Cryogenic refrigeration system and cryogenic pump

Abstract

A low temperature refrigeration system and method are disclosed. The low temperature refrigeration system includes a refrigeration unit including an expansion unit and a variable speed compressor configured to compress a refrigerant. The variable speed compressor is configured to receive refrigerant from the refrigeration unit via a lower pressure line and supply compressed refrigerant to the refrigeration unit via a higher pressure line. There is also a control circuit configured to control the variable speed compressor to maintain power consumption of the variable speed compressor below a predetermined threshold by controlling the compressor to initially operate at a reduced frequency during cooling and subsequently increasing an operating frequency of the variable speed compressor such that the variable speed compressor operates at a higher frequency.

Description

Technical Field

[0001] The field of the invention relates to cryogenic refrigeration systems, cryogenic pumps and refrigeration methods. Background Art

[0002] Low-temperature refrigeration systems are well known. These systems utilize a refrigeration process such as the Gifford-McMahon process and typically include an expansion-type refrigeration unit containing a thermal storage material and an expansion chamber. A compressor compresses refrigerant gas and supplies the compressed refrigerant to the refrigeration unit, where it is cooled by the thermal storage material and then expanded. The decompressed refrigerant gas returns to the compressor, completing the refrigeration cycle. By repeating this refrigeration cycle, extremely low temperatures can be achieved. For low-temperature refrigeration, helium may be a preferred refrigerant.

[0003] The efficiency of the refrigeration process is affected by the pressure difference between the higher-pressure and lower-pressure refrigerants, as well as the mass flow rate of the refrigerants. However, these factors are themselves limited by the capacity of the compressor.

[0004] It would be desirable to provide an improved refrigeration system. Summary of the Invention

[0005] A first aspect provides a low-temperature refrigeration system, comprising: a refrigeration unit including an expansion unit; a variable-speed compressor configured to compress refrigerant, the variable-speed compressor configured to receive refrigerant from the refrigeration unit via a lower-pressure pipeline and supply the compressed refrigerant to the refrigeration unit via a higher-pressure pipeline; and a control circuit configured to control the variable-speed compressor to maintain the power consumption of the variable-speed compressor below a predetermined threshold by controlling the compressor to initially operate at a reduced frequency during cooling and then increasing the operating frequency of the variable-speed compressor so that the variable-speed compressor operates at a higher frequency.

[0006] The efficiency of the refrigeration process increases with increasing pressure differentials between the higher- and lower-pressure lines and increasing the mass flow rate of the refrigerant. These factors can be increased by increasing the system's fill pressure and by increasing the compressor speed. However, increasing the compressor speed and increasing the refrigerant pressure both increase the power consumption of the compressor, which is typically limited to prevent overheating and failure.

[0007] The inventors of the present invention recognized that the power consumption of a compressor varies during operation and, in particular, may be at its highest during cooling and lower during steady-state operation. In particular, as the temperature decreases, some refrigerant accumulation may occur in the refrigeration unit, and the refrigerant pressure will drop. This pressure drop allows the compressor to operate at a higher frequency without requiring additional power. Therefore, they realized that if a control circuit were provided to initially operate the compressor at a lower speed during cooling, thereby limiting the power supplied to the compressor, then the operating frequency could be increased as the refrigerant pressure drops. This would allow the compressor to be configured to operate at a higher frequency during steady-state operation while being protected from overheating during cooling and providing improved performance.

[0008] In some embodiments, the cryogenic refrigeration system is provided with additional refrigerant by at least one of: increasing the initial charge pressure so that operation of the variable speed compressor at full speed during cool-down of the refrigeration unit will cause the variable speed compressor to consume power above the predetermined threshold; or by providing a buffer volume of refrigerant in fluid communication with a low-pressure line that supplies lower pressure refrigerant from the refrigeration unit to the compressor.

[0009] The cooling power of a refrigeration process, such as the Gifford-McMahon process, is driven by the pressure differential of the working fluid, or refrigerant. Based on the thermodynamic principle that a gas expands from a high-pressure level (the supply pressure) to a lower pressure level (the return pressure), the system's cooling power increases the higher the pressure differential between these two pressures. Furthermore, the higher the refrigerant mass flow rate through the system, the higher the cooling power provided.

