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