You can see here that the fan is shrouded to prevent it from throwing the air in all directions and out of the intake, i assume the 5870 is the same, you can see the gap between the blower and chassis which is probley the draw point where the lip is cut out to allow intake... soo if you put ur finger in the intake and felt upwards there would be a small gap to feed air from the intake of the blower.. if not the fan shroud and the top chassis are two seperate pieces that dont even touch.. i assume they are air "intake" and not exhaust
You can see here the same principle, the shroud for the radial fan hides the fan, but i bet there is just enough room to force those inlets to draw if the top becomes blocked
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