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Boiler is a metal container in which a liquid is heated and changed into a vapor. Most boilers change water into the vapor steam. Steam is used to heat buildings and processes. It changes from vapor to liquid form as it delivers heat into a room or building, giving off even more heat as a result. Some heating systems, called hydronic systems, circulate hot water rather than steam. However, the heat source in these systems is still referred to as a boiler. Steam produced in boilers is also used in steam turbines and for refining oil or drying paper.The process of heating a liquid until it reaches it's gaseous state is called evaporation.  

Heat is transferred from one body to another by means of 

(1) radiation, which is the transfer of heat from a hot body to a cold body through a conveying medium without physical contact, 

(2) convection, the transfer of heat by a conveying medium, such as air or water and 

(3) conduction, transfer of heat by actual physical contact, molecule to molecule.  

The heating surface is any part of the boiler metal that has hot gases of combustion on one side and water on the other.   Any part of the boiler metal that actually contributes to making steam is heating surface.  The amount of heating surface a boiler has is expressed in square feet.   The larger the amount of heating surface a boiler has the more efficient it becomes.   

Sensible Heat

The heat required to change the temperature of a substance is called its sensible heat.  In the teapot illustration to the left the 70 oF water contains 38 Btu’s and by adding 142 Btu’s the water is brought to boiling point.


In the illustration to the left, to change the liquid (water) to its gaseous state (steam) an additional 970 Btu’s would be required.   This quantity of heat required to change a chemical from the liquid to the gaseous state is called latent heat.


The saturation temperature or boiling point is a function of pressure and rises when pressure increases.  When water under pressure is heated its saturation temperature rises above 212 oF. This occurs in the boiler. In the example below the boiler is operating at a pressure of 100 psig which gives a steam temperature of 338 oF or 1185 Btu’s. 


Latent Heat           

When heat is added to saturated steam out of contact with liquid, its temperature is said to be superheated.  The temperature of superheated steam, expressed as degrees above saturation, is referred to as the degrees of superheat.



There are virtually infinite numbers of boiler designs but generally they fit into one of two categories: 

Firetube or as an easy way to remember "fire in tube" boilers, contain long steel tubes through which the hot gasses from a furnace pass and around which the water to be changed to steam circulates.

In  the fire-tube boiler, gases flow through tubes surrounded by water. This type of boiler is used in most steam locomotives, in small factories, and sometimes in heating homes. 

In a firetube boiler the heat (gasses) from the combustion of the fuel passes through tubes and is transferred to the water which is in a large cylindrical storage area. 


s400.jpg (6225 bytes)Common types of firetube boilers are scotch marine, firebox, HRT or horizontal return tube. 


Firetube boilers typically have a lower initial cost, are more fuel efficient and easier to operate but they are limited generally to capacities of 50,000pph and pressures of 250 psig.  



  Watertube or "water in tube" boilers in which the conditions are reversed with the water passing through the tubes and the furnace for the hot gasses is made up of the water tubes.  

In the water-tube boiler, gases flow over water-filled tubes. Both ends of the water tubes are connected to large containers, called drums. All high-pressure and large boilers are of the water-tube type. The small tubes in the water-tube boiler can withstand high pressure better than the large vessels of a fire-tube boiler.

The steam generated in the tubes of a water-tube boiler collects at the top of a drum. It usually flows through a set of tubes called a superheater. The hot gases flow over the steam-filled tubes, increasing the temperature of the vapor. The combination of a boiler and a superheater is often called a steam generator.


 The more common types of watertube boilers are "D" type, "A" type, "O" type, bent tube, and cast-iron sectional. All firetube boilers and most watertube boilers are packaged boilers in that they can be transported by truck, rail or barge.  


Large watertube boilers used in industries with large steam demands and in utilities must be completely assembled and constructed in the field and are called field erected boilers.


Watertube Boiler D-Type 

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Watertube Boiler  "A Type"

Watertube Boiler  "O Type"

Miura Watertube Boiler

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Field Erected Boiler With Superheater By B&HES at Thomaston, GA



Vertical tubeless boilers are used for small loads but really do not fit into either category as they do not have tubes.




