1、12.1CHAPTER 12LUBRICANTS IN REFRIGERANT SYSTEMSTests for Boundary and Mixed Lubrication . 12.1Refrigeration Lubricant Requirements 12.2Mineral Oil Composition and Component Characteristics . 12.3Synthetic Lubricants 12.3Lubricant Additives 12.4Lubricant Properties 12.5Lubricant/Refrigerant Solutions
2、 12.8Lubricant Influence on Oil Return. 12.15Lubricant Influence on System Performance . 12.17Wax Separation (Floc Tests). 12.21Solubility of Hydrocarbon Gases . 12.22Lubricants for Carbon Dioxide 12.23Solubility of Water in Lubricants . 12.26Solubility of Air in Lubricants 12.27Foaming and Antifoam
3、 Agents . 12.27Oxidation Resistance 12.28Chemical Stability 12.28Conversion from CFC Refrigerants to Other Refrigerants 12.28HE primary function of a lubricant is to reduce friction andTminimize wear. It achieves this by interposing a film betweenmoving surfaces that reduces direct solid-to-solid co
4、ntact or lowersthe coefficient of friction.Understanding the role of a lubricant requires analysis of the sur-faces to be lubricated. Although bearing surfaces and othermachined parts may appear and feel smooth, close examinationreveals microscopic peaks (asperities) and valleys. Lubricant, insuffic
5、ient amounts, creates a layer thicker than the maximum heightof the mating asperities, so that moving parts ride on a lubricantcushion.These dual conditions are not always easily attained. For exam-ple, when the shaft of a horizontal journal bearing is at rest, staticloads squeeze out the lubricant,
6、 producing a discontinuous film withmetal-to-metal contact at the bottom of the shaft. When the shaftbegins to turn, there is no layer of liquid lubricant separating the sur-faces. As the shaft picks up speed, lubricating fluid is drawn into theconverging clearance between the bearing and the shaft,
7、 generatinga hydrodynamic pressure that eventually can support the load on anuninterrupted fluid film (Fuller 1984).Various regimes or conditions of lubrication can exist when sur-faces are in motion with respect to one another: Full fluid film or hydrodynamic lubrication (HL). Mating surfacesare co
8、mpletely separated by the lubricant film.Mixed fluid film or quasi-hydrodynamic (or elastohydrodynamic)lubrication (EHL). Occasional or random surface contact occurs.Boundary lubrication. Gross surface-to-surface contact occursbecause the bulk lubricant film is too thin to separate the matingsurface
9、s.Various lubricating oils are used to separate and lubricate con-tacting surfaces. Separation can be maintained by a boundary layeron a metal surface, a fluid film, or a combination of both.In addition, lubricants also remove heat, provide a seal to keepout contaminants or to retain pressures, inhi
10、bit corrosion, andremove debris created by wear. Lubricating oils are best suited tomeet these various requirements.Viscosity is the most important property to consider in choosinga lubricant under full fluid film (HL) or mixed fluid film (EHL) con-ditions. Under boundary conditions, the asperities
11、are the contactpoints that take much, if not all, of the load. The resulting contactpressures are usually enough to cause welding and surface deforma-tion. However, even under these conditions, wear can be controlledeffectively with nonfluid, multimolecular films formed on the sur-face. These films
12、must be strong enough to resist rupturing, yet haveacceptable frictional and shear characteristics to reduce surfacefatigue, adhesion, abrasion, and corrosion, which are the four majorsources (either singularly or together) of rapid wear under boundaryconditions.Additives (e.g., oiliness agents, lub
13、ricity improvers, antiwearadditives) have also been developed to improve lubrication underboundary and mixed lubrication conditions. They form a film onthe metal surface through polar (physical) attraction and/or chem-ical action. These films or coatings result in lower coefficients offriction under
14、 loads. In chemical action, the temperature increasefrom friction-generated heat causes a reaction between the additiveand the metal surface. Films such as iron sulfide and iron phosphatecan form, depending on the additives and energy available for thereaction. In some instances, organic phosphates
15、and phosphites areused in refrigeration oils to improve boundary and mixed lubrica-tion. The nature and condition of the metal surfaces are important.Refrigeration compressor designers often treat ferrous pistons,shafts, and wrist pins with phosphating processes that impart acrystalline, soft, and s
16、mooth film of metal phosphate to the surface.This film helps provide the lubrication needed during break-in.Additives are often the synthesized components in lubricating oils.The slightly active nonhydrocarbon components left in commer-cially refined mineral oils give them their natural film-forming
17、properties.TESTS FOR BOUNDARY AND MIXED LUBRICATIONFilm strength or load-carrying ability often describe lubricantlubricity characteristics under boundary conditions. Both mixed andboundary lubrication are evaluated by the same tests, but test con-ditions are usually less severe for mixed. Laborator
