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Furnace Blower Velocity Survey for
DIY Blower Door
In an attempt get an accurate flow rate for my furnace blower based blower
door, I built a short rectangular duct that I attached to the blower outlet, and
then measured the flow velocities in a grid pattern near the end of that duct.
This shows the short duct attached to the outlet of the furnace blower.
The Velometer Pitot proble was used to measure the velocity at 75 points in a
grid pattern.
I used an Alnor Velometer Pitot probe to measure the flow velocities in a
grid pattern near the outlet of the duct. I started out with about 25
points, but found that the velocity across the duct changed rapidly in places,
and to be sure of getting a good average velocity, I went up to 75 points (5
rows of 15 points).
Because the duct is short, the variations in velocity that result from being
so close to the fan are fairly large. The velocity profile across the
short dimension of the duct (perpendicular to the blower shaft) for a typical
section looks like this;
The 1 to 15 are points across the duct space about 0.6 inches apart, and the
bottom axis is flow velocity in fpm.
Anyway, these 75 point profiles were done for all four blower speeds, and
these are the resulting average velocities and flow rates.
|
speed |
Vavg |
Airlfow |
|
1 |
1427 |
1427 |
|
2 |
1650 |
1650 |
|
3 |
1946 |
1946 |
|
4 |
2540 |
2540 |
The velocities are in feet per minute and flows are in cubic feet per minute.
The duct outlet area is exactly 1 sf, so the exit velocity and flow rate are
equal to each other.
If you want to see more of the details on all of the velocity profiles,
download this Excel
Spreadsheet...
As noted before, I compared the flow rates I measured with the published flow
rates for a Fasco blower sold by Grainger. This is how they compare:
The blue diamonds are the flow rates I measured, and the red squares are the
Fasco blower published flow rates.
So, this is pretty good agreement and leads me to think that there is a good
chance that most half hp furnace blowers will have similar flows.
Another view of the setup for the velocity profile.
The air velocity is measured at 15 points for each one of the 5 holes in the
side of the duct.
The board with close fitting holes allows consistent placement of the Pitot
tube.
Back to the main page on the DIY Blower
Door...
Gary February 24, 2013
