Two questions about gases (within 8 hours)

1. Hazardous specialty gases are typically drawn from cylinders placed in gascabinets and connected through cross-purge systems to the facility supply lines.
Why do we use cylinder purge gas (e.g., cylinder nitrogen) rather than “house”
nitrogen for the purge operation? Hint: The house gas is fed throughout the
entire facility.
2. ANSI/AIHA Z9.5 states that “Each hood shall maintain an average face
velocity of 80-120 fpm with no face velocity measurement more than plus or
minus 20% of average.”
(a) If a typical exhausted wet bench is 12 ft in length with a 2 ft high opening,
what is the volume of air (in cfm) flowing into the exhaust?
(b) If we have 30 of such wet benches in a facility that has 240,000 cfm of air
flowing through the room, what is the recirc fraction (R) for this facility?