Fire-tube boilers

Steam produced from a packaged boiler (fire tube or water tube) should always contain less than 5% entrained water. 

There are four fundamental types of boiler available today—electric boilers, fire tube (shell or FT) boilers, water tube (WT) boilers, and nuclear reactor boilers. Electric boilers apart, all other types are essentially developments from shell and tube heat-exchanger designs. 

Economic boiler see also Boiler, fire tube, economic 30 ... 

Lack of softening or dealkalization capability Lancashire boiler, see Boiler, fire tube, Lancashire 193... 

Package steam boiler (Fire-tube boiler) CS Steam generation (kg-h-1) 50,000 4.64 x 105 50,000-350,000 0.96... 

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. 

Related Calculations. Use this procedure for selecting draft fans for all types of boilers—fire-tube, packaged, portable, marine, and stationary. Obtain draft losses from the boiler manufacturer. Compute duct pressure losses using the methods given in later Procedures in this Handbook. 

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... 

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. )... 

The original steam generators were simple pressure vessels that were prone to caiasirophic failures and loss of life. Due to better boiler design, tube-fired boilers, and boiler inspections, the incidence of catastrophic failure is now to a rare event (about once every 100,000 vessel-years). In Great Britain in 1866, there were 74 steam boiler explosions causing 77 deaths. This was reduced to 17 explo.sions and 8 deaths in 1900 as a result of inspections performed by the Manchester Steam User Association. In the United States, the American Society of Mechanical Engineers established the ASME Pressure Ves.sel Codes with comparable reductions. 

Fire tube boiler designs employ the upper internal portion of the boiler vessel as a compartment to hold the generated steam, while WT boilers... 

Table 1.2 Typical energy consumption and output ratings for a fire tube boiler...

The first three boiler types may be variously used for commercial, institutional, or industrial applications. Fire tube boilers also may be used for steam turbine generator duty at lower electrical outputs, whereas only WT and nuclear reactor boilers are employed for utility power generation. 

Fire tube boilers (shell boilers or shell and tube boilers) convert heat from burning fuel within a furnace (combustion chamber, firebox, or furnace tube) to generate either HW or steam. Fire tube boilers are designed to direct the combustion gases through tubes (held within tube sheets) that are surrounded by BW, thus providing for a greater heat-transfer surface area and improved efficiency. 

The hot combustion gases were directed through the heat exchanger tubes, and to this day FT boiler combustion gas tubes continue to be variously called fire tubes, smoke tubes, flue gas tubes (or simply tubes). 

Early vertical boilers were constructed in several different designs, including FT and tubeless, dry-top, and wet-top versions. Typically, however, they were single-pass FT units containing an inner, combined BW and steam shell—through which a number of small fire tubes passed—and an outer combustion chamber. 

The early vertical boilers of dry-top design (steam on one side and hot combustion gases on the other side) were subject to the risk of overheating in any fire tubes located above the waterline, but these boilers could provide relatively dry steam with some degree of superheat. 

Scotch marine boilers (SM boilers) derive their name from the Scottish shipyards that built marine vessels for the British Navy. They were the first design of FT boiler to incorporate both furnace tubes and fire tubes inside the shell and replaced the brick-set boilers that used to burn through the bottoms of ships. The SM boiler was a particularly versatile design and quickly became the boiler of choice for many stationary (land) applications as well as for marine duty. Land-based SM boilers (now commonly called Scotch boilers) were not simply marine boilers adapted for stationary duty but incorporated specific design modifications to meet the requirements of land-based industry. 

FB boilers are similar to SM boiler designs except that, instead of a fully immersed furnace tube, they have a bottom furnace (with a crowned combustion chamber) that sits on a refractory floor. Two-pass fire tubes (smoke tubes) connect the combustion chambers to the gas exit vent (smoke stack). 

Fire tube boilers usually do not have ID fans because they normally can obtain sufficient ID from the stack s natural draft. However, they do employ FD fans, which provide a positive furnace pressure. 

Contaminants such as oil, magnesium phosphate, and hematite have a natural binding action that exacerbates the fouling problem and may result in the rapid agglomeration of tube deposits in WT boilers and fire-tube bridging in FT boilers. 

Both types of boiler systems may incorporate finned copper heating coils, which are located above the furnace and gas-pass tubes (smoke tubes or fire tubes) and provide for indirect heating of domestic HW. Where coils are fitted and the boilers are only fired during winter months, domestic HW heating usually is provided via gas heaters for the summer. 

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