Any vacuum-assisted drainage element on the fourdrinier table that utilizes an exterior air/water separator discharging through a barometric leg to a seal tank.
Definition source:
EN 9
Context:
The importance of fabric tension uniformity across the machine is easy to appreciate because variations resulting from, say, uneven frictional forces and uneven fabric wear, will cause non-uniform drainage pressure and hence, non-uniform paper structure (6). Beyond the deflection blades, drainage is induced by suction boxes and a suction couch.
