Conference Agenda

The large capacity scroll compressor(25HP) we studied was successfully operated with the thrust-slide bearing fully loaded by the axial-compliance-equipped housing due to gas compression pressure. The thrust slide-bearing exhibited high performance lubrication with no serious trouble. The mechanism for such high performance has been a mystery for a long time. The mechanism of this high performance lubrication needs to be revealed to permit further development of large capacity scroll compressors with even higher performance.

The present study focusses on the recently uncovered lubrication mechanism, denoted as precession-rolling-induced oil film pressure (scheduled to be reported by Prof. Anami: New Understanding of Precession-Rolling-induced Oil Film Pressure in Thrust-Slide Bearings in Scroll Compressors). Our scroll compressor consists of the fixed scroll on the top, the orbiting scroll in the middle and the axial-compliance-equipped housing on the bottom, pressed upward by an intermediate pressure and high pressure. First, detailed FEM simulations were performed for the three compressor components under correct contact and gas compression conditions, thus identifying the 3-D elastic deformations and the attitude of the thrust plate against the upper surface of the housing. Subsequently, assuming a small mean lift of the thrust plate, the precession-rolling-induced oil film pressure was numerically calculated by applying the theoretical non-dimensional formulations from which the resultant oil film force pressing the thrust plate upward was calculated. As a result, the converged mean lift of the thrust plate was such that the calculated upward oil film force balanced the resultant downward axial force due to gas compression. Finally, the identified mean lift of the thrust plate was compared with the measured value for the actual machine under operation; the two values were in good agreement. In addition, the conventional wedge-induced oil film force was numerically calculated for the converged lift of the thrust plate, showing that the wedge-induced oil film force contributed a small percentage of the lifting force relative to that provided by the precession-rolling-induced oil film force.

This study presents an investigation carried out to build an understanding of the dissociative heat of low-GWP refrigerant/lubrication oil mixtures. The dissociative heat is the heat that is generated (or absorbed) when the refrigerant gas dissolves in (or dissociates from) the refrigeration oil. Understanding of this behavior is important because the lubrication property of the lubricant oils is influenced by the thermal interactions with their surroundings. A theoretical model that predicts the dissociative heat was derived by combining a solubility model with the Clausius-Clapeyron equation and solving for the enthalpy change at the refrigerant-gas/lubricant phase equilibrium. Once the dissociative heat model was built, solubility data of low-GWP refrigerant/lubrication oil mixtures was collected from the literature and the model was tested. The considered mixtures were namely, R1234yf/POE(POE75), R1234ze(E)/POE(POE75), R1243zf/POE(RL68H), and R290/mineral oil mixtures. The calculated dissociative heat values were found to be generally consistent for all the mixtures. It was found that the dissociative heat is strongly affected by the system pressure. Analysis of the results seem to suggest that the further the state of the refrigerant gas from its corresponding saturation state at a given temperature, the higher the dissociative heat will be. Therefore, decrease in the system temperature at constant pressure or decrease in the pressure at constant temperature will yield low dissociative heat values.

Refrigerants considered or being considered as lower GWP options in multiple types of compressor designs and system applications, cover a broad range of chemistries. With every refrigerant chemistry difference, there is most likely going to be different lubricant chemistries, viscosities and formulations required to meet demand for reliable and efficient operation. Looking at a Strengths, Weaknesses, Opportunities and Threats (SWOT) type of approach, typically used to understand business practices, can be used to help define combinations and eliminate unlikely candidates.

Market changes in refrigerant candidates has created gaps in maintaining success with some current lubricants used with current higher GWP refrigerants. In addition, demand for more applications, such as high temperature heat pumps or operation in higher ambient environments, call for investigation of thresholds in some combinations or options. Numerous refrigerant manufacturers and suppliers can have different approaches to what products they want to offer which increases the number of candidates to evaluate and can strain evaluation timetables and cost by all the groups involved in the testing and approval process.

