Relationship of backpressure and flow! 1

[Jigneshpadia2002]

Hi !

I was doing one experiment on Tangential Flow Filtration membrane and I was confused by the flow outputs with and without pressure.
My assumption was

For a close system: Input = Output

Feed flowrate = Permeate flowrate + Retentate flowrate

When I started the experiment without any retentate and permeate pressure at afixed pump flowrate of 50 ml/min. I was observing the feed pressure gauge reading to be 18 psi.

When I increased the pressure on retentate side to 20 psi. This reduced the retentate flow. It also increased the feed pressure gauge. The flowrate of the pump was not changed. If the above equation of Input = Output was correct than the total of permeate + retentate sould remain unchanged. However, I observed that the total flow was decreased by 12 ml/min.

I am assuming that the flow decay is due to increase in pressure on feed and retentate side but I am still not able to understand clearly. Can I quantify the flow decay and pressure increase relationship.

Thanks a lot....

 

[Cockroach]

But clearly the static pressure drop across the pump is a function of fluid velocity, this is simply the Bernoulli Equation. This measure, velocity of the flow, can be easily used in the Continuity Equation (Mass Conservation) to give you a volumetric rate. Therefore you can relate the velocity of flow to any other point in your loop noting pipeline bore cross sectional area.

Maybe your gauge is too close to the pump outlet and you are picking up turbulence? Also, there are head losses associated with your pump system. I think you need to look at the outlet efficiency as a scalar multiple of fluid velocity.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[zdas04]

If input mass flow rate does not equal output mass flow rate then there has to be something consuming or adding mass between the two points. There really is not a third option.

Did you have a calibrated flow meter on each of the legs of this system?

You didn't say what kind of pump you are using, but if it is a dynamic pump like a centrifugal then it is unlikely that a significant increase in discharge pressure at the same flow rate is possible.

I think we need more data to stop this infernal guessing.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[Jigneshpadia2002]

About pump:
I am using colepalmer pump - Peristaltic pump. It requires me to use silicone tubing. Filter area is 50 cm2. pressure gauges were calibrated before usage.

Flowrate: without putting the filter inline the pump flowrate was measured and it was 50 ml/min.

I am not using flowmeter. Flow measurement is done manually using timer and graduated cylinder. Inorder to get more reliable and accurate reading I am recording the results in triplicates. Moreover I take 2 minute for timer and take the average reading for the flow.

Location of pressure gauge:
Pressure gauges are located within 5-8 cm of inlet and outlet of the filters. I also have the pressure gauge on the retentate side of the filter.

I am still not able to understand the decrease in flowrate when I pressurserize the retentate side.

Without pressure retentate flow was 48 ml/min and permeate was 1.5 ml/min.

After applying pressure of 20 psi on the retentate side the retentate flow was observed to be 30 ml/min.

Ideally the rest should be diverted to the permeate but permeate flow was only 4 ml/min resulting in total flow of 34 ml/min

where is the rest of flow lost. I have checked the integrity of the tubing and filter. There was no leakage.

The feed pressure was increased from 18 psi to 30 psi. SO my guess is the loss in the flow could be responsible of increased system pressure????

Can I calculate how much pressure results in what % of flow loss??

Thanks for your previous suggestions and explanation. I hope you guys. Please clarify this for me.

Thanks a lot in advance

 

[MikeHalloran]

"The flowrate of the pump was not changed" is not a correct statement. You didn't change the pump drive's settings, which is not the same thing.

Any peristaltic pump's flowrate will change with increasing discharge pressure.

It doesn't matter how much or how little you paid for the pump or the tubing, or how many rollers or pressure shoes the pump has, or whether it has ball bearings or sleeve bearings or no bearings. When the system impedance changes, the flow changes.







Mike Halloran
Pembroke Pines, FL, USA

 

[Jigneshpadia2002]

Mike...

It does make some sense but I am still not able to understand it clearly. If you can elaborate it for me than that would be great.

I got your point that if the pressure on inlet was changes it is going to affect the flow and that is why the total flow is less. but is there any equation or formula to calculate these numbers.

Thanks for your usefull tip.

 

[MikeHalloran]

Ideally, the performance curve for constant shaft speed for a 'positive displacement' pump should be a straight vertical line in Q-H space, where Q is flow and H is total head. For an impeller pump, like a Jabsco, the line tilts just a little to the NorthNorthWest, i.e., flow drops off with increasing head. The curve changes a little as the impeller ages. The curve for a gear pump is more nearly vertical, and gets more so as the price goes up and clearances go down.

The performance curve for a peri pump is also generally vertical, but it wobbles around erratically, as the total head changes, as the tubing ages, as the tension on the tubing changes, as the rollers' bearings dry out, as the temperature changes, as the planets interact with each other, and in response to certain unknown random or pseudorandom influences. It's a perfectly good pump for metering stuff against a constant load to one significant digit of resolution. Your application apparently requires two significant digits.

It happens that I have been sucked into several projects for which the accuracy and repeatability of a peri pump was critical, and was assumed as a given by management types. The general attitude was that something so "mechanically simple" must be dead- accurate by definition. You have never heard of the projects because they were killed, but only after consuming obscene amounts of money and time.

You can set up a simple apparatus and measure the pump's performance for yourself. You will not replicate your own data.

This is where noobs and students try to get funding for a more expensive pump of the same type, or design their own with improvements of one sort or another. I have traveled both roads; I have paid more for a single peri pump than for a nice new car, and I have used or built most of the variants you could conjecture, and some you wouldn't dream of. They all worked about as well as the C-P unit.

I.e., you can develop your own equation, but you'll have to change it tomorrow.




Mike Halloran
Pembroke Pines, FL, USA

 

[Jigneshpadia2002]

Any other comments?

 

[FACS]

One more comment,
If you increase the pressure inside the peristaltic tubing, does not the roller have to press harder on the tubing to compress it?

If the roller has to press harder, would not this require more horsepower from the motor?

Charlie

http://www.facsco.com

 

[MikeHalloran]

Yes, increased backpressure would require more power, so a motor running open loop would slow down, nominally reducing the flow.

Increased pressure would also expand the tubing, so the flow might increase, or not decrease in proportion to the motor speed change.



Mike Halloran
Pembroke Pines, FL, USA