in two adjoining rows intersect to form an approximately 45 u00b0 angle with one another with holes of 0 024 inch diameter and a thickness of aluminium shell of 0 062 inch the minimum spacing of centres dimension b is of the order of 0 046 inch in each longitudinal row with a spacing of 0 023 inch half of dimension a from a centre in one row to the adjacent centre in the next circumferential row this gives a total of 913 holes per square inch and 41 hole area to obtain satisfactory air flow conditions a hole area of at least 25 is desirable and preferably greater than 30 30 hole area can be achieved using 0 020 inch holes at the same centre spacing as above the above percentages of hole area are calculated by measuring the area of each hole at its smallest cross section in this case the holes are cylindrical and hence the cross section is constant examples with reference to the drawing wherein 34 a 34 designates the distance between the centers of adjacent holes in a row thereof circumferential the perforated roller and 34 b 34 designates the distance between the centers of adjacent holes in a row thereof parallel the roll axis and 34 d 34 designates the diameter of the circular hole of suitable.