In natural ventilation systems, the drafts in the flues or ducts are caused by the difference in density between the air in the ducts and the outer atmosphere. The higher the temperature in the ducts, the more rapid will the draft become.

In the following table 50 per cent, (a fair average in good work) has been deducted from the theoretical flow, to offset all ordinary resistances in the flues, such as friction, change in direction, etc. The difference in temperature given in the table is that existing between the outer atmosphere and the average of the air in the flue. Knowing the velocity per minute, and the cubic feet of air per minute to be removed, the area of the flue in square feet is found by dividing the volume by the velocity.

Flow Of Air In Flues Per Square Foot Of Section. By Natural Draft, In Cubic Feet Per Minute

Difference in Temp. F°.

Height of Flue in Feet.

10

15

20

30

40

50

60

80

100

10

108

133

153

188

217

242

264

306

342

15

133

162

188

280

26S

297

325

375

420

20

153

188

217

265

806

342

373

4S6

486

25

171

210

242

297

342

888

420

485

680

30

188

230

265

325

375

419

461

530

604

40

216

265

305

374

431

482

529

608

680

50

242

297

342

419

484

541

594

680

768

60

266

327

376

460

582

595

650

747

842

70

288

354

407

498

576

644

703

809

910

80

806

379

435

668

616

688

751

866

90

326

401

460

565

652

728

706

918

1,029

100

842

419

484

598

684

765

835

965

1,080

125

884

470

541

664

766

857

939

1,086

1,216

150

419

514

593

726

838

937

1,028

1,185

1325