As an illustration of the application of the formula, suppose that the temperature in the house and in the flue is 70°F. and that the outside temperature is 20°F. The height of the chimney is 30 feet. The area of the flue is 1 square foot. Volume = 14,400 √(((T - t)/491)H)
= 14,400√(((70 - 20)/491) × 30)
= 25,140 cubic feet per hour.
Such a ventilating flue would be sufficient in size, under the conditions given, to furnish air at the rate of 25,140 cubic feet per hour or 30 cubic feet per minute to 13 persons, provided of course that the air could enter the building at the rate demanded. Where no provision is made for the air to enter the building it must find its way by the accidental openings. A common illustration of this effect may be noticed in the rate at which the fire of a stove will burn in a tightly closed room. The opening of a door or window causes an immediate increase of combustion, because of the extra air supply. It is evident that in well-constructed houses other means should be provided for admitting air than that of accidental opening.
The following table calculated by the above formula gives the quantity of air in cubic feet per hour discharged through a flue of 1 square foot cross-section. The table shows the calculated discharge from flues of heights varying from 15 to 40 feet, and with temperature differences from 10° to 100° between the outside air and that of the house.
| Height of flue in feet | Temperature of air in the flue above that of external air | ||||||
| 10 | 15 | 20 | 25 | 30 | 50 | 100 | |
| 15 | 7,980 | 9,720 | 11,280 | 12,550 | 13,800 | 17,820 | 25,140 |
| 20 | 9,180 | 11,180 | 13,080 | 14,520 | 15,900 | 20,520 | 29,040 |
| 25 | 10,260 | 12,600 | 14,520 | 16,260 | 17,820 | 22,980 | 32,460 |
| 30 | 11,280 | 13,800 | 15,900 | 17,825 | 19,500 | 25,140 | 35,580 |
| 35 | 12,180 | 14,880 | 17,160 | 19,200 | 21,060 | 27,180 | 38,400 |
| 40 | 13,020 | 15,900 | 18,360 | 20,520 | 22,500 | 29,040 | 40,980 |
In Fig. 163 is illustrated the form of chimney that is often used for the ventilation of dwellings. This is built with three flues. The flue to the left—marked A at the top—is intended to carry away the smoke and gases from the kitchen range. The flue to the right is that to which is connected the smoke pipe from the furnace. The flue in the middle marked B is for ventilation. Occupying as it does the space between the other two, it is kept warm by the heat of the other flues and the draft is thus increased. Openings to the flue are shown in the different floors at the points R and S. The openings are furnished with registers which may be regulated to suit the weather conditions.
The dimensions of such a flue may be calculated by the formula given or the area may be taken from the table to correspond with required conditions. In all cases flues should be made ample in size, as they must often do their maximum work under the poorest conditions for the production of good draft.
The amount of air discharged from the flue as given in the table is due to the gravitational effect alone. The suction produced by the wind adds in a very large degree to the amount of air discharged. The quantity of air that will flow from a 30-foot flue, by reason of the suction of the wind, blowing 7 miles per hour is equal to the same flue working by gravity with a temperature difference of 20°. With a wind velocity of 7 miles per hour and a temperature as given, the capacity of the flue is doubled. It is easy, therefore, to understand why the rate at which fires burn is so greatly increased by high winds. At the time of very high winds, a chimney flue will carry away three and even four times the volume discharged at the time of atmospheric calm.
Cost of Ventilation.
—The cost of good ventilation is often looked upon as prohibitive, because of the expense in heat necessary to keep the inside atmosphere at standard purity. Cost of ventilation is determined by analysis of the known conditions and calculations made of the amount of extra heat necessary to warm the greater volume of air.
The common practice of estimating the quantity of heat used in any form of heating or ventilation is by reference to the B.t.u. used in producing the desired condition. This unit, as has already been stated, is the amount of heat necessary to change a pound of water, 1°F.