Cochran boiler is a vertical, coal or oil fired, fire-tube boiler. It is the modification of a simple vertical boiler with increase in surface area.
Cochran boiler consists of a cylindrical shell with its top in a spherical shape. The furnace of the Cochran boiler is in hemispherical shape. The grate is placed at the bottom of the furnace and the ash-pit is located below the grate. The coal is fed into the grate through the fire door and ash formed is collected in the ash-pit and coal is removed manually. The furnace and the combustion chamber are connected through a pipe. The back of the combustion chamber is lined with fire bricks.
The hot gases from the combustion chamber flow through the nest of horizontal fire tubes. The passing through the fire tubes transfers a large amount of the heat to the water through convection. The flue gases exhausting out by fire tubes to the atmosphere through chimney.
The spherical top and spherical shape of firebox are the special features of Cochran boiler. For constructing this shapes requires least volume material. The hemi-spherical head of the Cochran boiler shell gives maximum strength to withstand the pressure of the steam inside the boiler. The hemi-spherical crown of the fire box is advantageous for resisting intense heat and for the absorption of the radiant heat from the furnace.
Coal or oil can be used as a fuel in Cochran boiler. If oil is selected as fuel, no grate is required but the bottom of the furnace is lined with fire bricks. Oil burners are fitted at a suitable location below the fire door. A manhole near the top of the spherical shell is made for cleaning of boiler. Also a number of hand-holes are provided around the outer shell for cleaning purposes. The smoke box in is provided inside the boiler with doors for cleaning inside the fire tubes.
The flow of air through the grate is caused by means of the draught created by the chimney. A damper is usually placed inside the chimney to manage the discharge of hot gases from the chimney and thereby the supply of air to the grate is controlled. The chimney may be additionally supplied with a steam nozzle to discharge the flue gases quicker through the chimney. The steam to the nozzle is provided from the boiler.
Boiler is provided with all required mountings. The function of each mounting is briefly described below:
- Pressure Gauge: Pressure gauge indicates the pressure of the steam in the boiler.
- Water Level Indicator: Water level indicator is used to indicate the water level in the boiler The water level in the boiler should not fall below a critical level otherwise the boiler will be overheated and the tubes may damage.
- Safety Valve: The function of the safety valve is used to prevent the increase of steam pressure in the holler above its design pressure. When the pressure increases above design pressure, the valve opens and discharges the steam to the atmosphere. When this pressure falls just below design pressure, the valve closes automatically. Usually the valve is spring controlled.
- Fusible Plug: If the water level within the boiler falls below a predetermined level, the boiler shell and tubes will be overheated. And if this is continued, the tubes may get damaged, as the water cover will be removed. It can be prevented by stopping the burning of fuel on the grate. When the temperature of the shell increases above a particular level, the fusible plug, which is mounted over the grate, melts and forms an opening. The high-pressure steam pushes the remaining water through this hole on the grate and the fire is extinguished.
- Blow-off Cock: The water supplied to the boiler always has impurities like mud, sand and, salt. Due to heating of water, they get deposited at the bottom of the boiler and if they are not removed, they are accumulated at the bottom of the boiler and reduces its capacity and heat transfer rates. Also the salt content in water can goes on increasing as a result of evaporation of water. These deposited salts are removed with the blow-off cock mounting. The blow-off cock is located at the bottom of the boiler as shown in the above figure and is operated only when the boiler is running. When the blow-off cock is opened during the boiler operation, the high-pressure steam pushes the water and the collected waste material at the bottom is sent out. Blowing some water out also reduces the concentration of the salt. The blow-off cock is usually operated for few minutes after every 5 to 6 hours of boiler working. Blow-off cock helps to keep the boiler clean.
- Steam Stop Valve: Steam step stop valve regulates the flow of steam supply outside. The steam from the boiler first enters into an ant-priming pipe where most of the water particles associated with steam are removed.
- Feed Check Valve: The high pressure feed water is supplied to the boiler through this valve. Feed check valve opens towards the boiler only and feeds the water to the boiler. If the feed water pressure is less than the boiler steam pressure then this valve remains closed and prevents the back flow of steam through the valve.
The outstanding features of Cochran boiler are listed below:
- In Cochran boiler any type of fuel can be used.
- It is best suitable for small capacity requirements.
- It gives about 70% thermal efficiency with coal firing and about 75% thermal efficiency with oil firing.
- The ratio of grate area to the heating surface area varies from 10: 1 to 25: 1.
Advantages of Cochran Boiler:
- Cochran boiler is very compact and requires minimum floor space.
- Construction cost is low.
- It is semi-portable and hence easy to install and transport.
- Because of self contained furnace no brick work setting is necessary.
Disadvantages of Cochran Boiler:
- It requires high head room space.
- The capacity is less due to its vertical design.
- Due to its vertical design, it often presents difficulty in cleaning and inspection.