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## Max. Face Velocity?

Hi,

I have conducted pressure drop testing for a blast damper, the data has given readings of pressure drop and velocity.
I need to produce a pressure drop and face velocity graph to represent the data and this is where I need a bit of help.

How do I calculate face velocity? And can someone kindly explain to me what face velocity is and why this information is important??

I have googled info on face velocity but I'm going round in circles and genuinely need some help.

Thanks.

2. Originally Posted by Twit
How do I calculate face velocity? And can someone kindly explain to me what face velocity is and why this information is important??
You can always measure it with an anemometer. One of the keys to obtaining a good face velocity measurement is by averaging many readings covering the entire face.

If you know the CFM and Ak of the register, then you can do the math:

Velocity (FPM) = CFM ÷ Ak (Effective Area of Opening in sqft)

The supply air velocity measured at the register face determines how far the throw of the air will be. The faster the air exits the face, the farther the air will travel into the room.

Eventually, the supply air will slow enough to become ineffective in mixing with room air. That point is called the air flows terminal velocity.

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Originally Posted by rundawg
You can always measure it with an anemometer. One of the keys to obtaining a good face velocity measurement is by averaging many readings covering the entire face.

If you know the CFM and Ak of the register, then you can do the math:

Velocity (FPM) = CFM ÷ Ak (Effective Area of Opening in sqft)

The supply air velocity measured at the register face determines how far the throw of the air will be. The faster the air exits the face, the farther the air will travel into the room.

Eventually, the supply air will slow enough to become ineffective in mixing with room air. That point is called the air flows terminal velocity.
So, the max. face velocity is the maximum speed the air exits the face?

CFM - I don't have this info.
Ak - This is the dimensions of the damper under testing?

4. Originally Posted by Twit
So, the max. face velocity is the maximum speed the air exits the face? Yes

The Max Supply Outlet Face Velocity should be 700 fpm (Manual D). If you exceed this, you will probably have noise issues.

Ak - This is the dimensions of the damper under testing?

Effective area (Ak), is the area of the register, grille, or diffuser in square feet that is utilized by the air flowing through it.

When air flows through the bars or louvers of a product, it is compressed slightly between the bars, and there is some friction as the air makes contact with the bars as it flows past. This has the result of reducing the total area available for the air to pass through. Effective area is usually less than free area for the same product because of these reasons.
.

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Originally Posted by rundawg
.
Thanks!
Any suggestions on calculating CFM?

6. Originally Posted by Twit
Thanks!
Any suggestions on calculating CFM?
I know this is going to sound redundant, but you have to measure the airflow.

If you use an anemometer, pitot tube, or some other means of measuring the velocity of the air passing through the vent, you will be able to calculate the CFM.

The Ak of the register or grill you are using will be needed.

Then put the 2 pieces of data into this formula:

CFM = Ak x Velocity(FPM)

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Isn't effective area also known as " free area"?

I haven't performed any of this in years so I won't try and comment in depth.

I remember some grilles,registers and dampers having this stamped or stickered on them in an inconspicuous area ,,,,,or getting this info from the product catalog or calling the manufacturer.

A damper or grilles effective area is not just its actual dimensions as blades etc reduce this ,correct,?

8. Originally Posted by mikeacman
Isn't effective area also known as " free area"? No it is not.

A damper or grilles effective area is not just its actual dimensions as blades etc reduce this ,correct,? Correct
Free Area is the sum of the areas of all the spaces between the bars or fins of a grille measured in square inches.

Effective area (Ak), is the area of the register, grille, or diffuser in square feet that is utilized by the air flowing through it.

The manufacturer calculates this by measuring the Airflow Rate in CFM (Ft³/Min), and the Face Velocity in FPM (Ft/Min). Then they divide the Flow Rate (Ft³/Min) by the Face Velocity (Ft/Min). The result is the effective area (Ak) in square feet (Ft²).

When air flows through the bars or louvers of a product, it is compressed slightly between the bars, and there is some friction as the air makes contact with the bars as it flows past. This has the result of reducing the total area available for the air to pass through. Effective area is usually less than free area for the same product because of these reasons.

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Thank you

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