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Unregistered
01-30-2006, 06:56 AM
I need to measure the amount of cfm in a 24" pipe, I can calculate the FPM but how can I convert to CFM.

Robert Fogt
01-31-2006, 05:28 AM
You need to multiply the feet per minute times the cross-sectional area of the pipe.

Make sure all units are the same base unit.

feet/minute * square feet = cubic feet/minute

The formula for the area of a circle is Pi*radius², with radius being 1/2 the diameter.

Is the 24" the inside diameter? If so then:
3.14 * (1 foot * 1 foot) = 3.14 square feet

FPM * 3.14 square feet = CFM

dr_rupa
05-30-2006, 12:57 AM
I need to know 500 FPM = how much CFM

Robert Fogt
05-30-2006, 03:54 PM
That depends on the cross-sectional area of the pipe. (or river or whatever it is)

feet/minute (FPM) * square feet = cubic feet/minute (CFM)

mflanagan
12-06-2006, 12:04 PM
How do you calculate the cross sectional?

Robert Fogt
12-06-2006, 03:38 PM
The formula for the cross-sectional area of a pipe is:

britt44
03-02-2009, 06:19 PM
I need to find out how many cfms i need to in a 1000 square foot space with 12 foot ceilings

Unregistered
03-04-2009, 09:00 PM
you will need
1200 cfm

Unregistered
03-28-2009, 04:52 AM
you will need
1200 cfm
According to ASHRAE you will need to know what the space is used for and or how many people are going to be in it (if in the usa). most office spaces require 20cfm/person of outdoor air alone! and then whatever u need for cooling, usually .5cfm/sq ft. you should call a hvac contractor as i've been out of the business for about 5 years and dont have the latest figures, but that's what it was when i was designing systems, good luck!

Unregistered
06-19-2010, 11:33 PM
how do you calculate the cross sectional?

1000fpm how mach cfm

SAJID
07-07-2010, 11:07 PM
i want to know if a package unit ac has 800 cfm how i know that what is its fpm

Unregistered
01-18-2011, 11:12 PM
21000ed;59945]you will need
1200 cfm[/QUOTE]

Kato
09-01-2011, 06:25 PM
6" pipe, 806 fpm, 885fpm, 649 fpm, 570 fpm what are the cfms?

JohnS
09-02-2011, 02:27 AM
6" pipe, 806 fpm, 885fpm, 649 fpm, 570 fpm what are the cfms?

If the inside diameter is 6" or 0.5 ft, area is (pi/4)*diameter² or 0.19635 ft²
Multiply by the flow rate for approximate cubic feet per minute.
806 ft/min gives 158 ft³/min

Unregistered
09-02-2011, 12:51 PM
I need to find out how many cfms i need to in a 1000 square foot space with 12 foot ceilings

Its one cfm per sqft. and 5cfm delutionary air (fresh air) per ocupent.

Unregistered
09-23-2011, 08:01 PM
i need to measure the amount of cfm in a 12'' round flexible duct, i can calculate the fpm but how can i convert to cfm.
pls give me the solutions

JohnS
09-24-2011, 03:17 AM
pls give me the solutions

Multiply the flow in feet per minute by the cross sectional area of the 12" duct in square feet (0.785 ft²)

Unregistered
12-28-2011, 03:42 AM
[QUOTE=dr_rupa;4617]I need to know 500 FPM = how much CFM[/QUOT
E]

Unregistered
03-22-2012, 07:58 AM
A 3 inch diameter high velocity duct putting out 18 feet per minute equals how many cfm?

Unregistered
05-07-2012, 07:24 PM
so if i'm reading all these posts correct;y.... a 12" round duct would be .785 cubic feet and a 18" round duct would be 1.766 cu feet?

So if i'm flowing 250 feet per minute on a 18" return duct that would equal 441.5 CFM's?

If this is right then i think i found the problem with a unit i'm looking at for a friend. One 18" return for a 998 square foot sandwich shop and a 5 ton York package unit.

