CA1063592A

CA1063592A - Lubrication of refrigeration systems

Info

Publication number: CA1063592A
Application number: CA239,092A
Authority: CA
Inventors: Wolf-Peter Trautwein; Anthony Harlow
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1975-01-29
Filing date: 1975-11-06
Publication date: 1979-10-02
Also published as: DE2503613C3; FR2299398B1; FR2299398A1; DE2503613B2; GB1508349A; DE2503613A1

Abstract

ABSTRACT OF THE DISCLOSURE

This invention relates to lubrication of a refrigeration system wherein the lubricant comes into direct contact with the refrigerant and particularly such type systems employing a screw compressor utilizing a halogenated alkane refrigerant and a lubricant con-stituting a hydrogenated polyalkylene, the alkylene monomeric units of which contain between about 6 and about 12 carbon atoms.

Description

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BACKGROUND OF THE INVENTION

1 Field of the Invention.
. _ . , This invention relates to lubrication o~ a re-frigeration system and compositions used in effecting such lubrication.

2. Description of the Prior Art.

It has heretofore been known to lubricate rotary piston pos~tive displacement compressors o~ the type ^om-monly referred to as screw compressors. Such compressors, which ~ind application in refrigeration systems and other apparatus, are characterized generally by the creation of compression chambers formed by the intermeshing action o~
the lands of two intermeshing helically threaded rotors acting in conjunction with a suitable enclosing casing structure provided with inlet and discharge ports, the compression chambers decreasine in volume as the rotors revolve between the time of cutoff of the chambers from the inlet port and the time of communication o~ the chambers with the discharge port, to thereby provide a built-in com-pression ratio within the compressor.
- The above type compressors may be classified into two categories The earlier developed category was the so-called dry compressor. Such compressors have their compres-sion chambers sealed by space packing, that is, packing af-~orded by extremely close clearances maintained between the intermeshing rotors through the intermediary of synchronizing or timing gears and equally close clearances between the peripheries and the end surfaces of the rotors and the enclosing casing structure. In such compressors, there is always leakage from the compression chambers through these small clearance spaces and characteristically such compres-sors are always operated at high tip speeds of the circum-~erences of the rotors in order to reduce to as small an amount as possible the leakage loss through such clearance spaces and thus provide the highest possible volumetric 0 efficiency for the compressor.
The second category of the above-described com-pressors is the so-called wet or flooded type, in which a liquid lubricant is introduced for the dual purpose of pro-viding a liquid seal for closing the clearance spaces and also for the purpose of cooling the gaseous fluid being compressed directly during the compression phase of the - cycle. It is with lubrication of the latter type compressor that the present invention is particularly concerned. Such type compressors have been described in a considerable number o~ previous patents exemplary of which are U.S. 3,073,514;

3,084,851; 3,o88,658; 3,138,320; 3,283,996; 3,307,777;
3,314,597; 3,423,017; 3,432,089; 3,467,300 and 3,756,753.
; In addition to screw compressors, other wet type compressors contemplated as being suitable for refrigeration systems have been employed including, ~or example, rotary ~tane or Wankel type compressors. These latter compressors are, for present purposes, considered as being direct oil in~ection lubricated compressors.
; In its simplest aspects, commercial refrigeration involves evaporation 3~ liquid refrigerant from a container?

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compression of the resulting vapor to condensation pressure, condensation of the vapor by cooling ~he gas and return of the liquid to the container. In actual operation, the liquid container becomes an evaporator. m e vapor is compressed in a mechanical compressor and the hot high-pressure refrigerant is changed to a liquid in a condenser. The liquid refrigerant i8 collected and stored in a receiver from which its flow to the evaporator is controlled by an expansion valve. The temperature at which refrigeration takes place is controlled by the pressure maintained on the evaporating refrigerant.
The pressure to which the vapor must be compressed is con-trolled by the condensing temperature maintained by the avail-able cooling medium.
Lubrication of the compressor cannot be regarded separately from the whole system, since the liquid lubricant, l.e. normally employed mineral oil, carried over with the refrlgerant to the condenser and evaporator can interfere with the heat transfer and with the operation of the expansion ,. valve, and hence influence the overall efficiency of the re-frigerator system.
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Whlle mineral oils have heretofore generally been used in the lubrication of refrigerator compressors, there are properties of such lubricants which have to be compensated for in obtaining the best overall performance Thus, low temperature fluidity characteristics in mineral oil lubricants can only be obtained by using relatively low viscosity oils, made from selected naphthenic crudes using special refinery treatment levels. For bearing and cylinder lubrication, the ~ designer would usually prefer a somewhat higher viscosity oil : ~ .

