TYPES OF FANS

Two types of fan are in general use in ships are, centrifugal fans and axial flow fans.

Centrifugal fans consist essentially of a 'wheel' or 'runner' made up of radial blades rotating in a casing of the scroll form. The wheel is rotated at high speed by an electric motor on the same shaft.' Air is drawn in at the 'eye' of the fan, i.e. into the centre of the wheel and thence between the blades which give it rotation. The centrifugal force due to its rotation expels the air outwards towards the circumference and into the delivery trunk. A fan is said to be 'right handed' or 'left handed' according to whether the rotation is clockwise or anti-clockwise when viewed from the motor to the wheel.

Axial flow fans work on the same principle as the propellers of aircraft. They consist primarily of an impeller (or propeller) with blades mounted on a 'nacelle' enclosing an electric motor inside a circular trunk having a small clearance from the blades. Provided a straight length of trunk can be arranged at the inlet and outlet ends they can be designed to be as efficient as the centrifugal type For a given volume output and pressure, and can be used to distinct advantage where economy of space is important. The modern practice is to use this type for 1 group systems and machinery spaces.

Fans are not placed in bathrooms, wash places, drying rooms or other compartments containing humid air, nor in compartments where sparking could cause an explosion. They are usually suspended from the deck head or seated on the deck in a position affording an efficient arrangement of trunking. Non-air conditioned spaces of ships serving in tropical climates are fitted with fans of the open propeller type to provide additional means of rapidly circulating the air. Three types of open propeller fan are used. Slow-running overhead fans with wooden blades about 3 ft 6 inch diameter are fitted in wardrooms, officers' messes, dining halls, recreation and similar spaces. Small table fans with rubber blades 10 inch diameter are fitted in cabins and with rubber or steel blades 12 inch diameter in offices and mess spaces. Larger 16-in diameter fans with aluminium alloy blades (Hurricane type) are fitted in dining halls and large mess spaces.