Boilers and thermal liquid heaters generate heat for use in other operations. They differ in the fluid medium used to transfer this heat.

Industrial Boiler

(Copyright Cici Boiler Rooms, Inc., Evansville, IN)


Steam boilers heat water to produce steam, which is then used to generate energy or heat for other processes.

Steam Boiler

(Copyright Cici Boiler Rooms, Inc., Evansville, IN)

General Information

Boilers are used to generate steam that then provides heat or power. Water is converted to steam in the boiler. This steam travels through the heating apparatus which can be any piece of equipment that requires steam for operation. The cooled steam is then condensed into water and returns to the boiler to start the cycle again.

Boiler Diagram

Equipment Design

There are three main types of steam boilers: Fire tube, water tube, and cast iron.

In fire tube boilers, the combustion gases travel within the tubes to heat the surrounding water.

Fire Tube Boiler

Water Tube Boiler

In water tube boilers the water travels inside the tubes and the heat on the outside, as shown in the video above.

Cast iron boilers are similar to water tube boilers, but the water is contained in cast iron sections instead of in tubes.

The diagram below shows the components of a fire tube boiler.

Fire Tube Boiler Diagram

(Copyright Cici Boiler Rooms, Inc., Evansville, IN)

All steam boilers have four independent systems. The feed water system supplies water to the boiler, the fuel system transports the fuel, the draft system controls air flow for fuel combustion, and the steam system collects and controls the produced steam.

The most commonly used fuels are coal, natural gas, and oil.

Usage Examples

Most fire tube boilers are portable and are used in oil fields, saw mills, and at construction sites to generate heat or power. Water tube and cast iron boilers may be solid-fuel fired, or they may operate on standard liquid and gas fuels. The water tube boiler below operates on gas fuel and is part of a cogeneration plant where heat from the fuel is simultaneously converted into electrical and thermal energy.

Water Tube Boiler

(Copyright SICESGROUP, Italy)



  • Water and fuel, the only two feeds for this type of boiler, are abundant and inexpensive.
  • Able to handle large capacities and high pressure.
  • Can corrode during shutdown periods.
  • Water must be treated to prevent build-up.
  • There is always some heat loss.

Waste Heat

The picture below shows the immense size of some industrial waste heat boilers.

Waste Heat Boiler

(Copyright Babcock & Wilcox Volund A/S, Esbjerg, Denmark)

General Information

Environmental and economic concerns promoted the development of a boiler that reuses by-products.

Waste heat boilers recycle the heat, steam, or combustible by-products of other processes to provide energy.

Equipment Design

Because of their unconventional fuel source, waste heat boilers have a unique design. Waste heat boilers generally have larger intake capacities than other types of boilers because the entering fuel is not very efficient.

The waste steam or other fuels enter through the inlet duct and travel to the reheater, where any combustible material is burned. The resulting gases travel through a series of high and low pressure tubes to heat the water held in the reservoirs. The resulting steam exits through the outlet stack.

Waste Heat Boiler Diagram

(Copyright Babcock & Wilcox Volund A/S, Esbjerg, Denmark)

Usage Examples

Waste heat boilers are used in many industries. In steel mills, two furnaces supply heat to one boiler. Energy sources are recovered from diesel engine exhaust, ore refineries, and cement kilns. These boilers are also common in the paper and wood industries. The boiler shown below is a tail gas fired waste heat boiler. Tail gas containing CO is burned in the combustor, along with fuel from the refinery to produce steam.

(Copyright Alstom Power Inc., Energy Recovery, Wexford, PA)

Waste heat boilers process the waste fuel from joining processes. The picture below shows an example of a waste heat boiler that can be used for waste-fired power plants as well as gas turbines.

Waste Heat Boilers

(Copyright Babcock & Wilcox Volund A/S, Esbjerg, Denmark)



  • Recycle waste.
  • Good for the environment and business.
  • Depends on other processes for fuel and heat.
  • By-products of other processes are not the most efficient, combustible fuel sources; therefore, waste heat boilers are less efficient than other types of boilers.

Thermal Liquid Heaters

General Information/Equipment Design

Thermal liquid heaters are similar to other boilers in that heat is generated for use in a remote application. In thermal liquid heaters, however, fluids other than water transfer the generated heat.

Thermal liquid heaters are usually liquid tube boilers, comparable to water tube steam boilers. The most common heat sources are gas and oil fuels, and electricity. The thermal fluid heater shown below is used at an asphalt terminal. The oil or gas-fired heater uses thermal fluids to pump generated heat to storage tanks, heat exchangers, or steam generators.

Thermal Liquid Heater

(Copyright Heatec Industries, Inc., Chattanooga, TN)



  • Better than steam boilers if process operates near or below the freezing point of water or above 700°F.
  • Thermal fluids typically do not transfer heat as well as water.
  • Serious containment problems with thermal fluids at high temperatures, include corrosion, erosion, and freezing. Other problems come up with toxicity, explosiveness, and odor.
  • Thermal liquids and equipment are expensive.


Aalborg Keystone, Inc. , Erie, PA

Alstom Power Inc., Energy Recovery , Wexford, PA

Babcock & Wilcox Volund , Denmark

Cici Boiler Rooms, Inc. , Evansville, IN

Heatec Industries, Inc. , Chattanooga, TN

SICESGROUP , Varese, Italy


Elonka, Stephen M., and Anthony L. Kohan. Standard Heating and Power Boiler Plant Questions & Answers . New York: McGraw-Hill, 1984. Print.

Ganapathy, V. Waste Heat Boiler Deskbook . Englewood Cliffs: Prentice-Hall, Inc., 1991. Print.

Jackson, James J. Steam Boiler Operation , 2nd ed. Englewood Cliffs: Prentice-Hall, Inc.,      1987. Print.

Kohan, Anthony L. Boiler Operator's Guide , 4th ed. New York: McGraw-Hill, Inc., 1998. Print.

Payne, F. William, and Richard E. Thompson. Efficient Boiler Operations Sourcebook , 4th ed.      Upper Saddle River: Prentice-Hall, Inc., 1996. Print.

Perry, Robert H., and Don W. Green. Perry's Chemical Engineers' Handbook . 7th ed. New      York: McGraw-Hill, 1997: 27-40 - 27-51. Print.

Shields, Carl D. Boilers . New York:F. W. Dodge Corp., 1961. Print.

Steingress, Frederick M. Low Pressure Boilers . Chicago, IL: American Technical Society, 1970.      Print.


Maureen Hindelang

Erica Mauter

Alex Wozniak

Matthew Robertson

Joseph Palazzolo

Keith Minbiole

Austin Potter