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Branch Pipe Flow Question

Branch Pipe Flow Question

Branch Pipe Flow Question

(OP)
Hi,

I am a new mechanical engineer and new to branched duct flows. I am confused about fan sizing for them.  Say I have a system like below

----------------> fan >---> to atm
|       |
|       |
|       |
|       |
^       ^
|       |

Say I have two different flow rates and resistance losses in each of the two branch lines.  The method I found in "Industrial Ventilation 17th edition" is that you take the higher of the two resistances and use that as the static pressure necessary for the governing static pressure in both lines.  Where I am confused though is say the header diameter is the same as the diameter of both of the branched lines.  Wouldn't I need add the resistances of both lines to get a necessary static pressure in the header?  Since the P=F/A, and since the diameters of the branches are equal to the header, wouldn't the header static pressure necessary be equal to the sum of the two static pressures of the branch lines?

I saw a method using the resultant velocity pressure in the duct accounting for the sum of the two flow rates into the header.  However, this method only added about 10% more static pressure than the largest resistance needed in the header, whereas if I added both of the static pressures together I would get around 80% more static pressure.

Any help would be appreciated.
 

RE: Branch Pipe Flow Question

I think that method is for two parallel lines that do not each T into headers.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

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