FIELD OF THE INVENTION
The present invention relates to impeller of centrifugal compressor with low
flow coefficient.
More particularly, the present invention relates to high efficiency low flow
shrouded 2D impeller of centrifugal compressor.
BACKGROUND OF THE INVENTION
In general, shrouded 2D impellers of centrifugal compressor comprises at
least a disk and a shroud, and a plurality of blades disposed between the disk
and the shroud and substantially equidistantly in a circumferential direction to
define the flow passage formed by the disk and the shroud, and the vanes or
blades. The geometric configuration of the rotor blading significantly affects
the aerodynamic efficiency, due to the fact that, the geometric characteristics
of the blade determine the distribution of the relative velocities of the fluid
flow along the rotor, diffusion of fluid within the impeller, blade to blade
loading at hub, shroud and intermediate planes and flow passage area
distribution that affects the flow behaviour in the flow path and various
internal & external losses.
In a centrifugal compressor, gas that has to be compressed to the desired
delivery pressure enters axially at the first stage and gets compressed in the
impeller and passes through a diffuser, “U” shaped 180 deg. bend and
through a row of de-swirl vanes in the return channel that reduce the
tangential or swirl component of flow and then enters the next stage impeller
for further compression. The pressurized gas from the last stage impeller
goes into a volute / collector chamber that is concentrically arranged inside
the interstage duct.

One of the main demands from all the industries is the high aerodynamic
efficiency that needs to be achieved in all the stages of multi stage
compressor and especially in the last stages where the 2D impellers are
employed. None of the prior arts discussed about the efficiency improvement
of low flow 2D impeller and current efficiency levels achieved are not
mentioned. It is required to improve the efficiency of low flow coefficient 2D
impeller of centrifugal compressor by adopting appropriate impeller geometry
to improve the overall performance of the compressor to meet the industrial
demand.
Patents related to the Impeller of centrifugal compressor are given below,
[1] US 6,340,291 B1 22/01/2002 Lothar Reckert, Natendorf (DE);
[2] US 6,715,991 B2 06/04/2004 Eugenio Rossi, Viareggio (IT);
[3] US 7,563,074 B2 21/07/2009 Cheng Xu, Huntersville, NC (US);
[4] US 8,308,420 B2 13/11/2012 Manabu Yagi, Tsuchiura (JP);
A cylindrical blade for a rotor of radial type of impeller that is 2D in nature
and typical coordinates with varying thickness from inlet to exit is reported in
Ref [1]. Thickness of impeller blade is continuously increasing from inlet to
exit and reaches to maximum at approximately 50% of meridional length and
comes to minimum at exit. The impeller is meant for very low pressure raise
in the order of 5000 Pa. Prior art Ref. [2] is 2D impeller of radial type and
typical coordinates with varying radius are reported. The suction & pressure
surface of the blade referred a convex & concave surface respectively. The
blade coordinates are fixed and are given as ratio of the impeller exit radius.
1) Prior art Ref. [1], is a two dimensional centrifugal compressor impeller
caters to air pollution control measuring instruments that are used for
determining the bacterial content of the air, the present invention is

applicable to multi stage compressor for industrial applications to handle
various gas mixtures including air.
2) Prior art Ref. [1], volume handled by the impeller is very low which is
maximum of 9 m3 /hr. with head raise of 5000 Pa. the present invention
handles relatively large volumes of flow 3300 m3/ hr. and pressure raise
will be 30,000 Pa.
3) Prior art Ref. [1], caters to a plurality of impeller blades passing in part
spiral manner from an outside circumferential edge to the bore defining
impeller blade channels between adjacent impeller blades, the present
invention bears has constant blasé thickness leaving the leading edge.
The difference between both the geometries has been clearly illustrated in
Fig.1 & Fig.2.
4) Prior art Ref. [2], caters to the two dimensional centrifugal compressor
impeller where the geometry is represented in Cartesian co-ordinates in
terms of impeller exit radius which is 200 mm as shown in Fig.3, the
present invention bears a geometry in form of Bezier polynomials and it is
parameterized.