The feedwater system provides water to the boiler and regulates it automatically to meet the demand for steam.  Valves provide access for maintenance and repair.  The steam system collects and controls the steam produced in the boiler. Steam is directed through piping to the point of use.  Throughout the system steam pressure is regulated using valves and checked with steam pressure gauges.   The steam and feedwater systems share some components.  The fuel system includes all equipment used to provide fuel to generate the necessary heat.  The equipment required in the fuel system depends on the type of fuel used in the system.  All fuels are combustible and dangerous if necessary safety standards are not followed.  Fuels commonly used are nuclear fusion, electricity, the wastes of certain processes and fossil fuels.  The approximate heat value of certain fossil fuels:


·         Natural Gas 1,000 Btu/Cubic foot

·         #2 Oil 142,000btu/gallon

·         #4 oil 148,000btu/gallon

·         #5 oil 149,000btu/gallon

·         #6 oil 152,000btu/gallon

·         Coal 12,500btu/ton

·         Wood (Dry) 8,000btu/ton

·         Wood (Wet) 4,000btu/ton


In a fuel oil fired boiler plant, fuel oil leaves the tank through a suction line and duplex strainer traveling then to the fuel oil pump.  The fuel oil is then forced through the pump and then through the discharge line.  From the discharge line some fuel oil is burned and some returned to the tank through a regulating valve.


In a natural gas fired plant gas is supplied at a set pressure which varies depending on the gas source.  Gas systems are low pressure or high pressure.  In a low pressure gas system city gas pressure is reduced from pounds to inches of pressure by passing through a gas regulator.  Through the regulator gas is drawn into the burner and mixed with air supplied by a blower.  This mixture is directed to the burner where it is ignited with the pilot light.  In a high gas pressure system, gas passes through the regulator and gas is reduced to the proper pressure for the burner.  Some boilers have combination burners which can burn gas or fuel oil or a combination of both gas and fuel oil.


Coal fired boilers use mechanical feeders or stokers to feed fuel to the burner at a consistent rate.  For example, in a chain grate stoker coal is fed through the hopper and regulated before passing under the ignition arch.   The coal continues on a conveyor which carries the ignited coal slowly under the heating surface. Ash, slag and unburned parts or clinkers are discharged at the other side of the conveyor.


The draft system regulates the flow of air to and from the burner.  For fuel to burn efficiently the right amount of oxygen must be provided.   Air must also be provided to direct the flow of air through the furnace to direct the gases of combustion out of the furnace to the breaching.  A forced draft system uses a fan to force (or push) air through the furnace.  An induced draft system uses a fan to draw (or pull) air through the furnace.  A combination or balanced draft system uses forced and induced draft fans.   Gases of combustion enter the stack from the breaching and are released to the atmosphere.



Is the method of combining the fuel and air systems in a source of heat at sufficient temperature to produce steam.  Combustion may be defined as the rapid chemical combination of oxygen with the combustible elements of a fuel.  Only three combustible, chemical elements are of any significance: carbon, hydrogen and sulfur.  The boiler combustion furnace in which the fuel burns provides a chamber in which the combustion reaction can be isolated and confined so that it can be controlled.  In a scotch marine boiler it is referred to as a Morrison tube or in other boilers the firebox area.  The convection surfaces are the areas to which the heat travels that is not transferred in the combustion furnace.  Here additional heat is removed.  The burner is the principal device for the firing of oil and/or gas.   Burners are normally located in the vertical walls of the furnace.  Burners along with the furnaces in which they are installed, are designed to burn the fuel properly.



In a steam heating system steam leaves the main steam line and enters the main steam header.  From the main header piping directs the steam to branch lines.  Branch lines feed steam through a riser to the steam heating equipment.  At the heating equipment heat is transferred to the building space.   As the steam releases heat to the building space and is cools it turns back to water or condensate.  The condensate is separated from the steam by a steam trap. The steam trap allows condensate to pass but not the steam.  The condensate passes through the condensate return line and is collected and directed back to the boiler to repeat the steam to water process.


Referring back to the teapot example, after repeated use it began to acquire a "buildup" of solids from the water.  The same separation of solids in the water occurs in the boiler but since it is operating continuously and at higher temperatures this "buildup" can occur very rapidly.   When this occurs the heat transfer can not be achieved as readily which requires more fuel to produce the steam. If continued unchecked damage to the metals in the boiler shell and tubes will result.


Pretreatment equipment such as softeners, de-mineralizes, etc. are used to remove as much of the dissolved solids as possible before they get to the boiler.  To remove the solids that continue to the boiler chemicals are added to react with the solids creating a sludge.  This sludge is then periodically removed by opening valves from the bottom of the boiler and relieving it to the drain.  This process is called blowdown.


Waterside problems can also shorten boiler life from corrosion brought on by the oxygen content in the feedwater.  Pretreatment for the removal of oxygen is performed in a deaerator but here again the removal is not complete and chemical additions are made to aid in improving the oxygen removal process.


The water supplied to the boiler that is converted into steam is called feedwater.  The two sources of feedwater are: (1) Condensate. or condensed steam returned from the processes and (2) Makeup water (usually city water) which must come from outside the boiler room and plant processes.  For higher boiler efficiencies the feedwater can be heated, usually by economizers.





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