Patent Description:
A cryocooler is used to cool various target objects such as superconducting equipment, measuring equipment, and samples used in a cryogenic environment (for example, <CIT> and <CIT>).

In order to cool a target object with a cryocooler, first, the cryocooler must be started and the cryocooler must be cooled from the initial temperature to the target cryogenic temperature. The initial cooling of such a cryocooler is also referred to as a cooldown. Since the initial cooling is merely a preparation for starting the cooling of the target object, it is desired that the required time is as short as possible.

It is desirable to shorten the initial cooling time of the cryocooler.

According to an aspect of the present invention, there is provided a method for operating a cryocooler as defined in claim <NUM>. A cryocooler includes a first compressor, an expander, and a high pressure line and a low pressure line connecting the first compressor to the expander. The method includes: connecting a second compressor in series with a first compressor on a high pressure line or a low pressure line; connecting a buffer volume to the low pressure line via a supply valve; executing initial cooling for cooling an expander from an initial temperature to a cryogenic temperature in a state where the second compressor and the buffer volume are connected to the cryocooler; and executing a steady operation of maintaining the expander at the cryogenic temperature after the initial cooling. The execution of the initial cooling includes supplying a working gas to the expander by using the first compressor and the second compressor, and controlling the supply valve to keep a pressure of the high pressure line within a preset appropriate pressure range based on the measured pressure of the high pressure line.

According to still another aspect of the present invention, there is provided a cryocooler as defined in claim <NUM>. It includes: an expander capable of executing initial cooling for cooling from an initial temperature to a cryogenic temperature and a steady operation of maintaining the cryogenic temperature after the initial cooling; a high pressure line and a low pressure line connected to the expander; a first pressure sensor that measures a pressure of the high pressure line; a second pressure sensor that measures a pressure of the low pressure line; a buffer volume for storing a working gas; a supply valve that connects the buffer volume to the low pressure line; and a controller that controls the supply valve to keep the pressure of the high pressure line within a preset appropriate pressure range based on the pressure of the high pressure line measured by the first pressure sensor during the initial cooling. The controller discontinues control of the supply valve based on the pressure of the high pressure line when the pressure of the low pressure line measured by the second pressure sensor falls below a preset low pressure threshold, and controls the supply valve to restore the pressure of the low pressure line to the low pressure threshold based on the pressure of the low pressure line measured by the second pressure sensor.

Any combination of the components described above and any replacement of the components and expressions of the present invention between methods, devices, systems, and the like are also effective. The scope of the present invention is only defined by the appended claims.

According to the present invention, the initial cooling time of the cryocooler can be shortened.

Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. In the description and drawings, the same or equivalent components, members, and processes will be assigned with the same reference symbols, and redundant description thereof will be omitted as appropriate. The scales and shapes of each illustrated part are set for convenience in order to make the description easy to understand, and are not to be understood as limiting unless stated otherwise. The embodiments are merely examples and do not limit the scope of the present invention. All characteristics and combinations to be described in the embodiment are not necessarily essential to the invention.

<FIG> and <FIG> are views schematically illustrating a cryocooler <NUM> according to a first embodiment. As an example, the cryocooler <NUM> is a two-stage Gifford-McMahon (GM) cryocooler. <FIG> schematically illustrates a compressor <NUM> and an expander <NUM> constituting the cryocooler <NUM> together with a control device <NUM>, and <FIG> illustrates the internal structure of the expander <NUM> of the cryocooler <NUM>.

The compressor <NUM> is configured to collect the working gas of the cryocooler <NUM> from the expander <NUM>, pressurize the collected working gas, and supply the working gas to the expander <NUM> again. The compressor <NUM> and the expander <NUM> constitute a refrigeration cycle of the cryocooler <NUM>, whereby the cryocooler <NUM> can provide desired cryogenic cooling. The expander <NUM> is also referred to as a cold head. The working gas is also referred to as a refrigerant gas and is usually a helium gas, but other suitable gas may be used. For the sake of understanding, the flow direction of the working gas is illustrated by an arrow in <FIG>.

In general, the pressure of the working gas supplied from the compressor <NUM> to the expander <NUM> and the pressure of the working gas collected from the expander <NUM> to the compressor <NUM> are both considerably higher than the atmospheric pressure, and are referred to as the first high pressure and the second high pressure, respectively. For convenience of description, the first high pressure and the second high pressure are also simply referred to as high pressure and low pressure, respectively. Typically, the high pressure is, for example, <NUM> to <NUM> MPa. The low pressure is, for example, <NUM> to <NUM> MPa, for example, approximately <NUM> MPa. For the sake of understanding, the flow direction of the working gas is indicated by an arrow.

The expander <NUM> includes a cryocooler cylinder <NUM> and a displacer assembly <NUM>. The cryocooler cylinder <NUM> guides the linear reciprocating motion of the displacer assembly <NUM>, and forms expansion chambers (<NUM>, <NUM>) for the working gas with the displacer assembly <NUM>. In addition, the expander <NUM> includes a pressure switching valve <NUM> that determines an intake start timing of the working gas into the expansion chamber and an exhaust start timing of the working gas from the expansion chamber.

In this specification, in order to describe the positional relationship between the components of the cryocooler <NUM>, for convenience, the side close to the top dead center of the axial reciprocation of the displacer is "upper", and the side close to the bottom dead center is "lower". The top dead center is the position of the displacer where the volume of the expansion space is maximum, and the bottom dead center is the position of the displacer where the volume of the expansion space is the minimum. Since a temperature gradient is generated in which the temperature drops from the upper side to the lower side in the axial direction during the operation of the cryocooler <NUM>, the upper side can be referred to as a high temperature side and the lower side can be referred to as a low temperature side.

The cryocooler cylinder <NUM> has a first cylinder 16a and a second cylinder 16b. As an example, the first cylinder 16a and the second cylinder 16b are members having a cylindrical shape, and the second cylinder 16b has a diameter smaller than that of the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are coaxially disposed, and a lower end of the first cylinder 16a is rigidly connected to an upper end of the second cylinder 16b.

The displacer assembly <NUM> includes a first displacer 18a and a second displacer 18b connected to each other, and these move integrally. As an example, the first displacer 18a and the second displacer 18b are members having a cylindrical shape, and the second displacer 18b has a diameter smaller than that of the first displacer 18a. The first displacer 18a and the second displacer 18b are disposed coaxially with each other.

The first displacer 18a is accommodated in the first cylinder 16a, and the second displacer 18b is accommodated in the second cylinder 16b. The first displacer 18a can reciprocate in the axial direction along the first cylinder 16a, and the second displacer 18b can reciprocate in the axial direction along the second cylinder 16b.

As illustrated in <FIG>, the first displacer 18a accommodates a first regenerator <NUM>. The first regenerator <NUM> is formed by filling a tubular main body portion of the first displacer 18a with a wire mesh such as copper or other appropriate first regenerator material. The upper lid portion and the lower lid portion of the first displacer 18a may be provided as members separate from the main body portion of the first displacer 18a, the upper lid portion and the lower lid portion of the first displacer 18a may be fixed to the main body by appropriate means such as fastening or welding, and accordingly, the first regenerator material may be accommodated in the first displacer 18a.

Similarly, the second displacer 18b accommodates a second regenerator <NUM>. The second regenerator <NUM> is formed by filling a tubular main body portion of the second displacer 18b with a non-magnetic regenerator material such as bismuth, a magnetic regenerator material such as HoCu<NUM>, or other appropriate second regenerator material. The second regenerator material may be formed in a granular shape. The upper lid portion and the lower lid portion of the second displacer 18b may be provided as members separate from the main body portion of the second displacer 18b, the upper lid portion and the lower lid portion of the second displacer 18b may be fixed to the main body by appropriate means such as fastening or welding, and accordingly, the second regenerator material may be accommodated in the second displacer 18b.

The displacer assembly <NUM> forms a room temperature chamber <NUM>, a first expansion chamber <NUM>, and a second expansion chamber <NUM> inside the cryocooler cylinder <NUM>. The expander <NUM> includes a first cooling stage <NUM> and a second cooling stage <NUM> for heat exchange with a desired object or medium to be cooled by the cryocooler <NUM>. The room temperature chamber <NUM> is formed between the upper lid portion of the first displacer 18a and the upper portion of the first cylinder 16a. The first expansion chamber <NUM> is formed between the lower lid portion of the first displacer 18a and the first cooling stage <NUM>. The second expansion chamber <NUM> is formed between the lower lid portion of the second displacer 18b and the second cooling stage <NUM>. The first cooling stage <NUM> is fixed to the lower portion of the first cylinder 16a to surround the first expansion chamber <NUM>, and the second cooling stage <NUM> is fixed to the lower portion of the second cylinder 16b to surround the second expansion chamber <NUM>.

The first regenerator <NUM> is connected to the room temperature chamber <NUM> through a working gas flow path 36a formed in the upper lid portion of the first displacer 18a, and is connected to the first expansion chamber <NUM> through a working gas flow path 36b formed in the lower lid portion of the first displacer 18a. The second regenerator <NUM> is connected to the first regenerator <NUM> through a working gas flow path 36c formed from the lower lid portion of the first displacer 18a to the upper lid portion of the second displacer 18b. In addition, the second regenerator <NUM> is connected to the second expansion chamber <NUM> through a working gas flow path 36d formed in the lower lid portion of the second displacer 18b.

