Types of Boilers


Different types of boilers are available in the market, which are manufactured by different manufacturers. The different types of boilers find their use in different applications. As the time has passed, more efficient types of boilers have replaced the old and inefficient ones.  This article elaborates on different types of boilers and the boiler classification. Boilers can be broadly classified as shell tube and water tube boilers.

Shell and tube boilers

Shell and tube boilers are also referred to as fire tube or smoke tube boilers. Fire tube boilers; contain long steel tubes through which the hot gasses from a furnace pass and around which the water to be converted to steam circulates. Fire tube boilers, typically have a lower initial cost, are more fuel efficient and easier to operate. Their capacities are up to 25tons/hr and 17.5 kg/ cm2.

Different types of fire tube boilers:

Cornish boiler

These are the earliest form of high pressure fire tube boiler. These consist of long horizontal cylinder with single large flue containing fire. Fuel is added in the grate area where it burn to produce hot gases.  The hot gases transfer the heat to the water.

Water takes heat and after some time it starts boiling to produce steam. Hot gases upon reaching at the end of the fire tube, divided into two section and each move into the one of two side flue which take them once again at the front section of the boiler where they are move into the bottom flue and bottom flue take them toward the chimney.

Chimney throws these gases out of the boiler into the atmosphere. Maximum heat transfer is taken place at fire tube and shell section then taken place at side flue and at last at bottom flue.
 
For efficiency, the boiler was commonly encased beneath by a brick-built chamber.

Lancashire boiler:   

The Lancashire boiler is similar to the Cornish, but has two large flues containing the fires. Pressure range of the boiler is about 0.7 MPa to 2 MPa and efficiency is 65 to 70%.  Fuel in these boilers is added into the grate which heats the gases.

Hot gases enter the front section of the boiler and leave the boiler from back and then enter the bottom flue and start moving to front section of boiler. At front section hot gases leave the bottom flue and enter in side flue and move again towards the back of the boiler and enter the main outlet. 85% of heat is transferred when hot gases are in fire tube while 15% is transferred when they are in bottom and side flue.
    

Locomotive boiler

A locomotive boiler has three main components:
1.     Double-walled firebox;
2.     Horizontal, cylindrical "boiler barrel" containing a large number of small flue-tubes; and
3.     Smokebox with chimney, for the exhaust gases.
Fuel is burned to produce the hot gases. Fuel is feed through fire hole. Hot gases are diverted to fire tube with the help of fire brick arch. Steam is collected in the steam drum which is placed at the top of the shell. 

The wet steam goes through inlet headers of super heater and after passing through tubes, it returns to the outlet header of super heater and is taken out for steam engine.
Locomotive-type boilers are also used in traction engines, steam rollers, portable engines etc.
On the basis of construction these can be classified wet back boilers and dry back boilers.

Reversal Chamber:
This is the posterior portion of the combustion chamber through which the flue gases travel from the first pass (furnace) to the second-pass tubes

Wet Back Boilers:

In wet back boilers as the name suggests the reversal chamber is completely surrounded by water. The combustion reversal chamber is surrounded by water and therefore the heat in the flue gases is optimally utilized. Radiation losses are reduced as none of the parts of the combustion chamber are open to atmosphere instead they are surrounded by water. That means fewer losses, and lesser fuel bills. Most efficient modern boilers supplied are wetback type.

Dry Back Boilers:

The reversal chamber in dry back boilers is not completely surrounded by water. The posterior part is exposed to the atmosphere. This leads to the increased radiation losses, as the radiant heat is lost to the atmosphere instead of going to the water as in wet back boilers. Earlier generation boilers used to be dry back.
Thus wet back boilers ensure lesser radiation losses and hence save fuel.

The layout of the tubes involves the number of passes the tube will make to pass the heat from the boiler furnace before being discharged. These can be two- pass and three pass boiler.

Depending upon the layout of tubes boilers can be two pass or three pass boilers.

Two pass boilers:

In two pass the combustion gases travels two times in the boiler.
 Combustion gases should be cooled before entering the reversal chamber. Excess temperature causes overheating and cracking of the tube. The heat transfer rate is maximum at the first pass, this rate decreases with the increasing passes.
 

Three pass boilers:

A three pass design provides three opportunities for heat transfer. The stack temperature of 3 pass will be lower than that of 2 pass boiler, of the same design and operating pressure. Efficiency is more than two pass boiler.
Each pass in boiler should be designed with cross sectional area to achieve optimal flue gas velocity, which in turn maximizes heat transfer while also minimizing performance robbing sooth build up within the tubes.


No. Of pass    Area of tube m2    Temperature 0C    Heat transfer
1st    11    1600    65%
2nd    43    400    25%
3rd    46    350    10%



Water tube boilers:

 In water tube boilers, water and steam flow inside the tubes and the hot gases flow over the outside surface. Modern high capacity boilers are of water tube type. The boiler circulation system is constructed of tubes, headers, and drums joined in arrangement that provide water flow to generate steam.

Water tubes have high pressures and capacity than shell tube boilers. These boilers can be of single- or multiple-drum type. These have higher efficiencies than fire tube boilers.

Depending on layout boilers can also be classified as:

1.    Longitudinal drum boiler.

The feedwater is feed in drum.  The drum is placed above the heat source. The cooler water goes to the inclined tubes and the water is heating eventually in the hot tubes. As the water boils its density decreases and there is circulation of hot water and steam. Steam is separated from water in steam drum and taken out. Longitudinal drum boilers range from 2250 kg/h to 3600 kg/h.

2.    Cross drum boiler.

The drum in this type is placed in cross to the heat source. The temperature obtained in this type of arrangement is more uniform. When the steam loads are high the upper tubes can become dry which cause them to fail.  The layout of tubes is made in such a way that large numbers of tubes are made available. The capacity of cross drum range from 700kh/h to 240000 kg/h


3.    Stirling boiler:

A Stirling boiler has near vertical, almost straight water tubes that zig-zag between a number of steam and water drum. Usually there are three banks of tubes in a four drum layout.
The feedwater enters the left upper drum, from where it falls to lower water drum. Water in pipes and two drums is heated, the steam produced rise in upper drum from where steam is separated and taken off.
 

Package Boiler:

These boilers come as complete package. It requires only the steam, water pipe work, fuel supply and electrical connections to be made for it to become operational. Package boilers are generally shell type with fire tube design so as to achieve high heat transfer rates  
The packaged boiler is so called because it comes as a complete package. Once delivered to site, it requires only the steam, water pipe work, fuel supply and electrical connections to be made for it to become operational. Package boilers are generally of shell type with fire tube design so as to achieve high heat transfer rates by both radiation and convection
 
The features of package boilers are:
1.     Small combustion space and high heat release rate resulting in faster evaporation.
2.     Large number of small diameter tubes leading to good convective heat transfer.
3.     Forced or induced draft systems resulting in good combustion efficiency.
4.     Number of passes resulting in better overall heat transfer.
5.     Higher thermal efficiency levels compared with other boilers.

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