PDA

View Full Version : static pressure to cfm?

drafty888
02-12-2007, 10:52 AM
Anyone have a calculation to convert static pressure to CFM?

Thanks.

leaflying
02-12-2007, 12:07 PM
You can only convert velocity pressure (Total pressure - static pressure) to speed (fpm). The equation will consider engineering units, fluid (air) density and duct shape etc. That's why different VAV box has different flow pickup factor.
V=K*sqrt(dP)
V: speed
K: conversion factor, often from testing
dP: velocity pressure

BobbyBJr
02-12-2007, 12:35 PM
Sounds like you need to look at the fan curves in the performance data for the unit you are checking the static pressure on. There you should be able to find what you are looking for.

Bobby

drafty888
02-12-2007, 01:44 PM
Actually, I've got some VP sensors in a duct that are poly tubed to a Serta device that turns VP into a 4-20ma static pressure signal. Problem is, I don't need static pressure, I need CFM.

codewriter
02-12-2007, 04:20 PM
http://www.kele.com/webservices/index/index.aspx?url=tech/trindex.html

lwarren
02-12-2007, 08:40 PM
If i remember correctly you take the free square foot area of your duct multiply that by velocity pressure and then divide by 144.

codewriter
02-12-2007, 08:55 PM
Exactly,
AIR FLOW (cfm) = V x A
where V = average duct velocity (ft/min) A = duct cross sectional area *(ft2)

Read the FLOW section of the link I provided, also do yourself a favor and read everything on that page, tons of good info and a quick place to refresh the mind.

leaflying
02-12-2007, 09:18 PM
I found some typo.

Velocity, not velocity pressure.
1 sq feet=144 sq inch. So the duct area is actually sq inch, not sq feet.

If i remember correctly you take the free square foot area of your duct multiply that by velocity pressure and then divide by 144.

leaflying
02-12-2007, 09:29 PM
If you really wanna simple and ignore all calibration stuff.

Use K=4004 in my post at the 2nd floor. Pressure in "inch water", and velocity in "fpm". Then apply AIR FLOW (cfm) = V x A. And it's not static pressure, but velocity pressure. The relationship between vp and fluid speed has been decided in Bernoulli Equation. You cannot get speed from sp no matter how hard you try.

twisted pair
02-12-2007, 09:53 PM
I have done it the way Codewriter described. I am not sure what type of control system you have, but I have a screen capture of a calculation I have used many times to convert Velocity Pressure to CFM.

codewriter
02-12-2007, 10:55 PM
I found some typo.

Velocity, not velocity pressure.
1 sq feet=144 sq inch. So the duct area is actually sq inch, not sq feet.

What typo? Where?

EDIT: Nevermind, I need to adjust the eyes.

propmanage
02-13-2007, 03:02 PM
The fan shape and size will create different condition and the design engineers provide the fan curve so you can look up how much air a fan will move for a given pressure. I think you will need that fan curve so you can cross reference your static.

leaflying
02-13-2007, 03:58 PM
In this case, we need to measure total static pressure (TSP) on AHU unit, not a single point static pressure on common duct. And normally, the air flow is checked from a table/diagram rather than calculated in a controller.

Also, you cannot use this method to measure air flow on branch ducts.

The fan shape and size will create different condition and the design engineers provide the fan curve so you can look up how much air a fan will move for a given pressure. I think you will need that fan curve so you can cross reference your static.

osiyo
02-13-2007, 09:18 PM
Anyone have a calculation to convert static pressure to CFM?

Thanks.

STATIC pressure has not a thing to do with how much air is flowing. Blow up a baloon, tie the end in a knot. Inside that baloon you WILL have static pressure, but no flow of air whatsoever. Untie the end and let the air out of the baloon while holding a finger close to the hole, what you finger feels is VELOCITY pressure. High at first, since the static pressure in the baloon is high, compared to the ambient air around you, and the air will be coming out very fast. But as time passes, and the static pressure inside of the baloon is less, and there is no incoming air to keep it high, you'll feel less "push" against your finger (velocity pressure). Now get a baloon several times larger, blow it up to the same pressure as the first (static pressure). Then open a hole the same size as the first one and feel. It'll be EXACTLY the same amount of force that you'll feel. Because flow will be the same. It'll just continue longer before you feel a decrease because there are more cubic feet of air in that bigger baloon.

To make an easy, ball-park calculation of CFM.