The working gas flow between the first expansion chamber <NUM>, the second expansion chamber <NUM>, and the room temperature chamber <NUM> is not the clearance between the cryocooler cylinder <NUM> and the displacer assembly <NUM>, but a first seal 38a and a second seal 38b may be provided to be guided to the first regenerator <NUM> and the second regenerator <NUM>. The first seal 38a may be mounted to the upper lid portion of the first displacer 18a to be disposed between the first displacer 18a and the first cylinder 16a. The second seal 38b may be mounted to the upper lid portion of the second displacer 18b to be disposed between the second displacer 18b and the second cylinder 16b.

As illustrated in <FIG>, the expander <NUM> includes a cryocooler housing <NUM> that accommodates the pressure switching valve <NUM>. The cryocooler housing <NUM> is coupled to the cryocooler cylinder <NUM>, thereby forming a hermetic container that accommodates the pressure switching valve <NUM> and the displacer assembly <NUM>.

As illustrated in <FIG>, the pressure switching valve <NUM> includes a high pressure valve 40a and a low pressure valve 40b, and is configured to generate periodic pressure fluctuations in the cryocooler cylinder <NUM>. The working gas discharge port of the compressor <NUM> is connected to the room temperature chamber <NUM> via the high pressure valve 40a, and the working gas suction port of the compressor <NUM> is connected to the room temperature chamber <NUM> via the low pressure valve 40b. The high pressure valve 40a and the low pressure valve 40b are configured to selectively and alternately open and close (that is, when one is open, the other is closed).

The pressure switching valve <NUM> may take the form of a rotary valve. That is, the pressure switching valve <NUM> may be configured such that the high pressure valve 40a and the low pressure valve 40b are alternately opened and closed by the rotational sliding of the valve disc with respect to the stationary valve body. In that case, an expander motor <NUM> may be connected to the pressure switching valve <NUM> to rotate the valve disc of the pressure switching valve <NUM>. For example, the pressure switching valve <NUM> is disposed such that the valve rotation axis is coaxial with the rotation axis of the expander motor <NUM>.

Alternatively, the high pressure valve 40a and the low pressure valve 40b may be valves that can be individually controlled. In this case, the pressure switching valve <NUM> may not be connected to the expander motor <NUM>.

The expander motor <NUM> is connected to a displacer drive shaft <NUM> via a motion conversion mechanism <NUM> such as a Scotch yoke mechanism. The expander motor <NUM> is attached to the cryocooler housing <NUM>. The motion conversion mechanism <NUM> is accommodated in the cryocooler housing <NUM> similar to the pressure switching valve <NUM>. The motion conversion mechanism <NUM> converts the rotary motion output by the expander motor <NUM> into a linear reciprocating motion of the displacer drive shaft <NUM>. The displacer drive shaft <NUM> extends from the motion conversion mechanism <NUM> into the room temperature chamber <NUM>, and is fixed to the upper lid portion of the first displacer 18a. The rotation of the expander motor <NUM> is converted into an axial reciprocation of the displacer drive shaft <NUM> by the motion conversion mechanism <NUM>, and the displacer assembly <NUM> reciprocates linearly in the cryocooler cylinder <NUM> in the axial direction.

In addition, the expander <NUM> may include a temperature sensor <NUM> that measures the temperature of the second cooling stage <NUM> (and/or the first cooling stage <NUM>) and outputs a measured temperature signal indicating the measured temperature.

The compressor <NUM> includes a high pressure gas outlet <NUM>, a low pressure gas inlet <NUM>, a high pressure flow path <NUM>, a low pressure flow path <NUM>, a first pressure sensor <NUM>, a second pressure sensor <NUM>, a bypass line <NUM>, a compressor main body <NUM>, and a compressor housing <NUM>. The high pressure gas outlet <NUM> is installed in the compressor housing <NUM> as a working gas discharge port of the compressor <NUM>, and the low pressure gas inlet <NUM> is installed in the compressor housing <NUM> as a working gas suction port of the compressor <NUM>. The high pressure flow path <NUM> connects the discharge port of the compressor main body <NUM> to the high pressure gas outlet <NUM>, and the low pressure flow path <NUM> connects the low pressure gas inlet <NUM> to the suction port of the compressor main body <NUM>. The compressor housing <NUM> accommodates the high pressure flow path <NUM>, the low pressure flow path <NUM>, the first pressure sensor <NUM>, the second pressure sensor <NUM>, the bypass line <NUM>, and the compressor main body <NUM>. The compressor <NUM> is also referred to as a compressor unit.

The compressor main body <NUM> is configured to internally compress the working gas sucked from the suction port and discharge the working gas from the discharge port. For example, the compressor main body <NUM> may be a scroll type, a rotary type, or another pump for pressurizing the working gas. In this embodiment, the compressor main body <NUM> is configured to discharge a fixed and constant working gas flow rate. Alternatively, the compressor main body <NUM> may be configured to have a variable flow rate of the working gas to be discharged. The compressor main body <NUM> may be referred to as a compression capsule.

The first pressure sensor <NUM> is disposed in the high pressure flow path <NUM> to measure the pressure of the working gas flowing through the high pressure flow path <NUM>. The first pressure sensor <NUM> is configured to output a first measured pressure signal PH indicating the measured pressure. The second pressure sensor <NUM> is disposed in the low pressure flow path <NUM> to measure the pressure of the working gas flowing through the low pressure flow path <NUM>. The second pressure sensor <NUM> is configured to output a second measured pressure signal PL indicating the measured pressure. Therefore, the first pressure sensor <NUM> and the second pressure sensor <NUM> can also be referred to as a high pressure sensor and a low pressure sensor, respectively. Further, in this specification, any one of the first pressure sensor <NUM> and the second pressure sensor <NUM>, or both of them may be collectively referred to as a "pressure sensor".

The bypass line <NUM> connects the high pressure flow path <NUM> to the low pressure flow path <NUM> to bypass the expander <NUM> and return the working gas from the high pressure flow path <NUM> to the low pressure flow path <NUM>. The bypass line <NUM> is provided with a relief valve <NUM> for opening and closing the bypass line <NUM> or controlling the flow rate of the working gas flowing through the bypass line <NUM>. The relief valve <NUM> is configured to open when a differential pressure equal to or higher than a set pressure acts between the inlet and outlet of the relief valve <NUM>. The relief valve <NUM> may be an on/off valve or a flow control valve, and may be an electromagnetic valve, for example. The set pressure can be appropriately set based on the empirical knowledge of the designer, an experiment or simulation by the designer, or the like. In this manner, it is possible to prevent the differential pressure between the high pressure line <NUM> and the low pressure line <NUM> from exceeding the set pressure and becoming excessive. In addition, it is possible to prevent the pressure of the high pressure line <NUM> from becoming excessive.

The relief valve <NUM> may be configured to work as a so-called safety valve, that is, may be mechanically opened when a differential pressure equal to or higher than a set pressure acts between the inlet and outlet. Alternatively, the relief valve <NUM> may be opened and closed under the control of the control device <NUM>. The control device <NUM> compares the differential pressure between the high pressure line <NUM> and the low pressure line <NUM> to be measured with the set pressure, the relief valve <NUM> may be controlled to open the relief valve <NUM> when the measured differential pressure is equal to or higher than the set pressure, and to close the relief valve <NUM> when the measured differential pressure is less than the set pressure. The control device <NUM> may acquire the measured differential pressures of the high pressure line <NUM> and the low pressure line <NUM> based on the first measured pressure signal PH from the first pressure sensor <NUM> and the second measured pressure signal PL from the second pressure sensor <NUM>. As another example, the control device <NUM> compares the measured pressure of the high pressure line <NUM> with the upper limit pressure based on the first measured pressure signal PH, and the relief valve <NUM> may be controlled to open the relief valve <NUM> when the measured pressure is equal to or higher than the upper limit pressure, and to close the relief valve <NUM> when the measured pressure is less than the upper limit pressure.

The compressor <NUM> may have various other components. For example, the high pressure flow path <NUM> may be provided with an oil separator, an adsorber, or the like. The low pressure flow path <NUM> may be provided with a storage tank or other components. Further, the compressor <NUM> may be provided with an oil circulation system that cools the compressor main body <NUM> with oil, a cooling system that cools the oil, or the like.

In addition, the cryocooler <NUM> includes a gas line <NUM> that circulates a working gas between the compressor <NUM> and the expander <NUM>. The gas line <NUM> includes the high pressure line <NUM> connecting the compressor <NUM> to the expander <NUM> to supply the working gas from the compressor <NUM> to the expander <NUM>, and the low pressure line <NUM> connecting the compressor <NUM> to the expander <NUM> to collect the working gas from the expander <NUM> to the compressor <NUM>. The cryocooler housing <NUM> of the expander <NUM> is provided with a high pressure gas inlet <NUM> and a low pressure gas outlet <NUM>. The high pressure gas inlet <NUM> is connected to the high pressure gas outlet <NUM> by a high-pressure pipe <NUM>, and the low pressure gas outlet <NUM> is connected to the low pressure gas inlet <NUM> by a low-pressure pipe <NUM>. The high pressure line <NUM> includes the high-pressure pipe <NUM> and the high pressure flow path <NUM>, and the low pressure line <NUM> includes the low-pressure pipe <NUM> and the low pressure flow path <NUM>. The bypass line <NUM> may be considered to be a part of the gas line <NUM>. The bypass line <NUM> connects the high pressure line <NUM> to the low pressure line <NUM> to bypass the expander <NUM> and return the working gas from the high pressure line <NUM> to the low pressure line <NUM>.

