US2229954A

US2229954A - Process and apparatus for refrigeration

Info

Publication number: US2229954A
Application number: US34605A
Authority: US
Inventors: Paul D Barton
Original assignee: Sun Oil Co
Current assignee: Sunoco Inc
Priority date: 1935-08-03
Filing date: 1935-08-03
Publication date: 1941-01-28
Anticipated expiration: 1958-01-28

Description

Jan. 28, 1941. i p, 9 2,229,954

PROCESS AND APPARATUS FOR REFRIGERATION Filed Aug. 3, 1935 l7 "vac.

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Patented Jan. 28, 1941 UNITED STATES PROCESS AND maaa'rns roa aararcsaa'rron Paul D. Barton, Narberth, Pa., assignor to Sun Oil Company, Philadelphia, Pa... a corporation of New Jersey Application August a, 19:5, sQlll-l No. 34,805

11 Claims.

The present invention relates to a process and apparatus for refrigeration wherein a series of chambers are maintained at progressively lower temperatures and more particularly to a system adapted. for use in petroleum refineries for conducting cloud" and pour tests on lubricating oils.

In the manufacture of lubricating oils, transformer oils, and other mineral oil products, it is necessary to maintain a close check on the pour points of the oil, that is, the lowest temperature at which the oil will flow. In the case of paraffin base oils it is also necessary to check the cloud point, that is, the temperature at which paraflin wax first crystallizes, forming a cloudy suspension in the oil. In order to do this accurately it is necessary to employ refrigerating apparatus having a number of chillers maintainable at successively lower temperatures varying from about +30 F. to -60 F. or even 100 F. The apparatus, to be satisfactory, must be capable of easy and quick adjustment.

It is therefore an object of the present invention to provide a refrigeration apparatus having a series of refrigerating chambers maintained at successively lower temperatures by means of pressure reducing valves alone.

It is also an object of the present invention to provide a refrigeration apparatus in which anyone of the chambers may be cut out of the system without affecting operation of any of the other chambers, and particularly to provide an apparatus in which the last chamber in the series may be held at the same temperature as that which is next highest in the series in order to relieve the apparatus of undue load when the lowest possible temperatures are not desired in the last chamber.

A further object of the present invention is to provide a refrigerating system with the expanders in two series, one of which operates at temperatures down :to that temperature at which the vapor pressure of the refrigerant is about atmospheric pressure, and a second series of one or more expanders operating at lower temperatures and consequently sub-atmospheric pressures.

Briefly, the present invention resides in a process and apparatus wherein a bulk supply of the liquid refrigerant is maintained at substantially atmospheric temperatures and under a pressure sufficient to maintain it in liquid phase at such temperatures.- From such bulk supply the liquid refrigerant is fed to the first refrigeration chamber in the series through a pressure reducing valve which is controlled by the pressure on the down-stream side alone. The liquid refrigerant after passing through the valve enters the first refrigerating chamber, filling it to the height of the outlet pipe therein. From the first chamber the liquid refrigerant and such vapors as are formed by evaporation in the first chamber are passed out through a common utlet or overflow pipe to the second chamber of the series. Within the pipe leading from the first to the second chamber there is placed a pressure reducing valve similar to that on the inlet of the first chamber. From the second chamber both liquid and vaporous refrigerant pass through a pressure reducing valve, controlled by the pressure on the downstream side, to the thirdchamber. In each of the first three chambers a portion of the refrigerant will be evaporated, due to successively lower pressures being maintained in the chambers, to maintain the body of liquid refrigerant therein at a temperature approximating that at which the liquid and vaporized refrigerant will be in equilibrium at the pressure set by the pressure reducing valve just preceding each chamber.

From the third chamber in the series the liquid and vaporous refrigerant will overflow to a separation chamber where the liquid refrigerant is separated from that which has been vaporized in extracting heat from the first three chambers. The separation chamber will normally be held at a pressure substantially that of the third chamber or slightly in excess of atmospheric pressure. The vapors separated in the separation chamber pass off through a pressure reducing valve to a reciprocating compressor which takes suction at substantially atmospheric pressure. The liquid refrigerant from the separating chamber is fed through a pressure reducing valve to the fourth chamber of the series which is maintained at a pressure below atmospheric. The

.pressure reducing valve to the fifth chamber,

from whence nothing but vaporous refrigerant is withdrawn, this being withdrawn through a vacuum pump of the rotary cam type, which maintains a vacuum on the fourth and fifth chambers in order to reduce the pressure in these chambers to the vapor pressure of the liquid refrigerant at the desired temperatures for these chambers. If desirable, the pressure reducing valve between the fourth and fifth chambers may be by-passed thereby maintaining both chambers at approximately the same temperature. The rotary cam vacuum pump which produces the sub-atmospheric pressure in the last series of chambers also raises the pressure of the evacuated refrigerant vapors to substantially atmospheric pressure. At this pressure they are picked up by the reciprocating compressor together with the vapors from the separating chamber, are compressed, then cooled to liquefy them, and passed to the bulk supply of liquid refrigerant.

