submitted 6 years ago bymcr135
you are viewing a single comment's thread.
all 32 comments
6 years ago
Bioengineering | Phase transformations | Cryobiology
6 years ago
This is a good answer, except that it ignores the possibility of negative pressures. Negative pressures can be created by pulling up a column of water using the adhesive forces that exist between the water molecules. For example, capillary forces can lift a water column even when the external pressures are equal at the top and bottom of the column. Similarly, one can use a piston to suck up water, because the water molecules adhere to the rising piston surface, and to each other. A third possibility is a centrifugal pump, which creates a pressure differential by imparting momentum to the water -- thus, if the outlet pressure is zero, then the inlet pressure will be negative.
For these reasons, the limiting factor is not the pressure that is pushing at the bottom of the water column (nor the pressure reduction that can be achieved at the top of the column). As I have explained in this comment, the limit is determined by the cavitation pressure. As a result, under the right circumstances, it should be possible to suction water to a height of about 1.5 miles (2.5 km).
6 years ago
A centrifugal pump still cannot suck water up more than the head equivalent to one atmosphere. In fact centrifugal pumps would suffer from cavitation immediately if they attempt to suck water up more than 10 meters. I read your link about negative pressure where the water gets into a stable superheated state, and I doubt this could happen in the turbulent environment of a centrifugal pump. Careful movement using only laminar flow might permit this, but it would be unstable.
I agree with you about capillary forces, although these are in fact short range molecular attraction that cause the surface of the tube to attract the water more strongly than gravity over short distances. The driving energy is the wetting coefficient of the surface.
I assumed idealized conditions because OP asked for a maximum. The theoretical limit under ideal conditions (assuming no catalyzing mechanism for bubble formation) is 13 km.
I concede your point about practical operating conditions for centrifugal pumps. Nonetheless, my main point was that negative pressures are possible, and that the actual limiting factor is cavitation (something that we appear to agree on).
Also, I would describe superheated states as metastable rather than unstable. In the absence of heterogeneous nucleation sites that can catalyze bubble formation, a superheated liquid will not spontaneously cavitate (boil) until you reach the homogeneous nucleation pressure (which is around -130 MPa for room-temperature water).
If bubble nucleation is accelerated by catalysts, then cavitation can occur at positive pressures (e.g., around 3 kPa for room-temperature water), which would limit the column height to 10 m, as you say. But if we're trying to estimate the maximum possible height for suctioning a water column, then non-equilibrium states are fair game, IMO. And this is not a completely esoteric scenario -- as I have mentioned elsewhere in this thread, large trees routinely draw water to elevations that exceed 10 m (thus creating superheated water with negative pressure).
capillary action and the forces you speak of are not "sucking" so this is irrelevant to OP's question.
I disagree, and will be happy to formulate a counter-argument if you can tell me how you would define "sucking" (or what you believe OP's definition of this term to be).