Should You Install an Axial Fan or a Centrifugal Fan?
An Axial Fans is one in which the extracted air is forced to move parallel to the shaft about which the blades rotate. Most domestic wall fans use an axial impellar and are ideal for bathrooms, toilets and smaller rooms where the air is to be ducted straight through the wall or installed in a window.
Axial fans are not good for long duct runs. Anything over 5m will seriously impair their ability to extract air because of the larger air pressures present in the duct. When installing an axial type one should also avoid bending the duct if possible as this will also add to the pressure in the duct. In these situations one should use a centrifugal fan.
Centrifugal fans extract air at right angles to the intake of the fan, and spin the air outwards to the outlet by deflection and centrifugal force. The impeller rotates, causing air to enter the fan near the shaft and move perpendicularly from the shaft to the opening in the fan casing.
A centrifugal fan produces more pressure for a given air volume, and is used in installations where duct run is over 5 metres. Indeed, such is their efficiency that some centrifugal fans, such as the Vent Axia Solo can be used in a duct run up to 50m!
Centrifugal fans come in two main types: As an Inline duct fan or as a wall mounted fan. The wall mounted centrifugal fan looks just like a standard axial fan. Since the impellar is hidden by the front grille and casing it can be hard to differentiate the two but the price will usually give it away: Centrifugal wall fans are usually more expensive than Axial Fans.
Wall mounted centrifugal fans are often see in rooms which do not have exterior walls. Such rooms do not have windows or any natural ventilation and these circumstances usually require a long duct run to reach the outside of the building. The most common problem when air is not being extracted properly is when an axial fan is installed in this type of room… They are simply not designed for this operation.
Inline extractor fans, as the name suggests are situated in the length of ducting. They therefore look completely different from wall fans but most importantly are not installed in the room from which they are extracting air. Ducting will run from one end of the fan to a ceiling grille in the extraction zone and from the exhaust end of the fan to the outside grille. Inline fans are excellent for use above bathrooms or kitchens where extra extraction power is required.