Boilers

View A Video on Boilers - A Quick Introduction

Almost all boilers can be placed into one of those three categories (there are exceptions: Central Boiler's systems are wood fired). Combi boilers are different from conventional models because they do not store hot water but rather heat it rapidly on demand. They have a second heating element to speed the heating process and maintain a steady supply for as long as the boiler runs. Condensing boilers are able to recover energy that would normally be expelled through the flue by condensing the escaping water vapor back into liquid water. Because the boiler uses latent heat to boost its efficiency to above 90%, it is considered a high efficiency boiler. Another way to increase efficiency is through a high pressure boiler, which maintains an operating pressure greater than 15 PSI; some models achieve pressures as high as 1,000 PSI. Low pressure boilers have pressure less than 15 PSI and are used mostly in smaller buildings for heating a space. Industrial boilers are used either as part of a heating system or to individually generate hot water or steam for various industrial and manufacturing processes. They are much larger in size and built from heavy duty materials to ensure safety, durability and performance.

Boilers provide residential, commercial and industrial heating and are a key part in hydronics, which is the use of water as a heat-transfer medium. Boilers are constructed of stainless steel, aluminum or cast iron and are composed of a furnace with various heat exchanger components and a pump or fan. The two basic designs include fire-tube and water-tube boilers. Fire-tube boilers work by distributing heat through tubes immersed in water; water-tubes circulate water through heated tubes within the enclosure. Hot flue gases are passed over the tubes, heating the water, and then are discharged through a stack. Both designs are comparable in efficiency but are not interchangeable, because of structural considerations. Boilers rely on radiant heat and the transfer of thermal energy to guide the movement of the water or steam. Because heat moves from areas of high temperature to low temperature, the furnace is able to heat the tubes that heat the water, which in turn heats the pipes or radiators that heat the room; virtually the entire process relies on the behavior of high temperature fluids.

The purpose of a boiler is to provide hot water or to generate steam. Steam boilers produce steam for heating large buildings such as factories and warehouses; in other industrial applications the steam can be used for power. Hot water boilers are an important part in hydronics because they heat the water that circulates through a series of pipes and into radiators, baseboards and other heat exchangers. Boilers generally use one of three sources of fuel: gas, oil or electricity. Gas boilers are fueled by natural gas and use a natural gas flame to heat water to a specified temperature or to its boiling point, thereby producing steam. They are the most common type of boiler used for heating applications in the United States. Oil boilers use oil either as fuel for the furnace or to replace the water generally found in boilers and produce hot gases that travel through the piping system. Electric boilers offer efficient, clean and safe water heating. Unlike appliances that provide heating through the process of combustion, electric boilers eliminate the problems with carbon monoxide, back drafting, pilot lights, gas piping and venting.

Boilers have historically been used to produce steam for power. The steam locomotive and steam engine are two examples of this application. They were used industrially and were fairly dangerous, as early operators did not fully understand the relationship between temperature and pressure. Concerned citizens and workers contemplated getting rid of steam boilers altogether until advances in safety and knowledge were made. Today, boilers operate in many different settings such as schools and office buildings. The materials, design and construction have all improved to the point that fear is no longer a factor. Accidents do still occur but with less frequency and with fewer injuries or fatalities. Boilers are now an extremely valuable part of life and can be found in almost any environment. Once quite large, boilers are becoming smaller and smaller in order to fit into tight spaces, though some boilers, like those used industrially, must remain their size in order to produce the necessary amounts of steam and water. Mobile boilers are now available and can be used in military, emergency or temporary situations to provide steam for heat or power. Even as technology continues to make significant advances, boilers will most likely remain a critical part of life for years to come.

Images Provided by Hurst Boiler & Welding Company, Inc.
Images Provided by Unilux Advanced Manufacturing, LLC

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Boiler Types

• Cast iron boilers are modular boilers that are limited to low-pressure steam or hot water applications.
  
• Central boilers provide steam and heat for a larger complex heating system. Central boilers are often housed in a special environment with cooler temperatures surrounding them to prevent overheating.
• Combi boilers are tanks or vessels used primarily to transfer thermal energy from a heat source to water or stream in accordance with the demand for hot water.
• Condensing boilers are tanks or vessels that transfer thermal energy from a heat source to water or steam while recovering energy that would normally be expelled through the flue. By condensing the escaping water vapor back into liquid water, the boiler uses latent heat to boost its efficiency up to 98%.
• Double boilers allow for fine-tuned control and provide high thermal efficiency.
  
• Electric boilers are water or water/steam units powered by electricity rather than gas or other fuel.
  
• Firebox boilers use tube attachment techniques that are similar to those of the firetube boiler, but their combustion chambers are not round. Firebox boilers are compact, economical units that are typically used seasonally in low pressure steam or hot water applications in which efficiency is not a primary factor.
  
