Watertube Boiler Designs PDF

Chapter 1 • Watertube Boiler Designs OBJECTIVE 1 Explain the difference between packaged, shop assembled, and field-erected watertube boilers. Explain how boilers are rated. PACKAGED WATERTUBE BOILER A packaged watertube boiler is assembled in a factory and leaves the factory as a complete package...

Chapter 1 • Watertube Boiler Designs OBJECTIVE 1 Explain the difference between packaged, shop assembled, and field-erected watertube boilers. Explain how boilers are rated. PACKAGED WATERTUBE BOILER A packaged watertube boiler is assembled in a factory and leaves the factory as a complete package. It is shipped complete with fuel burning equipment, mechanical draft equipment, and automatic controls and accessories. These units have water-cooled furnaces and the air for combustion is supplied under pressure from a forced draft (FD) fan. A steel casing covers the outside of the boiler and furnace and prevents combustion gas leakage into the boiler room. A simple skid-type steel foundation is included and the boiler is bottom-supported. Capacities range from 2300 kg to over 65 000 kg of steam per hour. Operating pressures usually range up to 1700 kPa, although they may reach over 6200 kPa in some cases. Figure 1 shows a cutawayview of a two-drum packaged watertube boiler. The water-cooled furnace construction and the compact design of the unit with its steel clad exterior are clearly shown. Figure 1 - Two-Drum Packaged Boiler Steam drum -^ Burner Mud drum 3rd Class Edition 3 • Part B1 Watertube Boiler Designs • Chapter 1 The main advantage of the packaged boiler is its low cost. The manufacturing costs are lower because the entire boiler is built and assembled in the factory rather than in the field. Shipping costs are also lower if the boiler is sent as a unit rather than in parts. Because the packaged boiler is shipped by truck or rail car, the width and height are somewhat standard for any particular design of boiler. Boiler length can be changed to create different capacities. Figure 2 shows a 100 000 kg/h shop-assembled boiler. Figure 2 - Shop-Assembled Boiler Shop-Assembled Watertube Boiler The term shop-assembled could easily be confused with the term packaged. The difference is mainly in the size and the amount of shop assembly that occurs. A simple distinction is that a shop-assembled boiler cannot be ordered "off the shelf"; they are generally assembled to the exact specifications provided by the purchaser. Shop-assembled units are generally larger and cannot necessarily be shipped from the assembly shop as a single unit. However, the larger, main sections of the boiler are assembled in the shop. The steam generating section is fully assembled, with all tube, drum, and header connections complete. The refractory and outer casings may or may not be installed. Burner and draft equipment, including the exhaust stack, may be constructed as separate assemblies. External controls and devices will usually be installed later. The various sections are usually shipped separately to the purchasers site, where final assembly occurs. Site construction time is limited to this final assembly, plus any refractory, casing, control, or auxiliary attachments. Testing and approval of the boiler then occurs on site. 3rd Class Edition 3' Part B1 ^ Chapter 1 • Watertube Boiler Designs Field-Erected Watertube Boilers Boilers too large for transport to the site in one piece must be field-erected. Large sections of water wall, superheaters, and economizers may be constructed in the manufacturers plant and then shipped to the site for assembly. This on-site assembly will include erecting all the support structures, hanging the drums and headers in place, and attaching all tube sections by welding, rolling, or other methods. Burners, air and exhaust ductwork, internal and external refractory and insulation, and outer casings must be installed. Steam drum internals are installed after all tubes have been attached. All external auxiliaries, such as draft fans, air heaters, safety devices, and controls are then attached. Finally, on-site testing, approvals and commissioning are done. Most field-erected boilers have outputs of 150 000 kg/h or more. However, there are some applications where a smaller field-erected boiler is the best solution. Figure 3 shows an example of a smaller field-erected watertube boiler. Arrows on the main body of the sketch show the gas path through the unit. The cross-flow design provides maximum heat absorption and virtual uniformity of temperature and flow of combustion gas across the width of the heating surface. Arrows to the bottom right show an air inlet that connects to a duct under the furnace floor tubes. This duct construction permits less overall unit height than is attainable in units that use overhead air ducts. Capacities range up to 163 000 kg/h of steam at 6550 kPa and 480°C. Fuel for this model of boiler can be natural gas or oil. However, other field-erected boilers may be designed for other fuels, including coal, biomass, and others. Figure 3 - Field-Erected Watertube Boiler 3rd Class Edition 3 • Part B1 Watertube Boiler Designs - Chapter 1 Boiler Ratings Regulations made under various provincial legislative acts specify the following three methods for calculating the output or rating of a boiler: 1. One square metre of heating surface equals 10 kilowatts. The heating surface is the total area of all of the surfaces through which heat is transferred from the furnace or from the combustion gases to the water. The heating surface of a boiler must be determined by computing the area of the surface involved in square metres. Where a computation is to be made of a curved surface, the surface with the greater radius is taken. 2. Where electric power is used as the heating source, the boiler rating is the maximum kilowatt rating of the heating element. 3. Where neither of the above methods is applicable, an hourly boiler output of 36 megajoules is equivalent to 10 kilowatts. Steam Capacity The steam capacity of a boiler is expressed in kilograms of steam per hour. This calculation is not a true measure of the thermal energy supplied by the boiler because other factors, such as steam quality, steam temperature, and feedwater temperature must be considered. Steam generator capacities are described in terms of maximum continuous steam output, in kilograms per hour, at design steam temperature and pressure. Where the boiler provides its entire steam supply to an electric generating unit (i.e., to a steam turbine driving a generator), the output of the boiler is often expressed in terms of the power output of the generator (for example, a 300 megawatt boiler). 3rd Class Edition 3 • Part B1