Therefore, the working gas collected from the expander <NUM> to the compressor <NUM> enters the low pressure gas inlet <NUM> of the compressor <NUM> from the low pressure gas outlet <NUM> of the expander <NUM> through the low-pressure pipe <NUM>, and further returns to the compressor main body <NUM> through the low pressure flow path <NUM>, is compressed by the compressor main body <NUM>, and is pressurized. The working gas supplied from the compressor <NUM> to the expander <NUM> exits from the high pressure gas outlet <NUM> of the compressor <NUM> through the high pressure flow path <NUM> from the compressor main body <NUM>, and further supplied to the expander <NUM> via the high-pressure pipe <NUM> and the high pressure gas inlet <NUM> of the expander <NUM>.

Furthermore, the cryocooler <NUM> includes a buffer volume <NUM>, a supply valve <NUM>, and a collection valve <NUM>. The buffer volume <NUM> is a volume for storing the working gas, and may be, for example, a buffer tank. The supply valve <NUM> connects the buffer volume <NUM> to the low pressure line <NUM>, and the collection valve <NUM> connects the buffer volume <NUM> to the high pressure line <NUM>. The supply valve <NUM> and the collection valve <NUM> may be an on/off valve or a flow control valve, and may be an electromagnetic valve, for example.

The pressure of the buffer volume <NUM> is a filling pressure of the working gas in the cryocooler <NUM> when the operation of the cryocooler <NUM> is stopped. When the cryocooler <NUM> is operating (for example, during initial cooling or steady operation), the pressure of the buffer volume <NUM> is a pressure intermediate between the pressure of the high pressure line <NUM> and the pressure of the low pressure line <NUM> (for example, average pressure of high pressure and low pressure).

Therefore, when the supply valve <NUM> is opened during the operation of the cryocooler <NUM>, the working gas is supplied from the buffer volume <NUM> to the low pressure line <NUM> through the supply valve <NUM>. When the supply valve <NUM> is closed, the supply of the working gas from the buffer volume <NUM> to the low pressure line <NUM> is stopped. Further, when the collection valve <NUM> is opened, the working gas is collected from the high pressure line <NUM> to the buffer volume <NUM> through the collection valve <NUM>. When the collection valve <NUM> is closed, collection of the working gas from the high pressure line <NUM> to the buffer volume <NUM> is stopped. By opening and closing the supply valve <NUM> and the collection valve <NUM> in this manner, the amount of the working gas circulating in the gas line <NUM> can be adjusted, and as a result, the pressures of the high pressure line <NUM> and the low pressure line <NUM> can also be controlled.

As illustrated in <FIG>, the control device <NUM> that controls the cryocooler <NUM> includes a controller <NUM> that controls the supply valve <NUM> and the collection valve <NUM>. The controller <NUM> is electrically connected to the first pressure sensor <NUM> and the second pressure sensor <NUM> to acquire the first measured pressure signal PH and the second measured pressure signal PL. As will be described later, the controller <NUM> receives the first measured pressure signal PH from the first pressure sensor <NUM>, and opens and closes the supply valve <NUM> and the collection valve <NUM> based on the measured pressure of the high pressure line <NUM> indicated by the first measured pressure signal PH. Further, the controller <NUM> is electrically connected to the temperature sensor <NUM> to acquire the measured temperature signal from the temperature sensor <NUM>.

In the illustrated example, the control device <NUM> is provided separately from the compressor <NUM> and the expander <NUM>, and is connected to these, but the present invention is not limited thereto. The control device <NUM> may be mounted on the compressor <NUM>. The control device <NUM> may be provided in the expander <NUM>, such as being mounted on the expander motor <NUM>. The controller <NUM> may be provided in the supply valve <NUM>, in the collection valve <NUM>, or in each of the supply valve <NUM> and the collection valve <NUM>.

The control device <NUM> is realized as a hardware configuration by elements or circuits such as a CPU or memory of a computer, and is realized by a computer program or the like as a software configuration. In <FIG>, these are drawn as functional blocks realized by their cooperation as appropriate. It is understood by those skilled in the art that the functional blocks can be realized in various forms by combining hardware and software.

When the compressor <NUM> and the expander motor <NUM> are operated, the cryocooler <NUM> generates periodic volume fluctuations and pressure fluctuations of the working gas synchronized with the periodic volume fluctuations in the first expansion chamber <NUM> and the second expansion chamber <NUM>. Typically, in the intake process, the low pressure valve 40b is closed and the high pressure valve 40a is opened, whereby the high pressure working gas flows from the compressor <NUM> into the room temperature chamber <NUM> through the high pressure valve 40a, is supplied to first expansion chamber <NUM> through the first regenerator <NUM>, and is supplied to the second expansion chamber <NUM> through the second regenerator <NUM>. In this manner, the first expansion chamber <NUM> and the second expansion chamber <NUM> are pressurized from the low pressure to the high pressure. At this time, the displacer assembly <NUM> is moved upward from the bottom dead center to the top dead center, and the volumes of the first expansion chamber <NUM> and the second expansion chamber <NUM> are increased. When the high pressure valve 40a is closed, the intake process ends.

In the exhaust process, the high pressure valve 40a is closed and the low pressure valve 40b is opened, whereby the high pressure first expansion chamber <NUM> and the second expansion chamber <NUM> are opened to the low pressure working gas suction port of the compressor <NUM>, and thus the working gas expands in the first expansion chamber <NUM> and the second expansion chamber <NUM>. As a result, the low pressure working gas is discharged from the first expansion chamber <NUM> and the second expansion chamber <NUM> to the room temperature chamber <NUM> through the first regenerator <NUM> and the second regenerator <NUM>. At this time, the displacer assembly <NUM> is moved downward from the top dead center to the bottom dead center, and the volumes of the first expansion chamber <NUM> and the second expansion chamber <NUM> are reduced. The working gas is collected from the expander <NUM> to the compressor <NUM> through the low pressure valve 40b. When the low pressure valve 40b is closed, the exhaust process ends.

In this manner, for example, a refrigeration cycle such as a GM cycle is configured, and the first cooling stage <NUM> and the second cooling stage <NUM> are cooled to a desired cryogenic temperature. The first cooling stage <NUM> can be cooled to a first cooling temperature in the range of, for example, approximately <NUM> to approximately <NUM>. The second cooling stage <NUM> can be cooled to a second cooling temperature (for example, approximately <NUM> to approximately <NUM>) lower than the first cooling temperature.

The cryocooler <NUM> can execute initial cooling and steady operation following the initial cooling. The initial cooling is an operation mode of the expander <NUM> that rapidly cools from the initial temperature to the cryogenic temperature when the cryocooler <NUM> is started, and the steady operation is an operation mode of the expander <NUM> that maintains a cryogenically cooled state by initial cooling. The initial temperature may be an ambient temperature (for example, room temperature). In addition, the initial cooling can also be performed when the cryocooler <NUM> is restarted after maintenance of a cryogenic device (for example, superconducting equipment such as a magnetic resonance imaging (MRI) system). During the maintenance, the object to be cooled in the cryogenic device may be kept at a relatively low temperature (for example, <NUM> to <NUM>) without being raised to an ambient temperature. In this case, the initial temperature may be such a low temperature.

The expander <NUM> is cooled to a standard cooling temperature by initial cooling, and is maintained within an allowable temperature range of cryogenic temperature including the standard cooling temperature in steady operation. The standard cooling temperature varies depending on the application and setting of the cryocooler <NUM>, but is typically approximately <NUM> or lower in, for example, a application for cooling a superconducting device. In some other cooling applications, the standard cooling temperature may be, for example, approximately <NUM> to <NUM>, or <NUM> or less. As described above, the initial cooling can also be referred to as a cooldown.

Incidentally, during the initial cooling, the density of the working gas increases in the expander <NUM> as the temperature is lowered from the initial temperature to the cryogenic temperature. Along with this, the amount of the working gas accumulated in the expander <NUM> increases, that is, the working gas is absorbed by the expander <NUM> from the gas line <NUM>. As a result, as the cooling of the expander <NUM> progresses, the pressure of the working gas circulating in the gas line <NUM> gradually decreases. Since a decrease in the pressure of the working gas causes a decrease in the cooling capacity of the cryocooler <NUM>, there is a concern that this may become a factor that lengthens the time required for the initial cooling. Since the initial cooling is merely a preparation because the cooling of the target object is started by the cryocooler, it is desired that the required time is as short as possible.

In order to cope with such a problem, in this embodiment, the controller <NUM> controls the supply valve <NUM> to keep the pressure of the high pressure line <NUM> within a preset appropriate pressure range based on the pressure of the high pressure line <NUM> measured by the first pressure sensor <NUM> during the initial cooling. More specifically, the controller <NUM> may compare the measured pressure of the high pressure line <NUM> with the lower limit value Pc of the appropriate pressure range during the initial cooling, and operate the supply valve <NUM> to repeatedly open and close the supply valve <NUM> such that the pressure of the high pressure line <NUM> does not fall below the lower limit value Pc.

