Knurling/ribbing

With tolerances of all dimensions External shape, gripping areas (beading, knurling, ribbing, etc.)... 

Cox et al. [101] used several kinds of enhanced tubes to improve the performance of horizontal-tube multiple-effect plants for saline water conversion. Overall heat transfer coefficients (forced convection condensation inside and spray-film evaporation outside) were reported for tubes internally enhanced with circumferential V grooves (35 percent maximum increase in U) and protuberances produced by spiral indenting from the outside (4 percent increase). No increases were obtained with a knurled surface. Prince [102] obtained a 200 percent increase in U with internal circumferential ribs however, the outside (spray-film evaporation) was also enhanced. Luu and Bergles [15] reported data for enhanced condensation of R-113 in tubes with helical repeated-rib internal roughness. Average coefficients were increased 80 percent above smooth-tube values. Coefficients with deep spirally fluted tubes (envelope diameter basis) were increased by 50 percent. 

Points of attachment using screws. The metal Insert is threaded internally, and knurled externally to provide grip, it j supported by a boss, which in turn is supported by ribs of various designs. 

The inside surface of the bowl can be plain machined. However, some effort is often made to encourage cake to stick to the bowl, to aid scrolling instead of slipping round with the conveyor. The means of doing this could be by knurling the inside of the bowl for instance. This can wear smooth relatively quickly. More often longitudinal ribs are welded, or a liner with similar ribs is fitted (see Section 2.2.4.3). 

Instead of a liner, many bowls have longitudinal ribs spot welded to the inside of the bowl. Another alternative to a liner is knurling or roughening of the bowl, which is a simple procedure, but would normally be done only as a temporary measure as such surfaces will be quickly worn smooth. 

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