You need a pitot tube that's made for sensing velocity pressure. Nothing really special or rocket science about em. Essentially two tubes. One inside the other. At each end gap between inner and outer tube is soldered or braized closed. Just at the ends. There is still a gap between the two along the lenght. Then this combo-tube is bent at 90 degrees at on end, maybe 6 inches from that end. That's the short side. Long side should be 2 or 3 foot.

At the short end. Looking end-on, you see opening into inner tube. Back around the circumference of the outer tube, a little behind where the bead is where solder/braize was made, there will be 2 to 4 holes drilled thru outer tube only. Holes let air into that gap between the two tubes. This is to the "static" pressure pickup. Pitot is held, when measuring for velocity pressure with innner tube hole facing INTO the air flow. Pressure inside that inner tube will be "total pressure". Static pressure plus velocity pressure.

At long end. Near, but before, the end ... where bead of solder/braize was made ... a hole is made thru wall of outer tube ONLY. And a little nipple is soldered on. So air can enter static air pressure holes at short end, flow thru gap between tubes and out that nipple.

Then yah just need a suitable dual port pressure gage. Attach tube from static nipple to low side, and a tube from long end opening of inner tube (total pressure port) to high side of gage.

You need to determine the cross-sectional area of the duct, in square feet (sqft).

For a round duct 12 inches in diameter you'd figure it out this way:
Cross-sectional area in inches = Pi times the radius squared.
Thus square inches = 3.1415 * (6*6) = 3.1415 * 36 = 113.094
Thus square feet = .785375

With the pitot tube and a suitable gauge such as a Dwyer Magnehelic, scaled to read in inches of water ("WC), and a
range of 0-1 "WC, or 0-2"WC (depending on system and how high of a pressure one might expect. Too large a range over what yah need just gives you more inaccuracy.) Some other suitable gauge will also work as long as yah can read it in inches of water column. Newer digitals work well and can be very accurate. But unless you are a professional balancer, such accuracy is probably more than is useful to you, since there is a LOT more to know in order to achieve accurate, professional results. I'm presuming that you like to just have a reasonable ball-park idea of what kind of air flow you're getting. Given the nature of your question.

In any event, you use the pitot tube, inserting it into the duct, tube opening pointed INTO the on-coming air stream. Then see what sort of reading you get. Best if you sample at multiple points. Then average the results. There is actually a methodology for determining how many points you'd want to test at, and where, but that's beyond the scope of this note. In a round duct of 12 inch diameter, the radius would be 6 inches, so the best place to test would be with tip of pitot tube inserted 3 inches into the duct. (Actually, best to test by making two test holes on opposite sides of duct and taking 2 readings, then average. Better yet, take 4 readings 90 degrees apart going around the duct, then average.)

Let's say yah arrive at an average velocity pressure of 1.0 inch.

Okay, now yah need to convert that to FPM, Feet Per Minute (of linear velocity).

The conversion is:

Velocity pressure (in inches of water) squared, times 4005. (The 4005 is a "constant", and the number is only good if your measuring AIR flow. Don't try to use the same number for water flow)

So in this example we'd have (1.0*1.0)* 4005, or 4005 as our answer. That'd be 4005 FPM.

Now we have the cross sectional area of the duct in square feet. And the velocity of the air inside it is 4005 FPM.

So the CFM would be approx 3145.

Got it?

Of course this will only be approx. To get really accurate you need:
-- GOOD instruments
-- Experience
-- Good place to take your readings. ie A point on the duct at a spot that's at least a distance equal to 6 times the duct diameter from a turn in the duct or a place where duct transitions from a large to a smaller size, etc. Often hard to find. If yah can't get such a spot, use what yah have, but accuracy gets worse.

This is just a rough and easy way to get an APPROXIMATE idea of what air flow you have, cheaply.

REAL air flow measurement, and system balancing, takes some expensive instruments, more than a little trainning, and plenty of experience. And a good calculator, some good reference books and tables. Etc. If yah need to be really accurate ... hire a balancer.

propmanage
02-14-2007, 05:55 AM
Then why could you not just use an Alnor and Traverse for CFM.
I have not had to change static to cfm and would like to know more about this. When or if you do finish could you up date for future reference.
Thanks

drafty888
02-14-2007, 02:12 PM
Turns out the job was installed with some of Kele's AMP probes in the duct, poly'ed to a Serta DP transducer. My mistake was to assume the output of the transducer was something other than velocity pressure, which it actually is.

Seems that the original installer was looking for a very low cost air flow station.

propmanage
02-14-2007, 03:01 PM
Thanks,Got us thinking