Steam boilers fire-tube

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

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

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... 

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... 

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. 

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. 

Example 23.1 A small package fire-tube boiler has makeup water that contains 500 ppm dissolved solids. The steam system operates with 50% condensate return. Estimate the blowdown rate. Assume that the maximum limit for the TDS is 4500 ppm. Assume that there are no solids in the evaporation or the condensate return. 

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

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. 

Fig. 3.4. Entrance to fire tube boiler tubes after Fig. 3.3 s sulfur burning furnace. 1400 K gas ( 11 volume% S02, 10 volume% 02, 79 volume% N2) leaves the furnace and enters the boiler. It turns 90° in the boiler and flows into the tubes. The tubes are surrounded by water. Heat is transferred from the hot gas to the water - cooling the gas and making (useful) steam. The tubes are typically 0.05 m diameter. Table 3.1 gives industrial furnace data. Sulfur furnace boilers are discussed by Roensch (2005).

This product has enough S02 and a high enough 02/S02 ratio for subsequent catalytic S02 + AOj —> SO3 oxidation. It is also cool enough for its heat to be recovered as steam in a simple fire-tube boiler (Thermal Ceramics, 2005) and steam superheater. 

Most Gasification Most, Czech Republic Shell January 1971 Cracked residue (1250 mt/d) Fire-tube boiler 3.6 H2, methanol, power, steam... 

In contrast, fire-tube boilers are much better suited for natural circulation and the steam drum can sit in piggyback-fashion, right on top of the boiler. This makes it possible to provide each boiler with its own separate steam drum, which allows greater flexibility in the plot plan. But it took some time before this boiler type was accepted in ammonia plants as the stress pattern is more complex and less predictable than in water-tube boilers. 

The synthesis loop boiler on the exit of the converter is also a very important piece of equipment. In some modern plants not equipped with an auxiliary boiler it supplies nearly half of the total steam generation. It may generate as much as 1.5 t of steam per tonne of ammonia, equivalent to about 90% of the reaction heat. Fire-tube versions have been also used, including Babcock-Borsig s thin-tubesheet design. But compared to the secondary reformer service, where the gas pressure is lower than the steam pressure, the conditions and stress patterns are different. In the synthesis loop boiler the opposite is the case, with the result that the tubes are subjected to longitudinal compression instead of being under tension. Several failures in this application have been reported [993], and there was some discussion of whether this type of boiler is the best solution for the synthesis loop waste-heat duty. 

Fire 1.4. Steam boiler and furnace arrangements. [Steam, Babcock and Wilcox, Barberton, OH, 1972, pp. 3.14, 12.2 (Fig. 2), and 25.7 (Fig. 5)]. (a) Natural circulation of water in a two-drum boiler. Upper drum is for steam disengagement the lower one for accumulation and eventual blowdown of sediment, (b) A two-drum boiler. Preheat tubes along the Roor and walls are cormected to heaters that feed into the upper drum, (c) Cross section of a Stirling-type steam boiler with provisions for superheating, air preheating, and flue gas economizing for maximum production of 550,000 Ib/hr of steam at 1575 psia and 900°F. 

Depending on the pressure and temperature of the produced hot water respectively steam two different types of boiler construction, fire-tube boiler and water-tube boiler, are distinguished. Within the following sections steam boilers of small and medium scale up to a heat capacityl2 MW will be described. 

Fire-tube boilers consist of a water filled vessel which is crossed by the fire tubes. The hot fumes flow inside these tubes and their heat is transferred to the water respectively steam. The arrangement of superheat assemblies in the flue gas channel is basically possible. 

For fire-tube boilers steam pressure typically lies within a range of 8-32 bar and a temperature between 170-450 C. Water-tube boilers of the given power range are designed for a steam pressure of 60 bar and superior with steam temperatures of up to... 

Vertical boilers generally save floor space, but not if very large units are contemplated. Many such boilers have poor gas circulation and consequently are inefficient. Water-tube boilers may be shipped knocked down. They usually contain less water, relatively, than fire-tube boilers and hence are safer as well as responsive more quickly to changes in load. Having no parts of large diameter, they are. the only boilers well adapted to high steam pressures. Highest rates of evaporation are realized on water-tube boilers. 

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