air exchanges

[freonjohn]

We are currently replacing/modifing a/c equipment for an operating room. The O.R room is fed by an ahu. there are 2 return ducts in the O.R. room and 1 outside air duct that is tied into the return plenum. The engineer who designed the job wants the following air exchanges
min. changes outdoor air per hour = 3
min. total air changes per hour = 15

the O.R. room is 21x18x9 = 3402 cubic feet
both return air cfm's = 764
764x60/3402= 13.47 air exchanges in the O.R.

I do not understand the outdoor air exchanges
or how to figure out the number of outdoor air exchanges.
Am i suppose to have 12 air exchanges in the O.R. and 3 outdoor air exchanges?

thanks for any help!

 

[freonjohn]

forgot to mention -- outside air into return plenum is 300 cfm

 

[SAK9]

Fresh air or out door air should be 170(3402x3/60) cfm and the AHU flow rate should be 850(3402x15/60) cfm.If you select an AHU which can handle 170 cfm fresh air with a total airflow of 850 cfm,you should be OK.