Apparatus and method for controlling a cryogenic cooling system

Abstract

Apparatus for controlling a cryogenic cooling system is described. A supply gas line (3A) and a return gas line (3B) are provided which are coupled to a compressor (1) and to a mechanical refrigerator (2) via a coupling element (4). The coupling element is in gaseous communication with the supply (2A) and return gas lines and supplies gas to the mechanical refrigerator (2). The pressure of the supplied gas is modulated by the coupling element in a cyclical manner. A pressure sensing apparatus (6) monitors the pressure in at least one of the supply and return gas lines. A control system (5) is used to modulate the frequency of the cyclical gas pressure supplied by the coupling element in accordance with the pressure monitored by the pressure sensing apparatus. An associated method of controlling such a system is also described.

Claims

1. A method of controlling a cool-down process of a cryogenic cooling system, the cryogenic cooling system comprising a supply gas line and a return gas line for coupling with a compressor, a rotary valve in gaseous communication with the supply and return gas lines that supplies gas to a pulse tube refrigerator and cyclically modulates the pressure of the supplied gas so that the pressure varies at a given frequency, and a motor that drives the rotary valve, the method comprising: storing predetermined relationships between each of a plurality of pulse tube refrigerator temperatures and an optimum frequency for maximizing the cooling power of the pulse tube refrigerator, obtaining feedback indicative of the temperature of the pulse tube refrigerator by monitoring the pressure in at least one of the supply and return gas lines; identifying the optimum frequency for maximizing the cooling power of the pulse tube refrigerator based on the feedback indicative of the temperature of the pulse tube refrigerator and the predetermined relationship; and controlling a speed of the motor, while reducing the temperature of the pulse tube refrigerator towards an operational base temperature, to modulate the frequency of the cyclical gas pressure supplied by the rotary valve to approach or obtain the identified optimum frequency.

2. The method according to claim 1, wherein the rotary valve is moveable in a rotational manner and wherein the frequency of the cyclical gas pressure supplied by the rotary valve is effected by moving the rotary valve at a corresponding rotational speed.

3. The method according to claim 1, wherein the optimum frequencies identified by the predetermined relationships reduce vibrations of the cryogenic cooling system while maximizing the cooling power of the pulse tube refrigerator.

4. The method according to claim 3, wherein if, in accordance with the predetermined relationship, the frequency of the cyclical gas pressure supplied by the rotary valve would be below a minimum threshold frequency then the frequency of the cyclical gas pressure supplied by the rotary valve is set to the minimum threshold frequency.

5. The method according to claim 3, wherein if, in accordance with the predetermined relationship, the frequency of the cyclical gas pressure supplied by the rotary valve would be above a maximum threshold frequency then the frequency of the cyclical gas pressure supplied by the rotary valve is set to the maximum threshold frequency.

6. The method according to claim 1, wherein the frequency of the cyclical gas pressure supplied by the rotary valve is modulated to maintain the monitored pressure within a predetermined pressure range.

7. The method according to claim 6, wherein the predetermined pressure range is set in accordance with a maximum operational pressure of the cryogenic cooling system.

8. The method according to claim 1, wherein the frequency of the cyclical gas pressure supplied by the rotary valve is in the range of 1 to 5Hz.

9. The method according to claim 1, wherein the monitored pressure is in the range of 1 to 40 MPa.

10. The method according to claim 1, wherein the gas is helium.

11. The method according to claim 1, wherein the predetermined relationships between each of the plurality of pulse tube refrigerator temperatures and an optimum frequency for maximizing the cooling power of the pulse tube refrigerator is a mathematical relationship.

12. The method according to claim 1, wherein the predetermined relationships between each of the plurality of pulse tube refrigerator temperatures and an optimum frequency for maximizing the cooling power of the pulse tube refrigerator is stored in a look-up table.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An example of a control system and method according to the present invention is now described with reference to the accompanying drawings in which:

(2) FIG. 1 shows a conventional cryogenic cooling system;

(3) FIG. 2 shows an example cryogenic cooling system according to the invention; and,

(4) FIG. 3 shows a flow diagram in accordance with the example of the invention.

