Pressure Drop through a low-pressure Air PVC Piping System 1

[Subystud]

Hello.

I my question is regarding sizing of a blower for an air flow application. The system is as shown in the attached.

My question is regarding my method of calculation. There is a 3way 3" valve in the system and the Air enters the branch portion and must either be directed left of right. Cv value = 130 for branch. The current blower is 366 SCFM @ 6" in H2O and the question is can this be reused. My method is as follows:

1. Converted Cv to K using h = K v^2/2g, The h = 2.3' (1 psi of drop at listed Cv value). v = 5.9 ft/s (which is the velocity of 155 gpm flow in 3" cross section). K value for valve = 4.25
2. Figured K values for 2-4" 90s and 2-4" 45s. Summed K's to get Ktot = 5.85
3. Used Darcy-Weisbach to figure Pdrop at 366 SCFM.

And the value I derived is 9.7" H2O and my conclusion is that this blower may not be reused.

I was a bit skeptical of the Cv conversion to K because specific correlations exist for gas flow systems for figuring pressure drop given Cv. But, a colleague has informed me that this is his preferred method and he has done numerous landfill gas Pdrop calcs with good comparison of calculated results to reality.

Your input is appreciated - thanks!

Rob

 

[Subystud]

Edit: the Cv value is 130 and the flow of 130 gpm through a 3" cross section is 5.9 ft/s, but I listed the flow as 155 gpm in the thread. This is a mistake.

Cv = 130 and v = 5.9 ft/s at 130 gpm is the correct values

 

[katmar]

Your conversion from Cv to K uses the right approach, but is wrong for this calculation. What you have done would be correct if the valve were to be used in a 3" line. But the line is actually 4" and when you use the derived K values in the Darcy-Weisbach equation you will be multiplying them by the velocity in the 4" line, although the K value was evaluated for a 3" line.

There are 2 ways around this. The first would be to use exactly the conversion method from Cv to K that you used before but use the velocity that 130 gpm would give in a 4" line (ID=3.83"). The fact that the valve is 3" is irrelevant. The important parameters are that it has a Cv of 130 and that it is installed in a 4" line. This gives 3.63 ft/s and makes the K value 11.24.

The second way to do it is to use Equation 2-11 from the Crane TP410 manual which allows a K value derived for one diameter to be converted to a K value to be used for a different diameter. This formula is K2 = K1 / (D1/D2)^4. This would make your K value for the 4" pipe = 4.25 / (3/3.83)^4 = 11.3

Now your K values are all on the same basis and you can add them to the K values for the bends and apply the sum to the Darcy-Weisbach equation.

The calculation summary is attached.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[Subystud]

Interesting. Thank you very much, katmar. I have built up a test rig from parts laying around. I will report back with the test results, though your preliminary results makes it seem like it's not going to work.

So, if you take a centrifugal blower to shut off head, what happens? Does the wheel just spin and no flow happens?

 

[bimr]

The condition with low flow is called surge. If the blower is left on, it may destroy the blower.

http://snydertechnical.com/Blog/?page_id=33