Commercial hood systems require air flow, and exhaust fans and make-up air units are necessary to make that happen. Exhaust fans come in varying power ratings for different operations, and can have either direct, upward, or downward exhaust streams to ensure to accommodate the needs of every operation.
Make-up air units, similarly, come in varying powers but have a much different purpose. Make-up air units are intended to replace the air that is being exhausted. Exhausting a kitchen creates a small vacuum, and it is often necessary to replace the air being drawn out so keep swing doors closed, and to keep fumes being exhausted moving out without disrupting the air elsewhere.
Tips from the Experts:"It is important to properly place a make-up air unit. If a unit is placed to close to an exhaust vent, it may end up pumping exhaust back into your kitchen." - Pete