A boiler or steam generator is a device used to create steam by applying heat energy to water. There are two general types of boilers: ''fire-tube'' and ''water-tube''. Boilers are classified as "high-pressure" or "low-pressure" and "steam boiler"

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. 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, and Limitation for high capacity steam generation.


Water-tube Boilers

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 boilers are

  • Available in sizes far greater than a fire-tube design

  • 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

Water tube boiler is more efficient than fire tube boiler

A boiler with more passes has a higher heat extraction rate and is more efficient then one with less passes. Once a fire tube boiler is started up and is operating at its desired pressure, a fire tube boiler can handle a sudden upward load surge better than a water tube boiler because of the large steam disengaging area.

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

Boiler System Major Components

Feedwater Heaters: Feedwater heaters are energy recovery devices generally found only in large steam generating plants where all of the steam generated is not reduced to condensate by the steam user. This "waste steam" is reduced to condensate for return to the boiler in the feedwater heater. The boiler feedwater is used as a cooling medium to reduce the steam to condensate, which increases the temperature of the feedwater and, thereby, increases the thermal efficiency of the boiler.

Fuel Heater: Many boilers firing heavy fuel oil require fuel heaters to reduce the fuel viscosity, so the fuel can be atomized by the burner system for complete combustion.


Deaerators: A deaerator is a special case of feedwater heater that is designed to promote the removal of non-condensable gases from the boiler feedwater.

Pumps: In most hot water systems, the system circulating pumps are electric motor-driven, end suction centrifugal pumps. In steam systems, the condensate return pumps are typically electric motor-driven, end suction, centrifugal or turbine-type pumps. Feedwater pumps are generally electric motor-driven, multiple-stage, end suction centrifugal pumps.


Combustion Air Blowers: In many packaged boiler installations, the combustion air fan is designed and provided by the boiler manufacturer and is integral with the boiler housing. In installations where a stand-alone fan is provided, low-pressure centrifugal blowers are commonly used.

Economizer: An economizer is an energy recovery device that uses the hot exhaust gases from the boiler (waste heat) to heat combustion air or feedwater.


Steam Traps: Steam traps are installed throughout steam systems to remove condensate (spent steam), air, and non-condensable gases from the steam system.