6.1 Basic properties
Fluids are classified into liquids, which are virtually incompressible, and
gases, which are compressible. A fluid consists of a collection of molecules
in constant motion: a liquid adopts the shape of a vessel containing it, while
a gas expands to fill any container in which it is placed. Some basic fluid
relationships are given in Table 6.1.
Table 6.1 Basic fluid relationships
Mass per unit volume. Units kg/m3 (lb/in3)
Specific gravity, s
Ratio of density to that of water, i.e. s = ρ/ρwater
Specific volume, v
Reciprocal of density, i.e. v = 1/ρ. Units m3/kg (in3/lb)
Dynamic viscosity, µ
A force per unit area or shear stress of a fluid. Units
Kinematic viscosity, ν A ratio of dynamic viscosity to density, i.e. ν = µ/ρ.
Units m2/s (ft2/s)
A perfect (or ‘ideal’) gas is one that follows Boyle’s/Charles’s law
pv = RT
Engineers’ Guide to Rotating Equipment
p = pressure of the gas
v = specific volume
T = absolute temperature
R = the universal gas constant
Although no actual gases follow this law totally, the behaviour of most gases
at temperatures well above their liquification temperature will approximate
to it and so they can be considered as a perfect gas.
Changes of state
When a perfect gas changes state its behaviour approximates to
pvn = constant
where n is known as the polytropic exponent.
The four main changes of state relevant to rotating equipment are:
isothermal, adiabatic, polytropic, and isobaric.
The extent to which a fluid can be compressed in volume is expressed using
the compressibility coefficient β.
∆v = change in volume
v = initial volume
∆p = change in pressure
K = bulk modulus
a = the velocity of propagation of a pressure wave in the fluid.
Fluid statics is the study of fluids that are at rest (i.e. not flowing) relative