[0010] Therefore, the pressure increase provided by the compressor should be as high as possible. The capacity of a compressor is limited by its compression ratio and the maximum permissible power consumption, which is often related to the maximum permissible current in the motor windings of the motor driving the compressor. The compression ratio depends firstly on the volume ratio between the high-pressure volume and the low-pressure volume of the system, secondly on the system's charge pressure, and thirdly, of course, on the balance between the refrigerant demand of the refrigeration unit and the pump capacity of the compressor.

[0011] Improved cooling capacity of a refrigeration unit can be achieved by increasing the refrigerant's charge pressure. By increasing the charge pressure, both the high and low pressures increase, thereby increasing the recirculating mass flow rate of the refrigerant. However, this also increases the electrical power required for refrigerant compression. Providing a control circuit to control the compressor's operating speed and limit the power consumed allows the system to be designed with additional refrigerant.

[0012] In summary, additional amounts of refrigerant can be provided by increasing the initial fill pressure (effectively overfilling the refrigeration system) and / or by providing an additional buffer volume in fluid communication with the lower pressure line, thereby changing the volume ratio between the high pressure volume and the low pressure volume. The initial fill pressure of the system is typically set so that when the compressor is operating at full speed, it will not consume more power than its specified power consumption value and therefore will not overheat or cut out. Having a control system that sets the maximum power that can be consumed by the compressor allows the initial fill pressure of the refrigerant to be increased without the risk of the compressor overheating and cutting out. However, in some cases, the compressor may not be able to operate efficiently at high pressure, and in this case, the additional refrigerant can be supplied in the form of an additional buffer volume so that the additional refrigerant is present without increasing the initial fill pressure.

[0013] It should be noted that the additional charge pressure is the pressure of the refrigerant in the system when the system is not in operation. This initial charge pressure may be specified for a particular refrigeration system. The full speed of the compressor operation may be the maximum operating speed or frequency of the compressor during a stable cooling operation phase, where low temperatures are maintained and the refrigeration system is not in power saving mode.

[0014] In some embodiments, the control circuit is further configured to control the variable speed compressor to maintain the pressures of the higher pressure line and the lower pressure line within predetermined limits.

[0015] The compressor can operate efficiently within certain pressure limits, for example, a scroll compressor can have a performance map defining high and low pressure limits within which operation is acceptable. These limits depend on the mechanical properties of the pump (i.e., casing strength) and the thermodynamic (heat balance) limits of the scroll unit. In some embodiments, a control circuit configured to control the speed of the compressor in order to limit power consumption can also be used to control the speed to keep the compressor operation within these predetermined pressure limits.

[0016] In some embodiments, the refrigeration system further includes at least one pressure sensor to sense a pressure difference between the higher pressure line and the lower pressure line.

[0017] The system may include a higher pressure line sensor and a lower pressure line sensor, or it may include a differential pressure sensor for measuring the pressure difference between the two pressure lines.The signals from these pressure sensors are sent to the control circuit.

[0018] In some embodiments, the cryogenic refrigeration system further comprises an inlet valve on the higher pressure line; and the control circuit is further configured to control the inlet valve to maintain the pressures of the higher and lower pressure lines within predetermined limits.

[0019] In some embodiments, the control circuit is configured to maintain a pressure difference between the higher pressure line and the lower pressure line within predetermined limits.

[0020] In some embodiments, the control circuitry may control both the speed of the compressor and the operation of the inlet valve in order to maintain the upper and lower pressure values ​​within predetermined limits.

[0021] To control operation during steady state operation, in addition to keeping power below a predetermined threshold, the control circuit may also control the inlet valve to the refrigeration unit to maintain a desired pressure differential between the higher and lower pressure lines and ensure efficient cooling.

[0022] In some embodiments, the control circuit is configured to increase the frequency of operation of the variable speed compressor in response to a detected decrease in power consumption of the variable speed compressor.

[0023] During cooling, three effects affect the refrigerant pressure in the system. As the compressor warms up to its operating temperature (typically around 60°C), the pressure increases. The warming of the refrigerant in the low-pressure volume due to the heat load of the refrigeration unit (which may increase the helium temperature from 20 to 50°C) also causes a pressure increase. However, there is also a pressure drop in the system caused by refrigerant accumulation in the cold end of the refrigeration unit. This third effect is greater than the first two; as the system cools, the refrigerant pressure within the system decreases, and this becomes particularly noticeable at lower temperatures. Therefore, the compressor will typically experience a pressure drop during cooling, and the compressor's power consumption will also decrease. In some cases, the control circuitry, in response to detecting this power drop, increases the compressor's operating frequency to keep the compressor's power consumption close to a threshold, in some cases within 10%.

