Cochran Boiler Working and Construction
Cochran boiler consists of a cylindrical shell with its crown having a spherical shape. The furnace is also hemispherical in shape. The grate is also 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 located just below the grate and it is removed manually. The furnace and the combustion chamber are connected through a pipe. The back of the combustion chamber is lined with firebricks.
The hot gases from the combustion chamber flow through the nest of horizontal fire tubes (generally 6.25 cm in external diameter and 165 to 170 in number). The passing through the fire tubes transfers a large portion of the heat to the water by convection.
The flue gases coming out of fire tubes are finally discharged to the atmosphere through chimney.
You can refer below Cochran Boiler Diagram.
- Cochran Boiler Diagram
The spherical top and spherical shape of firebox are the special features of this boiler. These shapes require least material for the volume. The hemi spherical crown of the 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. This shape is also advantageous for the absorption of the radiant heat from the furnace.
Coal or oil can be used as fuel in this boiler. If oil is used as fuel, no grate is provided but the bottom of the furnace is lined with firebricks. Oil burners are fitted at a suitable location below the fire door.
A manhole near the top of the crown of shell is provided for cleaning. In addition to this, a number of hand-holes are provided around the outer shell for cleaning purposes. The smoke box is provided with doors for cleaning of the interior of the fire tubes.
The airflow through the grate is caused by means of the draught produced by the chimney. A damper is placed inside the chimney (not shown) to control the discharge of hot gases from the chimney and thereby the supply of air to the grate is controlled.
The chimney may also be provided with a steam nozzle (not shown) to discharge the flue gases faster through the chimney. The steam to the nozzle is supplied from the boiler.
Features of Cochran boiler
- It is very compact and requires minimum floor area.
- Any type of fuel can be used with this boiler.
- It is well suited for small capacity requirements.
- It gives about 70% thermal efficiency with coal firing and about 75% with oil firing.
- The ratio of grate area to the heating surface area varies from 10: 1 to 25: 1.
Mountings used in Cochran Boiler
The function of each is briefly described below (you can also refer Cochran Boiler Diagram shown above):
This indicates the pressure of the steam in the boiler.
Water Level Indicator
This indicates the water level 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 burn out.
The function of the safety valve is 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.
If the water level in the boiler falls below a predetermined level, the boiler shell and tubes will be overheated. And if it is continued, the tubes may burn, as the water cover will be removed. It can he 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.
The water supplied to the boiler always contains impurities like mud, sand and salt. Due to heating, these are 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 will goes on increasing due to evaporation of water. These deposited salts are removed with the help of blow off cock.
The blow-off cock is located at the bottom of the boiler (as shown in the figure) and is operated only when the boiler is running. When the blow-off cock is opened during the running of the boiler, the high-pressure steam pushes the water and the collected material at the bottom is blown out.
Blowing some water out also reduces the concentration of the salt. The blow-off cock is operated after every 5 to 6 hours of working for few minutes. This keeps the boiler clean.
Steam Stop Valve
It regulates the flow of steam supply outside. The steam from the boiler first enters into an anti-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. This 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.
Advantages of Cochran Boiler
(i) Low initial installation cost.
(ii) It requires less floor area.
(iii) Easy to operate and handle.
(iv) Transportation of Cochran boiler is easy.
(V) It can use all types of fuel.
Disadvantages of Cochran Boiler
(i) Low rate of steam generation.
(ii) Inspection and maintenance is difficult.
(iii) High room head is required for its installation due to the vertical design.
(iv) It has limited pressure range.
Applications of Cochran Boiler
- Variety of process applications in industries
- Chemical processing divisions
- Pulp and Paper manufacturing plants
- Refining units
Besides, they are frequently employed in power generation plants where large quantities of steam (ranging up to 500 kg/s) having high pressures i.e. approximately 16 mega-pascals (160 bar) and high temperatures reaching up to 550 °C are generally required.