Refrigeration method and system

FIELD OF THE INVENTION

The present invention relates to a method of operating an adsorption refrigeration system.

BACKGROUND TO THE INVENTION

Adsorption refrigeration systems are well known in the field of refrigeration and particularly cryogenics, for providing very low temperatures in a region such as a chamber. Adsorption refrigeration systems operate by the provision of an amount of liquid coolant within a chamber to be cooled. This is placed in gaseous communication with an amount of an adsorbing “sorption” material such as charcoal, the entire system being closed such that the amount of coolant within the system remains constant.

Typically the coolant in liquid form is obtained by condensation of gaseous coolant in contact with the cold walls of a member pre-cooled by an external source. This is performed in a conventional adsorption refrigeration system by the use of a “1K pot”.

A second, alternative method of obtaining liquid coolant uses an expansion process, in which gaseous coolant is decompressed from a high pressure under adiabatic conditions. This decompression causes liquefaction of the gas thereby generating the liquid coolant. The provision of liquid coolant by this method has been used in an experimental adsorption refrigeration system. However, only a short hold time was achieved in this case with respect to commercial systems having a 1K pot.

The adsorption material of the system is arranged to adsorb the gas above the liquid coolant such that further evaporation of the liquid occurs due to the corresponding reduction in the pressure. The latent heat of evaporation causes a reduction in the temperature of the system.

However, one problem with such adsorption systems is that the sorption material itself can only adsorb a finite amount of gas for a given pressure. Such devices are therefore effectively of the single-shot type, with typical commercial systems being operable for a number of hours, although this is dependent upon the sorption capacity of the sorption material.

Adsorption systems are advantageous in that they are relatively simple devices which can be recharged by simply heating the sorption material so as to cause desorption of the gas-coolant and return it to the gaseous phase. Upon sufficient subsequent cooling, the adsorption material can be reused. As the system is enclosed, there is no loss of coolant and there are no moving parts. This is beneficial in that low temperature experiments can be performed at low levels of vibration for many hours.

The primary problem with conventional adsorption refrigeration systems is the single-shot nature of their operation and therefore, for experiments that require longer periods of operation, conventional adsorption pumps are impractical.

Conventional systems having a 1K pot tend to be expensive due to the provision of the 1K pot and its support systems. In addition, the use of a 1K pot does also produce some vibrations, which it is desirable to eliminate.

In the alternative experimental system described above the expansion of the gaseous coolant was used to generate the liquid phase coolant rather than using a 1K pot. Thus, although the 1K pot was replaced by using the expansion effect, unfortunately the hold time at operational temperatures was significantly reduced.

There is therefore a strong market demand to improve the operational hold time of such devices along with the elimination of vibrations.

SUMMARY OF THE INVENTION

In accordance with the present invention we provide a method of operating an adsorption refrigeration system, the system comprising an adsorption pump which, in use, is arranged in communication with a chamber containing liquid and gaseous coolant, the method comprising:

i) expanding the gaseous coolant into an auxiliary volume member so as to cause the removal of part of the gaseous coolant from the chamber, thereby reducing the temperature and pressure of the gaseous coolant in the chamber; and

ii) operating the adsorption pump so as to further cool the chamber by causing the evaporation of the coolant liquid within the chamber.

We have therefore adopted a new approach in improving the performance of single-shot adsorption devices by the use of an expansion process in which gaseous coolant is expanded into an auxiliary volume member. When a gas is allowed to increase its volume under certain conditions (for example adiabatic conditions), the reduction in pressure produced causes a corresponding reduction in the temperature of the gas. We have realised that, in this way, the expansion effect can be used to greater effect in association with the evaporation cooling of a known adsorption system.

This significantly increases the time at which the chamber can be maintained at operational temperatures, which are suitable for activities such as experimentation. Whereas the known experimental expansion process obtained only a short hold time of a few hours at operational temperatures, with the present invention, hold times of 50 hours or more are realisable without the expense and vibration problems associated with conventional systems having a 1K pot. This is due to the use of an auxiliary volume member which allows for a much improved expansion effect. An increased volume of liquid coolant can therefore be generated and a lower starting temperature is also attainable before the conventional evaporation cooling step begins.

