Ultra-low temperature refrigeration system and ultra-low temperature pump

Abstract

A very low temperature refrigeration system and method are disclosed. The very low temperature refrigeration system includes a refrigerator unit including an expansion unit, a variable speed compressor configured to compress a refrigerant. The variable speed compressor is configured to receive refrigerant from the refrigerator unit via a low pressure line and to supply compressed refrigerant to the refrigerator unit via a high pressure line. The very low temperature refrigeration system also includes a control circuit configured to control the variable speed compressor to maintain a power consumption of the variable speed compressor below a predetermined threshold, the control circuit configured to control the variable speed compressor during cool down to operate the variable speed compressor at an initial reduced frequency and thereafter increase an operating frequency of the variable speed compressor such that the variable speed compressor operates at a high frequency.

Description

Technical Field

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

Background Art

[0002] Cryogenic refrigeration systems are known. Such systems use refrigeration processes such as the Gifford-McMahon process and generally include an expansion-type refrigeration unit that houses a heat storage material and an expansion chamber. A compressor compresses the refrigerant gas, supplies the compressed refrigerant to the refrigeration unit, where the refrigerant expands after being cooled by the heat storage material. The decompressed refrigerant gas is returned to the compressor, thereby forming a refrigeration cycle. By repeating this refrigeration cycle, cryogenic temperatures can be obtained. Helium is a preferred refrigerant for cryogenic refrigeration.

[0003] The effectiveness of the refrigeration process is affected by the differential pressure between the high-pressure refrigerant and the low-pressure refrigerant and the mass flow rate of the refrigerant. However, these factors themselves are limited by the capacity of the compressor.

Summary of the Invention

Problems to be Solved by the Invention

[0004] It is desired to provide an improved refrigeration system.

Means for Solving the Problems

[0005] A first aspect provides an ultra-low temperature refrigeration system, the ultra-low temperature refrigeration system comprising a refrigeration unit including an expansion unit; a variable speed compressor configured to compress a refrigerant, the variable speed compressor receiving the refrigerant from the refrigeration unit via a low pressure line and supplying the compressed refrigerant to the refrigeration unit via a high pressure line; and a control circuit configured to control the variable speed compressor to maintain power consumption of the variable speed compressor below a predetermined threshold. The control circuit controls the variable speed compressor during cool-down such that the variable speed compressor operates at an initial reduced operating frequency and then increases the operating frequency such that the variable speed compressor operates at a higher frequency.

[0006] The effectiveness of the refrigeration process is increased by an increase in the differential pressure between the high pressure line and the low pressure line and an increase in the mass flow rate of the refrigerant. These increases can be achieved by increasing the filling pressure of the system and increasing the speed of the compressor. However, increasing the speed of the compressor and increasing the pressure of the refrigerant both increase the power consumption of the compressor, and the power consumption of the compressor is generally limited to avoid overheating and failure of the compressor.

[0007] The inventors of the present invention recognize that the power consumption of the compressor changes during operation, and in particular, may be highest during cool-down and lowest during steady-state operation. Specifically, as the temperature decreases, refrigerant accumulates in the refrigeration unit and the pressure of the refrigerant decreases. Due to this pressure drop, the compressor does not require additional power and can operate at a higher frequency. Therefore, this is achieved by providing a control circuit to first operate at a low speed during cool-down, thereby limiting the power supplied to the compressor, and then controlling the compressor to increase the operating frequency of the compressor when the pressure of the refrigerant decreases. Thereby, the compressor is protected from overheating during cool-down, while being configured to operate at a high frequency during steady-state operation, which can bring about improved performance.

[0008] In some embodiments, the cryogenic refrigeration system supplies additional refrigerant by at least one of increasing the initial charge pressure or providing a buffer volume of refrigerant in fluid communication with the low pressure line such that operating the variable speed compressor at full speed during cooldown of the refrigeration unit can lead to the variable speed compressor consuming power in excess of a predetermined threshold.

