CA1074750A - Rotary screw compressor and method of operation - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A rotary screw air compressor using as a recircu-lating coolant liquid a dimethylsilicone, preferably forti-fied with a chlorinated cyclic derivative, as dibutyl chlorendate. Water is easily removed from the coolant liquid, enabling operation under conditions of minimum air temperature rise, with high efficiency. The coolant separates readily and thoroughly from the compressed air, providing high quality air.

Description

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ROTARY SCREW COMPRESSOR AND METHOD OF OPERATION
SPECIFICATION
This invention relates to a li~uid coolant, sealant and lub-ricant for an air compressor and to a method for using the liquid with a compr~ssor.
A rotary air compressor, as the rotary screw compressor of Bailey U.S. Patent 3,073,513 or Nilsson et al u.S. Patent 3,129,877, commonly uses an oil, as mineral oil, for a coolant, lubricant and ~alant. The liquid is introduced into the machine at or near the air intake, travels throu~h the machine with the air as it is com-pressed and the major portion of the liquid is dischar~ed with the compressed air. The liquid is separated from the compressed air an~ is r~used. A small portion of the liquid is directed to and lubricates the compressor bearing. Mineral oil functions well in many situations but has several deficiencies.
A principal problem is encountered where the compressor operates under conditions of high humidity. When humid air is compressed, the water content exceeds the dew point even though the temperature may rise as much as 100F. The water vapor is condensed and the water is physically mixed with the oil, forming an unstable emulsion which does not perform satisfactorily as a lubricant. In some situations an oil-wate-r separator must be prov-ided. See, for example, Hirsch U.S. Patent 2,701,684.
The mineral oil separated from the compressed air is cooled b~fore it is reinjected into the compressor. However, the degree of cooling must be limited to minimize water condensation. The temperature rise o the air in the compressor reduces compressor eficiency.
A substantial portion of the cooling liquid discharged with the compressed air is in an aerosol form and must be mechanically ;~
separated from the air. It is difficult to remove all traces of oil vapor and some travels with the air to the apparatus in which ~;

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the air is utilized. The presence of oil in the air is unsatis-factory where the air is required to have a high degree of clean-liness or purity as in food processing or the manufacture of quality paper, for example. Multiple separation treatments are sometimes required, but these are expensive and often not com~letely ef~ective.
A further problem with oil is the danger~of a fire if the temperature in the compressor becomes excessive.
Difficulties with oil may be avoided by operating the compressor dry. Such operation, however, requires that the two ro-tors be synchronized as by timing years so that there is no contactbetween the rotpr surfaces. Further, the tolerances of the rotors and of the casing must be much less than in a liquid sealed unit.
A dry compressor must operate at a much higher speed than a flooded compressor in order to overcome air slippage and to achieve the desired compression. Such machines are initially more e~pensive, more likely to require service in operation, and generally have a shorter life expectancy than an oil flooded machine.
Kodra U.S. Patent 3,535,057 suggests the use of water as a cooling and sealing medium for a rotary screw compressor. However, water has little lubricity and Kodra thus requires that the rotors be coated with a deformable material, as TEFLO ~ which will deform so that the two rotors may run without interference or damage.
Kodra further recommends that the rotors be synchronized by timing gears.
In accordance with this invention a compressor is provided with a cooling, sealing and lubricating fluid which includes di-methylsilicone and, more particularly, dimethylsilicone fortified with an antiwear additive, as dibutyl chlorendate.
Dimethylsilicone has a high coefficient of heat transfer, providing adequate cooling for efficient oPeration. Fur-ther-more, dime-thylsilicone and water do not mix or emulsiEy, but are readily separable, enabling trouble-free operation under conditions of high humidity. ~

