DETECTING CONTAMINATION OF A CRYOGENIC REFRIGERANT IN A CRYOGENIC REFRIGERATION SYSTEM

Abstract

A sensor for detecting contamination of a cryogenic refrigerant in a cryogenic refrigeration system, a method and a refrigeration system are disclosed. The sensor comprises: an inlet for coupling to a cryogenic refrigerant flow path in the cryogenic refrigeration system and a thermal conductivity detector in fluid communication with the inlet. The thermal conductivity detector is configured to generate a signal indicative of a detected thermal conductivity of the cryogenic refrigerant received from the cryogenic refrigeration system when the sensor is coupled thereto. The sensor also comprises circuitry configured to convert the thermal conductivity signal to an indication of contamination of the cryogenic refrigerant; and an output configured to output the indication of contamination of the cryogenic refrigerant.

Claims

1. A sensor for detecting contamination of a cryogenic refrigerant in a cryogenic refrigeration system, said sensor comprising: an inlet for coupling to a cryogenic refrigerant flow path in said cryogenic refrigeration system; a thermal conductivity detector in fluid communication with said inlet, said thermal conductivity detector being configured to generate a signal indicative of a detected thermal conductivity of said cryogenic refrigerant received from said cryogenic refrigeration system when said sensor is coupled thereto; circuitry configured to convert said thermal conductivity signal to an indication of contamination of said cryogenic refrigerant; and an output configured to output said indication of contamination of said cryogenic refrigerant.

2. The sensor according to claim 1, said sensor further comprising control circuitry, said control circuitry comprising an input for receiving at least one signal indicative of a current state of said refrigeration system, said control circuitry being configured to control operation of said thermal conductivity detector in dependence upon said at least one received signal.

3. The sensor according to claim 1, said sensor further comprising at least one valve arranged to control flow of said cryogenic refrigerant to and from said sensor.

4. The sensor according to claim 2, said control circuitry being configured to control operation of said at least one valve.

5. The sensor according to claim 2, said control circuitry being configured to initiate said thermal conductivity detector to perform a thermal conductivity detection in response to determining said received signal indicating that said cryogenic refrigeration system is in a regeneration phase.

6. The sensor according to claim 2, said control circuitry being configured in response to determining that said cryogenic refrigeration system is below 200K preferably below 100K and said contaminants are frozen within said cryogenic refrigeration system to initiate said thermal conductivity detector to perform said thermal conductivity detection as a baseline thermal conductivity detection.

7. The sensor according to claim 2, said control circuitry being configured in response to determining that said refrigeration system is above 220K preferably above 270K to control said thermal conductivity detector to initiate said thermal conductivity detector to perform said thermal conductivity detection as a contamination thermal conductivity detection.

8. The sensor according to claim 7, said circuitry being configured to convert said thermal conductivity signal to an indication of contamination of said cryogenic refrigerant in dependence upon both said baseline thermal conductivity detection and said contamination thermal conductivity detection.

9. The sensor according to claim 1, wherein said thermal conductivity detector comprises a filament thermal conductivity detector.

10. The sensor according to claim 9, comprising a further reference filament thermal conductivity detector, said further reference filament thermal conductivity detector being isolated from said refrigeration system and comprising refrigerant of a predetermined purity, said circuitry being configured to convert said thermal conductivity signal to an indication of contamination of said cryogenic refrigerant in dependence upon a comparison of said thermal conductivity detection of said further reference filament thermal conductivity detector and said filament thermal conductivity detector.

11. The sensor according to claim 1, wherein said thermal conductivity detector comprises a microelectromechanical system (MEMS) device.

12. The sensor according to claim 1, wherein said circuitry is configured to receive signals indicative of at least one of temperature and pressure of said refrigerant and to convert said thermal conductivity signal to said indication of contamination in dependence upon said at least one temperature and pressure.