Further, in this embodiment, the controller <NUM> controls the collection valve <NUM> to keep the pressure of the high pressure line <NUM> within an appropriate pressure range based on the pressure of the high pressure line <NUM> measured by the first pressure sensor <NUM> during the initial cooling. More specifically, the controller <NUM> may compare the measured pressure of the high pressure line <NUM> with an upper limit value Pd of the appropriate pressure range during the initial cooling, and operate the collection valve <NUM> to repeatedly open and close the collection valve <NUM> such that the pressure of the high pressure line <NUM> does not exceed the upper limit value Pd.

<FIG> is a flowchart for describing a method for controlling the cryocooler <NUM> according to the first embodiment. This method is repeatedly executed by the controller <NUM> in a predetermined cycle in the initial cooling of the cryocooler <NUM>. This method may be continuously executed not only during the initial cooling but also during the steady operation of the cryocooler <NUM>.

First, the pressure of the high pressure line <NUM> is measured (S10). The first pressure sensor <NUM> measures the pressure of the high pressure line <NUM>, and outputs the first measured pressure signal PH indicating the measured pressure of the high pressure line <NUM>. The controller <NUM> receives the first measured pressure signal PH and acquires the measured pressure of the high pressure line <NUM>.

Next, the measured pressure of the high pressure line <NUM> is compared with an appropriate pressure range (S12). The lower limit value Pc of the appropriate pressure range is set such that the cryocooler <NUM> provides a sufficient cooling capacity. The upper limit value Pd of the appropriate pressure range is set not to generate excessive pressure in the high pressure line <NUM>. The upper limit value Pd of the appropriate pressure range may be set to a pressure value smaller than the above-described set pressure at which the relief valve <NUM> is opened. The appropriate pressure range can be appropriately set based on the empirical knowledge of the designer, an experiment or simulation by the designer, or the like. The appropriate pressure range may be stored in advance in the controller <NUM> as an initial setting of the cryocooler <NUM>, or may be set in the controller <NUM> by the user before the cryocooler <NUM> is operated.

As an example, the upper limit value Pd and the lower limit value Pc of the appropriate pressure range may be selected from, for example, a range of <NUM> MPa to <NUM> MPa or a range of <NUM> MPa to <NUM> MPa. The width of the appropriate pressure range, that is, the difference between the upper limit value Pd and the lower limit value Pc of the appropriate pressure range may be set to a certain value within <NUM> MPa, <NUM> MPa, or <NUM> MPa, for example. For example, the appropriate pressure range may be set to <NUM> ± <NUM> MPa. In this case, the width of the appropriate pressure range is <NUM> MPa, the upper limit value Pd is <NUM> MPa, and the lower limit value Pc is <NUM> MPa.

The controller <NUM> compares the measured pressure of the high pressure line <NUM> with the lower limit value Pc of the appropriate pressure range, and opens the supply valve <NUM> when the measured pressure of the high pressure line <NUM> falls below the lower limit value Pc (PH < Pc) (S14). In this manner, the working gas is supplied from the buffer volume <NUM> to the low pressure line <NUM> through the supply valve <NUM>. Since the amount of the working gas circulating in the gas line <NUM> increases, the pressure in the high pressure line <NUM> is restored.

The controller <NUM> closes the supply valve <NUM> when the measured pressure of the high pressure line <NUM> is restored to an appropriate pressure range (S16). For example, the controller <NUM> may compare the measured pressure of the high pressure line <NUM> with the lower limit value Pc of the appropriate pressure range, and close the supply valve <NUM> when the measured pressure of the high pressure line <NUM> exceeds the lower limit value Pc (PH > Pc or PH ≥ Pc). When the supply valve <NUM> is closed, the supply of the working gas from the buffer volume <NUM> to the low pressure line <NUM> is stopped. In this manner, the present method ends, and is executed again in the next control cycle.

The pressure threshold for closing the supply valve <NUM> may be different from the lower limit value Pc of the appropriate pressure range, and may be larger than the lower limit value Pc, for example. The pressure threshold may be set not to exceed the upper limit value Pd of the appropriate pressure range. For example, the pressure threshold may be a value obtained by adding a predetermined ratio of a width of an appropriate pressure range (upper limit value Pd - lower limit value Pc) to the lower limit value Pc. For example, the predetermined ratio may be <NUM>% or less, <NUM>% or less, or <NUM>% or less.

<FIG> is a flowchart for describing a method for controlling the cryocooler <NUM> according to the first embodiment. This method is repeatedly executed by the controller <NUM> in a predetermined cycle in the initial cooling of the cryocooler <NUM>. This method may be executed in parallel with the method illustrated in <FIG>. This method may be continuously executed not only during the initial cooling but also during the steady operation of the cryocooler <NUM>.

First, the pressure of the high pressure line <NUM> is measured using the first pressure sensor <NUM> (S20). The controller <NUM> receives the first measured pressure signal PH from the first pressure sensor <NUM>, and acquires the measured pressure of the high pressure line <NUM>.

Next, the measured pressure of the high pressure line <NUM> is compared with an appropriate pressure range (S22). The controller <NUM> compares the measured pressure of the high pressure line <NUM> with the upper limit value Pd of the appropriate pressure range, and opens the collection valve <NUM> when the measured pressure of the high pressure line <NUM> exceeds the upper limit value Pd (PH > Pd) (S24). As a result, the working gas is collected from the high pressure line <NUM> to the buffer volume <NUM> through the collection valve <NUM>, and the pressure of the high pressure line <NUM> decreases.

The controller <NUM> closes the collection valve <NUM> when the measured pressure of the high pressure line <NUM> is restored to an appropriate pressure range (S26). For example, the controller <NUM> may compare the measured pressure of the high pressure line <NUM> with the upper limit value Pd of the appropriate pressure range, and close the collection valve <NUM> when the measured pressure of the high pressure line <NUM> falls below the upper limit value Pd (PH < Pd or PH ≤ Pd). When the collection valve <NUM> is closed, collection of the working gas from the high pressure line <NUM> to the buffer volume <NUM> is stopped. In this manner, the present method ends, and is executed again in the next control cycle.

The pressure threshold for closing the collection valve <NUM> may be different from the upper limit value Pd of the appropriate pressure range, and may be smaller than, for example, the upper limit value Pd. This pressure threshold may be selected from an appropriate pressure range, that is, may be larger than the lower limit value Pc of an appropriate pressure range.

The appropriate pressure range may be changed during the operation of the cryocooler <NUM>. For example, the appropriate pressure range in the initial cooling may be different from the appropriate pressure range in the steady operation, and may be higher than the appropriate pressure range in the steady operation, for example. For example, the lower limit value Pc in the initial cooling may be higher than the lower limit value Pc in the steady operation, and/or the upper limit value Pd in the initial cooling may be higher than the upper limit value Pd in the steady operation.

In this case, the switching from the initial cooling to the steady operation and the change of the appropriate pressure range may be controlled by the control device <NUM>. For example, the control device <NUM> may compare the measured temperature of the second cooling stage <NUM> (and/or the first cooling stage <NUM>) with the above-described standard cooling temperature based on the measured temperature signal from the temperature sensor <NUM>, execute the initial cooling when the measured temperature is higher than the standard cooling temperature, and shift from the initial cooling to the steady operation when the measured temperature is equal to or lower than the standard cooling temperature. The controller <NUM> may change the appropriate pressure range with the shift from the initial cooling to the steady operation.

Further, as will be described later with reference to <FIG> and <FIG>, the switching from the initial cooling to the steady operation and the change of the appropriate pressure range may be performed based on the pressure of the buffer volume <NUM> or based on the differential pressure of the high pressure line <NUM> and the low pressure line <NUM>. In this manner, the control device <NUM> can complete the initial cooling of the cryocooler <NUM> without depending on the temperature sensor <NUM>.

Here, in order to ensure the supply of the working gas from the buffer volume <NUM>, a condition desired for the buffer volume <NUM> is considered. From the state equation of the ideal gas, while the operation of the cryocooler <NUM> is stopped (that is, before the initial cooling), <MAT> is established. Here, PI (MPa) indicates the working gas filling pressure of the cryocooler <NUM> at the temperature T (K), VH (L) indicates the volume of the high pressure line <NUM>, VL (L) indicates the volume of the low pressure line <NUM>, VB (L) indicates the volume of the buffer volume <NUM>, n (mol) indicates the amount of the working gas in the cryocooler <NUM>, and R represents the gas constant.

Similarly, during the steady operation of the cryocooler <NUM>, <MAT> is established. Here, PH (MPa) indicates the pressure of the high pressure line <NUM> in the steady operation at the temperature T, PL (MPa) indicates the pressure of the low pressure line <NUM> in the steady operation at the temperature T, and PB (MPa) indicates the pressure of the buffer volume <NUM> in the steady operation at the temperature T.

From Equations (<NUM>) and (<NUM>), <MAT> is established.

In order to supply the working gas from the buffer volume <NUM> to the low pressure line <NUM> at any timing during the operation of the cryocooler <NUM>, for any temperature T in a temperature range from the initial temperature of the cryocooler <NUM> to the cryogenic temperature, <MAT> is supposed to be satisfied.

When Equation (<NUM>) is solved for PB and substituted into Equation (<NUM>), the following relationship is obtained.

Therefore, in order to ensure the supply of the working gas from the buffer volume <NUM> to the low pressure line <NUM>, it is preferable that the buffer volume <NUM> satisfies Equation (<NUM>) for any temperature in the temperature range from the initial temperature to the cryogenic temperature.