5) The prior art Ref. [2] geometry is of fixed type, the present
invention can be scaled up and scaled down by 40% in diameter to
produce other sizes of impellers with same high aerodynamic efficiency.
6) Prior art Ref. [3], caters to centrifugal compressor impeller of three
dimensional twisted blade profiles as shown in Fig. 4, the present
invention has two dimensional blades of constant thickness.
Comparing the geometries of prior art [3] and present invention from

Fig.2 and Fig.3, it is clearly concluded that prior art [3] and present
inventions are entirely different.
7) The prior art Ref. [4], though it is of the same interest of improving the
aerodynamic performance of centrifugal impeller, the methodology
followed and the final geometry achieved in both the cases are entirely
different. In the prior art Ref. [4], is of three dimensional blade
impeller and the geometry is not defined neither in terms of
coordinates nor in the parameterized form. Also the blade angle
distribution, wrap angle distribution at various sections of blade from
hub to shroud are not mentioned. In case of the present invention, the
geometry is clearly defined in the form of blade angle distribution,
wrap angle distribution, hub & shroud contour slopes, and impeller
passage area distribution. This is clearly vindicated from Fig. 5 & Fig. 6
where both the cases are distinguished in terms of impeller blade
angle distribution from inlet to exit that completely defines the blade
shape.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose high efficiency low flow
shrouded 2D impeller of centrifugal compressor.
Another object of the invention is to propose high efficiency low flow
shrouded 2D impeller of centrifugal compressor, which provides a geometrical
parameter, i.e., blade angle distribution of low flow 2D impeller at hub &
shroud surfaces along the meridional flow path from the impeller inlet to
impeller exit to achieve high aerodynamic efficiency in the centrifugal
compressor.

A still another object of the present invention is to propose high efficiency low
flow shrouded 2D impeller of centrifugal compressor which provides
geometrical parameter, i.e., blade wrap angle distribution of low flow 2D
impeller along the meridional flow path from the impeller inlet to impeller exit
to achieve high aerodynamic efficiency in the centrifugal compressor.
Yet another object of the present invention is to propose high efficiency low
flow shrouded 2D impeller of centrifugal compressor which provides
geometrical parameter, i,e., impeller passage area distribution of low flow 2D
impeller to achieve high aerodynamic efficiency in the centrifugal compressor.
Another object of the present invention is to propose high efficiency low flow
shrouded 2D impeller of centrifugal compressor, which provide geometrical
parameter, i,e., blades circumferential pitch of low flow 2D impeller to
achieve high aerodynamic efficiency in the centrifugal compressor.
A still another object of the present invention is to propose high efficiency low
flow shrouded 2D impeller of centrifugal compressor, which provides
geometrical parameter, i,e., inlet hub radius of low flow 2D impeller to
achieve high aerodynamic efficiency in the centrifugal compressor.
A further object of the present invention is to propose high efficiency low flow
shrouded 2D impeller of centrifugal compressor, which provides geometrical
parameter, i,e., inlet shroud radius of low flow 2D impeller to achieve high
aerodynamic efficiency in the centrifugal compressor.

A still further object of the present invention is to propose high efficiency low
flow shrouded 2D impeller of centrifugal compressor, which provides
geometrical parameter, i,e., blade width at exit of low flow 2D impeller to
achieve high aerodynamic efficiency in the centrifugal compressor.
Yet further object of invention is to propose high efficiency low flow shrouded
2D impeller of centrifugal compressor, which provides circumferential pitch to
reduce the frictional losses with in the impeller and improve the efficiency.
SUMMARY OF THE INVENTION
Accordingly, there is provided high efficiency low flow shrouded 2D impeller
of centrifugal compressor. The present invention differs from prior art
reference 1, in that the present invention is applicable to multi stage
compressor for industrial applications to handle various gas mixtures
including air. The present invention handles relatively large volumes of flow
3300 m3 / hr. and pressure raise will be 30,000 Pa. The present invention
bears has constant blasé thickness leaving the leading edge. The difference
between both the geometries has been clearly illustrated in Fig.1 & Fig.2. In
contrast with the prior art reference 2, the present invention bears a
geometry in form of Bezier polynomials and it is parameterized, the present
invention can be scaled up and scaled down by 40% in diameter to produce
other sizes of impellers with same high aerodynamic efficiency. In contrast to
the prior art reference 3, the present invention has two dimensional blades of
constant thickness. Comparing the geometries of prior art [3] and present
invention from Fig.2 and Fig.3, it is clearly concluded that prior art [3] and
present inventions are entirely different. As compared to the prior art
reference 4, in case of the present invention, the geometry is clearly defined
in the form of blade angle distribution, wrap angle distribution, hub & shroud