18、y tests to evalu-ate lubricants measure the degree of scoring, welding, or wear.However, bench tests cannot be expected to accurately simulateactual field performance in a given compressor and are, therefore,merely screening devices. Some tests have been standardized byASTM and other organizations.I
19、n the four-ball extreme-pressure method (ASTM StandardD2783), the antiwear property is determined from the average scardiameter on the stationary balls and is stated in terms of a load-wearindex. The smaller the scar, the better the load-wear index. Themaximum load-carrying capability is defined in
20、terms of a weldpoint (i.e., the load at which welding by frictional heat occurs).The Falex method (ASTM Standard D2670) allows wear mea-surement during the test itself, and scar width on the V-blocks and/or mass loss of the pin is used to measure antiwear properties. Load-carrying capability is dete
21、rmined from a failure, which can becaused by excess wear or extreme frictional resistance. The Timkenmethod (ASTM Standard D2782) determines the load at whichrupture of the lubricant film occurs, and the Alpha LFW-1 machineThe preparation of this chapter is assigned to TC 3.4, Lubrication.12.2 2010
22、ASHRAE HandbookRefrigeration(Falex block-on-ring tester; ASTM Standard D2714) measures fric-tional force and wear.The FZG gear test method Institute for Machine ElementsGear Research Centre (FZG), Technical University of Munich pro-vides useful information on how a lubricant performs in a gear box.S
23、pecific applications include gear-driven centrifugal compressorsin which lubricant dilution by refrigerant is expected to be quite low.However, because all these machines operate in air, availabledata may not apply to a refrigerant environment. Divers (1958)questioned the validity of tests in air, b
24、ecause several oils that per-formed poorly in Falex testing have been used successfully in re-frigerant systems. Murray et al. (1956) suggest that halocarbonrefrigerants can aid in boundary lubrication. R-12, for example,when run hot in the absence of oil, reacted with steel surfaces toform a lubric
25、ating film. Jonsson and Hoglund (1993) showed thepresence of refrigerant lowers both the viscosity and pressure-viscosity coefficient of the lubricant, and thus the film thicknessunder EHL conditions. These studies emphasize the need for labo-ratory testing in a simulated refrigerant environment.In
26、Huttenlochers (1969) simulation method, refrigerant vaporis bubbled through the lubricant reservoir before the test to dis-place the dissolved air. Refrigerant is bubbled continually duringthe test to maintain a blanket of refrigerant on the lubricant sur-face. Using the Falex tester, Huttenlocher s
27、howed the beneficialeffect of R-22 on the load-carrying capability of the same lubricantcompared with air or nitrogen. Sanvordenker and Gram (1974)describe a further modification of the Falex test using a sealedsample system.Both R-12 (a CFC) and R-22 (an HCFC) atmospheres improveda lubricants bound
28、ary lubrication characteristics when comparedwith tests in air. HFC refrigerants, which are chlorine-free, contrib-ute to increased wear, compared to a chlorinated refrigerant with thesame lubricant.Komatsuzaki and Homma (1991) used a modified four-ball testerto determine antiseizure and antiwear pr
29、operties of R-12 and R-22in mineral oil and R-134a in a propylene glycol. Davis and Cusano(1992) used a high-pressure tribometer (HPT) fitted with a high-pressure chamber up to 250 psi to determine friction and wear ofR-22 in mineral oil and alkylbenzene, and R-134a in polyalkyleneglycol and pentaer
30、ythritol polyesters.More recently, Muraki et al. (2002) found a breakdown of fluo-rinated ether (HFC-245mc) over R-134a, using x-ray photoelectronspectroscopy (XPS) to study surface films generated in a ball-on-ring tribometer under boundary conditions. These films are moreeffective at preventing we
31、ar and friction. Nunez et al. (2008) used anHPT in a pin-on-disk configuration under a constant 203 psi pres-ence of CO2; XPS analysis showed that interactions between CO2and moisture in PAG lubricants formed carbonate layers.Advanced surface analyses (e.g., XPS) in the presence of refrig-erants can
32、 lead to a good understanding and correlation of lubrica-tion performance. Care must be taken, however, to include testparameters that are as close as possible to the actual hardware envi-ronments, such as base material from which test specimens aremade, their surface condition, processing methods,
33、and operatingtemperature. There are several bearings or rubbing surfaces in arefrigerant compressor, each of which may use different materialsand may operate under different conditions. A different test may berequired for each bearing. Moreover, bearings in hermetic compres-sors have very small clea
