How Watertube Boilers Work

How Watertube Boilers Work

While used throughout the boiler industry in a variety of shapes and sizes, watertube boilers are unique in the higher pressures and capacities they can provide. 

How it Works

Burning gasses travel through the heat exchange passes of the boiler, through the large open combustion chamber and back through the narrower paths of tubes. These gases heat the water inside the tubes. As water circulates in the tubes from the lower mud drum, up to the steam drum. Boiler water and steam rise and the steam is released as it enters the top steam drum.

Watertube Boilers vs Firetube Boilers

The difference between a watertube boiler and a firetube boiler is right in the name -- they swap how the boiler handles fire and water.

A watertube boiler has pipes (tubes) filled with water which are surrounded and heated by hot gases, whereas a firetube boiler’s pipes (tubes) are filled with fire and heat the water that surrounds them.

Because steam and mud drums of the watertube boiler are much smaller in diameter than the significantly bigger shell diameter of a firetube boiler, these units can be designed for far higher pressures. Many power plant designs operate beyond 3000psi.

Because there is far more surface area in a watertube boiler, the capacity can be increased to tens of times that of the very biggest firetube boilers.

The elimination of the large cylindrical furnace of the firetube boiler with a tube walled combustion chamber also makes the watertube boiler safer. Overheating will result in the failure of individual small tubes rather than a single massive furnace tube.

Are There Any Drawbacks?

Cost and availability. Large watertube boilers are built to order, because they are expensive and often specialized for their application, so they are not as rapidly available as many firetube boilers. Fortunately, rentals are available for sudden or short-term needs.

Water treatment is essential. Important for all boilers, the smaller water passes require proper water conditioning and chemical treatment to keep the vessel in efficient working order.

Repairs are expensive – swapping out a set of tubes on a large industrial water tube boiler is far more costly than changing the straight tubes in a firetube boiler.

Types of Watertube Boilers

There are three main types of watertube boilers, each named for their relative shape.


This boiler has the steam and water drums one above the other. The water pipes are curved between the drums, creating an O shape.


Water is sent from the water drum to the generating tubes, which form the fire box. The water is boiled into steam and rises into the steam drum, and then to the steam header.


This boiler has two water drums. The two water drums’ contents are boiled by a shared header, then sent up generating tubes, into the steam drum and from there, to the steam header.

What Type of Boiler is Right for You?

Whatever your needs, WARE has a boiler for you. Explore our rentals here, or if you’re still not sure what type of boiler suits your needs, contact us.

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