Similarly, in order to ensure gas collection to the buffer volume <NUM>, a condition desired for the buffer volume <NUM> is considered. In this case, in order to supply the working gas from the buffer volume <NUM> to the high pressure line <NUM> at any timing during the operation of the cryocooler <NUM>, for any temperature T in a temperature range from the initial temperature of the cryocooler <NUM> to the cryogenic temperature, <MAT> is supposed to be satisfied.

Therefore, in order to ensure the collection of the working gas from the high pressure line <NUM> to the buffer volume <NUM>, it is preferable that the buffer volume <NUM> satisfies Equation (<NUM>) for any temperature in the temperature range from the initial temperature to the cryogenic temperature.

<FIG> is a graph illustrating an example of time-dependent changes in temperature and pressure during the operation of the cryocooler <NUM> according to the first embodiment. The illustrated pressure change is acquired by an experiment, and in the upper part of <FIG>, the pressure PH of the high pressure line <NUM> measured by the first pressure sensor <NUM> and the pressure PL of the low pressure line <NUM> measured by the second pressure sensor <NUM> are illustrated. A temperature T1 of the first cooling stage <NUM> and a temperature T2 of the second cooling stage <NUM> are illustrated in the lower part of <FIG>. The horizontal axis represents time.

Before the cryocooler <NUM> starts (time <NUM>), both the pressure PH of the high pressure line <NUM> and the pressure PL of the low pressure line <NUM> are the filling pressures PI, and the temperature T1 of the first cooling stage <NUM> and the temperature T2 of the second cooling stage <NUM> are room temperature (approximately <NUM>). When the cryocooler <NUM> is started and the initial cooling is started, the compressor <NUM> and the expander <NUM> work, the pressure PH of the high pressure line <NUM> is increased from the filling pressure PI, and the pressure PL of the low pressure line <NUM> decreases from the filling pressure PI. Due to the initial cooling, the temperature T1 of the first cooling stage <NUM> and the temperature T2 of the second cooling stage <NUM> decrease. When the first cooling stage <NUM> and the second cooling stage <NUM> are each cooled to the above-described standard cooling temperature (for example, T1 ≤ <NUM>, T2 ≤ <NUM>), the initial cooling is completed and shifted to the steady operation.

<FIG> schematically illustrates an enlarged portion A illustrated in <FIG>, and <FIG> schematically illustrates an enlarged portion B illustrated in <FIG>. <FIG> illustrates the pressure PH of the high pressure line <NUM> immediately after the start of the initial cooling together with the open/closed state of the collection valve <NUM>, and <FIG> illustrates the pressure PH of the high pressure line <NUM> after the portion A together with the open/closed state of the supply valve <NUM>.

As illustrated in <FIG>, when the pressure PH of the high pressure line <NUM> exceeds the upper limit value Pd of the appropriate pressure range, the collection valve <NUM> is opened. Since the working gas is collected from the high pressure line <NUM> to the buffer volume <NUM> through the collection valve <NUM>, the pressure PH of the high pressure line <NUM> decreases. When the pressure PH of the high pressure line <NUM> falls below the upper limit pressure Pd, the collection valve <NUM> is closed. In this manner, excessive pressurization of the high pressure line <NUM> can be avoided. The risk of an emergency stop of the compressor <NUM> due to excessive pressurization is reduced. Further, since the buffer volume <NUM> is pressurized by collecting the working gas, which leads to effective utilization for supplying the working gas from the buffer volume <NUM> to the low pressure line <NUM>.

As illustrated in <FIG>, when the pressure PH of the high pressure line <NUM> falls below the lower limit value Pc of the appropriate pressure range, the supply valve <NUM> is opened. The working gas is supplied from the buffer volume <NUM> to the low pressure line <NUM> through the supply valve <NUM>. Since the amount of the working gas circulating in the gas line <NUM> increases, the pressure in the high pressure line <NUM> is restored. In this manner, when the pressure PH of the high pressure line <NUM> exceeds the lower limit value Pc, the supply valve <NUM> is closed.

As described above, the density of the working gas increases in the expander <NUM> due to the temperature decrease of the expander <NUM> during the initial cooling, which has the effect of lowering the pressure PH of the high pressure line <NUM>. Therefore, even when the pressure PH of the high pressure line <NUM> is restored once, the pressure PH falls below the lower limit value Pc again. The supply valve <NUM> is opened again, the pressure in the high pressure line <NUM> is restored, and the supply valve <NUM> is closed. In this manner, the supply valve <NUM> operates to repeatedly open and close to maintain the pressure PH of the high pressure line <NUM> within an appropriate pressure range.

When the working gas is not supplied to the gas line <NUM> during the initial cooling, the pressure PH of the high pressure line <NUM> may significantly decrease due to the temperature decrease of the expander <NUM>. Since the cooling capacity of the cryocooler <NUM> correlates with the pressure PH of the high pressure line <NUM>, the cooling capacity of the cryocooler <NUM> may decrease as the initial cooling progresses. This can be a factor that increases the time required for the initial cooling.

On the other hand, according to the embodiment, the pressure PH of the high pressure line <NUM> can be maintained within an appropriate pressure range by controlling the supply valve <NUM> during the initial cooling. Therefore, the cooling capacity of the cryocooler <NUM> can be appropriately maintained, and an increase in the initial cooling time can be suppressed. Further, by keeping the pressure PH of the high pressure line <NUM> substantially constant, the cryocooler <NUM> can provide a stable cooling capacity.

In order to maintain the pressure PH of the high pressure line <NUM> within an appropriate pressure range, a method for controlling the supply valve <NUM> and the collection valve <NUM> based on the pressure of the low pressure line <NUM> is also conceivable. The pressure of the low pressure line <NUM> is affected by the cooling temperature of the expander <NUM> (varies depending on the cooling temperature). Therefore, it is practically indispensable to set an appropriate pressure range of the low pressure line <NUM>, that is, a pressure threshold of the low pressure line <NUM> for opening and closing the supply valve <NUM> and the collection valve <NUM> to a different value depending on the cooling temperature, and the design of the control becomes complicated. Further, even when the low pressure line <NUM> is within an appropriate pressure range, there may be a case where the pressure of the high pressure line <NUM> is excessively high depending on the cooling temperature. Therefore, a method based on the pressure of the high pressure line <NUM> as in the embodiment is advantageous in that such inconvenience is alleviated or prevented.

<FIG> is a graph illustrating an example of changes in temperature and pressure during the operation of the cryocooler <NUM> according to the first embodiment. <FIG> is a view schematically illustrating the cryocooler <NUM> according to the first embodiment.

Similar to the above-described embodiment, the cryocooler <NUM> includes the compressor <NUM>, the expander <NUM>, the buffer volume <NUM>, and the control device <NUM>. The controller <NUM> controls the supply valve <NUM> to keep the pressure of the high pressure line <NUM> within a preset appropriate pressure range based on the pressure of the high pressure line <NUM> measured by the first pressure sensor <NUM> during the initial cooling. Further, the controller <NUM> controls the collection valve <NUM> to keep the pressure of the high pressure line <NUM> within an appropriate pressure range based on the pressure of the high pressure line <NUM> measured by the first pressure sensor <NUM> during the initial cooling.

The cryocooler <NUM> includes a buffer pressure sensor <NUM> connected to the buffer volume <NUM> to measure the pressure of the buffer volume <NUM>. The buffer pressure sensor <NUM> is electrically connected to the control device <NUM>, and is configured to output a measured buffer pressure signal PB representing the measured pressure to the control device <NUM>.

In the upper part of <FIG>, in addition to the pressure PH of the high pressure line <NUM> and the pressure PL of the low pressure line <NUM> illustrated in <FIG>, the pressure PB of the buffer volume <NUM> measured by the buffer pressure sensor <NUM> is illustrated. The temperature T1 of the first cooling stage <NUM> and the temperature T2 of the second cooling stage <NUM> are illustrated in the lower part of <FIG>. As will be understood from <FIG>, when the cryocooler <NUM> is sufficiently cooled by the completion of the initial cooling and the temperatures of the first cooling stage <NUM> and the second cooling stage <NUM> are stabilized, the pressure PH of the high pressure line <NUM> and the pressure PL of the low pressure line <NUM> are also stabilized. At this time, both the supply valve <NUM> and the collection valve <NUM> are closed, and the buffer volume <NUM> is disconnected from the gas line <NUM>. Therefore, the pressure PB of the buffer volume <NUM> is also constant (final buffer pressure PF illustrated in <FIG>).

Therefore, the completion of the initial cooling can be determined by detecting the stabilization of the pressure PB having the buffer volume <NUM>. When the working gas filling pressure PI of the cryocooler <NUM> and operating conditions (for example, high pressure PH, low pressure PL, temperatures T1, T2, and the like) are known, the final pressure of the buffer volume <NUM> when the initial cooling is completed can be predicted. In this case, the controller <NUM> may compare the predicted value of the final buffer pressure with the measured pressure PB of the buffer volume <NUM>, and based on the comparison result, the controller <NUM> may determine whether or not the measured pressure PB of the buffer volume <NUM> is equal to the predicted value of the final buffer pressure. The controller <NUM> may complete the initial cooling when a state where the measured pressure PB of the buffer volume <NUM> is equal to the predicted value of the final buffer pressure continues for a predetermined time (for example, several minutes).

