almost got it

[speedyd1]

i have grapsed the general understanding of how a flow bench works and am in the process of building my first unit. my question is - is there a simple formula for figuring cfm after i have done my testing. i have the figures from the manometers and so forth / i'm just not sure how to convewrt them into cfm.

[bruce]

We need to know what style of bench you are building before anyone can help you out with the "math", did you build an orifice or pitot bench?

[speedyd1]

pitot -

[bruce]

I'll try and show the formula I've used in the past:

Static pressure (ps) vertical manometer
Barometric pressure (pb) barometer gage
indicated temp (ti) thermometer

absolute temp (ta)=ti+460
Dry air density (d)=1.325*(pb/ta)
Correction factor (cf)=sqrt(.075/d)

Pressure velocity (vp) from your inclined meter
Velocity feet per minute (vc)=(4005*(sqrt[vp]))*cf
CFM's = (inside dia of your testing pipe in square feet)*vc

Hope this all makes sense, and u get a good idea of whatca need to do? Hopefully I copied this from my notes correctly also . . . Simply create a spreadsheet that does all the math for you and you're all set!

Good luck!!

[speedyd1]

okay bruce - i'm with you so far. what i'm having trouble with is converting the measurement from the inclined manometer to velocity pressure. my scale is in inches. how does that work? also - my original design used a vertical manometer and not an inclined. does one function better than the other and is one easier to convert? it doesnt have to be absolutely accurate because i'm only looking for improved flow and not cfm. i thought it would be nice to know what i was atually reading or close therebouts.i'm at that point where too much information is still not enough.

[bruce]

You take the rise of the inclined meter and divide the length of the inclined part by that number. So say you have a 3" rise and your inclined peice is 30" every 10" is equal to 1" on your rise. You just divide the rest of the rise equally. Its all base on one inch of water. Your number is then used in the calculations. The best thing to do is have a couple of calibrated orifices of different dias that you can set your incline to. You need to make the well part about a ratio of 1000:1 of the inside surface area of your inclined peice of tubing so it does not effect the readings accuracy. There are some indepth discussions on here about manometer design. Hope this all makes sense? Its tough to explain it sometimes.

This is the very basic concept/design, one can get more elaborate as you require more accuracy.



Edited By bruce on 1085589120

[speedyd1]

thank you

[gaz]

[color=#000000]Hey Bruce I

[bruce]

All flow numbers are in inch H2O. The Static Pressure is just a reference for testing from test to test. Temps are in F. The .075 and 4005 are constants. What I would do is look for the forumla for measuring flow through a pipe in metric on the net somewhere? You should be able to find that info out there.

My formulas are standard HVAC calculations. What I did was make an Excel spreadsheet that does all the math for me. Since going to the Casio PV setup from Mouse it does all the calculations and I don't use the spreadsheet anymore. Now I need to make a spreadsheet to chart all my numbers so I can compare flow from test to test at a glance. Any Excel gurus out there? Hope this helps?



Edited By bruce on 1099312797

[gaz]

now I cracked Bruce's formula . Barometric pressure (pb) should be [inHg] , and temperature (ti) should be [*F].
ti=70 F => ta=460+70=530
pb=29,9 inHg
pv=2 inH2O
d=1.325*(29.9/530)=0.0745 ~0.075 (standard air density if air temp=70F and air pressure=29.9 inHg)
cf=sqrt(0.075/0.075)=1
vc=4005*cf*sqrt(pv)=4005*1*sqrt(2)=5663.9 Feet/min
if we have 3" pipe then cmf=5663.9*{3.14*[(3/12)^2]/4}=278 cfm.

this way it is working or did I get it wrong because I've no idea what the number for (pv) in reality would be?
now I have to find a way to make it work in metric units.

[bruce]

I was half asleep last evening when I posted my reply and noticed an error on the temp.

I think your numbers for CFM are not quite right? The inside surface area should be 3.14*R2/144 for area in feet. Also the inside dia of PVC is not what the manufacturer sells it as. Standard PVC pipe inside diameters are 1-1/2 = 1.592, 2"=2.049 and 3"= 3.042. You need to also subtract from your area number the surface area that comes in contact with your flow of your pitot or annubar that sticks in the pipe.

Why not use a conversion from feet per minute to meters per minute? Then the area of your pipes inside in meters? Would that work to give you Cubic Meters per Minute if that is what you are after? I found this info posted on the web in a newsgroup:

"One meter is exactly 100 cm = 100/2.54 in = 100 / (2.54 * 12) feet =3.2808399...feet. One meter cubed is (3.2808399... feet)^3 = 35.31466... cubic feet.
One cubic meter per minute is thus about 35.31466 feet per minute."

I was kinda slow on math way back when I was in school, I knew I shoulda listened

Anyone correct me if I have errored in my numbers along the way, I haven't been thinking flowbench calculations for sometime now and I could be a tad fuzzy![I]



Edited By bruce on 1099324703