PDA

View Full Version : measuring airflow


dave1234
08-23-2010, 04:41 PM
This is more of a AC topic but I choose to post here because this is where the best responses come from. I have a 15 ton system that freezes up constantly. There is fan cycling and a steady load. It seems to be an airflow issue since it seems as though little air can be felt through the vents. My question is whats the relationship/formula for sheave sizing based on a given motor HP. I know you want 400 cfm/ton, but I couldn't help but notice that the drive sheave looks not very old and it crossed my mind that it could be oversized reducing cfm. I also realize that the ductwork plays into it but I dont have all that info. How does one figure out how much cfm is across the evap using the pulley sizes and motor HP. I guess one would have to know the exact model# of the AHU for the cage size too. In a nut-shell- 15 ton Trane AHU- 3 HP motor- 5 1/2" drive sheave- 10" driven. How does one (or can you) calculate cfm acros the evap with those #'s ?
jpsmith1cm
08-23-2010, 04:44 PM
If you have a fan curve, a manometer and a tachometer, you can easily plot the blower's performance on it and determine total CFM.

I am currently shopping for an anemometer that will allow more direct readings of airflow.

You can use pulley ratio to determine blower speed, but that doesn't tell you CFM
dave1234
08-23-2010, 05:59 PM
Thats right. You would have to know all factors like duct sizing to determine actual cfm.
itsiceman
08-23-2010, 07:31 PM
You can use pulley ratio to determine blower speed, but that doesn't tell you CFMThey got cfm charts for that too. ;)
The pressure drop across evap (clean/dry) and fan chart should get you close to what you need to know with actual cfm across the coil.
Are you in a high altitude location?
dave1234
08-23-2010, 07:37 PM
They got cfm charts for that too. ;)
The pressure drop across evap (clean/dry) and fan chart should get you close to what you need to know with actual cfm across the coil.
Are you in a high altitude location?

No
jpsmith1cm
08-23-2010, 09:50 PM
They got cfm charts for that too. ;)
The pressure drop across evap (clean/dry) and fan chart should get you close to what you need to know with actual cfm across the coil.
Are you in a high altitude location?

This assumes that this information is available.

Yes, with RPM, Brake HP and static, you can get CFM from a fan chart.

I've never been fortunate enough to work on a system with a fan chart handy.