Square or rectangular ducting

Until recently special tables or charts were not available for the resistance of square or rectangular ducting. Even now, the few charts which are do not show all the combinations of width and depth desirable. Accordingly, an equivalent table may be used (Table 3.6). This table shows the size of round ducting which is equivalent in frictional resistance to a square, or any rectangu­lar duct, when passing the same volumetric flowrate of air. The air velocity, of course, is not the same.

For example, from Table 3.6 a 535 mm x 280 mm rectangular duct is equivalent to a 405 mm diameter duct. A405 mm diame­ter duct is equivalent to a 370 mm square duct.

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