New duct airflow

[mech7981]

Hi.
I have a question about a duct modification. I have an OAHU that serves existing AHU-1 with around 1500 cfm. I'm wanting to put a tap somewhere on that duct and run a round duct to serve AHU-2 with about 100 cfm of OA.
My question is, what size of round duct do I need to get approximately 100 cfm to AHU-2? The data is below.

1500 cfm = amount of airflow to existing AHU-1
20x12 = duct size from OAHU to existing AHU-1
20 ft = distance from OAHU to existing AHU-1
0.5"wc = external static pressure seen by OAHU
40 ft = distance from new tap location to inlet of AHU-2

I understand that I'll have to increase the fan speed to account for the new airflow, 1500 + 100 = 1600 cfm.

Thanks in advance for your help!

 

[CodyK]

What is the maximum external static pressure that fan is rated for? This will tell you what your max pressure drop in the duct can be. Then you can use a ductulator or chart and find out the minimum size of duct required.

 

[mech7981]

The design ESP = 0.50" w.g. That's with the current as-built conditions. The fan is probably capable of a little higher ESP though.

 

[BronYrAur]

Just making sure I understand. OAHU is an outside air handling unit, correct? And you are taking air from the discharge side of the OAHU fan, which I'm guessing has some indication that it can handle 0.5" external static, correct?

If so, you are in the "normal" low pressure duct sizing range.

 

[GT-EGR]

To really answer we’d need a lot more info.

But likely you could go with a 6” duct, that’s low pressure drop less than 0.1” per 100’ at your 100 CFM, which would leave you plenty of static remaining if you only have 40 ft to run. That being said if you have a lot of elbows and offsets maybe you are better off with 8”. Be sure to put a volume damper in this duct as well.

This duct also likely terminated into a negative pressure mixing box of the other unit, all the more reason for the volume damper, but means even more pressure available.