Firetube Boilers

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

Vertical Firetube Boiler

There is one other type of firetube boiler we want to mention, it is the vertical firetube boiler. These packaged boilers come in two formats:

  • • Vertical Tubeless
  • • Vertical Tubed

These boilers are encountered on the smallest end of the boiler spectrum. They are used to produce steam for light industrial/commercial applications such as: • dry cleaning • laminating • sterilization • bottle washing • concrete curing • garment manufacturing • plating • dairies • cooking •
Vertical firetube boilers are fueled by either gas or oil. They are an inexpensive solution to a small industrial — commercial need for process steam.

Firebox Boilers

Firebox design boilers carry with them the tradition of a basic boiler design proven over the last hundred years. A second cousin to the brickset and locomotive steam engine designs of the past, these boilers feature a simple durable construction that provides low maintenance performance year after year.

The firebox boiler is typically manufactured to low pressure steam or hot water heating applications often in institutional or light commercial applications. The firebox boiler is a compact, economical unit and serves as a good fit for seasonal use and in applications where high efficiency is not a driving concern.
Firebox design boilers are very similar to scotch firetube boilers in design except they have combustion chambers instead of immersed internal furnace tubes.

These combustion chambers are large crowned combustion zones located at the bottom and center of the boiler. It is because of these large combustion chambers that firebox boilers are still appropriate for the burning of solid fuels. From the combustion chamber are tubes arranged as in a scotch boiler which carry the hot combustion gases through the boiler water on their way to the vent.

Cast Iron Boilers

Cast iron boilers are a type of firetube boiler as well. In most original boiler rooms, the building was built around or on top of the boiler room. When it came time to replace many of these boilers, there was no convenient way to bring in a new boiler. The cast iron sectional boiler provided a solution to this problem.

Cast iron sections are taken individually into the boiler room and field assembled at the jobsite. While the initial cost is low, much on-the-job labor is required to make the boiler fully operational.

Cast iron boilers are used for closed, low pressure heating systems only. These boilers also have small steaming areas so that they are very sensitive to changes in demand or water levels.

Cast iron boilers have rough surfaces that require a lot of water treatment to maintain proper operation. They are sensitive to scale and require care in blowing down. It is difficult to remove sediment and sludge that form in the waterside of this design. That debris becomes encrusted causing the surfaces to overheat and crack. If a section becomes cracked, the entire boiler must be disassembled.

Scotch Boilers

Scotch boilers (often referred to as scotch marine boilers) derive their name from the Scottish shipyards that built boilers for marine vessels for the British Navy. Old brick set boilers used to burn through the bottom of the ships. The “Scotch Marine” design featured a cylindrical vessel enclosing a totally immersed furnace so they could not do damage to the British naval vessels.

These were the first truly “packaged boilers” since they had to be created for shipboard installation with everything essential already attached. From their use on ships, this same type of boiler caught on in the 1920’s as an alternative to the brickset boilers that were commonly constructed at that time.
Scotch design boilers are typically used for low or high pressure steam or low water heating and process applications often in industrial or institutional applications. They have medium to high space requirements. They are compatible with a variety of building management systems and are appropriate in situations calling for high efficiency.

Scotch design boilers all have an immersed internal furnace with tubes which carry the hot combustion gases through the boiler water on their way to the vent. The differences between scotch boilers produced by various manufacturers are:

  • Number of Passes
  • Rear Portion Construction

We reviewed the implications of the Number of Passes in the previous section. And while we have stated that the same amount of heat will be transferred by a boiler of any number of passes as compared to any other boiler if the boilers have equal heating surface...differences exist in the “quality” of the heating surfaces.

For example: Three-pass type boilers have greater primary heating surface and require fewer tubes. Four-pass type boilers have lower primary heating surface and require more tubes. Three-pass boiler types are generally accepted as the design which provides optimum economy of material in a compact vessel with acceptable draft loss.

In dealing with Rear Portion Construction, the terms “wet-back” and “dry-back” come into play. These are graphic terms which describe varying construction of a boiler at the point where the gases exit from the furnace and reverse direction to enter the first pass tubes:

  • If the rear chamber is constructed so that its rear wall is water-backed, the boiler is a “wet back” boiler.
  • If the rear chamber is enclosed at the rear by a removable cover containing refractory, the boiler is a “dry-back.”

The major advantage of dry-back construction is an unrestricted access to the rear ends of the flue tubes when the cover is opened.

Major Disadvantages are: that valuable radiant heating surface is sacrificed during the hottest portion of the heat exchange chain. The maintenance costs, in both time and dollars, involved in maintaining the refractory door required in “dry-back” construction is also a major drawback.

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