axial or centrifugal pump??

[ginger1]

Hello all,

I hope someone here can help me! i am designing a pump to act as a jet pump to power a model boat, after much research on the internet I seem to have more questions than answers, so I would appreciate it if someone on here can answer my questions.

Firstly, I have a sample of a different boat in which someone has used a windscreen washer pump as the power unit and I think this is far from ideal as it is designed to pump a large volume of water rather than creating a high pressure outflow, it this correct? And can i increase the output pressure by simply adding a reduce to the output flow?

I have looked at both a centrifugal pump and a axial pump to create the water jet by i'm still confused as which to one will be best, does anyone have any thoughts on this?

Thanks in adavance

 

[cbrn]

Hi,
imho the confusion comes from thinking that you need a high output pressure, whether what you seem to need is a high outflow speed. The idea of the windscreen washer pump doesn't seem so bad after all. Generally speaking, it seems to me that an axial pump is more suitable than a centrifugal one, but due to very low efficiencies in these sizes, most probably the best is to find some sort of "mixed-flow" pumps (pumps with a runner shape "somewhere between" a centrifugal and an axial ones...)

Regards

 

[Artisi]

Axial flow is the way to go - especially for ease of configuration in getting the water in and out - in one end and out the other.

From memory, the early jet skis and are probably still in use were axial implellers.

 

[electricpete]

Axial pumps are generally better suited to high flow / low dp applications and centrifugal are generally better suited for low flow / hi dp applications. Examining the specific speed helps to quantify this.

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[Cheddarcaveman]

My full sized jet boat had a centrifugal pump - huge volume through a tapered nozzle. I don't think you'll get the volume from a positive displacement - they are more a pressure provider.

 

[Artisi]

"Cheddarcaveman (Mechanical) 6 Dec 07 15:39
My full sized jet boat had a centrifugal pump - huge volume through a tapered nozzle. I don't think you'll get the volume from a positive displacement - they are more a pressure provider. "

Where did the positive displacement come from in the discussion- seems to me we have discussed centrifugal and axial?

I would check your pump to confirm that is in fact a "centrifugal" and not axial which are used to pump "huge" volumes in relation to their diameter.

 

[Cheddarcaveman]

Hey guys, having a "senior moment"! Rather than axial/centrifugal I was looking at axial as meaning positive displacement, screw etc.
My drive is indeed "axial" with one propeller supported within the "jet" casing, pulling the water from the front and ejecting it at high velocity from the rear.
My apologies!

 

[hydtools]

Just to add a little more confusion, my jet boat has a mixed flow pump. Which pump is best is an ongoing discussion for jet boats with no conclusion.

Axial, mixed, and radial are all types of centrifugal pumps.

Propulsion is the result of change in momentum so I would think flow volume or velocity is the key characteristic. Pressure follows as the demand of the pump outlet requires for the final outlet velocity.

Ted

 

[electricpete]

Some people call axial flow pumps centrifugal, some don't.

In the literal sense, if the distinguishing feature is axial flow, it doesn't make sense to call them centrifgal (center fleeing). But since they share very similar characteristics (like the pump laws) and since there is a broad spectrum of mixed flow between radial and axial, many people just lump them into the centrifugal category.

I wouldn't argue with anyone picking one terminology or the other.. just wanted to point out the ambiguous terminology. If I am mistaken and there is a definitive right answer, I'd be interested to hear about it.



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[Artisi]

electricpete (Electrical)

I think it is easier to call radial vane pumps - centrifugal, mixed flow - mixed flow and axial flow - axial flow, saves a lot of confusion and most people with a basic understanding of pump terminolgy know what you mean.

To be pedantic the impeller between centrifugal and mixed flow you can call Francis Vane, although most would still use centrifugal for this shape.

Just my thoughts on it.

 

[Cheddarcaveman]

OK. Makes me feel a bit better to know there is so much confusion on the subject.
I do not believe that an axial flow anything, whether it's a pump or compressor, is "centrifugal" as they do not reply on centrifugal force to accelerate the pumped liquid/air.
Axial compressors are rotating, aerofoil based compressors in which the working fluid principally flows parallel to the axis of rotation. This is in contrast with centrifugal compressors where the air may enter axially but will have a significant or total radial component on exit.

 

[hydtools]

I think confusion, including mine at times, comes from individual preferences of definition rather than a common definition basis.

Here is a manufacturer's definition generally accepted in industry:

http://www.gouldspumps.com/cpf_0005.html

Ted

 

[Cheddarcaveman]

But this primarily addressing "pump specific speed" not the issue whether a pump is defined as centrifugal or axial. However it does make two statements, which support my last post:

Radial flow impellers develop head principally through centrifugal force.

An axial flow or propeller pump with a specific speed of 10,000 or greater generates it's head exclusively through axial forces.

 

[electricpete]

I'll repeat I don't think there is any single right answer. But reading this one reference, the heading of that page is "centrifugal pumps" and it includes a discussion of axial pumps as if it is a subset.

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[Artisi]

The term centrifugal is used as a generic to differentiate between the pump types, piston, diaphram, jet, positive displacement etc but once established what you are discussing, in my view, you should then further define to radial (centrifugal in the real sense) mixed and axial, this immediately identifies the pump configuration.

 

[Daovanbinh]

dear all,

which codes/standards design apply for axial pumps?

 

[vpl]

Daovanbinh

Please ask your question as a new one.

You might also try to give us more information, as the codes and standards that apply depend a lot on your application.

Also, as a new participant, you might want to read the FAQ that's in my tag line.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[d23]

All,

The strictest definition seems to be straightforward. Centrifugal implies centrifugal or radial force is added in a direction away from the shaft. Axial implies the force added is in the same direction of the axial or shaft.

As for the boat I think 50+ years of outboards, inboards etc. using axial flow "propellers" should answer the question. High volumes with lower pressures work.

D23