US3922881A - Helium 3-helium 4 dilution refrigerator - Google Patents

Abstract

A 3He-4He dilution refrigerator with a continuous, heat exchange of concentrated 3He and dilute 3He streams in parallel flow, in a downward direction.

Description

United States Patent Staas et al.

HELIUM S-HELIUM 4 DILUTION REFRIGERATOR Inventors: Frans Adrianus Staas; Adrianus Petrus Severijns, both of Eindhoven,

Netherlands Assignee: U.S. Philips Corporation, New

York, NY.

Filed: Nov. 6, 1974 Appl. No.: 521,278

Foreign Application Priority Data Nov. 13, 1973 Netherlands 7315487 u.s. Cl 62/502; 62/5 14 Int. Cl.F25B 1/00 Field of Search 62/1 I4, 476, 502, 514

[ Dec. 2, 1975 [56] References Cited UNITED STATESPATENTS 3,58l,5l2 6/l97l Staas et al 62/56 OTHER PUBLICATIONS Wood, M. F.; The He Dilution Refrigerator, Advanced Cryogenics, 1971, N.Y., pp. 245-260.

Primary Examiner-William F. ODea Assistant Examiner-Ronald C. Capossela Attorney, Agent, or Firm-Frank R. Trifari; J. David Dainow [57] ABSTRACT A l-lel-le dilution refrigerator with a continuous, heat exchange of concentrated He and dilute l-le streams in parallel flow, in a downward direction.

7 Claims, 2 Drawing Figures HELIUM 3-HELIUM 4 DILUTION REFRIGERATOR BACKGROUND OF THE INVENTION The invention relates to ahelium 3 helium 4 dilution refrigerator for temperatures below the A point of helium. The refrigerator has a supply pipe for a stream of concentrated refrigeratedhelium 3 which opens into a mixing chamber forhelium 3 and helium 4; the mixing chamber is connected by a connecting pipe for a stream ofdilute helium 3 to a distillation chamber for separatingdilute helium 3 intohelium 3 and helium 4, which chamber has an outlet mainly forgaseous helium 3. A continuous heat exchanger is provided which is included in the supply pipe and also in the connecting pipe so as to effect a heat exchange between theconcentrated helium 3 stream and the dilutecolder helium 3 stream.

A refrigerator of the above-mentioned type, sometimes is referred to as mixing refrigerator is described in Cryogenics, April l966, pages 80-88.

The term continuous heat exchanger is used herein to mean a heat exchanger in which in operation, viewed in the direction of flow, a temperature gradient exists along the heat transmitting partition wall between the two heat exchanging fluids, as distinct from a discrete heat exchanger (step exchanger) in which there is no temperature gradient in the flow direction along the heat transfer partition wall, that is to say in which heat exchange takes place between two discrete temperature levels.

In the operation of the dilution refrigerator concentratedliquid helium 3 is supplied to the mixing chamher which containshelium 3 helium 4. Below 0.87K, phase separation occurs in theliquid helium 3 helium 4 mixture in the mixing chamber, yielding a phase which is rich inhelium 3 and behaves as a liquid and a phase which is poor inhelium 3 and behaves as a gas. The concentratedhelium 3 phase floats on the dilute phase poor inhelium 3.

When concentratedhelium 3 supplied to the mixing chamber, passes the interface with thedilute helium 3 phase, a cooling effect is produced owing to the large difference between the molar enthalpies of concentrated anddilute helium 3. At the interfaceliquid helium 3 is as it were evaporated, the heat of evaporation producing the cooling effect.Helium 3 atoms which pass the interface are conveyed via the dilute phase to the distillation chamber of higher temperature owing to the high osmotic pressure of thehelium 3 in the dilute solution.

Nonnally the distillation chamber is connected to a pump system. In the distillation chamber the two helium isotopes are separated by'a distillation process. The gaseous phase drawn off is rich in helium 3 (for example 96%) while the dilute liquid solution contains verylittle helium 3. The substantially puregaseous helium 3 is condensed and is returned through the supply duct to the upper end of the mixing chamber, so that the cycle is closed.

