Synthetic oil in air compressors Environmental concerns and the awareness of limited resources accompanied the shift away from reciprocating air compressors, which required enormous amounts of lubricant, and discharged it into the air and the environment. The shift to rotary and centrifugal compressors addressed these issues and resulted in greatly reduced consumption of lubricants. Joseph L. Foszcz, Senior Editor, Plant Engineering magazine
Key Concepts
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Synthetic lubricants were developed to eliminate varnish buildup in compressors.
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Synthetic lubricants last up to 1 times longer than hydrocarbon lubricants.
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Synthetic lubricants reduce component wear.
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Environmental concerns and the awareness o! limited resources accompanied the shi!t away !rom reciprocating air compressors, which re"uired enormous amounts o! lubricant, and discharged it into the air and the environment. #he shi!t to rotary and centri!ugal compressors addressed these issues and resulted in greatly reduced consumption o! lubricants, did a better $ob o! %eeping !luid in the compressor, but also increased demands on the !luid.
&ydrocarbon lubricants in rotary air compressors were !re"uently changed to avoid varnish problems in the compressor. Synthetics were developed to resolve those problems. 'any synthetics are now biodegradable and even have limited solubility in water. #his usually means the small amount o! lubricant that leaves with the compressor condensate causes no problem !or wastewater treatment plants. (nother trend is the reduction o! labor !or maintenance. Longer)li!e !luids and compressors re"uiring less maintenance have accompanied this trend. #he megatrend toward energy conservation was met with a new generation o! rotary compressors, incorporating two stages, capacity controls, variable speed drives, and varnish)!ree synthetic lubricants that prevent the loss o! energy e!!iciency. *ecause o! the combination o! heat and concentrated o+ygen inherent to air compressors, this application pushes the limits o! what lubricants can withstand. #he service li!e o! synthetic lubricants could reach 1, hr, lasting about 1months in /0 operation. (ctual li!e will vary, depending on the conditions o! the speci!ic application. nder these severe conditions conventional mineral oils typically last only 12 hr. *y comparison, lubricants made !rom synthetic base stoc%s e+tend the drain interval. Inside compressors
3n a compressor, the !luid is recirculated in a closed system. 3t !lows !rom the compressor to the receiver tan%, which contains the oil separator. From there it !lows through a particulate !ilter, an oil cooler, and bac% to the compressor. 4arnish !ormation in the lubricant can plug coolers or oil separators, or bloc% oil !low to bearings, causing catastrophic !ailure o! the compressor. 'any !luids have di!!iculty surviving the combined attac% o! air and elevated temperatures, typically up to 1 F. 5+idation occurs rapidly because o! reactions between impurities in the lubricant, especially in mineral oil, and the o+ygen in the compressed air. 5+idation is accelerated by heat, causing an increase in viscosity and lubricant !ailure. Improved costperformance
Lubricating air compressors with synthetic compressor oils is a cost)e!!ective way to e+tend maintenance intervals and optimize compressor per!ormance. Savings are possible because synthetic oils greatly e+tend changeout intervals and eliminate varnish buildup inside compressors. For e+ample, a rotary screw compressor system re"uiring 6 gal o! lubricant typically needs only one !ill o! synthetic compressor oil, compared with !ive !ills o! conventional mineral oil in the same time period. #his represents a saving in oil cost o! about 167. (dd to this the savings in labor, !luid disposal, and less !re"uent !ilter changes and the total annual cost saving is 7. Keys to performance
(bove the operating temperature o! 1- F, the li!etime o! a mineral)based lubricant is cut in hal! by every 8F increase in temperature. (s a mineral oil degrades, byproducts o! its brea%down may cause a buildup o! a hard, stic%y varnish on internal metal sur!aces. #his buildup adversely a!!ects compressor per!ormance. Synthetic lubricants have greater resistance to o+idation at rated operating temperatures and reduce heat)generating !riction. #hey achieve !ull lubrication at startup and reduce component wear. #he synthetic9s resistance to o+idation and other properties leads to longer service li!e, e!!icient lubrication, and less !ormation o! undesirable byproducts in the lubricant. #he molecular structure o! the advanced synthetic !luids e!!ectively withstands short)term temperature spi%es as high as : F without damaging the !luid. #he result is improved lubricity and virtual elimination o! varnish !ormation. Compressor types
#here are currently three main types o! compressors used !or industrial air compression; reciprocating, rotary screw, and centri!ugal. #he lubrication needs and types o! lubricants commonly used in these machines, and the !actors that should be considered in evaluating them are di!!erent !or each application. !eciprocating compressor lubrication
Smaller, single)acting units are typically splash lubricated =Fig. 1>. #emperatures o! F in the compression cylinder can be e+pected. #here is typically no !iltration and the same lubricant !unctions throughout the unit.
