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Unwin Gas Change of Pressure and Calculator and Formulas

Fluids Engineering and Design Resources

Unwin Gas Change of Pressure and Calculator and Formulas

The Unwin formula has been successfully used in steam piping calculations for many years. It is quite satisfactory for most purposes. However, at high flow rates, the pressure drops predicted by the Unwin formula are found to be higher than actual values.

The Unwin formula in USCS (Imperial) units is as follows:

Equation 1
ΔP = ( 3.625 x 10^-8 ) ν L W² ( 1 + 3.6 / D) / D⁵

Where:

ΔP = pressure drop, psig
W = steam flow rate, lb/h
L = pipe length, ft
D = pipe inside diameter, in
ν = specific volume, ft³/ lb

In SI units, the Unwin formula is as follows:

Equation 2
ΔP = 675.2723 ν L W² ( 1 + 91.44 / D) / D⁵

Where:

ΔP = pressure drop, kPa
W = steam flow rate, kg/hr
L = pipe length, m
D = pipe inside diameter, mm
ν = specific volume, m³/ kg

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Reference:

• Piping Calculations Manual,
  E. Shashi Menon
  SYSTEK Technologies, Inc