Alternatively, the controller <NUM> may calculate a difference between the measured pressure PB of the buffer volume <NUM> and a reference pressure during the initial cooling, and detect the stabilization of the calculated pressure difference, to determine the completion of the initial cooling. The reference pressure may be the pressure of the previously measured buffer volume <NUM>, and may be, for example, the maximum value PM of the pressure of the buffer volume <NUM> measured during the initial cooling. It is understood from <FIG> that the pressure of the buffer volume <NUM> increases from the filling pressure PI and takes the maximum value PM immediately after the start of the initial cooling.

The controller <NUM> may compare the calculated pressure difference (that is, the difference between the measured pressure PB of the buffer volume <NUM> and the reference pressure) with the pressure difference target value, and based on the comparison result, the controller <NUM> may determine whether or not the calculated pressure difference is equal to the pressure difference target value. The controller <NUM> may complete the initial cooling when a state where the calculated pressure difference is equal to the pressure difference target value continues for a predetermined time. For example, the predetermined time may be selected from a range of <NUM> minute or more and <NUM> minutes or less. When the difference between the calculated pressure difference and the pressure difference target value is within a predetermined value (for example, <NUM> MPa), it can be considered that the calculated pressure difference is equal to the pressure difference target value. Since this pressure difference target value does not depend on the filling pressure PI, it is possible to determine the completion of the initial cooling even when the filling pressure PI is unknown.

As another example of the reference pressure, the pressure PH of the high pressure line <NUM> (or the pressure PL of the low pressure line <NUM>) measured at the same timing as the measured pressure PB of the buffer volume <NUM> may be used. The controller <NUM> may calculate a difference between the measured pressure PB of the buffer volume <NUM> and the measured pressure PH of the high pressure line <NUM> (or the measured pressure PL of the low pressure line <NUM>), and detect the stabilization of the calculated pressure difference, to determine the completion of the initial cooling. Similar to the above example, the controller <NUM> may compare the calculated pressure difference with the pressure difference target value, and when the calculated pressure difference is equal to the pressure difference target value over a predetermined time, the controller <NUM> may complete the initial cooling.

As a further alternative example, the controller <NUM> may calculate the difference between the measured pressure PH of the high pressure line <NUM> and the measured pressure PL of the low pressure line <NUM>, and detect the stabilization of the calculated pressure difference, to determine the completion of the initial cooling.

In the above-described embodiment, the supply valve <NUM> is controlled (hereinafter, also referred to as high pressure priority control) based on the measured pressure of the high pressure line <NUM> during the initial cooling, and the pressure of the high pressure line <NUM> is kept within an appropriate pressure range. At this time, since the pressure of the low pressure line <NUM> is not managed, there is a possibility that an undesired phenomenon such that the pressure of the low pressure line <NUM> becomes extremely low may occur in some cases.

In order to cope with such a problem, the controller <NUM> may be configured to discontinue the control (that is, high pressure priority control) of the supply valve <NUM> based on the pressure of the high pressure line <NUM> when the pressure of the low pressure line <NUM> measured by the second pressure sensor <NUM> falls below a preset low pressure threshold. The controller <NUM> may be configured to control the supply valve <NUM> to restore the pressure of the low pressure line <NUM> to the low pressure threshold based on the pressure of the low pressure line <NUM> measured by the second pressure sensor <NUM>. In the following, the control of the supply valve <NUM> based on the pressure of the low pressure line <NUM> will also be referred to as low pressure priority control. An example of the switching processing from the high pressure priority control to the low pressure priority control will be described later with reference to <FIG>.

First, the pressure of the low pressure line <NUM> is measured (S30). The second pressure sensor <NUM> measures the pressure of the low pressure line <NUM>, and outputs the second measured pressure signal PL indicating the measured pressure of the low pressure line <NUM>. The controller <NUM> receives the second measured pressure signal PL and acquires the measured pressure of the low pressure line <NUM>.

Next, the measured pressure PL of the low pressure line <NUM> is compared with a low pressure threshold Pe (S32). For example, the low pressure threshold Pe may be set as a lower limit value of the pressure of the low pressure line <NUM> from the viewpoint of guaranteeing stable operation of the compressor <NUM>. For example, the low pressure threshold Pe may be selected from a range of <NUM> MPa to <NUM> MPa. The low pressure threshold Pe can be appropriately set based on the empirical knowledge of the designer, an experiment or simulation by the designer, or the like. The low pressure threshold Pe may be stored in advance in the controller <NUM> as an initial setting of the cryocooler <NUM>, or may be set in the controller <NUM> by the user before the cryocooler <NUM> is operated.

The controller <NUM> compares the measured pressure of the low pressure line <NUM> with the low pressure threshold Pe, and when the measured pressure of the low pressure line <NUM> exceeds the low pressure threshold Pe (PL > Pe or PL ≥ Pe), the high pressure priority control is selected (S34). In this case, as described with reference to <FIG>, the controller <NUM> compares the measured pressure of the high pressure line <NUM> with the lower limit value Pc of the appropriate pressure range, and operate the supply valve <NUM> to repeatedly open and close the supply valve <NUM> such that the pressure of the high pressure line <NUM> does not fall below the lower limit value Pc. In this manner, the high pressure priority control is continuously executed.

On the other hand, when the measured pressure of the low pressure line <NUM> falls below the low pressure threshold Pe (PL < Pe), the controller <NUM> selects the low pressure priority control (S36). The high pressure priority control is discontinued, and the low pressure priority control is started. An example of the low pressure priority control will be described later with reference to <FIG>. In this manner, the switching processing from the high pressure priority control to the low pressure priority control ends.

In the low pressure priority control, first, as illustrated in <FIG>, the pressure of the low pressure line <NUM> is measured (S40). Next, the measured pressure of the low pressure line <NUM> is compared with the low pressure threshold Pe (S42). When the measured pressure of the low pressure line <NUM> falls below the low pressure threshold Pe (PL < Pe), the controller <NUM> opens the supply valve <NUM> (S44). In this manner, the working gas is supplied from the buffer volume <NUM> to the low pressure line <NUM> through the supply valve <NUM>, and the pressure of the low pressure line <NUM> is restored.

On the other hand, when the measured pressure of the low pressure line <NUM> exceeds the low pressure threshold Pe (PL > Pe or PL ≥ Pe), the controller <NUM> closes the supply valve <NUM> (S46). The supply of the working gas from the buffer volume <NUM> to the low pressure line <NUM> is stopped. The pressure threshold for closing the supply valve <NUM> may be different from the low pressure threshold Pe, and may be somewhat larger than, for example, the low pressure threshold Pe. In this manner, the present method ends, and is executed again in the next control cycle.

According to the low pressure priority control described above, the supply valve <NUM> is opened and closed based on the pressure of the low pressure line <NUM>, and the pressure of the low pressure line <NUM> can be restored to the low pressure threshold Pe. However, in the low pressure priority control, the pressure of the high pressure line <NUM> is not managed. Therefore, during the low pressure priority control, there is a possibility that an undesired phenomenon such that the pressure of the high pressure line <NUM> becomes extremely low may occur.

Therefore, the controller <NUM> may be configured to discontinue the control of the supply valve <NUM> based on the pressure of the low pressure line <NUM> when the pressure of the high pressure line <NUM> measured by the first pressure sensor <NUM> falls below the high pressure threshold. The controller <NUM> may be configured to control the supply valve <NUM> to restore the pressure of the high pressure line <NUM> to the high pressure threshold based on the pressure of the high pressure line <NUM> measured by the first pressure sensor <NUM>. An example of the return processing from the low pressure priority control to the high pressure priority control will be described later with reference to <FIG>.

<FIG> is a flowchart for describing a method for controlling the cryocooler <NUM> according to the first embodiment. During the execution of the low pressure priority control described above, this method is repeatedly executed by the controller <NUM> at a predetermined cycle.

First, the pressure of the high pressure line <NUM> is measured (S50). Next, the measured pressure of the high pressure line <NUM> is compared with a high pressure threshold Pf (S52). For example, the high pressure threshold Pf may be the lower limit value Pc within an appropriate pressure range. The high pressure threshold Pf can be appropriately set based on the empirical knowledge of the designer, an experiment or simulation by the designer, or the like. The high pressure threshold Pf may be stored in advance in the controller <NUM> as an initial setting of the cryocooler <NUM>, or may be set in the controller <NUM> by the user before the cryocooler <NUM> is operated.

The controller <NUM> compares the measured pressure of the high pressure line <NUM> with the high pressure threshold Pf, and when the measured pressure of the high pressure line <NUM> exceeds the high pressure threshold Pf (PH > Pf or PH ≥ Pf), the low pressure priority control is selected (S54). In this case, the low pressure priority control is continuously executed.

On the other hand, when the measured pressure of the high pressure line <NUM> falls below the high pressure threshold Pf (PH < Pf), the controller <NUM> selects the high pressure priority control (S56). In this manner, the switching processing from the high pressure priority control to the low pressure priority control ends. By switching to the high pressure priority control again, the supply valve <NUM> is controlled based on the measured pressure of the high pressure line <NUM>, and the pressure of the high pressure line <NUM> is maintained within an appropriate pressure range.

