mass flow = volume flow * density = liter*lb/hour/ft^3
one liter = 0.001 m^3
one lb = 0.453592 kg
one ft^3 = 0.0283168 m^3
one hour = 3600s
mass flow = 21*72.42*0.453592*0.001/0.0283168/3600 m^3*kg/s/m^3 = 0.006767 kg/s
I didn't do this by hand so make sure you check the unit conversions and the number crunching.
This post was edited by Dontrunaway on Nov 24 2013 11:19am