13. A cryogenic refrigeration system comprising a cryogenic refrigerant and a sensor for determining contamination of said cryogenic refrigerant according to claim 1.

14. The cryogenic refrigeration system according to claim 13, further comprising at least one compressor for compressing said cryogenic refrigerant, at least one pump, and a controller for controlling operation of said cryogenic refrigeration system, said controller being configured to control a mixing cycle by triggering operation of said compressor and said at least one pump for a predetermined time prior to transmitting a signal to said sensor for initiating detection of said contamination of said cryogenic refrigerant.

15. A method of detecting contamination of a cryogenic refrigerant in a cryogenic refrigeration system, said method comprising: coupling a sensor comprising a thermal conductivity detector to a cryogenic refrigerant flow path in said cryogenic refrigeration system such that cryogenic refrigerant flows into said thermal conductivity detector, measuring the thermal conductivity of said cryogenic refrigerant with said thermal conductivity detector, converting said measured thermal conductivity to an indication of an amount of contamination of said cryogenic refrigerant; and outputting said indication of contamination of said cryogenic refrigerant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

[0052] FIG. 1 shows a section through a sensor according to an embodiment;

[0053] FIG. 2 shows a sensor according to an embodiment;

[0054] FIG. 3 shows a MEMS style TCD according to an embodiment;

[0055] FIG. 4 shows a refrigeration system showing possible positions of sensors according to embodiments;

[0056] FIG. 5 shows a refrigeration system showing a sensor according to an embodiment in a bypass line;

[0057] FIG. 6 shows the difference in thermal conductivity measurements for pure helium as opposed to helium contaminated with 100 pmm CO.sub.2; and

[0058] FIG. 7 shows a flow diagram illustrating steps in a method according to an embodiment.

DETAILED DESCRIPTION

[0059] Before discussing the embodiments in any more detail, first an overview will be provided.

[0060] Embodiments provide a method and means for the monitoring of refrigerant, in particular helium stream purity using thermal conductivity measurements in order to determine the presence of gas contamination in a cryogenic refrigeration system such as a cryo pump system. The development uses an in situ method to monitor refrigerant in some cases helium stream purity to determine the presence of contaminants prior to adverse impact on the system using gas thermal conductivity measurements, thereby mitigating the need for off site RGA analysis.

[0061] Various thermodynamic cycles such as the GM (Gifford McMahon) or Stirling cycle are used to generate very low temperatures and use Helium as the working fluid or refrigerant. One application of this technology is in cryo-pumps used to generate high vacuums, other applications include MRI scanners or high temperature superconductor cooling. In order for the proper operation of these systems, it is important to maintain the Helium inside the system to a well-defined level of purity. As the Helium purity decreases, the performance of the system degrades, ultimately leading to failure and the need for service and maintenance. Embodiments provide a method to measure the purity of a refrigerant for example, helium inside a refrigeration system of a cryogenic refrigeration system (while it is running) so that impurities can be detected before they reach levels that would cause pump failure.

[0062] The detection system uses Thermal Conductivity Detectors (TCD) which are used in gas chromatography. A TCD contains an electrical resistor that is placed in a gas flow path/volume. The temperature of the resistor changes (changing its resistivity) as heat is carried away from the resistor because of a gas flowing across it. Since different gasses have different thermal conductivities, gasses can be detected based on the rate of heat loss (change in resistivity) of the resistor.

[0063] Embodiments broadly describe two ways of using TCDs to determine refrigerant purity. Applicability of one method over the other is based on a number of factors

[0064] including but not limited to system type, layout, sensitivity of detection required etc. The term helium or refrigerant environment can mean a stagnant volume or a stream of helium or refrigerant gas at any pressure and flow rate.

