Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/601893
ML www.machinerylubrication.com | November - December 2015 | 43 The majority of compressor lubricants are synthetic. This allows them to have a longer service life and handle the rigors of the system better than mineral-based fluids. Most home air conditioners now use a blended refrigerant known as R-410a. A polyolester (POE) base oil is employed to help lubricate the system, but this oil can also separate from the refrigerant. Although the compatibility between the refrigerant and the lubricant is perhaps the most pressing issue in terms of lubrication, there are many others as well. For instance, moisture contamination can be very detri- mental to some synthetic base oils that are hydrolytically unstable. Moisture reacts with the base oil to form acids, change the viscosity and impair the oil's lubricating properties. This can lead to premature compressor failure as well as improper system cooling. Lubricant problems are common in any system. One way to avoid having issues with the gases being compressed is to simply remove the lubricant from the equation. This is a frequent occurrence with the "dry" compressors becoming more widely used. "Dry" refers to the lack of oil in the compression chamber. If the lubricant is not in the compression chamber, it is much less likely to mix with the refrigerant and cause problems. However, in wet or flooded compressors, the oil is present in the compression chamber and mixes intimately with the refrigerant. In these systems, the lubricant's compatibility with the refrig- erant is paramount. Many large compressors utilize a forced lubrication system, which features an oil reservoir, piping and a pump. The pump forces the oil through the piping and into the compressor where it lubricates and cools and then returns back to the reser- voir. These systems enable you to filter, cool and separate gases and water from the oil while it is in service. Smaller compressors typically are static- housed lubricant systems in which the compressor holds the oil and the system is completely sealed. Provided it is cleaned and sealed before being put into use, this type of system has a low likelihood of lubri- cant failure. Most often these systems will run for years without needing an oil change. The oil sits in the compressor to lubricate it, but some of the oil will flow through the refrigerant lines. In certain cases, an oil trap or knockout must be employed to keep oil from clogging the lines and reducing the system's cooling capacity. Oil Sampling In industrial plants, compressor systems tend to be among the most critical machines. Therefore, it is important to obtain oil samples periodically to check the health of the lubricant and the machine. Among the oil analysis tests performed on these fluids include elemental analysis, viscosity analysis and wear debris analysis. The viscosity must be monitored because refrigerant dilution can lead to a decrease in viscosity and an increase in machine wear. In some cases, oil samples must be degassed before they can be shipped to the laboratory or analyzed. Since the gas expands with temperature, it can result in a pressure increase in the bottle, causing a leak or the oil to erupt upon opening the bottle. While pressure-relieving caps can be used with these bottles, remember that every time you open the bottle, you expose it to contamination, which can affect your particle count results. With proper care and attention, the compressors in your refrigeration systems can provide years of trouble-free service. When changing the oil in these systems, keep in mind that it must be compatible with the refrigerant and the fluid previously used in the system. Finally, try to keep each compressor sealed, clean, cool and dry. If you are able to achieve this, you will stay cool even when the temperature outside gets hot. About the Author Wes Cash is a senior technical consultant with Noria Corporation. He holds a Machine Lubrication Technician (MLT) Level II certifica- tion and a Machine Lubricant Analyst (MLA) Level III certification through the International Council for Machinery Lubrication (ICML). Contact Wes at wcash@noria.com. 67% of lubrication professionals say oil samples are taken periodically from the compressors at their plant, based on a recent poll at MachineryLubrication.com