HVAC Unit Conversion Factors (incl. Pipe Capacities, Fan Laws and Electrical Units)

Electrical units Volts

(E or V) = Amperes (I) x Resistance (R or Ω)

Conductance = Amperes ÷ Volts

Parallel Ω = Rt = 1 ÷ ( 1/R1 + 1/R2 + 1/R3…)

Frequency (hertz) = 1 ÷ Time (seconds)

Power (P) = Volts x Amperes = Watts (W)

Volts x Amperes ÷ 1000 = Kilowatts (KW)

Volts x Amperes ÷ 746 = Horsepower (HP)

Horsepower = .746KW = torque x RPM ÷ 5250

KW Hours = HP x .746 x run hours ÷ efficiency

Cost for time = KW Hours consumed x KW hr. rate

Pipe capacities

Area of a circle = diameter squared x .7854

Circumference = diameter x 3.1416

2 x pipe diameter increases capacity four times

Line capacity (gal.) = area (sq. ) x L ( ) ÷ 231

Velocity

Feet/sec. = 0.408 x GPM ÷ pipe dia. ( ) squared

Area pipe = GPM x 19.2 = cu. ft. H2O x 144 ÷ (ft/min)

Velocity (ft./min.) = 4004 P Velocity pres. (” H2O)

Velocity pressure (” H2O) = (Total-Static) pressure (”H2O)

Fan Laws

CFM varies in direct proportion to RPM

CFM2 ÷ CFM1 = RPM2 ÷ RPM1

Static pressure varies as square root of RPM

SP2 ÷ SP1 = (RPM2 ÷ RPM1)²

Horsepower varies as the cube of the RPM

HP2 ÷ HP1 = (RPM2 ÷ RPM1)³

Fan Static Pressure = SP (outlet) + TP (inlet)

(SP = Static pressure) (TP = Total pressure)

Fan Total pressure = TP (outlet) - TP (inlet)

Conversion Factors