How to get actual cfm with only static pressure probe. 1

[mnchapman]

I have a air handler serving a school kitchen. The air flow seems poor and cooling is weak. How can I derive the actual cfm being delivered?

I took a External Static Pressure reading midway down the ductwork,, it .21" for a 20x20 inch square duct. The supply fan is rated 1650 cfm. There is a return air fan on the return duct. I want to get a velocity pressure reading but I don't know where to take the before and after samples. If the samples must be taken from the same size duct, then I can't do it at all since the blower mounts in the cabinet, thus with duct work on one side, and a cabinet on the other side.

Actually, I did take this mis-matched reading, by tapping into the existing blower differential pressure switches 1/8" lines. I got 1.20" reading but I don't know what I am measuring here. When I put in new filters, I got a reading of 1.00".

Secondly, if I increase the airflow (say with new belts), then can I expect the static pressure reading midway down the supply duct, to increase or decrease from my current. .21"?

The blue prints state: esp=.87" and tsp=1.87" (as I recall). I wonder where on the system, they take their readings? i.e. -in the cabinet, before the raf, after the raf, , etc.

I am on the brink of understanding all this. Any insight would be appreciated.

 

[MintJulep]

The short answer is that you can't get cfm with only static pressure.

At least not from a single point reading.

You can obtain cfm from a differential reading across a component with a known pressure drop vs flow curve. An orifice or nozzle would be great, but are not common in real-world systems. Conceptually you can do it across a filter, but I wouldn't trust it. Pressure drop across a fan is a pretty good indicator...If you can get an accurate reading. Unfortunately entry and discharge conditions typically prevent that.

 

[Yorkman]

Another method would be to do a pitot tube traverse of the duct. This requires finding the velocity pressure in the duct with a pitot tube, taking multiple readings in an equally divided grid pattern, and then calculating the average velocity in F.P.M. based on these readings. The more readings the more accurate, min. about 16.

V = 1,096?Pv•v
Where:

V = velocity ft/min
Pv = velocity pressure in: inches of water
v = specific volume of air in ft³/lb
(The most accurate results are achived about 10 equivalent duct diameters down stream of any fittings or branches. In your case about 15 - 18 ft.)


Once you know the average velocity in F.P.M. multiply that by the area of the duct. Q = A x V

Q = quanity C.F.M.
A = area sq.ft
V = velocity in F.P.M.

How's that for a qick lesson on air balance!

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.

 

[Yorkman]

Sorry, I don't know the code to insert a square root symbol also called a radicand it came out as a ? mark.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.

 

[mnchapman]

You mention a velocity reading with a pitot tube, however, the pitot static pressure probe only yeild static pressure, not velocity. This is a large part of the problem I'm having.

p.s. I could decipher the sq root meaning.

 

[mnchapman]

Maybe I should buy the hot wire anenometer for $160. It that what I need to do circumvent all the measurement issues?

 

[Yorkman]

The pitot tube I'm refering to is an instument that air balancers use to find the velocity pressure in a duct.

It is a tube within a tube. It has an impact hole in the tip which supplies the total pressure (velocity pressure + static pressure) this is the inner tube, and it has a series of holes around the radius of the tube. These are sensing the static pressure of the duct, and are in the outer tube only.
Hook the total pressure port to the high side of a magnehelic or water gage, and the staic pressure port to the low side of the magnehelic or water gage.
The difference of the two pressures is the velocity pressure in inches of water column. These readings are then calculated using the first formula to produce the acutal velocity in F.P.M.
I think you can find the pitot tube in a Grainger catalog along with the magnehelic for about $100.00

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.

 

[Yorkman]

A hot wire anemometer works, to just a new twist on an old method, The new stuff takes all the fun out of being a master geek like me!! I think you'll still need to get a good sampling to insure the accuracy of your calculations. Good luck.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.

 

[Yorkman]

Correction I did'nt mean radius when refering to the location of the series of holes I meant circumfrence. Sorry about the geometric mistake!

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.

 

[mnchapman]

I went to dwyer web site and found dozens of pitot products. I can't tell which does what, which is portable, which is permanent, etc.

 

[quark]

Hotwire anemometer is handy and good. Take multiple readings across the filter face and then take the average of it.

The instrument suggested by Yorkman in his 9 Jun 06 21:33 post is generally what we use for a permanent installation. Check with any of the BMS agencies, for ex. Honeywell, Siemens and Sauters etc.

 

[MintJulep]

Hot wire anemometer is less usefull than a pitot tube. It gives you only velocity. They are sensative to alignment with the flow stream. The batteries die.

Pitot can give you static, total and velocity pressure, depending on how you connect it to the manometer.

Dwyer instruments has all that you will need, and some informative literature to go with it.

Log-Chebeychov is preferred to equal area traverse.

 

[mnchapman]

I am back to my previous concern. Dwyer has tons of pitot tubes for sale. Which ONE should I buy for field use, that yeilds velocity reading?

 

[Yorkman]

I got mine here it is listed in thier PDF catalog as testers and come in lenghts from 8" to 72" heres the website.
When you get there type in keyword search pitot tube and check out testers go to the PDF catalog page as they have a good selection there.

http://www.grainger.com

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.

 

[Yorkman]

Mnchapman,
Correction on the lengths that should be 48" as the longest these are designed as a field test instrument, and are standard issue in any air balancers tool kit. I would also suggest a magnehelic gage that reads out in inches of water column. Try to match the range to the system pressures you are going to see. Thier more portable for getting up and down ladders, but dont take to bouncing around in a service truck to well so handle accordingly.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.

 

[mnchapman]

I looked up the part you mentioned. I see it but I don't understand how that will fit through a single test hole in a duct. Compared to a static pressure probe, I don't understand how that pitot works with its 2 prongs.

 

[imok2]

mnchapman You need to read this:

http://www.dwyer-inst.com/htdocs/airvelocity/AirVelocityIntroduction.cfm

 

[mnchapman]

imok2, your last post hit the nail on the head, thanks! It is all too clear to me now! Those pictures were worth a thousand words. Also thanks to the others that are helping me polish my knowledge!

Now does anyone have any advice for me with regards as to what to buy. I currently own the UEI twin pipe manometer and one magnetic static pressure tip.

Now I want to buy 'something' to measure velocity pressure. My choices are a pitot tube (8" ?) for around 50 bucks, or a hot wire anemomneter ($160). Are they both equally useful? Does the hot wire anemometer require the 8 1/2 duct diameter margins for testing?

 

[Yorkman]

Mnchapman,
my choice would be the pitot tube, it's foolproof as far as calibration issues, it's pretty hard to damage just keep the impact tube protected when you store it. If you are going to use it on that 20 x 20 exhaust you mentioned I think you will need the 24" tube. I would also down load the article from Dwyer that Imok2 sited, I saw that and it had all the formulas and grid lay out that you would need. Goodluck let us (me) know how it went.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.

 

[mnchapman]

Thanks for all the help. I now feel empowered to go back and tackle that job on Monday. I'm leaning toward the pitot tube as well, partly because I'm a pureist and like shiny tools.

I'm gonna wait to buy it for now, cause I don't think I have the access to the needed duct run of 16 ft to use it. I do access to a short 8 ft run.

But first, Im gonna take a closer look at some other pressures (esp) that were printed in the blueprints. Maybe see if I can locate where they collected it from and use that as a benchmark. Thanks a million.