Note: Descriptions are shown in the official language in which they were submitted.
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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
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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.
. . .
~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.
. ~
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
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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
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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
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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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1~7g~75V
~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
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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,
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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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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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