512
14 Radial Compressors
When the blades are inclined more backward, the rotor work decreases. Weak backward sweep is thus favourable with regard to the realisable pressure ratio, but leads
to a high outlet velocity of the impeller. So, a diffuser becomes necessary. A diffuser
with vanes is sketched. Other diffuser shapes are possible and will be discussed
in a later section. A scroll-shaped collector comes around the diffuser. Diffusers
inherently feature great losses (see later discussion). Compressors with slightly
backward swept blades thus have a lower efficiency than compressors with more
inclined ones. Rotors with forward inclined blades are never used.
14.1.3 Comparison Between Radial and Axial Compressors
The fundamental difference between both types is that radial compressors, for equal
external diameters, handle a lower flow rate, but realise a larger stage work. With
an axial compressor, 75 % of the frontal area may be available for through-flow (at
a 0.5 radius ratio) with an axial velocity up to about 50 % of the blade speed on the
average radius (about 40 % of the tip speed). A flow coefficient defined with the
frontal area and the peripheral speed thus may reach about 0.30. The flow area with
a radial compressor maximally covers about 45 % of the frontal area with a throughflow velocity maximally about 50 % of the blade speed at the inlet tip (about 35 % of
the peripheral speed of the rotor). The flow coefficient, according to the above definition, is maximally about 0.15. Shrouded rotors (Fig. 14.1) often feature a much
lower flow coefficient. The flow factor varies between about 0.02 to 0.15, with the
largest values obtained by open rotors.
With an axial compressor, the work coefficient on the average radius is at maximum about 0.4 ( ψ = ∆W/u
2
). Related to tip speed it is maximally about 0.3. With a
radial compressor, the work coefficient may reach until about 0.9 when applying
radial end blades.
The peripheral speed is comparable for both machines; maximally until about
400–450 m/s with open rotors. Strength does not strictly limit this speed. With titanium alloys in axial compressors and high-quality aluminium alloys in open rotor
centrifugal compressors, 550 m/s is achievable with both types (with steel, it is
about 400–450 m/s). A speed limitation normally follows from choking prevention. At 400 m/s peripheral speed, the relative velocity on the average radius at the
rotor inlet of an axial compressor is slightly supersonic and the absolute velocity
at the rotor outlet of a radial compressor is slightly supersonic too. With shrouded
rotors made of steel, the maximum peripheral speed is limited by strength to about
350 m/s.
The efficiency of an axial compressor is significantly better than that of a radial
one: the isentropic total-to-total efficiency is about 0.92 at the maximum compared
to maximally about 0.84 for a centrifugal compressor with open rotor and slightly
backswept blades. The efficiency improves with more backward sweep and closed
2 2u
W
u v
D =
14 Radial Compressors
When the blades are inclined more backward, the rotor work decreases. Weak backward sweep is thus favourable with regard to the realisable pressure ratio, but leads
to a high outlet velocity of the impeller. So, a diffuser becomes necessary. A diffuser
with vanes is sketched. Other diffuser shapes are possible and will be discussed
in a later section. A scroll-shaped collector comes around the diffuser. Diffusers
inherently feature great losses (see later discussion). Compressors with slightly
backward swept blades thus have a lower efficiency than compressors with more
inclined ones. Rotors with forward inclined blades are never used.
14.1.3 Comparison Between Radial and Axial Compressors
The fundamental difference between both types is that radial compressors, for equal
external diameters, handle a lower flow rate, but realise a larger stage work. With
an axial compressor, 75 % of the frontal area may be available for through-flow (at
a 0.5 radius ratio) with an axial velocity up to about 50 % of the blade speed on the
average radius (about 40 % of the tip speed). A flow coefficient defined with the
frontal area and the peripheral speed thus may reach about 0.30. The flow area with
a radial compressor maximally covers about 45 % of the frontal area with a throughflow velocity maximally about 50 % of the blade speed at the inlet tip (about 35 % of
the peripheral speed of the rotor). The flow coefficient, according to the above definition, is maximally about 0.15. Shrouded rotors (Fig. 14.1) often feature a much
lower flow coefficient. The flow factor varies between about 0.02 to 0.15, with the
largest values obtained by open rotors.
With an axial compressor, the work coefficient on the average radius is at maximum about 0.4 ( ψ = ∆W/u
2
). Related to tip speed it is maximally about 0.3. With a
radial compressor, the work coefficient may reach until about 0.9 when applying
radial end blades.
The peripheral speed is comparable for both machines; maximally until about
400–450 m/s with open rotors. Strength does not strictly limit this speed. With titanium alloys in axial compressors and high-quality aluminium alloys in open rotor
centrifugal compressors, 550 m/s is achievable with both types (with steel, it is
about 400–450 m/s). A speed limitation normally follows from choking prevention. At 400 m/s peripheral speed, the relative velocity on the average radius at the
rotor inlet of an axial compressor is slightly supersonic and the absolute velocity
at the rotor outlet of a radial compressor is slightly supersonic too. With shrouded
rotors made of steel, the maximum peripheral speed is limited by strength to about
350 m/s.
The efficiency of an axial compressor is significantly better than that of a radial
one: the isentropic total-to-total efficiency is about 0.92 at the maximum compared
to maximally about 0.84 for a centrifugal compressor with open rotor and slightly
backswept blades. The efficiency improves with more backward sweep and closed
2 2u
W
u v
D =