The Basics of Steam Boilers
- Steam boilers come in two standard types. In a fire-tube boiler, water surrounds steel tubes through which the blazing-hot gases from combustion flow on their way to the exhaust chimney. As the water boils into steam, the steam collects in a drum-shaped chamber under pressure. As the water keeps boiling, steam pressure builds until there is enough pressure to do work. A safety valve on the steam chamber releases steam at pressures above normal operating pressure. This safety device is used on all boilers. Fire-tube boilers were used on steam locomotives and antique steam tractors and are still in use in smaller industrial boilers and heating boilers.
- With a water tube boiler, the water is in tubes surrounded by the blazing-hot combustion gases. Water tube boilers can produce larger amounts of high pressure steam than fire tube boilers and can produce that steam quickly. Express boilers and flash boilers generate steam almost instantaneously because their water tubes are heated to extremely high temperatures. Water tube boilers are used in steamships, large manufacturing plants and electric generating plants. Unlike fire-tube boilers, water-tube boilers are very unlikely to explode because steam is used as fast as it is produced.
- Boilers are described as one-pass, two-pass, three-pass or four-pass. Passes describe the number of times the boiler water is exposed to the heat released by combustion. Boiler power is expressed in terms of boiler horsepower. One boiler horsepower is the ability to boil 4.15 gallons of water into steam at normal atmospheric pressure. Water's volume expands by 1,700 times when it turns into steam, which is what gives steam its power. It requires 1.9 boiler horsepower to turn one cubic foot of water (7.48 gallons) into 1,700 cubic feet of steam at normal atmospheric pressure. Put another way, to run a 10 hp steam engine, a boiler must turn 13.36 cubic feet of water into 22,730 cubic feet of steam every hour. That equates to 25.4 boiler horsepower.
- Low pressure boilers operate at pressures of up to 15 pounds per square inch (psi). High pressure boilers operate at pressures exceeding 75 psi. Low pressure boilers mostly are used for heating buildings in winter. A heating plant will face large seasonal variations in steam demand but no sudden demand changes. Heating boilers should be sized to meet the most severe winter conditions. High pressure boilers are used for heating, driving machinery and for materials processing. Steam demand for industrial processing can be continuous or intermittent. A properly designed boiler can handle minimum loads, maximum load and any load in between.