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A ~urther Eeature of the invelltion is that the aerosol of dimethylsilicone is more readily separa-ted from the compressed air than is mineral oil. Air sufficiently pure for food manufacture, for example, may be provided without expensive multiple stage separators.
Yet another feature of the invention is -the method of cool-ing a rotary air compressor which comprises delivering to the eoMpressor casin~ a dimethylsilicone liquid wllich ccols the com~re-ssor and air and seals and lubricates the eompr~ssor ro-tor means.
Thus broadly, the invention contemplates a rotary screw air eompressor, including a casing having an intake por-t for at-mospheric air and an outlet port for compressed air. The casing has two rotors with meshing complementary parts. The rotors draw atmospherie air into the casing through the intake port, compress the air within the casing and discharge compressed air from the easin~ through the outlet port. A liquid coolant dimethylsilicone is used for the compressor and the air compressed therein. The dimethylsilicone ~urther seals the spaces between the rotors and the casing,and a means, connected with the outlet port of the easing, is provided for 5eparating liquid coolant dimethylsilicone carried by the compressed air from the compressed air. Al`so provid- -ed is a means for delivering the liquid dimethylsilicone coolant to the eompressor easing. A further ineans is provided for return-ing the separated liquid eoolant dimethylsilicone to the delivery means.
Another aspect of this invention pertains to the method of eooling a rotary screw air compressor. The rotary screw air comp~
ressor has two rotors with meshing complementary parts t rotatable in a easing with intake and discharge ports. Air is trapped and compressed while being moved by the rotors from the intake port to the discharge port of the easing. The method of cooling comprises delivering to the casing a dimethylsilicone liquid whieh eools the compressor and air. The liquid dimethylsilicone also - , :

~ 74~50 seals the rotor means. The li~uid is further separated from the compressed air and carried from the discharge port of the casing with the compressed air.

Further Eeatures and advanta~es of the invention will readily be apparent from th~ fo~lowing specification and from the drawin~s, in wh.ich:
Figure 1 is a dia~ram illustratin~ a typical com-pressor and coolant system;
Figure 2 is a longitudinal section throu~h a rotary scxew compressox;
Fi~ure 3 is a transverse section throùgh the rotary screw compressor taken generally along line 3-3 o~ Figure 2;
Fi~ure 4, appearing witll Figure 1, is a dia~rammatic illustration oE an apparatus for separating water from the coolant,appearing with Figs. 1 and 5: and Figure 5, appearing with Fi~ure 1, is a diagrammatic illustration of another separatin~ apparatus, with Figs. 1 and 4.
A typical rotary screw compressor coolant system is illustrated in Figure 1. Compressor 10 has an intake port 11 2Q and a discharge port 12. The compressor is driven by a suitable prime mover (not shown) which may be an electric motor or an internal combustion engine, for example.
The coolant liquid is introduced into the compressor through the conduit 13. The compressed air, wh.ich carries Wit}l ~, `
2S it ~ substantial portion of the coolant liquid, is conducted rom discharge port 12 through conduit 14 to a separation chamber lS in which most of the liquid drops by grav.ity forming a liquid pool 17 at the bottom. A final separator 18 at the outlet of the separation chamber removes suspended coolant from the compressed air. Conduit 19 is connected from the outlet at the top of the separation chamber 15 to ~;
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an air cooler 20. A heat e~changer within the air cooler 20 is provided with cooling wa~er through conauits 21, 22. As the air is coole~, moi~ture in it condense~. The condensate is collected in trap 24 through dra~n valve 25. The Gompre.~3ed S air i~ delivered ~hrough conduit 26 to the appaxatus in which it is util-~xed.
The coolant liquid ls-returned ~rom ~epara~ion chamber 15 to ccndui~ 13 by the pre~sure of the air wi~hin the separat~on chamber~ A condui~ 30 is connec~ed from ~he 0 8Ump at the lower pa~t of the sepaxation chamber through filter 31 and temperature sensitive dlverter valve 3~ to heat exchange~ 33. Cooling water ~3 a~rculated to ~he heat exchangex 33 through inlet 34 and outlet conduit 35. Valve 36 in the outlet conduit 35 con~rols the 1OW of cooling water in accordance w~th the t~mperature o~ the coolant li~uid at inle$ 37, detect~d ~y ~ensor 36a~ ~ the coolant temperature rises, the flow of water i3 increased. Temperature ~en~iti~e diverter val~e 32 bypasse4 a portion of ~he coolant li~uid .
around heat exchanger 33 through conduit 39 to aford urthex ~0 control of the~mpexaturs o t~e coolant liquid a~ inlet conduit 13. Thi~ temperature controls indirectly th~ heat rise which i3 experienced by the air between the ambient . .
temperature at the air intake po~t 11 and the kemperature o~ the compressed air at discharge port 12.
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~he coolant liquid from the heat exchanger outlet 35 and bypas~ 39 is dellvered to coolant inlet conduit 13 through valve 42 which close~ when the compxes~or 1~ not operating to prevent the coolant from flooding the ¢ompres~or.
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Pr~ssure sw~tch 43 shut~ the compre~sor down in the event coolant pres~ure i~ lostO
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A small portlon of the coolant l~quid i8 d$rected through filter 44 and a coolant-water separator 45 ~to be described below) to lubricate the rotor bearings, illus~ra~ea diagxammat~cally at ~6.
~he final separator 18, which removes co~lant liquid ~rom the aompres~ed airt is a cylindr~cal screen fitted over the outlet o ~eparation chamber 15 to conduit 19. The bottom i8 closed by a plate 18a. Coolant liquid which collects in the bottom of the separator is discharge~
through conduit 48 and filter 49, and i8 reintroduced into the ~y~tem at compre~or inlet 11.
The invention i~ preferably practiced wlth a com- ` :
pre~sor having two rotary screw~ with me~hlnq land~ and grooves, such a~ those described in the Bailey and Nil~on et al pa~ents identif~ed above. A represen~ative compre~sor construction i5 illustrated in Figuras 2 and 3. Compre3sor ca~ing 52 has an intake port 53 which ls open to the àtmo~-phexa and a discharge port 54 through whic~ compre~ed aix i~ delivered to ~he separation ch~mber 15 or o~her ~uitable ~0 utilization apparatus. ~he ca`31n~ has an interior barrel with t~o intex~ecting bores 52a, 52b, the axes o which are coplanar. A ~air o rotors 55, 56, each having meshing land~ and grooYe~ are mounted ~or rota~ion withln the ~oxes :
52a, 52b respect~vely of the aaslng S2. Male rotor 55 ha~
~5 convex. land~ while female rokor 56 ha~ concave land~. The intervening groove~ be~ween the lands o each rotor comple~
ment the land~ on the other rotor. A shaft 58, pre~erably an exten3ion of male rotor SS, i8 connected with the prime mover tnot ~hown) ~o that the rotor is driven directly~ The ~6 ~ :

