$e_k$	specific kinetic energy
$e_p$	specific potential energy
F	Lagrangian function
$\eta$	isentropic efficiency
${\eta }^{*}$	geometric mean of all isentropic efficiencies
i	pressure level/number of stage
k	polytropic constant
$\lambda$	lagrange multiplier/volumetric efficiency
n	total number of compression stages/r.p.m. revolutions per minute
P	pressure
$\psi$	specific dissipation
q	specific heat
r	compression ratio
$r_t$	total compression ratio
R	ideal gas constant
T	absolute temperature
v	specific volume
w	specific work
$w^R$	specific reversible work
W	total work
Reciprocating compressors:
a	ratio between clearance and swept volumes
$\lambda$	volumetric efficiency
n	r.p.m. revolutions per minute
$N_{cyl}$	number of cylinders
$V_{asp}$	aspirated volume
$V_{piston}$	volume swept by piston