A condensing boiler has a special heat exchanger to remove heat from the flue gases.  The flue gases are thus at a lower temperature.  The overall efficiency of the boiler is increased.  Condensing boilers can have an efficiency as high as 95%. 

In many countries, the use of condensing boilers is mandatory.  Many governments give financial incentives to install condensers in boilers.

Boilers which are used in power generation applications are called utility boilers. 

Utility boilers generally use coal as fuel.  Gas and Oil are also used by utility boilers.  These boilers are of large capacity of the order of hundreds of Megawatts.  These boilers are used to drive a turbine which is coupled to a generator.

Steam Jet Draught refers to the Draught created by using a jet of steam.  The steam generated by the boiler can be used for this purpose.  The jet of steam is used to create an airflow which will cause the flue gases to exit through the chimney.

If the steam jet is applied near the stack of the chimney, the negative pressure it creates draws the flue gases from the furnace into the Chimney.  This is known as induced draught.  If the jet is applied below the grate, the steam pushes the flue gases in the direction of the chimney.  This is a forced draught.

The Steam Jet draught is a simple mechanism.  No external equipment such as compressors or blowers are required.  The steam when used below the grate cools the firebars and prevents the clinkers from sticking to the bars.

Water tubes boilers are boilers where the water flows in pipes above a furnace or a burner.  The hot gases in the burner warm the water.  Watertube boilers are used for high capacity applications.  They can built for very high pressure designs. 

The water tubes are connected to a steam drum on top.  The steam rises to the steam drum from where it is collected.  High efficiency is possible with water tube boilers.  Since the water flows through the tubes, very good water chemistry needs to be maintained to prevent deposits and corrosion. 

Large heating surface can be obtained by adding more number of tubes.  The heat transfer is by convection.  The flow rate of the water in the tubes is higher and hence greater efficiency is possible. 

The water tube boiler can control its own pressure and is known as a self controlled boiler.  This is because if steam is extracted from the drum, the pressure inside falls.  This causes the water in the tubes to flow faster.  This results in more heat transfer and the original pressure is quickly attained.

In Fire Tube boilers, the hot gases from the furnace are passed through a series of tubes.  The tubes are placed in the shell of the boiler which contains the water.  The water is thus heated by the heat from the fire tubes. 

These boilers are used in low pressure applications as it is difficult to design the tubes which can withstand the higher pressure of the water in the boiler.  Firetube boilers have a larger water volume and can thus handle sudden load surges better.  They are easier to repair and maintain. 

Firetube boilers are suitable for applications where the load is fixed with little variations.  They may take a long time to warm up as they have a large quantity of water. 

Firetube boilers are not suitable for high pressure applications and for applications which require high steam output.

Based on the fuel used to produce heat, boilers can be classified into

Solid Fuel Fired boilers

These boilers are fired by either wood or coal.  The fuel may be crushed into fine powder or cut into chips before being fed into the furnace.  The combustion of these fuels will produce ash which can be used for other purposes.

Liquid Fired boilers

These boilers are fired by a liquid such as Furnace oil or diesel.  The Furnace oil may required to be processed to attain the desired viscosity and the temperature before being fed into the furnace.  The furnaces for these boilers have a higher flow rate and smaller volume. 

Gas Fired boilers

These boilers have gas as the fuel.  The gases have very little residue.  The flow rate is higher and the furnace volumes are very small. 

Waste Heat Recovery Boiler

These boilers do not have a separate heat source.  They recover the heat from another industrial process such as exhaust from an engine or the waste heat in a cement plant.  The recovered heat is used to power the boiler. 

Pressure is a very important parameter in boilers.  The boiler and all the connected equipment are designed to withstand the pressure developed by the steam.  Pressure, is an important criterion to classify boilers.

Boilers are classified into

Low to medium pressure boilers

These boilers have an operating pressure of less than 10 bar.  Natural circulation is sufficient for these boilers.  Typical application are in industries.

High pressure boilers

High pressure boilers have an operating pressure of 10 to 14 bar.  They have forced circulation.

Super high pressure boilers

Super high pressure boiler are also used for utility applications.  The operating pressure is above high pressure boilers but generally lesser than 17 bar. 

Super critical boilers

Supercritical boilers have an operation pressure higher than 22.5 bar