FI2225501T4 - Cryogenic refrigeration method and device - Google Patents

Claims (7)

1. CRYOGENIC REFRIGERATION METHOD AND DEVICE

   [0001] The present invention relates to a cryogenic refrigeration device and method.

   [0002] The invention more particularly relates to a cryogenic refrigeration device for transferring heat from a cold source to a hot source via a working fluid circu- lating in a closed working circuit, the working circuit comprising in series: a com- pression portion, a cooling portion, an expansion portion and a heating portion.

   [0003] The cold source can be for example liquid nitrogen to be cooled and the hot source can be water or air.

   [0004] Refrigerators known to cool superconducting elements generally use a re- verse Brayton cycle. These known refrigerators use an oil-lubricated screw com- pressor, a counter-flow plate exchanger and an expansion turbine.

   [0005] These known refrigerators have many drawbacks including: e low energy efficiency of the cycle and therefore of the refrigerator, e use of oil to cool and lubricate the compressor, this requires an operation of deoiling the working gas after compression, e use of rotating joints between the electric motor and the compressor, e low isothermal compression efficiency of the compressor, e frequency of maintenance operations.

   [0006] Document US-3,494,145 describes a refrigeration system using couplings via gears requiring oil bearings. This type of device uses rotating joints such as mechanical seals between the working gas and the casing of the gears and oil bearings. This architecture increases the risks of working gas leakage and possi- — ble pollution of the working gas with oil. This system additionally relates to a low speed type motor.

   [0007] Document US-4,984,432 describes a refrigeration system using liquid ring type compressors or turbines operating with a low speed motor using conventional bearings such as ball bearings. This technology concerns positive displacement compressors and turbines.

   [0008] Document Saji, N. et al. “Design of oil-free simple turbo type 65K/65KW helium and neon mixture gas cooler for high temperature superconducting power cable cooling” (Advances in Cryogenic Engineering, vol. CP613, 2002) describes a device in accordance with the preamble of claim 1. One purpose of the present invention is to overcome all or some of the drawbacks of prior art set out here- inabove.

   [0009] To this end, the invention is defined by claim 1.

   [0010] The embodiments make it possible to obtain an oil pollution-free and con- tact-free system. Indeed, the combination of centrifugal compressors, centripetal turbines and bearings according to the invention reduces or eliminates any contact with the fixed parts and the rotating parts. This prevents any risk of leakage. In- deed, the entire system is hermetically sealed and includes no rotating joints to the atmosphere (such as mechanical seals or “dry face seals”).

   [0011] Thus, the invention includes the following characteristics: the compressors are of the centrifugal compression type, the expansion turbine(s) is/are of the centripetal expansion type, the output shafts of the motors are mounted to bearings of the magnetic or gas dynamic type, said bearings being used to provide support for the compressors and turbines, the cooling portion and the heating portion comprise a common heat exchanger in which the working fluid passes in a counter-flow depending on whether it is cooled or heated, furthermore, particular embodiments may also include the fol- lowing characteristics: — the working circuit comprises a volume forming a working fluid storage buffer ca- pacity, the working fluid is in the gas phase and consists of a pure gas or a mixture of pure gases from: helium, neon, nitrogen, oxygen, argon, carbon monoxide, me- thane, or any other fluid having a gas phase at the temperature of the cold source.

   [0012] The invention further provides a cryogenic refrigeration method as defined in claim 4. Moreover, some embodiments of the invention may include one or more of the following characteristics: at the end of the second cooling step, the working fluid is brought to a low tem- perature of about 60 K and in that the working circuit comprises a number of com- pressors about three times greater than the number of expansion turbines, the working fluid is used to cool or keep cold superconducting elements at a tem- perature in the order of 65 K, the temperature drop of the fluid making up the cold source is substantially iden- tical to the increase in gas temperature in the exchangers.

   [0013] The invention may have one or more of the following advantages: e the working fluid cycle (reverse Ericsson type) allows a higher efficiency to be achieved than known systems without creating or increasing other drawbacks, e the expansion work in the turbines can be advantageously utilised, e it is possible to dispense with the use of oil for lubrication or cooling, thereby eliminating the need for deoiling downstream of the compressor, as well as waste oil treatment and recycling operations, e the system only reguires a small number of movable pieces, which in- creases simplicity and reliability. Thanks to the invention, it is possible for the compressor to dispense with a mechanical power transmission of the speed multiplier, universal joints type, etc. e Maintenance operations are reduced or virtually non-existent, e the system allows rotating joints to be avoided and a completely hermeti- cally sealed system to the outside to be used. This prevents any loss or pollution of the working cycle gas, e the refrigerator overall size can be reduced compared to known systems.

