A Study on the Oil Supply System of a Rotary Compressor

For a rolling piston type of rotary compressor, oil supply system consisting of individual lubrication elements such as pumps, oil passages, and sliding surfaces has been modeled by employing equivalent electric circuit. Numerical solutions of the oil network model include total oil flow rate into the shaft hole, oil flow rates to various sliding surfaces, and oil leaks through roller end clearances. Also, analysis has been made on the oil content in the suction and compression chambers and in discharge refrigerant. Validation of the numerical simulation has been made by measurement of the total oil flow rate into the shaft. NOMENCLATURE A area R resistance c bearing clearance, constant r radius c. discharge coefficient sf Sommerfeld Number D bearing diameter T temperature F correction factor for resistance X oil factor H height a solubility L bearing length P. roller angle m mass flow rate b roller end clearance N rps E bearing eccentricity n specific heat ratio, power index jJ viscosity p pressure (j) angular velocity P, pressure ratio q side leak flow coefficient Q volume flow rate s head coefficient INTRODUCTION Lubrication is crucial to good reliability and high performance in a rotary compressor, as in other types of hermetic refrigeration compressors. For vertical rotary compressor which is the main stream of the rotaries, oil pumping power is produced by the shaft rotation, and the lubricating oil is drawn into the oil gallery inside the shaft and the oil is distributed into various sliding surfaces via several radial feeding holes. Some experimental and analytical studies on the individual lubrication elements such as shaft pump, radial pumps and various types of sliding surfaces of vertical rotary compressors have been reported[ 1-5], but very few studies have been carried out on the integrated oil supply system[6]. Lubrication elements whose performance characteristics are determined individually may change their characteristic values when they are integrated as one system, since individual operating conditions for each elements are set only after they are combined together as a system under real compressor operating condition. This study aims to introduce a modeling method of the integrated oil supply system, and to present computer simulation program for predicting the oil distributions inside a vertical rotary compressor. Fifteenth International Compressor Engineering Conference at Purdue University, West Lafayette, IN, USAJuly 25-28, 2000 153 MODELING OF OIL SUPPLY SYSTEM A schematic oil supply system of a rotary compressor is shown in Fig. I. Inflow of the oil into the system is made at the oil cap hole of the shaft, and the oil flows out of the system through the main journal bearing, sub journal bearing, and the clearances between the roller end faces and the cylinder flat walls. Between the inlet and outlets, various lubrication elements such as pumps and oil passages and sliding surfaces constitute oil distribution network. To model the individual lubrication elements and the whole oil supply system, analogy between the pipe flow theory and the electrical circuit theory is employed. In the analogy, the pressure differential, volume flow rate and flow resistance of the flow system correspond to the voltage, electric current, and electric resistance, respectively. Shaft Pump As the shaft rotates, the flow inside the shaft is forced to rotate to yield a pumping head. The shaft pump head will be changed when there exist discharge flows out of the oil gallery inside the shaft through the radial discharge holes. The head variation with the flow rate is expressed in equation (1) (1) Here, P; is the theoretical total head, and head coefficient of the shaft, s s~o, and c and n are experimentally determined constants. At the inlet of the shaft, the oil cap is necessary to support the pump head. The flow resistance of the oil cap can be written as p Q 2 (C.Aj (2) Radial Pump The performance curve of the radial feeding holes and the resistance at the pumps are given by equations (3) and (4), respectively AP R _rp_ r p· c Qn-• rp_Q_';,rpE•rp (3), (4) Here, P; is the theoretical Euler head, and s,p, crp, n are to be determined by experiment. Roller End Clearance The oil flow through the roller end clearance and the associated flow resistance are given by equations (5) and ( 6), respectively. Q = 2Ho 3 Ap P. rb 6pln(R, I RJ 2H R = _Ap_ = _,6 p'---ln-'-( R_,_,_l R--'''-'-.) 27! rb Q 2H03 p ~ v (5), (6) Journal Bearing Oil flow in the journal bearing consists of pressure-driven flow and the side leakage flow due to viscous dragging of the rotating shaft. The pressure-driven flow, QP and the corresponding resistance are given in equation (7), and (8), respectively. 3 Q = 1lfJ.rc (1 + 1.5e') p 6pd ' (7), (8) The side leakage flow, Q., and the corresponding viscous pumping head, P. are given by equations (9) and (1 0), respectively. Fifteenth International Compressor Engineering Conference at Purdue University, West Lafayette, IN, USAJuly 25-28, 2000 154 (9),(10) Here side leak coefficient ; is a function of Sommerfeld number and bearing aspect ratio[7]. Spiral Groove Grooves at journal bearings are to improve the oil supply through the bearings. When the groove is inclined, viscous action takes place to drag the flow in addition to the flow due to the pressure differential. The flow resistance in the groove, the flow due to viscous pumping Qvp and the viscous pumping head, P vp are given by equations ( 11 )-( 13 ), respectively.