Simple Duct Design Method A. Fundamental Knowledge 1. Pressure Classification Static pressure – pressure acting vertically upon the duct wall surface, PS mmwg; Dynamic pressure – pressure generated by air velocity, Pd mmwg; Total pressure – sum of static pressure and dynamic pressure, Pt mmwg; Pd = (γ/2g)• V2 = (V/4.05)2 Pt = PS + Pd where, γ – specific gravity of air, kg/m3 V – air velocity, m/s
Static静压 pressure 气流 Air flow
动压
Total全压 pressure
Dynamic pressure
Duct 风管
细孔 Pore
Ps
Pt
Pd
Pt 型管 U-pipe
2. Air volume Common unit for air volume CMM — m3/min CMH — m3/H CFM — ft3/min
m3/min
m3/H
ft3/min
1
60
35.32
0.0167
1
0.5881
0.0283
1.6989
1
Air velocity in the duct can be measured in the following two ways: • Measure dynamic pressure with pitot, and then calculate out the air velocity; • Measure air velocity directly with a hot-wire anemometer; 3. Relations between frequency and the rotation speed of motor: Theoretically, the rotation speed of motor and the frequency of power supply have following relations: Rotation speed = frequency
╳
2
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60/number of poles, (rpm)