Boilers designs

R. C. Attig and A. E. Duzy, Coal Msh Deposition Stndies and Mpplication to Boiler Design, American Power Conference, Chicago, lU., 1969. 

Some PFBC boiler designs incorporate high-temperature, high-pressure (HTHP) filter devices in the flue-gas stream. These are installed primarily to protec t the gas turbine from erosion damage by the fine particles that escape the cyclones, but as the filters remove virtually all the suspended particulates, they also eliminate the need for back-end removal. The commonest HTHP filter elements used are rigid ceramic candles. 

Boiler Design Issues Boiler design involves the interaction of many variables water-steam circulation, fuel characteristics, firing systems and heat input, and heat transfer. The furnace enclosure is one of the most critical components of a steam generator and must be conservatively designed to assure high boiler availability. The furnace... 

Fuel Characteristics Fuel choice has a major impact on boiler design and sizing. Because of the heat transfer resistance offered by ash deposits in the furnace chamber in a coal-fired boiler, the mean absorbed heat flux is lower than in gas- or oil-fired boilers, so a greater surface area must be provided. Figure 27-42 shows a size comparison between a coal-fired and an oil-fired boiler for the same duty. 

Design Criteria Industrial-boiler designs are tailored to the fuels and firing systems involved. Some of the more important design criteria include ... 

The boiler designer must proportion heat-absorbing and heat-recovery surfaces in a way to make the best use of heat released by the fuel. Water walls, superheaters, and reheaters are exposed to convection and radiant heat, whereas convection heat transfer predominates in air preheaters and economizers. The relative amounts of these surfaces vary with the size and operating conditions of the boiler. 

Ellison has published an extremely important factor for steam drum design called the Drum-Level-Stability Factor. As manufacturers have learned how to increase boiler design ratings, the criteria for steam drum design have lagged. The three historical steam drum design criteria have been ... 

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. 

Improved boiler design/operation (cogeneration), and distributed control systems... 

Increasing superheated steam temperature has the direct result of increased efficiency. For example, an increase in steam temperature of 55°C can give around 4 per cent increase in efficiency. However, in practice the temperature is limited by metallurgical consideration with upper limits of around 540°C. This temperature can also be influenced by the boiler design and the fuel used. 

With steam generated at or close to the boiler design pressure it is inevitable that some of the steam-using equipment will have to be supplied at a lower pressure. In some cases the plant items themselves have only been designed to withstand a relatively low pressure. Sometimes a reaction will only proceed when the steam is at a temperature below a certain level or an unwanted reaction will occur above a certain level. For these and similar reasons, steam often is distributed at a relatively high pressure which must then be lowered, close to the point of use. Pressure-reduction stations incorporating pressure-reducing valves are fitted to perform this function. 

Lees and Whitehead" have shown that differences in boiler design lead to differences in furnace atmospheres, which are subsequently reflected in differences in scale morphology and corrosion performance. Hence they report that there is no unique scale morphology which is characteristic of furnace wall corrosion. They also warn that the scale that is examined during an investigation may not be an exact reflection of the scale on the tube surfaces during operation due to the possible hydrolysis of the scale on cooling (when hot flue gas is replaced by moist air) and the redistribution of phases in the scale due to the loss of the incident heat flux. 

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

In modem boilers, both FT steam spaces and WT steam drums incorporate various devices to promote the effective separation of steam from water and the production of dry steam. These devices include horizontal separators and baffle plates in the steam/water space, and chevron driers, cyclone separators, and secondary steam scrubbers in the steam space. Older or simpler boiler designs with steam release velocities of below 3 ft/s (0.9 m/s) may rely solely on the natural separation of steam from water. 

Overall boiler design, including steam and water space dimensions, boiler rating, pressure, and separation equipment arrangements. 

Boiler designers must be in a position to accurately predict the various heat release rates from furnace and burner designs and to match them to limitation standards for both heat input and boiler constructional steels. Some considerations are ... 

Furnace heat release coefficients For packaged FT boilers, designs vary from 60,000 to 125,000 Btu/hr/ft2/°F (340.7-709.8 kW/m2/°K) across the radiant surfaces (essentially the furnace tube). 

Nevertheless, prudently decreasing the total amount of blowdown per unit time (e.g., by consistently running boiler TDS levels closer to the maximum suitable for the boiler design and operation) saves money from reduced waste heat and generates a subsequent lower demand for chemical treatments and makeup (MU) water. 

Typically, electric boiler designs are limited to under 600 kWh (2,000 lb/hr) output. However, in some countries where electricity is of particularly low cost (say, where where supplied by hydroelectric generation) electric boiler designs of up to 12,000 to 24,000 kWh (40,000-80,000 lb/hr) or more are commonly used in large-process industry. The maximum continuous rated (MCR) output is about 175,000 lb/hr. 

Stay bolts are also used in many FT boiler designs. Typically, these bolts are provided with a drilled weep hole that leaks if deterioration of the stay bolt occurs due to waterside corrosion, thus providing an early warning to the operator. 

Further boiler design developments produced various other types of compact, self-supporting, externally fired FT boilers, with the shell mounted over a steel-encased furnace. These designs were loosely called economic boilers and were typically coal- or oil-fired, three-pass boilers with an arched top (the crown sheet) and stayed side-sheets and other flat steel surfaces. 

This type of boiler remained in regular use in many countries until the late 1970s. Occasionally, HRT boilers designed for HW heating duty and dating from the mid-1920s, still may be found in daily operation (especially in some metropolitan New York City apartment buildings). 

Other boiler designs in regular use since before or around the turn of the twentieth century included vertical boilers, locomotive boilers, firebox boilers, and Scotch marine boilers (Scotch boilers). All these designs were classified as internally fired FT boilers. 

NOTE Corrugation of self-supporting furnaces (as found in some SM boiler designs) is one method of resisting the compressive forces in FT boilers. Rings, flanges, and stays are also used. 

In wet-back boiler designs the rear wall of the furnace and the point at which the first-pass exit gases reverse direction is surrounded by water thus, no combustion chamber refractory is required and maintenance costs are reduced. 

In dry-back boiler designs the tubes pass through the rear tube sheet before returning to the steam and BW compartment, which provides for a relatively larger horizontal heating surface and thus quicker steaming. 

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). 

All modem, packaged horizontal FT boilers are basically descended from the SM internally fired, FT boiler design. They became immensely popular during the early 1960s because of their versatility, cost-effectiveness, and relative efficiency (this last factor became very important following the fuel price crises of the early to mid-1970s). 

Modem, packaged vertical boiler designs are also very popular and are widely used for smaller or specialist applications. Today s vertical boilers are the product of over 50 years of continuous development. 

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