Air Pressure Drop across Pump

[invent11]

We intend to put a number of pumps in our plant in lay-up state for a period of time. During this time we blow dry air thru piping system into these pumps from dehumidifier units. The air static pressure out of dehumidifier Unit is limited to 5" W.C. How much pressure drop one should expect air flowing thru each pump? Any rule of thumb /thoughts? Thanks for your inputs

 

[1gibson]

For a centrifugal pump, the max sphere size the pump can pass is typically 85% of the hydraulic passage of the impeller. If you know number of impeller vanes and number of stages, you could probably get a rough number. Max sphere size isn't always on the datasheets, but if it is then this should be a good start. It's an unusual request so I wouldn't expect you to find anything more detailed than this. Interested to see if there are any other responses though.

 

[zdas04]

Sphere??????????? he is just blowing dry air through the system, where do sphere's come into it?

Upstream pressure, downstream pressure, gas temperature, pipe size and length, pipe roughness, inserted elements (like pump impellers), and gas composition (including viscosity) determine a flow rate exactly. If you have a max upstream pressure of 0.5 inH20, flowing to atmosphere at a known temperature, then you get a flow rate. If that is more than your blower will put out then upstream pressure will drop. If it is less than your blower is putting out, then upstream pressure will try to rise (until you run out of blower hp). If you add more pumps and piping in parallel then your blower discharge pressure will drop. I don't think it can drop to the point where you get zero dP and zero flow, but it can get awfully close.

The lower the pressure, the more water vapor air can carry and the more liquid you can evaporate. If I was doing this, I would look at putting the blower on the outlet and letting the pressure fall as low as it could (maybe several inH2O of vacuum) by pulling air through the dehumidifier instead of pushing the air.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

 

[1gibson]

Sphere size comes in as described; it indicates 85% of the cross section of the smallest hydraulic passage in the pump (the impeller.) So it should allow you to come up with a rough model of restriction across the pump element, based on this and other information found in the pump datasheet. I assumed the OP doesn't need any help figuring out friction losses in pipe.

I know it seems a little strange, that's just how I would start.

 

[zdas04]

In laminar flow, the flow restriction of nearly any size of vena contracta modeled elements is extremely low. I'm betting that his blower will not keep up with a 5 inH20 pressure drop.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

 

[bimr]

Depends on the length and size of the pipe/pump doesn't it.

Forcing 450 CFM through 10 linear feet of 3-Inch diameter galvanized pipe has a head loss of about 5" W.C.

Forcing 135 CFM through 100 linear feet of 3-Inch diameter galvanized pipe has a head loss of about 5" W.C.

Forcing 2800 CFM through 10 linear feet of 6-Inch diameter galvanized pipe has a head loss of about 5" W.C.

Forcing 850 CFM through 100 linear feet of 6-Inch diameter galvanized pipe has a head loss of about 5" W.C.

 

[LittleInch]

For a centrifugal pump there are many paths through, but quite convoluted so my estimate is that you should use 5 feet of the discharge pipework to calculate your overall losses which is what I guess you're trying to do? With only 5 inches of wc available I really don't think you're going to get much flow once you add in non return valves, valves and pipework so either lower preassure at the far end or feed the dry air into another blower or use a dryer on the end of a compressed air line.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[invent11]

Thanks for all your comments. LittleInch, what is your basis for :5 feet of the discharge pipework to calculate your overall loss? Is it from any reference?

 

[Artisi]

If you have any NRV's in the line - is 5"W.C. sufficient to lift the valve flap/s?
Possibly David's suggestion re "sucking" instead of pulling could well be the solution.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

It's an estimate, based on experience, in other words a guess!.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way