Fired heaters tube wall temperature

Fired heater reboilers should be designed to keep the tube wall temperature below 300°F which limits the heat flux to 6,500 to 8,500 Btu/(hr)(ft2) (Manning and Thompson, 1991 Ballard, 1966). Bacon (1987) recommends that the fired heater tube skin temperature be limited to a maximum of 350°F and heat flux should be limited to 6,000 to 8,000 Btu/(hr)(ft2). These constraints may require inlet ferrules to restrict heat flux at the fired inlet. 

Fired heaters provide a major part of heat source for reaction and separation. Reliability is the major concern for furnace operation with heat flux for large heaters and tube wall temperature (TWT) for small heaters as the most important reliability parameters. Increasing either heat flux or TWT could increase furnace efficiency. When operating heat flux is much lower than the maximum limit, this is an indication of the furnace being underutilized and thus presents an opportunity for increased feed rate. Increase feed rate is a win-win operation since more feed also results in reduced energy intensity. 

A fired heater is not operated uniformly over the entire run as it eould run light in turndown operation and harder in full capacity and toward the end of run for reaction heaters. To estimate the effects of changing tube wall temperature, corrosion rates, and pressure, a metaUurgic examination can be applied to estimate the remaining life of tubes. Knowing the tube life not only prevents premature tube failure, but also identihes the need for metal upgrade if the operating skin temperature increases over time. 

In order to adequately describe the size of a heater, the heat duty, the size of the fire tubes, the coil diameters and wall thicknesses, and the cor lengths must be specified. To determine the heat duty required, the maximum amounts of gas, water, and oil or condensate expected in the heater and the pressures and temperatures of the heater inlet and outlet must be known. Since the purpose of the heater is to prevent hydrates from forming downstream of the heater, the outlet temperature will depend on the hydrate formation temperature of the gas. The coil size of a heater depeiuLs on the volume of fluid flowing through the coil and the required heat duty. 

Figure 1.6 shows a typical direct fired heater. Oil flows through an inlet distributor and is heated directly by a fire box. The heat may be supplied by a heating fluid medium, steam, or an electric immersed heater. Direct heaters are quick to reach the desired temperature, are efficient (75-90%), and offer a reasonable initial cost. Direct fired heaters are typically used where fuel gas is available and high volume oil treating is required. On the other hand, they are hazardous and require special safety equipment. Scale may form on the oil side of the fire tube, which prevents the transfer of heat from the fire box to the oil emulsion. Heat collects in the steel walls under the scale, which causes the metal to soften and buckle. The metal eventually ruptures and allows oil to flow into the fire box, which results in a fire. The resultant blaze, if not extinguished, will be fed by the incoming oil stream. 

Newer heaters typically have thin reflective tiles, rather than massive refractory brick walls. Such newer heaters will heat up more rapidly. Also, the process fluid outlet temperature responds more rapidly to changes in the firing rate. This improves the heater outlet temperature control. Perhaps for this reason, it seems that heaters with reflective refractory walls are less subject to process tube coking and shortened heater run lengths. There is also a process, called "alonizing," that increases the reflectivity of older brick refractory heater walls. 

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