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W =μF⋅2πrN TH Rotation direction Disk (6) TECHNICAL REPORTS 4.4 Thrust bearing loss model The friction loss WTH of the orbiting scroll and the thrust bearing is expressed by equation (6) below. In equation (6), F is the average gas load in the thrust direction, calculated from the pressure in the compres- sion chamber, μ is the friction factor, r is the orbital radius and N is the rotational speed. The value for the friction factor μ above was determined by experiment, using the method of measuring the friction coefficient shown in Fig. 4, with lubricant dropped in a CO2 refrig- erant environment. Table 2 indicates that the compressor input power levels in both the measurement and analysis agreed, albeit with a difference of approximately 3%, in the range of rotational speeds from 30 to 100 rps: the availability of the analysis method was thus verified. Figure 5 shows the results of loss analysis for each rotational speed level. Figure 5 indicates the values obtained by dividing each loss by the total loss. This figure shows that the rate of leakage loss is largest at 30 rps, which is approx. 2.7 times that at 60 rps. The rate of thrust bearing loss was the next largest. However, comparing the thrust load levels at respective rotational speed values revealed that the thrust load at 30 rps was more than 10% larger than those under conditions at 60 rps or higher. This could presumably be attributed to the fact that the pressure rise in the compression chamber due to leakage was larger than that at 60 rps. It means that thrust bearing loss can be reduced by improving the leakage conditions in the low rotational speed range. In addition, suction heating loss can be expected to decrease by reducing mechani- cal loss and increasing the amount of refrigerant circu- lating, thus improving the efficiency beyond the scale of leakage loss improvement. Leakage loss was small at 100 rps, at which good performance was demonstrated, as shown in the figure. Quantitative loss analysis of scroll compressors using CO2 refrigerant was realized by using a simplified model based on a basic test. The authors will improve the performance of systems using this technology and also apply it to other compressors. References (1) Fumiaki Sano et al., A High Reliability Study of the Scroll Compressor, Proceedings of the 1994 Pur- due Compressor Conference (1994.7), pp. 199-204. (2) NIST, NIST Reference Fluid Thermodynamic Transport Properties – REFPROP Version 7.0 (2002.8). CO2 refrigerant environment Pin Pressing load Fig. 4 Pin-on-disk type friction factor measuring equipment 5. Results of Loss Analysis Table 2 shows the prediction errors in the com- pressor input power values from analysis results under the evaluation conditions shown in Table 1. In Table 2, the prediction error in compressor input power is ex- pressed by the equation γ = (Analysis input − Meas- urement input)/ (Measurement input) x 100[%]. Table 2 Prediction error of input power Rotational speed Prediction error of input power: γ 30 rps +3.4 60 rps –3.0 100 rps –0.5 Leakage loss Suction heating loss Discharge loss Scroll wrap-side friction loss Thrust bearing loss Oldham coupling friction loss Radial bearing loss Motor loss Tip seal friction loss Fig. 5 Analysis of losses in the prototype [%] Mitsubishi Electric ADVANCE December 2007 19 Rotational speedPDF Image | Heat Pump with Natural Refrigerants 3041
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