Industrial boilers are closed vessels that use a fuel source or electricity to heat water or generate steam for industrial heating and humidification applications. The basic concept of a boiler involves a heat source (furnace) and a heat exchanger (pipes or tubes) or heat transfer medium which allows water to be heated above its boiling point. The type of heat source and the method of heat exchange are what primarily define different types of boilers.

Type

There are many different types of industrial boilers available. Selecting between these types can be simplified by looking at three separate aspects of boilers: function, technology, and heat source.

Function

The most fundamental decision when selecting a boiler is what its function will be. Boilers can either be used to produce hot water or produce steam.

Hot water boilers are used to heat water for hydronic or hot water heating systems. Typical applications include domestic and commercial hot water needs, comfort heating, freeze protection, commercial dishwashers, radiant floor heating, car washes, laundromats, etc.

Steam boilers heat hot water to boiling in order to generate steam for use in different applications. These include turbines for power generation, various industrial heating applications (e.g. cement production), and agricultural soil steaming.

Design

Boilers can also be distinguished based on their design and construction, which defines their method of heat exchange.

Fire-tube boilers feed hot gases from the heating source through tubes. These tubes are positioned inside a water filled drum in order to transfer heat to the water. Fire-tube boilers are more fuel efficient and easier to operate than water-tube boilers, and because of their simple construction also tend to have a lower initial cost. However, because the thickness of the outer shell (which contains and pressurizes the water) is limited by cost and practicality, fire-tube boilers are typically limited to low pressure and low to medium capacity applications. Fire-tube boilers are subdivided into three groups.

• Horizontal return tubular (HRT) boilers typically have horizontal, self-contained firetubes with a separate combustion chamber.

• Scotch, Scotch marine, or shell boilers have the firetubes and combustion chamber housed within the same shell.

• Firebox boilers have a water-jacketed firebox and employ at most three passes of combustion gases.

Water-tube boilers feed water (rather than combustion gases) through tubes. A large number of these tubes run through a boiler drum (casing) where heated gases or heating elements surrounding the tube bring the water to its boiling point. This heat transfer method makes water-tube boilers more thermally efficient than fire-tube boilers. However, they are more complex to construct and are less tolerant of poor water quality, requiring water treatment and/or solids filtering in order to operate effectively. These boilers are selected when steam demand and steam pressure requirements are high.

Cast iron (sectional) boilers are fabricated from a number of cast iron sections that are bolted together. In their simplest form, they consist of merely a firebox surrounded by a water chamber for heat to be transferred directly. They can be assembled inside a room with a conventional size doorway, making them convenient as replacement units and eliminating the need for temporary wall removal to provide access for larger package units. There is minimal need for feedwater, and the boiler water does not concentrate. However, these "boilers" are not typically used to produce steam, but hot water. The brittleness of cast iron makes it impractical for high pressure steam boilers.

Combination or hybrid boilers are boilers which combine design and operational concepts from multiple different boiler types to maximize performance and efficiency for certain applications and conditions.

Combustion Technology

Some boilers may be designed according to the technology used to burn or heat a specific fuel source.

Fluidized bed systems utilize a "bubbling" bed of solid particles (sand and fuel) in order to obtain a uniform heating temperature and effective fuel-air contact. This technology offers a number of advantages, including compact boiler design, fuel flexibility, higher combustion efficiency, and reduced emission of noxious pollutants. Fluidized bed combustion is the new standard approach for boilers using biomass as a fuel source.

Stoker systems typically use a lump coal feed and bed of burning coals as the heat source. The two common types are spreader stokers and travelling-gate or chain-grate stokers.

Spreader stokers utilize a continuous feed of coal above a burning bed of coals. A spreader is used to evenly distribute the feed. This method of firing provides good system flexibility to meet load fluctuations.

Travelling-grate or chain-grate stokers utilize a coal feed on one end of a moving steel grate. The grate moves through the boiler as the coal burns and drops off at the end as ash. A coal gate is used to control the rate at which coal is fed into the furnace by controlling the thickness of the fuel bed. Coal in these boilers must be of uniform size so the coal is not burning when it reaches the end of the grate.

