In addition to selecting a fan, there may be some choice in the location of the fan or fans, and in this regard, the illustration in Figure 4 may prove useful. The following comments should also be kept in mind with regard to fan location:
1). Locate components with highest heat dissipation near the enclosure air exits.
2). Size the enclosure air inlet and exit vents at least as large as the venturi opening of the fan used.
3). Allow enough free area for air to pass with velocity less than 7 meters/sec.
4). Avoid hot spots by spot cooling with a small fan.
5). Locate components with the most critical temperature sensitivity nearest to inlet air to provide the coolest air flow.
6). Blow air into cabinet to keep dust out, i.e. pressurize the cabinet.
7). Use the largest filter possible, in order to:
a. Increase dust capacity
b. Reduce pressure drop.
Figure 4: Cabinet Cooling Hints
Information provided by: http://www.electronics-cooling.com