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.

Square or rectangular ducting

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

Posted in Fans Ventilation A Practical Guide


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