Thermic fluid heaters can also be used in boilers. These devices use petroleum-based fluids as the heat transfer medium, providing consistent temperature control in the system. Because condensate drain and flash steam are non-existent in thermic fluid heaters, they also have good thermal efficiencies and operate with minimum losses in closed cycle operations.

Heat Source

Another key aspect that differentiates boilers is the heat source used.

Coal is the standard fuel source for industrial boilers. Most coal-boiler applications used pulverized coal (coal ground to a fine powder), which burns more completely than larger clumps of coal.

Biomass fuel includes all types of burnable organic material, including rice husk, wood chips, and other agricultural waste products.

Electric is used in the form of either resistance heating coils or electrode-type units to provide heat for boilers, typically those of very low capacities or those for commercial/domestic use. Electrode-type electric boilers require high-quality water and proper conductivity to function effectively. The unit must also be shut down periodically to clean the insulators to prevent arcing. Electric boilers operate quietly, are easy to service and maintain, and are often cheaper than combustion boilers for low-capacity operations.

Gas-fired boilers are designed to operate on either propane or natural gas. Some gas-fired boilers can be switched over to one or the other with some modifications to account for the different characteristics of the fuels.

Oil-fired boilers used gasoline or other petroleum-based fluids as the fuel source. Thermic fluid heaters (described above) use these fluids as the heat transfer medium.

Hydronic boilers use steam as the heat source in the system's heat exchanger. Heat is transferred from the steam as it cycles through the system to heat the target water for hot water generation or for creating steam to run turbines in power plant operations.

Wood is a fuel source used for some traditional boiler designs.

Design Tip: The burner and firebox (chamber where burning takes place) in a boiler are designed for a particular type of fuel. Conversion to a different type of fuel requires a different type of burner and could require changes to the firebox size and shape.

Specifications

There are a few key specifications to consider when sourcing an industrial boiler.

Heat output or capacity is the amount of heat energy the boiler outputs over time, typically expressed in Btu/hour. Sometimes capacity is expressed in units of boiler horsepower (BHP), where 1 BHP equals 33,472 Btu/hour.

Maximum temperature is the maximum operating temperature of the boiler, or the highest temperature the system is designed to reach.

Maximum pressure is the maximum operating pressure of the boiler, or the highest pressure the system is designed to reach.

Efficiency, sometimes referred to as thermal efficiency, defines what percentage of heat generated by the boiler is useful energy (i.e. is used to heat the water in the system). It is determined based on the energy input minus the stack loss (waste heat) divided by the energy input.

Features

Industrial boilers may possess a number of features and accessories which are important for users to consider.

Deaerators are vessels in which boiler feedwater is heated under reduced pressure in order to remove dissolved air.

Packaged boilers are delivered to a site as a complete package, only requiring the steam, electrical connections, fuel supply, and pipework to be operational. Package boilers are typically characterized by high heat release rates, good thermal efficiencies, and small footprints.

Safety valves are spring-loaded valves that open automatically when pressure attains the valve setting. They are used to prevent excessive pressure from building up in a boiler.

Superheaters are banks of tubes in the exhaust gas duct after the boiler. They are auxiliary pieces of equipment used to superheat the steam, or heat it above its saturation temperature.

Water treaters or conditioners remove minerals from boiler water that could lead to scaling on the tube or casing walls.

Standards

Industrial boilers are subject to a number of different standards which dictate different parts of boiler design and function.

ASME (American Society of Mechanical Engineers) standards and codes for boilers exist to certify manufacturer's design and ensure proper construction.

EPA (Environmental Protection Agency) Clean Air Act emissions standards and codes exist to regulate the allowable air pollution given off by boilers, including sulfur oxide, nitrogen oxide, and other emissions.

References

Type of Boilers - United Nations Environment Programme

Image Credit

Alstrom Energy Group | ACME Engineering Products, Inc.