Impeller design

[pomp]

We are trying to design an impeller which takes flow from a full pipe (DN250) and forces it in to a rotational motion. The flow leaving the impeller proceeds axially in a 400mm diameter rotating drum. This is actually a “water ring” with an air column in the middle. This means that the flow in the impeller has a free surface unlike that of normal centrifugal pumps. As there is an air column the impeller is not creating pressure. We are just trying to get the flow to rotate at high speed (actually only about 600 rpm) to put it under high centrifugal force. Does anyone know of an application which may in anyway resemble this? Thank you.

 

[ApC2Kp]

pomp,
Sounds much like an old BIRD centrifuge, except the bowl of centrifuge rotates , and the flow is enormous. How much residence time is needed at 600rpm ? If the residence time is very little, then a cyclone separator is essentially what is descibed. If solids are in the flow stream, then it becomes a matter of what are the requirements for percent moisture in the solids or the vapor stream.

 

[pomp]

How much residence time is needed is a good question. The unit is used in the paper industry on flows with different levels of fiber and mineral contents as well as different chemicals. Currently the retention time is <1 second at 300 lpm (4,750 GPM).

There may be some misunderstanding. The rotating drum is solid, no holes. We are only trying to get all the air to migrate to the center of the drum from which the air (gas) is removed axially from the device.

My problem is to design a better impeller to feed this drum which requires a low suction head, provides good accelecation to the flow and minimize energy requirments. Since we do not create pressure and there is an open surface in the impeller, I am not sure how to go about the design.

 

[ApC2Kp]

pomp,
The residence time and velocity is within the operation parameters of a cyclone separator. It would be a vertical vessel with a tangential entry nozzle, bottom cone discharge nozzle for liquids + solids, and top center line vapor exhaust. The energy is provided by the circulation pump or gravity flow.
The BIRD centrifuge is a horizontal solid bowl design with an internal screw to push solids out of the conical small end while liquid overflows out the large end of cone. You don't need this equuipment for air separation.
The fiber and solids probably must stay in suspension for your process, otherwise I could see using a settling vessel to allow the air to be extracted from the flow stream.
Have you considered putting flow guide tabs in the drum or in the drum inlet to induce the spinning motion? Usually the design problem is to eliminate vortex motion in vessel outlet lines to a pump suction to prevent air or vapor interrupting pump flow. Here it seems you are trying to separate the air by intentionally spinning the flow. There are centrifugal air compressors with inlet guide vanes to spin the flow to improve flow throttling of the compressor. Flow vanes might generate your flow pattern to accomplish air disengagement. How much air is in flow?

 

[sprintcar]

Adding air into a centrifugal impeller will cause flow problems. ApC2Kp has the right info for you. How about using the pump to accelerate your slurry mix then either go into a cyclone unit or pipe it into a rotating drum. I would consider spinning the drum with an external power source so the rpm could be established at the optimum speed and not vary.
Otherwise, you're looking to design an axial flow impeller which would likely clog rather quickly given what you're pumping.

Keep the wheels on the ground
Bob
showshine@aol.com

 

[pomp]

The flow vanes that ApC2Kp mentions is basically what I have in mind, but I do not believe that I need them to run down the whole 1.5-3m long rotating drum. I thought the first 300-500mm should be enough.

Air content varies from 2-7% at 200-300 l/s or 14-76 m3/h at atm. We however vacuum addition to improve air removal causes the air flows to increase by 2-5 times.

Sprintcar we are not adding any air we are trying to get it out. Actually we have two pumps connected together. The first pump impeller is thought to accelerate the flow. In an open silo you need a large surface and time for the bubbles to rise to the surface with only 1g. Rotating the flow up to say 100gs those bubbles really move and you don't need to wait or waste valuable floor space. At the other end is another impeller (which is connected to a variable speed drive). There is no problems of swirling motion when feeding this second impeller and actually the rotating flow reduces power requires on that impeller. Of course the energy was added at the first impeller by the same motor.

So I guess I am actually looking for some sort of axial flow impeller. There should not be any risk of plugging at >4m/s and with the large passages.

 

[Artisi]

Your answer might be in the Sala froth pump, you will probably need 2 units for the flow rate you're are talking about.
This is a real option for your application - I recommended and installed a unit ( smaller than your requirement)which was very successful in a paper mill in Australia quite a few years back to overcome an air entrainment problem.

go to this site for more detail.

http://www.metsominerals.com/inetMi.../mmcontent2.nsf/WebWID/WTB-020108-2256A-4C796

Naresuan University
Phitsanulok
Thailand