A better understanding of the invention will be had by reference to the accompanying drawing in which:

B, C, and D represent the refrigerating chambers of the first or super-atmospheric series;

E represents the separating chamber; F, and G represent the refrigerating chambers of the second or sub-atmospheric series;

H represents a rotary cam vacuum pump and compressor;

I represents a knock-out drum for any liquid refrigerant carried over and through vacuum pump H;

J represents a reciprocating compressor; and

K represents a condenser for the compressed refrigerant.

Liquid refrigerant is forced from the vessel A through line III to pressure reducing valve II by the vapor pressure of the liquid refrigerant. In passing through valve I I the pressure of the liquid refrigerant is reduced to the pressure desired in the first chamber B in super-atmospheric series, the refrigerant passing into chamber B through inlet line I2. Each of the refrigerating chambers is provided with a dome such as I3, on chamber B, the liquid refrigerant being admitted to the top of the dome. When chamber B has been filled with liquid refrigerant at the desired temperature, the liquid and vaporous refrigerant both outflow through line I4 to a second pressure reducing valve I5, from whence they pass under a reduced pressure through line I6 to dome II in the second chamber C. The liquid and vaporous refrigerant outflowing from chamber C pass to reducing valve I 9 through line I 8 and thence to inlet 20 of chamber D. From chamber D the liquid and vaporous refrigerant pass out through lines 2|, 22, 23 and a portion of line 24, to separating chamber E. From separating chamber E vaporous refrigerant is taken off overhead through line 25 to pressure reducing valve 26 which serves to reduce the pressure of the vapors to substantially atmospheric pressure. After being so reduced in pressure, the vapors pass through line 21 to compressor J wherein they are compressed sufficiently to condense them on cooling to approximately atmospherictemperatures. The compressed vapors pass through line 28 to condenser K and the cooled and condensed refrigerant then passes to storage chamber A through line 29.

From the lower portion of separating chamber E liquid refrigerant is withdrawn through line 30 to pressure reducing valve 3| and thence through line 32 to chamber F which is the first chamber of the sub-atmospheric series. The liquid and vaporous refrigerant outflowing from chamber F pass through line 33 to line 34 having pressure reducing valve 35 therein. From pressure reducing valve 35, the refrigerant passes through line 36 to chamber G which is the last chamber of the series. vaporized refrigerant from chamber G passes off through

lines

31, 31a and 38 to a vacuum pump. and compressor H of the rotary cam type, from which the partially compressed refrigerant vapors pass through line 39 having knock-out chamber I therein, which serves to remove any liquid refrigerant passed through pump H from the line. From line 39 the refrigerant vapors pass through line 40 to line 21 and thence to the reciprocating compressor J.

Since it may not be necessary to maintain refrigerating chambers F and G at temperatures lower than that in chamber D, there is provided valved line 22a which may be used to by-pass the refrigerant. In such case the valved by-pass line H around pressure reducing valve 85 would also be used and the liquid and vaporous refrigerant passing from chamber G would pass through

lines

31, 24a and 24 to separating chamber E rather than-to compressor and vacuum pump H. If it is desired to cut chamber G entirely out of the system, this may be done by closing the hand controlled valves in lines M and 34, opening the hand controlled valve in line 33a, closing the hand controlled valve in line 24, thereby passing the vaporized refrigerant from chamber F through lines 33a, 24a, and 38 to vacuum pump and compressor H.

It is to be understood that valved by-passes may be inserted between lines I2 and I4, lines I6 and I8, and lines 20 and 2I in order to cut any one of the refrigerating chambers from the system. It is also to be understood that pressure reducing valves I I, I 5, I 9, and 3| may be equipped with valved by-passes so that any one of the refrigerating chambers may operate at the temperature and pressure of that immediately preceding.