DESCRIPTION OF PREFERRED EXAMPLE

(5) In order to provide a full understanding of the invention, we firstly describe a known closed cycle refrigerator (CCR) system in accordance with FIG. 1.

(6) The system 100 comprises a scroll compressor 1 and a pulse tube refrigerator (PTR) 2. Two gas lines 3A and 3B connect the scroll compressor 1 to the pulse tube refrigerator 2. The gas lines 3A and 3B are essentially gas pipes which are capable of withstanding a high pressure. The gas line 3A is a supply line which contains a coolant gas at a high pressure when in use. The line 3B is a return line in the form of a low pressure line. A coupling element in the form of a rotary valve 4 is illustrated as an integral part of the PTR 2. The rotary valve 4 is driven by a motor controller 5 and the operational speed of the motor is fixed to ensure a constant rotational frequency of the rotary valve given by a F.sub.optimum. This frequency is designed to be the optimum frequency for use of the PTR once at its cold or steady-state operational temperature.

(7) Optionally, a pressure sensor 6 may be present within the compressor so as to detect an abnormal pressure within the high pressure line 3A. The scroll compressor 1 is also provided with a bypass system 7 which is caused to operate when a critical value of pressure within the high pressure line is detected. In known systems, the critical pressure within the high pressure line 3A is always reached at the beginning of a cool-down process and remains for a relatively long period of the cool-down process. Depending on the type of mechanical refrigerator, such period can be at least one third and up to one half of the full cooling time required to reach the low temperature regime.

(8) Whilst a critical value of the pressure exists, the bypass 7 remains open and allows coolant gas to pass between the high pressure supply line and the lower pressure return line. In this case the coolant gas is helium and the operation of the bypass 7 ensures that no helium is lost to the external atmosphere. This is important since helium is an expensive gas.

(9) The above described example represents a standard prior art CCR system in which a mechanical refrigerator (cryocooler) is driven by a compressor. The mechanical refrigerator may take various forms including GM coolers, Stirling coolers, pulse tube refrigerators, cold heads and cryopumps. In each of these types of CCR a rotary valve or other coupling element regulates the mass flow of the coolant gas transferred between the compressor and the mechanical refrigerator. In order to maximise the cooling power available at low temperatures, the mechanical refrigerator is designed such that, when in the steady-state or cold condition the PTR (or equivalent) helium mass flow matches the compressor's optimum working point. Therefore in each mechanical refrigerator an optimum frequency value F.sub.optimum for the rotary valve or other type of coupling element exists in order to maximum the cooling power.

(10) It is notable however that an important physical property of helium, and indeed of other gases, is that the density of the gas increases as the temperature decreases. In cryogenic systems with mechanical refrigerators, the temperature difference between room temperature and the operational temperature is approximately 290 Kelvin which is a very significant temperature difference. At an operational temperature of around 2 to 4 Kelvin, the density of the helium gas coolant is significantly higher than that at room temperature. With an operational pressure of some bars, the density value of the helium at 4K is more than 100 times higher than its equivalent density at room temperature (300K).

(11) In the conventional CCR system described above, at the beginning of the cool down process, the mass flow of coolant gas delivered by the compressor cannot be fully transferred via the rotary valve to the PTR. This is because the operational frequency of the compressor is too low (a few Hertz). As a result, pressure may build upon the high pressure side of the compressor. Depending upon the initial filling pressure value of the system, a critical limit value may be exceeded. Typically a safety valve is set to operate below a critical value for this pressure and such a safety valve is positioned within the high pressure line. It is known to either vent the excess pressure to the external atmosphere or, as is shown in FIG. 1, to provide the safety valve in the form of a bypass which effectively vents the helium to the low pressure side of the compressor.

(12) The coolant gas pressures in each of the high pressure supply line 3A and low pressure return line 3B are provided by power from a compressor motor 8. The bypass may therefore take the form of an over pressure valve and this is desirable in comparison with a valve which vents the helium to atmosphere since the helium is not lost from the system if a critical value of the pressure is reached. Nevertheless, during the initial cool down, the critical value is always reached at the beginning of the cool down procedure.