Incidentally, by increasing the operating differential pressure of the cryocooler <NUM> (the pressure difference between the high pressure line <NUM> and the low pressure line <NUM>), and by increasing the cooling capacity of the cryocooler <NUM>, the time required for initial cooling can also be shortened. However, in the above-described embodiment, the relief valve <NUM> bypassing the high pressure line <NUM> and the low pressure line <NUM> may be an obstacle. When the relief valve <NUM> is of a type that is mechanically opened when a differential pressure equal to or higher than a set pressure acts between the inlet and outlet of the relief valve <NUM>, the operating differential pressure of the cryocooler <NUM> may be limited to this set pressure. This is because, when the pressure difference between the high pressure line <NUM> and the low pressure line <NUM> exceeds the set pressure of the relief valve <NUM>, the relief valve <NUM> is mechanically opened, the working gas flows out from the high pressure line <NUM> to the low pressure line <NUM> through the relief valve <NUM>, and as a result, an increase in the operating differential pressure of the cryocooler <NUM> may be impeded.

In order to cope with this, additional compressors may be temporarily installed in the cryocooler <NUM> for initial cooling, as described below. In the following, for convenience of description, the main compressor <NUM> of the cryocooler <NUM> will be referred to as a first compressor <NUM>, and the sub-compressor to be added will be referred to as a second compressor <NUM>.

<FIG> and <FIG> are views schematically illustrating the cryocooler <NUM> according to a second embodiment. <FIG> illustrates the settings of the cryocooler <NUM> in the initial cooling, and <FIG> illustrates the basic settings of the cryocooler <NUM> in steady operation before or after the initial cooling. <FIG> is a flowchart for describing a method for operating the cryocooler <NUM> according to the second embodiment.

The cryocooler <NUM> and the method for operating the same according to the second embodiment can be the same as the cryocooler <NUM> and the method for operating method the same according to the first embodiment, except for the second compressor <NUM>. Therefore, in <FIG> and <FIG>, the same reference numerals will be assigned to the configurations common to the first embodiment, and detailed description thereof will be appropriately omitted in order to avoid redundancy.

The cryocooler <NUM> takes a basic setting including the first compressor <NUM> and the expander <NUM> as illustrated in <FIG> before the initial cooling is performed. The first compressor <NUM> and the expander <NUM> are connected by the high pressure line <NUM> and the low pressure line <NUM>. The second compressor <NUM> and the buffer volume <NUM> are not connected to the cryocooler <NUM>.

In the method for operating the cryocooler <NUM> according to the second embodiment, as illustrated in <FIG>, the second compressor <NUM> and the buffer volume <NUM> are connected to the cryocooler <NUM> as pretreatment for initial cooling (S60, S61). The attachment order of the second compressor <NUM> and the buffer volume <NUM> is not limited.

The second compressor <NUM> is connected in series with the first compressor <NUM> on the high pressure line <NUM>. More specifically, as illustrated in <FIG>, the high pressure gas outlet <NUM> of the first compressor <NUM> is connected to the suction port of the second compressor <NUM>, and the discharge port of the second compressor <NUM> is connected to the high pressure gas inlet <NUM> of the expander <NUM>. Therefore, the cryocooler <NUM> has a two-stage compressor configuration including the first compressor <NUM> and the second compressor <NUM>.

The buffer volume <NUM> is connected to the low pressure line <NUM> via the supply valve <NUM> and is connected to the high pressure line <NUM> via the collection valve <NUM>. The supply valve <NUM> and the collection valve <NUM> may be accommodated in one housing together with the buffer volume <NUM> to form a buffer volume unit. The buffer volume unit and the second compressor <NUM> may be brought to the site where the cryocooler <NUM> is operated, for example, by a serviceman and connected to the cryocooler <NUM>.

When the second compressor <NUM> and the buffer volume <NUM> are connected to the cryocooler <NUM> in this manner, the cryocooler <NUM> is started and initial cooling is started (S62). As described above, the initial cooling is a process of cooling the cryocooler <NUM> from the initial temperature to the target cryogenic temperature in preparation for steady operation of the cryocooler <NUM>. The initial temperature may be an ambient temperature (for example, room temperature), or may be a temperature lower than the ambient temperature and higher than a target cryogenic temperature (for example, a temperature selected from a range of <NUM> to <NUM>). By the initial cooling, the first cooling stage <NUM> of the expander <NUM> is cooled to the first cooling temperature, and the second cooling stage <NUM> is cooled to the second cooling temperature.

The initial cooling is executed in a state where the second compressor <NUM> and the buffer volume <NUM> are connected to the cryocooler <NUM>. Therefore, in the initial cooling, the working gas is supplied to the expander <NUM> by using the first compressor <NUM> and the second compressor <NUM>. The first compressor <NUM> pressurizes the working gas of the cryocooler <NUM> collected from the expander <NUM> through the low pressure line <NUM>, and supplies the pressurized working gas to the second compressor <NUM>. The second compressor <NUM> further pressurizes the working gas from the first compressor <NUM>, and supplies this to the expander <NUM> again.

Further, in the initial cooling, the buffer volume <NUM> is used to keep the pressure of the high pressure line <NUM> within an appropriate pressure range, as in the first embodiment. That is, the supply valve <NUM> performs control to keep the pressure of the high pressure line <NUM> within an appropriate pressure range based on the pressure of the high pressure line <NUM> measured by the first pressure sensor <NUM> during the initial cooling.

When the initial cooling is completed, the second compressor <NUM> and the buffer volume <NUM> are removed from the cryocooler <NUM> (S63, S64). The order of removal of the second compressor <NUM> and the buffer volume <NUM> is not limited. The cryocooler <NUM> is returned to the basic setting illustrated in <FIG>. Then, a steady operation of the cryocooler <NUM> is performed (S65).

According to the second embodiment, the operating differential pressure of the cryocooler <NUM> can be increased by adding the second compressor <NUM>. In particular, the operating differential pressure of the cryocooler <NUM> can be increased exceeding the limitation caused by the set pressure of the relief valve <NUM> of the first compressor <NUM> described above. An increase in the operating differential pressure causes an increase in the cooling capacity of the cryocooler <NUM>, and can shorten the time required for initial cooling.

As another method for increasing the operating differential pressure of the cryocooler <NUM>, it is conceivable to bring a large-sized compressor having a higher output than the first compressor <NUM> to the site from the outside and replace the first compressor <NUM> with the large-sized compressor. However, such a large-sized compressor is generally unsuitable for carrying because the large-sized compressor is large in size and weight. On the other hand, since the second compressor <NUM> is used in combination with the first compressor <NUM>, the second compressor <NUM> may be relatively small in size and is easy to carry.

In the above-described embodiment, the second compressor <NUM> is connected to the discharge side (outlet side) of the first compressor <NUM> in order to form a two-stage compressor configuration. However, other configurations are possible. For example, in principle, the second compressor <NUM> may be connected to the suction side (inlet side) of the first compressor <NUM>. That is, the second compressor <NUM> may be connected in series with the first compressor <NUM> on the low pressure line <NUM>.

It is not essential that the buffer volume <NUM> is removed. Also in the second embodiment, similar to the first embodiment, as illustrated in <FIG>, a steady operation of the cryocooler <NUM> may be performed in a state where the buffer volume <NUM> is connected to the cryocooler <NUM>. Alternatively, also in the first embodiment, similar to the second embodiment, the buffer volume <NUM> may be removed from the cryocooler <NUM> after the initial cooling.

<FIG> is a view schematically illustrating the cryocooler <NUM> according to a third embodiment. <FIG> and <FIG> are flowcharts for describing a method for operating the cryocooler <NUM> according to the third embodiment.

The cryocooler <NUM> and the method for operating the same according to the third embodiment can be the same as the cryocooler <NUM> and the method for operating method the same according to the first embodiment, except for the second compressor <NUM>. Therefore, in <FIG>, the same reference numerals will be assigned to the configurations common to the first embodiment, and detailed description thereof will be appropriately omitted in order to avoid redundancy.

Also in the third embodiment, similar to the second embodiment, the cryocooler <NUM> takes a basic setting including the first compressor <NUM> and the expander <NUM> as illustrated in <FIG> before the initial cooling is performed. The first compressor <NUM> and the expander <NUM> are connected by the high pressure line <NUM> and the low pressure line <NUM>. The second compressor <NUM> is not connected to the cryocooler <NUM>.

In the method for operating the cryocooler <NUM> according to the third embodiment, as illustrated in <FIG>, the second compressor <NUM> is connected to the cryocooler <NUM> as pretreatment for initial cooling (S70). The second compressor <NUM> is connected in series with the first compressor <NUM> on the high pressure line <NUM>. As illustrated in <FIG>, the high pressure gas outlet <NUM> of the first compressor <NUM> is connected to the suction port of the second compressor <NUM>, and the discharge port of the second compressor <NUM> is connected to the high pressure gas inlet <NUM> of the expander <NUM>. Therefore, the cryocooler <NUM> has a two-stage compressor configuration including the first compressor <NUM> and the second compressor <NUM>. The second compressor <NUM> may be brought to the site where the cryocooler <NUM> is operated by a serviceman, for example, and may be connected to the cryocooler <NUM>.

