The image shows the simplest form of an internally fired vertical fire-tube boiler. It does not require heavy foundation and requires very small floor area.

Cylindrical shell:

The shell is vertical and it attached to the bottom of the furnace. Greater portion of the shell is full of water which surrounds the furnace also. Remaining portion is steam space. The shell may be of about 1.25 metres diameter and 2.0 meters height.

Cross-tubes:

One or more cross tubes are either riveted or flanged to the furnace to increase the heating surface and to improve the water circulation.

Furnace (or fire box):

Combustion of coal takes place in the furnace (fire box).

Grate:

It is placed at the bottom of fire box and coal is fed on it for burning.

Fire door:

Coal is fed to the grate through the fire door.

Chimney (or stack):

The chimney (stack) passes from the top of the firebox through the top of the shell.

Manhole:

It is provided on the top of the shell to enable a man to enter into it and inspect and repair the boiler from inside it. It is also, meant for cleaning the interior of the boiler shell and exterior of the compbustion chamber and stack (chimney).

Hand holes:

These are provided in the shell opposite to the ends of each cross tube for cleaning the cross tube.

Ashipt:

It is provide for collecting the ash deposit, which can be removed away at intervals.

Working:

The fuel (coal) is fed into the grate through the fire hole and is burnt. The ashpit placed below the grate collect the ashes of the burning fuel.

The combustion gas flows from the furnace, passes around the cross tubes and escapes to the atmosphere through the chimney.

Water goes by natural circulation due to convection currents, from the lower end of the cross tube and comes out from the higher end.

The working pressure of the simple vertical boiler does not exceed 70 N/cm^2.

The following mountings are fitted in the boiler:

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

Water gauge (water level indicator): this indicates the water level in the boiler.

Safety valve: it prevents an increase of steam pressure in theboiler above its design pressure.

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