[0009] The refrigeration or cooling capacity of a refrigeration process, such as the Gifford-McMahon process, is provided by the differential pressure of the working fluid or refrigerant. According to the thermodynamic principle of the expansion of a gas from a high pressure level (supply pressure) to a low pressure level (return pressure), the cooling capacity of the system increases as the differential pressure between these two pressures increases. Furthermore, the greater the mass flow rate of the refrigerant through the system, the greater the resulting cooling capacity.

[0010] Therefore, this pressure increase provided by the compressor must be as large as possible. The capacity of the compressor is limited by the compression ratio and the maximum allowable power consumption, which is often related to the maximum allowable current flowing through the 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 charge pressure of the system, and of course, thirdly, on the balance between the refrigerant demand of the refrigeration unit and the pumping capacity of the compressor.

[0011] By increasing the charge pressure of the refrigerant, the cooling capacity of the refrigeration device can be improved. By increasing the charge pressure, both the high pressure and the low pressure increase, and thus the recirculation mass flow rate of the refrigerant increases. However, this also increases the power demand required for compressing the refrigerant. By providing a control circuit that controls the operating speed of the compressor and limits the power consumption, a system with additional refrigerant can be designed.

[0012] In summary, the additional amount of refrigerant can be provided by increasing the initial charge pressure and substantially overcharging the refrigeration system and / or by providing an additional buffer volume in fluid communication with the low pressure line, thereby changing the volume ratio between the high pressure volume and the low pressure volume. The initial charge pressure of the system is generally set such that when the compressor operates at full speed, it does not consume power in excess of a specified power consumption value and thus does not overheat or trip. By having a control system that sets the maximum power that the compressor can consume, the initial charge pressure of the refrigerant can be increased without the risk of the compressor overheating and stopping. However, there may be cases where the compressor cannot operate effectively at high pressure, in which case the additional refrigerant can be supplied in the form of an additional buffer volume, such that there is additional refrigerant present without increasing the initial charge pressure.

[0013] Note that the additional charge pressure is the pressure of the refrigerant within the system when the system is not operating. This initial charge pressure can be specified for a particular refrigeration system. The maximum operating speed of the compressor can be the maximum operating speed or frequency of the compressor in a steady-state cooling operating stage where low temperature is maintained and the refrigeration system is not in a power save mode.

[0014] In some embodiments, the control circuit is further configured to control a variable speed compressor to maintain the pressures in the high pressure line and the low pressure line within a predetermined limit.

[0015] The compressor can operate effectively within a specific pressure limit. For example, a scroll compressor can have a performance map that defines the high pressure and low pressure limits, and operation within these limits is acceptable. These limits depend on the pump mechanism, i.e., the strength of the housing, and the thermodynamic (thermal equilibrium) limits of the scroll unit. In some embodiments, a control circuit configured to control the speed of the compressor to limit power consumption can be used to control the speed to maintain the operation of the compressor within these predefined pressure limit values.

[0016] In some embodiments, the refrigeration system further comprises at least one pressure sensor for detecting the differential pressure between the high-pressure line and the low-pressure line.

[0017] The system can comprise a high-pressure line sensor and a low-pressure line sensor, or can comprise a differential pressure sensor for measuring the differential pressure between two pressure lines. Signals from these pressure sensors are sent to a control circuit.

[0018] In some embodiments, the cryogenic refrigeration system further comprises an inlet valve on the high-pressure line, and the control circuit is further configured to control the inlet valve so as to maintain the pressures of the high-pressure line and the low-pressure line within a predetermined limit value.

[0019] In some embodiments, the control circuit is configured to maintain the differential pressure between the high-pressure line and the low-pressure line within a predetermined limit value.

[0020] In some embodiments, the control circuit can control both the speed of the compressor and the operation of the inlet valve in order to maintain the higher pressure value and the lower pressure value within a predetermined limit value.

[0021] In addition to maintaining the power below a predetermined threshold value, in order to control the operation during steady operation, the control circuit can also control the inlet valve to the refrigeration unit so as to maintain a desired differential pressure between the high-pressure line and the low-pressure line and ensure effective cooling.

