Design of Centrifugal Compressor-2

• •
PEMP RMD510
Shroud relative velocity at impeller inlet, Wsh,1 should not be very high . W2 0.75 de Haller ratio in impeller, Wsh,1 (to minimise flow separation in impeller passages and consequently
Rotor Outlet Velocity Diagram: Let us choose c,2 = 60º (suggested)
Cu , 2,ac tan 2 1 u2 tan c , 2 w
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Number of Blades
Choose even number of blades, so that half may be designated ‘Splitter Blades’. Taking a g intermediate te ed ate loading oad g from range of number of blades graph, Z = 20 (10 being splitter blades)
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Number of Blades
An A
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i nt quantity i is i the h ratio i of f outlet l tangential i l flow fl velocity l i to blade speed, Cu,2,ac / u2. If there is no swirl in the flow at rotor inlet inlet, this ratio becomes loading coefficient, .
cos β2 Cu, u 2 ,ac ac li f t 1 σ w slipfactor Cu,2 ,tl Z 0.7
Z : number of blades
2 : Angle between radial direction and

tangent to the rotor blade at the periphery.
Inlet velocity diagram (Axial entry)
Outlet velocity diagram (Radial exit)
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Limiting Design Conditions
Number of blades at periphery
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Design Data
Given data: Atmospheric Air Mass flow rate = 1 bar, 300 K = 0.75 kg/s
and, preferably, should be chosen to be 60° to 65° at design point.
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Limiting Design Conditions
• The relative velocity ratio is beneficially increased for backswept
to minimise losses)
• c, g effect on the stability y of a radial vaneless diffuser. c 2 has strong
As a general rule, for vaneless and vaned diffusers
αc,,2 70
Therefore
1 η R p,c p cp 0 ,2 1 u2Cu,2 u1Cu,1 g c c pT0 ,1 p0 ,1
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PEMP RMD510
Design of Centrifugal Compressor Compressor-2 2
Session delivered by: Prof Q. Prof. Q H. H Nagpurwala
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Max. rotational speed of a backswept impeller should be lower than for a radial impeller of the same tip diameter to contain stress levels
• Blade sweep back angles of up to 50 50º can be used, used if there is no call for
Cu,2 ,ac tan β2 1 ψ u2 tan αc,2 σ w
The
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number of blades and the blade angle at exit are not totally independent. independent
those of straight radial vanes (zero back sweep).
• However, , there is a reduction in max. p possible work p per stage g due to

Reduction in work coefficient (~0.9 for radial impellers to 0.5 or less for backswept impellers)
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Session Objectives
To introduce the delegates g to
PEMP RMD510
• • •
the procedure for aerodynamic design of centrifugal compressors based on specific speed numerical simulation of a typical centrifugal compressor presentation of compressor performance and flow behaviour through impeller blade passages
PEMP RMD510
2 = Blade angle at outlet is 45
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Slip Factor and c,2
Slip Factor:
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Cu , 2,ac cos 45 w 0.8967 1 0.7 20 Cu , 2,tl
Notations
C1 Wsh,1 uhb,1 ush, h1 u2 Cu,2 W2 : Absolute Velocity at entry : relative velocity at shroud at entry : Peripheral velocity at hub : Peripheral velocity at shroud : Peripheral velocity at exit : Whirl Whi l velocity l it at t exit it : Relative velocity at exit
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Schematic of Radial Compressor
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Compressor stagnation-to-static stagnation to static pressure ratio = 3:1 Choose a speed to give maximum efficiency (polytropic, (p y p , stagnation g to static) ) To find: Rotor diameter Blade axial width at outlet rpm
impellers.
PEMP RMD510
• A smaller proportion of stagnation enthalpy rise is in the form of
velocity.
• Hence, Hence polytropic efficiencies of backswept impellers are higher than
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Slip Factor