equation for fluid flow in pipes with multible inlets.
equation for fluid flow in pipes with multible inlets.
(OP)
Hi I would like to calculate the flow and pressure distribution in a pipe with multible inlets.
The pipe is basically a 25 m long straight pipe, Ø500mm, with 10 gaps (Ø110) drilled in to its side. at the end of the pipe a ventilator is sucking the air through the pipe at a flow of 2000 m3/h.
I would like to calculate the corresponding flow through each gap and the pressure below each gap. I know i need to use some kind of iteration process (proably based on Bernouilli's equation and the minor losss coeficient for the gaps and the loss coeficient for the tube) but it would be nice to se some one who has descriebed how this is done in practice.
Can anyone refer me to where i can see such description? a book/ homepage or alike.
Thanks in advance





RE: equation for fluid flow in pipes with multible inlets.
http://www.pdhcenter.com/courses/m246/m246content....
RE: equation for fluid flow in pipes with multible inlets.
RE: equation for fluid flow in pipes with multible inlets.
RE: equation for fluid flow in pipes with multible inlets.
RE: equation for fluid flow in pipes with multible inlets.
With air systems, the ports should be sized to obtain a desired velocity. It is usually around 2-4 meters/sec. An addition, if you have particulate present, your piping should also be sized for a velocity that will carry the particulate.
The friction loss through the ports will be negligible. Size the fan for the loss in the straight pipe which appears to be oversized and will cause an expansion loss.
RE: equation for fluid flow in pipes with multible inlets.
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: equation for fluid flow in pipes with multible inlets.
For ease of reference let us number the inlets from 1 to 10, with #1 being the furthest from the fan and #10 being the closest. Let the absolute pressures inside the duct in line with each inlet be numbered P1 to P10, and the flows through each of the inlets be numbered F1 to F10.
The only number you have to guess is the pressure P1. Everything else after that is easily calculated. Once you have specified P1 you can calculate F1 using a simple orifice equation. The flow along the duct towards the fan between inlet 1 and inlet 2 is the F1 you have just calculated. From the flow F1, plus the dimensions of the duct, you can calculate the pressure drop due to friction between inlets 1 and 2. Call this ΔP1-2. Now P2 = P1 - ΔP1-2.
This puts you in a position to repeat everything you did for inlet 1. You have P2 so you can calculate F2. The flow between inlets 2 and 3 is F1 + F2 so you can calculate the pressure drop between inlets 2 and 3 - and therefore you can calculate the pressure P3.
You keep stepping along the duct like this until you have solved for each of the flows F1 to F10. If you add them all up you should get 2000 m3/h. If the total flow is not 2000 m3/h adjust P1 and repeat. Goal Seek takes all the pain out of this.
You will find that for a given number and diameter of inlets, as the diameter of the duct increases the flows into each inlet become more uniform. But of course at the cost of a larger duct.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: equation for fluid flow in pipes with multible inlets.
Right now things look simple I'll let you know if things work out as I expect.
RE: equation for fluid flow in pipes with multible inlets.
These problems are usually calculated using the velocity pressure method. Here is a link to a spreadsheet:
http://www.google.com/url?sa=t&rct=j&q=&am...
The background for the calculations can be downloaded at:
https://law.resource.org/pub/us/cfr/ibr/001/acgih....
RE: equation for fluid flow in pipes with multible inlets.
Thanks
RE: equation for fluid flow in pipes with multible inlets.
But it is unlikely that these flows will match exactly because underlying this whole problem is the assumption that the given fan will move 2000 m3/h of air through the given duct. In reality, you need to find the point on the fan curve that also satisfies the system (duct flow) curve. This means that the method proposed by bimr is also a trial and error method because when you get to the end of the duct and find that the flows do not match you will have to re-run the calculation with a new combination of pressure and flow rate selected from the fan curve.
The method I proposed suffers from the same problem. After performing the iterative solution I outlined earlier you would obtain a pressure at the suction of the fan and if this did not match the fan curve you would be forced to select a new flow rate. In the end, both my and bimr's methods would be iterative trial and error solutions. It does not matter which end you start from - if the pressures and flows all balance in the end you should get the same answer from both methods. Maybe do it both ways and tell us how close the 2 answers came. That would be very interesting and would certainly increase your confidence that you have found the correct solution.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: equation for fluid flow in pipes with multible inlets.
The original post states the air is "sucking the air through the pipe at a flow of 2000 m3/h", which gives you the static head (velocity head) at the outlet.
One works through the exhaust system adding up the head losses until you determine how much air flow is being pulled through the last opening. The static head remaining at the last opening will determine how much air is being pulled in at the last opening.
Conservation of energy means that all energy changes must be accounted for as air flows from one point to another.
The pipe velocity is low in this problem, so there does not seem to be any reason that the pipe would not be able to easily pass the volume.
This method is supposed to be accurate to 5%. No guesswork is required. Refer to Figure 5-7 for an example.
RE: equation for fluid flow in pipes with multible inlets.
I also agree with bimr's statement "The static head remaining at the last opening will determine how much air is being pulled in at the last opening." My concern is that this quantity of air is not guaranteed to balance with the assumed total of 2000 m3/h unless the static pressure used for the outlet end was correct.
It seems to me that Example 1 in Fig 5-7 in bimr's attachment starts its calculation from the end away from the fan and that the static pressure at the fan suction is the result of the calculation rather than an input to the calculation. BTW, this attachment is an excellent resource - thanks very much for that.
So the essence of our disagreement is how we determine the static pressure at the outlet. I am saying that it can only be determined by trial and error, but bimr is saying that it can be calculated from the known information. Hopefully RuneRRA will run both versions of the calculation and give us a report back.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: equation for fluid flow in pipes with multible inlets.
The orifices will have a pressure drop of 0.178 inches of water at 200 m3h. The head loss for 2000 m3/h through 2.5 m of Ø500mm straight pipe to the first orifice is only 0.0019 inches of water. The total headloss through the straight pipe will be around 1% of an orifice headloss.
You should have equal distribution through the orifices since the headloss through the straight pipe is minimal.
You can calculate it with the spreadsheet. Unfortunately, it is in American units.
RE: equation for fluid flow in pipes with multible inlets.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"