[0024] In some embodiments, the maximum operating frequency during steady-state operation is between 50 and 70 Hz, and the initial operating frequency during cool-down is lower, between 30 and 50 Hz.

[0025] The initial operating frequency can be significantly less than the steady-state operating frequency, and in some cases it can increase from 35 Hz until it reaches a maximum operating frequency, which in some embodiments is 60 Hz. This is the operating frequency at which the variable speed compressor operates in steady state. In some embodiments, steady-state operation may be slightly below the maximum operating frequency.

[0026] In some embodiments, the refrigerant includes helium.

[0027] In some embodiments, the refrigeration unit is configured to cool to 80K, preferably to 50K, more preferably to below 10K, more preferably to 4K.

[0028] The refrigeration unit can be a low-temperature refrigeration unit capable of cooling to 80K, or in some embodiments it can be an extremely low-temperature refrigeration unit (cooling down to 4K in some embodiments). Where the refrigeration unit operates at very low temperatures, the effect of refrigerant accumulation in the refrigeration unit and the corresponding reduction in pressure within the refrigeration system is particularly pronounced. Therefore, embodiments are particularly effective for such refrigeration systems.

[0029] In some embodiments, the initial pressure of the refrigerant is 5% higher, preferably 10% higher, than the pressure specified for the refrigeration unit without power control of the variable speed compressor during cooling.

[0030] Without power control of the compressor during cooling, then typically it will be driven at a constant speed and the speed / pressure of the refrigerant will be set so that the compressor does not overheat during cooling when there is peak power consumption. With power control during cooling, then the refrigerant pressure may increase because the power consumption during cooling is controlled and is no longer a limiting factor.

[0031] In some embodiments, the refrigeration system further comprises the buffer volume of refrigerant, the buffer volume comprising more than 20%, preferably more than 50%, and in some cases more than 90% of the total amount of refrigerant in the refrigeration system.

[0032] The refrigeration system can be provided with additional refrigerant, in some cases 20% additional refrigerant by volume or mass and in other cases 50% or 90% additional refrigerant or more. The refrigeration system can be configured to operate with a certain amount of refrigerant so that when the variable speed compressor is operated at the maximum operating speed from startup, the power threshold consumed by the compressor is not exceeded. This is a standard refrigerant amount for the refrigeration system. Embodiments that provide compressor power control can provide the additional amount of refrigerant by increasing the initial charge pressure and / or supplying it to a buffer volume associated with the lower pressure line, which buffer volume changes the volume ratio between the high pressure volume and the low pressure volume.

[0033] A refrigeration system may be specified to operate with a refrigerant at an initial fill pressure of between 13-17 bar. In cases where the lower end of the range is specified, then the pressure may be increased by overfilling with refrigerant and increasing the fill pressure in the system. This is acceptable for refrigeration systems according to embodiments because the compressor power supply is controlled to remain below the threshold. In cases where the refrigerant supplied by the system is at the upper end of the range, then the compressor may not be configured to operate at a pressure higher than the specified pressure, in which case a buffer volume in the low-pressure line may be used to supply additional refrigerant, thereby changing the volume ratio between the high-pressure volume and the low-pressure volume. In some cases, a combination of increasing the pressure of the refrigerant and adding a buffer volume may be employed.

[0034] In some embodiments, the control circuit is further configured to control the variable speed compressor in the power save mode to maintain power consumption of the variable speed compressor below a predetermined reduced threshold.

[0035] The control circuitry can also be configured to operate a power saving mode in which, in response to determining that there is a reduced load on the refrigeration system during steady-state operation, it sets a reduced threshold for compressor power. The same control circuitry used for cooling can then control power consumption in this reduced-load, power saving mode.

[0036] A second aspect provides a cryopump comprising the cryogenic refrigeration system according to the first aspect.

[0037] A third aspect provides a method of operating a low-temperature refrigeration system, which includes a refrigeration unit having an expansion unit and a variable speed compressor configured to compress refrigerant so that higher pressure refrigerant is supplied to the refrigeration unit and lower pressure refrigerant is received from the refrigeration unit, the method comprising: operating the compressor at an initial lower frequency during initial cooling of the refrigeration unit to keep the power consumed by the variable speed compressor below a predetermined threshold; and subsequently increasing the operating frequency of the compressor to full speed operation.

