Fire-tube and Water-tube Boilers

Fire-tube Boilers

In fire-tube boilers, combustion gases pass through the inside of the tubes with water surrounding the outside of the tubes. The advantages of a fire-tube boiler are its simple construction and less rigid water treatment requirements.

The disadvantages are the excessive weight-per-pound of steam generated, excessive time required to raise steam pressure because of the relatively large volume of water, and inability to respond quickly to load changes, again, due to the large water volume.

The most common fire-tube boilers used in facility heating applications are often referred to as ''scotch'' or ''scotch marine'' boilers, as this boiler type was commonly used for marine service because of its compact size (fire-box integral with boiler section).

The name "fire-tube" is very descriptive. The fire, or hot flue gases from the burner, is channeled through tubes ('''Figure 2''') that are surrounded by the fluid to be heated. The body of the boiler is the pressure vessel and contains the fluid. In most cases, this fluid is water that will be circulated for heating purposes or converted to steam for process use.

Fire-tube Boiler Gas Flow

Every set of tubes that the flue gas travels through, before it makes a turn, is considered a "pass." So, a three-pass boiler will have three sets of tubes with the stack outlet located on the rear of the boiler. A four-pass boiler will have four sets and the stack outlet at the front.

Fire-tube boilers are:

• Relatively inexpensive
• Easy to clean
• Compact in size
• Available in sizes from 600,000 btu/hr to 50,000,000 btu/hr
• Easy to replace tubes
• Well suited for space heating and industrial process applications

Disadvantages of fire-tube boilers include:

• Not suitable for high pressure applications 250 psig and above
• Limitation for high capacity steam generation

Water-tube Boilers

In a water-tube boiler ('''Figure 3'''), the water is inside the tubes and combustion gases pass around the outside of the tubes. The advantages of a water-tube boiler are a lower unit weight-per-pound of steam generated, less time required to raise steam pressure, a greater flexibility for responding to load changes, and a greater ability to operate at high rates of steam generation.

Water-tube Boiler

A water-tube design is the exact opposite of a fire-tube. Here, the water flows through the tubes and is encased in a furnace in which the burner fires. These tubes are connected to a steam drum and a mud drum. The water is heated and steam is produced in the upper drum.

Large steam users are better suited for the water-tube design. The industrial water-tube boiler typically produces steam or hot water primarily for industrial process applications, and is used less frequently for heating applications. The best gauge of which design to consider can be found in the duty in which the boiler is to perform.

Water-tube boilers:

Are available in sizes far greater than a fire-tube design, up to several million pounds-per-hour of steam Are able to handle higher pressures up to 5,000 psig Recover faster than their fire-tube cousin Have the ability to reach very high temperatures Disadvantages of the water-tube design include: High initial capital cost Cleaning is more difficult due to the design No commonality between tubes Physical size may be an issue