Influence of Volumetric Displacement and Aspect Ratio on the Performance Metrics of the Rotating Spool Compressor Craig Bradshaw, PhD Manager of Research and Development, Torad Engineering Greg Kemp, Torad Engineering Joe Orosz, Torad Engineering Eckhard A. Groll, Purdue University
Overview The spool compressor Fundamental parameters of performance for the spool compressor Modeling Methods Aspect and eccentricity ratio studies Scaling rules Friction and efficiency trends Scaling studies (displacement) Scaling rules Efficiency trends Conclusions 2
The Spool Compressor 3
Problem Statement What are the most fundamental parameters to improve the performance of the spool compressor? Orosz et al. (2012) presented the Zsoro number, given as the ratio of seal loss to displaced volume Yielded good insight into further prototype development Does not include any parameters of length Add fundamental length parameters which includes friction and leakage influence of: Tip seal Top dead center Vane 4
Modeling Methods 1 1 Presented in Bradshaw and Groll (2013), IJR 5
Fundamental Geometry Fixed operating condition on R410A, 905 kpa, 2282 kpa, and 11K, suction, discharge, and superheat Maintaining a fixed displacement of 54 cm 3 Porting is fixed and valve dynamics are ignored External heat transfer area is held fixed, internal scales with geometry Vane size scales with eccentricity Fundamental dimensions of sealing elements remain the same Rr L h R D 2R s stator s 6
Relative Friction Seal Friction Vane Friction 7
Leakage 8
Overall Isentropic Efficiency Relative change in overall efficiency is expected to be on the order of 5 percentage points due to changes in geometry (62% 67%) 5 th generation has a significantly different displaced volume, used as a reference 9
Capacity Scaling Studies Utilized eccentricity ratio and length-to-diameter ratio as 5 th generation prototype = 0.89 L/D=0.95 Porting Valve dynamics removed Port areas scaled with square of diameter (limited) Vane width scaled with eccentricity Seals Tip seal width scaled with eccentricity Side seal width fixed Surface area of shell scaled with square of diameter 10
Capacity Studies Results Assumes same aspect ratio and length to diameter ratio as the 5 th generation prototype Efficiency increases roughly 5 points between 3.5 and 5 tons Efficiency increases another 6 points from 5 tons to 25 tons 11
Conclusions The spool compressor performance is highly sensitive to the basic geometry and the displaced volume Studies have shown the potential for an 11 point increase (5 from geometry + 6 from increase in displacement) in overall isentropic efficiency of a 5 ton, 6 th generation prototype A 7 th generation 25 ton shows a potential for 16 point overall isentropic efficiency improvement over the 5 th generation 12
Loss Analysis of Rotating Spool Compressor Based on High-Speed Pressure Measurements Craig Bradshaw, PhD Manager of Research and Development, Torad Engineering Greg Kemp, Torad Engineering Joe Orosz, Torad Engineering Eckhard A. Groll, Purdue University
Overview Diagnosis of the internal losses of the spool compressor Experimental methods Resulting indicator diagram Loss analysis Pareto of compressor losses Conclusions 14
Experimental Methods Instrumented 5 th generation prototype spool compressor (39 cm 3 displacement) Operated at 3550 rpm, 905 kpa, 2282 kpa, and 11K suction, discharge pressures and superheat, respectively 15
Experimental Methods, cont. Instrumented using Endevco 8530B-500 high-speed pressure sensors Sampled at 30,000 samples per second Roughly 20 cycles averaged per cycle 95% confidence interval presented as uncertainty of indicated losses 16
Experimental Methods - Placement Sensor 1 Sensor 2 Sensor 3 17
Sensor Locations and Orientation 18
Correlating Volume Curves to Pressure Data Vane position is inferred from pressure data An algorithm to filter, determine inflection point, and assign vane position is developed 214.1 215.1 19
3550 RPM Indicator Diagram 20
Loss Analysis Indicator diagram displays minimum specific power requirements Deviations from ideal represent losses The indicator diagram cannot identify external loss sources Tip/side seal friction Vane friction Viscous/bearing losses 21
Indicated Losses Discharge process Represents a combination of port and valve losses Compression process Represents porting losses during suction process Calculated using a similar procedure as discharge losses 22
Indicated Losses Compression process losses Represents leakage and backflow into closed suction pocket causing additional work Compared against ideal compressor process with polytropic exponent of 1.11, calculated using a best fit with model presented by Bradshaw and Groll (2013) 23
The Curtain Area Potential flow restriction caused by the limited area between the rotor and cylinder at the leading edge of the discharge port Hypothesized this loss can be a significant relative to other port losses 24
External Losses - Tip and Side Seal Friction 25
Loss Pareto 26
Conclusions A methodology was developed to diagnose the different losses present in the spool compressor Compressor analysis presented is of particularly poor performance Part of the poor performance is likely to be attributed to the pressure sensors Side seal losses contribute the largest proportion to the losses Compression losses indicate recompression and leakage which should be addressed Discharge losses suggest the port location/areas should be adjusted Curtain area losses are low in this case but should be considered for optimum efficiency 27
Future Efforts Additional port locations, speeds, and operating conditions have been examined but require further rigor 28
Supplemental
The Spool Compressor 30