Fire-TUbe Size

The area of the fire tube is normally calculated based on a heat flux rate of lO.OOO Btu/hr-ft-. The fire-tube length can be determined from  

A burner must be chosen from the standard sizes in Tabic 2-12. For example, if the heat duty is calculated to be 2.3 MMBtu/hr, then a standard 2.5 MMBtu/hr fire tube should be selected. 

Any combination of fire tube lengths and diameters that satisfies Equation 5-7 and is larger in diameter than those shown in Table 2-12 will be satisfactory. Manufacturers normally have standard diameters and lengths for different size fire tube ratings. 

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FIG. 27-45 Location and relative size of each of four passes of the flue gas through a fire-tube boiler. From Cleaver Brooks, Inc. Reproduced from Gas Engineer s Handbook, Industrial Press, New York, 1965, with permission. )... 

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. 

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. 

Bath-type heat exchangers can be either direct or indirect. In a direct bath exchanger, the heating medium exchanges heat directly with the fluid to be heated. The heat source for bath heaters can be a coil of a hot heat medium or steam, waste heat exhaust from an engine or turbine, or heat from electric immersion heaters. An example of a bath heater is an emulsion heater-treater of the type discussed in Volume 1. In this case, a fire tube immersed in the oil transfers heat directly to the oil bath. The calculation of heat duties and sizing of fire tubes for this type of heat exchanger can be calculated fom Chapter 2. 

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. 

Minimum coil length Minimum fire tube area Shell size... 

Table 8-2 can be used for an initial approximation of reboiler duties, If the reboiler is heated with a fire tube, the fire tube should be sized for a maximum flux rate of 8,000 Btu/hr-ft . 

Boilers are available in two basic designs a fire tube, in which water circulates in tubes heated externally by fire and a water tube, in which hot gases from fire pass through the tubes in the boiler. Fire-tube boilers are generally limited in size to approximately 12,000 kg/h (25,000 lb/h) and to about 20 bar (250 psig). Their size and pressure limitations preclude their use in large industrial facilities or in power plants. 

Industrial and utility boilers are broadly classified as fire-tube or water-tube. In fire-tube boilers, the hot combustion gases pass through tubes, and heat is transferred to water outside the tubes. Most steam locomotives had this type of boiler. The most common and least expensive boiler of this type is the horizontal return tubular (HRT) boiler. However, because of the design and construction of fire-tube boilers, there is a definite limitation to their size and the pressure that they can tolerate. 

The tube size, the number of tubes per row, the area per row and the minimum cross-sectional area for flue gas flow are considered in this rating procedure. Usually, in fired heaters the tube size in the radiant section is the same as in the convection section, and the number of tubes per row is such that the flue gas rate is about or below 15 feet per second. The area per row is the exposed area per tube times the number of tubes. The minimum cross-section area for flue gas flow is the total cross-sectional area of the convection section less the projected area of one row of tubes. 

Fig. 4.27. Fire-tube boilers with packaged automatic gas, oil, or dual-fuel burners having integral fans. These three-pass boilers have a large Morrison tube into which the burner fires as the first pass (radiation), and two banks of many small tubes (convection) for the second and third passes. Fire-tube boilers are more compact and less expensive than water-tube boilers, but they are limited in steam pressure and size, typically 150 psig (1030 kPa) maximum steam pressure and 33 kk Btu/hr (35 000 MJ/h) maximum input.

Boiler, fired, package gas-oil fired, water tube with fire tube for smaller size,... 

Developments in design. The sulphur burner is almost invariably a fire-tube boiler, but there is a choice of acid coolers depending on the size of plant for which they are used, the area of the land and capital costs. 

Tube sizes Tubing sizes are entirely different from pipe sizes. Tubing is often used in heat exchangers and fired equipment like furnaces. 

Shell size Fire-tube Fire-tubes Oil Free Gas (MM... 

The above procedure allows the production facility engineer to choose the major sizing parameters of heater-treaters when little or no laboratory data are available. This procedure does not give the overall dimensions of the treater, which must include inlet gas separation and FWKO sections. However, it does provide a method for specifying a fire-tube capacity and a minimum size for the coalescing section (where the treating actually occurs) and provides the engineer with the tools necessary to evaluate specific vendor proposals. 

Fire tube OD can be finalized based on the nearest higher standard size. The length can be adjusted accordingly. 

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