Mini-Encyclopedia of Papermaking Wet-End
Part Three: Equipment & Unit Operations
The tray of a paper-forming machine is the collection point for white water that drains from the wet web before any application of vacuum. The tray is often filled with foam, punctuated by the "rain" of white water coming from hydrofoils. Tray water typically has a higher fines concentration when compared to the lesser amounts of white water collected from vacuum boxes, the couch roll, and wet presses. Modern systems for control of retention aid flows require accurate measurement of tray water solids. This can be a difficult number to obtain with accuracy due to the tendency of foam bubbles to float some of the fine materials, and due to the fact that the early parts of the dewatering process yield white water that is much richer in fines than the later parts. The greater clarity of white water farther down the paper machine is due to the filtering effect of the wet web itself.
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