final report on the multiport dryer - Argonne National Laboratory

4A B C D EF G HA - Steam convection heat transfer resistanceB - Condensate layerC - Scale inside the dryerD - Dryer shellE - Dirt and air between the outside surface of the dryer and the wet paperF - Paper driedG - Dryer felt and fabricH - Air boundary layerFig. 1. Resistances to heat transfer on cylinder dryer• Sheet-to-dryer contact, including- Dirt and air between the outside surface of the dryer and the wet paper.- Paper.- Dryer felt or fabric.- Air boundary layer.Of these, the primary contributors to overall thermal resistance are the condensate layer, dryershell, and sheet-to-dryer contact [6].• Condensate Layer. As steam and liquid water flow through a series of conventionalpaper dryers, some condensate always remains in the steam drums as a condensate layeron the inner wall of the shell. Heat transferred from the steam to the wet sheet must passthrough this film of condensate. The condensate is thus the first barrier or resistance toheat transfer. The thickness of the condensate layer is greatly affected by dryer speed; asthe rotational speed of the cylinder increases, the contribution of the condensate layer tooverall resistance also increases.• Dryer Shell. The second primary barrier to heat transfer is the dryer shell, whereresistance is affected by both shell thickness and shell thermal conductivity. Thickwalledshells with low thermal conductivity produce greater resistance to heat transferthan thin-walled shells with high thermal conductivity.