In order to reduce to a minimum the transport of heat by concentratedhelium 3 to the mixing chamber, this concentratedhelium 3 during its return is caused to exchange heat with the colder,dilute helium 3 which is flowing from the mixing chamber to the distillation chamber. To realize temperatures of about 0.025K in the mixing chamber a continuous heat exchanger alone is sufficient. If the required temperature is near or even 2 below 0.0l0l(, it is known to include in the supply and connection pipes one or more discrete heat exchanges (sintered copper heat exchangers, thin foil-plate heat exchangers) arranged in series between the continuous heat exchanger and the mixing chamber.

In the known dilution refrigerators the continuous heat exchanger is used as a counterflow heat exchanger in which theconcentrated helium 3 and thedilute helium 3 meet in counterflow. This gives rise to problems primarily due to gravity. If theconcentrated helium 3 stream runs down and thedilute helium 3 stream runs up, instabilities are produced by the occurrence of convection in thedilute helium 3 stream. In the path from the lower temperature range in the mixing chamber to the higher temperature range in the distillation chamber a temperature gradient occurs in the flow direction in the heat exchange region of the dilute solution. To satisfy the condition of constant osmotic pressure in the superfluid phase the concentration ofhelium 3 in the dilute solution decreases towards the distillation chamher. A decrease of thehelium 3 concentration, however, means an increase in density of the dilute solution, that is to say the dilute solution has a higher specific weight near the distillation chamber than near the mixing chamber. The force of gravity in conjunction with the density gradient gives rise to convection in the dilute solution, so that the condition of constant osmotic pressure is disturbed and flow instabilities involving energy losses occur. If theconcentrated helium 3 stream runs up and thedilute helium 3 stream runs down, no disturbing convection is produced in thedilute helium 3 flow.

No difficulties would arise in the concentrated helium 3 stream if it should consist ofpure helium 3. Normally, however, a certain percentage (for example 4%) of helium 4 still is contained in theconcentrated helium 3 stream. When this stream is cooled to a temperature below 0.3 to 0.4K in the continuous heat exchanger, phase separation occurs in the said heat exchanger (as in the mixing chamber). By gravity the dilute phase (helium 4) collects in the lower part of the relevant flow channel of the heat exchanger. In analogy with the process in the mixing chamber a cooling effect is produced in this lower part of the heat exchanger channel on the passage ofhelium 3 through the accumulated dilute superfluid phase, so that the temperature of this channel part falls. As a result, there is substantially no temperature difference anymore between the said channel part containing dilute phase and the other channel part which also containsdilute helium 3 and is in heat exchanging contact with the first mentioned part.

Thus there is substantially no heat transfer between the two channel parts so that the lower part of the heat exchanger is almost inoperative. In addition, the rate of flow ofhelium 3 may become high in the twodilute 3 He regions, which results in considerable pressure gradients due to the viscous behavior of thehelium 3 moving through the superfluid helium. The frictional heat produced thereby also contributes to a reduction of the thermal efficiency of the continuous heat exchanger.

It is an object of the present invention to provide a solution of the above-described problems.

SUMMARY OF THE INVENTION According to the invention thehelium 3 helium 4 dilution refrigerator is characterized in that the continuous heat exchanger is arranged in the supply and connection pipes as a parallel flow heat eschanger for the concentrated anddilute helium 3 streams, the continuous parallel flow heat exchanger being positioned in operation so that the two heat exchanging streams run down in a vertical sense. This ensures that both disturbing convection in thedilute helium 3 stream and accumulation of dilute phase (superfluid helium) in the part of the continuous heat exchanger through whichconcentrated helium 3 flows are prevented.

In an advantageous embodiment of the helium 3- helium 4 dilution refrigerator according to the invention the continuous parallel flow heat exchanger is divided into a plurality of series connected continuous parallel flow heat exchange elements, the heat transferring wall surface area between the concentrated anddilute helium 3 streams of each continuous parallel flow heat exchange element satisfying the relation:

where r: the number of moles ofhelium 3 which is circulated per second,

T input temperature in K of the concentratedhelium 3 flow for the parallel flow heat exchange element,

0 =the Kapitza conduction coefficient of the heattransferring wall defined as where Q heat flux in watts/cm through the heat-transfer ring wall,

T temperature in "K of theconcentrated helium 3 stream in the parallel flow heat exchange element,

T temperature in K of thedilute helium 3 stream in the parallel flow heat exchange element.

