From the factory the fans are orientated to blow air out. Here are some videos that someone put together demonstrating the difference. You can use the video to see how the fans are orientated.  Old fans: [URL] New fans: [URL] 
Hello everyone,  I decided to replace the fans on my 6224 so that I could use that switch near my desk.  After some hit and miss I have fans that work with this but in the time delay from when I started this I did not keep track as to which direction should the fans go in (blow out and pull air out or blow in and push air in).  My other switches are no help as some go one way and others go the other. Thank you for your time!
Dell 6224 Fan Air Flow