It is a multi-tubular vertical fire tube boiler having a number of horizontal fire tubes. T is the modification of a simple vertical boiler where the heating surface has been increased by means of a number of fire tubes.

It consists of

1.    Shell

2.    Crate

3.    Fire box

4.    Flue pipe

5.    Fire tubes

6.    Combustion chamber

7.    Chimney

8.    Man-hole

Shell

It is hemispherical on the top, where space is provided for steam.

Grate

It is placed at the bottom of the furnace where coal is burnt.

Fire box (furnace )

It is also dome-shaped like the shell so that the gases can be deflected back till they are passed out through the flue pipe to the combustion chamber.

Flue pipe:

It is a short passage connecting the fire box with the combustion chamber.

Fire tubes:

A number of horizontal fire tubes are provided, thereby the heating surface is increased.

Combustion chamber:

It is lined with fire bricks on the side of the shell to prevent overheating of the boiler. Hot gases enter the fire tubes from the flue pipe through the combustion chamber.

Chimney:

It is provided for the exit of the flue gases to the atmosphere from the smoke box.

Manhole:

It is provided for inspection and repair of the interior of the boiler shell.

Normal size of a Cochran boiler:

Shell diameter – 2.75 meters:

Height of the shell – 6 meters.

Working of the Cochran boiler:

Coal is fed into the grate through the fire hole and burnt. Ash formed during burning is collected in the ashpit provided just below the grate and then it is removed manually.

The host gases from the grate pass through the flue pipe to the combustion chamber. The hot gases from the combustion chamber flow through the horizontal fire tubes and transfer the heat to the water by convection.

The flue gases coming out of fire tubes pass through the smoke box and are exhausted to the atmosphere through the chimney.

Smoke box is provided with a door for cleaning the fire tubes and smoke box.

The following mountings are fitted to the boiler:

Pressure gauge: this indicates the pressure of the steam inside the boiler.

Water gauge: 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 over heated and the tubes may burn out.

Safety valve: the function of the safety valve is to prevent an increase of steam pressure in the boiler above its normal working pressure.

Steam stop valve: it regulates the flow of steam supply to requirements.

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 (drains) out the impurities like mud, sand, etc., in the water collected at the bottom.

Fusible plug: it protects the fire tubes from burning when the water level in the boiler falls abnormally low.

Salient features of Cochran boiler:

1.    The dome shape of the furnace causes the hot gases to deflect back and pass through the flue. The un-burnt fuel if any will also be deflected back.

2.    Spherical shape of the top of the shell and the fire box gives higher area by volume ratio.

3.    It occupies comparatively less floor area and is very compact.

4.    It is well suited for small capacity requirements.