The control of emissions from open surface tanks is usually perfomed using simple exhaust systems or combined with push (1). In the first case the exhaust entries generate omnidirectional speed fields within the tank, which rapidly reduce efficiency as the distance increased and are recommended for tanks of less them 1 metre in length. In the push-pull systems, a curtain of air sweeps over the surface of the tank and drags emission towards the exhaust causing a jet wall which (2), when well formed, permits high capture efficiency (3).
Before starting to design an HVAC installation for treatment, supply and exhaust of air to and from a building the needs should be listed, transformed to requirements and their consequences analysed. Requirements lead to different amounts of airflow for their fulfilment. These needs for airflow should be calculated. The demand leading to the highest call for airflow will decide the airflow for which the equipment should be dimensioned - the dimensioning airflow.