AIR COMPRESSOR AND SYNTHETIC COMPRESSOR LUBRICANT SELECTION GUIDE 2 Compressed Air Fundamentals 2 Types of Compressors 2 Reciprocating Compressors 3 Rotary Vane Compressors 3 Screw Compressors 3 Equipment Selection and System Design 4 Compressor Selection Criteria 5 Energy wasted by compressed air system 6 Synthetic Compressor Lubricant Fundamentals 6 Table 1 - Synthetic Lubricants Used In Compressor Applications 7 The primary advantages of synthetics are: 8 Long life (extended drain intervals) because of high oxidation resistance 8 Reduced Maintenance Cost and Production Downtime 8 Reduced Product Cost by using up to 30% Less Oil Consumption and Carryover 8 Better Anti-wear Protection 8 Savings on Electric Power Costs 8 Safety 8 One Common Oil for Crankcase and Cylinder Lubrication 9 Compressor Efficiency 9 Inherent Cleanliness 9 Lower Sump Temperature 9 Better Water Condensate Separation 10 3-4 Times Longer Oil Separator Life 10 Improved Cold Weather Operation (wide operating temperature range) 10 Lower Acidity and Corrosivity 10 Low Toxicity 10 Oiltech synthetics are Completely Compatible with Petroleums 10 Excellent chemical inertness 10 Changeover procedure: 11 Start-up of new and rebuilt compressors 12 Condition Monitoring 12 Air Compressor and Synthetic Compressor Lubricant Selection Guide Compressed Air Fundamentals In order to understand the operation of a compressed air system, it is useful to know something about Boyle's law, which states that, provided the temperature is kept constant, the volume of a given mass of gas is inversely proportional to its pressure. This is true for perfect gases and for most purposes air can be considered to be a perfect gas. When we compress a gas, we do work by reducing the volume and increasing the gas pressure. If the compression is carried out slowly and the temperature kept constant, which requires cooling (isothermal compressor) the work input will be minimal. However, if a gas is compressed quickly, it gets hotter, the temperature rises and causes an additional increase in pressure which in turn increases the work input. Thus in order to minimise the work input, it is important to keep the working curve as close to an isothermal curve, by designing the compressor so that heat can be removed fairly readily as the air is compressed. This can be achieved by incorporating an efficient compressor cooling system. Alternatively, if compressed air is required at very high pressures, work input can be minimised by splitting the compressor into two or more stages, and cooling air between stages. Although it was suggested above that air could be treated as a perfect gas, air is in effect a mixture of gases. One of these gases is in fact water vapour. This can cause problems if it is allowed to condense and is not removed. Hence before factory air is distributed it should be cooled in the after-cooler to cause the water to condense, and a separator should then be used to remove the condensed water. Types of Compressors In most factories the compressors will usually be one of the following types. * Reciprocating * Rotary Vane * Screw Reciprocating Compressors This type of compressor operates in a similar way to a car engine. Air is drawn through an inlet valve into the cylinder by the descending piston. On the return stroke, the piston initially compresses the air, and finally pushes it out through the exhaust valve. It may be further divided into two broad classifications namely Water-cooled and Air-cooled. Other methods of classification include Cylinder/piston configuration: i.e. Horizontal or Vertical and Lubricated or Non lubricated cylinders types. Rotary Vane Compressors This type of compressor operates by means of a rotor fitted with sliding vanes. The rotor is mounted eccentrically within the stator, and when it rotates the sliding vanes form cavities which expand during one half of the cycle and contract during the second. Thus air can be drawn in, compressed and expelled as the rotor revolves. Screw Compressors This type of compressor has two matching helical rotors, rotating in opposite directions. The design of the screw rotor is such that volume between the rotors decreases axially and this decrease in volume compresses the air trapped between the rotors. Each compressor type has its own advantages and disadvantages, and the choice of which type to use will depend upon both the site details and the compressed air usage pattern. Fresh air is drawn into the compressor via an air filter and is usually compressed to a pressure of 7-8bar for distribution around the factory. The compression process generates a lot of heat and the compressor itself may either be air, oil or water cooled. An intercooler is fitted between the stages of a multi-stage compressor. Its purpose is to cool the air between the stages and also to condense out surplus water vapour before it reaches the next compression stage, to approach the ideal isothermal compression cycle. When the air leaves the compressor it is very hot, and if passed immediately to the receiver and pipework it will cool and the water vapour within it will condense. The condensed water may cause problems through rusting of the cylinders, valves, pipework, air tools and products. It