Non-Destructive Assessment of Wrist Journal Bearings in Hermetic Reciprocating Compressors

A. D. Kelly General Signal Laboratory Equipment 275 Aiken Road Asheville, NC 28806 Low temperature refrigeration mechanically and thermally stresses a compressor's wrist journal bearing. Knowledge of bearing condition would be useful for self-diagnostics or predictive maintenance, particularly in freezers used for storage of valuable biomedical products. This initial investigation considers external temperatures, power consumption, and compressor housing acceleration as possible indicators of bearing condition. A simple mathematical compressor model based on slider-crank kinematics and isentropic compression relationships provides insight into the effects of bearing wear. The subject is also studied experimentally by comparing normal systems to systems that were artificially forced to develop wear. One practical means to identify bearing wear is an audible rattle during balanced pressure compressor start-up. B cylinder bore diameter c piston clearance at IDC D capillary inner diameter f friction factor k ratio of specific heats, Cplc. L capillarylength NOMENCLATURE m mass flow rate n polytropic compression exponent P pressure Re Reynolds number (} Crank angle U capillary flow velocity INTRODUCTION f.J refrigerant viscosity V cylinder volume v refrigerant specific volume w compressor rotational speed X piston position from TDC The refrigeration system studied was used in a cascade freezer designed to maintain -130°F (-90°C). The high temperature stage operates with higher compressor loading and is therefore of primary interest. The high stage evaporates at -40°F (-40°C) and condenses at 70-100°F (20-38°C). Major system components include a nominal Yz hp (550W) hermetic reciprocating compressor, finned-tube condenser, capillary expansion device, a tube-in-tube cascade evaporator, and high-pressure HFC refrigerants. The subject freezers are commonly equipped with microprocessor controls that continuously monitor operation and alert users to unfavorable conditions. The goal was to identify potential indicators for detecting connecting rod wear during the early stages. Nonintrusive measurements were preferred to avoid introducing potential leaks or accidental contamination. The more ambitious goal was to find a performance parameter measurement that would allow technicians in the field to determine connecting rod condition. Figure 1 illustrates the type of wear that was of concern. The wear usually occurs first on the wrist pin journal bearing, as shown. This investigation was both analytical and experimental. First, a simple mathematical compressor model was constructed to explore the relationships between rod bearing wear, rod bearing forces, and compressor performance. An experimental investigation then compared the performance of normal freezers with freezers known to have worn bearings. External system temperatures, energy consumption, and vibration were considered as indicators of possible damage. System pressures, although intrusive to measure, are included in this report to increase understanding of system behavior.