# does anyone know free air conversions for round ducting?

Posted By TCaldwell, Jan 28, 2008 at 11:57 PM

Not open for further replies.
1. #1

### TCaldwell Minister of Fire 2. ```NULL ```

Oct 26, 2007
711
61
Loc:
860-868-9014 h 203 948 0864 c nw corner ct.
I am trying to calculate with a 7'' round duct the air velocity is 2107 ft/min . what does that equate to for cfm, and a 6'' duct with a velocity of 2823 ft/min , I am told that you multiply the fpm x the free air number? to equal cfm, I hope somebody knows how to calculate free air for round ducts, thanks tom ps. these numbers are from the garn

2. #2

### heaterman Minister of Fire 2. ```NULL ```

Oct 16, 2007
3,354
622
Loc:
Falmouth, Michigan
Trying to determine CFM??

Multiply FPM x PI x radius squared and divide by 12

Take your 6" pipe......... radius is 3" x 3 x 3.14 = 28.26 sq inches in that 6" circle. The velocity is 2,823 FPM so 2,823 x 28.26 =79,777

divide by 12 to get feet and you have 6,648 CFM

This sounds REALLY high, are you sure about the velocity?

Then again.................I shouldn't attempt any math at all this time of the day. Time for this kid to hang it up.

3. #3

### vik455 New Member 2. ```NULL ```

Jan 26, 2008
1
0
Loc:
sw mi
take the 79777and divide by 144. one square foot = 144 square inches. 79777/144=554 cfm

4. #4

### eekster New Member 2. ```NULL ```

Oct 24, 2007
51
0
Loc:
southeast michigan
I looked at my air duct calculator at work here and I think you be around that 550 cfm mark also. That"s alot of cfm"s through a 7" pipe.

5. #5

### TCaldwell Minister of Fire 2. ```NULL ```

Oct 26, 2007
711
61
Loc:
860-868-9014 h 203 948 0864 c nw corner ct.
heaterman, vik and eekster are correct, duct dia squared x 3.14=square inches, square inches divided by 144= sqft take sqft x fpm=cfm. yes it sounds high but the burn rate is 425,000btu/hr. heaterman , in your testo garn results what differential pressures have you experienced , i am running 500cfm at 2 inches water column, 3/4 hp @ 3450 rpm thanks tom

6. #6

### atlarge54 New Member 2. ```NULL ```

Dec 3, 2007
185
0
Loc:
Hoosier
Without looking it up I think it's the radius squared times 2pi for area, not diameter squared. I've been wrong before, so it wouldn't hurt to double check.

7. #7

### TCaldwell Minister of Fire 2. ```NULL ```

Oct 26, 2007
711
61
Loc:
860-868-9014 h 203 948 0864 c nw corner ct.
you are correct i got lost, it is the radius squared not the diameter squared,who thought being a pyro would become so complicated!

8. #8

### Jersey Bill Member 2. ```NULL ```

Jan 21, 2008
132
0
Loc:
Central NJ
2800 fpm seems awefull high for a chimney, if thats what it was.
normally in ductwork design I keep the velocity below 1000 fpm,
except for powered exhaust with entrained solids. in that case i try
not to exceed 1500 fpm. At about 1000 fpm you can hear the air whistle as it
goes through.

Was the 2800 fpm a measured value.

I have a ductulator which replaces those messy calculations. they don't use radius either.

9. #9

### TCaldwell Minister of Fire 2. ```NULL ```

Oct 26, 2007
711
61
Loc:
860-868-9014 h 203 948 0864 c nw corner ct.
jersey bill this was a measured reading at the invert of the discharge of a 6'' flue stack, the air supply reading was 2107 fpm through a 7 inch duct . these were generated by a 3/4 hp draft inducee with a shrouded radial blade also static at 2 inch w/c this boiler fires at 425000btu/hr+ does that calculator know where i could find a blower for a 1 hp motor that would up the cfm to 650+/-?

10. #10

### Jersey Bill Member 2. ```NULL ```

Jan 21, 2008
132
0
Loc:
Central NJ
OK, i get it. big stuff there.
I ran thise numbers on the ductulator again and it came out to 590 cfm measured.
No help on finding a bigger blower. You are right on with the horsepower though.

If you reduce the resistance, like by increasing the duct size, the pressure drop would decrease,
and the flow would increase, using the same fan. I dont know the big picture and what you are up against.
You might be able to swap some 90's for some 45's, or change them to long sweep turns.
good luck