Volume vs velocity with an air manifold

[cityjack]

Afternoon all,

Forgetting friction and air leaks. Theoretically, if I have an air manifold that has one input port(2") and five output ports(1"), if I put say 500 cfm into the input port, I know my velocity will be higher on the outputs due to smaller diameter, but will I divide the input cfm(500) by 5 on the 5 output ports? 100cfm per port?

Thank you all for your help. I am using an anemometer measuring velocity at the individual output ports to then calculate cfm to see if my blower is up to spec.

Thank you again.

Sid

 

[MintJulep]

Well, if you forget friction then yes. Divide by the number of ports.

But friction is the controlling factor. So forgetting it is kind of a bad idea.

 

[EnergyProfessional]

You know to measure the flow you need to traverse. there is a velocity distribution. You also should measure in a straight duct or pipe and use an actually good meter appropriate for the application.

If you do it wrong, you get 5 numbers you can add up to get another wrong number. To validate fan, you also need the pressure it generates without system effect. With a given fan, flow could be anything from 0 to max flow. there is no friction-free or leak-free reality.

Manage your expectations.

 

[cityjack]

Thank you guys for your help. The velocity measuring tool I am using is an anemometer from Omega, handheld. It is flexible tubing I am measuring on the 5 ports for my fpm. I am also measuring right at the output of my blower which is a larger radius. I am already seeing a discrepancy due to the length of hose and it being flexible and NOT smooth.

I need to manage the expectations as you say I am seeing. [highlight #FCE94F]"If you do it wrong, you get 5 numbers you can add up to get another wrong number."[/highlight][highlight #FCE94F][/highlight]

Just trying to learn and would really like to get close or even in the same ballpark honestly. I'll keep working at this.

Sid

 

[LittleInch]

Well all you really have is conservation of mass.

So 500cfm in, 500 cfm out measured at the same reference pressure.

Now your 5 outlets could be 100 each, but equally could vary from 250 to 50 depending on the orientation, size of the manifold, distance between the out ports and whether the downstream pressure in all the ports is identical.

Also your outlet velocities, if the flow is equally split, will be lower than the inlet. It's basically square area.

So SA of the inlet is proportional to 2^2 = 4. outlet is 5 x 1^2 = 5. So a bigger square area in total on the outlet equals lower velocity, by about 25%

Flow velocities vary across the diameter so using a spot anemometer smaller than the diameter then you won't get a uniform velocity.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cityjack]

Little Inch, this is good stuff. If you care, I'll give you more about exactly what I'm working on. I have an issue with a dust collector I need to tend to right now. I'll come back and share more.

Thank you so much for your reply and ear.

Sid

 

[EnergyProfessional]

Traversing a duct
You also need sufficient straight duct up and downstream of the flow probe. Obviously if you add duct, you influence the flow.

There is a velocity profile. This profile will be highly disturbed after an abrupt change in flow, like an exit from plenum to your opening. that's why you need to measure flow when it is stabilized (hence the minimum straight duct up and downstream requirement, which will differ based on type of disturbance)

No balancer ever does it correctly.

Another problem you run into is if your probe is relatively large compared to the opening. Then you restrict the flow for the one opening you measure at the time of measuring. So you need a tiny probe, and/or measure all 5 openings simultaneously. That's why those diffuser flow measurements with flow-hoods are a guess at best.

then obviously the calibration of your device. But most error comes from the measuring method. Just because you see a number, doesn't mean it is a correct number.

Check the instrument manual on what they require and what limitations are.

 

[MintJulep]

With 1" and 2" diameter flexible tube there is really no such thing as a traverse.

With small diameters the introduction of a measurement tool likely alters the flow by a non-negligible amount.

I'm understanding correlated flexible tube. Flow is not likely to be laminar, which tends to make measurement unreliable.

"If you do it wrong, you get 5 numbers you can add up to get another wrong number."

Since you seem to be comparing inputs and output more likely you will get 5 wrong input numbers that do not add up to the wrong output number.

What are you trying to do and why? What level of accuracy do you really need? I just can't imagine getting any meaningful results from a handheld anemometer.