[0038] In some embodiments, the method further comprises an initial step of providing an increased amount of refrigerant to the refrigeration system by at least one of: increasing the charge pressure of the refrigerant within the system such that operation of the variable speed compressor at the maximum operating frequency during cooling will exceed the predetermined threshold of power consumption; or providing a buffer volume of additional refrigerant in fluid communication with the lower pressure line.

[0039] It should be noted that the efficiency of the system generally improves with lower compressor speeds, so overfilling the refrigeration system with refrigerant can allow it to operate efficiently at lower speeds during cooling. When the pressure of the refrigerant drops due to the accumulation of refrigerant at lower temperatures in the refrigeration unit or cold head, the speed of the compressor can be increased and cooling efficiency can be maintained.

[0040] Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly listed in the claims.

[0041] Where an apparatus feature is described as being operable to provide a function, it will be understood that this includes apparatus features providing that function or being adapted or configured to provide that function. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Embodiments of the present invention will now be further described with reference to the accompanying drawings, in which:

[0043] Figure 1 A low temperature refrigeration system according to a first embodiment is shown;

[0044] Figure 2 The difference between a conventional low-temperature refrigeration system and a low-temperature refrigeration system according to an embodiment is shown.

[0045] Figure 3 shows a cryogenic refrigeration system according to a further embodiment; and

[0046] Figure 4 A flow chart illustrating steps in a method according to an embodiment is schematically shown. DETAILED DESCRIPTION

[0047] Before discussing the embodiments in more detail, an overview will first be provided.

[0048] Embodiments provide cryogenic cooling devices. Embodiments can be used in cryogenic cooling systems, which can be used for superconducting magnets / coils. They can be used in cryogenic condenser systems and cryogenic pump systems for resublimation, desublimation, solidification, or deposition of gases.

[0049] This cryogenic device initially operates in cooling mode and then in steady-state refrigeration mode. During the cooling process of the refrigeration unit, refrigerant accumulates significantly in the colder parts of the refrigeration unit (e.g., in the coldhead piston system). This results in a decrease in both high and low pressures in the refrigeration system. This reduction in refrigerant consumption reduces the electrical power consumption of the compressor. Therefore, in the steady-state, unregulated state at cooling temperatures, the compressor will operate with a reduced refrigerant quantity.

[0050] However, if the amount of refrigerant in the system is increased to compensate for the amount of refrigerant accumulated in the refrigeration unit at steady state, there is a risk of the compressor becoming overloaded during the initial cool-down period. During the cool-down period, there is a relatively short period in which the pressure in the system is at its highest, and during this period there is a risk of the compressor thermally overloading.

[0051] The embodiments resolve these conflicting issues and attempt to achieve improved cooling power in steady-state operation and improve the refrigerator cooling process by using a control scheme that controls the winding current and electrical power of the compressor motor. A threshold power value is set and the motor is controlled to rotate at a frequency such that the power consumed is below the threshold power.

[0052] The control mechanism can support both initial cooling and system startup during compressor warm-up or restart in case of mismatch in cold head cooling or hot system restart or restart under non-steady-state conditions.

[0053] This control mechanism allows additional refrigerant to be added to the system. This is accomplished by increasing the system's charge pressure. The refrigerant pressure in the refrigeration unit, or coldhead, connected to the compressor is fixed by the initial charge. During low-temperature system startup (opening the coldhead refrigerant valve actuator and then turning on the compressor), the coldhead typically cools within 20 to 60 minutes. This cooling process is influenced by three factors that affect the system's refrigerant pressure.

[0054] 1) Pressure increase by heating the compressor volume consisting of the vortex volume and the oil pre-separator volume to an operating temperature of 60°C.

[0055] 2) The pressure increase of the refrigerant low-pressure volume flow caused by the cold head heat load (the refrigerant temperature increases from 20 to 50°C).

[0056] 3) Pressure drop in the system caused by refrigerant accumulation in the cold end of the cold head.

[0057] Effect 3 is significantly greater than effects 1 and 2, resulting in lower "equivalent" refrigerant system pressures throughout the system, especially for low-temperature applications. Consequently, both high and low pressures are reduced, which limits cold head performance. Furthermore, the compressor cannot realize its full refrigerant mass flow potential because the suction pressure is too low.

