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I'm trying to gauge the strength of aquarium air pumps, for use in a vertical hydroponic farm. A curious question arose: is it easier to pump water upwards vertically, or is it easier to pump it downwards (assuming a volume like an aquarium)?

Upwards, you fight with gravity -- downwards you are aided by gravity but fight water pressure.

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up vote 5 down vote accepted

Assuming both ends of a pipe are submerged in the same body of water, pumping upwards requires exactly the same force as pumping downwards, at least if the flow is relatively slow. If this wasn't the case, the water would move through the tube by itself if the pump was removed.

This means that when the water isn't moving, the force of gravity pushing the water down must exactly balance the force due to the pressure difference between the two ends. Because these forces cancel each other out, when you start to pump the water through the tube, the only force you have to overcome is friction, and that doesn't depend on whether you're pumping up or down. (Though if you're pumping at a high speed turbulence will occur, and then the difficulty of pumping might be asymmetrical, depending on the geometry of the pipe and the tank, in which case pumping in one direction might be harder than pumping in the other.)

Things are different if the ends of the tube are in different tanks. In this case the gravitational and pressure forces don't balance, and water can flow through the tube by itself - this is how siphoning works.

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The other answers are fine, but this is the most-complete – New Alexandria Apr 9 '13 at 13:43

Pumps are used to achieve three things maintain the flow rate, and overcome the head and the resistance in the pipe line, I will assume the pipe line is the same in both cases. If you pump upwards you have to over come the head (pressure) which increases with the height you are trying for the water to reach. If you pump downwards you don't have to overcome the pressure. I don't know how you have the two options, in one the water will go upwards and the other downwards, how do you compare both since the system is ready and you will only place the pump. So please explain further about what the system will look like in both options so that I can help you.

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Upwards you still have water pressure, consider the weight of a column of water on the exit of your pump

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