How do I determine air flow in a duct?

In summary, to determine the air flow rate in a Ø160mm duct that is 10m in length and open at each end, you need to construct a system curve for the duct and find where it intersects with the fan output curve. This can be done using various assumed air flow rates and can be checked using a spreadsheet. As long as the air velocity in the duct is kept below 330 ft/s, air can be treated as an incompressible fluid for pressure drop calculations. Additional resources, such as an HVAC handbook and engineering websites, can provide more information and guidance on this topic.
  • #1
TSN79
424
0
Picture a Ø160mm duct 10m in length, open at each end. Then I install an in-line fan at one end so that it blows into the duct and just let it run at full power. How can I determine what the air flow will be? I startet thinking about it and was just surprised that I didn't know how to approach it as I'm doing duct sizing and such pretty much every day. The fan has a capacity diagram, and the operating point will be somewhere along the outline close to maximum flow, but where exactly? Do I need to assume some pressure drop in order to get anywhere?
 
Engineering news on Phys.org
  • #2
TSN79 said:
Picture a Ø160mm duct 10m in length, open at each end. Then I install an in-line fan at one end so that it blows into the duct and just let it run at full power. How can I determine what the air flow will be? I startet thinking about it and was just surprised that I didn't know how to approach it as I'm doing duct sizing and such pretty much every day. The fan has a capacity diagram, and the operating point will be somewhere along the outline close to maximum flow, but where exactly? Do I need to assume some pressure drop in order to get anywhere?
You have to construct a system curve for your duct for various assumed air flow rates, and then find out where the duct system curve intersects the fan output curve.
As long as the air velocity in the duct is kept below about 330 ft/s, air can be treated as an incompressible fluid for the purposes of making pressure drop calculations.

If you have an HVAC handbook, this should be explained somewhere.

http://nptel.ac.in/courses/112105129/pdf/R&AC Lecture 38.pdf
 
  • Like
Likes russ_watters
  • #3
SteamKing said:
You have to construct a system curve for your duct for various assumed air flow rates, and then find out where the duct system curve intersects the fan output curve.
And if you do this with a spreadsheet, you can check a bunch of different airflows in a short period of time, until you get to the one that falls on the fan curve.
 

Related to How do I determine air flow in a duct?

1. What is air flow in a duct?

Air flow in a duct refers to the amount of air that is moving through a duct system at a given time. It is typically measured in cubic feet per minute (CFM).

2. Why is it important to determine air flow in a duct?

Determining air flow in a duct is important for several reasons. It helps ensure proper ventilation and air quality, allows for efficient heating and cooling, and can help identify any issues or blockages in the duct system.

3. How do I measure air flow in a duct?

Air flow in a duct can be measured using an anemometer, which is a device that measures the speed and direction of airflow. The anemometer is placed at a specific point in the duct and the air flow is measured in cubic feet per minute (CFM).

4. What factors can affect air flow in a duct?

Several factors can affect air flow in a duct, including the size and shape of the duct, any obstructions or blockages, and the type of fan or blower being used to move the air. Temperature, humidity, and pressure differentials can also impact air flow.

5. Can air flow in a duct be changed or adjusted?

Yes, air flow in a duct can be changed or adjusted by adjusting the fan speed or the damper settings in the duct system. It is important to regularly check and adjust air flow to ensure proper ventilation and efficiency.

Similar threads

Designing an Air Float Conveyor: Calculating Pressure and Flow for Moving Objects
    • Mechanical Engineering
Replies
3
Views
384
Please tell me the difference in the size of the air volume
    • Mechanical Engineering
Replies
3
Views
270
What Size Pipe is Needed for Optimal Air Flow in a Coen Duct Burner System?
    • Mechanical Engineering
Replies
1
Views
968
Air Friction energy loss on a trampoline
    • Mechanical Engineering
Replies
13
Views
1K
Volumetric flow rate of a pipe
    • Mechanical Engineering
Replies
19
Views
2K
How to determine mdot for choked airflow through a valve, given T,P
    • Mechanical Engineering
Replies
3
Views
1K
Where to place a fan along a duct?
    • Mechanical Engineering
Replies
4
Views
1K
Flow rate of air through constriction
    • Aerospace Engineering
Replies
10
Views
758
Determining dimension of exhaust pipe from an air duct
    • Mechanical Engineering
Replies
2
Views
1K
Optimizing Flow Rate for Above Ground Pool Heater Installation
    • Mechanical Engineering
Replies
20
Views
3K

Hot Threads

Recent Insights

Back
Top