It was found that thus optimum use of the heat transferring wall surface area between the concentrated anddilute helium 3 streams and hence a heat exchange system of very high thermal efficiency are obtained. For metals the Kapitza conduction coefficient is of the order of 2.5 times W/cm "K and for synthetic materials it is of the order of 17.5 times l0 W/cm K A further advantageous embodiment of thehelium 3 helium 4 dilution refrigerator according to the invention is characterized in that each continuous parallel flow heat exchange element comprises two tube elements arranged in coaxial spaced relationship, the wall of the inner tube element forming the heat transfer wall. Having regard to thermodynamic considerations and owing to the comparatively small liquid contents and consequent small heat capacity, the described construction provides the advantage that each continuous parallel flow heat exchange element in itself has a high thermal efficiency.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing of FIG. I which shows schematically and not to scale a helium 3-helium 4 dilution refrigerator having a continuous parallel flow heat exchanger which comprises three series connected elements.

FIG. 2 is another embodiment of the heat exchanger 2a of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1,reference numeral 1 denotes a supply pipe which opens in amixing chamber 2, which chamber is connected by a connection or returnpipe 3 to a distillation chamber 4 having anoutlet 5. Theoutlet 5 is connected by asuction pipe 6 to a diffusion pump 7 which in turn is connected to a rotary pump 8. Adelivery outlet 9 of the rotary pump 8 is connected by apipe 10 to thesupply pipe 1. Thepipe 10 includesheat exchangers 11, 12, I3 and 14 which are accommodated incontainers 15, 16, 17 and distillation chamber 4 respectively.

In said containers condensation and further precooling ofconcentrated helium 3 takes place. For example, thecontainer 15 is filled with liquid nitrogen (78K) while thecontainers 16 and 17 contain liquid helium of, say, 4.2K and 1.3K respectively. The refrigerator further comprises a continuous parallel flow heat exchanger 20 which is composed of threeelements 20a, 20b and 200 which are included in series arrangement in thesupply pipe 1 and also in theconnection pipe 3. The elements are vertically positioned so that in operation the two streams in the supply and connection pipes both pass through the heat exchange elements downwardly, i.e. in the direction in which the force of gravity acts.

In operation substantially puregaseous helium 3 delivered to the pipe I0 by the rotary pump 8 is condensed in the heat exchangers B1 to 14 and cooled to a temperature of about O.7K. The condensedconcentrated helium 3 is further lowered in temperature in the continuous parallel flowheat exchange elements 20a, b, c and then enters the mixingchamber 2 which contains twophases 22 and 23 ofconcentrated helium 3 and superfluid dilute helium 3 (helium 3 dissolved in helium 4) respectively which are separated by aninterface 21. The passage ofhelium 3 from thephase 22 via theinterface 21 to thephase 23 produces a cooling effect. Thehelium 3 which has passed theinterface 21 is conveyed in the dilute phase through the connectingpipe 3 to the distillation chamber 4 and, during this transport, by parallel flow heat exchange in the continuous heat exchange elements cools theconcentrated helium 3 on its way to the mixingchamber 2. In the distillation chamber 4 thedilute helium 3 is separated intohelium 3 and helium 4. The substantiallypure helium 3 is drawn off through theoutlet 5 and thesuction pipe 6 by the pump system comprising the diffusion pump '7 and the rotary pump 8, and then is returned to thepipe 10.

The heat transferring wall surface area 0- between the concentrated anddilute helium 3 streams of each of the three parallel flowheat exchange elements 20a, b, c satisfies the relation.

where h the number of moles ofhelium 3 which is circulated per second, 0 the Kapitza conduction coefficient,

T the input temperature of theconcentrated helium 3 stream for the relevant heat exchange element.

