Compressed air systems, which are found in virtually every manufacturing plant, are used in countless applications. In fact, there are more than 100,000 air compressors in the U.S. today, providing compressed air for tools, pneumatic equipment and instruments, among other things. In some cases, loss of compressed air in a facility can actually cause a plant shutdown. It’s no wonder, then, that keeping air compressors running is so critical in plant operation. Doing so, however, calls for special types of lubricants, as many compressors must run in severe environments.Currently, there are more than 90 suppliers of air compressor lubricants. Proper selection—and subsequent monitoring—of the products they offer is essential to the reliability of your equipment.
Air compressor types
Table I lists the various air compressor types and lubricants used.
In the past, reciprocating compressors were used extensively to provide compressed air. These units have been largely replaced by flooded rotary screw compressors—which currently account for more than 80% of the market. A small percentage of rotary vane compressors also are utilized. Centrifugal and dry screw compressors are in service where oil-free air is required. The following discussion will focus on the main compressor types:
Flooded rotary screw compressors…
Flooded rotary screw type compressors (Fig. 1) are the most common. Compact in size, they run very efficiently and quietly. A single-stage model will produce 100-200 psig of air; with two stages, pressures of 400 psig and a capacity of 1150 cfm can be reached. Sump volumes range from 10-60 gallons, with 15-30 gallons being typical.
Lubricated components, including radial and tapered roller thrust bearings and the helical speed gear, are lubricated separately from the screws where lubricant is injected with the incoming air supply to also provide sealing between the male and female rotors, as well as for cooling and rust protection. Air discharge temperatures are usually 100 F or more over the ambient temperature.
Because of the high oxidative environment consisting of moisture and high temperatures, rotary screw units usually require synthetic lubricants as discharge temperatures approach 200 F. OEMs have their own customized synthetic lubricants, which are supplied for their respective compressors. Many OEMs require use of these lubricants during the warranty period.
Table II gives the approximate life of a lubricant based on type and temperature. Remember that the oxidative life of the lubricant is directly affected by temperature. As a general rule, the life decreases by 50% for every 18 F rise in temperature. Different manufacturers may have higher or lower numbers based on the formulation of the finished product.
Flooded screw compressors are directly affected by the quality of the incoming air supply—which can have a major effect on the oil. Acid gases can cause severe corrosion of the screws and other components. Many compressor oils are formulated with neutral sulfonates (either calcium or barium) as corrosion inhibitors to neutralize acid gas intrusion. Monitoring the pH and acid number will indicate the depletion of the corrosion inhibitor. In some instances, monitoring of the incoming air source with the use of coupons to measure corrosivity is recommended.
At one time, reciprocating compressors (Fig. 2), were the most popular type. That’s because of their efficiency and ability to attain high pressures. Over time, however, they have been replaced in popularity by the more compact—and easier to maintain—rotary screw compressor.
Reciprocating compressors range from one to three stages and typically operate between 100 and 250 psig. These recip units use splash lubrication for the cylinders, while larger ones have separate lubrication for the frame and cylinder region. At lower temperatures, mineral oil may be used—but carbonization of the exhaust valves can be a problem with paraffinic mineral. This condition can result in hot gases being recompressed, resulting in ignition of the deposits. Naphthenic oils may have been used in the past because of their formation of soft deposits and solvency. However, in light of their low viscosity index and low oxidative stability, they are no longer recommended.
Today, the recommendation for reciprocating compressors at high temperatures is to use diesters because of their high thermal stability and excellent solvency for any deposits. Although PAGs have also been used successfully, there may be compatibility issues. While PAOs have high thermal stability, they are not recommended because of a tendency to form hard deposits upon oxidation. Typical ISO VG recommendations are 100-150. Some manufacturers offer an intermediate grade with a viscosity of ~125 cSt, which falls outside of the normal ISO Viscosity Grades of 100 and 150.
Centrifugal compressors (Fig. 3) typically are available in two- to four-stage models, with normal operating pressures of 100-125 psig. These dynamic units can operate at speeds up 50,000 rpm, producing large volumes of oil-free air.
At the high speeds and tight clearances in which these compressors operate, selection of the correct viscosity ISO VG 32 is critical. Any particulates entering with the air can cause severe damage to the gears and impellers. The tight bearing clearances require lubricants that will not varnish or form deposits. Ingersoll Rand requires the use of a specially formulated PAG/POE synthetic blend called Techrol Gold with a viscosity ~32 cSt with their centrifugal units. PAOs and high-quality turbine oils also are used by other centrifugal compressor OEMs.
Small, quiet rotary vane compressors operate at low pressures of ~100 psig. This type of unit is flooded with oil; the lubricated components are the vanes, ring and bearings. Depending on the OEM, viscosity grades 32-150 are used—ISO VG 100 is the most common. Because of boundary lubrication between the vanes and ring, anti-wear additives are required. High temperatures associated with rotary vane compressors require the use of synthetics—which will most commonly be a diester because of its thermal stability and solvency.
Dry screw compressors…
Dry screw compressors, which supply oil-free air at low to moderate pressures, are not as common as flooded screw units. The lubricated components are the timing gears and bearings—usually lubricated with ISO VG 32-68 mineral oil with good anti-wear properties.
