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Ewan · 12-02-2003, 09:51 AM

In order to tackle the problem one has to have an understanding of how centrifugal pumps work and why they are designed the way they are.
Going by the name, a centrifugal pump works by applying a centrifugal force to the water. i.e water at the edge of the impeller has a centrifugal force which flings it ******ds away from the impeller. Since the pump is full of water, water is centrifugally forced towards the edge of the pump casing which in turn forces the water out of the outlet.
Centrifugal force is proportional to v^2/r where v is velocity and r is the radius. Since v in turn is proportional to rotation speed (W) and radius (the larger the radius the higher the speed) this boils down to centrifugal force being proportional to W^2*r.
Thus as radius increases, force increases linearly.
In other words a large diameter wheel will produce the most force at the tip of the impeller which in turn will cause a higher pressure at the outer pump casing and outlet.

Why the closed impeller?
If you have an open impeller then the water can leave the impeller and loose it's centrifugal force and thus it's pressure. Having a closed impeller makes sure that all the water that enters the pump leaves the pump at the outer edge of the impeller and thus has the most force (and resulting pressure). To maximise pressure you have to make sure the water stays in the impeller all the way.
Thus you should be able to increase the pressure of the pump by closing the impeller in the manner that bigben2K describes.
However when you have a closed impeller you don't ahev a lot of room for the water to flow whihc will limit how much they can pump. You notice how thin the gap is in the closed impeller for the water to pass through? That's why the max flow is so much lower.
High flow pumps have an open design with plenty of room for water to move through the pump. If water slips of the edge of the impeller, that's OK because in the short time it was in the impeller it may have developed enough pressure anyway. In this case, the aim is to simply get the water in and out.

But can't we have it both ways? Have high flow and high pressure by having a closed impeller with a much larger gap instead of the realy thin gap? Yes you can. Increasing the gap will allow more water into the impeller without realy negatively effecting the pressure since the impeller radius is unchanged.
But now the problem is that you've got tons of flow and heaps of pressure. Can you're little motor handle the extra load? Maybe.

As has been pointed out, the AC motor outputs at a constant rotation speed due to the frequency of the AC it's being fed (it rotates at the same frequency as the AC, thus 50Hz will yeild 3000 rpm). If it can't keep up, it will simply stall. So an AC powered pump has to be designed in such a way that the flow through the pump will never overwork it. This means that more often than not, a closed impeller type pump has already been sized in such way as to minimise this risk. You could try to improve upon it but you risk overloading the pump motor.

As one could guess, increasing rotational speed would also increase both flow and pressure, but then you have to change AC frequency which ain't so easy. Note that with a DC motor the problem isn't as bad. It will simply slow down if the load increases. Thus, you could overclock a DC pump by overvolting it. You might burn out your DC motor this way though.

correct me if I'm wrong.

12-02-2003, 09:51 AM	#14
Ewan Cooling Neophyte Join Date: Jun 2003 Location: Sweden Posts: 30	In order to tackle the problem one has to have an understanding of how centrifugal pumps work and why they are designed the way they are. Going by the name, a centrifugal pump works by applying a centrifugal force to the water. i.e water at the edge of the impeller has a centrifugal force which flings it *****ds away from the impeller. Since the pump is full of water, water is centrifugally forced towards the edge of the pump casing which in turn forces the water out of the outlet. Centrifugal force is proportional to v^2/r where v is velocity and r is the radius. Since v in turn is proportional to rotation speed (W) and radius (the larger the radius the higher the speed) this boils down to centrifugal force being proportional to W^2r. Thus as radius increases, force increases linearly. In other words a large diameter wheel will produce the most force at the tip of the impeller which in turn will cause a higher pressure at the outer pump casing and outlet. Why the closed impeller? If you have an open impeller then the water can leave the impeller and loose it's centrifugal force and thus it's pressure. Having a closed impeller makes sure that all the water that enters the pump leaves the pump at the outer edge of the impeller and thus has the most force (and resulting pressure). To maximise pressure you have to make sure the water stays in the impeller all the way. Thus you should be able to increase the pressure of the pump by closing the impeller in the manner that bigben2K describes. However when you have a closed impeller you don't ahev a lot of room for the water to flow whihc will limit how much they can pump. You notice how thin the gap is in the closed impeller for the water to pass through? That's why the max flow is so much lower. High flow pumps have an open design with plenty of room for water to move through the pump. If water slips of the edge of the impeller, that's OK because in the short time it was in the impeller it may have developed enough pressure anyway. In this case, the aim is to simply get the water in and out. But can't we have it both ways? Have high flow and high pressure by having a closed impeller with a much larger gap instead of the realy thin gap? Yes you can. Increasing the gap will allow more water into the impeller without realy negatively effecting the pressure since the impeller radius is unchanged. But now the problem is that you've got tons of flow and heaps of pressure. Can you're little motor handle the extra load? Maybe. As has been pointed out, the AC motor outputs at a constant rotation speed due to the frequency of the AC it's being fed (it rotates at the same frequency as the AC, thus 50Hz will yeild 3000 rpm). If it can't keep up, it will simply stall. So an AC powered pump has to be designed in such a way that the flow through the pump will never overwork it. This means that more often than not, a closed impeller type pump has already been sized in such way as to minimise this risk. You could try to improve upon it but you risk overloading the pump motor. As one could guess, increasing rotational speed would also increase both flow and pressure, but then you have to change AC frequency which ain't so easy. Note that with a DC motor the problem isn't as bad. It will simply slow down if the load increases. Thus, you could overclock a DC pump by overvolting it. You might burn out your DC motor this way though. correct me if I'm wrong.