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~7~7~) interengagement bet~een the two rotors causes the female rotor to turn as the male is turned by the prime mover. Sui~able radial and thrust bearings (not shown in detail) are provided for each rotor. Casing 52 is preferably double walled and has passages 59 through which the cooling liquid is circu-lated. In accordance with Nilsson et al U.S. patent 3,129,877, the coolant liquid is prefexably introduced into the interior of the casing 52 at the point where the land of the female rotor enters the grooves of the male rotor, the start of the com-pression phase, as through port 60, Figure 3. Alternatively,the coolant may be introduced through the air intake.
A rotary screw compressor may operate dry ox with an injected liquid as a coolant and sealant. If the two screws are synchronized by timing gears, or if they are coated to avoid abrasion, a lubricant is not necessar~. ~ screw com-pressor operated dry requires extremely high rotor speeds and generally has low efficiencyO Such compressors are generally used only where cleanliness of the air is a principal re~uire-ment. The coolant and sealant liquid may also serve as a lubricant and timing gears or rotor coatings are not necessary.
Kodra U.S. pa~ent 3,535,057, for example, suggests the use of water as the coolant liquid. Water, however, has little lubricity and Kodra requires a rotor coating, as of TEFL0 ~, and recom-mends synchronization by timing gears.
rrhe most common practice with rotary screw compres-sors where a high degree of cleanliness of the air is not essential, is to use a mineral oil as a coolant, sealant and lubricant. There are several disadvantages in using mineral oil~ Some have been suggested above. The most troub]esome are discussed below.

