Types of Boilers

Fire-Tube Boiler (aka Scotch Marine Boiler)

A boiler in which hot gases pass through tubes that are surrounded by water.

These boilers are typically used in low- & high-pressure steam or hot water applications and are known for their ease of maintenance.

Water-Tube Boiler

A boiler in which water circulates inside tubes that are heated externally by combustion gases.

These boilers are used in low- & high-pressure steam or hot water applications and power generation due to their ability to handle higher capacities.

Package Boiler

A factory-assembled boiler system that includes the burner, control systems, and accessories.

These boilers are compact and designed for quick installation.

Scotch Marine Boiler

A type of fire-tube boiler that is commonly used in marine applications and industrial heating.

It features multiple passes of hot gases to improve efficiency.

Vertical Boiler

A small, space-saving boiler with a vertical orientation, often used in limited-space industrial applications.

Electric Boiler

A boiler that uses electric resistance heating elements instead of combustion to generate steam or hot water.

These are used in locations where emissions regulations prohibit fossil fuel combustion.

Condensing Boiler

A high-efficiency boiler that captures and recycles waste heat from the exhaust gases, increasing energy efficiency by utilizing latent heat from condensation.

Steam Generator

A small, high-pressure boiler is used in applications requiring rapid steam production, such as medical sterilization, food processing, and power plants.

Waste Heat Boiler

A boiler that recovers heat from exhaust gases of industrial processes or engines to generate steam, reducing energy waste.

Biomass Boiler

A boiler that burns organic materials such as wood pellets, agricultural waste, or other renewable sources to generate heat or steam.

HRSG (Heat Recovery Steam Generator)

A specialized boiler is used in combined-cycle power plants to recover heat from gas turbine exhaust and generate steam for additional power generation.

Pulverized Coal Boiler

A boiler that burns finely ground coal particles in suspension to improve combustion efficiency and power output. It is commonly used in power plants.

Fluidized Bed Boiler

A boiler that uses a bed of sand-like material in which fuel is burned in a suspended state, improving combustion efficiency and reducing emissions.

Modular Boiler

A system consisting of multiple smaller boilers that operate in sequence to meet varying steam or hot water demands efficiently.

Supercritical Boiler

A high-pressure boiler that operates above the critical point of water (3,200 psi and 705°F), improves efficiency by eliminating the phase change from water to steam.

Boiler Components and Key Terms

Absolute Pressure

The total pressure exerted on a system, equal to the sum of the gauge pressure and atmospheric pressure.

Boiler Efficiency

A measure of the fuel-to-steam efficiency, often used interchangeably with combustion or thermal efficiency.

Boiler Horsepower

The evaporation of 34.5 pounds of water per hour from a temperature of 212°F, equivalent to the generation of 33,475 BTU per hour.

Blowdown

The process of removing water from a boiler to control concentrations of dissolved and suspended solids.

Condensate

Water that forms when steam cools and condenses back into liquid.

Feedwater

Water introduced into a boiler, deaerator, feed tank, during operation to compensate for water lost through evaporation and blowdown.

Saturated Steam

Steam at boiling temperature that contains no superheat.

Superheated Steam

Steam heated above the boiling point of water at a given pressure, increasing energy content and efficiency.

Glossary of Boiler Parts

Air Preheater (APH)

A heat exchanger that warms combustion air using waste heat from flue gases, improving efficiency.

Burner

A device that mixes fuel and air to produce combustion for heat generation.

Combustion Chamber

The part of a boiler where fuel combustion occurs, generating heat for steam production.

Control Panel

A centralized unit containing switches, sensors, and displays used to monitor and regulate boiler operations.

Damper

A movable plate used to regulate airflow in the boiler or flue gas system.

Deaerator

A device used to remove dissolved gases, such as oxygen and carbon dioxide, from feedwater to prevent corrosion.

Downcomer

A pipe that directs water from the steam drum to the mud drum in a water-tube boiler.

Draft Fan

A fan used to regulate air supply and exhaust gases in the combustion process.

Economizer

A heat exchanger that recovers waste heat from flue gases to preheat the feedwater, increasing efficiency.

Expansion Tank

A vessel that accommodates the expansion and contraction of water due to temperature changes.

Flue

A duct or pipe that channels combustion gases from the boiler to the chimney or exhaust system.

Fusible Plug

A safety device that melts at a predetermined temperature to prevent overheating and potential explosions.

Gage Glass (Sight Glass)

A transparent tube that shows the water level inside the boiler.

Low-Water Cutoff

A safety device that shuts down the burner if the water level drops too low, preventing damage.

Mud Drum

A lower boiler drum where sediment and sludge accumulate for removal through blowdown.

Pressure Relief Valve (Safety Valve)

A valve that automatically releases excess pressure to prevent over pressurization and explosions.

Refractory

Heat-resistant materials are used to line the combustion chamber and protect it from high temperatures.

Soot Blower

A device used to remove soot and ash from heat exchanger surfaces to maintain efficiency.

Steam Drum

A boiler component that collects and separates steam from the boiling water in a water-tube boiler.

Superheater

A heat exchanger that increases steam temperature beyond the boiling point, improving energy efficiency.

Turbine

A rotary engine that converts steam energy into mechanical power for electricity generation.

Vent Stack

A vertical pipe or chimney that safely expels combustion gases from the boiler.

Water Column

A component that connects the boiler to the sight glass, providing a clear water level reading.

Also used to employ safety controls for excessively high or low water levels. Can also be used to control the flow of water into the boiler. 

Water Softener

A system that removes hardness-causing minerals from feedwater to reducing scale formation inside the boiler.