I must vent air through an 8 inch duct from one area to another using an 8 inch line fan. One goal is to vent the maximum amount of air possible through the duct. Is there any different between putting the fan at the beginning of the duct and pushing the air, or at the end of the duct and pulling the air?
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$\begingroup$ related: physics.stackexchange.com/q/71620 $\endgroup$– Kyle OmanAug 12, 2014 at 17:09
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3 Answers
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Although this may be a small effect, I think pushing is better than pulling. If you pull air, the you are pumping air that starts a at a pressure (and density) less than atmospheric. This means that the pressure differential across the length of the duct cAn never be more than one atmosphere.
On the other hand if you push air you can easily generate a larger pressure differential with appropriate fan design: air pressure at the inlet of the duct could be 3 atmosphere, for instance.
So I think you want the fan at the inlet.
See my earlier answer to a related question for more information and a diagram.
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If you wanted to pump water then it would be better to "push" and not "pull", since otherwise the pressure in the pump could drop too low such that cavitation could occur. But you are dealing with "pumping" a gas, for which this will not be a problem.
An effect which could alter the performance would be the angle of attack of the blades of the fan. If the fan would be placed at the beginning of the duct, and therefore pushing the air into the duct, then the air outside of the duct will be sucked in from all direction (assuming there is no duct in front of the fan). In the other situation the air will be sucked through the duct and therefore the velocity of all the air will be parallel to each other. The best option in this case I think would depend on design of the fan blades, such that the angle of attack of a blade (which also varies along the radius) would closest to optimal, however this might also change by increasing the rotational speed of the fan.
Another effect which might influence the performance would be the extra friction caused by a rotating air flow inside the duct (so for the push configuration). I do not have a good estimate on the order of magnitude of this friction, so it might be neglectable, however it would not occur in the "pull" configuration. Is therefore for this effect I would argue that "pull" configuration would be best.
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In theory, pushing will move more air - all other things being equal.
Look at it from a mass flow perspective. Higher pressure air has higher density. When you push, the air in the duct will have higher pressure than if you were pulling form the other end. This means that for a given unit of time, more air flows though a pushed duct than a pulled one (again- assuming all other things equal).
In practice, best choice likely depends on why you are moving the air.
If, for example the air has something in it that you are trying to get rid of (odors, humidity), then it is better to "pull" since the inside of duct will be at lower pressure than ambient, so any small leaks will let in more fresh air rather than blow out the stuff you are trying to get rid of.
If, on the other hand, you are trying to get conditioned air into a space (cooled, fresh) then you want to push so you do not suck up stuff you don't want and bring that to the end of the duct.
So... Air conditioner duct running though hot attic - push. Bathroom vent duct running though conditioned space - pull.
