Power Required to Drive Cone Fans.-The following table shows the approximate horsepower of motors required to drive cone fans at stated speeds. The efficiency of the fan is taken as 40 per cent.; efficiency of motor, 80 per cent. POWER REQUIRED TO DRIVE CONE FANS.

The horsepower required at other speeds than those stated will be proportional to the cube of the speeds. The horsepower in the blast, which is the basis of this table, was computed for an area just within the capacity of the fan, that is, the maximum area of opening through which the fan will discharge air at a velocity corresponding to the speed of the tips of the blades.

Properly designed motors of the medium and larger sizes

should give over 80 per cent. efficiency. The approximate horsepower will therefore be less than stated in column 3. For example, a motor with 85 per cent. efficiency need be only, or about 94 per cent. of approximate horsepower stated.

Capacity of Centrifugal Fans.--The capacity of centrifugal fans under usual working conditions, expressed in cubic feet of air delivered, may be obtained approximately by mu!tiplying the cube of the diameter of the fan wheel, in feet, by the number of revolutions, and by a coefficient having a value, depending on the back pressure, of from .4, with a 1-oz. back pressure, to .6 for single-inlet fans under free discharge, a coefficient of .5 being used with a back pressure of 1 in. of water column.

The approximate capacities of blowers under ordinary working conditions in connection with heaters and distributing ducts, and the approximate indicated horsepower of engines, or nominal horsepower of motors, to drive them are given in the following table:

CAPACITY OF CENTRIFUGAL FANS.

Diam. of Wheel.
Feet.
Width of Housing.

18

60

390 4,800 1.9 435 5,300 2.6 475 5,800 3.4 21 330 6,200 2.4 370 7,000 3.4 400 7,600 4.4 24 290 8,000 3.0 325 9,000 4.3 360 9,800 5.7 28 260 11,000 4.2 290 12,000 5.2 320 13,500 7.7 32 230 13,000 5.0 260 14,500 7.0 36 210 17,000 6.6 240 19,000 9.3 42 195 23,000 8.8 220 25,000 12.4 170 30.000 11.8 185 34,000 16.4 48 150 33.000 13.0 165 37.000 18.1 54 130 42,000 16.6 145 47,000 23.2 115 54,000 20.9 130 60,000 29.2

285 16,000 9.3

260 21,000 12.2 240 28,000 16.1

200 36,000 20.0

180 41.000 23.8

160 52,000 30.6

140 66,000 38.4

The speeds given in the table, "Capacity of Centrifugal Fans," cover the range ordinarily used in heating and ventilating work. The velocities through properly proportioned fan outlets are approximately 1,830, 2,030, and 2,260 ft. per min. for the several columns in the order stated.

The horsepowers are based on efficiencies of 40 per cent. for the fans and 80 per cent. for the motors. With motors of higher efficiency, the horsepower will be lower than stated in the table; that is, a motor of 88 per cent. efficiency need be only gg, or about 90 per cent. of the horsepower given. An inspection of the table will show the economy of power gained by running fans at relatively low speeds; for example, to deliver 28,000 cu. ft. of air per min. a 6' fan at 240 rev. may be used, requiring 16.1 H.P. Column four shows that 30,000 cu. ft. may be delivered by a 7′ fan at 170 rev. with a consumption of only 11.8 H. P. The difference in cost between the two sizes would not be great and would soon be made up by the saving in power. As a rule, it is not wise to run fans slower than the speeds stated in the third column.

The free-delivery capacity of cased centrifugal fans having wheels whose width at the blade tip is one-half their diameter, may be found, approximately, by multiplying the coefficient .000303 by the product obtained by multiplying the cube of the fan-wheel diameter, in inches, by the number of revolutions per minute. Expressed as a formula, in which

C= capacity, in cubic feet per minute;

D = diameter of fan wheel, in inches;

R =

we have C

revolutions per minute,

.000303 D3 R, from which it is evident that

In order to render capacity calculations for fan wheels of different widths of blades and diameters easy, a table of factors made up from a table published by Mr. M. C. Huyett, is presented here. To obtain the capacity of centrifugal cased fans by the aid of this table, multiply the tabulated factor

corresponding to the diameter and width of fan wheel, in inches, by the number of revolutions of the fan per minute. The product will be the free-delivery capacity of the fan, in cubic feet per minute. Thus, a fan 100 in. dia. and 38 in. wide, at 150 rev. per min. will deliver 230 × 150 = 34,500 cu. ft. of air per min.

43.6 48.0 52.3 56.6

61.0

65.4

69.8

74.1 78.5 82.8

65.2

119.8

51.3 55.9 60.6 69.9 74.5 | 54.5 59.5 64.5 69.5 74.5 79.5 58.2 63.4 68.6 73.9 79.2 84.5 67.2 72.9 78.5 84.0 89.6 95.2 100.8 71.3 77.2 83.2 89.2 95.0 75.3 81.6 87.9 94.2 100.5 106.7 113.0 86.2 92.8 99.5 106.0 112.6 119.3 91.0 98.0 105.0 112.0 119.0 126.0 133.0 96.0 103.1 110.6 118.0 125,3 132.8 140.1 108.6 116.4 124.1 132.0 139.6 147.5 114.2 122.4 130.5 138.6 146.8 128.3 136.8 145.5 154.0 134.3 143.2 152.1 161.1 170.1 140.5 150.0 159,3 168.7 178.1

79.2

83.9

88.5

84.5

89.4

94.3

89.7 95.0

100.0

106.4

101.0 106.9

112.9

119.3

126.0

155.0

162.6

80

155.2 163.0

82

163.2 171.4

84

86

88

90

92

94

214.0 224.4

96

223.6 234.6

98

233.5

100

242.5 254.5

102

104

106

108

170.7 178.5 186.2 179.5 187.8 196.0 204,0 171.1 179.7 188.2 196.8 205.3 214.0 179.0 188.0 197.0 206.0 215.0 224.0 187.5 196.8 206.1 215.5 225.0 234.3 243.6 253.0 196.0 206.0 215.8 225.8 235.5 245.0 255.0 264.6 274.3 284.0 205.0 215.5 225.8 236.0 246.2 256.3 266.4 277.0 287.0 297.5 235,4 246.0 256.6 267.5 278.0 289.0 299.5 310.0 246.0 257.0 268.2 279.6 290.3 302.0 313.0 245.0 256.5 268.0 280.0 292.0 303.5 315,0 327.0 266.6 278.8 291.0 303.0 315,0 327.0 339.5 253.0 265.0 277.5 290.0 303.0 315.5 328.0 341.0 353.0 262.4 275.5 288.6 302.0 315.9 328.0 341.0 354.0 367.0 272.5 286.5 300.0 313.5 327.0 341.0 354.5 368.0 382.0 395.0 283.0 297.5 311.5 326.0 340.0 354.0 368.0 382.0 396.5 411.0

Horsepower Required to Drive Blowers.-The power required to drive an encased fan, or blower, may be found by multi