The square of the said input temperature appears in the denominator of the right-hand part of the relation. A lower input temperature of theconcentrated helium 3 flow permits an increase of the heat transferring wall surface area of the associated heat exchange element. This is expressed in FIG. 1. The temperature of theconcentrated helium 3 stream in thesupply pipe 1 decreases in the downward direction. The heat transferring wall surface area of theelement 20c is greater than that of 20b, which in turn is greater than that of 20a, because the elements have different flow passage lengths.

Since in the parallel flow heat exchanger 20 the concentrated anddilute helium 3 streams both run down in the direction of the force of gravity, the difficulties described hereinbefore are obviated. The construction of the parallel flow heat exchanger from discrete series connected elements having different heat transferring wall surface areas results in a heat transfer system of high thermal efficiency. Preferably the elements each comprise two concentric tubes la and 3a, as shown in FIG. 2, the wall of the inner tube being used for the heat transfer between the two streams. Obviously the continuous parallel flow heat exchanger may be divided into two or more than three elements while satisfying the above relation.

Condensation and precooling of theconcentrated helium 3 may be effected by means other than a bath of liquid nitrogen and two baths of liquid helium, and also other pump systems, which may or may not operate at room temperature, may be used. If desired, further discrete heat exchangers (made for example of sintered copper) may be included in the supply and connection pipes l and 3 respectively between the continuous heat exchange elements 200 and the mixingchamber 2.

What is claimed is:

1. In a He He dilution refrigerator for temperatures below the A point of helium, including first means for condensing and cooling gaseous helium into con- .centrated liquid He, a mixing chamber containing con- 'centrated He and superfluid dilute He, a distillation chamber for separating dilute He into He and He, and a supply duct for flowing concentrated liquid He from said first means to said mixing chamber, anda return duct for flowing dilute He from said mixing chamher to said distillation chamber, the improvement in combination therewith of a continuous heat exchanger comprising adjacent and parallel, concurrent oriented lportions of said supply and return duct in heatexchange relationship.

2. A refrigerator according toclaim 1 wherein said heat exchanger comprises a plurality of series-connected, continuous, parallel-flow heat exchanger elements, each element having a heat-transferring wall surface-area 0' between the concentrated He in the supply duct and the dilute He in the return duct, said 0' satisfying the relation:

where it the number of moles ofhelium 3 which is circulated per second,

T input temperature in K of theconcentrated helium 3 stream for the parallel flow heat exchange element,

0 the Kapitza conduct coefficient of the heat transfer wall, defined as where Q heat flux in Watts/cm through the heat transferring wall,

T temperature in "K of theconcentrated helium 3 stream in the parallel flow heat exchange element,

and

T temperature in K of thedilute helium 3 stream in the parallel flow heat exchange element.

3. A refrigerator according toclaim 2 wherein each of said heat-exchange elements comprises inner and outer tubes in coaxial relationship with a bore through the inner tube and an annular space between said tubes, said bore and annular space corresponding to said supply and return ducts and the wall of the inner tube forming said heat-transfer wall.

4. A refrigerator according toclaim 1 wherein said first means comprises a heat exchanger for reducing the temperature of the helium to approximately l.3K.

5. A refrigerator according to claim 4 wherein said first means comprises three heat-exchangers in series containing liquefied gases at temperatures of approximately 78K, 4.2K, and 1.3K.

6. A refrigerator according toclaim 1 wherein said continuous heat exchanger comprises a plurality of series-connected, continuous, parallel-flow heat exchanger elements for cooling to successively lower temperatures, and said return duct provides a flow path from said mixing chamber first to the heat exchanger element having the lowest of said temperatures, and then to heat exchanger elements having successively higher temperatures.