Air Compressor Lubricant Properties
1. Mineral Oil:
2. Polyalphaolefin (PAO):
4. Polyalkylene Glycols (PAGs)
5. Polyol Esters (POE)
OEM compressor oils
Tables III and IV describe the base stock composition of rotary screw and centrifugal air compressor oils for the most common OEM equipment.
All PAOs have a small amount of diester or some other synthetic to increase solvency and to counteract slight seal shrinkage. PAGs for air compressors are formulated with POEs or diesters to counteract PAG seal shrinkage. The above table lists only base stock types. A very important consideration when switching from OEM oils to another supplier is the total formulation package, including the additives which enhance the performance properties of the fluid. There are many suppliers of air compressor fluids that perform well in most rotary screw compressors. The only fluid that requires the same type of lubricant base stock is the silicon variety—because of its incompatibility with all other synthetic fluids. When switching from PAG/POE type fluids to PAOs, flushing is required because of the incompatibility of the two fluid types.
Various flushing procedures are provided by the compressor fluid suppliers. For example, when switching from a PAG/POE to PAO, or vice versa, use a diester to flush the system and then fill with the new fluid.
Polyol Esters have the best performance properties of the common air compressor fluids—and are being recommended by some OEMs for extended warranty. They are compatible with all fluids except silicones and have the highest thermal stability. They also can be used in temperatures up to 240 F. (Most rotary screw compressors will not exceed 230 F before shutdown occurs.) The only issue is cost, but the differences are not as large as one might expect. Table V reflects the relative cost of the various fluids in five-gallon pails. These are general prices and may vary from supplier to supplier. These prices also may differ significantly from OEM compressor fluids—which in some cases may be 30-50% higher based on the fluid type and the OEM. Clearly, if consideration is given to switching from an OEM fluid to a lower-cost outside supplier fluid, one must consider the warranty implications. In addition, the user must be satisfied that the new fluid will perform as well or better than the existing lubricant.
Oil condition monitoring
It is recommended that oil analysis be performed on flooded rotary screw compressors on a routine basis to identify potential wear problems and to evaluate the oil condition. Many times, the life of the oil can be extended based on oil analysis data.
The remaining oxidative life of a fluid is determined by running the acid number, also referred to as TAN, which stands for total acid number. As oils oxidize they produce weak organic acids, which are measured by titrating with KOH. The results are expressed as milligrams of KOH required to neutralize one gram of sample. The new oil acid number is compared to the used oil acid number to determine the degree of oxidation.
Viscosity changes are also monitored to indicate oxidation problems and also the introduction of a heavier or lighter contaminant fluid. Most of the time, other indicators will condemn the fluid before viscosity changes do.
Some air compressors operate in a potentially corrosive air environment. If this is the case, pH is a good indicator of the introduction of acid gases and also the depletion of the corrosion inhibitors.
Table VI presents some guidelines for acid number oil monitoring of the three most common fluids for screw compressors.
Some OEMs condemn acid numbers at 1.0; some labs condemn at 1.5. Table VI reflects the common values in the industry—but you need to follow the guidelines of your OEM, based on its fluid type and warranty conditions.
Table VII illustrates several other oil condition tests, and the same limits apply to the three most common air compressor fluids.
OEMs and some outside suppliers provide oil analysis services for condition monitoring of their fluids. There are many outside oil analysis laboratories who also evaluate oil and equipment condition.
Keep in mind that an oil analysis program is recommended for any compressors that are critical in their service. It allows changing the fluid at an optimum time and identifying any equipment problems before they become serious.
Compressors often operate under severe conditions and require high-performance fluids—many of which are synthetics specifically designed for compressor use. Because of warranty requirements, OEMs supply a large amount of fluid to the market. There are also a large number of outside suppliers who have formulated high-quality air compressor fluids who actively compete in the market with OEM fluids, which can be 30-50% higher in price. Careful consideration is required when switching from one supplier to another. Even when switching to the same fluid type, formulations may differ enough to affect performance. Be particularly careful when switching to a different fluid type, and adhere to the flushing procedures recommended by the new supplier.
There are many high-quality fluids in the marketplace that provide excellent air compressor performance. In the past, diesters were the fluids of choice for rotary screw compressors—the most common compressor type today. Now, PAOs and PAG/POEs are used extensively. Use of POEs is growing rapidly because of their excellent performance. They are recommended by some OEMs for extended warranty.
Routine oil analysis is an important consideration to monitor fluid life for optimum change intervals and to identify early any potential equipment problems.
For a more in-depth analysis on the mechanical aspects of rotary screw compressors, please refer to the article by Heinz Bloch entitled "Screw Compressors: Operating Principles & Sealing," which was published as a two-part series in this magazine (Jan./Feb. and Mar./Apr. 2008). LMT
The author wishes to thank Tim Taylor, Bob Sturdy and Kent Brandon of Summit Industrial Products for providing valuable assistance in the preparation of this article.
Special thanks also go to Dow Chemical for information used in this article, and to Daryl Beatty, formerly of Dow Chemical, for his mentoring on lubrication of compressed air equipment.