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

[0023] During cooldown, there are three effects that affect the refrigerant pressure within the system. There is a pressure increase due to the compressor warming up to its operating temperature (typically around 60 °C). There is also a pressure increase due to the heat load of the refrigeration unit raising the helium temperature from 20 °C to 50 °C, causing the refrigerant in the low-pressure volume to warm. However, the pressure within the system can also decrease due to the refrigerant accumulating within the system. This third effect is greater than the first two, so as the system cools, the refrigerant pressure within the system decreases, which is particularly pronounced at low temperatures. Thus, the compressor during cooldown generally experiences a pressure drop, and the power consumption of the compressor also decreases. In some examples, the control circuit increases the operating frequency of the compressor in response to detecting this power decrease in order to maintain the power consumption of the compressor near a threshold value, and in some cases within 10% of the threshold value.

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

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

[0026] In some embodiments, the refrigerant includes helium. In some embodiments, the refrigeration unit is configured to cool to 80 K, preferably to 50 K, more preferably to less than 10 K, and even more preferably to 4 K.

[0027] The refrigeration unit can be a cryogenic refrigeration unit capable of cooling down to 80K, and in some embodiments, can be a particularly cryogenic refrigeration unit capable of cooling down to 4K. The effect of the accumulation of the refrigerant in the refrigeration unit and the corresponding pressure drop in the refrigeration system is particularly significant when the refrigeration unit operates at a very low temperature. Therefore, the embodiments are particularly effective for such refrigeration systems.

[0028] In some embodiments, the initial pressure of the refrigerant is 5%, preferably 10% higher than the pressure defined for a refrigeration unit without power control of the variable speed compressor during cooldown.

[0029] When there is no power control of the compressor during cooldown, generally the compressor is driven at a constant speed, and the speed / pressure of the refrigerant is set so that the compressor does not overheat during cooldown with peak power consumption. When there is power control during cooldown, the power consumption during cooldown is controlled and is no longer a limiting factor, so the refrigerant pressure can be increased.

[0030] In some embodiments, the refrigeration system further comprises a buffer volume for the refrigerant, and the buffer volume includes a volume exceeding 20%, preferably exceeding 50%, and in some cases exceeding 90% of the total amount of the refrigerant in the refrigeration system.

[0031] The refrigeration system can optionally additionally contain 20% by volume or mass of the refrigerant, and in other cases can additionally contain more than 50% or 90% by volume of the refrigerant. The refrigeration system can be configured to operate with a predetermined amount of refrigerant, and when operating at the maximum operating speed of the variable speed compressor from startup, the power threshold consumed by the compressor is not exceeded. This is the standard amount of refrigerant for the refrigeration system, and embodiments providing power control of the compressor can provide an additional amount of refrigerant either by increasing the initial filling pressure and / or by supplying with a buffer volume associated with the low pressure line, and the buffer volume changes the volume ratio between the high pressure volume and the low pressure volume.

[0032] The refrigeration system can be defined to operate with a refrigerant having an initial filling pressure of 13 - 17 bar. When defined at the lower limit of this range, the pressure can be increased by overfilling the refrigerant and increasing the filling pressure within the system. This is acceptable in the refrigeration system according to the embodiment because the power supply of the compressor is controlled to be maintained below a threshold value. When the system is supplied with refrigerant at the upper limit of this range, the compressor may not be configured to operate at a pressure higher than the defined pressure. In such a case, a buffer volume in the low - pressure line that changes the high - to - low pressure volume ratio can be used to supply additional refrigerant. In some cases, it is possible to combine increasing the pressure of the refrigerant and adding a buffer volume.

[0033] In some embodiments, the control circuit is further configured to control the variable - speed compressor in a power - save mode to maintain the power consumption of the variable - speed compressor below a predetermined reduced threshold value.

[0034] Also, the control circuit can be configured to operate in a power - save mode that sets a reduced threshold value for the power of the compressor in response to a determination that the load of the refrigeration system is decreasing during steady - state operation. The same control circuit used for cooling can control this load reduction, i.e., the power consumption in the power - save mode.

[0035] A second aspect provides a cryopump including the cryogenic refrigeration system according to the first aspect.

