A water tube boiler is a variety of boiler which circulates water in tubes. The water in these tubes is heated by a fire, located outside them. Commonly used in high pressure boilers, water tube boilers burn fuel inside a separate furnace. This creates heated gas, which transfers the heat to the water inside the tubes. This water becomes steam, which can be used for heating, to drive turbines, and for all kinds of other day to day uses.

Smaller boilers use multiple tubes separate from the furnace, while larger utility water tube boiler installations actually build the tubes directly into the walls of the furnace itself.

When the water is heated, it rises into a part of the boiler called the steam drum. Saturated steam is removed from the top of the drum. It may then re-enter the furnace via a superheater, creating what’s called superheated steam. This is the kind of steam used to drive turbines, such as those used to make electricity.

This is because ordinary steam is full of water droplets that can cause serious damage to the blades in a conventional turbine. Superheated steam is heated to a temperature of over seven hundred degrees Fahrenheit, or nearly four hundred degrees Celsius. It can even be heated to a higher temperature than this to ensure that there are no water droplets in the steam. This makes it easier to operate the turbine correctly.

The cooler water at the bottom of the steam drum eventually returns to the feedwater drum. This happens through large tubes, referred to as downcomer tubes. This water preheats the water already in the drum, so that less energy is required to turn it into steam again. The exhaust gases from the furnace are also used to increase efficiency. They preheat the air being blown into the water tube boiler furnace and enable more efficient burning. Exhaust gases are also used to help warm the water supply. In power stations, these water tube boilers might be called steam generating units. In your home, they’re the furnace and system that drives your radiators.

Older types of boiler, called fire tube boilers, surround the heat source with water and pass the gases from burning the fuel through the water via tubes. Unfortunately, this structure is much weaker than a water tube boiler, and is almost never used for pressures over three hundred and fifty PSI. That’s because of the danger of explosion. Water tube boilers have a much lower chance of failure, since there’s never a large volume of water in the boiler at any one time.