78.54

factor 3.4, the required area will be, approximately, 3.4 =267 sq. in., and the corresponding velocity of the flow will be 3,670

3.4

1,079 ft. per min. The frictional resistance offered by the piping to the passage of air therein depends on the extent of the surface with which the air comes in contact, being directly proportional to the length and inversely to the diameter of the pipe, also varying as the square of the velocity of the moving air.

FACTORS FOR DUCT AND FLUE AREA INCREASE.

With carefully made galvanized-iron pipe, erected with all internal laps extending in the direction of the air movement,

practical use may be made of the following formulas, in which

p loss of pressure, in ounces per square inch;

The horsepower lost in friction in a pipe 100 ft. long may be expressed by the formula

wherein a is area of pipe, in square inches, and the other letters have the same meaning as in the previous formulas.

Resistance of Bends.-The resistance caused by bends in the piping may be approximately determined by using the

factors given in the table "Frictional Resistance of Elbows," which gives the resistance of elbows in equivalent lengths of straight pipe, expressed in terms of the pipe diameter. For example, the resistance offered by an elbow having an inner radius of 2 diameters will, approximately, be the same as that of a straight piece of the pipe 28 diameters in length. The necessity of avoiding short-radius bends is made apparent by the tabulated figures, which show that the resistance increases rapidly as the inner radius of the bend decreases.

Air Velocities.-Experience has shown that for flues in the walls of such buildings as schools, churches, office buildings etc., a flue velocity from 500 to 800 ft. per min., with a corresponding velocity through registers into rooms of from 250 to 600 ft. per min., and a floor-register velocity of 300 ft. per min., are desirable. Velocities of from 1,800 to 2,500 ft. per min. are customary in the main air duct. The branch ducts to flues should be figured at velocities of from 1,000 to 1,500 ft. per min., depending on their distances from the fan. The intermediate main ducts should be proportioned for velocities gradually reducing from that in the main duct near the fan to 1,000 to 1,500 ft. at the extreme ends, the extreme end being taken as a branch duct. Except in cases of great lengths of pipe and a multiplying of elbows and turns, the lesser velocity of the extremes given above should be used. The velocities for factory buildings may be practically twice those given above.

Capacity of Natural-Draft Vent Ducts.-The quantity of air, in cubic feet per minute, discharged by a ventilating duct 1 sq. ft. in area, under different conditions, with an external air temperature of 320 F., is given in the table "VentilatingDuct Capacity." The quantity of air discharged by ventilating flues having an area other than 1 sq. ft. may be found by multiplying the discharge taken from the table by the area of the ventilating flue in square feet.

The amount of air discharged when the external temperature is above 320 may be obtained approximately by using the tabulated factors corresponding to the difference between the internal and external temperatures.

VENTILATING-DUCT CAPACITY.

Excess of Temperature of Air in Vent Duct, Above
That of External Air.

5o

10°

100 150

20° 25° 30° 50° 100°

150°

Weight of Piping.-In estimating the weight of the round, galvanized-iron pipes that are commonly used for the ducts and flues of blower systems, the accompanying table, by the E. M. Link Machinery Company, will be found convenient, as the weights therein given represent the weight of the pipes made up complete, proper allowance having been made for laps and trimmings. The elbows have an inner radius equal to the diameter of the pipe. Owing to the variations in the thickness of the material, galvanizing, etc., the weights given in the table are not absolutely exact, but are sufficiently accurate for all practical purposes.

The weight of rectangular pipes is best obtained by first finding the surface of the pipe, in square feet, and multiplying this by the weight of the iron, in pounds per square foot, corresponding to the gauge number, as given in the table "Weight of Galvanized Sheet Iron."

Ducts and flues of galvanized iron should not, as a rule, be lighter than the following gauges: No. 28, for pipes under 8 in. in diameter; No. 26, for pipes 8 in. to 12 in.; No. 24, for pipes 13 in. to 18 in.; No. 23, for pipes 19 in. to 24 in.; No. 22, for pipes 25 in. to 30 in.; No. 20, for pipes 31 in. to 40 in.; No. 18, for pipes 41 in. to 50 in.; No. 16, for pipes 51 in. to 60 in.