is good practice, therefore, to cool the air immediately after it leaves the compressor, so that most of this water is condensed out before it reaches the receiver. Cooling of the air is achieved by an aftercooler which in itself can be either of the air or water cooled type. If the compressed air is required to be of high quality and extra dry, i.e. for instrument use, then it will be passed through a filter and a refrigerated or desiccant dryer before being distributed by the pipework system. Equipment Selection and System Design In order to select the correct type of compressor, several factors need to be taken into account. Firstly, the quality of air required by the plant the compressor is to serve, and secondly, the pressure at which the air is required. Air demand for each item of equipment can be determined from the manufacturers handbooks. This is then totalled for all the equipment which might be operating at one time. An allowance for air leaks should be added which even for a well maintained system, can be approximately 5%. The pressure at which the compressor must deliver air, is the highest pressure that any single item of equipment requires, plus an allowance for the pressure drop up to the point of use (ideally no more than 50kPa) Centrifugal and axial flow compressors are rarely used for general industrial applications as they are designed to produce large quantities of compressed air at generally lower than normal factory air pressure. Compressor Selection Criteria Criteria Reciprocating Rotary Vane Rotary Screw Noise Level Noisy Less Noisy Quiet if enclosed Size Least compact Compact Compact Oil Carry Over Moderate, except with Low to medium Low Oil Free models Vibration High Almost none Almost none Maintenance Many wear parts Few wear parts Very few wear parts Capacity Low to high Low to medium Low to high Pressure Medium to Low to medium Medium to high very high Efficiency at High, Shows Medium to high High Full Load significant advantage for large capacity 2 stage machines Efficiency at High Poor below Poor below Partial Load 60% full load 60% full load System cost ratio 1.0 0.6 0.6 (e.g. 100l/Sec at 700 kPa) The cost of operation should take in to account the average demand for air, compared with the compressor capacity. A more efficient compressor at partial loads can be more economical over a five year period than a cheaper less efficient model. Therefore individual assessment is required for each installation. Reciprocating Vane Screw Compressor Compressor Compressor Full Load Single Stage 0.40-0.45kw/l/s 0.35- 0.40kw/l/s Power 0.38-0.43kw/l/s Requirement Two Stage (at 700kPa) 0.30-0.35kw/l/s No Load 10%-25% 30%-40% 25%-60% Power as % of Full Load A typical Full Load and No Load Characteristics Energy wasted by compressed air system Approximately 10% of all the electrical energy used in industry is for compressed air. This is because it has an unlimited number of applications and is safe, reliable and versatile. Compressed air allows for the use of light and robust pneumatic tools and is widely used in the spray painting industry. Compressed air is also used as a power source for advanced production machinery by driving all pneumatic control systems. However, by nature of its production and distribution, this can be a rather inefficient form of energy if not carefully controlled. It is estimated that approximately 15% of energy is unnecessarily wasted in a typical compressed air system. A further 80% of the remaining energy is discarded as heat, which could be easily reclaimed for space or process heating. Such enormous energy waste is costing industry many millions of dollars every year, money which could be realised as profit or invested in productivity improvement. Synthetic Compressor Lubricant Fundamentals Since the development of a machine by man to compress air for his usage, petroleum has been utilized as the lubricant for the compressor. Today, many air compressors require lubricants with a higher performance level than can be satisfied with petroleum products. Conventional mineral oil based lubricant base-stocks are a complex mixture of naphthenic and paraffinic compounds containing nitrogen, oxygen, sulfur, as well as aromatic and other unsaturated hydrocarbons. Some of these are susceptible to thermal, chemical, and oxidative breakdown which results in sludge, varnish and polymerisation within the compressor. As these by-products accumulate, lubricant flow is disturbed. Efficiency and heat transfer are reduced, causing higher operating temperatures. If polymerisation occurs, the compressor could seize up on shutdown. Finally , sludge formation can trap abrasive dirt and wear particles causing equipment wear. Synthetic lubricants are manufactured in chemical reactors under controlled conditions. The result is a consistent lubricant base-stock free from any chemical contaminants Type of synthetics used in the lubrication of air compressors include: * Silicones * Phosphate esters * Diesters * Polyalkylene glycols * Synthesized hydrocarbons. Table 1 - Synthetic Lubricants Used In Compressor Applications LUBRICANT OXIDATION LUBRICITY VISCOSITY ELASTOMER COST/LITRE STABILITY INDEX COMPATIBILITY UNIT Petroleum Fair Good Fair Very