generally... and depending upon equipment type (i.e. centrifugal, axial, etc.). would help to know application consideration.
igv's are design to pre-rotate or spin the air in the direction of fan/compressor rotation. this has a flow-modifying and a resistance effect. this allows operating a constant speed fan at different process conditions. the igv's should be built as part of the fan/compressor by the mfg.
some factors to consider are power consumption & turndown.
there are several web sites - try "inlet guide vanes" at the google search site and you will note the multiple applications.
I am assuming here that you are referring to variable inlet guide vanes. I am also presuming that you are not referring to Inlet Louvre Dampers which produce a similar effect but are much less efficient.
As pmover states inlet guide vanes spin the gases or air in the direction of rotation and by doing so are a much more efficient method of turndown than a throttling device.
Variable speed is the only method that is more efficient than IGVs by a significant margin but is not suitable for every application. The big difference is that variable speed also reduces the fan peak pressure in the ratio of square of the speed. This can be a problem if the system resistance curve is relatively flat. Inlet guide vanes have the advantage in this case as the peak pressure capability remains more or less unchanged with turndown.
The subject is a very broad one and if you can make the question more specific I can give you a more focussed answer. There is a good AMCA publication covering the various control systems and their applications among other things.
Centrifugal compressors develop head as a result of the change in angular momentum of a gas. Imparting a swirl on the inlet gas therefore changes the incomming angular momentum and therefore will reduce the head developed. Actually, guide vanes also introduce a pressure drop, so their total effect is a combination of swirl and inlet thrittling such as you would get with a throttle valve. AIGV's (Adjustable inlet guide vanes) will allow more efficient operation than an inlet throttle valve at reduced head requirements for a constant speed driven single stage compressor, but also introduce additional complexity and maintanence issues due to linkages, leak paths, etc. The decision as to whether the long term benefits favor AIGV or just a throttle valve must include estimates of operation time at different conditions and the power savings associated with an AIGV vrs straight throttle valve.
While this thread being posted in the gas compression forum would indicate that it might be a fan/compressor application, the answers took the topic to much broader range of applications.
In light of that, I will give another link that will take the discussion one step further.
