From: TOListserver@theatreorgans.com on behalf of ASeflPipes@aol.com
Sent: Friday, May 23, 2003 4:58 PM
To: Theatreorgans-L List Member
Subject: [Theatreorgans-L] CFM Orifice
In a message dated 5/20/03 11:35:10 AM Pacific Daylight Time,
<< Can any of you engineers help me with the following information. I am
>sizing a blower for an European dance organ and need to know the size
>orifice that will pass 600 CFM at 10.5" W.P? I suspect the relationship
>is linear, so if you can furnish me with CFM flow through various size
>orifices at 10.5"W.P., it would be most appreciated. I can the plot it
>out on a graph and fill in the intermediate sizes. As the measurement
>of CFM is not real easy, I suspect in most cases this figure is
>calculated from orfice and static pressure.
>Dick Leis in Florida
How about if I furnish you with math "back of the envelope" examples. You
don't say whether the orifice is a free standing plate, a linear pipe, or what.
It does make a difference. If the orifice is on a blower plate of thin metal
then the "vena contracta" effect really comes into play whereby the air
self-limits its own flow as it tries to squeeze into the opening and flow through.
Pressure roughly converts to flow speed (velocity) under most circumstances.
The formula is Velocity in feet per second is equal to 66 times the square
root of Pressure in Water Column Inches. Thus, 3"WC will give you a 114 feet
per second flow into free air. 114 fps = 66 * (3)^.5 To convert to feet per
minute flow you would need to multiply by 60 seconds for 6480 feet per minute.
Assuming a 4" diameter orifice of a cross sectional area of A = pi*r^2 then
with 12.56 square inches or 0.087 square feet you get what is called a blast
area. Now the problem comes in that the orifice can restrict flow ranging from
a coefficient of discharge of 0.60 to 0.94 depending on how the flow is
funneled into the orifice. A thin metal plate with sharp edges will give you the
0.60 times the 0.087square feet blast area for an *effective* orifice of 0.052
square feet. Multiplying the effective orifice of 0.052 square feet times the
flow of 6480 feet per second and you have about 337 cubic feet per minute at
Taking your figure of 10.5"WC then V = 66*(10.5)^.5 = roughly 214 fps.
214 fps times 60 seconds gives 12,840 feet per minute.
Using a 4" diameter opening you get (12.57"^2/144"^2) or 0.087 square feet.
Multiplying 0.087 square feet times the 0.60 factor gives an effective
orifice of 0.052 square feet.
0.052 square feet times 12,840 fps will give you a delivery of 668 cubic feet
Thus, a 4" diameter thin plate orifice will allow you the 600 cubic feet per
minute at 10.5"WC that you were seeking and give you about 10% more flow for
Hope that helps,
A guy known for his wind...
PS: Air flow is also temperature sensitive since air viscosity is directly
effected. The above is calculated in the mid 60s Fahrenheit.
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