Calculating CFM hole size and delta P known

In summary, The conversation is discussing the fabrication of a smaller version of a device used to measure airflow through a residential bath fan. The desired measurements are to determine air leakage through recessed can lights when the house is depressurized. The conversation also mentions a potential formula and alternative methods for measuring CFM.
  • #1
David Meiland
2
0
I'm looking to fabricate a small version of this device http://energyconservatory.com/products/products7.htm , which is used to measure airflow through a typical residential bath fan. It adjusts to three different known hole sizes, and typically connects to a manometer that has algorithms built in for displaying CFM directly. Low end of the range for this device is 10 CFM, which is too high. What I want to do is get approximate measurements of air leakage through recessed can lights when a house is depressurized to a specific level (-50 pascals with respect to outside is standard). I can use this information to determine how much improvement can be expected if the fixture is modified or replaced.

Is there a good formula to use for this purpose? I am expecting to drill a small hole (maybe 3 square inches) and measure in pascals.

Thank you for your help!
 
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  • #2
Wow
Laptop Computer Stand $ 130.00
Re-Usable Fan Shipping Box. $ 50.00
Kinda pricy

Their system has a problem of accuracy, which is only my gut instinct, or rather they do on this one. If you notice in the manual the formula for CFM from the pressure reading matches the chart only at the low end.

Try this
http://woodworking.com/forum/showthread.php?20658-Measuring-CFM

It actually is the same thing as the damper on the stove pipe for a wood burning stove or the interior oil stoves i- e space heater. The cfm is fairly low and may suit your purpose.
 
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Related to Calculating CFM hole size and delta P known

1. How do I calculate the CFM (cubic feet per minute) of a hole with a known size and delta P (pressure drop)?

To calculate CFM, you can use the following formula: CFM = (Area of hole in square feet) x (Velocity of air in feet per minute). To find the area of the hole, use the formula: Area = pi (3.14) x (radius of hole in feet)^2. Delta P can be measured using a manometer or pressure gauge.

2. Can I use a standard CFM calculator to determine the CFM of a hole with a known size and delta P?

No, standard CFM calculators are typically used for larger air systems and may not be accurate for smaller holes. It is best to use the formula mentioned in question 1 or consult a professional for accurate calculations.

3. What is a typical delta P value for a hole in a ventilation system?

The delta P value for a hole in a ventilation system can vary depending on the size and type of the hole, as well as the air flow rate. However, a typical value can range from 0.01 to 0.05 inches of water column (in. WC).

4. Can I use the same formula to calculate the CFM of multiple holes in a ventilation system?

Yes, you can use the same formula mentioned in question 1 for multiple holes. Simply add the individual CFM values for each hole to determine the total CFM of the system.

5. How can I adjust the CFM of a hole in a ventilation system?

The CFM of a hole can be adjusted by changing the size of the hole or adjusting the air velocity. A larger hole or higher air velocity will result in a higher CFM, while a smaller hole or lower air velocity will result in a lower CFM. It is important to carefully calculate and adjust the CFM to ensure proper air flow in the system.

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