Types of Boilers
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A wide range of options
Boiler systems are classified in a variety of ways. They can be classified according to the end use, such as foe heating, power generation or process requirements. Or they can be classified according to pressure, materials of construction, size tube contents (for example, waterside or fireside), firing, heat source or circulation. Boilers are also distinguished by their method of fabrication. Accordingly, a boiler can be pack aged or field erected. Sometimes boilers are classified by their heat source. For example, they are often referred to as oil-fired, gas-fired, coal-fired, or solid fuel –fired boilers.
Firetube boilers consist of a series of straight tubes that are housed inside a water-filled outer shell. The tubes are arranged so that hot combustion gases flow through the tubes. As the hot gases flow through the tubes, they heat the water surrounding the tubes. The water is confined by the outer shell of boiler. To avoid the need for a thick outer shell firetube boilers are used for lower pressure applications. Generally, the heat input capacities for firetube boilers are limited to 50 mbtu per hour or less, but in recent years the size of firetube boilers has increased.
Firetube boilers are subdivided into three groups. Horizontal return tubular (HRT) boilers typically have horizontal, self-contained firetubes with a separate combustion chamber. Scotch, Scotch marine, or shell boilers have the firetubes and combustion chamber housed within the same shell. Firebox boilers have a water-jacketed firebox and employ at most three passes of combustion gases.
Most modern firetube boilers have cylindrical outer shells with a small round combustion chamber located inside the bottom of the shell. Depending on the construction details, these boilers have tubes configured in either one, two, three, or four pass arrangements. Because the design of firetube boilers is simple, they are easy to construct in a shop and can be shipped fully assembled as a package unit.
Watertube boilers are designed to circulate hot combustion gases around the outside of a large number of water filled tubes. The tubes extend between an upper header, called a steam drum, and one or more lower headers or drums. In the older designs, the tubes were either straight or bent into simple shapes. Newer boilers have tubes with complex and diverse bends. Because the pressure is confined inside the tubes, watertube boilers can be fabricated in larger sizes and used for higher-pressure applications.
Small watertube boilers, which have one and sometimes two burners, are generally fabricated and supplied as packaged units. Because of their size and weight, large watertube boilers are often fabricated in pieces and assembled in the field. In watertube or “water in tube” boilers, the conditions are reversed with the water passing through the tubes and the hot gases passing outside the tubes. These boilers can be of a single- or multiple-drum type. They can be built to any steam capacity and pressures, and have higher efficiencies than firetube boilers.
Almost any solid, liquid or gaseous fuel can be burnt in a watertube boiler. The common fuels are coal, oil, natural gas, biomass and solid fuels such as municipal solid waste (MSW), tire-derived fuel (TDF) and RDF. Designs of watertube boilers that burn these fuels can be significantly different.
Electric boilers can use electric resistance heating coils immersed in water and are normally very low-capacity units. Other types of electric boilers are electrode-type units that generate saturated steam by conducting current through the water itself. Boiler water conductivity must be monitored and controlled. If the conductivity is too low, the boiler will not reach full operating capacity. When the conductivity is too high, over-current protection will normally shut off the power.
Proper conductivity and high-quality water as well as effective water treatment is required. Solids from the saturated steam tend to accumulate slowly on the insulators supporting the electrodes from the grounded shell. The unit must be shut down periodically so that the insulators can be washed off to prevent arcing. Finally, voltages of up to 16 kV may be used. Protection is needed for ground faults, over-current and, for three-phase systems, loss of phase. The main electrical disconnect switch must be locked out before performing maintenance on the boiler.
Other boiler classifications
Cast Iron boilers
Cast iron boilers are fabricated from a number of cast iron sections that are bolted together. The design of each section includes integral water and combustion gas passages. When fully assembled, the interconnecting passage create chambers where heat is transferred from the hot combustion gases to the water. These boilers generally produce low-pressure steam (15 psig) or hot water (30 psig) and burn either oil or natural gas.
Because of their construction, cast iron boilers are limited to smaller sizes. Because the components of these boilers are relatively small and easy to transport, they can be assembled inside a room with a conventional size doorway. This feature means that cast iron boilers are often used as replacement units, which eliminate the need for temporary wall removal to provide access for larger package units. They consist simply of a firebox surrounded by a water chamber for heat to be transferred directly from the firebox to the boiling water or to tube-type water heaters, while there are no boiler tubes. There is minimal need for feedwater, and the boiler water does not concentrate.
Another boiler type that is sometimes used to produced steam or hot water is the tubeless boiler. The design of tubeless boilers incorporates nested pressure vessels with water located between the shells. Combustion gases are fired into the inner vessel where heat is transferred to water located between the outside surface of the inner shell and the inside surface of the outer shell. For oil-fired and natural gas-fired vertical tubeless boilers, the burner is typically located at the bottom of the boiler.
Some special applications of boilers require specific designs and operating procedures. These include waste to steam (trash to steam), waste heat recovery and heat recovery steam generators (HRSG).