A Short Term, Accelerated Test Method for Rotary Compressors

Compressor reliability and durability testing has historically required significant time and a variety oftest methods, including basic materials and lubricants testing using Falex test methods or equivalent, long term compressor bench tests, end product reliability tests and final field trials before production release. Reducing the total design cycle time to a minimum, while maintaining a high confidence level in reliability test results as they relate to expected compressor quality and reliability, is always a major industry goal. A short term (200 hour) accelerated test protocol has been developed for rotary compressors used in air -conditioning and small heat pump applications, to provide quick understanding of expected bearing performance and compressor quality. This test procedure can be used to compare bearing performance and manufacturing quality of compressors from different manufacturers, to evaluate new refrigerants, lubricants and materials, and to compare design modifications to base designs. INTRODUCTION Qualification of compressors for reliability and durability historically has required many months and often years of material, component, and product tests before production release. From a business perspective, reducing the design cycle time to a minimum while maintaining a high level of confidence in the reliability of the test results, has significant economic advantage. While it is possible to run parts of a total test program in parallel, most often each test phase, from basic material and lubricant screening, to the final field trial, is conducted in series. Convention compressor bench tests using load stands, generally require from 2000 hours to 4000 hours (12 to 24 weeks). A short-term (200 hours), accelerated test protocol has been developed for rotary compressors used in air -conditioning and small heat pump applications. This test protocol has been used to compare quality and reliability of different compressors and to evaluate the effect on bearing performance of compressor design changes, new refrigerants such as R407C and R41 OA, alternative lubricants and lubricant additives, and material modifications. TEST PROTOCOL The 200hour test protocol is designed to heavily stress the vane to vane slot bearing, shaft journal bearings, and the roller o.d. to vane nose bearing. The roller o.d. to vane nose bearing is the most sensitive bearing in the rotary compressor and performance of that bearing is usually indicative of expected mechanical reliability, given the compressor designer has followed good design practice regarding material selection, lubricant selection, and journal bearing loading. The compressor load condition of500 psig (35.2 kg/cm) discharge pressure and 125 psig (8.80 kg/cm) suction pressure for R22, was selected to provide high overall bearing loads (See graph #1). It is desirable to be able to test many different rotary compressor designs from a wide range of manufacturers using a single test protocol. The load condition selected is compatible with generally available motor maximum running torque so that any rotary compressor can be tested with it's designed motor, as manufactured. At operating test pressures, a certain level of liquid floodback is provided and monitored by sight glass observations and by tracking convergence of accumulator-in versus evaporator-mid temperatures. The level of liquid floodback to provide appropriate test acceleration, within the 200 hour test period, was developed from designed experiments and correlated to actual field return compressor cut-apart analyses, to ascertain that the bearing wear modes were consistent with field experience. Even at the high test pressures, it is the level ofliquid floodback which primarily determines the acceleration factor. If the return gas is superheated, gas pressure loading plays the predominant role in bearing stress and wear. A small amount of liquid flood back reduces the quality of lubrication at the vane nose roller interface and simulates actual field conditions created by dirty evaporator filters, dirty condensers, and some abnormal heat pump operating conditions. A large amount of liquid floodback does not normally duplicate bearing performance observed from field returns, even though the acceleration factor is greatly increased. To demonstrate the sensitivity of compressor bearing performance to load conditions and level of liquid floodback after 200 hours on test, standard compressors were ran using a marginal lubricant. The results provide an example of sensitivity to test conditions which has been confirmed over many years and several hundred compressor tests. This test series provides a qualitative and quantitative comparison of bearing performance under conditions of superheated return gas, small amount of :floodback, and large amount of floodback at the standard test protocol operating pressures of 500 I 125 psig (35.2 I 8.80 kg/cm). Within the 200-hour test period, significant differences can be seen and measured in bearing wear at each of the three test conditions. See figure # 1 for photo and metrology comparisons. The compressor tested with superheated return gas showed only polish bands on the roller o.d., no indicated wear on the vane nose, and very light polish on the shaft journals. The