An axial fan is the type you are most likely to see, and have a builder or electrician install if you ask them to install a fan for you. Countless bathrooms and kitchens across the UK will have some form of axial fan installed. The fan blade itself is mounted on a motor shaft, a bit like an airplane propeller, and it draws air in through the blade in a linear fashion and pushes it out the back of the fan body through what which is known as the spike.
Most average fans of this type have a flow rate of around 75 m3/h (cubic meters per hour), which is fine for most small bathrooms with a short length of duct through the wall, not more than a meter. There are some that can extract up to 97m3/h and can be ducted up to 5m, but all axial fans work best in a shorter duct run. By design, axial fans cannot handle long runs of ducting, as they lack the pressure to push air a long distance and reach a stop condition where the air in the duct no longer moves. This can be the reason for premature failure when installed incorrectly in a long conduit.
Standard sizes for household axial fans are four and six inches, four inches for a bathroom, and six inches for a kitchen. UK building regulations require that 60 liters per second (equivalent to about 245 m3/h) be extracted from a kitchen, which will provide a six-inch axial fan. There are nine and even twelve inch versions available, but these are more for commercial use, providing much higher flow rate but also much more noise.
A centrifugal fan is a bit different. Its blade is a circular drum or plate configuration with blades or ribs attached around its circumference with a gap in between, this is called an impeller. Moves air by pushing it down an outlet duct in the fan housing at 90 degrees to the impeller itself as it rotates, the air then rushes to equalize the lack of air pressure through the center of the impeller, which is then pushed out of the duct and the cycle continues. This creates a much higher air pressure, with higher efficiency.
Due to the higher pressures involved, centrifugal fans are better able to cope with longer runs of ductwork. Some domestic centrifugal fans can be ducted up to fifty meters and still provide ample extraction. These fans will only deliver about the same 90 m3/h that a four-inch axial fan can achieve, but the pressure is held at a much greater distance. If you have a duct that runs a long distance, through a duct system in a flat, for example, or through a ceiling with a couple of bends, a centrifugal fan will work much better than an axial fan. .
Axial fans, when used as intended, will provide excellent results with minimal noise; however, a centrifugal fan reaches greater distances, but will produce more noise in operation.