7. A He He dilution refrigerator operable with a supply of gaseous helium, comprising, pump means for circulating said helium, first means for cooling and condensing said helium from said pump means, a continuous heat exchanger for further cooling said liquid helium into concentrated He, a mixing chamber containing dilute He and receiving said concentrated He with an interface between said concentrated and dilute He and resulting refrigeration, a supply duct for flow ing said liquid He from said first means through said heat exchanger to said mixing chamber, a return due for flowing said dilute He from said mixing chamber through said continuous heat exchanger where it provides refrigeration for said liquid He flowing there through, and thence to said mixing chamber, a distilla tion chamber for separating gaseous He from said di lute liquid He received via said return duct from saic heat exchanger, and a suction pipe for flowing said gas eous He from said distillation chamber to said purnr means, said heat exchanger comprising portions of sait return duct and said supply duct which are adjacen and situated in heat transfer relationship for paralle flow in a concurrent direction.

Claims (7)

1. IN A 3HE - 4HE DILUTION REFRIGERATION FOR TEMPERATURES BELOW THE $ POINT OF HELIUM, INCLUDING FIRST MEANS FOR CONDENSING AND COOLING GASEOUS HELIUM INTO CONCENTRATED LIQUID 3HE, A MIXING CHAMBER CONTAINING CONCENTRATED 3HE AND SUPERFLUID DILUTE 3HE, A DISTILLATION CHAMBER FOR SEPARATING DILUTE 3HE INTO 3HE AND 4HE, AND A SUPPLY DUCT FOR FLOWING CONCENTRATED LIQUID 3HE FROM SAID FIRST MEANS TO SAID MIXING CHAMBER, AND A RETURN DUCT FOR FLOWING DILUTE 3HE FROM SAID MIXING CHAMBER TO SAID DISTILLATION CHAMBER, THE IMPROVEMENT IN COMBINATION, THEREWITH OF A CONTINUOUS HEAT EXCHANGER COMPRISING ADJACENT AND PARALLEL, CONCURRENT ORIENTED PORTIONS OF SAID SUPPLY AND RETURN DUCT IN HEAT-EXCHANGE RELATIONSHIP.

2. A refrigerator according to claim 1 wherein said heat exchanger comprises a plurality of series-connected, continuous, parallel-flow heat exchanger elements, each element having a heat-transferring wall surface-area sigma between the concentrated 3He in the supply duct and the dilute 3He in the return duct, said sigma satisfying the relation:

3. A refrigerator according to claim 2 wherein each of said heat-exchange elements comprises inner and outer tubes in coaxial relationship with a bore through the inner tube and an annular space between said tubes, said bore and annular space corresponding to said supply and return ducts and the wall of the inner tube forming said heat-transfer wall.

4. A refrigerator according to claim 1 wherein said first means comprises a heat exchanger for reducing the temperature of the helium to approximately 1.3*K.

5. A refrigerator according to claim 4 wherein said first means comprises three heat-exchangers in series containing liquefied gases at temperatures of approximately 78*K, 4.2*K, and 1.3*K.

6. A refrigerator according to claim 1 wherein said continuous heat exchanger comprises a plurality of series-connected, continuous, parallel-flow heat exchanger elements for cooling to successively lower temperatures, and said return duct provides a flow path from said mixing chamber first to the heat exchanger element having the lowest of said temperatures, and then to heat exchanger elements having successively higher temperatures.

7. A 3He - 4He dilution refrigerator operable with a supply of gaseous helium, comprising, pump means for cirCulating said helium, first means for cooling and condensing said helium from said pump means, a continuous heat exchanger for further cooling said liquid helium into concentrated 3He, a mixing chamber containing dilute 3He and receiving said concentrated 3He, with an interface between said concentrated and dilute 3He and resulting refrigeration, a supply duct for flowing said liquid 3He from said first means through said heat exchanger to said mixing chamber, a return duct for flowing said dilute 3He from said mixing chamber through said continuous heat exchanger where it provides refrigeration for said liquid 3He flowing therethrough, and thence to said mixing chamber, a distillation chamber for separating gaseous 3He from said dilute liquid 3He received via said return duct from said heat exchanger, and a suction pipe for flowing said gaseous 3He from said distillation chamber to said pump means, said heat exchanger comprising portions of said return duct and said supply duct which are adjacent and situated in heat transfer relationship for parallel flow in a concurrent direction.

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