#he lubricant in these units sees relatively severe thermal stress. ?ue to small sump sizes, high temperatures, moisture accumulation, and lac% o! !iltration, !luids with good antiwear properties are normally employed. 'ost large, reciprocating compressors are o! the double)acting type. ( crosshead isolates the cylinder, where compression occurs, !rom the sump, allowing the utilization o! two di!!erent lubricants to meet the speci!ic needs o! each location. #he sump o! the double)acting machine is a mild environment and hydrocarbon oils are typically used there. #emperatures are typically in the 1@2)F range, with little aeration, resulting in a very mild o+idizing environment. 3n this application, a lubricant may thrive !or several years be!ore analysis indicates a need !or service. !otary screw compressor lubrication
. Small sump sizes mean a small !luid capacity and increased demands on the !luid. Fluid
replenishment rate is a %ey !actor in the !luid li!e in these units. #here is also a trend to higher pressures and higher discharge temperatures.
Aentri!ugal compressors di!!er !rom rotary and reciprocating compressors because the lubricant is not in contact with the air that is being compressed =Fig. 6>. #he !luid serves only as a bearing and gear lubricant. ( centri!ugal compressor !luid has less severe o+idative stability re"uirements.
3t is critical that the lubricants in these units do not !orm deposits or varnish on bearing sur!aces. *earing clearances typically may run as little as .1 in., and any signi!icant reduction in clearances can result in overheating and damage to bearings. #he !luid condition o! synthetics in centri!ugal compressors is easily monitored. Proper viscosity, viscosity increase, and particulate levels are a critical concern in these units, as bearing designs and rotor stability are dependent on proper !luid viscosity. "hen not to use a synthetic
Bearly all compressor manu!acturers now use and recommend long)li!e synthetic !luids. E+ceptions to those recommendations are generally made a!ter the !act, when an application is !ound to have e+cessive levels o! acid gases and other air contaminants that cause early !luid !ailure, or accumulate in the !luid to cause corrosion in the compressor. 3n those cases, hydrocarbon !luids are generally recommended because o! low cost, and the !luid is changed on a 1)hr or more !re"uent schedule. #naly$ing compressor fluids
Chat are the goals o! an oil analysis programD #he most basic goal is to establish a !luid change interval, and to ensure the !luid in use is still suitable. Second, and uni"ue to rotary compressors, analysis serves to identi!y
problems and contamination in the inlet air, which is mani!ested as chemical and particulate contaminants in the !luid. Finally, various tools !or particulate measurement are use!ul in trac%ing and alerting to wear conditions. %luid change interval
#he change interval !or rotary compressors is typically dictated by contamination or the o+idation state o! the lubricant as measured by several metrics. #he typical change interval !or synthetic rotary compressor !luids suggested by manu!acturers is - hr. #his number is usually conservative, as many users do not per!orm any analysis at all, using only the change interval recommendations. nder good conditions with no contamination, the actual li!e o! synthetic !luids, as measured by analysis, can reach 1, hr. ( conservative change interval may still be wise to avoid contaminating the new oil, due to the di!!iculty o! getting all the old oil out o! the compressor. For centri!ugal compressors, actual !luid li!e with polyglycol and other advanced synthetics may e+ceed :, hr under normal conditions, due to the e+tremely mild nature o! the o+idative environment, and limited opportunity !or contamination. #ypical recommendations, in the absence o! analysis to veri!y !luid condition, range !rom - to 1@, hr. #he lower the operating temperature, the longer the synthetic !luid will last. PLANT ENGINEERING extends its appreciation to the Dow Chemical Co. for its assistance in the preparation of this article.
!ecycling compressor lubricants
For the name o! a waste lubricant recycler in your area, contact the Bational 5il or visit their website at "hat do they do, what does it cost&
'ost recyclers collect the !luid, charging a nominal !lat !ee !or pic%up plus about : per drum. Some have minimum amounts. #ypically, these companies then !ilter the !luid, dehydrate it, and sell it as bun%er !uel !or utility
companies, asphalt plants, cement %ilns, or industrial boilers. #his practice reduces the use o! new !uel oil while eliminating a waste product. "hat types of fluid will they ta'e&
#ypically, oil companies do not care whether the !luid is petroleum oil or synthetic, $ust so certain items are not mi+ed with it. #hey do not want the oil to be mi+ed with anti!reeze, trans!ormer oil =PA*9s>, solvents, or e+cessive water. *ut it usually does not matter i! petroleum oil is mi+ed with the synthetic, since it will be used as !uel. #he recycler may want to %now what the !uel value =*tu/lb> is. (rum (isposal
?rum recycling companies are located in most ma$or cities. #he recyclers re"uire that the drums, which have contained products such as lubricants, be as empty as possible by simply tipping them. ?rums, which have contained hazardous materials, must be triple rinsed. Bote; #he rinse solvent !rom hazardous materials is also considered to be hazardous. 'ost common types o! synthetic lubricants are not considered hazardous material !or purposes o! drum disposal. )ualification Caution* *e!ore using any oil or drum recycling company, it is highly
recommended that you con!irm that they are properly disposing o! the product. For e+ample, do they have a state or EP( permit numberD #he generator o! the waste =user o! the product> is responsible !or its proper disposal, even though an oil hauler may have carried it away. Typical synthetic benefits
Fewer oil changes Fewer !ilters Fewer replacement parts Fewer maintenance interruptions Lower waste lube disposal costs
Lower labor costs Power and energy savings E+tended e"uipment li!e