[0058] In order to increase the recirculating refrigerant mass flow and adjust the system to increase the differential pressure, the speed of the compressor can be increased (for example, from 50 to 70 Hz). It can be determined that the efficiency of the compressor is lower the higher the speed. The reason for this is that by increasing the compressor speed, energy dissipation is increased, which will increase the gas velocity in the piping and compressor housing, resulting in gas pressure loss and causing low suction pressure. Therefore, for a given differential pressure, running the compressor at its highest possible speed may not be optimal. On the contrary, perhaps increasing the amount of refrigerant by increasing the fill pressure can provide both improved performance and increased efficiency.

[0059] With conventional systems, such overcharging is not possible because the required higher motor current of the compressor (probably a scroll pump) (to pump the denser refrigerant) would cause winding overheating (which could cause a trip due to the motor protection switch).

[0060] The problem of thermal shutdown of the compressor has been solved in an embodiment by a power control of the compressor, which can be adjusted to the maximum allowed current in the motor windings that will not cause the winding temperatures to become too high. The power control will affect the frequency of the compressor drive (motor). This means that a substantially constant level of electrical power consumption will cause the compressor motor speed to initially be lower than the rated speed of the compressor motor, but in some embodiments this effect is compensated by increasing the refrigerant charge pressure. This feature allows the system to be overcharged with a defined refrigerant pressure without the risk of thermal shutdown or overload. With such a power control, the cooling process will start at a medium or low compressor speed, supplying the necessary pressure difference and refrigerant flow required for cold head operation.

[0061] Additionally and / or alternatively, this effect can be compensated by providing a buffer volume of refrigerant associated with the low-pressure line. In this regard, the filling pressure and mass flow rate may be affected not only by the actual filling pressure of the stopped system, but also in the same way by creating a permanent low-pressure volume excess, which transfers the refrigerant mass to a smaller high-pressure volume, thereby increasing the total system pressure in running operation. This option may be necessary to bypass the low-pressure strength limitations of the compressor components.

[0062] In summary, if additional refrigerant (e.g., helium) is used, the reduced pressure difference between the high pressure (supply) and low pressure (return) is compensated in the performance of the cold head by providing a higher helium pressure when the system is initially charged and / or by providing a helium buffer volume attached to the low pressure line (both of which result in an increased helium mass flow rate).

[0063] Figure 1A cryogenic system according to an embodiment is shown. The cryogenic refrigeration system includes a control circuit 10 configured to control a motor 20 that drives a compressor 30 that compresses refrigerant in the refrigeration system. The control circuit 10 also controls a motor 70 that drives an inlet valve (not shown) that controls the supply of higher-pressure refrigerant to a refrigeration unit or cold head 40. In this embodiment, a temperature sensor 60 is associated with the cooler portion of the cold head 50.

[0064] The control circuit 10 comprises a data memory storing threshold values ​​for setting maximum values ​​of power and / or current supplied to the compressor.

[0065] In this embodiment, the refrigeration unit 40 comprises a Gifford McMahon refrigeration unit (the Gifford McMahon piston system is hereinafter referred to as a "cold head") and the compressor 30 supplies pressurized helium as the refrigerant or coolant. The system is a hermetically sealed system with a closed refrigerant cycle. To improve the performance of the cryogenic system, a control system 10 is provided that allows the variable frequency drive output current and / or electrical power supplied to the compressor motor 20 to be measured and regulated.

[0066] Keeping the power consumption of the compressor 30 below a threshold allows the refrigeration system to be configured with a higher initial charge pressure of refrigerant, as it is no longer limited by the power consumed by the compressor running at maximum speed during the cooling process.

[0067] In this embodiment, the compressor motor 20 is directly connected to a variable frequency drive (VFD) 25 that supplies power to the compressor and is controlled by the control circuit 10. This current and / or electrical power control will affect the output frequency of the VFD and therefore the rotational speed of the compressor motor. The control circuit 10 attempts to regulate the compressor speed to approach the maximum allowable electrical load of the motor 20 windings without risking thermal shutdown.

[0068] The system operates as follows:

[0069] Start the program:

[0070] 1) Turn on the cold head 40 and the compressor 30.

[0071] 2) The VFD 25 ramps up the output frequency of the power supplied to the compressor motor 20 until a threshold value of winding current and / or power is reached, in which way the control circuit 10 limits the VFD 25 output frequency.

[0072] 3) During the warm-up period of the compressor-cold head system, the most critical load condition is through the control of the VFD 25 by limiting the frequency, which will limit the speed of the compressor 30 and result in lower winding current and / or power.

