# Calculating Air Pipe Flow System Rates & Velocities

• matt1996
In summary, the person is working in an engineering internship and has been assigned to find flow rates and velocities in a suction system in their factory. The system has a main branch with a diameter of 30" and is attached to a suction system that draws 30,000 cfm at full capacity. There are also several other branches and sub-branches with varying diameters attached to machines that create dust. The person is wondering how to find the cfm and velocity in each branch and if more information is needed. They are considering using the ratio of the areas of the branches to calculate the flow, assuming that the flow through each sub-branch is proportional to its area.
matt1996
I'm working in an engineering internship after my first year and we haven't covered fluid dynamics in my program yet. I've been assigned to find flow rates and velocities in several branches of a suction (dust collection) system in our factory. The main branch has a diameter of 30" and is attached to a suction system "sucking" 30 000 cfm total, and there are several other branches and sub-branches with varying diameters ultimately attached to machines which create the dust to be sucked up (rough diagram attached, colour-coded for different diameters.) How can I find the cfm and velocity in each branch? Do I need more information?
Thanks

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Welcome to PF!

If you are sure the total is 30,000 CFM, you can just use the ratio of the areas of the branches...

...but I suspect you actually aren't sure the total is 30,000 CFM. Where did you get that number?

Thanks for the reply. the fan draws (approximately) 30 000 cfm at full capacity according to the manufacturer. So since the head loss due to the pipe lengths and minor sources is minimal (I calculated it to be about 5% or so for the entire system) I can just assume the flow drawn through each sub-branch is proportional to its area relative to the areas of other sub-branches on the same main branch? For example in the diagram if FL4 sander's pipe has twice the area of FL4 finisher's, 2/3 of the flow from branch 3 will come from the sander?

## 1. How do I calculate the flow rate of air in a pipe system?

The flow rate of air in a pipe system can be calculated using the following formula:
Q = (π * d2 * v) / 4
where Q is the flow rate in cubic feet per minute (CFM), d is the diameter of the pipe in inches, and v is the velocity of the air in feet per minute (FPM).

## 2. What is the relationship between pipe diameter and flow rate?

The flow rate of air in a pipe system is directly proportional to the pipe diameter. This means that as the diameter of the pipe increases, the flow rate also increases. This is because a larger pipe has more space for air to flow through, resulting in a higher flow rate.

## 3. How do I determine the velocity of air in a pipe system?

The velocity of air in a pipe system can be calculated using the following formula:
v = (Q * 4) / (π * d2)
where v is the velocity in feet per minute (FPM), Q is the flow rate in cubic feet per minute (CFM), and d is the diameter of the pipe in inches.

## 4. What is the maximum recommended velocity for air in a pipe system?

The maximum recommended velocity for air in a pipe system is typically around 4000 FPM. This is to prevent excessive pressure drop and turbulence, which can affect the performance and efficiency of the system.

## 5. How do I ensure the correct flow rate and velocity for my specific pipe system?

To ensure the correct flow rate and velocity for your specific pipe system, you should consult with a professional engineer or use specialized software that takes into account factors such as pipe material, length, fittings, and pressure drop. It is also important to regularly monitor and maintain the system to ensure optimal performance.

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