Any excess liquid in chamber E or knock-out chamber I may be withdrawn therefrom through valved lines 43 and 44 respectively which communicate with line 45 having pump 46 therein to force the liquid back into the refrigerant supply tank A.

In Figs. 2 and 3 there is shown one form of v In the modification refrigerating chamber. shown, the chamber is made up of a section of large diameter, say twelve or fourteen inch, pipe 50', having a bottom head 5| welded therein, and a top head 52' also welded therein. The top head 52' has drilled therein a number of holes in which are welded or rolled cups 53' in which the bottles containing the oil to be tested are placed. There is also provided on each top plate 52' a dome 54 into which the inlet and outlet tubes, 55' and 56 respectively, lead, the top of the outlet tube 56' being placed above the level of the top sheet 52' so that the entire chamber with the exception of the dome will be filled with liquid.

It is preferred to use a rotary cam pump such as pump H, although other types of pumps will operate successfully. It is preferred to use that type of pump having two cams so arranged that their peripheral faces are always in contact. Any one of many types of pressure reducing valves may also be employed, it being necessary, however, to use a valve in which the flow is controlled by the pressure on the down-stream side, that is, a valve which will open when the pressure on the down-stream side is raised above a certain minimum.

Any one of a number of refrigerants may be used. However the use of propane (CBHB) is preferred as this is available in practically all large refineries and may be purchased very cheaply. It is preferred to use propane since. when repairs are necessary to the system, the refrigerant may be vented to the refinery gas system through valved line 42. Some of the refrigerill ants which may be used and their low temperature characteristics are as follows:

As a specific example, where propane is used as the refrigerant, it is maintained in chamber it. at substantially 80 F. and at a pressure of M213 pounds per square inch absolute, its vapor pressure at that temperature. It is passed under this pressure through line l to valve II which serves to reduce its pressure to approximately W3 pounds per square inch absolute, which is the pressure corresponding to the vapor pressure of propane at +30 F., the temperature desired to be maintained in refrigerating chamber 13.

Liquid propane and any propane vapors formed in removing heat from'the chamber B will pass out through line I l to pressure reducing valve l5 which is set to reduce the pressure on the propane liquid and vapors to about 38.2 pounds absolute, which is the vapor pressure of propane at 0 F. The propane liquid and vapors then pass to refrigerating chamber C and flow therefrom through line III to pressure reducing valve it whichis set to reduce pressure to 20.3 pounds per square inch absolute which is the vapor pressure of propane at 30 F. The liquid and vaporous propane then outflow from chamber D through line 2|. Since the pressure in chamber D (20.3 pounds per square 'inch absolute) is only 5.6 pounds per square inch above atmospheric pressure, no substantial reduction in temperature can be effected by further reducing the pressure to atmospheric pressure, it is therefore necessary to reduce the pressure to considerably below atmospheric pressure by means of a vacuum pump. If vaporous and liquid refrigerant both were passed directly to a. further refrigerating chamber maintained under a vacuum, it would be necessary for the vacuum pump to bandle not only the vapors liberated at the lower pressure, but also the vapors which had been liberated in the chambers B, C and D. In order to avoid placing such load on the vacuum pump, the liquid and vaporous refrigerant from chamber D are passed through

lines

22, 23, and 24 to chamber E which is maintained at the same pressure as refrigerating chamber D. Within the chamber E, the propane vapors are separated from the liquid propane and pass out through line 25 to pressure reducing valve 26 which serves to reduce the pressure to substantially atmospheric. This last pressure reduction may seem unnecessary but it is in fact necessary, since it obviates the necessity of the vacuum pump in the latter part of the system compressing the vapors from the sub-atmospheric chambers up to the pressure existent in chamber E, thereby lessening the load on vacuum pump H. From the lower portion of chamber E liquid refrigerant at substantially -30". F. is withdrawn and passed through line 30 to pressure reducing valve 3| which serves to reduce the pressure to 9.74 pounds absolute, or substantially of mercury vacuum, which is the vapor pressure of propane at -60 F. The liquid and vapors overflowing from.

chamber F pass out through lines 33 and 3| to expansion valve 35 and thence through line 36 to chamber G. Within the chamber G a pressure of 6.37 pounds absolute (17" Hg) is maintained by pressure reducing valve 35, this being the vapor pressure of propane at --'75 F. Vapors generated in chamber G are removed through

lines

31, 31a, and 38 to vacuum pump and compressor H which serves to maintain sub-atmospheric pressures on the chambers F and G and to raise the pressure of the evacuating vapors to the pressure in line 21, which is regulated by pressure reducing valve 26.