Not enough flow through the return

JohnS
05-08-2012, 02:22 AM
so if i'm reading all these posts correct;y.... a 12" round duct would be .785 cubic feet and a 18" round duct would be 1.766 cu feet?

So if i'm flowing 250 feet per minute on a 18" return duct that would equal 441.5 CFM's?

If this is right then i think i found the problem with a unit i'm looking at for a friend. One 18" return for a 998 square foot sandwich shop and a 5 ton York package unit.

Not enough flow through the return

In both cases, the area is in square feet, but numerically correct. When multiplied by a velocity in feet/minute, you get cubic feet per minute.

Unregistered
10-15-2012, 11:23 AM
Hello,

I'm trying to calculate CFM of exhaust. I have 51 FPS with 280F exhaust temp and 29.7 in hg for the barometric pressure.

How do I do the conversion while also correcting for the temperature and pressure?

Thanks for the help!

JohnS
10-15-2012, 12:40 PM
Hello,

I'm trying to calculate CFM of exhaust. I have 51 FPS with 280F exhaust temp and 29.7 in hg for the barometric pressure.

How do I do the conversion while also correcting for the temperature and pressure?

Thanks for the help!

First you need the cross sectional area of the exhaust pipe, in square feet. Multiply by the velocity in feet per second then by 60 s/min. That will give you actual flow in cubic feet per minute. If you need "standard" flow at some temperature, convert by ideal gas law or a variant P1*V1/T1 = P2*V2/T2
(note both temperature and pressure must be absolute, and you have to know the "standard" conditions for "standard.")

Unregistered
12-13-2012, 10:17 AM
i have 1000 fpm need to covert to c f m

Unregistered
05-05-2013, 11:36 PM
How do i find the air velocity in cfm for a 12x12 duct if the air is traveling at 2000 fpm?

JohnS
05-06-2013, 01:45 AM
How do i find the air velocity in cfm for a 12x12 duct if the air is traveling at 2000 fpm?

Just multiply by the area, which is 1 ft², so 2000 cfm.

Unregistered
08-10-2013, 06:59 PM
I just did a balance report on a house and used a balometer at each register. There were three toe kicks 2-1/4 x 10 i could not get the balometer under the cab so i used my feildpiece but it reads ft/min i have to send the report to energystar how do i convert to cfm is it 2-1/4*10 = 22.5*fpm/460 =10350 but the other cfm readings are two digit numbers

JohnS
08-11-2013, 01:04 AM
I just did a balance report on a house and used a balometer at each register. There were three toe kicks 2-1/4 x 10 i could not get the balometer under the cab so i used my feildpiece but it reads ft/min i have to send the report to energystar how do i convert to cfm is it 2-1/4*10 = 22.5*fpm/460 =10350 but the other cfm readings are two digit numbers

Not sure where the 460 comes from, and it looks like you multiplied, not divided. You need the area of the duct in square feet
2.25" x 10" x 1 ft²/144 in² = 0.15625 ft². Multiply the fpm flow by that area.

jcofini@460
10-04-2013, 06:25 AM
[QUOTE=britt44;59856]I need to find out how many cfms i need to in a 1000 square

Mrs X
10-04-2013, 02:09 PM
[QUOTE=britt44;59856]I need to find out how many cfms i need to in a 1000 square

Please can you describe your set up? I'm so sorry, but it is not obvious from the question you have written what you need to know. Thanks.

Unregistered
10-10-2013, 07:30 AM
According to ASHRAE you will need to know what the space is used for and or how many people are going to be in it (if in the usa). most office spaces require 20cfm/person of outdoor air alone! and then whatever u need for cooling, usually .5cfm/sq ft. you should call a hvac contractor as i've been out of the business for about 5 years and dont have the latest figures, but that's what it was when i was designing systems, good luck!

At standard 4 to 8 air changes per hours you need 1200 CFM for 6 air changes.

1000 x 12 = 12,000 Cubic Feet
12000 x 6 air changes = 72,000 Cubic Feet to change in 1 hour
72000 / 60 minutes = 1,200 CFM