Selecting Lubricant for the Ammonia Refrigeration System

When selecting a lubricant for your ammonia refrigeration system, several requirements should be considered. 1. Is the lubricant approved by the manufacturer of the compressors? Most refrigeration compressor manufacturers and packagers provide a list of lubricants that they recommend. Some do not consider the application of the compressor or the type of system in which it is used. For ammonia compressors, the usual viscosity recommendations are 68 centistokes (300 saybolt seconds universal, SSU). The normal question might be: “Would my compressor operate more efficiently using a less viscous lubricant?” I have asked the question of several screw compressor manufacturers, and their usual answer is “no.” Always follow the manufacturers’ recommendations and never use any additive that is advertised to increase your system capacity by 15% or 20%. 2. Is it readily available? The lubricant supplier should maintain a stock near your plant or should keep it in your plant. Note that lubricant should not be kept in the equipment room. 3. What is its vapor pressure? Besides the viscosity requirement the lubricant vapor pressure is most important. All lubricants have a volatile component that becomes a vapor at elevated temperatures and pressures. These vapors cannot be separated in a coalescing element, which is the normal lubricant separation device used in screw compressors. These coalescers are usually about 99% effective while separating lubricant aerosols (a fine mist resembling tobacco smoke) from the ammonia discharge gas. To keep lubricant carryover to a minimum, the selection for mineral lubricants should have a vapor pressure of at least 7.7 × 10 mm of mercury at 150°F. This ensures around 2 ppm carryover provided the manufacturer of the separator supplied an efficient coalescer system. Coalescing separators are almost this efficient for reciprocating compressors. However, the carryover will be slightly higher because of the much higher discharge temperature, which will increase the vapor pressure. 4. Is it hydroscopic? Some lubricants such as esters are hydroscopic. They attract moisture from the atmosphere. Normally this lubricant type is not used in ammonia refrigeration systems. Hydroscopic lubricants should not be considered. 5. Is it soluble with ammonia at all operating temperatures and pressures? Mineral lubricants are only slightly soluble in ammonia. At 95°F (35°C) the solubility is 170 ppm and at –30°F (–34°C) it is only 10 ppm. This accounts for the lubricant accumulating at the lowest temperature component of the refrigeration system. Polyalphaolefin (PAO) lubricant, another lubricant that is excellent with ammonia refrigeration systems, has similar solubility characteristics. This property of ammonia refrigerants dictates the piping and equipment locations of the system. 6. What is the pour point of the lubricant? The pour point is important if the system operates at temperatures below –20°F (–29°F). It is best to have a lubricant with a pour point below the operating suction temperature. This pour point is controlled by the amount of wax left in the lubricant after it is dewaxed at the refinery. Many lubricants that indicate a –30°F (–34°C) pour point do not meet these criteria when subjected to the ASTM test, so ask for ASTM test results. Mineral lubricants are available that have undergone rigid processing (hydrotreated), have minimal wax content, and low pour points. The PAO lubricants have a pour point that approaches –100°F (–73°C). 7. Does it have the proper additives to make it compatible with the system? A reliable ammonia compressor mineral lubricant will have several approved additives, which provide better lubrication and system performance. a. Defoaming. The screw compressor lubricant separator is designed to separate the lubricant that is injected into the compressor during operation from the ammonia gas. Even with a change of direction when the lubricant falls into the sump, some ammonia will remain in the lubricant. It will gradually separate. However, this tends to cause foaming. A defoaming agent specifically designed for this duty is a must. b. Pour point extension. If the system is to operate at low temperatures, the lubricant should have a pour point depressant additive. Without the additive, wax will separate from the lubricant and “foul” the evaporators and the control valves. The evaporators then lose efficiency as does the entire system. A mere 1.5 mils of wax and lubricant coating an evaporator surface can cause a 23% loss in the heat transfer rate. c. Oxidation inhibitor. An oxidation inhibitor also is helpful in ammonia refrigeration systems. Where the system operates in a vacuum, air often is drawn into the system along with the moisture it contains. Maintenance procedures that require opening the system often introduce air into the system. Moisture air and any gases that might be generated by the breakdown of the ammonia or be a part of the plant charge can cause corrosion. This antioxidant is an excellent additive.