The temperatures herein stated for chambers B, C, D, and F are those set forth in the -A. S. T. M. Standards on Petroleum Products and Lubricants for the Bath Temperatures to be used in Determining the Pour and Cloud Points of Petroleum Products. If the refrigerating system is to be used for other purposes, the temperatures of the various baths may be varied, and can be closely regulated bymerely adjusting the various pressure reducing valves throughout the system. Other refrigerants may be substituted for propane if desired. For instance, if temperatures as low as -150 F. are desired in the last chamber of the series, ethane may be used, as this refrigerant has a vapor pressure at that temperature of seven pounds absolute, or about 15.6" Hg vacuum. 011 the other hand, if higher temperatures are desired, it may be more practical to use other commercial refrigerants having lower vapor pressures at higher temperatures, thus eliminating mechanical load on the system.

As before stated, refrigerating chambers F and G may not always be in use, in which case it would be unnecessary to maintain the vacuum pump and compressor H in constant use. These chambers may be entirely cut out of thesystem by closing the hand-controlled valves in lines 32 and 31a, or they may be permitted to ride at the same temperature as refrigerating chamber D and separating chamber E, by closing the handcontrolled valves in

lines

23 and 32, and opening the valve in line 22a, thereby passing vaporous and liquid refrigerant directly from chamber D to chamber F. From chamber F the vaporous and liquid refrigerant may be withdrawn through lines 33a, Ma and 24 to separating chamber E or the valves in lines 33a and 34 may be closed, the valvein line 4| opened, and the vaporous and liquid refrigerant passed to chamber G from whence it will flow through

lines

31, 24a and 24 to separating chamber E,th e valve in line 31a being closed. In such case chambers D, F, and G will all be maintained at substantially the same temperature, that is, -30 F. When it is desired to lower the temperatures in chambers F and G,

the valves in lines 22a, and 4| will be closed, and

the valves in

lines

23, 34, and 31a will be opened, thereby restoring normal circulation in the system.

Should it be desired to have both chambers r" various combinations may be resorted to as desired, without departing from the spirit and scope of the present invention.

From the foregoing it is readily apparent that a refrigeration system has been provided wherein the temperatures may be controlled through wide ranges solely by pressure reducing valves, it being only necessary to set each pressure reducing valve at the vapor pressure of the refrigerant being used for the temperature desired in the refrigerating chamber next following. A system has also been provided wherein the low temperature (sub-atmospheric) chambers may be temporarily out out of the system or may be temporarily retained at the temperature existing in the last chamber of the super-atmospheric series.

What I claim, and desire to protect by Letters Patent, is as follows:

1. In a refrigeration system, a refrigerant supply, a series of refrigerating chambers at successively lower temperatures, each containing a body of liquid refrigerant, a refrigerant inlet to each chamber, a liquid and vaporous refrigerant outlet from each chamber at substantially the liquid level therein, a refrigerant supply line to the inlet of the first chamber in the series, a line connecting the outlet of each chamber with the inlet of the next chamber, and pressure reducing valves in said supply and connecting lines responsive to the pressure on the downstream sides thereof, whereby the temperature in each chamber is controlled solely by the vapor pressure of the liquid refrigerant therein.

2. In a refrigeration system, a refrigerant supply, a series of refrigerating chambers at successively lower temperatures, each containing a body of liquid refrigerant, a refrigerant inlet to each chamber, a liquid and vaporous refrigerant outlet from each chamber at substantially the liquid level therein, a refrigerant supply line to the inlet of the first chamber in the series, a line connecting the outlet of each chamber with the inlet of the next chamber, pressure reducing valves in said supply and connecting lines responsive to the pressure on the downstream sides thereof, a chamber for separating liquid and vaporous refrigerant connected to the outlet from the last chamber of the series, a second series of refrigerating chambers at successively lower temperatures, each containing a body of liquid refrigerant, a refrigerant inlet to each chamber, a liquid and vaporous refrigerant outlet from each chamber at substantially the liquid level therein, a refrigerant supply line to the inlet of the first chamber in the second series from said separating chamber, a line connecting the outlet of each chamber of said second series with the inlet of the next chamber, pressure reducing valves in said last mentioned supply and connecting lines responsive to the pressure on the down stream sides thereof whereby the temperature in each chamber of the second series is controlled solely by the vapor pressure of the liquid refrigerant therein, and means for producing sub-atmospheric pressure in the charm bers of said second series.

