US2952139A - Refrigeration system especially for very low temperature - Google Patents

Description

Sept. 13,- 1960 REFRIGERATION SYSTEM ESPECIALLY FOR VERY LOW TEMPERATURE Filed Aug. 16, 1957 3 Sheets-Sheet 1 IN ENTORS.

PATRICK 8. KENNEDY BY HUGH R. SMITH ATTORNE Y.

Sept. 13, 1960 P. B. KENNEDY ETAL REFRIGERATION SYSTEM ESPECIALLY FOR VERY LOW TEMPERATURE- Filed Aug. 16, 1957 3 Sheets-Sheet 2 IN VEN TORS.

PATRICK B. KENNEDY HUGH R. SMITH ATTORNEY.

Sept. 13, 1960 P. B. KENNEDY ETA;

REFRIGERATION SYSTEM ESPECIALLY FOR VERY LOW TEMPERATURE Filed Aug. 16, 1957 3 Sheets-Sheet 3 rllllllL.

United States Patent REFRIGERATION SYSTEM ESPECIALLY FOR VERY LOW TEMPERATURE Patrick B. Kennedy, Berkeley, and Hugh R. Smith, Jr., Oakland, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Aug. 16, 1957, Ser. No. 678,750

8 Claims. (Cl. 62- 35) This invention relates to refrigeration systems and particularly to such systems employing a plurality of refrigerants and producing very low end temperatures.

Refrigeration systems have always made use of certain well-known physical principles, one of the most important being that which finds embodiment in the process of expanding a fluid from a higher to a lower pressure, and from liquid to vaporous state. When such a process is executed, the fluid becomes cooler without giving up any heat to its environment and is thus placed in condition to absorb heat from any object of higher temperature that it may be desired to refrigerate. In ordinary refrigerators, such as those employed in the home for food storage, the end temperature desired is not very low and a single refrigerant is usually adequate for producing the contemplated effect. But when it is required to produce a very low temperature on the order of say -20() degrees Fahrenheit, it is ordinarily more convenient to use two or more refrigerants, one being expanded to precool the second, the second being expanded to precool the third, and so on. A system well known in the art for embodying such a method is that known as the cascade system, wherein a'series of refrigerants, selected in some descending order of boiling points, are one by one in turn vaporized by expansion, each being then used to condense the next in the chain before that onefs expansion.

A defect in the cascade system arises from the circumstance that each refrigerant is circulated in a separate closed piping system. It will be understood that a large portion of each refrigerant, when the system is operating, is under compression and proceeding'in a downstream direction between its compressor and its expansion valve (that is, through the high-pressure side of the system), where it is at a sufliciently low temperature that the pressure involved is not excessive. Likewise, the remaining portion of the refrigerant, on the low-pressure side of the system, is at about ambient temperature and at some pressure less than the high side pressure, which condition is maintained as long as the quantity of fluid in the volume enclosed by the low side is a comparatively small proportion of the total amount. When, however, the system ceases to operate (i.e., when it is shut down or brought to an at-rest state), either through accident or deliberate intent, the cooling process stops and the fluid tends slowly to warm up on the high side and to expand into the low side until it is homogeneously distributed at ambient temperature on both sides. But at this temperature, if the total quantity of gas is too great in relation to the total volume enclosed by the system, an excessive pressure may result. In the low temperature stages of a cascade system such shut-down pressures may reach extremely dangerous figures, such as two or three thousand pounds per square inch.

- Heretofore in the art, several methods have been attempted to effect relief of, to obviate, or merely to contain excessive shut-down pressures. Containment has been effected by constructing the entire system of strong and massive components, but such apparatus is expensive 2,952,139 Patented Sept. 13, 1960 and wasteful, and it does not entirely remove the danger of explosion. Relief has been provided by the obvious expedient of releasing the gas into the atmosphere, which is also wasteful. Massive, large-volume emergency expansion chambers have been used, also expensive; and balloons, which leak and pop. Besides, none of these devices is intrinsically capable of rendering the system fail-safe.

To these disadvantages, it may be listed that the cascade system is additionally expensive in requiring the installation of a separate compressor for each refrigerant system; and that it is additionally dangerous when any of the fluids used are inflammable.

