Unfortunately single phase induction motors are tricky to speed-control and many don't support it. This one
appears to support speed control and dual windings are hinted at. I'd try to find more information from
the manufacturer, or go for a DC motor fan which are always speed-controllable (and often
more efficient)
I note the same fan manufacturer does 24VDC brushless fans: http://www.justfans.co.uk/catalogues/ECO.pdf