[0073] 4) The progress of the cold head 40 cooling will cause more and more helium to accumulate in the cold head 40. This will cause the system pressure to drop. The VFD 25 will provide an output frequency that is controlled to ramp up the current and / or power supplied to the motor.

[0074] 5) Achieve steady-state operation. The current and / or power control will advise the VFD to deliver the maximum possible frequency of the VFD to the motor 20. At this point, control of the system can be transferred to the control of the inlet valve by the stepper motor 70, which can be controlled to provide the desired pressure difference between the supply and return pressure lines.

[0075] Surprisingly, it was discovered that it's not necessary to run the compressor at the highest possible speed to achieve maximum cooling power at the cold head in steady-state operation. Instead, increasing cooling power in steady-state operation can be measured by using a system configuration in which the VFD output frequency is limited compared to the grid frequency while maintaining full winding current / power. The optimal operating point is found between the mass flow rate of recycled helium and the pressure difference between the supply and return pressures in the system. In this regard, the power consumed depends on the compressor's frequency / speed as well as the suction and high pressures. By combining controls based on the system's pressure and compressor speed, the optimal operating point can be found. This may be due to the increased energy dissipation in the system at higher compressor speeds, which immediately leads to performance losses. Therefore, it may not be desirable to run the compressor at the highest possible speed allowed by a certain threshold power and the maximum possible differential pressure in the system, and with the help of appropriate measurements and controls, a preferred operating point with a lower frequency can be found.

[0076] In some embodiments, the current / power control system 10 can also be used to limit the energy demand of the cryogenic system in steady state during partial load conditions. In this case, additional set points or thresholds corresponding to lower winding currents / electrical powers can be used. In this way, the same control circuitry can be used to support energy conservation modes.

[0077] Figure 2 The operation of the embodiment is shown compared to a conventional refrigeration system. The left graph shows the operation of the conventional system, while the right graph shows the operation of the refrigeration system according to the embodiment.

[0078] The upper left figure shows how, in a conventional system, pressures are initially low before startup, then gradually decrease after startup in both the high- and low-pressure lines as temperatures drop and refrigerant accumulates in the coldest parts of the system. With a system according to an embodiment, initial refrigerant pressures are higher as the system "overcharges." The compressor speed is limited at startup, so the high- and low-pressure lines need time to reach steady-state values. Once reached, these pressures are maintained, and due to the initial overcharge, the higher pressures are higher than in a conventional system.

[0079] In the second set of graphs, compressor power is shown decreasing over time after startup in a conventional refrigeration system as the pressure in the system decreases due to the accumulation of refrigerant in the coldest part of the cold head. The right graph shows how the power supplied to the compressor initially decreases during the cool-down period, corresponding to a lower compressor speed. Power gradually increases to a maximum power, which can be maintained once the system reaches steady-state. In both examples, the maximum power that the compressor can safely handle is 8.3 kW; however, in the examples, by controlling the compressor during the cool-down period, this higher power can be applied to the compressor at steady-state without overloading the compressor during the cool-down period.

[0080] The third pair of graphs shows how the compressor operating frequency varies for a conventional refrigeration system and a refrigeration system of an embodiment. The refrigeration system of the embodiment initially has a reduced compressor operating frequency, which increases to full maximum speed or near full maximum speed once cooling is complete. The prior art compressor has a single operating speed.

[0081] The last two graphs show how embodiments provide improved cold head power due to increased mass flow of refrigerant, which embodiments may provide by overfilling the system or providing additional volume.

[0082] Figure 3 An alternative embodiment of a refrigeration system is shown having a low pressure buffer volume 80 connected to a low pressure line 62 that returns refrigerant from the refrigeration unit 40 to the compressor 30. There is a pressure sensor 65 for sensing the pressure of the low pressure line 62 and an additional pressure sensor 67 for sensing the pressure of the high pressure line 64. In some embodiments, a differential pressure sensor may be used instead of the high and low pressure sensors.

[0083] During operation, the control circuit 10 controls both the compressor speed and the rotational frequency of the inlet valve used to supply high pressure refrigerant from the higher pressure refrigerant line 64 to the refrigeration unit or coldhead 40 by controlling the rotational frequency of the motor driving the compressor according to a power threshold.

