Rather than fixing an airchange,I would suggest you to calculate the airflow requirement based on the heat emission from the turbine.
Heat emission in Kw= air flow(m3/s)x1.2(specific heat) X1.02(density)x5 (Temperature rise)
If you do not have turbine heat emission data,you may assume an efficiency of around 30%
and claculate the heat output based on the turbine out put.