# 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.

 Round duct mm dia Square duct mm x mm Rectangular duct — depth mm d 230 J 305 380 455 535 610 685 Width mm w 155 140 90 75 — — — — — 180 160 115 100 75 — — — — 205 185 155 125 100 90 — — — 230 210 205 155 125 115 100 90 — 255 230 230 180 155 125 115 100 90 305 280 370 265 215 190 165 140 125 355 325 495 355 280 240 215 190 165 405 370 660 470 370 315 280 255 215 455 420 865 620 470 395 345 305 265 510 465 1030 760 585 485 420 355 330 560 510 1245 940 710 585 510 430 380 610 560 1450 1120 840 685 595 520 455 660 605 — 1270 1015 815 710 610 535 710 650 — 1485 1180 965 815 710 610 760 700 — — 1370 1120 940 785 710 815 745 — — 1525 815 865 795 — — — 915 915 840 — — — — 1015 965 885 — — — — 1575 1295 1170 1015 935 — — — — — 1475 1295
 Table 3.6 Equivalent dimensions of round, square and rectangular ducts for equal friction and flowrate

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