[0036] A third aspect provides a method of operating an ultra-low temperature refrigeration system comprising a refrigeration unit including an expansion unit and a variable speed compressor configured to compress a refrigerant, wherein a higher pressure refrigerant is supplied to the refrigeration unit and a lower pressure refrigerant is received from the refrigeration unit. The method includes operating the variable speed compressor at an initial reduced frequency during an initial cool-down of the refrigeration unit to maintain the power consumed by the variable speed compressor below a predetermined threshold, and then increasing the operating frequency of the variable speed compressor to a maximum operating speed.

[0037] In some embodiments, the method further includes 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 in the system such that operation of the variable speed compressor at the maximum operating frequency during cool-down exceeds a predetermined threshold for power consumption, or providing an additional refrigerant buffer volume in fluid communication with the low pressure line.

[0038] It should be noted that since the efficiency of the system generally improves as the rotational speed of the compressor decreases, by overfilling the refrigeration system with refrigerant, it is possible to operate efficiently at a low rotational speed during cool-down. When the pressure of the refrigerant decreases at a low temperature due to the accumulation of the refrigerant in the refrigeration unit or the cold head, the rotational speed of the compressor can be increased to maintain the cooling efficiency.

[0039] Further specific preferred aspects are set forth in the independent and dependent claims. The features of the dependent claims can be combined with the features of the independent claims as appropriate and in combinations other than those explicitly defined in the claims.

[0040] When a feature of an apparatus is described as being operable to provide a certain function, it is to be understood that this includes a feature of the apparatus that provides or is adapted or configured to provide that function. Embodiments of the present invention are described in detail below with reference to the accompanying drawings.

Brief Description of the Drawings

[0041]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0042] Before explaining the embodiments in detail, an overview will first be presented. The embodiments provide an ultra-low temperature cooling device. The embodiments can be used in an ultra-low temperature refrigeration system for a superconducting magnet / coil. These can be used in an ultra-low temperature condenser system and an ultra-low temperature pump system for the re-sublimation, condensation, solidification or deposition of a gas.

[0043] Such an ultra-low temperature device initially operates in a cool-down mode and then in a steady-state refrigeration mode. During the cool-down process when the refrigeration unit gets cold, the refrigerant inside the cooling part of the refrigeration unit, such as inside a cold head piston system, accumulates significantly. As a result, the high pressure and low pressure in the refrigeration system will decrease. This reduction in refrigerant leads to a decrease in the power consumption of the compressor. Therefore, without adjustment in the steady state at the cooled temperature, the compressor will operate with a reduced amount of refrigerant.

[0044] However, increasing the amount of refrigerant in the system to compensate for the refrigerant accumulated in the refrigerant unit in the steady state poses a risk of overloading the compressor during the initial cool-down. During the cool-down, there is a relatively short period when the pressure in the system is at its highest, and there is a risk of the compressor becoming thermally overloaded during this period.

[0045] Embodiments seek to address these competing issues and achieve improved cooling capacity in steady-state operation and an improved cool-down process for a refrigerator by using a control mode that controls the winding current and power of a compressor motor. A threshold power is set, and the motor is controlled to rotate at a frequency such that the power consumption is less than this threshold power.

[0046] This control mechanism can support both initial cool-down and restart during warm-up operation of the compressor, mismatches in cool-down of the cold head, restart of the warm-up system, and start-up of the system in the case of restart in an unsteady state.

[0047] By having this control mechanism, refrigerant can be added to the system. This can be done by increasing the filling pressure of the system. The pressure of the refrigerant in the refrigeration unit or cold head connected to the compressor is made constant by initial filling. At the start of a cryosystem (switching on the drive part of the cold head refrigerant valve and then switching on the compressor), the cold head is typically cooled within 20 to 60 minutes. This cool-down process is affected by three factors that affect the refrigerant pressure in the system, namely 1) Pressure increase due to the temperature rise of the compressor volume, which consists of the scroll volume and the oil pre-separator volume, up to an operating temperature of 60°C, 2) Pressure increase due to the temperature rise of the low-pressure refrigerant volume flow caused by the heat load of the cold head (rise in refrigerant temperature from 20°C to 50°C), 3) Pressure drop in the system due to refrigerant accumulation at the low-temperature end of the cold head, and is affected by these.

