How do we move water?

Water is extraordinarily cohesive, and that means we can move water in any direction. Listen to Professor Søren Rud Keiding explain the intricacies of water flow and pressure.

Water can be made to move in any direction we choose. It's often a matter of which materials we use, how much energy we need and ultimately also practicality and price when we move water.

A company like Grundfos is naturally very interested in understanding these basic characteristics of water that enables us to control the flow and the pressure when we pump liquid water. If you want to compress liquid water, when you push liquid water, for example it's one of the really, really fascinating properties of liquid water that it is almost impossible to compress. If you go to the bottom of the ocean, the deepest point in the ocean 10 km below sea surface the weight down there of the water on top of you is roughly 10,000 tons. In spite of that, the water is only compressed by a few percent. So compressing water is very, very difficult. If water could not be compressed at all then the surface of the ocean would be roughly 50 m higher all over the globe. Actually, it is to some extent easier to compress a rock than to compress liquid water. Another example of the compressibility of water, which is very, very low is the water jet cutters. They are small nozzles where you have water coming out at extremely high pressure. They are very, very efficient in cutting materials in very, very clean cuts. And they are actually only working because the compressibility of liquid water
is as low as it is.

Pumping height is a term familiar to many pumping engineers around the world. So it's basically the height of the column of water that you can pump. And that is actually an interesting scientific question that is heavily disputed in literature nowadays. People disagree on the background of the pumping height of water. Some say it's mainly due to the influence of atmospheric pressure and gravity and others say it has to do with the cohesion of liquid water. The fact that liquid water is actually very, very strongly connected to itself. So if you want to lift liquid water, you have to understand the hydrogen bonds. So again, as before, hydrogen bonds are the key to understanding why water has these unusual properties and in particular, why the cohesion of liquid water is so high. So whether you are pushing water, compressing water, pumping water pulling in water, or whether you are boiling water it all comes down to the hydrogen bonds and how much energy is used. So if you're pulling on a column of water the cohesion of water, the stickiness of water depends on the hydrogen bond and how much energy is used to break these hydrogen bonds.

And the energy used to break hydrogen bond is actually also the energy used to make a liquid boil. So boiling a liquid, thereby evaporating the liquid is basically breaking the hydrogen bonds. So when we study water we have to study how much energy it takes to break the hydrogen bond. Once we know that, we also know how much energy it takes to make water boil.

Course overview

Modules: 6
Completion time
Completion time: 30 minutes
Difficulty level
Difficulty level: Intermediate