The expansion stage of the method may either be performed once in a single step, or in a number of separate sub-steps in a multistage process with, for example, consecutive pressure reductions, in order to further reduce the temperature in the chamber prior to performing step (ii). Typically in this case step (i) further comprises expanding the gaseous coolant separately into a number of additional auxiliary volume members. Prior to the expansion step (i), the quantity of gaseous coolant supplied to the adsorption refrigeration system is preferably in excess of the saturation limit of the adsorbent material within the adsorption pump when operating under normal working conditions.

Prior to step (i), the coolant may be provided either as a gas or a liquid from any suitable source. However, preferably the coolant is provided as a gas from an auxiliary volume member used in step (i).

The auxiliary volume is preferably a static volume provided by a storage reservoir or a second adsorption pump. The auxiliary volume member may be arranged to have a constant geometrical volume or a variable volume. The use of a variable volume member allows the pressure within the chamber to be controlled and therefore the degree of cooling can be controlled accordingly.

In either case, the expansion of the gaseous coolant may be effected by allowing the gas to expand into the auxiliary volume member. This is generally performed using a controllable valve.

Typically the capacity of the auxiliary volume member is greater than the adsorption capacity of the adsorption pump and this ensures that the single-shot operational time is maximised.

Although the adsorption pump may be separated from the chamber using an appropriate valve such that steps (i) and (ii) of the method are separable, typically the adsorption pump remains in communication with the chamber during the steps of providing and/or expanding the cooling gas to and from the chamber respectively. The operational simplicity of the method is therefore improved.

In such a case, prior to step (i) the adsorption pump is cooled to the first temperature such that the adsorption material contained therein adsorbs the gaseous coolant so as to become substantially saturated for pressures higher than the ultimate pressure obtained at the lowest temperature. Preferably the adsorption pump is then disconnected from the storage vessel and heated so as to desorb gaseous coolant and thereby increase the gas pressure in the chamber. This increase in pressure may be in addition to a positive pressure of gas provided when the gaseous coolant is initially supplied to the chamber prior to step (i).

Typically the adsorption pump is also heated during step (i) so as to maximise the effect of the first stage expansion to the auxiliary volume member.

Following the expansion step (i), the adsorption system is typically isolated from the auxiliary volume member during the subsequent step (ii) so as to maximise the single-shot operational time.

Advantageously in some cases the expansion effect may also be used when the adsorption pump system is no longer in communication with the auxiliary volume member. This may be achieved by cooling the adsorption pump prior to step (ii), thereby further reducing the pressure of the gaseous coolant within the chamber. This effectively expands the gaseous coolant further and causes further cooling in a second expansion step. It was an analogous step of this kind, using only the internal volume of an adsorption pump system, that was used in the known experimental expansion cooling method (described earlier) to generate all of the liquid coolant.

In contrast, the present invention preferably uses this additional expansion process and/or that of step (i) to cause the partial liquefaction of the gaseous coolant.

The method can be used with many known coolants such as helium-4, nitrogen, neon or hydrogen although it is particularly suitable for use with helium-3 as this provides the capability of attaining the lowest temperatures for experimental purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

Some examples of a refrigeration method according to the present invention will be now be described with reference to the accompanying drawings, in which:

FIG. 1

is a schematic illustration of a refrigeration system according to a first example;

FIG. 2

is a flow diagram of the method according to the first example;

FIG. 3

shows a second example system;

FIG. 4

is a schematic representation of a third example system having a heat pipe;

FIG. 5

is an illustration of a fourth example;

FIG. 6

is a graph of experimental data measured in accordance with the third example;

FIG. 7

is an example of a quasi-continuous refrigeration system in accordance with a fifth example;

FIG. 8

is a sixth example showing the cooling of a dilution refrigeration device; and,

FIG. 9

is an image of part of a practical device according to the fourth example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of a refrigeration system for use in accordance with the invention is generally indicated at

1

in FIG.

