Most front-to-rear cooled chassis platforms have tube-axial fans in the rear of the enclosure, pulling the air in an S-pattern.  The typical CFM in this approach is about 80 per fan and can cool about 110W per slot at max speeds.  This could be effective for many of the medium power OpenVPX and other applications.  However, if you have multiple boards over 115W each, you may need another approach. 

Another design technique is using powerful air-pulling RiCool fans placed directly above the boards in the card cage.   With fans that pull in one axis (to the top of the chassis) and blow the air out in another axis (through the rear of the chassis), the air bends to reach the fans are cut in half.    This patented reverse-impeller configuration reduces the backflow of the air and the backpressure that hampers performance.   

 The reverse impeller approach creates up to 191 CFM per fan and 3.6" (H20) of static pressure.  By comparison, the tube-axial approach generates only about 0.22"-0.40" (H20) of static pressure. 

The standard tube-axial cooling configuration is effective in most embedded system applications.  However, for high-power systems, as many OpenVPX applications are trending, having powerful and hot-swappable RiCool reverse-impeller blowers is an excellent option.