This paper approaches some refrigerants and lubricants with a modified SWOT tactic for design, picking candidates and initiating a screening processes which utilizes several techniques to understand acceptability. Both synthetic and natural refrigerants will be discussed along with evaluating some applications that are current topics of interest. Some lubricants used with current refrigerants are not the best candidates and, in some cases, not suitable with particular lower GWP refrigerants, resulting in needed changes or variations to lubricant chemistries. The lower stability of some newer refrigerants along with some applications looking to minimize carbon footprint by utilizing electrification and higher temperature limits creates issues with lubricant and refrigerant stability, compatibility, lubrication, and proper interaction that impacts compressor and system reliability and efficiency.

Refrigerant-oil foaming is common in compressors found in many HVAC systems. Foam generation is caused by the degassing of refrigerant from oil during compressor startup. Excessive foaming can cause lubrication loss, insufficient lubrication, oil leakage and other problems impacting vapor compression cycle performance. Low-GWP refrigerants, particularly HFOs and blends, have un-characterized foaming behavior. Characterizing and understanding foaming behavior is necessary for the integration of new refrigerants in vapor compression cycles so as to prevent poor system performance due to insufficient lubrication and oil migration. In this work, an apparatus is designed and fabricated to study refrigerant-oil pairs in three manners: 1) physical foaming characterization, 2) measurement of dynamic surface tension between refrigerant-oil media, and 3) measurement of solubility data. The apparatus is designed such that all experiments can be performed in the same device for the purpose of rapidly generating data. Foaming is generated through induced pressure drop by charging and heating a chamber of refrigerant and oil and opening it to a low-pressure chamber. Surface tension data is measured in post-processing via a force balance of pressures, surface tension, viscous and inertia effects in a schema commonly known as the “maximum bubble pressure method”. Foaming experiments are measured by appropriate instrumentation and visually recorded with a custom made, high-pressure sight glass which utilizes pressurized water to attain higher internal refrigerant-oil pressures. Physical characterization of foaming and bubbles is carried out in post-processing with camera-tracking software, OpenCV. Dynamic viscosity and density data is measured directly with a circulation loop containing a viscometer and a densometer. To validate methods and experimental results of this project, foaming data is generated and compared to previous foaming studies on R-22/mineral oil and R-134a/polyoloester oil. Solubility data of oil-refrigerant mixtures is validated with external experimental data.

The growing global need for cooling systems exacerbates sustainability challenges by increasing energy consumption and environmental impacts. In compressors, thermal properties of the preference lubricant play a critical role in reducing energy consumption and enhancing efficiency. The thermal stability of lubricant is a critical factor determining the performance and efficiency of compressors in refrigeration systems. High thermal stability means the ability of the lubricant to remain chemically intact under high temperature and pressure conditions, which increases durability during long-term use. Furthermore, thermally stable lubricants maintain their quality by strengthening resistance to oxidation and minimize wear and failures in the system. This improves energy efficiency, ensures low maintenance and supports the long life and reliable operation of cooling systems such as refrigerators. With increased thermal conductivity, the heat transfer capacity of the lubricant improves, enabling efficient heat dissipation within the system, resulting in consistent cooling performance. Therefore, the thermal properties of lubricant are an important parameter to consider in order to optimize the overall efficiency of refrigeration systems.

In this study, it is aimed to investigate the thermal properties of lubricants and to examine their effect on compressor performance. Within the scope of the study, the weight loss stages of lubricants under high temperatures and their degradation reactions at high temperatures were investigated. The study investigated the thermal stability of lubricants through Thermogravimetric Analysis (TGA), oxidation resistance using Differential Scanning Calorimetry (DSC), and reological behavior by creating viscosity-temperature curves. The contribution of the lubricant to energy efficiency was compared with compressor calorimeter tests at the end of the study. Thus, compressor efficiency has been improved by using a lubricant with stronger thermal stability.