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than can be obtained ~s dictated by the low temperature re-qulrements Furt~er, an important phenomenon in refrigerator compressor lubrication is the diluting effect on vlscosity of the lubricant by the tendency of the refrigerant to dissolve therein.
In refrigeratlon equipment of the direct oil in-~ection lubricated compressor type, the lubricant comes into contact wlth the refrigerant and in addition to lubricating the compressor, it serves as a seal between the low pressure and high pressure sides of the system. As the refrigerant flows from the c~mpressor through the condenser and evaporator and back to the intake side of the compressor, it carries some o~ tbe lubrlcant with it. Thus, while the lubricant is re-quired only at the compressor, it circulates throughout the ~15 system.
Problems have heretofore been encountered in the compressors forming part of a refrigeration cycle using a rerrigerant of a halogenated alkane, such as those normally referred to as "Freons" , which are soluble to a considerable extent with mineral oil lubricants. Thus the mineral oil lubricant supplied to the chambers for bearing lubrication, sealing, thrust balancing and similar purposes, normally has a pressure exceeding the pressure in the high pressure section oP the compressor and the amoun~ of "Freon"* refrigerant dissolved therein is considerable. This dissolved "Freon" refrigerant has ;
a diluting effect on the viscosity of the mineral oil lubricant which further compounds the disadvantage inherent in these attributable to their relatively low viscosities necessary to obtain a low pour point and "Freon" floc point. Th~ latter refers to the temperature at which a heavy precipitate of : *Trademark ~ 5 ~

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wax occurs ln a mlxture o~ Freon and 10 weight percent of the oll. The decreased viscosity of the oil ~ilm can lead to extreme thln film lubrication conditions~ which will -~
enhance wear. The tenacity of thls lower viscosity oil rllm wlll also be reduced and the oll may no longer be able to resist the blasting effect of liquid refrigerant droplets, carried;over with the gas stream under some conditions, causing rupture of the oil film whlch leads to increased wear. The lower viscosity of the oil film further results . . ..
ln a less efflcient seallng effect wlth resultant high com-pressor efficiency losses, In addition to mineral oil lubricants, synthetic r hydrocarbon lubrlcants, such as alkylated aromatics, have been proposed ~or use in refrigeration systems employing a ~'Freon"-type refrigerant. m ese synthetic lubricants, however, are completely miscible, as described in u.s. Patents 3,092,891 and 3,169,928, with preferred halogenated alkane refrigerants such as chlorodifluoromethane~and hence present some of the same problems noted above in connection with the use of ;; 20 mineral oil lubricants. Other prior art of interest includes . U.S. Patent 3,642,634 and U.S. Patent 3,733,85~ involving use of refriger- :
atlon lubricants, in combination with halogenated alkane re-frigerants, of an alkylbenzene having one or more side chains j .
of 1 to 25 carbon atoms and containing a total of from lO to 25 carbon ato~s in the alkyl groups and from about 2 to about 50 percent by weight of polyisobutylene. Such mixtures have the additional disadvantage of requiring careful control of proportions in the lubricant mixture.
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SUMM~RY OF THE INVENTION
In aooordan oe wqth the present invention, there is provided, a lubricant composition consisting essentially of a mixture of a minor proportion of a halogenated alkane refrigerant and a major proportion of a hydrogenated polyaIkylene, the aLkylene monomeric units of which oontain between about 6 and about 12 carhon atoms.
me present inventian also provides an improvement in a method of lubricating equipment of a refrigeration system in which a halogenated alkane ; is utilized as the refrigerant, the aforesaid improvement oomprising introducing into said refrigeration system a lubricant co~prising a hydrogenated poly-., aLkylene, the aLkylene monomeric units of which contain ~etween about 6 and about 12 carbon atoms.
As oo~pared with a mineral oil based refrigeration lubricant, the use of a hydrogenated polyaLkylene base lubricant possesses the advantages of a higher viscosity index, lower volatility and a greater thermal and chemical stability. Sinoe the hydrogenated polyaLkylene base lubricants do not contain wax-like materials, their pour points are naturally low.
FbrthermDre, the visoosity of the hydrogenated polyaLkylene lubricants i can be higher than for mineral oils sinoe there is no "natural" link with the pour point. In additian, the "Freon" refrigerants do not dissolve to .i the same extent as is the case with mineral oils, so that the effect of ; dilution is appreciably lower. m ese characteristics have been found to -~ render the hydrogenated polyaIkylene base lubricants eminently suitable for oompressors handling refrigerants of the"Freon" type. In systems where the viscosity has a pronounoe d effect on efficiency such as in screw oompressors, where the lubricant has a major function in sealing, the hydrcgenated polyaIkylene base lubricants afford a very substantial ; improvement in compressor efficiency.
ffle refrigerants used herein are the halogenated alkanes, oontaining fluorine, preferably having one or two carbon atoms, and - oommLnly called "Freons". Typical refrigerants include trifluonDchlDrometh:ne (R 13), trifluorobromo-:'~ . - ': ' , . . , . .. ~ . .. . :