[0048] Effect 3) is significantly greater than effects 1) and 2), and in particular for low-temperature applications causes a lower "equivalent" refrigerant system pressure throughout the system. As a result, both the high pressure and the low pressure decrease, the performance of the cold head is limited, and furthermore, the compressor cannot obtain the full potential of the refrigerant mass flow because the suction pressure is too low.

[0049] To increase the recirculating refrigerant mass flow and adjust the system to increase the differential pressure, the speed of the compressor can be increased (e.g., from 50 Hz to 70 Hz). It is known that the efficiency of the compressor decreases as the speed increases. The reason is that as the speed of the compressor increases, the energy dissipation increases, the gas flow rate in the piping and the compressor housing rises, leading to gas pressure loss and causing a decrease in the suction pressure. Therefore, it may not be optimal to operate the compressor at the maximum possible speed with a predetermined differential pressure as the target. Instead, according to this, increasing the refrigerant charge by increasing the filling pressure can enable both performance improvement and efficiency improvement.

[0050] In a conventional system, overfilling is not possible because in a compressor that can be a scroll pump, a larger motor current is required (to deliver a higher density refrigerant), which leads to overheating of the winding (which may lead to shutdown by a motor protection switch).

[0051] In the embodiment, the problem of compressor temperature switch-off is addressed by the power control of the compressor, which can be adjusted to the maximum allowable current of the motor winding that cannot lead to too high a winding temperature. The power control will affect the frequency of the compressor drive unit (motor). That is, a substantially constant level of power consumption will initially result in a compressor motor speed below the rated speed of the compressor motor, but this effect is compensated in some embodiments by increasing the refrigerant filling pressure. This feature enables the specified refrigerant pressure overfilling of the system without the risk of temperature switch-off or overload. With such power control, the cool-down process will start at a medium or low compressor speed that supplies the required differential pressure and the refrigerant flow rate required for the operation of the cold head.

[0052] Additionally and / or alternatively, this effect can be compensated for by providing a predetermined buffer volume of refrigerant associated with the low-pressure line. In this regard, the filling pressure and mass flow rate are not only affected by the actual filling pressure of the stopped system, but can also be similarly affected by creating a permanent excess of low-pressure volume, which causes the mass of the refrigerant to move to a smaller high-pressure volume, resulting in an increase in the overall system pressure during the operating operation. This option may be necessary to address the low-pressure strength limitations of the compressor components.

[0053] In summary, when additional refrigerant such as helium is used, the reduction in differential pressure between the high pressure (supply side) and low pressure (return side) is compensated for with respect to the performance of the cold head by providing a high helium pressure when the system is initially filled, which in both cases results in an increase in helium flow rate, and / or by providing a buffer volume of helium connected to the low-pressure line.

[0054] FIG. 1 shows a cryogenic system according to an embodiment. The cryogenic refrigeration system includes a control circuit 10 configured to control a motor 20 that drives a compressor 30 that compresses refrigerant within the refrigeration system. Further, the control circuit 10 controls a motor 70 that drives an inlet valve (not shown) that controls the supply of high-pressure refrigerant to the refrigeration unit or cold head 40. In this embodiment, there is a temperature sensor 60 associated with the low-temperature portion 50 of the cold head.

[0055] The control circuit 10 includes a data storage unit that stores a threshold value for setting the maximum value of the power and / or current supplied to the compressor.

[0056] In this embodiment, the refrigeration unit 40 includes a Gifford-McMahon type refrigerator unit (hereinafter, this Gifford-McMahon type piston system is referred to as a "cold head"), and the compressor 30 supplies pressurized helium as a cryogenic agent or refrigerant. This system is a closed system having a closed refrigerant cycle. In order to improve the performance of the cryogenic system, a control system 10 is provided, which enables measurement and adjustment of the variable frequency drive output current and / or power supplied to the compressor motor 20.

[0057] By maintaining the power consumption of the compressor 30 below the threshold value, the refrigeration system can be set at a high initial filling pressure of the refrigerant because it is no longer limited by the power consumed by the compressor operating at maximum speed during the cool-down process.