   [0014] Further features and advantages will become apparent upon reading the description hereinafter, made with reference to the figures, wherein: e Fig. 1represents a schematic view illustrating the structure and operation of a first exemplary embodiment of a refrigeration device according to the invention, e Fig. 2 schematically represents a detail of Fig. 1 illustrating an arrange- ment of a drive motor for a compressor-compressor or compressor-turbine assembly, e Fig. 3 schematically represents an exemplary working cycle of the working fluid of the refrigerator of Fig. 1, e Fig. 4 represents a schematic view illustrating the structure and operation of a second exemplary embodiment of a refrigerator according to the in- vention, e Fig. 5 schematically represents a second example of a working cycle of the working fluid of the refrigerator according to Fig. 3.

   [0015] Referring to the exemplary embodiment of Fig. 1, the refrigerator according to the invention is provided for transferring heat from a cold source 15 at a cryo- genic temperature to a hot source at room temperature 1, for example.

   [0016] The cold source 15 can be, for example, liquid nitrogen to be cooled and the hot source 1 can be water or air. To perform this heat transfer, the refrigerator illustrated in Fig. 1 uses a working circuit 200 of a working gas comprising the components listed below.

   [0017] The circuit 200 comprises several centrifugal compressors 3, 5, 7 arranged in series and operating at room temperature.

   [0018] The circuit 200 comprises a plurality of heat exchangers 2, 4, 6 operating at room temperature arranged at the outlet of the compressors 3, 5, 7 respectively. The temperatures of the working gas at the inlet and outlet of each compression stage (i.e. at the inlet and outlet of each compressor 3, 5, 7), are kept by heat exchanges at a substantially identical level (see zone A in Fig. 3 which represents a gas working cycle: temperature in K as a function of entropy S in J/kg). In Fig. 3, the rising parts of the saw-toothed zone A each correspond to a compression stage while the falling parts of this zone A each correspond to exchanger cooling.

   [0019] This arrangement makes it possible to approach isothermal compression. The inlet and outlet temperatures of each compression stage are substantially the same.

   [0020] The exchangers 2, 4, 6 may be separate or consist of separate portions of a same exchanger in heat exchange with the hot source 1.

   [0021] The refrigerator comprises several so-called high-speed motors (70, see

   Fig. 2). High-speed motors usually refer to motors whose speed of rotation allows direct coupling with a centrifugal compression stage or a centripetal expansion stage. The high-speed motors 70 preferably use magnetic or dynamic gas bear- ings 171 (Fig. 2). A high-speed motor typically rotates at a speed of rotation of 10,000 revolutions per minute or several tens of thousands of revolutions per mi- nute. Instead, a low-speed motor rotates at a speed of a few thousand revolutions per minute.

   [0022] Downstream of the compression portion comprising the compressors in series, the refrigerator comprises a heat exchanger 8 preferably of the counter- flow plate type separating the elements at room temperature (in the upper part of the circuit 200 represented in Fig. 1) from the cryogenic temperature elements (in the lower part of the circuit 200). The fluid is cooled (corresponding to zone D of

   Fig.3). Cooling the gas from room temperature to the cryogenic temperature is performed by counter-flow exchange with the same cryogenic temperature work- ing gas returning from the expansion portion after heat exchange with the cold source 15.

   [0023] Downstream of this cooling portion formed by the plate exchanger 8, the 5 circuit includes one or more expansion turbines 9, 11, 13, preferably of the cen- tripetal type, arranged in series. The turbines 9, 11, 13 operate at cryogenic tem- peratures, the inlet and outlet temperatures of each expansion stage (turbine inlet and outlet) are kept substantially identical by one or more cryogenic heat exchang- ers 10, 12, 14 arranged at the outlet of the turbine(s). This corresponds to the zone C of Fig. 3, the falling portions of the zone C each corresponding to an ex- pansion stage while the rising portions of this zone correspond to heating in the exchangers 10, 12, 14. This arrangement makes it possible to approach isother- mal expansion. The inlet and outlet temperatures of each expansion stage are substantially the same. In addition, and in order to increase efficiency of the re- — frigerator, the increase in temperature of the working gas in the exchanger(s) (10, 12, 14) may be substantially identical (in absolute value) to the decrease in tem- perature of the fluid to be cooled (15) (cold source).

   [0024] These heating exchangers 10, 12, 14 may be separate or consist of sepa- rate portions of a same exchanger in heat exchange with the cold source 15.

   [0025] Downstream of the expansion portion and the heat exchange with the cold source 15, the working fluid exchanges heat again with the plate heat exchanger 8 (zone B of Fig. 3). The fluid exchanges heat in the exchanger 8 in counter-flow with respect to its passage after the compression portion. After heating, the fluid returns to the compression portion and can start a new cycle.

   [0026] The circuit may further comprise a working gas capacity at room tempera- ture (not represented for the sake of simplification) to limit pressure in the circuits, upon stopping the refrigerator for example.