3. In a refrigeration system, a refrigerant supply, a series of refrigerating chambers each containing a body of liquid refrigerant at successively lower temperatures, a refrigerant inlet to each chamber, a liquid and vaporous refrigerant outlet from each chamber at substantially the liquid level therein, a refrigerant supply line to the inlet of the first chamber in the series, a line connecting the outlet of each chamber with the inlet of the next chamber, pressure reducing valves in said supply and connecting lines responsive to the pressure on the downstream sides thereof, a chamber for separating liquid and vaporous refrigerant connected to the outlet from the last chamber of the series, a liquid refrigerant feed line from said separating chamber to a further refrigerating chamber, a vapor outlet from said last mentioned refrigerating chamber, vacuum producing means in said line, and a pressure reducing valve in the feed line from the separating chamber controlled by the pressure on the down-stream side thereof, whereby the temperature in said last mentioned chambar is controlled by the vapor pressure of the liquid refrigerant therein.

4. A refrigerating process wherein the temperatures are controlled solely by pressure comprising establishing a bulk supply of liquid refrigerant, a series of refrigerating zones operating at superatmospherlc pressures, a separating zone and a series of refrigerating zones operating at sub-atmospheric pressures, passing liquid refrigerant from the bulk supply to the first refrigerating zone of the super-atmospheric series and during its travel reducing its pressure to that at which its vapor and liquid will be in equilibrium at the temperature 'desired in the first zone, passing the partially chilled liquid and vaporous refrigerant from the first zone to and through the remaining zones of the-super-atmospheric series and reducing the pressure previous to passing the refrigerant into each zone to the pressure at which its vapors and liquid willbe in equilibrium at the temperature desired in such zone, passing the liquid and vaporous refrigerant to the separating zone and separating the vaporized refrigerant from the liquid, passing the liquid refrigerant to and through the refrigerating zones comprising the sub-atmospheric series and reducing the pressure previous to entering each zone to that pressure at which its vapor and liquid will be in equilibrium at the temperature desired in each zone, and compressing and condensing the vapors removed from the separating zone and the last refrigerating zone of the sub-atmospheric series and returning the condensed refrigerant to the bulk supply.

5. A refrigeratingprocess wherein the temperatures are controlled solely by pressure comprising establishing a bulk supply of liquid refrigerant, a series of refrigerating zones operating at superatmospheric pressures, a separating zone and a refrigerating zone operating at sub-atmospheric pressures, passing liquid refrigerant from the bulk supply to the first refrigerating Zone of the super-atmospheric series and during its travel reducing its pressure to that at which its vapor and liquid will be in equilibrium at the temperature desired in the first zone, passing the partially chilled liquid and vaporous refrigerant from the first zone to and through the remaining zones of the super-atmospheric series and reducing the pressure previous to passing the refrigerant into each zone to the pressure at which its vapors and liquid will be in equilibrium at the temperature desired in such zone, passing the liquid and vaporous refrigerant to the separating zone and separating the vaporized refrigerant from the liquid, passing the liquid refrigerant to and through the refrigerating zone operating at subatmospheric pressure and reducing the pressure on the refrigerant previous to entering such zone to that pressure at which its vapor and liquid will be in equilibrium at the temperature desired in said zone, and compressing and condensing the vapors removed from the separatin zone and till aaeaott the last refrigerating zone and returning the com densed refrigerant to the bulk supply.

6. A refrigerating process wherein the temperatures are controlled solely by pressure, comprising establishing a bulk supply of liquid refrigerant, a refrigerating zone operating at super-atmospheric pressure, a separating zone and a refrigerating zone operating at sub-atmospheric pressure, passing liquid refrigerant from the bulk supply to the refrigerating zone operating at superatmospheric pressure, and during its travel reducing its pressure to that at which its vapor and liquid will be in equilibrium at the temperature desired in the super-atmospheric zone, passing the partially chilled liquid and vaporous refrigerant to the separating zone and separating the vaporous refrigerant from the liquid, passing the liquid refrigerant to the sub-atmospheric refrigerating zone and during its passage reducing the pressure to that pressure at which the vapor and liquid will be in equilibrium at the temperature desired in the sub-atmospheric zone, removing the vapors from the separating chamber and the sub-atmospheric zone and compressing and condensing them and returning the condensed refrigerant to the bulk supply.