[0058] In this embodiment, the compressor motor 20 is directly connected to a variable frequency drive (VFD) 25 to supply power to the compressor and is controlled by the control circuit 10. This current and / or power control will affect the output frequency of the VFD and thus the rotational speed of the compressor motor. This control circuit 10 attempts to adjust the rotational speed of the compressor to approach the maximum allowable electrical load of the windings of the motor 20 without the risk of temperature switch-off.

[0059] The system operates as follows. (Startup procedure) 1) Switch on the cold head 40 and the compressor 30. 2) The VFD 25 increases the output frequency of the power supplied to the compressor motor 20 until the threshold values of the winding current and / or power are reached. In this way, the control circuit 10 limits the output frequency of the VFD 25. 3) During the heat-up of the compressor cold head system, the most critical load situation is passed at a limited frequency by the control of the VFD 25, which limits the rotational speed of the compressor 30 and results in a lower winding current and / or power. 4) As the cooldown of the cold head 40 progresses, helium accumulates in the cold head 40 more and more. This will result in a decrease in the system pressure. The VFD 25 will result in a controlled increase in the current supplied to the motor and / or the output frequency of the power. 5) Steady operation is achieved. The current and / or power control notifies the VFD to release the maximum possible frequency of the VFD to the motor 20. At this point, the control of the system can shift to the control of the inlet valve by the stepping motor 70, which can be controlled to provide a desired differential pressure between the supply pressure line and the return pressure line.

[0060] Surprisingly, it has been found that in order to maximize the cooling capacity of the cold head in the steady state, it is not necessary to operate the compressor at the highest possible rotational speed. Rather, by using a system configuration that limits the output frequency of the VFD compared to the grid frequency while maintaining the full winding current / power, an increase in the cooling output in steady operation can be measured. A preferred operating point has been found between the mass flow rate of the recirculating helium and the differential pressure between the supply pressure and the return pressure in the system. At this point, the power consumption depends on the frequency / speed of the compressor, as well as the suction pressure and the high pressure. By combining the control based on the pressure in the system and the speed of the compressor, a preferred operating point can be found. The reason may be that at a higher speed of the compressor, the energy dissipation in the system, which will immediately cause a performance loss, increases. Therefore, it may not be desirable to operate the compressor at the maximum possible speed allowed by a specific threshold power and the maximum possible differential pressure in the system, and by appropriate measurement and control, a preferred operating point at a lower frequency can be found.

[0061] In some embodiments, this current / power control system 10 can be used to limit the energy demand of a cryogenic system in a steady state under partial load conditions. In this case, additional set points or thresholds corresponding to lower winding current / power can be used. In this way, the same control circuit can be used to support an energy-saving mode.

[0062] FIG. 2 shows the operation of an embodiment compared to a conventional refrigeration system. The left graph shows the operation of the conventional system, and the right graph shows the operation of a refrigeration system according to an embodiment.

[0063] The upper left graph shows that the pressure of the conventional system is low in the initial state before the switch is turned on, and after the switch is turned on, both the high-pressure line and the low-pressure line have a pressure that gradually decreases as the temperature drops and the refrigerant accumulates in the coldest part of the system. In the system according to an embodiment, the initial refrigerant pressure is higher because the system is "overfilled". Since the speed of the compressor is limited at the time of switching on, the high-pressure line and the low-pressure line take time to reach a steady state. Once the pressure is reached, it is maintained and, due to the initial overfilling, becomes higher than the high pressure of the conventional system.

[0064] The second set of graphs shows that 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 compressor power after startup decreases over time. The right graph shows how the power supplied to the compressor decreases at the beginning of the cooldown, which corresponds to a lower compressor speed. The power gradually increases and reaches the maximum power that can be maintained when the system reaches a steady state. This maximum power that the compressor can safely handle is 8.3 KW in any embodiment, but through the control of the compressor during cooldown in one embodiment, this high power can be applied to the compressor in a steady state without overloading the compressor during cooldown.

[0065] The third pair of graphs shows how the operating frequency of the compressor changes in a conventional refrigeration system and in a refrigeration system of one embodiment. The refrigeration system of one embodiment has an initially low compressor operating frequency, which increases to the maximum speed or nearly the maximum speed when the cool-down is complete. The prior art compressor has a single speed.

