Fire tube

Fire retardant treatments Fire-tube furnace Fireworks... 

In fire-tube furnaces developed in the nineteenth century, such as typified by the Scotch-Marine boiler (Fig. 1), thin currents of water contact a multiplicity of tubes thus, the hot gases transmit heat simultaneously to aH regions of the bulk of the water. Therefore, this boHet—furnace combination steams readily and responds promptly to load changes, and is, for a given amount of heating surface, the least expensive of aH furnace—boHet instaHations... 

Furnaces of this type, such as the steam locomotive furnace—boHet design, had the obvious disadvantage that pressure was limited to ca 1 MPa (150 psi). The development of seamless, thick-waH tubing for stationary power plants (ie, water-tube furnaces) and other engines for motive power, such as diesel—electric, has in many cases ecHpsed the fire-tube boHet. For appHcations calling for moderate amounts of lower pressure steam, however, the modern fire-tube boHet continues to be the indicated choice (5). 

Figure 27-43 shows the amount of energy available for power by using a fire-tube boiler, an industrial bofler, and subcridcal- and supercritical-pressure boilers. Condensing losses decrease substantially, and regeneration of air and feedwater becomes increasingly important in the most advanced central-station boilers. 

FIG. 27-44 A four-pass packaged fire-tube boiler. Circled numbers indicate passes. (From Cleaver Brooks, Inc. Reproduced from Gas Engineer s Handbook, Industtial Press, New York, 1965, with permission. )... 

A griod example of convection in a process application is the transici of heat from a fire tube to a liquid, as in an oil treater. A current is set up between the cold and the w arm parts of the water transferring the heal from the surface of the fire tube to the bulk liquid. 

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. 

The required fire tube area is thus given by ... 

For example, if total heat duty (sensible heat, latent heat duty, heat losses to the atmosphere) was 1 MMBtu/hr and water was being heated, a heat flux of 10,000 Btu/hr-ft would be used and 100 ft of fire tube area would be required. 

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. 

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 an indirect bath heat exchanger, the heating medium provides Iil u to an intermediary fluid, which then transfers the heat to the fluid h)cuig heated. An example of this is the common line heater used on many gas well streams to keep the temperature above the hydrate formal ion lem perature. A fire tube heats a water bath, which provides heat to tlie v.all siieam flowing through a coil immersed in the bath. Details pertaining to dcsi jit of indirect bath heaters are presented in Chapter 5. 

In fire tube type heaters, the coils are immersed in a bath of water. The water is heated by a fire tube that is in the bath below the coils. That is, the fire tube provides a heat flux that heats the water bath. The water bath... 

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. 

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. 

Assuming a 10-ft shell, then four passes of 3-in. XXH are required. This will require a 30-in. OD shell for the coils and fire tube. 

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

The gas from the glycol/condensate separator can be used for fuel gas. In many small field gas packaged units this gas is routed directly to fire tubes in the reboiler, and provides the heat for reconcentrating the glycol. This se[>.irator is sometimes referred to as a gas/glycol separator or pump gas separator. 

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 . 

Fire tubes, especially in heater treaters, where they can be immersed in crude oil, can become a source of ignition if the tube develops a leak, allowing crude oil to come in direct contact with the flame. Fire tubes can also be a source of ignition if the burner controls fail and the tube overheats or if the pilot is out and the burner turns on when there is a combustible mixture in the tubes. 

Fire tubes can lead to fire or explosion if there is a leak of crude oil into the tubes or failure of the burner controls. An explosion could be sudden and lead directly to injury. Therefore, a high level of safety is required. 

The hazard tree also helps identify protection devices to include in equipment design that may minimize the possibility that a source will develop into a condition. Examples would be flame arrestors and stack arrestors on fire tubes to prevent flash back and exhaust sparks, gas detectors to sense the presence of a fuel in a confined space, and fire... 

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