An Axial fan is the type you are most likely to see, and have fitted by a builder or electrician if you ask them to fit a fan for you. Countless bathrooms and kitchens across the UK will have an axial fan of some kind fitted. The fan blade itself is mounted on an axle from the motor, a bit like an aircraft propeller, and draws air through the blade linearly and pushes it out the back of the fan body through what is known as the spigot.
Most average fans of this type have a flow rate of around 75 m3/h (metres cubed per hour) which is fine for most small bathrooms with a short run of duct through the wall, no more than a metre. There are some that can draw as much as 97m3/h and can be ducted up to 5m, but all axial fans work best on a shorter duct run. By design, axial fans do not cope with long runs of duct as they lack the pressure required to push the air a long distance and reach a stall condition where the air in the duct no longer moves. This can be a reason for premature failure when incorrectly installed on a long run of duct.
Standard domestic axial fan sizes are four and six inch, four inch for a bathroom and six inch for a kitchen. UK building regulations require that 60 litres per second (which equates to around 245m3/h) is extracted from a kitchen, which a six inch axial fan will provide. Nine and even twelve inch versions are available, but these are more for commercial use, providing a much greater flow rate but also much more noise.
A centrifugal fan is a little different. It’s blade is a drum or circular plate configuration with blades, or ribs attached around its circumference with a space in the middle, this is called an impeller. It moves the air by pushing it down an exit duct in the fan casing at 90 degrees to the impeller itself as it rotates, air then rushes to equalise the lack of air pressure through the centre of the impeller, which is then pushed out of the duct and the cycle continues. This creates much greater air pressure, with greater efficiency.
Due to the greater pressures involved, centrifugal fans are better able to cope with longer runs of duct. Some domestic centrifugal fans can be ducted up to fifty meters and still provide ample extraction. These fans will still only provide around the same 90m3/h that a four inch axial fan can achieve, but the pressure is maintained over a much greater distance. If you have a duct run that is over a long distance, through a ducting system in a flat for example, or through a roof space with a couple of bends, a centrifugal fan will cope far better than an axial fan.
Axial fans, when used as intended will provide excellent results with minimal noise, however a centrifugal fan achieves greater distances, but will produce more noise in operation.