1 0 6 3 ~92 methane (R 13Bl), trlchlorofluoromethane (R 11), dlchloro-dlfluoromethane (R 12), dichlorofluoromethane (R 21), chloro-- dlfluoromethane (R 22), trlchlorotrifluoroethane (R 113), ~ hexa~luoroethane (R 116), dichlorotetrafluoroethi~ne (R 114), - 5 azeotroplc mlxture of dichlorodifluoromethane and dlfluoro-ethane (R 500), azeotroplc mixture o~ monochlorodifluoro-methane and monochloropentafluoroethane (R 502) and chloro-. ~, ~ fluoromethane (R 31). Of these, the lower boiling "Freons"
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are preferred. These include chlorodifluoro~ethane (R 22), --10 dichlorofluoromethane (R 21), trichloro~luoromethane (R 11) ~ and dichlorodifluoromethane (R 12).
; The hydrogenated polyalkylene lubrlcant employed may be manufactured by techniques well known in the art such as described, for example, in U.S. Patent 3,199,178. Briefly, ,j .
-15 these entail polymerizing normal alpha-monoolefins (C6 to C12) either thermally or catalytically in the presence of a di-~ tertiary alkyl peroxide or a Friedel-Crafts catalyst such as ;~ aluminum chloride or boron trifluoride. Normal alpha-mono-,'-I
olefins having between about 6 and about 12 carbon atoms can be used, It is generally preferred that the mean value of the olefin chain length be about 10 carbon atoms. Rëpresentative olefins include l-hexene, l-octene, l-nonene, l-decene and 1-dodecene. Of these preference is accorded l-octene and l-decene ~ or olefin mixtures rlch ln either or both of these two olefins.
`~ 25 Generally~ t}e relative amounts of the halogenated . ~ .
alkane refrigerant and the hydrogenated polyalkylene lubricant are such that the resulting mixture coming into contact with ~, the compressor of thé refrigeration system contain a ma~or proportion and more particularly from about 60 to about 90 .
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weight percent hydrogenated polyalkylene and a minor pro-portion and more particularly ~rom about 10 to about 40 weight percent of halogenated alkane re~rigerant.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates miscibility behavior of mixtures of hydrogenated polydecene and chlorodifluoro-methane with temperature variation.
Figure 2 depicts miscibility behavior of mixtures o~ various mineral oil and synthe~ic lubricants and chloro difluoromethane with change in temperature.

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, DESCRIPTION OF SPECI~IC ~ODI~ENTS

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To illustrate physical characteristics of mixtures of a typical hydrogenated polyalkylene and a halogenated alkane refrigerant, determination of density~ viscosity, ." .
solubility and miscibility properties were carried out.
Density measurements on mixtures of (1) a hydrogenated poly-decene synthetic oil containing an antioxidant and an anti-wear additive and (2) chlorodifluoromethane were made. These measurements are useful for con~ersion of the dynamic viscosity to kinematic viscosity. Also, these measurements are use~ul for the calculation o~ ~low rates when such mixtures are mechanlcally pumped. The hydrogenated polydecene synthetic oil formulation had the following properties:

