[oly77]

Hi'' I only whant to know,the formula for cfm,, Example: I have a York Curbpak Air Handling Unit 50hp, 1750rpm 460volt pulleys sizes (motor p 6'') shaft p 10'' how i obtein the cfm thanks

 

[onetime]

CFM = Velocity x Area
CFM = BTUH / 1.08 x TD
CFM = E x I x 3.14 / 1.08 x TD

 

[air2spare]

get an IOM for that unit from York and remove the guess work

 

[marter]

CFM = Velocity x Area
CFM = BTUH / 1.08 x TD
CFM = E x I x 3.14 / 1.08 x TD

wont these formulas tell you what the cfm should be and not is ?
where does the 1.08 come from sorry im still a sorta n00b

 

[just_opinion]

OL77,

With such a large unit, formula #1 cannot get the total CFM correctly since the duct will be too large to get good velocity of the air. The answer will be way off

If by a slim chance it has a gigantic electric heater attached to it, than a qualified technician can use the BTU formula to get an approximated CFM

Factory blower curve is you BETTER bet on the total CFM - no formula. How to do this, please call qualified technician.

Call a company out to measure each supplying grills and add them up. Now that is the BEST way.

Good luck

 

[tnhikr44]

OL77,

With such a large unit, formula #1 cannot get the total CFM correctly since the duct will be too large to get good velocity of the air. The answer will be way off

If by a slim chance it has a gigantic electric heater attached to it, than a qualified technician can use the BTU formula to get an approximated CFM

Factory blower curve is you BETTER bet on the total CFM - no formula. How to do this, please call qualified technician.

Call a company out to measure each supplying grills and add them up. Now that is the BEST way.

Good luck

just_opinion,

Not trying to be an ass, but, in my opinion you have it completely backwards.

1. Size and velocity are relative. If the system is large the ductwork is large... but the velocity is close to the same. For instance, the velocity across the face of a coil (cooling) is pretty much the same in all size systems.

2. When ever applying temperatures to formulas it has been my experience that obtaining very accurate temperatures is not as easy as it seems. A temperature traverse is required to obtain accurate enough temperature rises to back into cfm via the heat tranfser formulas. If you are off by a degree or two it really screws that formula up.

3. Fan curves are created in perfect conditions in a lab. Those conditions rarely exist in the real world. Actual conditions create their own "system" curve... which almost always deviates from the published curve.

4. A summation of the supply grilles does not include any leakage that may exist. I have seen 25% leakage before... thats 75% of what the unit is putting out actually reaching the grilles.


oly 77,

I would suggest that you find out what the unit is suppose to be delivering and, based on conditions, subtract AT LEAST 15% from design. If you have some means to read velocity I will gladly give you more info as to how to obtain actual airflow.

 

[ac/dc]

I would measure the cfm at supply defusers to see what is being delivered.
I would also traverse the supply duct to see what the air handler is actually
putting out. I would assure the filters and coil were clean and return/fresh air
was adequate and not restricted. Of course fan laws and formula's can be used to see what it should be but that doesnt tell you what it actually is.

 

[chiller rob]

1.08 is a constant.... its unit solution is....

1.08 BTU * MIN/ SQ FT * DEG F * HR

 

[just_opinion]

Tnkihr44,

I some what agree with you. (of course there will be a) But, with such large system, it is very difficult to measure the return air velocity in the duct.

As for the duct leakage, if it leaks that bad, then measure CFM is a waste of time until the leaks are repaired. At the 25% leakage, the building will be in such a nagative pressure that the grills and walls are mold-up and it will sucks you in as soon as you open the door.

Let's be specific on the answer to the post. And not be too generic.

I do agree with Marter, that with those formulas we can get in the ball-park figure.

 

[DPinst]

1.08 is a constant . it is calculated from density of standard air (.075 lbs /cu.ft)x specific heat of air (0.24 btus/lb) x minutes in hour (60min/hour)=1.08

 

[chiller rob]

dpinst thanks for that...

in the denominator i had sq ft... that is wrong and should be cu ft...

but there should also be a Deg f in the denomonator as well to cancel out the difference in temp multiplier to end up with clean BTUH on the other side of the equation...

so looking thru my notes i cant find how 1/ deg F gets into the constant... How does this work?

 

[chiller rob]

sorry... i think i just answered my question...

the specific heat = 0.24 btu / lb * deg F

 

[tnhikr44]

Tnkihr44,

I some what agree with you. (of course there will be a) But, with such large system, it is very difficult to measure the return air velocity in the duct.

As for the duct leakage, if it leaks that bad, then measure CFM is a waste of time until the leaks are repaired. At the 25% leakage, the building will be in such a nagative pressure that the grills and walls are mold-up and it will sucks you in as soon as you open the door.

Let's be specific on the answer to the post. And not be too generic.

I do agree with Marter, that with those formulas we can get in the ball-park figure.

just_opinion,

1.

with such large system, it is very difficult to measure the return air velocity in the duct.

As I stated previously, size and velocity are relative. On that return duct you mention the typical velocity will be somewhere between 500 f.p.m. and 1000 f.p.m. on nearly every size system. Reading 500 f.p.m. is the same if the duct is 1 sq. ft. or 50 sq. ft.(free open area).

2.

As for the duct leakage, if it leaks that bad, then measure CFM is a waste of time until the leaks are repaired. At the 25% leakage, the building will be in such a nagative pressure that the grills and walls are mold-up and it will sucks you in as soon as you open the door.

Duct leakage has little to no effect on bldg. pressurization. The difference between exhaust air and make-up air is where bldg. pressures are determined.

3.

Let's be specific on the answer to the post. And not be too generic.

Well, I have tried to be specific. I measure airflow (and water flow) and calculate coil performances (BTU's) for a living. I have all the latest equipment designed specifically for such. Using heat transfer formulas to back into an airflow is the very least accurate method. Hands down.

 

[DCSCO]

oly77,
Use an anemometer to measure air flow in feet per minute. Take five readings in a diagonal line accross an accessible coil. Average the readings and multiply by the area in square feet to calculate CFM. You can calculate CFM within 10% this way. For an accurate reading, do a Google search on Duct Traverse. Their is plenty of information available.
dc

 

[chillerguy2002]

Oly77,
Good replies above, but why do you ask? Your info is not enough to tell anyone how to calculate it, or why you need to know it. Typical reverse engineering of an existing unit is to measure the coil to obtain the square foot dimension ( fin height inches x fin length inches divided by 144) and assume the unit was set up for 500 feet per minute face velocity max (typical). Example: 30 sq ft x 500 fpm =15000cfm nominal. Real operating cfm will then depend on the fan static curve and could vary widely.

What are you looking for?