caculating CFM

[trialanderror]

did some googling, and didn't come up with anything, maybe i wasn't useing the right syntax, google isn't that smart....tried finding something to use the dimensions of a squirrel cage fan and rpm to caculate the CFM.

if you could post your fan size, width height depth and rpm and cfm, maybe i can figure out a formula to get close.

 

[mga]

here's a ton of them:

http://www.google.com/search?rls=ig&hl=en&q=calculating+CFM&aq=f&oq=


this one has a formula:

http://contractingbusiness.com/enewsletters/cb_imp_46087/

 

[habanero]

I don't think you can reliably calculate the exact cfm of a blower based only on the cage size and rpm. You can probably get close, though, by researching other blowers and finding one of the same size and rpm.

No matter what the cfm rating of the blower, the duct properties will also greatly influence the overall cfm.

 

[trialanderror]

here's a ton of them:

http://www.google.com/search?rls=ig&hl=en&q=calculating+CFM&aq=f&oq=


this one has a formula:

http://contractingbusiness.com/enewsletters/cb_imp_46087/

exactly what i'm not looking for.

show me how to use one of those formulas to calculate CFM with a squirrel cage fan thats 11" in diameter, 6" wide, turning 1050 rpm?



and yah habanero, always factors, but, like we think, it'll give ballpark area....

http://www.heatexchangersonline.com/airtowater.htm
i'm using that for a base reference to pick out a heat exchanger. looking for one that'll easily meet my BTU requirments with a lower temp, but if acutally ran at 180F, easily once again or double my heat requirement. i think chooseing one that way would be failsafe.

 

[Davej_07]

Rpm and squirrel cage size would only be part of the equasion. What is the outlet size on the fan itself? Inlet diameter?

Dave

 

[AOD]

I think CFM = Velocity in FPM x area of the opening in sq. ft. It has little to do with the fan itself. Calculating velocity is tricky, it usually requires a vane anamometer or a pitot tube manometer, and a set of conversion tables.

 

[Biker Dude]

The manufacturer of the blower should have those specs for you as the design engineers would have already done the calculations. Without that info you would be able to do it yourself with a vane anemometer and once you got airspeed you could use the continuity equation to estimate the flow: (velocity) x (Area) = flow [fpm x sq ft = cfm]. Of course this is all academic because duct work sizing and flow restriction across the heat exchanger affect actual cfm out the vents.

AOD you beat me to it!

 

[trialanderror]

sweet, thanks all.

 

[AOD]

A blower manufacturer will only specify wide open CFMs at a certain speed, that is, the CFM's a blower will move sitting outside of a furnace running on the floor with zero restrictions. This is the most air that blower will ever move. It is up to the duct system designer to make a duct system that will allow the blower to move enough CFM's needed to heat/cool each room, which is usually done through sizing ducts and is based off of static pressure loss inside the duct system.

 

[mga]

A blower manufacturer will only specify wide open CFMs at a certain speed, that is, the CFM's a blower will move sitting outside of a furnace running on the floor with zero restrictions. This is the most air that blower will ever move. It is up to the duct system designer to make a duct system that will allow the blower to move enough CFM's needed to heat/cool each room, which is usually done through sizing ducts and is based off of static pressure loss inside the duct system.

very true.

however, the OP didn't ask for that. he simply stated he could develop a forumula for a blower only. hell, anytime you buy one, it states what that is...no special formula needed.

on the other hand, as you said, the ducting is critical and simply throwing pieces of sheet metal together isn't the way to do it.

 

[AOD]

The same method should work to calculate the WOCFM of a blower that it out of the furnace running on the floor, you still need an anemometer.

 

[mga]

i got one...rather old tho....lol...sits on the book shelf:

 

[AOD]

i got one...rather old tho....lol...sits on the book shelf:

Whoa, that is old school