1

. An adsorption pump

2

is provided in the form of a chamber connected via a pumping line

3

to a pot

4

. The adsorption pump

2

contains an adsorbing material which in this case is charcoal, indicated at

5

. The adsorption pump

2

, pumping line

3

and the pot

4

are conventional components of a single-shot adsorption refrigeration system. In the present example helium-3 is the coolant liquid/gas.

In a conventional system, these components form a closed system in which gaseous coolant

10

within the volume of the pumping line and pot is adsorbed by the charcoal

5

such that liquid coolant

9

in the bottom of the pot

4

is progressively evaporated causing the pot

4

to cool.

In the present case, however, the internal volume of the adsorption pump

2

, pumping line

3

and pot

4

is connected via a pipe

6

to an auxiliary volume member

7

in the form of a reservoir such as a gas storage vessel. A valve

8

within the pipe is selectively operable by a user so as to allow or prevent communication of gaseous coolant

10

(helium-3) between the reservoir

7

and the remainder of the system.

For the purposes of this example, the charcoal

5

is capable of adsorbing about four litres of coolant gas, the reservoir

7

having a volume of about ten litres.

FIG. 2

is a flow diagram of a method of operating the apparatus of FIG.

1

. At step

200

, the refrigeration system indicated at

1

, other than the storage reservoir

7

is cooled by conventional means such as a helium-4 cryostat to about 4K. Prior to this operation, an amount of helium-3 is provided as a gas to the adsorption pump

2

, pumping line

3

and pot

4

. At this stage there is no liquid coolant within the pot

5

and the valve

8

is closed.

At step

201

, when the system has been cooled to the cryostat temperature of 4K, the valve

8

is opened and the helium-3 coolant gas passes from the reservoir

7

, due to its higher pressure, into the adsorption pump

2

, pumping line

3

and pot

4

. It should be noted that during this step, the charcoal

5

becomes saturated with helium-3 and the pressurising continues until a positive pressure of, for example, about 0.5 Bar (absolute pressure) is attained and the relative pressure between the adsorption pump system and reservoir has been reduced. The use of an absolute pressure lower than that of atmospheric pressure prevents the loss of the relatively expensive helium-3 through leaks to the surrounding environment. The valve

8

can be operated for a sufficient time to equalise the pressure between the adsorption pump system and the reservoir if necessary.

Following pressurisation at step

202

, the valve

8

is closed at step

203

and the charcoal

5

in the adsorption pump is then heated at step

104

to a temperature of about 100 K. This heating causes the desorption of the helium-3 coolant

10

from the charcoal

5

and therefore causes a further increase in the gas pressure within the system, typically up to 10 Bar. During this step, the pot is maintained at a low temperature, for example at 4K.

Once the desorption is complete, the pressure within the adsorption pump system and the pot

4

is higher than prior to heating due to the desorption of the helium-3 from the charcoal

5

.

At step

205

, the valve

8

is then opened. This causes the expansion of the gas at step

206

as the gas flows so as to equalise the pressure between the two systems. Therefore, the gas within the adsorption pump system expands into the additional volume provided by the reservoir

7

. The magnitude of the expansion depends upon the volume of the reservoir

7

and the pressure of the gas within the reservoir prior to the valve

8

being opened. This expansion causes the partial liquefaction of the coolant gas and, unlike in conventional adsorption refrigeration systems where a “1K pot” is used to provide the liquid helium-3, here the helium-3 is conveniently produced by the expansion process itself.

If it is not sufficient to use the same reservoir

7

for initial delivery of the gaseous coolant and to act as an auxiliary volume member, then two or more separate reservoirs could be used in series, to provide a higher saturation pressure for the adsorption pump.