   [0027] The refrigerator preferably uses a gas phase fluid circulating in a closed circuit as the working fluid. This consists, for example, of a pure gas or a pure gas mixture. The most suitable gases for this technology especially are helium, neon, nitrogen, oxygen and argon. Carbon monoxide and methane can also be used.

   [0028] The refrigerator is designed and controlled in such a way as to obtain a fluid working cycle close to the reverse Ericsson cycle. This means: isothermal compression, isobaric cooling, isothermal expansion and isobaric heating.

   [0029] According to one advantageous feature, the refrigerator uses several so- called high-speed motors 70 to drive at least the compressors 3, 5, 7 (i.e. to drive the compressor impellers).

   [0030] As depicted in Fig. 2, each high-speed motor 70 receives, on one end of its output shaft, a compressor impeller 31 and, on the other end of its shaft, an- other compressor impeller or a turbine impeller 9. This arrangement provides many advantages. This configuration allows in the refrigerator direct coupling be- tween the motor 70 and the compressor impellers 3, 5, 7 or between the motor 70 and the turbine impellers 9, 11, 13. This eliminates the need for a speed increaser or reducer (thereby limiting the number of movable pieces required). This config- uration also makes it possible to utilise the mechanical work of the turbine(s) 9, 11, 13 and consequently increase the overall energy efficiency of the refrigerator. According to this configuration, the refrigerator exhibits an oil-free operation, which ensures purity of the working gas and eliminates the need for a deoiling operation.

   [0031] The number of high-speed motors mainly depends on the desired energy efficiency for the refrigerator. The higher this efficiency, the higher the number of high-speed motors should be.

   [0032] The ratio of the number of compression stages (compressors) to the num- ber of expansion stages (turbines) is a function on the target cold temperature. For example, for an unclaimed refrigerator, the cold source of which is 273 K, the number of compression stages will be substantially egual to the number of expan- sion stages. For a refrigerator with a cold source of 65 K, the number of compres- sion stages is about 3 times greater than the number of expansion stages.

   [0033] Fig. 4 illustrates another embodiment that can for example be used to cool or keep superconducting cables at a cryogenic temperature of about 65 K. For this temperature level, the number of compression stages (compressors) should be about three times greater than the number of expansion stages (turbines). This can be made in several possible configurations. For example, three compressors and one turbine or six compressors and two turbines, etc.

   [0034] The choice of the number of members will depend on the desired energy efficiency. Thus, a solution using three compressors and one turbine will have less energy efficiency than a solution using six compressors and two turbines.

   [0035] In the example of Fig. 4, the refrigerator comprises six compressors 101, 102, 103, 104, 105, 106 and two turbines 116, 111 and four high-speed motors

   107, 112, 114, 109. The first two compressors 101, 102 (i.e. the compressor im- pellers) are mounted to both ends of a first high-speed motor 107 respectively. The following two compressors 103, 104 are mounted to both ends of a second high-speed motor 112 respectively. The following compressor 105 and the turbine 116 (i.e. the turbine impeller) are mounted to both ends of a third high-speed motor 114 respectively. Finally, the last turbine 111 and the sixth compressor 106 are mounted to both ends of a fourth motor 109 respectively.

   [0036] The path of the working gas during a cycle in the closed loop circuit can be described as follows.

   [0037] In a first step, the gas is gradually compressed by successively passing through the four series compressors 101, 102, 103, 104, 105, 106.

   [0038] At the end of each compression stage (at the outlet of each compressor), the working gas is cooled in a respective heat exchanger 108 (by heat exchange with air or water for example) to approach isothermal compression. After this com- pression portion, the gas is isobarically cooled through a counter-flow plate ex- changer 103. After this cooling portion, the cooling gas is gradually expanded in the two series centripetal turbines 116, 111. After each expansion stage, the work- ing gas is heated by heat exchange in an exchanger 110 (for example by heat exchange with the cold source), so as to achieve substantially isothermal expan- — sion. At the end of this isothermal expansion, the working gas is heated in the exchanger 113 and can then start a new cycle with compression.

   [0039] Fig. 5 shows the cycle (temperature T and entropy S) of the working fluid of the refrigerator of Fig. 5. As previously for Fig. 3, six saw teeth corresponding to the six successive compression and cooling operations are distinguished in the compression zone A. In zone C, the expansion zone, two saw teeth can be iden- tified, corresponding to the two successive expansions and heating.

   [0040] The invention improves cryogenic refrigerators in terms of energy effi- ciency, reliability and overall size. The invention makes it possible to reduce maintenance operations and eliminate the use of oils.

   [0041] Of course, one or both ends of the output shafts of the motor(s) can directly drive more than one route (i.e. several compressors or several turbines).