[0084] Operation of the refrigeration system begins with the compressor being driven at an initial, lower frequency while the refrigerant pressure is high. The operating frequency is set by a threshold power limit on the compressor motor, controlled by control circuit 10. As the refrigerant pressure drops due to the accumulation of refrigerant in the cooler portions of the refrigeration unit 40, the power used by the compressor also drops. The control circuit detects this and increases the frequency and speed, which results in an increase in motor winding current and power consumption, such that power consumption remains close to a threshold level, in some embodiments, within 2% of that value, in other embodiments, within 5% of the threshold power level, and in yet other embodiments, within 10% of the threshold power level. In this manner, as the refrigerant pressure drops, the motor speed increases while the power consumed by the motor remains below, but close to, the threshold. Once the system reaches steady state, with the compressor operating at or near its maximum frequency, the refrigeration system can be controlled by control circuit 10 controlling the inlet valve of the cold head 40. Control of the inlet valve controls the differential pressure of the refrigeration system, which in turn affects the cooling of the refrigeration system. In this regard, the compressor can operate effectively within certain high and low pressure limits, which, for a scroll compressor, can be defined in a scroll performance map that sets limits for the high and low pressure lines within which the compressor can operate effectively. In some embodiments, the control unit 10 can be used to control the operating speed of the compressor, ensuring not only that the maximum power consumption is not exceeded, but also that the high and low pressure limits of the scroll performance map are not exceeded. Thus, the control unit 10 receives signals from pressure sensors 65 and 67, and in response to any of these indicating that the pressure in the high and / or low pressure lines is moving towards the limits, the control unit can correct the operating speed of the compressor to move the pressure away from these limits. In some embodiments, the control unit 10 can control one or both of the inlet valve and the speed of the compressor to keep the operation of the compressor within the desired pressure limits.

[0085] In this embodiment, there is a low-pressure buffer volume 80 that allows additional refrigerant to be supplied to the system without increasing the pressure within the system beyond the pressure at which the scroll compressor 30 can operate efficiently. In this way, this embodiment adjusts the volume ratio between high and low pressure and increases the amount of refrigerant in the system without increasing the charge pressure. This is an alternative option to increasing the refrigerant charge pressure (although it can be used in conjunction with this option).

[0086] Both options for increasing the refrigerant quantity (i.e., increasing the fill pressure and providing an additional buffer volume) can be used to adjust the operating conditions of the refrigeration unit to increase the cooling power and / or efficiency of the system. A technical limitation for increasing the cooling power is the electrical power consumption of the compressor, which results in high currents in the compressor motor windings. If these currents become too high, the thermal protection switch (circuit breaker) will operate and shut down the compressor motor. For safe and stable operation of the system, this should be avoided.

[0087] Figure 4 A flow chart illustrating steps in a method according to an embodiment is schematically shown.

[0088] Startup occurs at step S0, which may be an initial start-up or a start-up after an interruption in the refrigeration cycle. At step S10, the compressor motor is powered to operate at a first initial reduced frequency. The power consumed by the motor may be continuously evaluated, as indicated in steps D5 and D15, and if it is below a threshold value P t Exceeding a certain amount ΔP, the rotation frequency of the motor is increased in step S20. If it is higher than the threshold value P t , then the rotation frequency is reduced in step S30. If it is within the required range, the rotation frequency is not changed.

[0089] After the rotation frequency may increase to or approach the maximum rotation frequency at step S20 , it is determined at step D25 whether the cooling process is completed, and if so, steady-state operation begins.

[0090] Although illustrative embodiments of the present invention have been disclosed herein in detail with reference to the accompanying drawings, it should be understood that the invention is not limited to the precise embodiments and that various changes and modifications may be made therein by those skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

[0091] Reference numerals

[0092] 10 Control Circuit

[0093] 20 Compressor motor

[0094] 25 Variable Frequency Drive

[0095] 30 compressor

[0096] 40 Refrigeration unit or cold head

[0097] 50 Low temperature point of cold head

[0098] 62 Lower pressure line

[0099] 64 Higher pressure lines

[0100] 65, 67 pressure sensors

[0101] 80 buffer volume

Claims

1. A low-temperature refrigeration system comprising: a refrigeration unit, the refrigeration unit comprising an expansion unit; a variable speed compressor configured to compress refrigerant, the variable speed compressor configured to receive refrigerant from the refrigeration unit via a lower pressure line and supply compressed refrigerant to the refrigeration unit via a higher pressure line; as well as A control circuit is configured to control the variable speed compressor to maintain power consumption of the variable speed compressor below a predetermined threshold by controlling the variable speed compressor to initially operate at an initially reduced operating frequency and subsequently increasing the operating frequency of the variable speed compressor so that the variable speed compressor operates at a higher frequency during cooling.