7. A refrigerating process wherein the temperatures are controlled solely by pressure, comprising establishing a bull: supply of liquid refrigerant and a series of successively cooler refrigerating zones each containing a body of liquid refrigerant, feeding liquid refrigerant from the bulk supply to the first zone of said series, maintaining in each zone of the series a body of liquid refrigerant and a vapor space above the liquid body, outflowing at approximately the liquid level therein vaporous and liquid refrigerant from one zone of the series to the next zone of the series, maintaining successively lower pressures in the successive chambers, thereby effecting partial evaporation of the liquid therein, and as the pressure in each zone falls below a predetermined value supplying refrigerating fluid thereto to replace refrigerating fluid discharged therefrom to thereby re-establish the pressure therein to the predetermined value.

8. A refrigerating process wherein the temperatures are controlled solely by pressure, comprising establishing a bulk supply of liquid refrigerant and a series of successively cooler refrigerating zones each containing a body of liquid refrigerant and a vapor space above the liquid body, feeding liquid refrigerant from the bull: supply to the first zone of said series, maintaining the refrigerant in said first zone at a predetermined temperature below that in the bulk supply by reducing the pressure during the flow of the refrigerant from the bulk supply to the first zone to that corresponding to substantially the vapor pressure at said lower predetermined temperature, feeding partially chilled liquid and va porous refrigerant from approximately the liquid level in the first zone to the next zone of the series and maintaining the refrigerant in the latter zone at a predetermined temperature below that in the first zone by reducing the pressure during the flow of the refrigerant from the first zone to the second zone to that corresponding to substantially the vapor pressure at the predetermined temprature in the second zone.

9. A refrigerating process wherein the temperatures are controlled solely by pressure, comprising establishing a. bulk supply of liquid refrlgerant and a series of successively cooler refrigerating zones, fwding liquid refrigerant from the bulk supply to the first zone of the series and during its travel chilling said refrigerant by reducing the pressure thereon to substantially the vapor pressure corresponding to a predetermined temperature of the chilled refrigerant in the first refrigerating zone, then feeding the partially chilled liquid and vaporous refrigerant from each refrigerating zone to the next refrigerating zone maintained at a lower temperature than the preceding refrigerating zone and reducing the pressure between adjacent zones to substantially the vapor pressure corresponding to a predetermined temperature of the cooled refrigerant in the second of said adjacent zones, maintaining in each zone a body of liquid refrigerant and a vapor space above the liquid body and outfiowing the refrigerant from each refrigerating zone at approximately the liquid level therein to thereby effect the outflow of the refrigerant in both liquid and vapor phase.

iii. In a refrigeratingsystem, a high pressure liquid refrigerant supply tank, a series of similar enlarged low pressure refrigerating chambers, a pipe connecting said refrigerant supply tank with the first of said series of refrigerating chain bers and adapted to supply liquid refrigerant thereto, means for conducting mixed liquid and vapor refrigerant from each of said refrigerating chambers except the last to the next succeeding chamber in said series, said means comprising pipes the inlet end of each of which extends from a point near the top of the corresponding refrigerating chamber, a valve in said pipe connecting said supply tank to said first chamber and a similar valve in each of the other pipes, said valves being responsive to the pressure on the downstream side thereof and adapted to open when the pressure falls below a predetermined minimum, a discharge pipe from the last chamber of said series, means for withdrawing mixed liquid and vaporous refrigerant from the last named pipe, means for separating from the last named mixed fluids said liquid from said vaporous refrigerant, and means for compressing and condensing said vaporous refrigerant and returning the same to said refrigerant supply tank.

il. in a refrigerating system, a liquid refrigerant supply tanlr, a series of refrigerating chambers, a pipe having its inlet end opening into said supply tank and its discharge end opening into the first chamber of said series and aflording a passage for the liquid refrigerant from said supply tank to the first refrigerating chambers of said series, pipes one connecting each chamber of said series except the last with the next chamber of the series, each pipe determining the liquid level in the corresponding chamber and being so near the top thereof that such chamber must nearly fill with liquid before it can outflow therefrom, and means to establish a relatively high pressure in said supply tank and successively lower pressures in said series of chambers, said means including means to withdraw refrigerant from the last chamber of the series and pressure reducing valves one in each pipe responsive to pressure on the downstream side thereof and adapted, when the pressure differential between connected chambers rises above a predetermined value, to open and admit sumcient refrigerant fluid to the lower pressure chamber to re-establish said pressure differential.

' PAUL D. BARTON.