A fan is simply a machine for moving air and other gases by means of a rotating impeller using centrifugal or propeller action, or both. There are four main types of fan used for general ventilation work: centrifugal, propeller, mixed flow and axial flow. Sub-division of these main types need not concern us here, but a few brief notes will help you to recognise them and to know what they are doing.

Centrifugal fans

A centrifugal fan has an impeller with a number of blades around the periphery, and the impeller rotates in a scroll or volute shaped casing, and it is this casing which identifies the centrifugal fan. As the impeller rotates, air is thrown from the blade tips centrifugally into the volute shaped casing (snail shell) and out through the discharge opening, and at the same time more air is drawn into the ‘eye’ of the impeller through a central inlet opening in the side of the casing, thus creating a continuous flow of air through the fan impeller and casing.

The volute shape of the casing helps to transform some of the velocity pressure of the air leaving the impeller into useful static pressure to overcome resistance to airflow in the ducting system to which the fan is connected. In normal ventilation work, a centrifugal fan would be used for static pressures (system resistances) up to about 750 Pa(=N/m²). A point to note is that the air flow through a centrifugal fan cannot be reversed.

In-Line Centrifugal fans

Instead of using a volute casing to collect the swirling air from a centrifugal impeller, it may be allowed to spin forwards into a concentric annular casing. Guide vanes will then convert the swirl velocity pressure into fan static pressure, and an outlet duct can be fitted in line with the inlet duct as for axial fans. Performance tends to be somewhat inferior to the corresponding volute model, and the chief advantage is avoidance of the transverse bulk and right angle direction change associated with a standard centrifugal fan.

Propeller fans

A propeller fan usually has a curved sheet metal-bladed impeller fitted to the motor spindle, the motor being mounted on a ring for wall fixing, or in a short length of duct for duct fixing. The air is drawn into the impeller in a fairly smooth pattern from all directions, and discharged in a direction approximately parallel to the axis of the fan, but with a helical twist.

Its main use is for moving large volumes of air against low system resistances, say up to 65 Pa, and is very popular in ventilation work in diameters from 300mm to 900mm, due to the robust construction and comparatively low price. However, where appearances matter, such as in offices, shops and hotels,this type is not usually acceptable. It can give reversed air flow at reduced volume andpressure by reversing the direction of rotation. The power required to drive this type of impeller continues to increase as the resistance to airflow increases (i.e., an overloading power characteristic), and the motor must have sufficient power to deal with the heaviest load possible at the designed fan speed to prevent it being overloaded.

Mixed Flow fans

A mixed flow fan combines the characteristics of the large volume of air moved by the propeller fan, (axial flow intake), and the higher pressure of the centrifugal fan, (radial flow discharge). This type of fan with its peripheral centrifugal discharge, fits in very well for roofmounting, say over an exhaust duct system serving a tall block of offices or flats. The airflow through the fan cannot be reversed in direction. It will operate against static pressures up to about 750 Pa and has a non-overloading power characteristic.

Axial Flow fans

An axial flow fan is a development of the propeller fan, but is more efficient (70% - 80%) due mainly to the aerofoil section blades and finer clearances between the impeller blade tips and the cylindrical fan casing. It has a non-overloading power characteristic which enables the correct motor horsepower to be used for any particular fan or application without causing a burn-out due to overload.

It is less bulky than a centrifugal fan for the same output and has the advantage of straight-through airflow, but for static pressures higher than about 250 Pa its higher running speed tends to make it noisier than the centrifugal fan unless special precautions are taken. To increase its performance against higher resistances two or more impellers can be used, forming a multi-stage fan, usually with guide vanes or contra-rotating impellers so that the air leaves the last impeller in an axial direction without helical twist, thereby increasing considerably the possible maximum pressure available. The airflow through the non-guide vane fan can easily be reversed by reversing the direction of the rotation of the impeller, but as the aerofoil section of the blades would then be running back to front, i.e., with the trailing edge of the aerofoil leading, the fan output would be reduced by 25% or more.

There are, however, special reversible fans available which give equal volumes in either direction, by arranging alternate blades on the impeller to face in opposite directions, i.e., one correctly fitted for extract and the next for intake, or by having flat blades. These inevitably reduce the output by 15% or more below that of the normal arrangements. Another point worth noting in axial fan application is that concerning hot or moist fumes. For such cases the centrifugal fan can have the advantage of a motor outside the airstream, so that a standard motor can be used.

To overcome this disadvantage in the case of the axial flow fan, it can be made with a bifurcated casing, so that a motor with slightly extended shaft can be mounted outside the airstream, which passes on either side of the motor through the special casing.

Fan Performance

While the selection of a ventilation unit from the output tables to perform under free air conditions is a simple matter, it is useful to know the rudiments of fan performance against some resistance to airflow, such as ducting and filters, or even sufficient free area for the passage of replacement air into a room from which the unit is extracting.

Characteristic Curve

Any particular fan design has its own characteristic curve, which is a graph made by plotting a number of test points showing volume delivered against different resistances. Volumes are measured in cubic metres per second (m³/s), which for the sake of convenience in calculations in our class of work is converted to cubic metres per hour (m³/h, and pressures are measured in Pascals (Pa.).

Pressure

The total pressure produced by a fan is made up of the static pressure, that is the useful working pressure available for overcoming the resistance of a ventilating system and velocity pressure, which is the pressure due to the speed of the air. These fan pressures are of a very low order and that is the reason for using the unit of Pascals instead of kg/m² or g/cm².

Pascals

One Pascal is a pressure equal to a force of one Newton applied over an area of one square metre (= 1 N/m²). As one Newton approximately equals a force of 0.1 kg, one Pascal = 0.1 kg/m² or 0.0000 1 kg/cm² (=0.000 145 lb/in²). Other expressions used for resistance are inches water guage (1Pa = 249.1 in.wg) and mm waterguage, an intriguing combination of metric and imperial, 1mm wg = 9.8Pa.