In the present invention the refrigerants are first mixed and vaporized at less than atmospheric pressure, then heated and compressed together, and then separated by a fractionating condensation process, the first to liquefy being expanded to precool the others in the chain, and so on. The flow systems for all of the refrigerants are interconnected so that each refrigerant is freely distributed in the system when it comes to rest, and the refrigerants are especially selected so that at least one of them is in liquid form at ambient temperature and at a selected moderate at-rest pressure, and is consequently constrained to occupy a lesser volume, thus providing pressure relief for the remaining refrigerants.

Accordingly, it is an object of this invention to provide a very low temperature refrigeration system characterized by improved safety and economy in operation and at rest.

It is a further object of this invention to provide a very low temperature refrigeration system in which the shut-down pressures are in the same order as the maximum working pressures.

It is a still further object of this invention to provide a very low temperature refrigeration system in which one compressor is used to compress all of the refrigerants.

It is another object of this invention to provide a very low temperature refrigeration system in which the natural inflammability of the refrigerants is substantially reduced.

Further objects and advantages of the invention will become clear with reference to the drawing, of which:

.Figure 1 is a schematic diagram showing the arrangement of the apparatus of the invention;

Figure 2 is a sectional elevation view of a portion of the apparatus shown in Fig. 1; and

Figure 3 is a combination schematic diagram and graph illustrating the operation of the invention.

Referring now to the drawing and particularly to Fig. 1 thereof, there is shown a pair of firstand second-stage compressors 11 and 12, the output or high pressure side of the first being connected to the inlet or low pressure side of the second by means of a conduit 13. Incoming refrigerants enter the low side of compressor 11 by means of a conduit 14 and leave the high side of compressor 12 by means of a conduit 16 leading to a first-stage condensation tank 17.

In the present embodiment of the invention, two refrigerants are used, these being Freon-12 (dichlorodifiuoromethane or CCI F and ethylene (C H They pass through line 14, through the compressors 11 and 12 and into the tank 17 in mixed vaporous state. A cooling water propelling means 18 circulates cooling water through a coil 19 disposed partly within the tank 17, condensing out a major portion of the Freon, which collects in the sump of the tank and is drained off by a main Freon conduit 21. Branching from the main Freon conduit 21 are two subsidiary Freon conduits 22 and 23 which pass separately but in a parallel manner through heat exchangers 24 and 26, to expansion valves 27 and 28, and are coiled, conduit 22 within the lower portion.

tail hereinafter. Leaving the condenser 29, the conduits 22 and 23 are tapped in heat-exchanging relationship by control bulbs 31 and 32 which govern the operation of valves 27 and 28 in a manner well known to the art; and the conduits 22 and 23 then pass again through respective heat exchangers 24 and 26, the conduit 22 being returned to the conduit 13 between the two compressors 11 and 12, and the conduit 23 being returned to conduit 14 on the low side of compressor 11.

Of all the components of the system, the secondary fractionating condenser 29 is the only one not of standard commercial manufacture. As will be seen in the drawing, condenser 29 is entered at the bottom by a conduit 33 leading upward from the tank 17 and terminating in an open end within the condenser at a point spaced substantially above and away from the sump of the condenser; and the sump is drained by a conduit 34 leading back to the tank 17. As described previously, the major portion of the gaseous Freon is condensed out of the Freon-ethylene mixture in the tank '17, but a small percentage of the Freon remains in the flow of ethylene as it passes upward through the conduit 33. The major portion of the remaining Freon is then condensed out by action of the expanded Freon in the coiled conduits 22 and 23, and refluxes through the conduit 34 to the tank 17. A main ethylene conduit 36 drains the still gaseous ethylene (about 99.8 percent pure) from the top of the condenser 29.

