Installation Fee: HK$1,500 (for window size less than 60cmx120cm) Model F390 Fresh AirFlow Purifier is quiet and equipped with a heavy duty centrifugal fan with its backward turning impeller which is more powerfull and more effective in raising the CADR (Clean Air Delivery Rate) to a higher level. It could be installed both on floor and on bay window. Installation is done inside the unit and air purifier is not seen from outside of the building.