Just to be clear, I'm assuming that in your pressurized tank vs vaccum tank example, that the fan assembly, including any duct inlets/outlets, are completely contained within the hypothetical tank. Basically simulating extremes of the ambient pressure the fan system is operating in.
Originally Posted by Core_d
It would have static pressure pushing out on the walls, but that pressure does not work directly against the fan paced within the tank.
The increased density of the air in your hypothetical pressurized tank is what would cause the fan to operate under a higher load than the same fan placed in your hypothetical vacuum tank.
To simplify things for better understanding of the air density thing, imagine that your standard motor type fan is a device made for throwing boxes of air, 1 cubic foot in size.
If the air in the 1 cubic foot boxes is very dense, it will weigh more than if the inside of the boxes were in a vacuum.
Since the fan has to work harder to throw the heavier boxes of very dense air, it will have higher amp draw than it does throwing the lighter boxes that are in a vacuum.
The static pressure the fan is moving air against is a measure of the flow resistance of the fan assembly, and all associated components it has to move air through.