Couch Pit Agitator The Couch Pit is situated below the couch roll of the machine. During normal machine operation, this pit receives the edge trim off the sheet. During a "break" the entire sheet enters the pit. The purpose of the Couch Pit Agitator is to re-pulp the sheet so it can be pumped to the broke system. Mixer design is based primarily on the machine tonnages at the couch roll, the sheet consistency and the type of sheet.
The pit is slightly wider than the machine width (to accommodate sheet entry). The machine-direction width just below the machine varies, but invariably necks down at the bottom of the pit. Optimum conditions are achieved with a residence time during break of 3 - 5 minutes. Generally, the pit is divided into cubes, with one agitator per cube recommended. This ensures a rapid and uniform re-pulping action. Normally, a 45° fillet, width equal to 1/3 of the pit width in the machine direction, is used.
A shower installation, consisting of two headers - one on the fillet side of the pit and the other on the side - with 1/2" spray nozzles equally spaced at 6" intervals, is located so that their spray discharge intersects at the center of the pit at the liquid surface. The water pressure creates a spray that will both keep the sheet away from the walls and also break the sheet into ribbons. After resuspensioning the consistency would normally be in the 4-5% range.