Use of a Two Sensor SystemFilament TCD

[0065] This method used two separate TCD sensors where one sensor is placed in a high purity refrigerant environment and the other is placed in a potentially contaminated refrigerant environment. In this example the refrigerant is helium and because of the difference in conductivities of the pure and impure Helium environments, the resistivity of the TCD sensors is significantly different. A Wheatstone Bridge electrical circuit is used to convert the two sensor resistances to an output voltage that can be read by the system indicating how different the two streams are from one another. An increased difference between the voltage of the pure helium system and the system being tested indicates an increased contamination of the system.

[0066] All gasses, excluding hydrogen, have a degree of thermal conductivity lower than that of helium. For this reason, helium is often used as a reference gas to compare thermal conductivities of gasses to, it is also a common refrigerant used in cryogenic systems. The standard or reference environment for Pure Helium is at 200 PSIG and the reference voltage observed at this environment is 5.2 mV. This voltage is collected over a set time and used in a zero-point calibration. The reference voltage is then subtracted from the obtained voltage when the impure gas in question is passed though the sensing TCD. This allows the user to calculate the change in voltage due to contamination of the helium stream as compared to pure helium. An example showing the difference in these voltages is shown in FIG. 6.

Use of One Sensor SystemMems Style TCD

[0067] The use of a MEMS (micro-electromechanical system) TCD can allow for the accurate contamination concentration prediction with the use of a single sensor. MEMS have a much higher signal to noise ratio than their filament counterparts. This in turn allows a single MEMS sensor assembly to sense changes in gas purity at sub 100 ppm quantities.

[0068] A MEMS solution is more elegant than a filament-based design as a single sensor can be zeroed in the pure gas refrigerant. Any change from the zero point at a fixed pressure and temperature can be attributed to a change in purity of the gas. Use of a TCD to monitor the purity of a cryogenic helium loop will allow for detection of contamination prior to adverse impact to the system. This monitoring can occur in-situ on a warm system. The TCD can be integrated directly into the refrigerant loop.

[0069] FIG. 1 shows a section through a filament type thermal conductivity detector TCD 5 according to an embodiment. The TCD 5 comprises an inlet coupling 10 and outlet coupling 20 for coupling to the refrigeration system of for example a cryo pump. When coupled to the system refrigerant flows through the TCD 5 from inlet 10 to outlet 20. The TCD comprises filament 30 which is heated and whose resistance depends on its temperature, which in turn depends on the thermal conductivity of the refrigerant. There is a passage 25 for receiving the wires for sending current to the filament 30 and allowing changes in the resistance to be detected. Circuitry not shown determines the resistance of the filament and in some embodiments compares this with the resistance of a corresponding filament in pure refrigerant and from the difference in the values a measure of contamination of the refrigerant is devised and output.

[0070] FIG. 2 shows the TCD 5 of FIG. 1 not in section, with inlet coupling 10 and outlet coupling 20.

[0071] FIG. 3 schematically shows a MEMS style TCD 5, with inlet and outlet couplings 10 and 20 and with an integrated temperature sensor 40 and pressure transducer 50. The thermal conductivity of the refrigerant gas will vary with the temperature and pressure and thus, some TCDs will have these sensors integrated into them, values from these sensors being used in the conversion of the detected thermal conductivity measurement to amount of contamination.

[0072] FIG. 4 schematically shows a refrigeration system and potential sites for TCDs 5 according to embodiments. The refrigeration system comprises a compressor 60 and a plurality of refrigeration units 72. Sensors according to embodiments may be used in this system for the monitoring of impurities. These TCD sensors 5 may be placed within the refrigerant lines themselves and four example locations for TCDs are shown. That is 5A in the compressor refrigerant supply line, 5B in the compressor refrigerant return line, 5C in the refrigerator unit supply or 5D in the refrigerator unit return line 5D.

[0073] Operation of the TCDs may be triggered by control circuitry (not shown) to take measurements at appropriate times. The thermal conductivity measurements may be converted to an indication of contamination of the refrigerant and this can be used in servicing decisions to avoid contaminants rising above critical levels. In some embodiments the TCDs may be filament style TCDs and may operate in conjunction with a reference filament TCD that contains pure refrigerant, differences in the thermal resistance of the reference and other TCD being used to determine the level of contaminants.

