Convert Pressure Drop from SCFM for Water

I don't think it works that way.

Let's see, 20 SFCM=2PSI drop. How many psi drop will 5 GPM produce?

I'd like to see a picture of this filter.

Is this thing substantial enough to withstand water instead of air?

What are the wetted materials?

What does the equipment manufacturer have to say on the topic? They have been telephoned, haven't they?

You are a bit on the extreme.

If you are working on two different fluids, it is possible through the standard manufacturer graphs you can find the dp (differential pressure) for two flows and then there are equations available to take care of the viscosity effect (the result will be approximate)

However from a compressible (air) to incompressible is a bit too complex to calculate. If the filter does not complain then the only way to find it is by doing it.

UD15

Thank you for your replies so far. Well, I have the manufacturer's manual and it only gives the pressure drop for air as the fluid at a specific flow rate. For example, it says that when the flow rate is 20 SCFM at a line pressure of 100 psi, the pressure drop will be 2 psi. I wanted to know if I could use this information to get the pressure drop if water is used as the fluid.

And to address some of the concerns raised here, yes, the filter is rated for use both with gases and liquids. The housing material is SS 316 and it is a high-pressure filter.

I thought that there have to be pressure drop data both for the liquids and gases as the fluids. That way if the fluid is a gas, then the data from gases is used and if the fluid is a liquid, then the pressure drop data for liquids is used.

MrGeneRall, could you please elaborate on how you got your value?

Thanks

It was a joke....sorry.

I think there are too many unknowns to do any calculations. Screen/mesh size, density and number if multi layered. Without knowing the area of the filter medium relative to the amount of open space you have as well as the inlet/outlet sizes it will be impossible to do.

Either call the supplier (if it's rated for water, surely they would know) or, if practical, plumb a couple of pressure gauges in and test it yourself.

Good luck.

Quite often compressed air filters are for "trapping" water i.e. moisture from the compressed air stream. Which would cause a lot of pressure drop if the water even passed thru without destroying the filter.

I believe that the pressure drop- assuming that the filter CAN handle water- would be the pressure drop for air times the ratio of water density to air density times the appropriate volume ratio (P2 = P1 x (V2 / V1)squared).

The density of "standard" water is 62.4 Lbs per CuFt, the density of Standard Air is 0.075 Lbs per CuFt, so- in your stated case- the pressure drop would be

2 x 62.4 / 0.075 x ((5 / 7.5) / 20) ; (5 / 7.5) is gallons divided by gallons per CuFt to establish amount of CuFt of water passing through filters.

Any way, that would be 55.469 PSI.

If anyone else has a suggestion, I am open to hear it.

You can't use a direct ratio due to the compressibility of air. Air will be compressed inside the filter, and expand as it exits ... kinetic to potential and then back to kinetic. Water won't do that kinetic potential kinetic dance.

The drop in pressure of a non-compressible will be exponentially greater.

BUT ... for a very thin filter profile, your approach would be a fair estimate of the minimum drop. The thinner the filter the fairer the estimate.

At least he knows that the drop will be at least your answer.

784 is a fairly large difference in density.

I was clearly not thinking a moment ago. Not only is air compressible in the filter ... it is compressible ahead of the filter ... I had a blonde moment.

Therefore, I have to ask that you disregard my previous comment beyond the first paragraph, and state that your approach it flawed due to comparisons between compressible and non-compressible fluids.

The OP does NOT state that the filter is for compressed air, just the DeltaP with air. It could be the filter for the INLET to an air compressor.

You are right in your "position", assuming that the air being filtered is compressed.

Assuming that the air is at 7 BAR (105 PSIG), its density would be (7 x 0.075) Lbs. per CuFt, so the resulting "solution" would then be 55.469 / 7 = 7.924 PSIG.

Again- the OP did not state a pressure drop with COMPRESSED air, so IF the air is pressurized, the actual DeltaP value will definitely vary from my OP. If the initially defined air is/was NOT pressurized, I think my initial answer is correct.

It was very late at night and somehow I tagged post 12 as OFF-TOPIC. It is NOT- it is a direct response to post 11 and relates to post 9 and it totally ON-TOPIC.

Please forgive my screw-up and read the post.

