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.
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.
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.