A：a. Unidirectional flow: elevated perforated plate is used, its “effective” return air area (Total area of holes) is ≤50%; to maintain the effect of unidirectional flow, wind speed at return air inlet ≤wind speed at HEPA air outlet is preferred.
∵Return air amount=air intake － forced air displacement － gap leakage.
∴Return air amount＜Air intake.
b. Non-unidirectional flow: in most cases, stainless steel grille or honeycomb return air inlet is adopted, installation height is about ≤50cm above the floor; optimal wind speed at return air inlet is ≤ 3m/s; then, air flow (Q M³/min) = effective section area (A㎡) × wind speed (V m/s) × 60, select the size and number of honeycomb plate or return air inlet.