Good 1 Silicones Very Good Fair Excellent Good 33 Phosphate esters Good Good Poor Poor 3 Diester Excellent Very Good Excellent Good 5 Poly Alkylene Glycols Fair Good Good Fair 5 Synthesized Very Good Good Good Good 5 Hydrocarbons Note: Compatibility with diester is good with exception of neoprene and low nitrile Buna "N". Elastomer supplier should be consulted in case of doubt. The primary advantages of synthetics are: Long life (extended drain intervals) because of high oxidation resistance Approximately eight fewer oil changes compared with petroleum oil. Use of the proper synthetic compressor lubricants will provide 8000 hours in screw compressor, 4000 hours in vane compressors and an eight fold extension of the required change intervals when using standard petroleum based oils in reciprocating compressors. These extended drain intervals are made possible because of the vastly superior oxidation resistance combined with much better thermal stability of the synthetic base fluid. All this results in less oil to buy, less stock, fewer change outs and reduces your waste oil problem. Reduced Maintenance Cost and Production Downtime Oiltech synthetics have high natural solvency and superior thermal stability which results in cleaner operation. Fewer valve inspections are required and a dramatic reduction in the hours or days normally required for the messy job of cleaning deposits from valves, inner and after-coolers. Reduced Product Cost by using up to 30% Less Oil Consumption and Carryover Very low volatility (about 30 times lower than petroleum oils at 100oC) results in a reduced level of consumption. The higher density of synthetics contributes to more efficient oil separation and lower carryover. Better Anti-wear Protection Unlike petroleum oils, the synthetic fluids are polar and have a natural affinity to adhere to the compressor's metal surfaces. This, combined with the practical elimination of abrasive carbon deposits and optimised anti-wear packages results in a substantial saving in replacement parts and resultant labour costs. Savings on Electric Power Costs 2-7% less amperage is required because synthetics have up to 20% lower coefficients of friction and superior metal wetting properties compared with petroleum oils. Safety The combination of rust and carbon deposits and excessive oil in the cylinders, discharge lines, and after-coolers can lead to explosions. The rust acts as a catalyst and lowers the spontaneous ignition of oil-soaked carbon deposits to about 140oC in a 689 kPa air system. Also, the greater amount of oil present, the greater the chance of an explosion. In addition, pressure lowers the auto- ignition point of an oil. For example, a conventional petroleum oil having an auto-ignition temperature of 350oC at atmospheric pressure will have a reduced auto-ignition of 230oC at a pressure of 931 kPa. Discharge temperatures of 149oC and above are common for reciprocating air compressor cylinders and with faulty valves, high-ambient temperatures, or fouled inter-coolers, compressor cylinder temperatures can reach 232oC and above. With these temperatures and the presence of oil-soaked deposits or high pressures, a fire can ignite spontaneously. If there is an excessive oil accumulation in the compressed air system, the fire can propagate, feeding on the vaporized excess oil. The resulting shock wave develops extreme pressures and temperatures which , in turn, can cause an explosion at some weak spot in the compressed air piping, coolers or receivers. Synthetic compressor lubricants have flash points 40oC higher and auto-ignition temperatures 65oC higher than most petroleum based compressor oils, resulting in a reduced risk of explosive ignition and fire. This is further enhanced by the lower volatility/low vapour pressures of the synthetics and the reduction of carbon deposit hot spots - the most frequent source for air system ignitions. More information on compressed air system explosion may be obtained from Oiltech on request. One Common Oil for Crankcase and Cylinder Lubrication Eliminates the possibility of using the wrong oil for either application requirement. Compressor Efficiency The virtual elimination of valve carbon deposits ensures that recompression and loss of rated pressure are problems of the past. Inherent Cleanliness The excellent oxidation resistance of the synthetic base fluids substantially reduces the possibility of viscosity increase, thickening and the development of gums, varnishes and strong corrosive acids as are typical with degrading petroleum based compressor oils. All of this keeps the entire compressed air system clean. Lower Sump Temperature Typically 50 to 10oC cooler because of a 12% better specific heat (Oiltech synthetics hold more heat without a temperature rise) and 15% better thermal conductivity. Better Water Condensate Separation In sumps, traps and air/oil separators, due to the superior demulsification properties of Oiltech synthetics. This contributes to better lubrication and less damaging rust downstream. 