2. The low-temperature refrigeration system according to claim 1, wherein: The cryogenic refrigeration system is provided with additional refrigerant by at least one of: increasing the initial charge pressure so that operation of the variable speed compressor at full speed during cooling of the refrigeration unit will cause the variable speed compressor to consume power above the predetermined threshold; or By providing a buffer volume of refrigerant in fluid communication with the lower pressure line.

3. The low-temperature refrigeration system according to claim 1 or 2, wherein: The control circuit is further configured to control the variable speed compressor to maintain the pressures of the higher pressure line and the lower pressure line within predetermined limits.

4. The low-temperature refrigeration system according to claim 1 or 2, wherein: The control circuit increases the operating frequency of the variable speed compressor in response to a detected decrease in power consumption of the variable speed compressor.

5. The low-temperature refrigeration system according to claim 1 or 2, wherein: The initial reduced operating frequency of the variable speed compressor is less than 70% of a maximum operating frequency during steady-state operation of the refrigeration unit.

6. The low-temperature refrigeration system according to claim 1 or 2, wherein: The initially reduced operating frequency of the variable speed compressor is less than 60% of a maximum operating frequency during steady-state operation of the refrigeration unit.

7. The low-temperature refrigeration system according to claim 5, wherein: The maximum operating frequency during steady state operation is between 50 and 70 Hz, and the initial reduced operating frequency during cool down is lower.

8. The low-temperature refrigeration system according to claim 7, wherein: The initial reduced operating frequency is between 30 and 50 Hz.

9. The low-temperature refrigeration system according to claim 1 or 2, wherein: The refrigerant includes helium.

10. The low-temperature refrigeration system according to claim 1 or 2, wherein: The refrigeration unit is configured to cool to 80K.

11. The low-temperature refrigeration system according to claim 1 or 2, wherein: The refrigeration unit is configured to cool to 50K.

12. The low-temperature refrigeration system according to claim 1 or 2, wherein: The refrigeration unit is configured to cool to below 10K.

13. The low-temperature refrigeration system according to claim 1 or 2, wherein: The refrigeration unit is configured to cool down to 4K.

14. The low temperature refrigeration system according to claim 2, wherein: The initial pressure of the refrigerant is 5% higher than the pressure specified for the refrigeration unit without power control of the variable speed compressor during cooling.

15. The low temperature refrigeration system according to claim 2, wherein: The initial pressure of the refrigerant is 10% higher than the pressure specified for the refrigeration unit without power control of the variable speed compressor during cooling.

16. The low temperature refrigeration system according to claim 2, further comprising the buffer volume of refrigerant, the buffer volume containing more than 20% of the total amount of refrigerant in the refrigeration system.

17. The low-temperature refrigeration system according to claim 16, wherein the buffer volume contains more than 50% of the total amount of refrigerant in the refrigeration system.

18. The low-temperature refrigeration system according to claim 17, wherein the buffer volume contains more than 90% of the total amount of refrigerant in the refrigeration system.

19. The low-temperature refrigeration system according to claim 1 or 2, wherein: The control circuit is further configured to control the variable speed compressor in a power saving mode to maintain power consumption of the variable speed compressor below a predetermined reduced threshold.

20. A cryogenic pump comprising a cryogenic refrigeration system according to any preceding claim.

21. A method of operating a low temperature refrigeration system comprising a refrigeration unit having an expansion unit and a variable speed compressor configured to compress refrigerant such that higher pressure refrigerant is supplied to the refrigeration unit via a higher pressure line and lower pressure refrigerant is received from the refrigeration unit via a lower pressure line, the method comprising: operating the variable speed compressor at an initial lower frequency during an initial cool-down of the refrigeration unit to maintain power consumed by the variable speed compressor below a predetermined threshold; as well as The operating frequency of the variable speed compressor is then increased to full speed operation.

22. The method according to claim 21, comprising: Initially, an increased amount of refrigerant is provided to the refrigeration system by at least one of: increasing the charge pressure of the refrigerant within the system such that operation of the variable speed compressor at a maximum operating frequency during cooling will exceed the predetermined threshold value of power consumption; or A buffer volume of additional refrigerant is provided in fluid communication with the lower pressure line.

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