(13) Later, as the low temperature steady-state regime is approached, the pressure reduces and the bypass closes. Once the low pressure has reduced to the operational pressure in the steady-state, the frequency of the rotary valve and the pressure which it controls (having a frequency of F.sub.optimum) attain the optimum for the operational temperature.

(14) An example of a CCR system according to the invention is now described with reference to FIG. 2. In FIG. 2, apparatus having analogous features with that in FIG. 1 is provided with similar primed reference numerals.

(15) In FIG. 2, the CCR system according to the invention is illustrated at 200. A scroll compressor 1 is connected via high (3A) and low (3B) lines to a PTR 2. A coupling element in the form of a rotary valve 4 again controls the PTR 2. In this example the rotary valve 4 is operable at a variable frequency F. In this case, the modified motor controller 5 receives a signal from the pressure transducer 6. This transducer is a pressure sensor which provides a monitoring signal which can be related to the pressure magnitude sensed by the transducer. The signal is provided to the motor controller 5. The motor controller 5 contains a processor and associated programmable memory. The processor samples the signals from the pressure transducer 6 and, using an appropriate algorithm or look-up table, converts these to a suitable control signal which is outputted to the rotary valve 4. This is illustrated in FIG. 2 by the lines linking the pressure transducer 6 to the motor controller 5, and the motor controller 5 to the rotary valve 4. The motor controller 5 therefore provides a control mechanism for operating the CCR 200. It will be appreciated that the components shown in FIG. 2 are illustrated schematically and therefore other ordinary equipment which is not specifically shown such as safety valves, oil separates, filters, heat exchangers, sensors and so on, is nevertheless present.

(16) The example apparatus as shown in FIG. 2 therefore has the same benefits as the apparatus in FIG. 1 during steady-state low temperature operation of the mechanical refrigerator in the form of the PTR 2. However, it also allows improved efficiency to be achieved during the cool down procedure. This is achieved by varying the rotary valve mechanism frequency so as to dynamically accommodate the helium mass flow exchange between the PTR 2 and the compressor 1. At high temperatures, such as those close to room temperature, the rotary valve 4 is operated with a corresponding frequency regime F that is significantly higher than the optimum design frequency F.sub.optimum which is associated with the PTR 2 at its steady-state low temperature. Due to the high frequency regime F, the pressure within the high pressure side of the compressor is reduced in comparison with prior art systems and therefore the mechanical refrigerator is able to operate without losing efficiency at the initial high temperature. Later, when the PTR cools, the frequency regime can be reduced in order to approach and then obtain F.sub.optimum as the steady-state temperature is reached.

(17) The overall efficiency of the CCR 200 is therefore considerably improved in comparison with that of known systems such as 100 in FIG. 1. In this particular example, the frequency F is electronically controlled in accordance with a signal from the pressure transducer in accordance with an automatic feedback mechanism which is regulated by the motor controller 5. It is notable that no temperature sensors or, in this particular example, that no more than one pressure transducer 6 is used. The key parameter is the maximum pressure allowed in the system, because this is typically the design limitation of the compressor and this governs the possible cooling efficiency of the mechanical refrigerator.

(18) Thus the efficiency of the PTR 2 is maximised. It will be appreciated that an algorithm to optimise the frequency F plus the function of the pressure experienced may be derived by calculation or by experimental measurements. A further variable for consideration in deriving for such an algorithm (or equivalent) is a consideration to ensure that overall vibrations are reduced.

(19) The practical benefit of the example apparatus is that the CCR system 200 reaches the low temperature regime more quickly than the equivalent CCR 100 shown in FIG. 1. The available cooling power at high temperatures is also considerably enhanced such that an overall improvement of the key parameters of the system by at least 35% is observed.

(20) Referring now to FIG. 3, the operation of the system as shown in FIG. 2 is described in more detail. At step 300 the compressor 1 is started and the compressor motor 8 is initiated. At step 301 the motor controller 5 rotates the rotary valve 4 at a speed (SL) which is a maximum for the PTR 2 in question. This value is denoted Qmax in FIG. 3. At step 302 the signal from the pressure transducer 6 is sampled and averaged by an algorithm denoted Routine1, the sampling being at a rate of a few milliseconds. At step 303 a first pressure reading is evaluated by converting an averaged pressure signal over a number of counts into a pressure reading, denoted Pactual. At step 304 Pactual is compared with a predetermined set point value (denoted SPMax). If the pressure Pactual is greater than SPmax (which might be typically 410 psi or 2.83 MPa) then the compressor is automatically stopped at step 305 and a fault code is displayed. Such failure typically occurs when the high pressure line is not connected to the rotary valve 4 or is blocked.

