I would like to know, how can I calculate the loss coeficient of the vents in Airpak

if you don't have the manufacturer's data for the exhaust vent, you can either have airpak calulate the loss coefficient based on correlations where you supply the geomtetry type and the free-area ratio of the exhaust vent. alternatively, you could use something like i'delchik's handbook of hydraulic resistance and do a hand calculation based on the geometry and input the number yourself.