Boiler is a metal vessel inside which water is heated. At one time, most boilers changed water into steam through boiling. The steam was used to heat buildings or to power such machines as steam shovels or train locomotives. Some steam-producing boilers are still in use today, and many large cities have underground pipes that deliver steam from boilers to buildings. However, most boilers installed today for use by homes and small businesses do not produce steam. Instead, these boilers heat water that is circulated through hydronic systems. In these systems, hot water is pumped through a network of pipes to provide heating for rooms and hot water for bathing, cooking, and other purposes.
The heat used by a boiler usually comes from the burning of oil, natural gas, coal, or wood. Some boilers use heat generated by electric power. Some boilers capture waste heat from other processes, such as the operation of a turbine (type of rotary engine).
Boilers must be strong enough to hold the high pressures inside them without bursting. Law requires that each boiler have a safety valve. When the pressure in the boiler reaches the danger point, the steam or water opens the valve and escapes (see Safety valve ).
Most boilers are made of copper, steel, or cast iron. They vary widely in size. Boilers in large electric power stations may be taller than a 10-story building.
A conventional boiler has an efficiency that usually ranges from 78 to 83 percent. Efficiency refers to the percentage of heat produced by the fuel-burning process that is transferred to the water or steam in the boiler. In the 1990’s and early 2000’s, as fuel costs rose, boiler manufacturers began to produce more high-efficiency condensing boilers. These boilers are designed to capture more heat from the fuel-burning process. They usually have efficiencies of 90 percent or higher. Many high-efficiency condensing boilers are small and lightweight, and are made of stainless steel or aluminum.
A high-efficiency condensing boiler captures heat from water vapor and other by-products of the fuel-burning process. This heat would otherwise escape from the building through a chimney. The boiler reduces the temperature of the by-products, causing them to condense from gases to liquids and release heat. After this heat is extracted, the condensed by-products are discharged via the building’s sewage pipes.
Some impurities in water can cause corrosion and weaken a boiler. Others cause a mineral coating called scale to form on the inside of the boiler tubes. Scale lowers a boiler’s efficiency by reducing the transfer of heat. Chemicals can be added to the water to prevent corrosion or scale (see Water softening ).
See also Furnace ; Heating ; Locomotive (Steam locomotives) ; Steam engine ; Turbine ((Steam turbines)) .
