Industrial boilers are used to make steam or to heat water or other fluids and are fired by a variety of different fuels including coal, natural gas, electricity, and oil. The three basic types of industrial boilers can be configured in several different ways and incorporate a variety of features. They are water tube boilers, fire tube boilers and fire box boilers.
Water Tube Boilers
In water tube boilers the fluid flows through tubes that are encased in a furnace and connected to a steam drum and a mud drum. The fluid is heated in the steam drum, and the mud drum collects solids precipitated out of the fluid because of the high pressure and temperature in the boiler. In order to maximise the amount of steam per unit of feed water, the feed water is cycled through the steam drum several times. As the water from each cycle evaporates the concentration of solids increases. When the concentration of solids becomes great enough to interfere with the boiler's efficiency, the solids are automatically blown into the mud drum with compressed air.
Water tube boilers can handle greater pressures and temperatures, and they are available in larger sizes than fire tube or fire box boilers. However, the initial cost of water tube boilers is higher and they are more difficult to clean than the other two main types of industrial boiler.
Fire Tube Boilers
In fire tube boilers the hot gases from the heat source are channelled through tubes that are surrounded by the fluid being heated. The heated fluid may remain liquid or may produce low-pressure steam (below 250 psi).
Fire tube boilers have lower initial cost than water tube boilers and they are easier to clean. It is also easier to replace tubes. They are not suited for producing high pressure steam, but do well for space heating and industrial process heating. When space is an issue their relatively small size is an asset.
Fire Box Boilers
The fire box is the space or "box" where the fuel is burnt or where the heat source is. In a fire box boiler, the hot gases from the fire box are directed into the fire tubes where they heat the fluid. From that point it behaves just as a fire tube boiler, and fire box boilers can arguably be called a type of fire tube boiler. The standard locomotive steam boilers were fire box boilers.
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