(21) If however the pressure is lower than the set point pressure of 410 psi (2.83 MPa) then at step 306 a second algorithm (Routine2) is used in which the motor controller 5 begins taking monitored pressure readings at a predetermined sampling rate. Routine2 converts a rolling average of pressure values from the pressure transducer 6 and assigns the evaluated value to Pactual.

(22) At step 307 Pactual is compared with a set point pressure SP1. SP1 is a pressure value slightly less than the maximum pressure (SPmax) allowed by the compressor design (SP1 is for example 400 psi, 2.76 MPa). It is desirable to operate the PTR, when possible, at the highest safe pressure which can be thought of as SP1, this allowing the maximum cooling power of the PTR 2. As the PTR 2 cools the speed of the rotary valve 4 required to maintain the high pressure close to SP1 gradually decreases. For this reason a gradual slowing of the rotary valve 4 is desired. This is achieved by monitoring the pressure Pactual.

(23) At step 308, which occurs if the average pressure Pactual is less than the set point pressure (SP1), then a reduction in speed of the rotary valve 4 is desirable. At step 308 an evaluated speed Ev is calculated. This is calculated as the current speed (SL) modified by an amount f representing a decremental change in the speed. This evaluated speed is compared with a speed Qmin at step 309. Qmin is the optimal speed in the cold condition for the PTR 2 (that is the speed used at the base temperature). If the evaluated speed Ev is not less than Qmin then the reduction in speed is assigned as the new speed SL at step 310. Having reduced the speed the algorithm returns to step 303 and repeats.

(24) If the evaluated speed Ev at step 308 is less then Qmin, then at step 311, the speed SL is set to Qmin and the algorithm loops back to step 303.

(25) The other alternative at step 307 is that the pressure Pactual is not less than SP1. In this case it is desirable to increase the speed of the rotary valve 4. A similar calculation is then performed at step 312 to that performed at step 308, namely, calculating the evaluated speed, Ev. Here the evaluated speed is then compared with a speed Qmax at step 313. Qmax is the maximum speed of operation of the rotary valve 4 which in turn is set by the maximum operational speed of the PTR 2.

(26) At step 314, if the evaluated speed Ev is not greater than Qmax then an increase of the speed (SL) to Ev is effected. The algorithm then loops back to step 303.

(27) If the evaluated speed Ev is greater than Qmax, than a step 315, the speed SL is set at Qmax and the algorithm again loops back to step 303.

(28) This process is repeated throughout the operation of the PTR 2 and in particular during the cooling cycle.

(29) The global effect of this is that the actual pressure Pactual is maintained closer to SP1 by reducing the speed until Qmin is reached. It is the nature of the operation of the system the Qmin is reached before the PTR 2 reaches the base temperature. Once Qmin is actually reached then in practice Pactual reduces due to the further cooling but the speed SL remains unchanged at the Qmin value.

(30) Whilst the focus of the present example is in the cooling cycle of a closed cycle refrigerator such as the PTR 2, it is also notable that such a process as described above also works during a warming procedure from the base temperature.

(31) There are a number of different practical means by which the algorithm which governs the process of FIG. 3 may be implemented. In FIG. 3, values for f may be calculated by the equation: f=c (PactualSP1) where c is a constant. This ensures that the magnitude of change in the speed which may be effected during each process loop is proportional to the difference between the actual pressuce (Pactual) and the desired pressure (SP1).

(32) It will be appreciated that the illustrative example of FIG. 3 can be easily effected via look-up tables. A more advanced system having effectively a continuum of temperature-pressure regimes can of course be contemplated and effected either via a corresponding number of table entries in a look-up table or via a calculation according to a linear or polynomial approximation for example. This may include the use of additional considerations for optimising the performance of the system, such as in reducing vibrations.