Cochran boiler is a vertical, multi-tube boiler generally used for small capacity steam generation. The Cochran boiler is made in different sizes of evaporative capacities ranging from 150 to 3000 kg per hour and working pressure up to 15bar.
The Cochran boiler consists of an external cylindrical shell and a firebox. The shell and firebox are both hemispherical, the hemispherical crown of the boiler shell gives maximum space and strength to withstand the pressure of steam inside the boiler. The hemispherical crown of the firebox is also advantageous for resisting intense heat.
The firebox and a combustion chamber are attached through a short pipe. The flue gases from the combustion chamber flow to the smokebox through a number of tubes. The gases from the smokebox escape to the atmosphere through the chimney. The combustion chamber is lined with firebricks to withstand intense heat.
A manhole near the top of the crown on the shell is provided for cleaning. At the bottom of the firebox, there is a grate and the coal is fed in through the fire hole, which is kept closed by a fire door.
Parts of a Cochran Boiler
The following are the main parts of a Cochran Boiler:
- Boiler shell
- Combustion chamber
- Fire tubes
- Fire hole
- Man hole
- Flue pipe
1. Boiler Shell
It is made up of steel plates bent into a cylindrical form and riveted or welded together. The ends of the shell are closed by means of endplates. A boiler shell should have enough capacity to include water and steam.
2. Combustion Chamber
The combustion chamber is the space, usually below the boiler shell, meant for burning fuel in order to generate steam from the water contained in the shell.
The grate is a platform, in a combustion chamber, upon which fuel is burnt. The grate generally consists of cast iron bars which are spaced apart so that air (required for combustion) can pass through them. The surface area of the grate, over which the fire takes place, is called the grate surface.
It is the space, above the grate and below the boiler shell, in which the fuel is actually burnt. The furnace is also called firebox.
5. Fire Tubes
It is a horizontal tube placed between the combustion chamber. The flue gases from the combustion chamber flow to the smokebox through the number of fire tubes. These fire pipes help in the exchange of heat from hot flue gases to water.
6. Fire Hole
The fire hole is provided at the bottom of the combustion chamber to fire the fuel inside the furnace.
A chimney is provided at the top of the boiler connected to the smokebox. The exhaust gases coming out of fire tubes pass through a smokebox and escape to the atmosphere through a chimney.
8. Man Hole
A manhole is provided for the cleaning, repairing and inspecting of the boiler shell.
9. Flue Pipe
The firebox and the combustion chamber are connected through a short pipe called flue pipes. The hot flue gases from the grate insert into the combustion chamber through the flue pipe.
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Working of Cochran Boiler
- Coal is supplied into the grate through a fire hole and burnt.
- Ash formed during burning is collected in the ash-pit provided just below the grate and then it is removed manually.
- The hot gases from the grate pass through the combustion chamber to the horizontal fire tubes and transfer the heat to the water by convection.
- The exhaust gases coming out of fire tubes pass through smokebox and escape to the atmosphere through a chimney.
- The smokebox is provided with a door for cleaning the fire tunes and smokebox. The working pressure and steam capacity of Cochran boiler are 6.5 bar and 3500 kg/hr respectively.
Cochran Boiler Mountings and Accessories
Following are the Important mounting of cochran boiler:
- Water level indicator
- Pressure gauge
- Safety valve
- Stop valve
- Blow off valve
- Feed check valve
- Fusible Plug
1. Water Level Indicator
The water level indicator is an important fitting, which indicates the water level inside the boiler. It is a safety device, upon which the safe working of the boiler depends.
2. Pressure Gauge
The pressure gauge is used to measure the pressure of the steam inside the steam boiler. The pressure gauge is fixed in front of a steam boiler.
3. Safety Valve
The safety valve is an instrument attached in the steam boiler for preventing explosions due to excessive internal pressure of steam. Thus, a safety valve ensures safety to a boiler from being damaged due to excessive steam pressure.
4. Stop Valve
The function of a stop valve is to control flow within the boiler and to stop it completely when required.
5. Blow off Valve
The function of a blow-off valve is to remove periodically the sediments deposited at the bottom of the boiler while the boiler is in operation and to empty the boiler while it is to be cleaned or inspected.
6. Feed Check Valve
The function of a feed check valve is to control the flow of water from the feed pump to the boiler and to prevent the backflow of water from the boiler to the pump when the pump pressure is less than the boiler pressure or when the feed pump ceases to work.
7. Fusible Plug
The function of the fusible plug is to put-off the fire in the furnace of the boiler when the water level falls below an unsafe level and thus avoids the explosion, which may take place due to overheating of the tubes and the shell.
Advantages and Disadvantages of Cochran Boiler
- Low initial installation cost.
- It requires less floor area.
- Easy to operate and handle.
- Transporation of Cochran boiler is easy.
- It can use all types of fuel.
- Low rate of steam generation.
- Inspection and maintenance are difficult.
- High room head is needed for its installation due to the vertical design.
- It has a limited pressure range.
As you know Cochran boiler is a vertical, multi-tube boiler commonly used for small capacity steam production. It is low in cost and easy to operate and handle. This boiler uses any type of fuel.
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