contour slopes, and impeller passage area distribution. This is clearly
vindicated from Fig. 5 & Fig. 6 where both the cases are distinguished in
terms of impeller blade angle distribution from inlet to exit that completely
defines the blade shape.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure.1: Impeller of prior art Ref [1] with variable impeller blade thickness
Figure.2: Impeller of present invention with constant impeller blade
thickness leaving the leading edge distinguishing from the
prior art.
Figure.3: Impeller blade of prior art Ref. [2] where the 2D impeller is
defined in terms Cartesian ordinates and is fixed type not
scalable and other information not furnished.
Figure.4: Impeller of prior art Ref. [3] which is 3D in nature distinguishing
from present invention.
Figure.5: Impeller blade angle distribution of prior art Ref. [4]
Figure.6: Impeller blade angle distribution of present invention
distinguishing from prior art.
Figure.7: Impeller blade wrap angle “ θ “ distribution of present invention from
impeller inlet to exit.
Figure.8: Impeller passage area distribution of present invention.
Figure.9: Impeller cross-section of present invention
DETAIL DESCRIPTION OF THE INVENTION
The main objects of invention are parameterised impeller geometry and their
definitions that control the entire impeller geometry are described in the
following.

a) Impeller blade angle “β” distribution at impeller hub & shroud surfaces
along the impeller inlet to impeller exit is defined in Fig. 6. Blade angle
distribution at hub & shroud is same in the case of present invention.
This blade angle distribution completely controls the flow physics
within the impeller like diffusion, pressure recovery coefficient,
pressure loss coefficient, relative velocity distribution, impeller passage
area distribution, flow pattern at impeller exit and along the
downstream of the compressor stage and thereby efficiency. The
impeller blade angle variation along the percentage length of
meridional flow path “B” of the present invention is shown in Fig.6 and
it is governed by Equation.1.
β = 23.542 + 18.7561 B + 19.5363 B2 - 145.631 B3 + 255.449 B4 - 586.712 B5 +
1037.48 B6 - 891.247 B7 + 301.761 B8 - 13.3295 B9 --- Equation.1
b) Impeller wrap angle “θ” distribution at impeller hub and shroud
surfaces from impeller inlet to impeller exit influences blade loading /
relative velocity distribution, the pressure recovery, flow behaviour and
thereby efficiency. The present invention has the wrap angle
distribution which increases uniformly from impeller inlet to impeller
exit for achieving higher efficiency as in Fig.7. Numerically the wrap
angle variation is (-) 33 deg. to (-) 110 deg. for both hub & shroud
with respect to meridional plane. The impeller wrap angle variation of
the present invention along percentage length of meridional flow path
“A” is shown in Fig. 7 and it is governed by Equation.2.
θ = (-) 33.9091 - 112.76 A + 95.8141 A2 - 309.13 A3 + 1192.6 A4 - 3006.49 A5
+ 4592.82 A6 - 4156.95 A7 + 2056.9 A8 - 429.339 A9 ---- Equation.2
c) Impeller blades circumferential pitch in terms of degrees depends of
the total number of blade in the impeller. The number of blades in the
impeller shows major impact on the peak blade loading and total

frictional losses. It is observed that low flow stages requires lesser
number of impeller blades whereas high flow stages demands more
number of impeller blades to achieve proper blade loading which
results in efficient flow behaviour in the entire compressor stage. For
the present invention which is a low flow impeller, the circumferential
pitch is 27 deg. to 24 deg. for achieving better blade loading and
lesser friction losses. This has ultimately resulted higher efficiency of
compressor.
d) Impeller inlet hub radius ‟R1h” shown in Fig.9, influences the efficiency
of compressor stage. Higher inlet hub radius demands higher inlet
shroud radius which increases the inlet relative velocity at shroud.
Increase in inlet shroud velocity reduces the impeller diffusion and
there by pressure recovery and impeller efficiency. Inlet hub radius
also influences the multi stage compressor shaft diameter and
compressor rotodynamic behaviour. Considering above all
requirements, for the present invention which is of 500 mm impeller
diameter with flow range of 70% to 130% of design flow, the inlet hub
diameter is 37% to 39% of impeller exit diameter.
e) Impeller inlet shroud radius ‟R1s” shown in Fig.9, is free to increase or
decrease whereas the hub radius is fixed by the rotodynamic
requirement. This is basically controlled by operating range of
compressor stage as reducing the inlet radius drastically will result in
chocking at higher flow. Inlet shroud radius of the present invention
that has enabled to achieve higher compressor efficiency is 61% of
impeller exit diameter for the flow range of 70% to 130% of design
flow.