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Density (15C), g ml 1 0.832 Klnematic Viscosity at 37,8-C (100F~, cSt 61.0 50.0-C cSt 37.6 98.9-C (210F), cSt 9.3 Viscosity Index 149 ; ~ Pourpoint, C -60 Flashpoint, C 240 Ash (oxide), wt.-% < 0.01 Specific Heat (50C) cai g-l oC-l ~ o.48 The method used for density determination was similar to that described b~ H. J. ~ f~ler in Kaltetechnik, 11 (1959), 70/4, mis method involved use of spherical , 15 glass vessels with graduated capillaries (0.02 ml/division) ; and a total volume of about 25 ml. The vessels were cali-brated volumetrically with bromobenzene at t = 20C. before the experiments. m e required amount of hydrogenated poly-decene synthetic oil is drawn into the test vessel through a metal capillary. The test vessel is weighed to + 0.1 mg and the test refrigerant distilled over under vacuum. The ; test mixtures are frozen out in liquid nitrogen and after pumping down to P ~ 10~2 torr the vessels are sealed o~f with a prop~ne-oxygen torch. The exact` amount Or re~rigerant in the mixture is determined by reweighing the test vessel plus the pieces of drawn-off capillary.
Tubes with different mixture concentrations are prepared and placed in an unsilvered Dewar vessel. This is coupled to a circulating thermostat and controlled to ~ 0.01C.

The volume changes are determined over a range o~ te~per-- atures, thereby enabling the density of the mixture to be - calculated.
The density of the mixtures was determined in the temperature range of -20 to 70-C. ~or mixtures having hydro-, genated polydecene synthetic oil concentrations of 70, 80 ... .
~ and 90 weight percent. The results are shown in the follow-ing Table I:

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TABLE I

Density gm/ml Mixture Concentration TemperatureWt.$ Hydrogenated Polydecene Synthetic Oil t, C 7o. o 80.0 go. o loo . o -20 0.9573 0.9223 o,8867 o.8s35 -lo 0.9492 0.9146 0.8806 0.8480 o 0.9413 0.9075 o.8743 0.8422 o o. ~335 o. goog 0.8680 o.8364 o 20 0.9262 0.8935 o .8617 o .8307 0.9188 o.886s 0.8553 0.8249 o~ 9117 o.87g6 0.8490 0.8191 o. go46 0.8727 0.8426 0.8133 0.8979 o.8663 o.8364 0.8076 7o o.8go6 o.8sg8 0.8302 0.8018 Viscosity measurements on the above mixtures of hydrogenated polydecene synthetic oil and chlorodifluoro-méthane were made.
A falling ball viscometer similar to the type des-cribed by H. J. Lo'ffler in Karltetechnik, ~ (1960), 71/5 was used. The dynamic viscosity of the mixture is calculated from fall time, calibration constant of the falling ball and densities of the ball and mixture. Viscosities were measured in the temperature range of -20 to ~70C. at mixture concen-trations of 70, 80 and 90 weight percent hydrogenated poly-decene synthetic oil. Knowing the mixture density, the measured dynamic viscosity was converted to kinematic vis-cosity. The experimental results are shown in the following Table II:

TABLE II
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Klnematic Viscosity Centistokes Mixture Concentration TemperatureWt,% Hydrogenated Polydecene Synthetic Oil t, C 70,0 80,0 90.0 100,0 . -20 51.5 142.5 510 2950 -10 31.3 78.5 250 1010 0 20.2 47,5 138 482 13,8 30,9 82,0 253 9~9~ 20.~ 51.8 143 .
.: 30 7,41 15.0 34.4 86.o

4 5.77 11.~ 24.2 55.0 50 4.60 8.53 17.5 37.6 . 3,60 6,69 13,3 26.5 2.88 5.42 10.4 19.4 106359Z -!

