Page 61 - Compression Machinery for Oil and Gas

52 SECTION II Types of Equipment

Another requirement is, that mass cannot appear or disappear, thus for any
flow from a point 1 to a point 2:
_ m 1 ¼ ρ Q 1 ¼ _ m 2 ¼ ρ Q 2
2
1
ρ Q ¼ ρ c A
This requirement is valid for compressible and incompressible flows, with
the caveat that for compressible flows the density is a function of pressure and
temperatures, and thus ultimately a function of the velocity.
These two concepts explain the working principles of the vanes and dif-
fusers used. Due to the requirement for mass conservation, any flow channel
that has a wider flow area at its inlet and a smaller flow area at its exit will
require a velocity increase from inlet to exit. If no energy is introduced to
the system, the Bernoulli’s law requires a drop in static pressure
(Fig. 3.23A). Examples for flow channels like this are turbine blades and noz-
zles, inlet vanes in compressors, and others (Fig. 3.23B). Conversely, any flow
channel that has a smaller flow area A at its inlet and a larger flow area at its exit
will require a velocity decrease from inlet to exit. If no energy is introduced to
the system, the Bernoulli’s law requires an increase in static pressure
(Fig. 3.23C). Examples for flow channels like this are vaned or vaneless dif-
fusers, flow channels in impellers, rotor and stator blades of axial compressors
volutes, and others (Fig. 3.23D).

Velocity
Velocity
Pressure
Pressure
Mass flow
Mass flow
(A)

Compressor
blades
Turbine
blades
(B) (D)

Velocity
Velocity
Pressure
Pressure
Mass flow
Mass flow
(C)
FIG. 3.23 Acceleration and diffusion [3].