Radiant tubes surface area

Find the needed tube surface area from the heat absorbed and the radiant flux. When a process-side calculation has been made, the required number of tubes will be known and will not be recalculated as stated here... 

In preliminary design, the heat duty and furnace efficiency are the prime considerations. However, if the tube area needs to be specified, a preliminary estimate can be obtained from an assumed flux. In the radiant section, this usually lies in the range of 45,000 W m-2 to 65,000 W m 2 of tube surface, with a value of around 55,000 W m 2 most often used. The heat flux is particularly important if a reaction is being carried out in the furnace tubes. Overall heat transfer coefficients in the convection section are in the range 20 to 50 W m-2 K-1. 

A = surface area of the radiant tubes in the firebox, ft2 e = emissivity factor... 

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-... 

As 41 temperature measurement points were located with a different length between them, the temperature in every place after displacement T av, i was assumed to be the same in each surface area A,. The difference between temperatures on the circumference was neglected, because of low value and because in practice the radiant tube emitted energy mainly from the right and left sides. Calculations were done assuming different... 

It is now necessary to check the computed radiant absorption to be sure it meets the design limitations. First, divide the heat absorption by the total exposed tube area to get the average heat flux. If it is higher than the allowable maximum, a new furnace with more tube surface must be selected and the rating repeated. If the actual heat flux is considerably below the allowable value, a smaller furnace should be considered. 

Larger-si2ed heaters are usually hori2ontal box heaters. The radiant coils can be located either on the side walls so that the units are fired from underneath, or in a center row of tubes in which the heater is fired from both sides to provide a higher heat flux for reducing the radiant surface. An access area at one end of the box is required in order to remove the tubes. Sometimes multiple coils are included in the same box, which may require access to both ends of the box. 

A fire tube contains a flame burning inside a piece of pipe which is in turn surrounded by the process fluid. In this situation, there is radiant and convective heat transfer from the flame to the inside surface of the fire tube, conductive heat transfer through the wall thickness of the tube, and convective heat transfer from the outside surface of that tube to the oil being treated. It would be difficult in such a simation to solve for the heat transfer in terms of an overall heat transfer coefficient. Rather, what is most often done is to size the fire tube by using a heat flux rate. The heat flux rate represents the amount of heat that can be transferred from the fire tube to the process per unit area of outside surface of the fire tube. Common heat flux rates are given in Table 2-11. 

Several types of photon transducers are available, including (I) photovoltaic cells, in which the radiant energy generates a current at the interface of a semiconductor layer and a metal (2) phototubes, in which radiation causes emission of electrons from a photosensitive solid surface (3) photomultiplier tubes, which contain a photoemissive surface as well as several additional surfaces that emit a cascade of electrons when struck by electrons from the photosensitive area (4) photoconductivity transducers in which absorption of radiation by a semiconductor produces electrons and holes, thus leading to enhanced conductivity (5) silicon photodiodes, in which photons cause the formation of... 

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