Air Flow Through a Wye 2

[Genie0501]

Hello,

A coworker asked me today if we could determine the flow of air through a pipeline when the air coming into the pipe is 15000 cfm. I told him that Q = V*A, flow rate does not change regardless of diameter, and that we could easily calculate this value. The more I think about it though, the more I am unsure if this is the case.

Specifically, we have two 7500 cfm blowers, each pumping air into separate 20-inch pipes. These pipes come together at a wye and the single pipe expands to 24 inches. He asked me if the velocity of the air would increase or decrease when it entered the 24-inch pipe and if there would be any significant losses.

My first though for these questions is to go back to what I learned in fluid mechanics about flow rate and Bernoulli's equation, but I am not sure if these are valid equations for air. I am pretty sure that Flow in = Flow Out (so Q1 + Q2 = Q3) but beyond that I am unsure if I can use the equations that I know for incompressible fluids (water, oil, etc) for air.

Any help or insight (even just some information about where I can look to find more information) would be greatly appreciated!

 

[LittleInch]

You need to do some searching on this site about flow of air and gases. Your initial assumption is not correct and therefore all the rest doesn't work. At steady state, it is mass flow which remains constant at any point, not volume. Air is compressible therefore your missing parameter for working out velocity is pressure. At 14.5psig, volume is approx half compared to standard conditions.

In your example I would expect the velocity to increase as the internal area of the 24 inch pipe decreases compared to the two 20 inch pipes. As velocity goes up, friction losses increase proportional to velocity squared.

You'll find a lot out there on gas flow. Try this site and engineering toolbox or look back at your notes for compressible flow...

My motto: Learn something new every day

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

 

[Engdoitbetter]

Volume flow is constant at steady state if fluid is incompressible.
There are some engineering situations where air flow can be treated as incompressible flow (this is the case of, if I remember correctly, axial compressors). This happens since pressure variation is negligible compared to the initial pressure.

I strongly suggest you to try a hand calculation according to Bernoulli equation, which should yield approximate pressure variation between 20" pipe and 24" one. Then look for calculations for gas flow and see the differences.

My fast calculation with Bernoulli resulted in a doubled kinetic energy downstream, so pressure is about a half of upstream value.

Regards,

Stefano

 

[katmar]

You have not given enough information to be able to give a definitive answer. LittleInch is correct that (strictly speaking) volume is not conserved in gas flow. But I suspect that in this case you can assume that it is conserved with very little error. It all depends on your system pressure. The way you describe the situation, and the fact that you call your prime movers "blowers" rather than "compressors", makes the application sound like HVAC work. If it is, then the pressure drops you will get will be very small compared with the system pressure and the density of the air can be regarded as constant. You should describe your flows as acfm or scfm - and define the temperature and pressure.

Putting some numbers to it - if your pressure drops are of the order of 20 inches of water column and your system pressure is close to atmospheric (14.7 psia = 407 "WC) then your density will change by less than 5%. Making the assumption that the volumetric flow is constant will therefore lead to errors in the velocity calculations of less than 5%. There are probably other areas where the uncertainty is the same or worse, so assuming constant density is often acceptable in HVAC work.

A note to Stefano - The combining of the flows into a single 24" pipe may double the kinetic energy, but this does not mean the pressure is halved. The velocity head increases from 0.7 "WC to 1.4 "WC, but since you probably started with about 420 "WC the decrease in air pressure is very small.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[LittleInch]

Point taken and very well explained by katmar. However I've just done a velocity calc based on near atmospheric pressure and came up with 18m/sec (58ft/sec) in the 20"pipe. Sounds rather high to me for an hvac system, but some feedback would be good genie to know what sort of pressures we're talking about.

My motto: Learn something new every day

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

 

[Engdoitbetter]

I'm sorry katmar, you're right.
This is one of the cons of "fast calculations"!

Anyway, velocity is indeed high as LittleInch pointed out.

Regards,
Stefano

 

[Genie0501]

Thank you so much for your responses!

We are using these blowers (I'm not sure how else to refer to them....basically they are big fans) to blow the air outside a pipeline (just open air) into a pipeline so that we will have breathable air in the pipe. As far as I know, we should be blowing air at atmospheric pressure into the pipeline where it should also be atmospheric pressure (or close to it). I think the two 20" pipes are flexible tubing and the 24" is the actual pipeline. We're not looking to really pressurize the air at all....we just want to move outside fresh air into the pipeline.

Thanks!

 

[miningman]

What you really have here is an example ot two fans operating in parallel. When you say your blowers are rated at 7500 cfm each, this is meaningless unless you specify at what point on the fan curve this 7500 cfm refers to. Point of maximum efficiency?? Maximum flow at zero pressure?? Point at perhaps 1 " WG?? What I can guarantee is that the flow in the 24 inch line with two fans operating will not be anywhere near double the flow with one fan running. You need to get the appropriate fan curves, stack them together to get the curve for two paralell fans and then superimpose the system resistance curve.

The high velocities previously alluded to mean you are dealing with high pressure fans, not low pressure blowers. As a coarse check on yiour previous assumptions use Fan HP = Volume ( cfm) x pressure ("WG) / 6350 x Fan Efficiency.

This will give you an initial estimate of the actual volume that can be achieved by whatever fans you are proposing to utilize.