f) Impeller blade width at exit ‟B2” shown in Fig.9, plays major role in
achieving the overall efficiency of corresponding compressor stage.
Impeller exit width influences diffusion, flow associated problems like
re-circulation, separation and low momentum zones. Impeller exit
blade width is directly related to impeller exit blade angle ‟β2b” and
flow coefficient. Increasing the impeller width demands higher
pinching in the diffuser width in order to avoid flow associated
problems like recirculation, separation and low momentum zones in
the diffuser and further downstream. Diffuser pinching in the width
more than 25% can cause flow disturbance at the diffuser inlet. For
the present invention with 500 mm impeller exit diameter where the
flow co-efficient is low with the impeller exit blade angle (-) 71 deg.
the impeller exit blade width is 21.65 mm.
g) Impeller passage area distribution is mainly responsible for diffusion
with in the impeller and influences the compressor efficiency. This
passage area distribution varies from high flow to low flow stages.
Continuous increase in the impeller passage area from impeller inlet to
exit along the meridional flow path ensures conversion of kinetic
energy in to pressure and also eliminates recirculation zones with in
the impeller. The impeller passage area distribution for achieving the
higher efficiency for present invention which is of low flow is shown in
Fig. 8. The impeller passage area in mm2 “PA” distribution along the
percentage length of meridional flow path “A” is governed by
Equation.3.
PA = 582.116 - 95.1008 A + 9089.97 A2 - 60484.7 A3 + 230152 A4 - 542110 A5
+ 790938 A6 – 694189 A7 + 335438 A8 - 68554.6 A9 --- Equation. 3
In the present invention, impeller geometrical parameters along the
meridional flow path from impeller inlet to impeller exit like Blade angle, Wrap
angle, Passage area, Inlet hub radius, Inlet shroud radius, Impeller blade exit

width, Impeller blade circumferential pitch are finalised based on systematic
design approach with the rich experience of compressor design & extensive
CFD analysis and performance testing of prototype where the efficiency
improvement achieved is very close to the theoretical efficiency improvement.

WE CLAIM :
1. High efficiency low flow shrouded 2D impeller of centrifugal
compressor, the 2D-impeller comprising;
at least a disk and a shroud, and a plurality of blades disposed
between the disk and the shroud and substantially equidistantly in a
circumferential direction to define the flow passage formed by the disk
and the shroud, and the vanes or blades, characterized in that the
blade angle distribution at inlet and exit of impeller is (-) 66 deg. and
(-) 71 deg. respectively with maximum blade angle of (-) 61 deg. at 40
% of meridional flow path length.
2. 2D impeller of centrifugal compressor as claimed in claim 1, wherein
the impeller blade wrap angle distribution at inlet and exit of impeller
is (-) 34 deg. and (-) 110 deg. respectively which continuously
increases from inlet to exit.
3. 2D impeller of centrifugal compressor as claimed in claim 2, wherein
the impeller passage area continuously increases from inlet to exit.

4. 2D impeller of centrifugal compressor as claimed in claim 3, wherein the
impeller diameter is about 500 mm and impeller exit width is about
21.65 mm with scalability of 40 % in diameter both in upper and lower
side of the diameter.
5. 2D impeller of centrifugal compressor as claimed in claim 4, wherein the
outer diameter of the impeller ranges between 300 mm to 700 mm with
an inlet hub radius of 37% - 39% of the impeller outer diameter and an
inlet shroud radius of 60% - 62% of impeller outer diameter.

6. 2D impeller of centrifugal compressor as claimed in claim 5, wherein the
outer diameter of the impeller ranges between 300 mm to 700 mm with
an impeller exit blade width of 3.8 % to 4.5 % of impeller exit diameter.
7. 2D impeller of centrifugal compressor as claimed in claim 5, wherein the
circumferential pitch is between 27 to 24 deg.