513
14.1 Construction Forms and Applications
rotors to maximally about 0.87. The mean reason is the very inhomogeneous flow
at the rotor outlet of a radial compressor, caused by turbulence segregation by the
Coriolis force, as with radial pumps and fans (Sect. 3.3.4 in Chap. 3). The Coriolis
force drives high-energy cores within the turbulent motion from the suction side to
the pressure side. This leads to a lowered velocity level near the suction side (see
Sect. 14.5.1). The inhomogeneity of the rotor outlet flow causes a significant mixing loss downstream of the rotor. Further, rotors with radial end blades or with slight
backward sweep, i.e. with machines intended for high work, require a significant
velocity reduction in a diffuser downstream of the rotor. A strong deceleration inevitably creates large loss. A third reason is rather important leakage loss with an
open impeller.
14.1.4 Examples of Radial Compressors
Some industrial centrifugal compressors resemble centrifugal pumps. Figure 14.1
is an example. Rotors have backward swept blades, as pumps do. After each rotor,
there is a ring-shaped vaneless chamber, functioning as a diffuser, followed by a
chamber with return channels leading the flow to the next stage. After the last stage,
the gas is collected with a scroll. Work per rotor is not very high in compressors as
in Fig. 14.1. The stage work coefficient is maximally about 0.5. So, the rotor outlet
velocity is maximally about 50 % of the peripheral speed. Therefore, a vaneless annular chamber is sufficient as diffuser (see Sect. 14.5). A compressor as in Fig. 14.1
is intended to allow significant flow rate variation. The flow range is limited by
rotating stall or surge at low flow rate and choking at high flow rate (similar as with
axial compressors). The operating range will be discussed in Sect. 14.6. With moderate backward sweep of the blades and vaneless diffusers, the design flow rate has
a large margin against the limiting phenomena.
The air compressor shown in Fig. 14.4 features a first open rotor stage (but there
are no splitter blades), followed by three shrouded ones. With this type, the first
stage has an open rotor because the volume flow rate is the largest in the first stage.
The compressor shown in Fig. 14.4 is intended for a relatively high flow rate. Similar machines exist with only shrouded rotors, intended for lower flow rates. The air
is cooled after each stage (called an isothermal compressor). Coolers are integrated
into the machine. The supply to the coolers requires a rather special vaned chamber,
functioning as diffuser. A simpler vaned chamber guides the flow from the outlet
of a heat exchanger to the inlet of a rotor. Figure 14.5 shows cross sections of the
chambers. The successive radial stages feature increasingly smaller through-flow
areas. The rotors have moderately backswept blades, resulting in a higher work
coefficient than typical for pump rotors. This implies the need for more blades.
Typically, rotors have 16–20 blades (see also Fig. 14.1). Construction mostly encompasses milling of the back disc together with the blades from a massive block,
lathing of the shroud and welding of the shroud onto the blades. As compressor rotors run much faster than pump rotors, finishing must be far more accurate. These
14.1 Construction Forms and Applications
rotors to maximally about 0.87. The mean reason is the very inhomogeneous flow
at the rotor outlet of a radial compressor, caused by turbulence segregation by the
Coriolis force, as with radial pumps and fans (Sect. 3.3.4 in Chap. 3). The Coriolis
force drives high-energy cores within the turbulent motion from the suction side to
the pressure side. This leads to a lowered velocity level near the suction side (see
Sect. 14.5.1). The inhomogeneity of the rotor outlet flow causes a significant mixing loss downstream of the rotor. Further, rotors with radial end blades or with slight
backward sweep, i.e. with machines intended for high work, require a significant
velocity reduction in a diffuser downstream of the rotor. A strong deceleration inevitably creates large loss. A third reason is rather important leakage loss with an
open impeller.
14.1.4 Examples of Radial Compressors
Some industrial centrifugal compressors resemble centrifugal pumps. Figure 14.1
is an example. Rotors have backward swept blades, as pumps do. After each rotor,
there is a ring-shaped vaneless chamber, functioning as a diffuser, followed by a
chamber with return channels leading the flow to the next stage. After the last stage,
the gas is collected with a scroll. Work per rotor is not very high in compressors as
in Fig. 14.1. The stage work coefficient is maximally about 0.5. So, the rotor outlet
velocity is maximally about 50 % of the peripheral speed. Therefore, a vaneless annular chamber is sufficient as diffuser (see Sect. 14.5). A compressor as in Fig. 14.1
is intended to allow significant flow rate variation. The flow range is limited by
rotating stall or surge at low flow rate and choking at high flow rate (similar as with
axial compressors). The operating range will be discussed in Sect. 14.6. With moderate backward sweep of the blades and vaneless diffusers, the design flow rate has
a large margin against the limiting phenomena.
The air compressor shown in Fig. 14.4 features a first open rotor stage (but there
are no splitter blades), followed by three shrouded ones. With this type, the first
stage has an open rotor because the volume flow rate is the largest in the first stage.
The compressor shown in Fig. 14.4 is intended for a relatively high flow rate. Similar machines exist with only shrouded rotors, intended for lower flow rates. The air
is cooled after each stage (called an isothermal compressor). Coolers are integrated
into the machine. The supply to the coolers requires a rather special vaned chamber,
functioning as diffuser. A simpler vaned chamber guides the flow from the outlet
of a heat exchanger to the inlet of a rotor. Figure 14.5 shows cross sections of the
chambers. The successive radial stages feature increasingly smaller through-flow
areas. The rotors have moderately backswept blades, resulting in a higher work
coefficient than typical for pump rotors. This implies the need for more blades.
Typically, rotors have 16–20 blades (see also Fig. 14.1). Construction mostly encompasses milling of the back disc together with the blades from a massive block,
lathing of the shroud and welding of the shroud onto the blades. As compressor rotors run much faster than pump rotors, finishing must be far more accurate. These