Recently I've been looking at a range of network server cabinets with a mesh door at the rear. At the top of the cabinet are ceiling mounted fans with dust filters attached. These fans suck air out of the cabinet - presumably to remove the hot air that has risen to the top.
My question is why do these fans have dust filters? What point do they serve? If the air flow is pulled in by these fans through the mesh door, then surely they are not there to protect the hardware inside from dust?
Also, there are other minor grilled gaps around the enclosure, how does this kind of ventilation design work?