Although the charcoal

5

is maintained at the elevated temperature (for example 100K) during this stage, the expansion of the helium-3 coolant gas causes a significant reduction in the temperature within the pot

4

(in this example by about 2.8K). This is a sufficiently low temperature for helium-3 to exist in the liquid phase. Partial liquefaction of some of the coolant gas within the system therefore occurs due to the expansion and this liquid helium-3 collects in the pot

4

.

Ideally the reduction in the pressure is as large as possible and in this case, on completion of the expansion, the gas pressure is similar to the lowest pressure attained having completed cooling the system at step

202

. As mentioned earlier, when using the reservoir

7

, this pressure would be the final equalised gas pressure for the volume and adsorption refrigerator, (generally below 1 atmosphere to minimize leaks).

Returning to

FIG. 2

, once the pressure has equalized between the adsorption system and the reservoir

7

, the valve

8

is then closed at step

207

to isolate the adsorption pump

2

, pumping line

3

and pot

4

. The adsorption pump is then cooled at step

208

.

During step

208

as the adsorption pump cools from 100K to about 20K, a second expansion effect occurs due to the pressure reduction of the gas in the vicinity of the pump. It should be remembered that the pot

4

at this stage is at the low temperature produced by the earlier expansion process. Therefore, the coolant gas in the pot

4

is caused to effectively expand by the pressure reduction and this causes further cooling within the pot

4

and further liquefaction of the helium-3.

At step

209

, the adsorption pump begins to operate in a conventional manner as it is cooled below about 20K. As the gaseous coolant is adsorbed, evaporation of the liquified helium-3 in the pot

4

occurs and this process continues such that a temperature of about 0.3K is attained for many hours.

The pre-cooling caused by the expansion of the coolant gas into the storage reservoir

7

therefore provides a lower starting temperature for the conventional operation of the adsorption system. This in turn produces a significant increase in the single-shot operational time and particularly in the case of helium-3 where the ordinary single-shot time is increased from between 5 and 6 hours to between 20 and 50 hours.

Rather than using a single auxiliary volume member, it will be appreciated that the cooling by the initial expansion effect can also be achieved by expanding the gas separately into a number of separate auxiliary volume members. This can be performed in a number of steps so as to maximize the gas expansion effect.

FIG. 3

shows a second example of apparatus for performing the method. Here, similar apparatus to that of

FIG. 1

is denoted by primed reference numerals of similar value. In this case, the storage reservoir

7

′ is only used to add the coolant under pressure to the adsorption system at step

202

. Three cooling loops

15

′ are also indicated in FIG.

3

.

A second adsorption pump

11

′ is provided to act as the volume into which the gaseous coolant

10

expands during step

205

under the control of a second valve

12

′. In this alternative system, the method of

FIG. 2

is modified slightly. Following the heating of the adsorption pump at step

204

, the valve

8

′ remains closed and the second valve

12

′ is opened which again causes a reduction of the coolant gas pressure within the adsorption system.

Unlike the main adsorption pump

2

′, during this step the adsorption pump

11

′ is cooled so as to maximise the adsorption of the gaseous coolant

10

, thereby reducing its pressure and causing a corresponding drop in temperature. Once sufficient cooling has been achieved or saturation of the adsorption pump

10

′ has been reached, the valve

12

′ is closed at step

207

, again isolating the main adsorption system and allowing it to be operated in a conventional manner at step

209

.

As a further modification to the apparatus and methods described herein, the temperature of the additional volume(s) such as the storage reservoir

7

of

FIG. 1

, can be controlled in a variable manner during the performance of the method so as to enhance the cooling function.

Some further examples of apparatus for performing the method according to the invention are now described. These each involve the use of a “heat pipe” to act as a thermal diode and increase the operational performance of the refrigeration system.

A third example of refrigeration apparatus comprises:

a sealed chamber containing coolant and having an upper region and a lower region, the upper and lower regions being arranged in communication with one another; and

an adsorption refrigeration system as described above in association with any of the earlier examples. The adsorption refrigeration system is here arranged to cool the upper region with respect to the lower region of the sealed chamber.