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~C)7~750 A principal difficulty with mineral oil is its ten-dency to ~orm an emulsion with water that condenses from the compressed air. This is a particularly seriou~ probl~m with compressors operating in a humid atmosphere. A machine in S Geoxgia in the summertlme, or example, may condense three ox ~our gallons of water per hour. With the pre~sure condi-tions in the compressor and the separator 150 ana the mixing action of the rotary screw~, th~ oil and water are thoroughly m~xed ~orming an emul~ion which separa~es rather slowly. ~he oil-water emulsion cannot ef~ecti~ely be separatea in the con stant 10w coolant sy~tem of ~he com~re~sor. The emulsion ha~
little lubricity and the compressor will be dama~ed if operated with it as a lubricant. Accordingly, i~ i~ neces.sary that ~he ~ystem operation be controllea ~n such a manner ~hat excesslve 15 condensat~on does not occur. This is usually achieved by allow~ng the air dischArged from ~he compressor to have an ele~a~ed temperature 80 that the water vapor pres~ure approaches the pre~sure of ~he compre~sed air and li~tle wa~er condense~.
An appropriate d~charge temperature i~ e~ablished by regulat-2~ ~ng the ~empexature o~ the ~njected coolant. Rather than : aooling ~h2 coolant liquid to the ex~ent po~sible (a~ 10 above ~he tempexature o~ the cooling water~ the heat exchan~er water 10w is throttled and a portion o~ the cool~n~ dlvertea . :
~hrough b~pa~s 39. For example, the mineral oil may be in-2~ jected into th~ compres~or at a temperatuxe of the order o 140 ~. rather than at 70~ or 80 which w~uld be easible wlth cool~ng water at 60~, :
Operat~on at an elevated tempera~ure is, however, undesirable ~or several reasons~ First, eficiency o~ ~he - 8 - :

