Types of Boilers

Condensing and Non Condensing Boilers

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. 

Fired and Unfired Boilers

Fired Boilers are boilers in which steam is generated by the direct application of heat.  Heat is generated by burning fuels such as coal, fuel or gas

In Unfired Boilers, steam is generated by an indirect source such as hot water or steam from another process.  No fuel is burnt in these kind of boilers.  

Utility 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.

Water Tube Boilers

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.  Water tube 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.

Velox Boilers

A Velox boiler is a supercharged boiler.  The Velox boiler is based on the principle that when the velocity of the exhaust gases exceed the velocity of sound, the heat transfer increases greatly. 

The flue gases are used to drive a gas turbine coupled to a compressor which compresses the gas from the atmospheric pressure to the furnace pressure. 

The Velox Boiler has a very high combustion rate.  It can be quickly started. 

Supercharged Boilers

In a Super charged boiler, compressed air is fed to the furnace.  The Super charged boiler has better heat transfer capacity.  The heat transfer required is lesser compared to a conventional boiler.  The exhaust gases which come out with high velocity are used to rotate a gas turbine.  This gas turbine in turn can rotate other auxiliaries.

A compressor is used to produce the compressed air for the furnace.  This compressor is powered by the gas turbine. 

The boiler is easier to control. It is also easier to start and requires lesser personnel to operate it. 

Firetube Boilers

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. 

Package Boilers

A Package Boiler is a boiler that has its own burner.  It is small in capacity and is fired by gas or oil.  It has a short start-up time.  It is able to generate a large quantity of steam at high pressure and temperature.

The Package Boiler can be used for different applications.  In some cases, the package boiler is used to generate steam for industrial applications.  The package boiler can also be used for power generation when used with a steam turbine. 

In some cases, the package boiler is used to supplement the output of another large boiler as a peak load source.  In some cases, the boiler can also be used as an emergency generator.  

Electrically Powered Boilers

Electrically Powered Boilers are boilers that are powered by electricity.  These boilers are used for applications such as heating, laundry and in medical applications such as sterilization.  Electrical boilers are not economical at higher capacity.

The advantages of Electrical Boilers are that they are compact and require very few parts.  They are easy to control and use.  They do not produce any residue or exhaust.

Electrical Boilers are usually powered by a three phase supply. 

The Electrical Boiler has a resistor as its heat source.  When electric current flows through the resistor, heat is produced.  This heat can be used to raise the temperature of the water. 
Very precise control of the temperature is possible with modern electronic controls.