Air Flow

Selection of AIR FLOW to cool an apparatus
P : represents the power to be dissipated (in watts)
Δt : represents the temperature difference between internal temperature of the equipment and ambient temperature
Q : represents the air flow of the fan = P(w)1.2XΔt L/s
THE AIR FLOW IS DETERMINED BY THE PROJECTION ON THE GRAPH BELOW FROM THE INTERSECTION POINT OF THE LINES AND Δt ON THE AIR FLOW SCALE.
Example given a power to dissipate of 400 W and t of 10℃
REQUIRED AIR FLOW TO COOL = 34 L/s = 72 CFM
Conversion Chart
(CFM = L/s x 0.0353 x 60)
L/s 1 2 3 4 5 6 7 8 9 10 15 20
CFM 2.2 4.24 6.35 8.48 10.6 12.7 14.8 16.9 19 21 31.8 42.2
CMM 0.06 0.12 0.18 0.24 0.3 0.36 0.42 0.48 0.54 0.59 0.9 1.2
CMH 3.73 7.2 10.8 14.4 18 21.6 25.1 28.7 32.2 35.6 54 72
L/s 25 30 35 40 45 50 60 70 80 90 100
CFM 53 63.5 74 85 95 106 127 148 169 190 212
CMM 1.5 1.8 2.09 2.4 2.69 3 3.59 4.19 4.78 5.37 6
CMH 90 108 126 144 161 180 216 251 287 322 360
CONVERSION°C = °F-321.8F° = °C X 1.8 + 32°F
C 10 15 20 25 30 35 40 45 50 60 70 75
F 50 68 77 86 95 104 113 122 140 149 158 167


Air Measurement
Determination of the air performance curves is obtained by using the double chamber method based on AMCA standard. The difference between the pressures before and after the nozzle (differential pressure Pn) is measured so as to obtain the air flow at the nozzle and the different pressure between those in the twochambers (static pressure Ps). The air flow is calculated from the differential pressure by using equation (A). The auxiliary blower enables to cancel out the aerodynamic resistance.
Q = 60 AV…..(A)
where
Q: Air flow rate (m3/ min)
A: Nozzle sectional area = π D2 / 4
V: Average flow velocity from nozzle
r: Specific gravity (kg / m3) of air (r = 1.2 kg / m3 at 20℃, 1atm)
g: Gravitational acceleration = 9.8 m/s2
Pn: Differential pressure mm H2O)
Ps: Static pressure (mm H2O)


Table of Temperature Vs. Air Flow
P(KW) 0.1 0.25 0.5 1.0 1.5 20 2.5 3.0 4.0 5.0
ATc°C ATt°F
50 122 3.5 9 18 35 53 70 88 105 141 176
45 113 4 10 20 39 59 78 98 117 156 195
40 104 6.6 11 22 44 66 88 110 132 176 220
35 95 7.5 12.5 25 50 75 100 125 151 201 251
30 86 9 15 29 59 88 117 146 176 234 293
25 77 11 18 35 75 105 141 176 211 281 351
20 68 13 22 44 88 132 176 220 264 351 439
15 59 18 29 59 117 176 234 293 351 469 586
10 50 26 44 88 176 264 351 439 527 704 879
5 41 53 88 176 351 527 704 879 1055 1406 1758

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