Boiler tubes are constructed of carbon steel or alloy steel and are seamless. They are widely utilized in power generating steam boilers, fossil fuel plants, industrial processing facilities, electric power plants, and so on. Boiler tubes can be either medium-pressure or high-pressure boiler pipes. The primary function of a boiler tube is to conduct heat from the furnace to the water that will produce steam for driving machinery.
Carbon steel is used because it is resistant to corrosion when exposed to water at temperatures between 200 and 500 degrees Celsius (392 and 932 degrees Fahrenheit). However, due to its susceptibility to stress cracking when exposed to dry heat, alloy steels are preferred for use in high-temperature applications where there is little liquid water present such as with natural gas.
Alloy steels contain several additional elements in addition to carbon and iron. These additional elements may include chromium, molybdenum, nickel, manganese, and titanium. Each of these additional elements serves a specific purpose within the steel structure to make it more resistant to corrosion or stress cracking under different conditions. For example, chromium increases resistance to corrosion when exposed to water at low temperatures while nickel increases resistance to stress cracking when exposed to dry heat.
Carbon steel boiler tubes must occasionally be replaced due to excessive wear caused by the hard particles in coal dust.
A boiler's pressure vessel is often built of steel (or alloy steel), or traditionally, wrought iron. Wrought-iron boilers are very heavy and require strong foundations; they are also expensive to fabricate. Steel boilers are lighter and can be mass-produced at low cost, so they are the standard for industrial applications.
The other main material used for boilers is aluminum, which is both lightweight and durable. It can be formed into any shape and can be painted to look like steel or other materials. Aluminum has a natural resistance to corrosion because it contains small amounts of other elements that protect it from oxidation. However, like any other metal, an aluminum boiler can become corroded if not treated properly. Corrosion can be prevented by using an anti-corrosive treatment on the surface of the boiler every few years.
Boilers also contain many other components such as burners, heat exchangers, valves, and gauges. Most of these components are made of steel or aluminum. A boiler's pressure vessel must be thick enough to withstand the pressures inside it, but this extra thickness increases its weight significantly. For this reason, boilers use hollow shapes instead of full barrels most of the time. There are two types of hollow boilers: shell and tube.
Fire tube and smoke tube boilers are other names for shell and tube boilers. Fire tube boilers are made up of long steel tubes through which hot gases from a furnace travel and water to be turned to steam circulates. Fire tube boilers are often less expensive to install, more fuel efficient, and easier to run. They do, however, require more regular maintenance than flat plate boilers.
Smoke tube boilers work on the same principle as fire tube boilers but use metal plates instead of tubes. This means they are less affected by corrosion and damage caused by burning materials with high sulfur contents. Smoke tube boilers are also less expensive to manufacture and install than their fire tube counterparts.
Flat plate boilers are generally larger than either type of tube boiler and include an array of flat plates attached to a cold side or header section. Hot gases pass between the plates, where they heat the plates until they glow red-hot. The heated plates then transfer their heat to water that flows over them from the cold side. Flat plate boilers are most commonly used in large power plants where they can supply enough steam to operate large turbines.
The choice of boiler should consider quality, durability, efficiency, and cost. In general, fire tube boilers are higher quality, more durable, and more efficient than smoke tube or flat plate boilers. However, smoke tube boilers are less expensive to purchase and install than fire tube boilers.