PDA

View Full Version : CFM of Outdoor air

r404a
08-30-2004, 11:08 PM
A/C Guys ( and Gals)

I will be starting up a system with a rooftop AHU and the plans call for 165 CFM outdoor air to be brought in at all times. Is there a temp mixture formula or should I figure it with the area of the manual damper? Any input is appreciated greatly. Thanks in advance.

R404a

NormChris
08-30-2004, 11:17 PM
Set the outside air cfm by the mixed air temperature of the airstream between the outside air and the return air. Here is the equation to determine the correct mixed air temperature that represents your mixture of outside air.

MAT = (CFM oa X Temp oa) + (CFM ra X Temp ra) divided by the CFM total.

Adjust the OA damper until you reach the calculated MAT of the outside and return air and you are done.

Norm

NormChris
08-30-2004, 11:21 PM
Using the equation and method I just posted you need to know both the CFM of the outside air and of the return air. Together they add up to the total.

There is another equation you can use if you know the percentages of outside and return air instead of the actual CFM of each. Both methods work equally well.

Where did you get the required CFM that you posted? Did they also give you the return and total CFM?

r404a
08-30-2004, 11:29 PM
Norm,
Thanks for the reply. I got the required outdoor CFM from the plan drawings spec'd by the engineer. The other information stated on the drawing calls for 1000 cfm at 1/2" ext static pressure and that sounds right, as the A/C sytem is 3 ton. A small steam heating coil is installed as well.

Thanks
R404a

NormChris
08-30-2004, 11:37 PM
You can turn those numbers into percentages and do it that way as well. Here is the equation.

MAT = OAT - ((OAT - RAT) X % RA)

Again, you first calculate the MAT (mixed air temp) that represents the correct mixture of outside and return air. Then, you put an accurate thermometer in the mixed air path and adjust the outside air damper until you reach that temperature. Then lock down the damper in that position and you are done.

The trick is getting a good set of temperatures especially for the mixed air.

Norm

NormChris
08-30-2004, 11:52 PM
You can also check a system to determine the actual percentage of outside air that the system is bringing in by using the following equation.

%OA = MAT - RAT divided by OAT - RAT

A Fluke 52 thermocouple thermometer works best for all of these measurements. I always have two of them handy so I can take 4 temperatures at the same time.

sonc
08-31-2004, 04:39 AM
http://www.heatincorporated.com/~technical_information/Troubleshooting/Freshair.pdf

NormChris
08-31-2004, 09:12 PM
R404, how about a report on how you made out setting that outside air?

Norm

r404a
09-01-2004, 12:02 AM
NormC and Sonc

Thanks for all of the help, hopefully I will get all of the controls mounted and get the unit lit off this week. I appreciate the information. thank you.

R404a

twcpipes
09-01-2004, 04:41 PM
Could you also let us know if the blower will overcome the static of the hot water coil when you start this up?
Thanks, tom

Carnak
09-01-2004, 07:06 PM
Norm

On your next article you should combine calculating mixed air temp with the bypass factor. Contact air at ADP temp mixes with bypassed air at entering air temp to give LDB.

Its the way Willis viewed how a coil worked and puts a little meaning into the whole concept of bypass factor.

NormChris
09-01-2004, 07:15 PM
Originally posted by Carnak
Norm

On your next article you should combine calculating mixed air temp with the bypass factor. Contact air at ADP temp mixes with bypassed air at entering air temp to give LDB.

Its the way Willis viewed how a coil worked and puts a little meaning into the whole concept of bypass factor.

Actually I already wrote such an article. It is one chapter in a manuscript for a book I wrote on "Applied Psychrometrics For HVAC Technicians". I just have never published it.

Norm

DaleP
09-02-2004, 07:37 AM
the best way to determine how many cfm of outdoor air you have is to do a proper test and balance on the system. Granted the temp formulas will give you an educated guess but if to do them properly you have to know the system cfm why not just measure the OA cfm while you are at it and adjust it. Correct me where I am going wrong here - please. My .02 DaleP

NormChris
09-02-2004, 07:53 AM
Originally posted by DaleP
the best way to determine how many cfm of outdoor air you have is to do a proper test and balance on the system. Granted the temp formulas will give you an educated guess but if to do them properly you have to know the system cfm why not just measure the OA cfm while you are at it and adjust it. Correct me where I am going wrong here - please. My .02 DaleP

These equations are what balancers use. You can't always get air instruments in the often tight spots where you need to make a reading.

DaleP
09-03-2004, 08:06 AM
I understand the formulas and use them often but r404a is wanting a specific cfm. I am assuming "probably falsly" he has no system cfm information. If the system would have been balanced then the OA would have been set by the balancer. Obviously from the formulas you posted you need to know the total equipment cfm to even begin to use %'s of air flow. If you know that information chances are it has had a T&B or someone is guessing anyway. If this is a RTU I find it hard to believe you can't traverse a duct or damper assembly and do it properly. If he has the system cfm info than the formulas are a good answers to go by otherwise it is just a guess. My .02 again - DaleP

Carnak
09-03-2004, 12:14 PM
He could set the outside air percentage like Norm said, especially if the air was well mixed and mixed temp was measured before it picked up heat from the blower and motor.

Without measuring total airflow, he would only have it proportional to the amount of air moving.

Hard to estimate total CFM from ATR from a steam coil. Not easy like electric heat, or flue gas analysis/clocking gas meter.

He would have to do a traverse or look at ESP/CFM charts.

Sometimes there is pressure drop infor for O/A dampers in manuals, but like Norm said it is pretty tough to get those readings.

hvacker
09-03-2004, 07:09 PM
Trane has a little calc, one of those cardboard dial things that works with temp differences that is sort of accurate if there is enough temp difference between air temps otherwise it's not too close. On t&b work the calcs have been accepted as a percentage of cfm.