Under working conditions, the chamber is at least partially filled with coolant gas, so that when the upper region is cooled with respect to the lower region, the coolant gas is caused to condense into coolant liquid in the upper region, the coolant liquid then moving, under gravity, into the lower region such that the lower region is cooled.

In contrast to known methods in which target apparatus to be cooled is placed directly adjacent the lowest temperature region of the refrigeration system, we have realised that by also incorporating a sealed chamber containing a coolant, it is possible to maintain the desired low temperature in a region spatially displaced from the refrigeration system.

This is achieved by providing a fast transport of low temperature coolant liquid at the desired low temperature from the upper region to the lower region. By placing the apparatus to be cooled in contact with the lower region rather than the upper region, improved temperature stability is attained. This is because the intermediate cooling process (provided by the condensing coolant) dampens any temperature instability within the refrigeration system. The poor thermal conductivity of the vapour helps to break the link between the upper and lower regions. Known apparatus arrangements attempt to make good thermal contact with the refrigeration system and therefore are exposed to any instability within them.

Particular advantage is provided with single-shot adsorption refrigerators, in that the apparatus described effectively acts as a heat diode. However, “the heat pipe” described here could be used with other forms of refrigeration systems.

Under working conditions, when the coolant gas is condensed by the adsorption refrigeration system, this cooling is very quickly and effectively applied to the lower region by the physical passing of the liquid from the upper region to the lower region rapidly.

However, in the reverse direction, should the upper region increase in temperature with respect to the lower region, then the main mechanisms by which the heat may transfer between the two regions is via conduction within the walls of the chamber or through the vapour. This reverse heat transfer can be reduced by minimising the conductivity of the chamber walls, thereby increasing the thermal isolation of the lower region.

This is particularly advantageous with single-shot adsorption refrigeration systems as a thermal barrier is provided between the apparatus to be cooled and the refrigeration system. As the lower region accordingly rises in temperature much more slowly than the upper region, the speed at which the lower region may be returned to the ultra-low temperature desired is significantly increased.

Preferably therefore the upper and lower regions are arranged to be spatially separated by an elongate intermediate region as this increases the thermal barrier between the two regions in the reverse direction. Although either of the upper or lower regions may be of similar form and cross-section as the intermediate region, preferably at least one of the upper and lower regions is arranged as a sub-chamber, having a diameter in excess of that of the intermediate region.

The coolant used depends upon the application and in particular upon the working temperature desired to be attained and typically will be a similar coolant to that used in the adsorption refrigeration system. Typically these coolants are capable of evaporation and condensation at the approximate working pressures and temperatures involved. As mentioned earlier, suitable coolants include nitrogen, helium-4, neon, hydrogen and, for ultra-low temperatures, helium-3. Mixtures of two or more of these coolants can also be used.

The apparatus desired to be cooled preferably further comprises a cold platform arranged in thermal contact with the lower region for use in mounting further devices or further apparatus to be cooled, such as experimental devices or the still of a dilution refrigerator.

As a further advantage of the apparatus of this third example, the temperature stability and the thermal barrier effect can be used in producing a “quasi-continuous” cooling system. At least two sets of cryogenic cooling apparatus according to the third example can be provided with a common cold platform arranged in thermal contact with each lower region of the sealed chambers of the apparatus. An example of this is described below in the fifth example where each single-shot refrigeration system is arranged such that the common cold platform is continuously cooled by at least one of the sets of refrigeration systems.

Each set of the refrigeration systems may have their own individual lower regions in contact with the cold platform although preferably they share a common lower region of their chambers, such that the coolant may pass between the chambers of the sets of apparatus.

A variety of experimental apparatus may be mounted to the cold platform, although one alternative use of the system is in cooling part of a dilution refrigeration device to provide continuous dilution refrigeration as is illustrated below in the sixth example.