3-4 Times Longer Oil Separator Life Prolong the life of costly air/oil separators elements by the practical elimination of oil sludging and varnishing. Improved Cold Weather Operation (wide operating temperature range) Outdoor units start easily because of lower pour points (some as low as -60oC), eliminating the need for seasonal oil changes. The very high VI property allows it to operate at a larger temperature range without significant changes in viscosity. This will ensure that the compressor receives optimum lubrication and anti-wear protection. The thermal conductivity of synthetic compressor oil of about 15% greater than mineral oils of equivalent viscosity allows it to cool more effectively, warm up faster and cool more rapidly at soak-back. Lower Acidity and Corrosivity Oiltech synthetic compressor lubricants exhibit far greater resistance to build up of strong acids (as is typical of degrading petroleum oils) ensuring maximum life from non-ferrous inner and after-cooler components. Low Toxicity Oiltech synthetic compressor lubricants are as low or lower in toxicity than petroleum based compressor lubricants. Additionally they are biodegradable. Oiltech synthetics are Completely Compatible with Petroleums However, significant dilutions with petroleum oil will reduce the typical high performance capabilities. Excellent chemical inertness Synthetic lubricants have provided successful solutions to compressor problems associated with chemically hostile gases. Reactions may occur with conventional mineral oils. Many chemical plant applications prohibit the use of petroleum based lubricants due to possible contamination of catalysts. Typical gases include: * Methylene Chloride * Sulfur Dioxide * Hydrochloric Acid * Ammonia Miscellaneous Process Gases * -CO2 (with acids, H2S, or oxidizer contaminants) * Chlorosiloxanes * Chlorinated Hydrocarbons * Gases containing trace mineral acids When a reactive gas is compressed, not only is the compressor equipment subjected to a potentially corrosive situation, but the lubricant itself is suseptible to chemical attack which can be intensified by the elevated temperatures and pressures present. Gas purity is crucial when the gas is used in a chemical process. Catalyst poisoning is also a consideration for these gases. For these reasons, only the highest quality base fluids can be used to lubricate compressors handling the following 'inert' process and industrial gases: * Hydrogen * Helium * Carbon Dioxide * Nitrogen The low volatility of synthetic compressor lubricants results in negligible contamination of the process gas by volatile components. Process gases are usually pure and anhydrous, so lubricant additives usually are not needed. Changeover procedure: * While all Oiltech synthetic air compressor lubricants are compatible with petroleum based oils, their high solvency properties and excellent dispersing capability means an initial flush is recommended. * Drain all old oil from the unit while the oil is warm. * Be certain that all condensate drain valves are emptied of all fluids and sludge. * Remove all pipe plugs in the compressor, oil reservoir, separator and oil lines and drain completely. * Clean carbon from discharge valve * Remove the heads of inter-cooler and inspect tubes on water side for scale. Remove latter by rodding and flushing. Also clean the air side of shell to remove any remnants of previous lubricant. This may be done by steaming thoroughly or by using one of the following agents, depending on the extent of the deposits: methylene chloride, mild caustic, carbon remover, or acid. Consult your air compressor representative for advice on best cleaning method. * Insure that lubricant check valves are in operating condition * Reassemble, pre-lubricating parts with synthetic compressor fluid where necessary. Note: Due to the continuing deposit cleaning characteristics of Oiltech synthetics, filters, traps and air/oil separators should be regularly monitored in the early stages for fouling by loosened gums and resins. Start-up of new and rebuilt compressors Running in of new or rebuilt compressors can take 50 to 100% longer with synthetics than with petroleum oils due to better anti-wear properties of synthetics. It is therefore recommended that new or rebuilt units be run for 150-300 hours on mineral oil before upgrading to the synthetic. Condition Monitoring During long term operation, it is advantageous to monitor the condition of the lubricant. This procedure enables proper maintenance schedules to be developed for each individual plant and applications. Normal tests include: * Viscosity * Acidity * Particulate count * Moisture In addition, filtergram is also utilized in order to determine: * Wear elements * Contaminant types Spectrographic analysis is utilized to determine: * Additive levels Service life of diester based lubricants will range from 4000 to 8000 hours, depending upon compressor design and ambient conditions. Vane type compressors experience a lubricant service life of 4000+ hours. Screw compressors show a typical service life of 8000+ hours. Air cooled reciprocating compressors achieve 8000 hours when operating on diester fluids. Air Compressor and Synthetic Compressor TECHNICAL BULLETIN Lubricant Selection 6 Oiltech Australia Pty. Limited 10 KYLIE PLACE, CHELTENHAM NORTH, VICTORIA 3192, AUSTRALIA PHONE:+61 3 9553 2544 FAX:+61 3 9553 4846 E-mail: info@oiltech.com.au 08 Oct., 99