Difference between Dampers and IGV

Louver opens to allow air to pass, Inlet guide vanes provide a 'pre-whirl' to the air, it sort of get the air spinning before it hits the fan.

What does this mean? I don't know.

But some time our customer ask for Inlet multi louver damper and some time they ask for inlet guide vane...

So i just wanted to know what is the difference between both at inlet. Can this change charact. properties of fan ?

Inlet guide vanes allow flow modulation similar to changing the speed of the fan. While not as efficient as speed change, guide vanes afford lower brake horsepower at the fan when compared to an inlet damper alone at the same air volume.

As Enginerd alluded to, fans with inlet guide vane and dmaper flow control have a different load profile based on percent flow. A guide vane fan is more efficient.

Having said that, where ever you are specifying a flow control device, you should investigate a VSD instead. The VSD will be far more energy efficient, be more reliable, and give better flow control.