Axial fans extract air through the impeller which is parallel to the shaft around which the blades spin. These are the most common of domestic wall fans and are perfect for use in the bathroom, toilet or smaller rooms where it needs to be installed in a window or ducted straight through the wall. The standard size impeller for this type of extractor is normally around four inches or possible six inches if it is a large space. They usually come in two different voltages, 240 volt for mains or 12 volt for low voltage applications. Different switching options are common with features like pullcords, himidistats and timers which can be used to activate the fan.
A centrifugal fan is one in which the air is extracted at right angles to the intake of the fan, the air is then spun outwards to the outlet by centrifugal force and deflection. The rotating impeller causes the air to enter the fan near the shaft, this then moves perpendicularly from the shaft to an opening in the fan casing. This type of action produces a higher pressure which allows the extracted air to be pushed along much longer ducting than an axial fan. The wall mounted version looks just like an ordinary axial fan and since the impeller is discreetly hidden behind a grille and casing it can be difficult to spot the difference. The only tell tale sign is usually the price as centrifugal wall fans are generally more expensive than axial fans. They are ideal for rooms where there aren't any exterior walls because they tend not to have natural ventilation or windows and the ducting will more than likely be very long.
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