Pressure drop along a pipe with a branch

[adam_pedro]

Hi,

I'm trying to determine if a compressed air system has sufficient system to feed a new load
The minimum pressure required for existing loads and the current load is 7 bar.

In order to determine the pressure at E i did a pressure loss calculation for the following
A to B - △P start of pipe to end of reducer
B to C - △P end of reducer to just before tee at C
At C i took out the 3 m3/h for load at G
C to D - △P from point just before tee at C to just before tee at D using 27 m3/h as flow
Then repeated the same process as C to D fro D to E

What i wanted to confirm is if the thought process I used is correct for estimating capacity?

 

[BvU]

hello pedro,

What did you do with the fixtures ?
What pressure drops did you find ?

If p is 7 at the inlet, requiring p_min = 7 at the outlet is hopeless ...

 

[adam_pedro]

hello pedro,

What did you do with the fixtures ?
What pressure drops did you find ?

If p is 7 at the inlet, requiring p_min = 7 at the outlet is hopeless ...

I made a copy and paste error there. It's 8 bar at the inlet and 7 bar min for users.
We can ignore other fittings for the purpose of this discussion. Howere when i calculated pressure drop across the tee I use flow through tee as one of the fitting.
What i wanted to check is if the methodology is right?

 

[BvU]

$\Delta p\propto {1\over 2} \rho v^2\$, so the main pressure drop is determined by the 24 m³/h flow. Then CG and DE are pretty small corrections. If you want to do it in detail, some iterations may be necessary.

 

[jrmichler]

What i wanted to confirm is if the thought process I used is correct for estimating capacity?

You have it right. Start at the beginning, and calculate the pressure drop one section at a time for the flow rate in that section. Just remember to calculate the pressure drop for each section using the pressure at the beginning of that section and correcting for the increased volume as the pressure decreases. Then, when you get to the end, start over using the average pressure for each section. This is the iteration that @BvU mentioned.

A one bar pressure drop is high for a compressed air distribution system. If your pressure drop is too much, consider adding a separate line from the source to a point near the far end of the system to make a big loop.

 

[adam_pedro]

You have it right. Start at the beginning, and calculate the pressure drop one section at a time for the flow rate in that section. Just remember to calculate the pressure drop for each section using the pressure at the beginning of that section and correcting for the increased volume as the pressure decreases. Then, when you get to the end, start over using the average pressure for each section. This is the iteration that @BvU mentioned.

A one bar pressure drop is high for a compressed air distribution system. If your pressure drop is too much, consider adding a separate line from the source to a point near the far end of the system to make a big loop.

I forgot to account for the volume increase as pressure drops.
What I don't understand is the iteration aspect. What I am trying to achieve when I repeat the calculation using the average pressure for each section?

 

[BvU]

Good point. Since you only have branching off, one pass may be enough.

 

[jrmichler]

You are calculating compressible flow using an equation based on incompressible flow. The normal procedure is to find the pressure drop for a length of pipe, then divide the pipe into shorter lengths, recalculate for each shorter section, and add up the pressure drops. Repeat with shorter lengths until the total pressure drop is calculated to your desired accuracy.

 

[BvU]

Agree. Turns out $\Delta p$ is not negligible for BD to get an accurate answer.

 

[adam_pedro]

I plan on uploading a worked example soon for review and for the benefit of future forum users