[0074] In other embodiments the TCD may be a MEMS style TCD and reference baseline measurements may be made, by taking measurements at cryogenic temperatures when contaminants are captured in the coldest part of the system and the refrigerant is therefore pure and comparing these with measurements taken at warmer temperatures, in some cases following a mixing cycle, where the contaminants are present in the refrigerant. The differences in the thermal conductivity of the baseline and warmer measurements are used to determine the level of contamination. These two measurements may be taken during a period where the refrigerant is not flowing, the baseline measurement being taken at the start of such a period where the temperatures are low and the other measurement being taken when the system has warmed.

[0075] In some embodiments measurements from pressure, temperature and in some cases flow sensors, either associated with the TCD itself or as separate components in the refrigeration system, may be used in the conversion of the thermal conductivity measurement to contamination indications.

[0076] FIG. 5 shows an alternative system where the refrigeration units of FIG. 4 are replaced by cryopumps 70. In this embodiment the TCD 5 is within a bypass line. Flow within the bypass line is controlled by valves 75 and 76 which in turn are controlled by control circuitry 80. Control circuitry 80 also controls operation of the TCD 5 and receives signals indicative of conductivity from the TCD 5 along with pressure and temperature measurements from other sensors not shown. The control circuitry 80 also receives signals from and sends signals to the refrigeration system controller 90, that controls the operation of the refrigeration system. Thus, in some embodiments control circuitry 80 may receive a signal indicating a regeneration cycle is about to start from the refrigeration system controller 90 and in response it may control valves 75 in the bypass lines to open and valve 76 to close. Refrigerant will then flow into TCD 5 and a baseline thermal conductivity measurement may be taken along with a pressure and temperature measurement at the start of the regeneration cycle. Control circuitry 80 may then control valve 76 to open and valves 75 to close and after a predetermined time or when the refrigerant reaches a predetermined temperature, may request a mixing cycle from refrigeration controller 90. Refrigeration controller 90 may initiate the mixing cycle by turning on the compressor 60 and cryopumps 70 for a minute or so and then turning them off. Control circuitry 80 may then control valves 75 in the bypass lines to open and valve 76 to close. The warmer mixed refrigerant will then flow into TCD 5 and a thermal conductivity measurement may be taken along with pressure and temperature measurements. It should be noted that during the regeneration cycle the compressor and pumps are not generally operational and the refrigerant is stagnant, which may improve the accuracy of the measurements by removing flow effects. Processing circuitry 82 within control circuitry 80 may then determine the amount of contamination of the refrigerant from the respective thermal conductivity measurements and the temperature and pressure measurements.

[0077] FIG. 6 shows the difference in the corrected Voltage measured from a sensor at different sample points for pure refrigerant and refrigerant contaminated with 100 ppm CO.sub.2. These measurements are taken at different times, with the system being flushed between the measurements.

[0078] FIG. 7 shows a flow diagram illustrating steps in a method according to an embodiment. Initially at step S10 the TCD is coupled to a cryogenic refrigerant flow path in a cryogenic refrigeration system. This may involve opening some valves, or it may involve an initial step of mounting the TCD to the system. Once coupled then at step S20 cryogenic refrigerant flows into the thermal conductivity detector, and at step S30 the thermal conductivity of the cryogenic refrigerant is measured. At step S40 the measured thermal conductivity is converted to an indication of an amount of contamination of the cryogenic refrigerant. This may involve a comparison with a thermal conductivity measurement for non-contaminated refrigerant and/or adjustments for measured temperature, pressure and potentially flow rate of the refrigerant at the time of the thermal conductivity measurements. At step S50 the calculated indication of contamination is output either directly by display (not shown) to a user, and/or as a signal to the control circuitry of the refrigeration system.

[0079] Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

[0080] Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.

[0081] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.