Referring now to Fig. 2, there is shown a detailed drawing of the condenser 29, comprising a tubular shell 37 permanently closed at the bottom and bearing at the top a peripheral flange 38 to which is sealed a removable cover plate 39. Inlet conduit 33, sump-draining conduit 34, and ethylene outlet conduit 36 are disposed as previously described. The remainder of the interior apparatus is suspended from the removable cover plate 39; the conduit 23 entering centrally and leading immediately to a manifold 41, where it is divided into seven branches that are rejoined in a manifold 42 disposed slightly above the middle portion of the condenser cavity. The outlet portion of the conduit 23 then leads from the manifold 42 and out through the top cover plate 39. Centrally suspended from the manifold 42 is a detachable baflle retainer 43 transpierced by a downward extending bafllemounting bolt 44. Three circular concave bafiles 46, each pierced by numerous drain holes 47, are mounted concavity-upwards upon the bolt 44, are spaced apart by tubular spacers 48, and are reteained below by a manifold 49 into which the bolt 44 is tightly threaded. Conduit 22, entering through the cover plate 39 at a int displaced from the center thereof, passes through the baffles 46 and enters the manifold 49, where it likewise divides into seven branches and is rejoined at a manifold 51 disposed near the sump of the condenser. The return loop of conduit 22 likewise leaves through the cover plate 39. Now it will be seen that the baffles 46 are each refrigerated to a degree and form cooled surfaces to aid in the condensation of Freon as it passes upward in the flow of ethylene vapor through the holes 47. The baflles 46 additionally act as a heat partition, dividing the interior cavity of the condenser into two heat compartments '52 and 53. It will be shown later how the upper compartment 53 is maintained at a slightly lower temperature than the lower compartment 52, thus increasmg the efficiency of Freon distillation and providing a decreasing temperature gradient for further cooling the ethylene vapor as it passes upward therethrough.

Referring now again to Fig. l, the main Freon conduit 21, after it is tapped by subsidiary Freon conduits 22 and 23, continues through two heat exchangers 54 and 56 to a bulb regulated expansion valve 57 and is coiled within an ethylene condenser 58. Leaving the condenser 58, it passes again through heat exchanger 56 and 18 returned to the compressor-input line 14 on the low pressure side of the compressors. Likewise 9 ethylene conduit 36, leaving the fractionating condenser 29, passes through a heat exchanger 59 and opens into the ethylene condenser 58. Leaving this, it passes through a heat exchanger 61, through a bulb regulated expansion valve 62, and is coiled within an insulated box 63 thermally connected to the object to be refrigerated, in this case a large mercury-diffusion vacuum pump 64. Leaving the box 63, conduit 36 returns through heat exchangers 61, 59, and 54 to the compressor-inlet line 14. To complete the system, suitable oil separators 66, driers 67, and pressure regulating valves 68 are disposed in appropriate places, all as standard in the art.

To assist in more clearly describing the functioning of the system, the circuits of Fig. l are shown schematically in Fig. 3, which is a combination flow and heat exchange diagram, as well as a graph of the temperatures obtaining throughout the system when it is in operation. For instance, the path of the ethylene refrigerant (indicated by shading lines inclined from lower left to upper right) has been laid out with all the apparatus that the ethylene passes through in horizontal straight alignment so that the rising and falling temperatures both above and below 0 F. are shown as the curve that bounds the shaded area. That portion of the path in which the ethylene is in liquid form is additionally shaded with dashed lines; the pressures obtaining are written within the blocks representing portions of apparatus; and all heat exchanging relationships are indicated by heavy solid arrows. The Freon paths are likewise shown by areas oppositely shaded from upper left to lower right; and the return paths are also shown.

Beginning now with compressor-inlet conduit 14 on the left hand side of the figure, and assuming that the system is in full operation, it is shown how the incoming mixture of gaseous ethylene and Freon approaches the firststage compressor 11 at ambient temperature and at a pressure of about 3.5 pounds per square inch absolute (p.s.i.a.). Inside the compressor 11, the pressure is raised to 12.7 p.s.i.a. and heat is added as shown by the arrow, to raise the temperature to a figure in the neighborhood of 200 F. The mixture proceeds by conduit 13 to second-stage compressor 12 where the pressure is raised to about p.s.i.a. and more heat is added to bring the temperature to about 300 F. Continuing through conduit 16, the mixture enters condensing tank 17 where considerable heat is extracted by the cooling water from circulator 18, and the temperature drops to slightly less than ambient; and as the temperature passes through 117 P. (which is the condensation temperature of Freon for 165 p.s.i.a.), the Freon becomes liquefied as indicated and separates from the mixture, to be drained away in conduit 21. Reflux of Freon from the fractionating condenser 29, having already been explained, is not shown in the graph. But the Freon, continuing in the first subsidiary Freon conduit 22, gives up heat in the heat exchanger 24 and then is cooled without heat exchange in the expansion valve 27, where the pressure drops suddenly to 12.7 p.s.i.a. (as drawn by the conduit 13 between the two compressors) and the temperatm'e drops to about 30' F. The vaporized Freon then absorbs heat in passage through the first heat compartment 52 of condenser 29 and is retm'ned to ambient temperature in second passage through the heat exchanger 24. It then returns to conduit 13. a

