**
**
d = pipe internal diameter (in.)

D = pipe internal diameter (ft.)

f = Chen friction factor

fD
= Darcy friction factor

G = mass flow rate (lb/h)

*
**k* = ratio of specific heat capacities (Cp/Cv)

K = resistance coefficient due to friction loss

LEQ
= equivalent length of piping system (ft.)

Mw
= molecular weight of fluid (lb/lb-mol)

NRe
= Reynolds Number

P1
= upstream pressure (psia)

P2
= downstream pressure (psia)

t = fluid temperature (°F)

V = fluid velocity (ft/s)

Vs
= fluid sonic or critical velocity (ft/s)

yi
= mole fraction of component *i*

*
**
**r* = fluid density (lb/ft3)

µ = fluid viscosity (cP)

e = absolute pipe roughness (ft)