Vapor pressure measurements on the above mixtures - of hydrogenated polydecene synthetic oil and chlorodifluoro-methane were carried out to determine the solubility of the latter in the synthetic oil, ;~
The procedure and apparatus used in determination of such measurements was that described by K. Mall in Kaltetechnik-Klimatislerung, 22 (1970), 257/9. Vapor pres-sure measurements were made at lO-C. intervals in the temperature range -20 to 70-C. for mixtures having hydro-genated polydecene synthetic oil concentrations of 70, 80 and 90 weight percent. The results are shown in the follow-ing Table III:

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TABLE III

Absolute Vapour Pressure P~ atmospheres : Mlxture Concentration ;~ TemperatureWt.% Hydrogenated_Polydecene Synthetic Oil ; 5 t, C 0.0 70.0 80.0 90.0 -20 2.50 2.44 2.18 1. 60 -lo 3.62 3.56 3. o7 2.11 + 0 5.08 4.95 4.21 2.88 o 6.94 6.72 5.56 3.62 o 20 9.26 8.81 7.10 4. 53 12.11 11.25 8. 94 5.61 15. 57 13~99 11.08 6.80 19.71 17. 25 13.50 8.12 24.62 21,05 16.27 9.57 30.42 24.96 19.25 11.05 . ' .

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A qualitative representation of the miscibility behavlor of the above mixtures of hydrogenated polydecene and chlorodifluoromethane is shown in Figure 1 where the temperature mixture concentration relationship is depicted. It will be seen from this figure that under the temperature range shown, i.e.
the normal temperatures to which the mixture of hydrogenated polyalkylene and halogenated alkane refrigerant will be sub-~ected, the components making up such mixture are partially miscible at hydrogenated polydecene concentrations o~ approx-~mately 1 to 50 weight percent. As the temperature is lowered, the degree of partial immiscibility increases up to a concen-tration of approximately 75 weight percent.
The complicated miscibility behavior-of the halo-genated alkane refrigerant with various oils is shoT~n in Figure 2 ~or chlorodifluoromethane with a naphthenic mineral oil (A) a naphthenic mineral oil/alkyl aromatic hydrocarbon combination (B), an alkyl aromatic hydrocarbon (C), hydro-~^ genated polydecene (D) and polyglycol (E) It will be seen by reference to Figure 2 that the miscibility characteristics for the hydrogenated polydecene lubricant (D) are quite dis-tinct ~rom comparable characteristics of the other illustrated mineral oll and synthetic lubricants. Thus, the miscibility 4~ .
of the halogenated alkane refrlgerant is lower in the hydro-genated polydecene lubricant than in other available refri-gerator lubricants, - l6

Claims

The embodiments of the invention in which an ex-clusive property or privilege is claimed are defined as follows:

1. A lubricant composition consisting essentially of a mixture of a minor proportion of a halogenated alkane refrigerant and a major proportion of a hydrogenated poly-alkylene, the alkylene monomeric units of which contain between about 6 and about 12 carbon atoms.

2. The lubricant composition of Claim 1 wherein the halogenated alkane refrigerant constitutes about 10 to about 40 weight percent of said mixture and the hydrogenated polyalkylene component constitutes about 60 to about 90 weight percent of said mixture.

3. The lubricant composition of Claim 1 wherein said hydrogenated polyalkylene is hydrogenated polyoctene, hydrogenated polydecene or mixtures rich in either or both of these hydrogenated polyalkylenes.

4. The lubricant composition of Claim 1 wherein said hydrogenated polyalkylene is hydrogenated polydecene.

5. The lubricant composition of Claim 1 wherein said halogenated alkane refrigerant is chlorodifluoromethane and said hydrogenated polyalkylene is hydrogenated polydecene.

6. In a method of lubricating equipment of a refrigeration system in which a halogenated alkane is utilized as the refrigerant, the improvement which comprises introducing into said refrigeration system a lubricant comprising a hydro-genated polyalkylene, the alkylene monomeric units of which contain between about 6 and about 12 carbon atoms.

7. The method of Claim 6 wherein said hydrogenated polyalkylene is hydrogenated polyoctene, hydrogenated polydecene or mixtures rich in either or both of these hydrogenated polyalkylenes.

8. The method of Claim 6 wherein said hydro-genated polyalkylene is hydrogenated polydecene.

9. A method for lubricating a compressor of a refrigeration system in which a halogenated alkane is utilized as the refrigerant which comprises introducing into said refrigeration system as a lubricant a hydrogenated polyalkylene, the alkylene monomeric units of which contain between about 6 and about 12 carbon atoms.

10. The method of Claim 9 wherein said hydro-genated polyalkylene is hydrogenated polyoctene, hydrogenated polydecene or mixtures rich in either or both of these hydrogenated polyalkylenes.

11. The method of Claim 9 wherein said hydro-genated polyalkylene is hydrogenated polydecene.