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compre~or is recluc~d. The mos~ efficient compxession opera-tion is achieved as an isothe~mal xeduction of volwne. For an increase of 2S ~. in the discharge temperature, efficiency is reduced about 2.5~. ~ccordingly, to the exten~ that the coolant liquid is not cooled as far as possible, compressor e~iciençy is sacrificed ~o avoid emulsifying the oil.
` ~Soxeover, the mineral oil is adversely afected by the elevated temperatùre at which it operates. ~ven at temperatures as low as 15~ F., the oil will in time form shellacs, varnishes and acids which oul the cooling system and attack ga~kets, seals and diaphragms in the air system.
It is necessary that the oil be changed periodically, that the cooling system be cleaned and that the seals, gaskets and diaphragms be replaced.
lS Mineral oil tends to vaporize at the ele~ated operat-ing temperatures re~uired and a si~nificant portion of the oil vapor is carried out o~ the machine with ~he compressed air. Oil cont~mination of the air cannot be tolera~ed for 30mo uses, as or food processing or in the manu~acture of quali~y papers, for e~ample. Either the oil vapor must be r~moved ~y suitable iltering or preaipitating apparatus or a dry compressor must be used.
Another problem o hi~h ~emperature operation with ~ineral oil is that ~iscosity and lubricity of the oil de-~5 cr~ases as temperature increases. ~his results in a short~nedbearing life and adds to the expense of the compressor opera-tion.
In accordance with the invention, a dimethylsilicone is usea as the compressor coolant. This material has several ~ g _ .:
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p~ysical and chemical characteri~tic~ which contribute to clean, efficient and reliable compressor operation.
The dimethyl~ilicone coolant base material may be a Dow Corning 1uid sold t~der the numbers 200, 210 or 210H~
In a compres~or wit}l timing gears or a coating on ~he rotor~, where lubricat~on of the compressor i~ unnece~sary, these ~luids are sati~actory. For the typical rotaxy screw com-pre~sor where the coolant liquid also provides lubricakion, the dimethylsilicone base material i~ preferably foxtified with a chlorinated cyclic deriva~iYe. More speci~ically, the fortif~ing mater~al is a dibutyl chlorendate. The ai-methylsilicone lubricant is described in detail in Groenho~
et al 3,757,827 and i~ ~old by Dow Corning under the numbQr 4-3600. In addition to the ant1wear or lubricant ~ortlfica-t~on provided by the dibutyl chlorendate, the oil may be ~urther supplemented ~or extreme pressure operation by the addltion o~ a thiadiazole der~vative ta dialkyl derivative of 2,5-dimercapto-1,3,4-thiadizaole).
The dibutyl chlorendate and the thiadia~ole are bo~h added in ~u~ficiant quantity.to form a ~aturated solu-tion in the dimethylsilicone. The composition and lubricant characteri~tics of the liquid ar~ de~cribed in a paper pr~-sented by George J. Quaal.at the 4th Mational SAMPE Irechnlca Con~erence, Oc~ober, lg7~, Palo Alto, Cali~ornia.
2S ` In the event foaming is a probl~m, a ~ultable an~
~oaming age~t may be added. A fluo.ros~licone compound as Dow Corning FS12~ has been ~ound to be sati fac~ory~
A pxincipal characteri~t~c of the dimethyl~ilicone which make~ it sui~abl~ as a coolant to ~nhance ~he compre~^
sor operation, i~ ~hat it ha~ a negligi~le a~i~ity fox waterO
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~ater and dimethyls~licone do not mix ~o faxm an emulsion even under conditions o~ compressox dischaxge pr~ssure and the turbulence associa~ed with the rota~y s~rew action.
Although violen~ mixing may occur, the wa~er and ~h~ di-S meth~lsil~ cone separate rapidl~O ~ccordingl~, it is un-necessaxy that an eleva~ed air discharge temperature ~e.
m~intained to minimi~e condensation. Rather, the ~lmethyl-silicone li~uid coolant may be cooled as far as pract~cablewith the cooling me~ium used. Fox example, i~ cooling water is available at 60, the temperature of tha dimethylsilicone ~luid may be reduced to 70 be~ore it is injected into the compre~sor~ In~ection o~ the coolant at such a low tempera-ture enables a close approach to isothexmal compression o~
the air, a desixed condltion ~or maximum e~ficiency9 Wate~
conaensed ~uring compression separates rapidl~ from the imethylsilicone coolant and can be automatically eje~ted from the sy~tem as will be describad ~elow. . .
A s~cond important character~ tic o~ tlle dimethyl~
s~licone cool2nt is that it ~oes not remain as an aerosol in 2q the compressed air, bu~ separates out quic~ly ana thorou~hly withou~ specia1 separation apparatus or treatment. This characterlstic is believed to be related to the low sur~ace .. :, , .
tension of ~he dimethylsilicone~whîch leads to the formation o~ large drops which all out of the air in~ the ~ump of separation tan~ 15. In add~tion, the coolant remalnin~ in aerosol~orm we~s the element~ o the ~inal separator 18, further en~ancin~ the efficiency o separation of the coolant from the compressed air. In a typical mineral oll compressox system, there may be 15 parts pex million of mlneral oil in \ \
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the compressed air. With dimethylsilicone coolant in a com-pressor system using the same separator, less of the co~lant escapes.
As a convexse to the tendency o the dimethylsili~
S cone to separate from air, such air as may be mixed with the coolant escapes xapidly and is not carried with the coolant thro~gh the system to be injected lnto the compressor with the coolant. When air is mixed wi~h the coolan~, the air expands upon injection into the compressor, reducing the volumetric ei~iency of the compressor.
Several characteris~ics of th~ dimathylsil~cone coolant contribute to maintenance-free operation of the system and to long life of the seals, gaskets and d~aph~agms with which the coolan~ and the compressed aîr come in contact.
The dimethylsilicone liquid is t subject to ox~dat~on at the same rate as mineral oil. ~urthermora, such minimal oxidation as occurs does not produce shellacs, varnishes and acids, which attack the seals, gaskets and diaphra~m~. The -~
xelative quantity of the coolant wh~ch escapes the compres-sor and is tran~mitted through the compres~ed air system i~
much leRs than with mineral oil.
The dimethylsilicone has low volatility a~ comparea with mineral oil and has little tendenc~ to decompose when heated. Fur~hermore, the compre~sor operating tempera~ure 2~ is lower. All of these factors cnhance the seal, ~a~ket and diaphragm lie.
The viscosity and shear characteristics o~ the di-methylRilicone are relati~ely ~table with tempe~ature. Thus, ; proper lubrication of the system is malntain0d throughout the operating temperatuxe range, ~L~7~7SO

Dimethylsilicone is a much safer coolant fluid to use than is mineral oil. Fire~ some~imes occur in a compressed `
air system as when there is a failure of the coolant supply system and the temperature within the compressor rises causing S an expansion of the rotors so that ~hey rub on each other or on the casin~, striking sparks. ~Oil vapor in ~he compres~ed air ignite.q and the ~ire may be transmi~ted ~hroughout. the compressed air system. Dimethylsilicone ignite~ only reluc-~antly and when.it burns does not generate ~uffi~ient heat to ~ustain combu~tion. Thu3, even i a ire starts, it rap~dly dies out.
Where the recirculated coolant liquid is utilized to lubxicate the compres~or bearings, it i~ par~icularly im-portant that the amount o~ wa~er carried wi~h the liqu~d ba minimized~ Accordingly, the coolant~water ~eparator 45 i9 connected in th~ syst~m at a poLnt ahead o~ rotor bearln~
45. Two forms of separator are illustrated in ~ ures 4 and 5.
Th~ apparatu~ shown in Figure 4 make~ use of the low ainity between the dimet~yl3ilicone and water to achieve sepaxa~ionO The coolant liquid ~rom fil~er 44 enters a separator chamber 65 through inlet 66 and ~s di~charged through coolant outlet 67. Any water with the dime~hylsili-COnQ se~tl~ to the bottom of cham~er 65, with a claarly de-~ined boundary 68 bet~Jeen the watsx and the dimethyl~ilicone.
A level cvntrol 70 has probes 71, 72 extendin~ into the chamber 65, one abo~re and the other below the boundary 6 8 .
A watex discharge ~ralve 73 is actuated by level contrc~l 70 to open and drain water from the system when the boundary 68 reache~ the upper probe 71 and to close when the ~undary drop~ to lower probe 72D

