Cochran Boiler is a modified form of the simple vertical boiler which has various firetubes which can exchange heat to water through convection
The efficiency of this boiler is much better than the efficiency of simple vertical boiler.
In this boiler flue gases will travel in the tubes which will be surrounded by water and water will be heated by convection from this tubes.
Cochran boiler is used in many industries. It is used in many power plants to produce electricity where high quanity of steam having high pressure and temperature is required.
Properties of Corchan Boiler:-
1) It is a vertical drum boiler.
2) It is a horizontal firetube boiler means firetubes in this boiler are placed horizontally.
3) It is a multi tubular boiler.
4) It is a natural circulation boiler.
5) It is an internally fired boiler means that furnace will present inside the boiler shell.
6) It is a natural circulation boiler means water will be circulated naturally and no pump will be used for water circulation.
Parts of Corchan Boiler:-
Main Parts of Corchan Boiler are :-
1) Shell :
It is the outermost part of the cochran boiler which is cylindrical in shape and has a hemispherical dome type crown at the top. Other parts of the boiler are present inside this shell.
2) Furnace or Fire Box:
Furnace is present in the bottom of the boiler. It is dom-shaped shell inside which all the fuel is burnt and heat is produced. It is dom-shaped so that it can deflect back the unburnt charge to furnace and the flue gases to the combustion chamber.
It is present at the bottom of the furnace. It is the platform in which solid fuels like coal are placed. It is provided in the boiler for supporting solid fuel in the furncae.
This grate in the furnace should have openings of right size.The hole should not be so large that they allow unburned fuel particles to fall through and should not be so small that they obstruct sufficient amount of air to pass through the fuel.
4) Combustion Chamber :
It is present above the furnace. Flue gases from the furnace enters the combustion chamber through flue pipe and these flue gases flow through the horizontal firetubes.
Combustion Chamber is fire bricks on the side of the shell to prevent overheating of the boiler.
5) Flue Pipe :
It is a small passage connecting the firebox or furnace and the combustion chamber. Hot gases reach the combustion chamber from furnace through this flue pipe.
6) Fire tubes:
Cochran boiler has multiple firetubes. The hot flue gases from the combustion chamber flow through these fire tubes to reach the smokebox. This firetubes transfer heat to water by convection.
7) Smoke Box:
After passing through the firetubes the flue gases reaches the smokebox. Smokebox provides a convenient collection point for ash and cinders drawn through the firetubes which can be cleaned easily at the end of the working day. Smokebox is provided with a door for cleaning the firetubes and smokebox.
Without smokebox all char must pass up the chimney or will collect in the tubes, gradually blocking them.
8) Chimney :
Chimney is the part where the smoke reaches after passing through the smoke box. Chimney transfers smoke to the environment. The size of chimney in this boiler is small as compared to chimneys of other boilers.
9) Manhole :
It is present in the boiler for the inspection and repair of the interior of the boiler shell.
Apart from the parts there are various mountings fiited to the boiler :
1) Pressure Guage:
This measures and indicates the pressure of the steam inside the boiler.
2) Water gauge:
Water gauge indicates the level of water in the boiler. The water level in the boiler should not fall below a particular level otherwise the boiler will be overheated and the tubes may burnt out.
3) Safety Valve:
The function of the safety valve is to prevent an increase of steam pressure in the boiler above its normal working pressure.It blows off extra steam when the steam pressure inside the boiler reaches above safety valve.
4) Steam Stop Valve:
It regulates the flow of steam supply to the requirements. It transfer the steam to the required area otherwirse it stops the steam in the boiler.
5) Fusible Plug: It protects the firetubes from burning when the water level in the boiler falls abnormally low. If the water level below a predetermined level, the firetube get overheated as no heat is transferred to water and ramians in the tube. and if this continues tubes may burn.
When the temperature of shell increases above a particular level.the fusibleug which mounted on the grate, melts and form an opening. The high pressure steam pushes water through this hole on the grate and the fire is extinguised and the tubes are prevented from burning.
6) Blow off Cock:
It is located at the bottom of the boiler. When the blow-off cock is opened during the running of the boiler, the high pressure steam pushes or drains the impurities like mud,sand etc in the water collected at bottom. It is also sometimes used to drain off whole water from the boiler.
Working of Cochran Boiler:
At first, the fuel is inserted in the furnace and placed in the grate.The fuel is ignited through the fire hole present at right bottom side of the boiler.The air enters in the combustion chamber through the atmosphere.
The fuel is burnt in the furnace and due to burning of fuel hot gases and smoke are produced. These hot flue gases and smoke reaches the combustion chamber through the flue pipes.
The ash produced due to combustion is collected in the ash pit present in the bottom of the the grate. Ash pit also functions as supply chamber of air through the grate.
The heat is produced due to combustion is absorbed by the boiler surface at the top of the combustion chamber. Different fire bricks wall and baffles are provided in the combustion chamber for proper mixing of air with combustible gases.
After reaching the combustion chamber, the hot flue gases enter into the fire tubes which are surrounded by water. Heat is transferred from hot flue gases in fire tubes to the water which surrounds the fire tube.
As the heat is transferred to water, the temperature of water increases and it gets converted into steam. The steam generated is collected in upper side of the boiler in the hemispherical dome. An anti-priming pipe is installed at the top of boiler that sperates wet steam to dry steam i.e remove water particles from the steam and make it dry steam. This dry steam is supplied where it is required through the steam stop valve.
After exchanging heat with water the hot flue gases and smoke move to the smokebox. In the smokebox convenient collection point is provided for ash and cinders collection drawn from the firetubes which can be easily cleaned.
After passing through smokebox the smoke is discharged to the atmosphere through the chimney.
The ash which are collected in the ashpit is cleaned and removed from the boiler using the blow off cock which is present at the bottom left of the boiler which remove the ash and mud collected at bottom with help of pressure of water.
Various mountings and accessries are attached to the boiler for safety and efficient working of boiler.
Advantages of Cochran Boiler :-
- It requires less floor area.
- Easy to operate and handle.
- Low initial installation cost.
- It has higher volume to area ratio.
- It is easy to transport from one place to another.
- It can use all types of fuel.
Disadvantages of Cochran Boiler:-
- Low steam generation rate.
- High room head is required for its installation due to vertical design.
- It has limited pressure range.
- Difficult to inspect and maintain.