Firetube boilers are used in applications that require moderate pressures and moderate demand. Firetube boilers generally have 2-inch diameter or larger tubes. They are usually straight and relatively short so that the hot gases of combustion experience a relatively low pressure drop while passing through them.

Firetube boilers are generally built similar to a shell and tube heat exchanger. The water space surrounding the tubes is usually contained by a large cylindrical or flat surfaced vessel. For this reason, firetube boilers are rarely designed for more than 300 psi as the required wall thickness would be too excessive.

Firetube boilers have a fairly large amount of contained water so that there is a considerable amount of stored heat energy in the boiler. This also allows for load swings where large amounts of steam or hot water are required in a relatively short period of time, as often happens in process applications.
Firetube boilers can take a great deal of abuse and inattention and still function at competent levels. Firetube boilers have a life expectancy of 25 years or more. You can find boilers 75 years old and older are still known to be in operation.

Consistent maintenance and careful water treatment (as we will discuss in the next segment on the third boiler element — water) go a long way towards insuring the long life of these boilers.

Packaged Firetube Boilers

There was a time when an end-user would purchase a boiler shell from one place and firing equipment from another and go down to the hardware store to get the controls and wiring in hopes of making it work...however, in the past 70 years standardized packaged boiler units have taken the place of those “one-of-a-kind” units. Packaged boilers are simply boiler/burner packages complete with all trim components that make them ready from the factory to be shipped and installed as a complete unit.

Packaged boilers are self-contained units that save their owners in installation cost at the time of delivery. Their only requirements after delivery are for electrical, water, fuel and chimney hook-up. These units are also fire tested at the factory before shipment. Those fire test reports are supplied to the owners and can be helpful with start-up settings and efficiency data.

There are two major types of packaged firetube boilers:

Scotch | Firebox

Two other types of firetube boilers are:

Cast Iron | Vertical Firetube