That portion of the Freon proceeding through the second subsidiary conduit 23 undergoes a similar transformation, giving up heat in heat exchanger 26 and being vaporized to 2.5 p.s.i.a. in expansion valve 28. But because the suction pressure on the low side of this valve is considerably less than the suction pressure on the low side of valve 27, the temperature drop is correspondingly greater for the Freon in valve 28, say to about 40 F. Thus is the heat compartment 53 maintained at a lower temperature than compartment 52 as hereinbefore described. The Freon in conduit 23 then absorbs some heat from compartment 53 and is returned to ambient temperature in its second passage through heat exchanger 26. It then returns to conduit 14.

The Freon in the main Freon conduit 21 is now shown giving up heat in passage through heat exchangers 54 and 56,- and then being vaporized to 2.5 p.s.i.a. in expansion valve 57. Although this is the same suction pressure to which the Freon in conduit 23 is vaporized, yet it will be seen that the Freon in conduit 21 has been precooled to a lower temperature, and that its expansion temperature is therefore considerably lower, e.g., about 60 F. Since this temperature is below the condensation temperature of ethylene (for 165 p.s.i.a.), the Freon in this line is useful for liquefying the ethylene in condenser 58. The Freon is thus shown absorbing heat in passage through condenser 58. It is then returned to ambient temperature in heat exchanger 56 and is returned to the conduit 14.

Now the ethylene that was separated from the liquefied Freon in condensing tank 17 is shown giving up heat in passage through compartments 52 and 53 of fractionating condenser 29, in heat exchanger 59, and in the ethylene condenser 58, where it is at last liquefied. It is then further cooled in heat exchanger 61; it is expanded through expansion valve 62 to 2.5 p.s.i.a. and is vaporized to a final refrigerating temperature of about -200 F. in passage through the coil in insulating box 63. There it absorbs heat from the vacuum pump 64 and flows on to be returned to ambient temperature in three steps, i.e., while passing through the heat exchangers 61, 59, and 54. It is then returned to the line 14 where it mixes with the returned Freon before reenter-ing the compressors.

Now it is'known that the ethylene, while the system is operating, has about 98% of its total quantity liquefied and concentrated at low temperature and at 165 p.s.i.a. pressure in the liquid part of the high side of the system, which lies between fractionating condenser 29 and expansion valve 62; and that the remainder of the ethylene is a vapor sparsely distributed at very low suction pressure on the low side of the system, and at 165 p.s.i.a. in the relatively large volume of the tank 17 and condenser 29. However, when the compressors stop operating and the system warms up, then the liquid ethylene entirely vaporizes. In such a case the contained volume of the ethylene system alone is too small for the quantity of vapor that results, and it is necessary to make extra space for the ethylene to expand into. This extra space is provided partly by the structure of the invention, which has numerous cross communications between the contained volumes of the ethylene and the Freon circulation systems, and partly by the selection of Freon as the second refrigerant to be used. At a temperature between 60 F. and 110 F. (for ambient), Freon-12 has a condensation pressure of from 72 to 150 p.s.i.a. When the system stops operating and warms up, if the pressure in the system becomes greater than the Freon condensation pressure, the Freon must all condense into liquid form, thus shrinking substantially in volume and leaving capacious quantities of space in its own system for expansion of the ethylene. As a result, as has been observed in actual operation of the invention, shut-down pressures of less than 250 p.s.i.a. assuredly obtain at all stages after the compressors cease to operate. It will be obvious that no problem is involved in restarting the system, since the Freon begins to cool the ethylene at the same time as it begins to expand in a vapor, and the stage of full operation is attained by degrees in a comparatively short time, i.e., half an hour.