Typically in this case, the system further comprises a dilution refrigeration device having a still, wherein the still is in thermal contact with each lower region of the sealed chambers of the refrigeration apparatus. In this case the upper part of the still is preferably cooled such that the distillate vapour is condensed into a distillate liquid and passed to the mixing chamber of the dilution refrigerator.

This apparatus may be preferably used in cooling the upper part of a still to around 0.4 kelvin thus enabling the continuous operation of the dilution refrigerator. The coldest temperature in such a system, as in other dilution refrigerators, is attained in the mixing chamber.

Turning now to specific examples of these “heat pipe” systems,

FIG. 4

is a schematic representation of the third example of the invention. A heat pipe

101

is provided in the form of an elongate hollow cylinder fabricated from a suitable cryogenic alloy such as stainless steel. The heat pipe

101

is vertically orientated and has an upper region

102

and a lower region

103

as indicated in FIG.

4

. An intermediate region

104

separates the upper

102

and lower

103

regions. The heat pipe

101

is of circular cross-section and is sealed at the ends of the upper region

102

and lower region

103

by end plates

105

. These are formed for example from stainless steel disks having a similar cross-section to that of the cylinder. The heat pipe

101

is therefore enclosed by the end plates

105

to form a sealed chamber

106

.

Prior to sealing, an amount of coolant such as helium is added to the chamber, the coolant having a boiling point appropriate for the application under working conditions. As in general the coolant is added when the heat pipe is at temperatures above the operating temperature, coolant is normally added as a gas to the chamber

106

prior to the chamber being sealed.

The lower region

103

is placed in thermal contact with apparatus generally indicated at

108

, for example by attaching the apparatus

108

to the end plate

105

of the lower region

103

. The apparatus

108

generally represents equipment or devices desired to be cooled and examples of these include experimental apparatus and indeed certain components in other refrigeration systems, as will be described later.

The upper region

102

of the heat pipe

101

is placed in thermal contact with a refrigerated component

109

which in turn is refrigerated by an adsorption refrigeration system

110

, examples of which have been described above with reference to

FIGS. 1 and 3

.

In conventional systems, the apparatus

108

would be provided in good thermal contact with the refrigerated component

109

. However, in the present example the heat pipe

101

is placed between these components and this serves to increase the time period during which the apparatus

108

can be maintained at a desired low temperature.

Under normal operating conditions, the amount and type of coolant

107

is chosen such that an amount of coolant vapour

112

is present within the heat pipe when the refrigeration system

110

is operated to begin refrigerating the component

109

. Some of the coolant

107

may also be present in the liquid phase as coolant liquid

113

positioned in the lower region

103

at the beginning of this operation. This will depend upon the pressure and temperature within the sealed chamber

106

.

As refrigeration of the refrigerator component

109

begins, the temperature of the end plate

105

in the upper region

102

is reduced with respect to the vapour

112

inside the chamber

106

. This causes the condensation of the vapour on the surface of the upper end plate

105

within the chamber

106

. As condensation continues, a number of droplets

114

begin to form on the inner surface of the end plate

105

in the upper region. As the nucleation and growth of these droplets continues, they eventually detach from the end plate

105

and fall due to gravity into the lower region

103

.

The temperature of the droplets is substantially the same as that of the refrigerated component

109

and the gradual accumulation of the coolant

107

droplets in the lower region

103

causes that lower region

103

to attain a temperature close to that of the refrigerated component

109

, such as 0.30±02K. In turn, the apparatus

108

is cooled by a thermal contact with this region

103

through the lower end plate

105

.

The condensation of the vapour serves to reduce its pressure which encourages further evaporation from the surface of the coolant liquid

113

gathered in the lower region

103

. The rapid movement of the droplets

114

from the upper region

102

to the lower region

103

ensures that the apparatus

108

is cooled to a similar temperature as that of the refrigerator component

109

.

