation in a Furnace Chamber A furnace chamber of rectangular paraUelepipedal form is heated hy the combustion of gas inside vertical radiant tubes hningthe sidewalls. The tubes are of 0.127-m (5-in) outside diameter on 0.305-m (12-in) centers. The stock forms a continuous plane on the hearth. Roof and end walls are refractory. Dimensions are shown in Fig. 5-20. The radiant tubes and stock are gray bodies having emissivities of 0.8 and 0.9 respectively. What is the net rate of heat transmission to the stock by radiation when the mean temperature of the tube surface is SIG C (1500 F) and that of the stock is 649 C (1200 F) ... 

Alloy 800 (32% nickel, 20% chromium and 46% iron) is used for furnace equipment such as muffles, trays and radiant tubes and in oil and petrochemical plants as furnace coils for the reforming and pyrolysis of... 

Figure 17.15. A fired heater as a high temperature reactor, (a) Arrangement of tubes and burners (1) radiant tubes (2) radiant panel burners (3) stack (4) convection chamber tubes (Sukhanov, Petroleum Processing, Mir, Moscow, 1982). (b) Radiant (surface-combustion) panel burner (1) housing (2) ceramic perforated prism (3) tube (4) injector (5) fuel gas nozzle (6) air throttle Sukhanov, Petroleum Processing, Mir, Moscow, 1982). (c) Fired tubular cracking furnace for the preparation of ethylene from naphtha.

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. 

ASTM E 84 Standard Test Method for Surface Burning Characteristics of Building Materials ASTM E 108 Standard Test Methods for Fire Tests of Roof Coverings ASTM E 119 Standard Test Methods for Fire Tests of Building Construction and Materials ASTM E 136 Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750°C ASTM E 162 Standard Test Method for Surface Flammability of Materials Using a Radiant Heat Energy Source... 

Cost of process furnaces, box type with horizontal radiant tubes. 

Example 10 Furnace Simulation via Zoning The furnace chamber depicted in Fig. 5-20 is heated by combustion gases passing through 20 vertical radiant tubes which are backed by refractory sidewalls. The tubes have an outside diameter of D = 5 in (12.7 cm) mounted on C = 12 in (4.72 cm) centers and a gray body emissivity of 0.8. The interior (radiant) portion of the furnace is a 6 x 8 x 10 ft rectangular parallelepiped with a total surface area of 376 ft2 (34.932 m2). A 50-fL (4.645-m2) sink is positioned centrally on the floor of the furnace. The tube and sink temperatures are measured with embedded thermocouples as 1500 and 1200°F, respectively. The gray refractory emissivity may be taken as 0.5. While all other refractories are assumed to be radiatively adia-... 

The thermal system model for radiant-tube continuous furnace involves integration of the mathematical models of the furnace enclosure, the radiant tube, and the load. The furnace enclosure model calculates the heat transfer in the furnace, the furnace gas, and the refractory walls. The radiosity-based zonal method of analysis [159] is used to predict radiation heat exchange in the furnace enclosure. The radiant-tube model simulates the turbulent transport processes, the combustion of fuel and air, and the convective and radiative heat transfer from the combustion products to the tube wall in order to calculate the local radiant-tube wall and gas temperatures [192], Integration of the furnace-enclosure model and the radiant-tube model is achieved using the radiosity method [159]. Only the load model is outlined here. 

H. Ramamurthy, S. Ramadhyani, and R. Viskanta, A Thermal System Model for a Radiant-Tube Continuous Reheating Furnace, J. Mat. Eng. Performance, 4, pp. 519-531,1995. 

The obtainable NO, emissions depend not only on the burner, but are also affected by the particular application. For instance, in a small radiant tube it is clearly more difficult to obtain an efficient recirculation of flue gases as required by flameless oxidation (Figure 23.17). The lowest NO values down to 1- digit ppm could be reached with burners, for which the combustion chamber had been optimized on purpose for flameless oxidation (combustor). In applications of direct firing in industrial furnaces with air preheating in the range... 

Several thousand high velocity FLOX burners (especially recuperative natural gas burners with efficient, built-in air preheating) are in fact satisfactorily firing in free flame or in radiant tubes in heat treatment and or heating furnaces [14,18,19]. In other circumstances, however, air and fuel jets from the burner nozzles cannot follow the basic formulae of the free jet and recirculation of flue gases is not unconditional. 

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... 

Experimental investigations of radiant tubes in laboratory furnaces and under practical conditions in field tests are important in order to develop and improve radiant tubes. This chapter will give an overview about the different radiant tube technologies and its application in industry and will describe measurement techniques for experimental investigations of radiant tubes in laboratory furnaces. 

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