Transcript Kevin Finney (11/04/2003 Room 307 08:30 AM)

Development, Fabrication, and
Testing of a Miniature Centrifugal
Compressor
Thesis Defense
by
Kevin Gregory
Finney
November 4, 2003
Presentation Content
•Project Specifications
•What is a Centrifugal Compressor?
•What makes this compressor unique?
•Components and procedures for development
–Motor
–Bearings
–Coupler
–Compressor Part Modeling
–Compressor Assembly
•The testing environment / Acquisition of data
•Results and conclusions from testing
Project Requirements
• Reverse Turbo-Brayton cryogenic cooling system
for space applications
• Goal was to develop a compressor capable of
system specifications
– Designed inlet conditions
•
•
•
•
•
Working fluid was air
Inlet Pressure = 1 atm (14.7 psig)
Temperature = 25o C
Total-to-static pressure ratio of 1.7 required
Mass flow rate of 4.5 grams per second
• Led to the design of a centrifugal compressor
– Restrictions imposed by entire system
• Size of a cola-can / impeller diameter of 4.5 mm
• Required the compressor to operate at 150,000 rpm
Definition of Centrifugal
Compressor
• Fundamentals
Dynamic compressor
of a dynamic machine
–
–
–
Geometry
Closed
volume
of Flow
of fluid
Pathdoes not exist
Achieves of
Direction
pressure
Energyrise by a dynamic transfer of energy
to
a continuously
flowing fluid stream
Fluid
State
• Liquid = Pump
• Gas = Fan, Blower, Compressor
Characteristics of a
Centrifugal Compressor
= 14.7 psig
• 1 to 2 =
actual
compression
process
• 1 to 2s =
isentropic
compression
Unique Design Characteristics
• 150,000 rpm
operating speed
• Axial flow exiting
the diffuser
• 4.5 millimeter
diameter impeller
Requirements for the
Development of Compressor
• Material Selection
–
–
–
–
Weight (inertial forces at high speeds)
Strength (withstand centrifugal stress, hoop stress, and torque)
Feasibility for manufacturing
Cost
• Motor Selection
– Design required 300 Watts of power at 150,000 rpm
– Motor for testing purposes built by Koford Engineering
• Bearing Selection
– Ball bearings for testing purposes (ceramic balls and cage)
– Air foil/journal bearings implemented in future design
• Minimal losses in bearing necessary for minimizing power required
• Coupler Selection
–
–
–
–
Alignment (angular, axial, lateral)
Maximum speed
Ability to modify for speed
Availability
• Rapid Prototype models
– Visual aid during design and
machining stages
• Manufacturing methods
– Parts capable of machining
– Manufacturing methods available
• Balance of rotating components
– Internal parts of motor balanced
– Impeller balanced
– Balanced at American Hofmann
Motor Characteristics
•Torque required at maximum speed was 0.02Nm
•3-phase DC motor via controller with AC supply
–Voltage controlled speed
–Current drawn determined by torque
Design Expectations:
150,000 rpm no-load requires 33 Volts and 6.8 amps
150,000 rpm with 300 Watts output requires 27.89 amps
Therefore, to obtain the desirable speed and power…
Voltage Current
Electrical_Power
33V 27.89A  920.37 W
Electrical_Power
Shaft_Power  Heat
920.37W  300W  620.37 W
Heat
620.37W
A method to remove this heat had to be implemented…
Parallel Flow
Motor
Chosen due to simplicity in
manufacturing the cooling
jacket.