[0066] The last two graphs show how one embodiment provides an improved cold head output by an increase in the mass flow rate of the refrigerant that can be provided by overfilling the system or providing additional volume.

[0067] Figure 3 shows an alternative embodiment of a refrigeration system 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 in the low-pressure line 62 and an additional pressure sensor 67 for sensing the pressure in the high-pressure line 64. In some embodiments, a differential pressure sensor can be used instead of the high-pressure and low-pressure sensors.

[0068] During operation, the control circuit 10 controls the compressor speed by controlling the rotational frequency of the motor driving the compressor depending on a power threshold, and also controls the rotational frequency of an inlet valve for supplying high-pressure refrigerant from the high-pressure refrigerant line 64 to the refrigeration unit or cold head 40.

[0069] The operation of the refrigeration system starts with a compressor that is initially driven at a low frequency while the refrigerant pressure is high, and the operating frequency is set by the threshold power limit of the compressor motor controlled by the control circuit 10. When the refrigerant accumulates in the low-temperature part of the refrigeration unit 40 and the refrigerant pressure drops, the power used by the compressor also drops, and the control circuit detects this and increases the frequency and rotational speed, which causes an increase in the motor winding current and power consumption, and the power consumption is maintained near the threshold level, within 2% of the threshold level in some embodiments, within 5% in other embodiments, and within 10% in still other embodiments. In this way, when the refrigerant pressure drops, the speed of the motor increases, but the power consumed by the motor is maintained near but below the threshold. When the system reaches a steady state, the compressor operates at its maximum frequency or near it, and the refrigeration system can be controlled by the control circuit 10 that controls the inlet valve of the cold head 40. The control of the inlet valve also controls the differential pressure of the refrigeration system that affects the cooling of the refrigeration system in the same way. In this regard, the compressor can operate effectively within specific high-pressure and low-pressure limits, which can be defined by a scroll performance map that sets the limit values of the high-pressure and low-pressure lines within which the compressor can operate effectively in the case of a scroll compressor. In some embodiments, the control unit 10 can be used to control the operating speed of the compressor to ensure not only that the maximum power consumption is not exceeded but also that the high-pressure and low-pressure limits of the scroll performance map are not exceeded. Therefore, the control unit 10 receives signals from the pressure sensors 65 and 67 and, in response to indicating that the pressure in the high-pressure line and / or low-pressure line is approaching either limit value, the control unit can modify the operating speed of the compressor to keep the pressure away from these limit values. In some embodiments, the control unit 10 can control one or both of the inlet valve and the speed of the compressor to maintain the operation of the compressor within the desired pressure limits.

[0070] In this embodiment, there is a low-pressure buffer volume 80, which enables 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 effectively. In this way, this embodiment adjusts the volume ratio between high pressure and low pressure and increases the amount of refrigerant within the system without increasing the filling pressure. This is an alternative to increasing the filling pressure of the refrigerant (however, it can be used in combination with this option).

[0071] Both options for increasing the amount of refrigerant, namely the option of increasing the filling pressure and the option of providing an additional buffer volume, can be used to adjust the operating conditions of the refrigeration unit to enhance the cooling capacity and / or efficiency of the system. The technical limit for enhancing the cooling capacity is the power consumption of the compressor where a high current flows through the windings of the compressor motor. If this becomes too high, the temperature protection switch (circuit breaker) will operate and the compressor motor will stop. This needs to be avoided for the safe and stable operation of the system.

[0072] FIG. 4 is a flowchart showing the steps of a method according to an embodiment. Startup occurs in step S0, which can be an initial startup or a startup after interruption of the refrigeration cycle. In step S10, the compressor motor is powered to operate at a first initial reduced operating frequency. The power consumed by the motor can be continuously evaluated, as indicated in steps D5 and D15. If it is below the threshold Pt and exceeds a certain amount ΔP, the rotational frequency of the motor is increased in step S20. If it exceeds the threshold Pt, the rotational speed is decreased in step S30. If it is within the required range, the rotational speed is not changed. After the rotational frequency has increased to approximately the maximum rotational frequency or near it in step S20, it is determined in step D25 whether the cooling process is complete. If it is complete, steady operation is started.