Single-shot adsorption refrigerator systems such as those represented by

110

are not capable of continuous operation. Therefore single-shot cooling is provided for a limited period and should this period be insufficient for the purposes of using the apparatus

108

, then this process must be restarted after a period in which the refrigeration system

110

is regenerated. Such a regeneration period is often lengthy and this causes the apparatus

108

to rise in temperature.

In the present example, the heat pipe

101

effectively operates as a heat diode in which, when the refrigerator component

109

is at a low temperature with respect to the apparatus

108

, the cooling of the apparatus

108

is rapid due to the action of the falling droplets

114

.

However, when the refrigerator component

109

is no longer cool with respect to the coolant vapour

112

, the elongate shape of the heat pipe

101

and the presence of the vapour

112

provide a thermal barrier between the upper

102

and lower

103

regions of the heat pipe

101

and therefore also between the refrigerator component

109

and the apparatus

108

.

The rise in temperature of the apparatus

108

is therefore significantly slower than that of the refrigerated component

109

and this allows experiments to continue in some cases after the refrigeration system

110

has ceased to function. In addition, this means that upon attempting to cool the apparatus

108

again at a later time following regeneration of refrigeration system

110

, the apparatus

108

is already at a temperature much closer to the desired low temperature suitable for operation of the apparatus

108

.

A fourth example is shown in FIG.

5

. Here similar components are given similar reference numerals as in FIG.

4

.

The upper region

102

of the heat pipe

101

is provided as two connected sub-chambers

116

,

117

, the upper sub-chamber

116

being in direct thermal contact with the evaporation chamber

109

and is of similar diameter, whereas the second sub-chamber

117

below the upper

116

is of a reduced diameter. Beneath this, the intermediate region

104

of the heat pipe

101

is again of narrower diameter than that of the lower sub-chamber

117

and this in turn is connected to a sub-chamber forming the lower region

103

having again a larger diameter.

The intermediate region

104

of the heat pipe

101

is, for example, 3 mm in diameter with a length of 100 mm. Again the heat pipe

101

is formed from a suitable stainless steel, the top volume of which in the combined sub-chambers

116

and

117

is about 0.5 cm

3

with a similar volume forming the chamber of the lower region

103

.

The coolant

107

in this example is helium-3 and a typical volume of helium-3 used in this case is 200 cm

3

at standard temperature and pressure. This equates to approximately 0.3 cm

3

of liquid under working conditions.

In this fourth example, the heat pipe

101

operates in a similar manner as that described in the third example using the helium-3 to cool the apparatus

108

(not shown in FIG.

5

).

Using the apparatus schematically shown in

FIG. 5

, following an initial cooling stage taking approximately 20 minutes, a temperature of approximately 0.36 kelvin has been maintained for up to 10 hours with a temperature stability of around 1 millikelvin. This indicates that the heat pipe

101

can effectively cool the apparatus

108

to the low temperature desired and maintain this with a great degree of temperature stability for a number of hours. During a desorption procedure which takes about 20 minutes, the thermal barrier effect of the heat pipe ensures that the temperature of the lower region only increases by about 0.5 Kelvin.

Some experimental data relating to the heat pipe of this fourth example are presented in

FIG. 6

in the form of a graph of temperature (in Kelvin) against time (seconds×1000). Temperature data are plotted in

FIG. 6

as two traces A, B. The trace A represents the temperature measured at the evaporation chamber

109

within

FIG. 5

, whereas the trace B represents the temperature measured within the apparatus

108

in thermal contact with the lower region

103

.

It should be noted that the traces A and B have been shifted in temperature (ordinate) with respect to one another and therefore these temperatures do not represent absolute values with respect to one another or with respect to the temperature scale. However, each trace A,B represents relative changes in measured temperature. The region of interest regarding the temperature response of the system is broadly indicated at

118

. The parts of the traces A,B outside this region merely show various test responses.

