extractor fans

[nospam]

Hello,

I'm a bit confused because I though centrifugal fans were better than axial ones. I was looking in a catalogue and it said that for four inch fans the axial moved 80-odd cubic metres per hour but the centrifugal only managed 60-odd.

Is it that in optimum conditions axial fans will move more air? Is it that centrifugal fans are better only once the ducting becomes longer? If so, at what length of ducting should the switch from axial to centrifugal take place?

Or is it that these are maximum values when there is nothing behind the fan, used only for marketing? Is it that in all practical applications the centrifugal will move more air?

Thanks.

[leighhales]

Hello,

There are different applicati> Hello,

[David Hansen]

On Sat, 27 Jan 2007 13:51:30 GMT someone who may be [e-mail] wrote this:-

"Better" is a word which when used prompts the question, "better at what?".

Axial fans are cheaper and generally work fine with short ducts with few bends. Centrifugal fans are more expensive, but work with longer ducts with bends because they develop higher pressures. Which is better? It depends on what you want to do.

[nospam]

Getting steam out as quickly as possible!

In long ducts: no question, it has to be centrifugal, but which moves the most air along a short duct?

[David Hansen]

On Sun, 28 Jan 2007 10:01:56 GMT someone who may be [e-mail] wrote this:-

Neither. It depends on the design specification. However, for a given flow rate along a short duct people tend to go for the cheapest (and neatest if mounted on the surface).

Also note that if you get a fan with integral back draught shutters, generally the more expensive ones, then the outlet can be a simple grille.