Dispersed Parallel
Flow
Motor
Cooling
Jacket
• Parallel Flow Concept
– Pressure loss occurred
• one inlet line and six
outlet lines
• Eliminated possibility of
leaks
– Turning of fluid
occurred in plate
• Sealed with Silicone
sealant
Bearing Selection
• Few bearings available capable of 150,000 rpm
– DN Limit (occurred when rotating components
involved)
• Inner shaft diameter (millimeters) multiplied by the speed
(rpm)
10 mm X 150,000 rpm =1,500,000
• Type of lubricant determined by the DN Limit
• Maximum DN Limit of 2 million for ‘Barden’ ceramic ball
bearing
– Expected power loss per bearing important to the
amount of power required by motor
• Advantages of ceramic bearings
– Ceramic Balls versus Steel Balls
Ball Bearing
•Ceramic balls and cage allowed
for higher speed capability
•Same bearing throughout
assembly
•Expected 50 Watts of loss in
each bearing
•Handled large radial loads
compared to axial loads
Air Foil Bearing
•Very expensive to incorporate
•Integrated part of entire design
•Only a few manufacturers:
Mohawk Innovative Technology,
Inc. (MITI)
R&D Dynamics
Schematic of Foil Bearing courtesy of R&D Dynamics
Current Coupler Selection
Maximum
Speed
30,000 rpm
Lateral
Misalignment
±0.13mm
±0.005in
Axial
Misalignment
±0.8mm
±0.032in
Angular
Misalignment
± 1o
Mass
20 grams
Coupler Modifications
• Modification of
coupler for increased
speed capability
– Stainless Steel Sleeve
Retainer
Compressor Modeling and
Drawings
• Modeling of Parts
– Pro Engineer
– Finite Element Analysis on blade shape to determine
loading effect caused by maximum pressure
• Assembly of Parts in Design Stage of Development
– Interference between components / clearances
• Drawings of Parts for Machinist
– Complicated curvatures
• required the coordinates of the curve
– Tolerance of bearing bores critical to the radial stress
placed on the balls
– Prototyped parts developed
• assisted the machine shop with visual aid of complicated
geometry
Part Modeling
•Complex geometry
–Undercut of impeller
blades
–Complicated
manufacturing methods
Max Stress = 1ksi
Yield Stress = 75ksi
Hub
Shroud
Concluded that blade will not fail from fluid loading.
Load does not include the centrifugal forces.
Compressor Assembly
• Compressor Bearing
Placement
– Bearing Jig Fixtures
• Bearing in Diffuser
• Bearing in Top Cap
• Specific order of assembly
–
–
–
–
–
Collector to Diffuser (bearing)
Collector to Housing
Impeller to Diffuser Bearing
IGV to Housing
Top Cap (bearing) to IGV and
Impeller
Exploded View of Compressor
Bearing Jig Fixtures
•Interference fit (force)
–Arbor Press
–Fixtures to hold part
–Bushings to press bearings
•Expansion fit (heat)
–Used if excessive force required
and for disassembly
•Jig fixtures designed for
disassembly
Impeller bottomed
out in bore
Rapid Prototyping of Parts
• Allowed a visual aid
during design
– Correction of
assembly issues /
interferences
• Supplied the
machinist with an aid
– Blade shape and
complex geometry
more understandable
– Provided a visual to
CNC code
Comparison of prototype parts to manufactured parts
Impeller (cast)
Inlet Guide Vane
Diffuser
Manufacturing of Parts
• Impeller cast in Aluminum A356
– Properties of A356
• Used for aircraft and missile components requiring high
strength, ductility, and corrosion resistance.
• Used for intricate castings such as cylinder blocks, cylinder
heads, fan blades, and pneumatic tools
• Contains 7% Silicon and traces of Magnesium and Iron. These
alloying elements assist in the strength and corrosion
properties.
• Tensile Strength37ksi
• Yield Strength27ksi
Properties taken from “Structure and Properties of Engineering Alloys” by Smith
Straight Blade Impeller
•Similar blade shape except there
was no undercut
•Only required 4-axis CNC
•More homogeneous material
•More naturally balanced
Balancing
• American Hofmann (Lynchburg, VA)
– Balanced to a g-level (ANSI Standard) equivalent of
150,000 rpm
– Material was removed in order to re-skew the axis of
the hub to the axis of the shaft
• Two of the three ‘Curved Blade’ Impellers
• ‘Straight Blade’ Impeller
Photo courtesy of American Hofmann
Alignment of Assemblies
• Motor shaft alignment to the compressor shaft
– Axial
– Lateral
– Angular
• Run-out restricted by the radial play in the bearings