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~0~4750 The coolant-waker separator of Figure 5 ~ases its operation on the difference in size of the molecules of the dimethylsilicone coolant and the water. A separator chamber 75 has an inlet 76 and an outle~ 77 connected in series with the coolant flow path. The wall-of chamber 75 is a mem~xan~
having pa~sages of a size large enough to allow flow of water molecules but so small that molecules of the dimethylsiliaone coolant cannot pas~. ~ore specifically, the membrane pre- :
erably has pores with a diameter of the order of 5 ko 50 angstrom units. Under the pressure exerted by the compressed air on the coolant fluid in the system, water molecule~pass through the membrane and foxm drop~ on the outside ~ cham-ber 75, which run off at ~he bo~tom, as ;ndicated at 78 The dimethyl~ilicone coolant passes through chamber 75 to outlet 77. ~urbulence of the liquia flow within the chamber prevents dirt blockage of the mémbxane pore~.
Coolant-water separator 45 is illustrated as con-nected in the system immediately ahead o~ bearings ~6. Water ~epara~ion could be effected at an earlier point, as in con-duit 30. ~

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Claims (11)

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

1. A rotary screw air compressor, comprising:
a casing having an intake port for atmospheric air and an outlet port for compressed air;
two rotors in said casing with meshing complementary parts, rotatable to draw atmospheric air into the casing through the intake port, to compress the air within the casing and to discharge compressed air from the casing through said outlet port;
a liquid coolant dimethylsilicone for said compressor and the air compressed therein and for sealing the spaces between the rotors and the casing;
means connected with the outlet port of said casing for separating liquid coolant dimethylsilicone carried by the compressed air from the compressed air;
means for delivering said liquid dimethylsilicone coolant to said compressor casing; and means for returning the separated liquid coolant dimethylsilicone from the separating means to said delivery means.

2. The air compressor of Claim 1 in which the dimethylsilicone is fortified with an antiwear additive and lubricates the interengaging surfaces of the rotor means and casing.

3. The air compressor of Claim 2 in which said dimethylsilicone is fortified with dibutyl chlorendate.

4. The air compressor of Claim 1 including means for removing water from at least a portion of the separated liquid.

5. The air compressor of Claim 4 in which the rotors have bearings and including means for delivering separated liquid from which water is removed to the bearings.

6. The air compressor of Claim 1, 3 or 5 wherein the separating means includes:
an air receiving chamber having an inlet connected with the outlet port of the compressor casing and an outlet at the top of the receiving chamber for delivering compressed air, free of said liquid;
a liquid separator screen between the chamber and the air outlet therefrom; and a liquid outlet connected with a sump at the lower portion of the receiving chamber.

7. The method of cooling a rotary screw air compressor having two rotors with meshing complementary parts, rotatable in a casing with intake and discharge ports, air being trapped and compressed while being moved by the rotors from the intake port to the discharge port of the casing, which comprises:
delivering to the casing a dimethylsilicone liquid which cools the compressor and air and seals said rotor means;
and separating liquid, carried from the discharge port of the casing with the compressed air, from the compressed air.

8. The method of Claim 7 in which the dimethyl-silicone is fortified with an antiwear additive and lubricates said rotor means.

9. The method of Claim 8 in which said dimethyl silicone is fortified with dibutyl chlorendate.

10. The method of Claim 7 including the step of returning the separated liquid to the compressor.

11. The method of Claim 10 including the step of removing condensed water from liquid returned to the compressor.