Fire tube heat density

Fire tube heat density depends on the type of draft used for design. Most TEG dehydration units are designed for natural draft, and for such conditions, the maximum fire tube heat density used is 24 kj/(mm2 h) [1]. The general equation of heat density is 

Q = total reboiler duty, kj/h A = cross-section area of the fire tube, mm  

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For natural draft fire tubes, the minimum cross-sectional area oi liu-fire tube is set by limiting the heat release density to 21,000 Biu/lir nr. At heat relea.se densities above this value, the flame may become utr.ia ble becriuse of insufficient air. Using this limit, a rninimuin fire eiv diamelei is established by ... 

In applying Equation 2-22 note that the burner heat release density will be somewhat higher than the heat duty, including losses used in Equation 2-21, a standard burner size will be chosen slightly larger than that rer uircd. Standard burner sizes and minimum fire tube diameters are inciuded in Table 2-12. 

The primary objectives, in the operation of a fired heater are to avoid excessive heat density in the firebox, maintain a small negative pressure below the bottom row of convective tubes, and obtain complete combustion of the fuel in the firebox with minimum oxygen in the flue gas. Regardless of the flue-gas oxygen content, however, the point of absolute combustion is the combustion air rate that maximizes process-side heat absorption for a given amount of fuel. 

An intended application, although never commercially realized, was in fire transfer lines referred to as Flashtrack [12]. In a particular example, three 45 tm thick PTFE tapes with a coarse 16 (Am thick Mg coating on both sides would be cut into 2 mm wide strips and were confined in a 4 mm inner diameter Viton tube. After heat shrinking of the tube, it was read for use. The loading density was 1 g pyrolant per meter. When the tube is ignited, the pyrolant deflagrates with a velocity of the order of 100 m s but leaves the tube intact... 

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