[0073] Exemplary embodiments of the present invention are disclosed in detail herein with reference to the accompanying drawings, but the present invention is not limited to the exact embodiments, and it should be understood that various changes and modifications can be made therein by those skilled in the art without departing from the scope of the present invention defined by the appended claims and their equivalents.

Description of Reference Numerals

[0074] 10 Control circuit 20 Compressor motor 25 Variable frequency drive 30 Compressor 40 Refrigeration unit or cold head 50 Low temperature point of cold head 62 Low pressure line 64 High pressure line 65, 67 Pressure sensors 80 Buffer volume

Claims

1. A cryogenic refrigeration system, comprising a refrigeration unit including an expansion unit, a variable-speed compressor configured to compress a refrigerant, the variable-speed compressor receiving the refrigerant from the refrigeration unit via a low-pressure line and supplying the compressed refrigerant to the refrigeration unit via a high-pressure line, a control circuit configured to control the variable-speed compressor to maintain power consumption of the variable-speed compressor below a predetermined threshold, and during cooldown, the control circuit controls the variable-speed compressor such that the variable-speed compressor operates at an initial reduced operating frequency and then increases the operating frequency so that the variable-speed compressor operates at a higher frequency, wherein additional refrigerant is supplied by increasing an initial filling pressure such that if the variable-speed compressor is operated at full speed during cooldown of the refrigeration unit, the variable-speed compressor may consume power exceeding the predetermined threshold. Cryogenic refrigeration system.

2. The cryogenic refrigeration system according to claim 1, wherein the control circuit is further configured to control the variable-speed compressor to maintain pressures in the high-pressure line and the low-pressure line within a predetermined limit value.

3. The cryogenic refrigeration system according to claim 1 or 2, wherein the control circuit is configured to increase the operating frequency of the variable-speed compressor in response to a detected decrease in power consumption of the variable-speed compressor.

4. The cryogenic refrigeration system according to any one of claims 1 to 3, wherein the initial reduced operating frequency of the compressor is less than 70% of a maximum operating frequency during steady-state operation of the refrigeration unit.

5. The cryogenic refrigeration system according to any one of claims 1 to 4, wherein the maximum operating frequency during steady-state operation is between 50 and 70 Hz, and the initial reduced operating frequency during cooldown is lower.

6. The cryogenic refrigeration system according to any one of claims 1 to 5, wherein the refrigerant contains helium.

7. The cryogenic refrigeration system according to any one of claims 1 to 6, wherein the refrigeration unit is configured to cool to below 80K.

8. The initial pressure of the refrigerant is 5% higher than the pressure established for the refrigeration unit without power control of the variable speed compressor during cooldown, the cryogenic refrigeration system according to any one of claims 1 to 7.

9. The refrigeration system further includes the buffer volume of the refrigerant, and the buffer volume includes a volume exceeding 20% of the total amount of the refrigerant in the refrigeration system, the cryogenic refrigeration system according to any one of claims 1 to 7.

10. The control circuit is further configured to control the variable speed compressor in a power save mode to maintain the power consumption of the variable speed compressor below a predetermined reduced threshold, the cryogenic refrigeration system according to any one of claims 1 to 9.

11. A cryogenic pump comprising the cryogenic refrigeration system according to any one of claims 1 to 10.

12. A method of operating a cryogenic refrigeration system, comprising a refrigeration unit including an expansion unit and a variable speed compressor configured to compress a refrigerant, wherein a higher pressure refrigerant is supplied to the refrigeration unit and a lower pressure refrigerant is received from the refrigeration unit, during an initial cooldown of the refrigeration unit, operating the variable speed compressor at an initial reduced operating frequency to maintain the power consumed by the variable speed compressor below a predetermined threshold; subsequently, increasing the operating frequency of the variable speed compressor to a maximum operating speed; including, the method including an initial step of supplying an increased amount of refrigerant to the refrigeration system by increasing the filling pressure of the refrigerant in the cryogenic refrigeration system such that the operation of the variable speed compressor at the maximum operating frequency during cooldown exceeds the predetermined threshold regarding power consumption. Method.

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