34、rances. Permissible bearing wear is mini-mal because wear debris remains in the system and can cause otherproblems even if clearances stay within working limits. Compressorsystem mechanics must be understood to perform and interpret sim-ulated tests.Some aspects of compressor lubrication are not sui
35、table for lab-oratory simulation; for instance, return of liquid refrigerant to thecompressor can cause lubricant to dilute or wash away from thebearings, creating conditions of boundary lubrication. Tests usingoperating refrigerant compressors have also been considered (e.g.,DIN Standard 8978). The
36、 test is functional for a given compressorsystem and may allow comparison of lubricants within that class ofcompressors. However, it is not designed to be a generalized test forthe boundary lubricating capability of a lubricant. Other tests usingradioactive tracers in refrigerant systems have given
37、useful results(Rembold and Lo 1966).Although most boundary lubrication testing is performed at ornear atmospheric pressure, testing some refrigerants at atmosphericpressures yields less meaningful results. Atmospheric or low-pressure sealed operation with refrigerant bubbled through thelubricant dur
38、ing the test has yielded positive results for refrigerantswith a normal evaporation pressure within 145 psi of the testingpressure under the normal compressor operating temperature range.Refrigerants that operate at high pressure, such as CO2, and zeo-tropic refrigerant blends, such as R-410A, requi
39、re testing at near-operation elevated test pressures.REFRIGERATION LUBRICANT REQUIREMENTSRegardless of size or system application, refrigerant compres-sors are classified as either positive-displacement or dynamic. Bothfunction to increase the pressure of the refrigerant vapor. Positive-displacement
40、 compressors increase refrigerant pressure by reducingthe volume of a compression chamber through work applied to themechanism (scroll, reciprocating, rotary, and screw). In contrast,dynamic compressors increase refrigerant pressure by a continuoustransfer of angular momentum from the rotating membe
41、r. As the gasdecelerates, the imparted momentum is converted into a pressurerise. Centrifugal compressors function based on these principles.Refrigerant compressors require lubricant to do more than sim-ply lubricate bearings and mechanism elements. Oil delivered to themechanism serves as a barrier
42、that separates gas on the dischargeside from gas on the suction sides. Oil also acts as a coolant, trans-ferring heat from the bearings and mechanism elements to thecrankcase sump, which, in turn, transfers heat to the surroundings.Moreover, oil helps reduce noise generated by moving parts insidethe
43、 compressor. Generally, the higher the lubricants viscosity, thebetter the sealing and noise reduction capabilities.A hermetic system, in which the motor is exposed to the lubri-cant, requires a lubricant with electrical insulating properties. Re-frigerant gas normally carries some lubricant with it
44、 as it flowsthrough the condenser, flow-control device, and evaporator. This lu-bricant must return to the compressor in a reasonable time and musthave adequate fluidity at low temperatures. It must also be free ofsuspended matter or components such as wax that might clog theflow control device or d
45、eposit in the evaporator and adversely affectheat transfer. In a hermetic system, the lubricant is charged onlyonce, so it must function for the compressors lifetime. The chemi-cal stability required of the lubricant in the presence of refrigerants,metals, motor insulation, and extraneous contaminan
46、ts is perhapsthe most important characteristic distinguishing refrigeration lubri-cants from those used for all other applications (see Chapter 6).Although compression components of centrifugal compressorsrequire no internal lubrication, rotating shaft bearings, seals, andcouplings must be adequatel
47、y lubricated. Turbine or other types oflubricants can be used when the lubricant is not in contact or circu-lated with the refrigerant.An ideal lubricant does not exist; a compromise must be made tobalance the requirements. A high-viscosity lubricant seals gas pres-sure best, but may offer more fric
48、tional resistance. Slight foamingcan reduce noise, but excessive foaming can carry too much lubri-cant into the cylinder and cause structural damage. Lubricants thatare most stable chemically are not necessarily good lubricants.Moreover, because refrigerant dilutes the lubricant and travels withit,
49、the lubricant exists in refrigeration system as a refrigerant/lubricant solution. This mixture dictates the lubricants ability toLubricants in Refrigerant Systems 12.3lubricate a compressor, and can affect other properties, such as oilreturn, in the rest of refrigeration system. It also ultimately deter-mines the lubricants effect on system performance in terms of heattransfer and system efficiencies.Although a precise relationship between composition and per-formance is not easily attainable, standard ASTM bench tests areuseful to provide quality control information on lubrican