It will be obvious that as many different refrigerants and as many refrigerant circulation systems as are desired may be incorporated in the device, and that the objects of the invention will still be attained as long as at least one of the refrigerants is by character a liquid at ambient temperature under the at-rest pressure obtaining. To ensure that such will be the case, the system, when it is originally charged with refrigerant, is charged 20 the at-rest pressure desired.

It will also be obvious that, particularly with more complex systems, considerable savings in both power expended and capital installation may be made by having all of the refrigerants compressed in the same main compressor or in a series of limited capacity compressors as in the described embodiment of the invention.

It has furthermore been observed that the mixing of inflammable gases with others that are not inflammable renders the mixture also not inflammable.

While this description has been directed toward a preferred embodiment of the invention, it will be apparent that many modifications and other embodiments may be devised without departing from the spirit and scope of the teachings set forth; and it is intended to limit the invention only by the appended claims.

What is claimed is,

1. In a binary refrigeration system, a closed circuit comprising first and second compressors connected in series; a fractionating tower connected in series with and downstream from said compressors; first, second, and third evaporators connected in parallel to the liquid outlet of said fractionating tower, the outlet of said first evaporator being connected between said first and second compressors and the outlets of said second and third evaporators being connected to the inlet of said first compressor; a condenser connected to the vapor outlet of said fractionating tower; a fourth evaporator connected between said condenser and the inlet of said first compressor; and means external to said closed circuit for exchanging heat between said fractionating tower and said first and second evaporators, between said condenser and said third evaporator, between the inlets and outlets of each of said evaporators, between the inlet of said third evaporator and the outlet of said fourth evaporator, and between the inlet of said condenser and the outlet of said fourth evaporator.

2. The system as defined in claim 1 wherein the binary refrigerant of said system comprises a mixture dichlorodifluoromethane and ethylene.

3. A closed circuit refrigerating system including a binary refrigerant having high and low boiling componcnts and comprising first and second compressors connected in series; first fractionator means receiving discharge from said second compressor for condensing higher boiling point component from the compressed binary refrigerant; first and second paralleled intake heat exchangers coupled to receive condensate from the first fractionator; a second dual series chamber fractionator coupled to receive refrigerant from the first fractionator and return condensed higher boiling component thereto with first and second cooling coils disposed in each of said chambers coupled to receive high boiling component from said first and second exchangers and return such component countercurrently through the first exchanger to the connection between said compressors and through the second exchanger to the intake of the first compressor, respectively; a third heat exchanger coupled to receive lower boiling component from the second fractionator; a condenser receiving lower boiling component from the third exchanger; a fourth exchanger receiving condensate from said condenser; refrigerating expansion coil means receiving lower boiling component from the fourth exfrom the first fractionator; a sixth heat exchanger receiving higher boil-ing component from the fifth exchanger to be circulated therefrom through the coil of said condenser and returned countercurrently therethrough to the intake of the first compressor.

4. Thesystem asdefinedinclaim 3 whereinthebinary refiigerant comprises a mixture of dichlorodifluoromethane and ethylene.

5. The system as defined in claim 3 wherein said second iractionator is constructed with said dual series chambers disposed one above the other with a battle therebetween.

6. The system as defined in claim 3 wherein said second fractionator is consmlcted with said dual series chambers disposed one above the other with a baflle therebetween and said cooling coils are constructed to provide a cluster of cooling coil surfaces.

7. The system as defined in claim 3 wherein said second fractionator is constructed with said dual series chambers disposed one above the other in a single cylindrical shell with a bathe therebetween and said cooling coils are constructed to provide a cluster of cooling coil surfaces.

8 8. The system as defined in claim 7 wherein said binary refrigerant comprises a mixture of dichlorodifluoromethane and ethylene.

References Cited in the file of this patent UNITED STATES PATENTS 950,436 Claude Feb. 22, 1910 1,607,320 Van Nuys Nov. 16, 1926 2,352,581 Winkler June 27, 1944 2,458,894 Collins Jan. 11, 1949 2,680,956 Haas June 15, 1954 2,682,756 Clark et al. July 6, 1954 2,697,922 Schilling Dec. 28, 1954 2,770,951 Morrison NOV. 20, 1956

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