You can always go back and change your vote, but you can't nullify it.

the difference in viscosity will make the water far slower

the water is 60 times as viscous = 60 times the drag = 60 times slower or 60 times the pressure needed, more or less. Water and air are simple molecules, so this is a reasonable generalization.

http://physics.info/viscosity/

Air at 15 C .0179

Water at 20 C 1.00

Kinematic viscosity is a measure of the resistive flow of a fluid under the influence of gravity. It is frequently measured using a device called a capillary viscometer — basically a graduated can with a narrow tube at the bottom. When two fluids of equal volume are placed in identical capillary viscometers and allowed to flow under the influence of gravity, a viscous fluid takes longer than a less viscous fluid to flow through the tube. Capillary viscometers are discussed in more detail later in this section.

As I stated in #3, the matter is not so simple due to the

a) Compressibility of air

b) An additinal parameter is the difference in Re (reynold's Number) which will be very unstable for the air flow (It is easy to visualise that the mesh area is highly turbulent, more so in case of gaseous fluid)

UD15

Depending on the filter type it may either be hydrophobic (water hating) or hydrophilic (water loving). If the filter is hydrophobic then it will block when trying to pass water (depending on the pressure applied). Have you checked with the supplier if the filter can be used for both air and water?

Thus, translating into numbers:

20 SCFM = 576 in^3/s; 5 GPM = 19,25 in^3/s; density of water: (3,61x10^-2) lb/in^3; density of air: (4,44x10^-5) lb/in^3

DeltaPw = [((3,61x10^-2) x (19,25)^2)/((4,44x10^-5) x (576)^2)] x 2 psi = 1,82 psi

1,82 psi x 55 = 100 psi

This result is approximate and provides only a rough order of magnitude. Other factors must be analyzed as the geometry of the porosity and surface tension. For a more accurate value, it would be important to test the filter at the desired conditions, as already mentioned.

As many others have noted, this is not a simple conversion issue. First, the pressure drop vs. flow for air handling is not a linear relationship, but a power function, and this power function can only be determined by pure testing. That is, doubling the flow may increase the differential loss by more than double. In addition, changing the motive pressure (OP cited 100 psig) will affect the differential pressure loss across the filter as well. The filter characteristics are set by the type of filter material and the relative pore size presented to the flowing media.

OP states the filter is rated for both liquid use and air use. Liquid filters require very special requirements not often found in air filters. So if the user manual for this filter does allow use for both liquid and air use, then it should also present suitable charts for determining the loss. If none are included, then I would doubt the validity of the liquid/air claim.

Next, the viscosity issue is certainly a barrier to easy conversion. As has been noted, pushing compressible air (at elevated pressure) through a fine filter is relatively easy. (Think blowing through a window screen.) However, OP doesn't relate what driving pressure the water is under, only the requested flowrate. Due to the surface tension and viscosity of the liquid (assuming water), we would also need to know how much pressure the water is under. (Think pushing Silly Putty, modeling clay, or Pla-Doh through the same screen as above.) If this pressure is under the barrier pressure presented by the liquid surface tension, then no water will pass the filter. Basic bottom line is that the filter manufacturer would have suitable curves showing performance for this filter if, in fact, it is rated for both liquid and air use.

Using the Cv method, what is the pressure drop that you found?

To convert pressure drop from one fluid to another we need to do a few things. I don't know about a mesh screen but this is how you do it for a tube.

Here is a definition of pressure drop from Darcy-Weisbach

For fluid1:

DP=f*L/D*1/2*rho*v^2

Where f1, the friction coefficient is dependent on the Reynolds number and surface roughness.

For 4000<Re<100.000 Turbulent flow in smooth pipes, the blasius equation can be used. (ALWAYS CHECK THIS! otherwise use Moody or laminar correlation)

f=0.3164/(Re^0.25)

Now to calculate DP2/DP1 the pressure drop ratio, we can write:

DP2/DP1=f2/f1*L/D*D/L*1/2*2/1*rho2/rho1*v2^2/v1^2

Simplify:

DP2/DP1=f2/f1*rho2/rho1*v2^2/v1^2

Where f2/f1 = (Re1/Re2)^(0.25)=((rho1*u1*mu2)/(rho2*u2*mu1))^(0.25)

If you put all of these together and consolidate you get:

DP2/DP1=(rho1/rho2)^(-0.75)*(mu1/mu2)^(-0.25)*(u1/u2)^(-1.75)

Since you are doing equal flow rate (GPM) u1=u2 and the last term drops out.

Hope this helps, Cheers