Why 10 meters is the technical limitation of water lifting in non-pump irrigation now?

Question

I readed a Deutsche news about bionics applications in non-pump irrigation online in July, 2006.

In this news, it is mentioned that the current
technical non-pump water lifting height is 10 metres, and the example in natural reality is about 100 metres. I want to know if the major principle of non-pump water lifting is about the surface tension of water and capillary elevation? I am wondering what are the major obstacles for artificial technique to reach the natural one.

From the formula of capillary elevation, it seems that the water lifting height could reach about 1000 metres (order of magnitude) if the redius of the capillary was about 10 nanometres (order of magnitude). Am I right or wrong in using the formula to make such a theoretical estimation? Are there any limited conditions for using the formula, which may not allowe me to apply it in such a huge scale? May be that there is difficulty in practice to achieve the theoretical one?

Thanks for helps!

Tom

Answer 1

Those first two answers were basically correct, but didn't appear very well explained to me.

When trying to lift water, it only appears you are trying to suck the water up through a pipe. What actually happens when you do this is you are creating a vaccum in the pipe, and at the end of the pipe in the water, the water has atmospheric pressure pushing against it.

It is actually the atmospheric pressure pushing down on the water that pushes it up into the end of the tube where you have created a vacuum.

Since the amount of atmoshpheric pressure is basically fixed, you can only lift as high as the force given by the atmospheric pressure equals the weight of water in the pipe, with is at about 10 meters.

I have no idea what is meant by "the example in natural reality is about 100 meters". This makes absolutely no sense.

Answer 2

By non-pump lifting, you mean not placing a pump at the bottom. Instead the pump is at the top and lifts by suction. Liquids move by the action of differential pressure. When lifting water against gravity, you need to counter the pressure produced by the weight of the water column itself (about 10 kPa/m) so the pressure at the top of the column must be reduced by 10 kPa for each meter of height. Since atmospheric pressure is 100 kPa, an absolute pressure of 0 (a complete vacuum) corresponds to 10 m of water. Since you can't suck any harder than a complete vacuum, 10 m is the physical limit.

If you were to pump a higher density liquid, the maximum height is proportionally less. Mercury has a density of 13.4 times that of water so the maximum lift is only 760 mm. You have heard of 760 mm mercury as a barometer reading. A barometer uses a column of mercury with a vacuum on top since the height of the column is a direct measure of the current atmospheric pressure.

Your capillary question is interesting. The action of surface tension in a very small tube will lift the water to greater heights and I am not completely sure but I believe that it will also suppress the formation of vapor bubbles.

Regardless of the height of the water, the princlple of conservation of energy makes the capillary pump not very useful. The water is lifted to a height in the capillary but this is done by the adhesion of the water (via surface tension) to the tube wall. It requires energy to create the water surface needed to remove the water from the tube. This energy is precisely equal to the energy it woudl take to raise the water to this height. In addition, considerable extra energy is needed to overcome the fluid friction caused by using a very small tube. Thus, it makes more sense just to put a pump at the bottom, the power requirements are identical and the flow losses are much less.

Answer 3

Ten meters is the maximum height atmospheric pressure can lift a column of water. This pressure is the ONLY factor causing "non-pump" lifting, which is not lifting at all, but the net lowering of the mass of water through a siphon.

Answer 4

it is more the mass and weight of the water againts gravity

Answer 5

Pretzels says you can't suck any harder than a vacuum. He obviously has not met my girlfriend.