Within the region

118

of

FIG. 6

, a rise in temperature of approximately 60 millikelvin is deliberately caused in the region of the evaporation chamber

109

. In trace A, the measured increase in temperature can be seen to be very rapid and take a matter of only a few seconds. However, a response to the temperature rise in trace B is shown to be much slower, representing the thermal barrier that the heat pipe

101

provides upon the heating of the evaporation chamber

109

. The initial rapid rise in trace B is caused by dew evaporation.

It can also be seen that the lower region of the heat pipe only attains this approximate 60 millikelvin increase in temperature over a period of about 10 minutes.

A fifth example of the invention is shown in

FIG. 7

in which two identical sets of refrigeration apparatus (as described in connection with

FIG. 5

) are used in parallel to cool a common set of apparatus

108

. Each of these only differs from the system described in

FIG. 5

by a lateral displacement of the lower region

103

of the heat pipe

101

. The corresponding reference numerals of the second set of refrigeration apparatus are denoted with similar numerals with the addition of a “′”.

By using two single-shot adsorption refrigeration systems

110

,

110

′, and two corresponding heat pipes

101

,

101

′ a quasi-continuous cooling offset effect can be achieved in the apparatus

108

. As the time required for desorbing either of the adsorption refrigeration systems

110

,

110

′ is significantly less than a single-shot operation period, whilst one system is cooling the apparatus

108

, the other may be desorbed. The corresponding heat pipe attached to the adsorption refrigeration system

110

,

110

′ during desorbing prevents this from heating the apparatus

108

significantly.

A slight reduction in the single-shot operation time may result, although the desired low temperature within the apparatus

108

is maintained permanently by the repeated switching of each of the refrigeration devices

110

,

110

′ into adsorption and desorption modes.

In

FIG. 7

, the heat pipes

101

,

101

′ each have separate lower regions

103

,

103

′ although these can be joined to form a common lower region and therefore a single sealed chamber. This may provide greater extensive cooling over a larger surface area.

FIG. 8

shows an example of this in which the lower region

103

of the two heat pipes

101

,

101

′ is connected. Again, as in

FIG. 7

two single-shot adsorption refrigeration systems

110

,

110

′ are used along with corresponding evaporation chambers

109

,

109

′ and heat pipes

101

,

101

′. However, rather than using the lower region

103

to directly cool experimental apparatus, in this case the combined lower region

103

is placed in thermal contact with an upper surface of a still chamber of a dilution refrigerator.

The still

119

and a mixing chamber

120

of the dilution refrigerator are indicated in FIG.

8

. With this system, a cold platform is maintained at about 0.4 kelvin which is sufficient to cause condensation on an upper surface

121

of the still

119

such that the mixture

122

of helium-3 and helium-4 distill and present as the liquid within the still, can be distilled into helium-3. The helium-3 distillate is reintroduced into the mixing chamber via a conduit

123

. A simple funnel device

124

is provided to collect the dripping helium-3 from the upper surface

121

of the still. The funnel

124

is connected to an upper end of the conduit

123

and the liquid helium-3 distillate is returned to the mixing chamber

120

.

FIG. 9

shows a prototype apparatus according to this example. An adsorption pump is indicated at

130

, at one end of the apparatus. A 4K heat exchanger

131

and an IVC flange

132

are also shown. Further along the apparatus a pumping line

133

and helium-3 pot

134

are provided, with the heat pipe being positioned at the opposite end of the apparatus with respect to the sorb

130

. The upper

135

and lower

136

regions of the heat pipe are indicated in FIG.

6

.

This apparatus is capable of attaining a temperature of 0.35 kelvin and has a cooling power of about 100 microwatts at 0.4 kelvin, that is a suitable temperature for providing helium-3 distillation in the still.

Number	Name	Date	Kind
4993239	Steidl et al.	Feb 1991	A
6502419	Pfister et al.	Jan 2003	B2

Refrigeration method and system

Information

Patent Number

Date Filed

Date Issued

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Original Assignees

Examiners

CPC

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US

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Abstract

Description

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Priority Claims (1)

US Referenced Citations (2)