If you’ve ever had to work with marine boilers, you’ve probably come across the terms “water-tube” and “fire-tube”. These are 2 basic ways to classify marine boilers. But what do they really mean and how are they different?

What they are: Whether a boiler is described as water-tube or fire-tube (also commonly known as Smoke-tube), it refers to the boiler’s tube design.

In Water-tube boilers, water is heated inside the boiler tubes with hot gases surrounding them. In contrast, Fire-tube boilers contain hot flue gases (smoke) inside the boiler tubes with water surrounding them.

Image source:  Midwest Machinery

Differences in application: Water-tube boilers are chosen for applications that require large quantities of high-pressure steam. This is because water-tube boilers can generate higher pressures of steam of up to 5,000 psig and produce up to several million pounds-per-hour of steam. Most main propulsion boilers are water-tube boilers due to the larger quantities and pressures of steam required to drive the steam turbines of vessels.

On the other hand, Fire-tube boilers are best suited for applications that only require small quantities of low-pressure steam, such as for auxiliary machinery. Water-tube boilers are also chosen for applications that require steam on demand as steam generation occurs in a much shorter period compared to Fire-tube boilers.

Due to their safer operation and lower maintenance requirements, Water-tube boilers are gaining popularity in the modern age.