110. The carrying capacity of a flue or duct is controlled by the temperature of the hot air and the height of the flue, in the same manner as a chimney. A flue which extends to the third story of a building will discharge more air per minute than a similar flue which discharges at the second or the first floor. This is because the column of hot air extending to the third floor is higher than the others, and consequently the upward pressure is greater. In Table 17, which gives the velocity of air in flues, in feet per minute under natural draft, an allowance of 50 per cent. over the theoretical flow has been made for friction and other common resistances. This will be sufficient for all ordinary circumstances. It should be borne in mind that the volume of flow shown in the table cannot be attained unless the air in the room is permitted to escape freely and as rapidly as the fresh warm air is inclined to come in.

Table 17

The difference in temperature given in the table is that existing between the outer atmosphere and the average of the air in the flue.

Each register should be supplied by an independent vertical duct.