• Accurate to the accuracy of the
measuring tools
– 0.0005 inch accurate dial indicators
Adjusting the Alignment
• Adjust assembly
alignment with shims
– By using shims of 0.0005”
thickness, shafts were
adjustable
• More accurate the
alignment, more rigid the
coupler could act
– Resulted in higher
operating speeds with less
power consumption
The Testing Environment
Voltage versus
Speed Curve
Mass
Flow
Meter
Pressure
Transducer
Curve
Controller
Fan
Reinforced Cage
Controller Case
Components Required for Testing
• Motor and Compressor Assemblies
– Assembly support brackets
– Common base
• Instrumentation
– Temperature
• Thermocouples
– Pressure
• Calibration curve for pressure range
– Flow (mass flow measurement)
– Operating Speed
• Digital reader, Oscilloscope, Frequency counter
– Input Power
• Data Acquisition
Areas of Desired Measurements
Pressure and Temperature at Inlet
Mass Flow Controller
Power Out of Motor
Motor Case Temperature
Motor Bearing Temperature
Pressure and
Temperature
after Mixer
Bearing Temperature
Pressure and
Temperature at Diffuser
Exit
Motor Bearing Temperature
Bearing Temperature
Power In
Order of Testing
• Motor Test
– Determine ‘Free-spin’ motor data
– Compare the speed measurements for accuracy
– Develop Voltage versus speed curve
• ‘Blank Shaft’ Test
– Determine the efficiency of the motor
– Determine the loss per bearing
• Compressor Test
– Determine the efficiency of the compressor
– Determine the work of the impeller on the fluid
Motor Test
• ‘Free-spin’ operation
– Motor shaft spun only
– Input variables and shaft
speed recorded
‘Blank Shaft’ Test
• Motor efficiency = 40% to 70%
– 90,000 rpm = 65% with load
• Loss per bearing = 105 Watts
at 90,000 rpm
Purpose of Blank Shaft
• Blank shaft (no hub nor blades) machined for use
in determining the power loss in the bearings
– Run motor without any attachments and record power
supplied to motor.
– Assemble entire unit with blank shaft and operate at
150,000 rpm and record power supplied to motor.
Psupply_with_shaft  Psupply_no_attachments
P supply_1_bearing
P supply_2_bearings
2
Psupply_2_bearings
View of Assembly with Blank
Shaft
Compressor Test
• Curved Blade Impeller
– 89,485 rpm, 3.13 g/sec, 2.70 psig
• Straight Blade Impeller
– 93,984 rpm, 5.14 g/sec, 5.05 psig
• Video of Compressor Test
Cast Impeller
Pow er versus Speed
1400.0
Straight
Blade
Impeller Test
1
1200.0
Power (Watts)
1000.0
800.0
Straight
Blade
Impeller Test
2
600.0
400.0
Straight
Blade
Impeller w/
Data
Acquisition
200.0
0.0
0
20000
40000
60000
Speed (rpm )
80000
100000
120000
Gage Pressure Versus Speed
12.0
Cast Impeller
Gage Pressure (psig)
10.0
Straight Blade
Impeller Test 1
8.0
Straight Blade
Impeller Test 2
6.0
Straight Blade
Impeller w/ Data
Acquisition
Design Point
4.0
2.0
0.0
0
20000
40000
60000
80000 100000 120000 140000 160000
Speed (rpm )
Compressor Efficiency
• Power consumption curves
Speed Impeller Work
rpm
Watts
50000
60
60000
90
70000
150
80000
200
90000
225
Mass Flow Rate
g/s
3.849
3.159
3.389
3.952
5.139
Compressor Efficiency
0.571
0.376
0.355
0.381
0.612
• Actual output conditions:
–
–
–
–
93,984 rpm
1.29 pressure ratio
61.2% isentropic efficiency
5.1 grams per second
mass flow rate
Dimensional Analysis Plots
Conclusion
• Straight Blade Impeller more effective than Curved Blade
Impeller
• Compressor was on way to design conditions
– Pressure ratio of 1.7
– Mass flow rate of 4-8 grams per second
– Operating speed of 150,000 rpm
• Reduce losses
– Improve alignment
• Implement laser aligning procedures
• Introduce rigid coupler
• Incorporate one shaft throughout the assembly
– Incorporate air foil bearing / air journal bearing
• Only if power consumption remains high
References
Barden, “Precision Bulletin-The Effects of High Speed on Ball Bearings”
MMG 2.5 5/94.
DellaCorte, C. “Performance and Durability of High Temperature Foil
Air Bearings for Oil Free Turbomachinery” NASA/TM-2000209187/REV1. Glenn Research Center, 2000.
Koford, Stuart. “MK-Koford Brushless and Brush Motors.” Website.
2003. http://www.koford.com
Rimtec, “Motion Control” A Couple of New Ideas. Vic Jha.
January/February, 2000.
Smith, William F. Structures and Properties of Engineering Alloys,
Second Edition. New York: McGraw-Hill, Inc., 1993.
Questions and Comments