• Fire-tube boilers are cylindrical vessels with the flame in the furnace and the combustion gases inside the tubes. The furnace and tubes are within a larger vessel, which contains the water and steam.
  
• Flexible water-tube boilers, also called "bent tube boilers," are a common type of boiler valued for their resistance to thermal shock. Flexible water-tube boilers are used in low pressure steam or hot water applications and can be a part of a field-erectable package.
  
• Gas boilers use natural gas to heat the water and generate the steam necessary for heating applications.
• High efficiency boilers are tanks or vessels that transfer thermal energy from a heat source to water or steam and then recover heat that would typically escape through the flue. Condensing boilers are the most efficient boilers with efficiencies above 90%.
• High pressure boilers are tanks or vessels that facilitate the transfer of heat from its source to water or steam while maintaining a high internal pressure.
• Hot water boilers are tanks or vessels that transfer thermal energy from a heat source to water that is then circulated to provide residential, commercial and industrial heating. Hot water boilers are an important part in hydronics.
• Industrial boilers are used either as part of a heating or HVAC system or to individually generate hot water or steam for various industrial processes. Boilers may be powered by oil, natural gas, or electricity.
• Low pressure boilers are tanks or vessels that facilitate the transfer of heat from its source to water or steam at pressures below 15 pounds per square inch.
  
• Membrane watertube boilers are compact boilers with high outputs that are used in high or low pressure or hot water applications. Membrane watertube boilers are ideal for applications in which space is limited.
  
• Mobile boilers are used often in temporary shelter environments, such as military camps or for emergency boiler breakdown situations. Mobile boilers can vary greatly in size and capacity and be towed to a location via semi-trucks or small vehicles.
• Oil boilers are tanks or vessels that either use oil as fuel for the furnace which in turn heats water that circulates through radiators, baseboards or other heat exchangers or as a replacement for the water generally found in boilers and produce hot gases that travel through the piping system.
• Packaged boilers are units that produce both heat and hot water to an environment.
  
• Steam boilers are boilers whose primary function is to produce steam. Steam boilers are a general type of boiler.
  
• Tubeless boilers are vertical boilers that have the burner located either at the bottom, middle or top. Tubeless boilers are easily operated boilers that have no tubes but collect the steam over the water in a large jacket or "U" tube.
  
• Vertical boilers are commonly used in steam trucks, buses, trams and portable equipment, such as donkey engines utilized for sawmilling and dock work. The design of the vertical tube boiler lends itself to rough handling while in steam, making it ideal for use in situations in which the work plant has to be moved frequently over rough terrain.
  
• Waste-heat boilers make use of the rejected heat from other processes, such as gas turbines.
  
• Water tube boilers are safe boilers that consist of a header drum to which the water tube pipes connect. The drum and the tubes are usually surrounded by an insulating jacket or brickwork, and the fire and products of combustion are directed to pass through the tubes a multiple number of times, horizontally or vertically.

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Boiler Terms

Accumulation Test - A test that measures the relieving capacity of boiler safety relief valves.

Ambient Air - The air that surrounds the equipment.

Aspirating Burner - A burner in which the fuel, in either a gaseous or finely divided form, is burned in suspension. The air for combustion is supplied by bringing it into contact with the fuel as it is drawn through one or more openings by the lower static pressure created by the velocity of the fuel stream.

Boiler Lay-Up - The removal of a boiler from service for a length of time. A boiler may be laid-up dry or wet.

Boiler Pressure - Pressure of the steam of water in a boiler, generally expressed in pounds per square inch (psi) and corresponding temperature.

Boiler Vent - A valved port used to vent air from a full boiler and to prevent a vacuum from forming when the boiler is drained. Boiler vent openings are located on the highest part of the waterside of the boiler.

Bottom Blowdown - The draining of a portion of the water in the boiler in order to remove the heavy sludge that tends to settle at the bottom. This process is done periodically.

BTU (British Thermal Unit) - Amount of heat needed to raise or lower the temperature of one pound of water by one degree Fahrenheit under standard pressure.

By-Pass Line - A pipeline that passes around a control in order to allow the boiler to be operated manually without having to use the control.

Continuous Blowdown - A small, continuously draining stream of water that controls the quantities of impurities in a boiler.

Cut-In Pressure - A pressure control setting at which the boiler automatically turns on.

Equalizer - Connections between parts of a boiler to equalize pressures.

Explosion Door - A door in a furnace or boiler setting that is designed to be opened by a pre-determined gas pressure.

Flash Point - The lowest temperature at which, under specified conditions, fuel oil gives off enough vapor to flash into a momentary flame when ignited.

Furnace - An enclosed space of a boiler in which the fuel undergoes combustion.

Pilot - A flame which is utilized to ignite the fuel at the main burner or burners.

Stack - A vertical conduit that, due to the difference in density between internal and external gases, creates a draft at its base.

Steam - The vapor phase of water, unmixed with other gases.

Vaporization - The change from liquid or solid phase to the vapor phase.

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