The second compressor <NUM> includes the compressor main body <NUM> similar to the first compressor <NUM>. In addition, the second compressor <NUM> includes a compressor motor <NUM> in which an operating frequency (that is, a rotation speed) is variable, and the compressor main body <NUM> is driven by the compressor motor <NUM>. The compressor motor <NUM> may be, for example, an electric motor, or may be any other suitable type of motor. By increasing the operating frequency of the compressor motor <NUM>, the discharge flow rate of the compressor main body <NUM> is increased, and as a result, the pressure of the high pressure line <NUM> can be increased. On the contrary, by reducing the operating frequency of the compressor motor <NUM>, the discharge flow rate of the compressor main body <NUM> is reduced, and as a result, the pressure of the high pressure line <NUM> can be reduced.

The control device <NUM> includes an inverter <NUM> that controls the operating frequency of the compressor motor <NUM>. The compressor motor <NUM> and the inverter <NUM> are supplied with power from an external power source <NUM> such as a commercial power source (three-phase AC power source). As will be described later, the inverter <NUM> is configured to adjust the frequency of the power input from the external power source <NUM> under the control of the controller <NUM> and output the frequency to the compressor motor <NUM> at any frequency. The operating frequency of the compressor motor <NUM> corresponds to the output frequency of the inverter <NUM>, and can be adjusted in the range of <NUM> to <NUM> or <NUM> to <NUM>, for example.

In a state where the second compressor <NUM> is connected to the cryocooler <NUM>, the cryocooler <NUM> is started and initial cooling is performed (S72). The working gas is supplied to the expander <NUM> by using the first compressor <NUM> and the second compressor <NUM>. The first compressor <NUM> pressurizes the working gas of the cryocooler <NUM> collected from the expander <NUM> through the low pressure line <NUM>, and supplies the pressurized working gas to the second compressor <NUM>. The second compressor <NUM> further pressurizes the working gas from the first compressor <NUM>, and supplies this to the expander <NUM> again. As will be described later, the operating frequency of the compressor motor <NUM> of the second compressor <NUM> is controlled to keep the pressure of the high pressure line <NUM> within an appropriate pressure range based on the measured pressure of the high pressure line <NUM>.

When the initial cooling is completed, the second compressor <NUM> is removed from the cryocooler <NUM> (S73). The cryocooler <NUM> is returned to the basic setting illustrated in <FIG>. Then, a steady operation of the cryocooler <NUM> is performed (S75).

An example of the control processing of the operating frequency of the compressor motor <NUM> based on the measured pressure of the high pressure line <NUM> will be described with reference to <FIG> which is not part of the present invention. This processing is repeatedly executed by the controller <NUM> in a predetermined cycle in the initial cooling of the cryocooler <NUM>.

First, the pressure of the high pressure line <NUM> is measured (S80). Next, the measured pressure of the high pressure line <NUM> is compared with a high pressure target value Pg (S82). For example, the high pressure target value Pg may be the lower limit value Pc of an appropriate pressure range. The high pressure target value Pg can be appropriately set based on the empirical knowledge of the designer, an experiment or simulation by the designer, or the like. The high pressure target value Pg may be stored in advance in the controller <NUM> as an initial setting of the cryocooler <NUM>, or may be set in the controller <NUM> by the user before the cryocooler <NUM> is operated.

The controller <NUM> compares the measured pressure PH of the high pressure line <NUM> with the high pressure target value Pg, and outputs a magnitude relationship between the two as a comparison result. That is, the comparison results obtained by the controller <NUM> are any of the following three states: (i) the measured pressure PH is smaller than the high pressure target value Pg, (ii) the measured pressure PH is larger than the high pressure target value Pg, and (iii) the measured pressure PH is equal to the high pressure target value Pg.

The inverter <NUM> is controlled based on the comparison result of the controller <NUM>, and the operating frequency of the compressor motor <NUM> is controlled according to the output frequency of the inverter <NUM>. Specifically, (i) when the measured pressure PH is smaller than the high pressure target value Pg, the controller <NUM> controls the inverter <NUM> to increase the operating frequency of the compressor motor <NUM> (S84). Accordingly, the pressure of the high pressure line <NUM> can be increased.

When the operating frequency of the compressor motor <NUM> is increased or decreased, the controller <NUM> may increase or decrease the operating frequency by a predetermined amount from the value of the current operating frequency of the compressor motor <NUM>. When the current operating frequency value already reached the upper limit value when the operating frequency is to be increased, the controller <NUM> may maintain the upper limit value without increasing the operating frequency. For example, in a case where the operating frequency of the compressor motor <NUM> is within a range of <NUM> to <NUM> and the current value is already the upper limit value of <NUM>, the controller <NUM> does not further increase the operating frequency from <NUM>, and <NUM> is maintained. Similarly, when the current operating frequency value already reached the lower limit value when the operating frequency is to be reduced, the controller <NUM> may maintain the lower limit value without reducing the operating frequency.

Alternatively, the controller <NUM> may control the inverter <NUM> to adjust the operating frequency of the compressor motor <NUM> to minimize the deviation of the measured pressure PH from the high pressure target value Pg (for example, by feedback control such as PID control). In this manner, the controller <NUM> may compare the pressure of the high pressure line <NUM> with the target pressure, and control the inverter <NUM> to reduce the operating frequency of the compressor motor <NUM> when the pressure of the high pressure line <NUM> exceeds the target pressure and increase the operating frequency of the compressor motor <NUM> when the pressure of the high pressure line <NUM> falls below the target pressure.

According to the third embodiment, the operating differential pressure of the cryocooler <NUM> can be increased by adding the second compressor <NUM>. In particular, the operating differential pressure of the cryocooler <NUM> can be increased exceeding the limitation caused by the set pressure of the relief valve <NUM> of the first compressor <NUM> described above. An increase in the operating differential pressure causes an increase in the cooling capacity of the cryocooler <NUM>, and can shorten the time required for initial cooling. In addition, since the second compressor <NUM> is used in combination with the first compressor <NUM>, the second compressor <NUM> may be relatively small in size and is easy to carry.

Similar to the second embodiment, also in the third embodiment, as illustrated in <FIG>, the buffer volume <NUM> may be connected to the cryocooler <NUM> for initial cooling together with the supply valve <NUM> and the collection valve <NUM>. As pretreatment of the initial cooling, the buffer volume <NUM> is connected to the low pressure line <NUM> via the supply valve <NUM> and is connected to the high pressure line <NUM> via the collection valve <NUM>. During the execution of the initial cooling, the buffer volume <NUM> is used to keep the pressure of the high pressure line <NUM> within an appropriate pressure range, as in the first embodiment. That is, the supply valve <NUM> performs control to keep the pressure of the high pressure line <NUM> within an appropriate pressure range based on the pressure of the high pressure line <NUM> measured by the first pressure sensor <NUM> during the initial cooling. After the initial cooling, the buffer volume <NUM> may be removed from the cryocooler <NUM> together with the supply valve <NUM> and the collection valve <NUM>. In this manner, by using the buffer volume <NUM> for the initial cooling, the time required for the initial cooling can be shortened similar to the above-described embodiment.

Further, as illustrated in <FIG>, in order to form the two-stage compressor configuration, the second compressor <NUM> may be connected to the suction side (inlet side) of the first compressor <NUM>. That is, the second compressor <NUM> may be connected in series with the first compressor <NUM> on the low pressure line <NUM>. In this case, the operating frequency of the compressor motor <NUM> that drives the compressor main body <NUM> of the first compressor <NUM> may be variable. The operating frequency of the compressor motor <NUM> may be controlled by the controller <NUM> and the inverter <NUM> to keep the pressure of the high pressure line <NUM> within an appropriate pressure range based on the measured pressure of the high pressure line <NUM>. Along with or instead of this, the operating frequency of the compressor motor <NUM> of the second compressor <NUM> may be controlled by the controller <NUM> and the inverter <NUM>, similar to the embodiment illustrated in <FIG>.

Above, the present invention was described based on examples.

For example, the control processing described with respect to the first embodiment (for example, the completion processing of the initial cooling based on the buffer pressure, the switching processing from the high pressure priority control to the low pressure priority control, and the switching processing from the low pressure priority control to the high pressure priority control) may be applied to the second and third embodiments.

The pressure sensors such as the first pressure sensor <NUM> and the second pressure sensor <NUM> are not essential to be provided in the compressor <NUM>, and may be provided at any place where the pressure of the gas line <NUM> and the expander <NUM> can be measured. For example, the first pressure sensor <NUM> may be provided at any place on the high pressure line <NUM>, and the second pressure sensor <NUM> may be provided at any place on the low pressure line <NUM>.

In the above-described embodiment, the supply valve <NUM> and the collection valve <NUM> are prepared as separate valves, and each of the supply valve <NUM> and the collection valve <NUM> are connected to the buffer volume <NUM>. However, the present invention is not limited thereto. For example, the supply valve <NUM> and the collection valve <NUM> may be integrated, or may be, for example, a three-way valve connected to the buffer volume <NUM>. By switching the three-way valve, a supply state where the buffer volume <NUM> is connected to the low pressure line <NUM> and a collection state where the buffer volume <NUM> is connected to the high pressure line <NUM> may be switched.

In the embodiment described above, the buffer volume <NUM> is a single buffer tank. However, in a certain embodiment, the buffer volume <NUM> may be a plurality of buffer tanks. One buffer tank may be connected to the low pressure line <NUM> by the supply valve <NUM>, and another buffer tank may be connected to the high pressure line <NUM> by the collection valve <NUM>. Further, in the above-described embodiment, the buffer volume <NUM> is disposed outside the compressor <NUM> and the expander <NUM>, but the present invention is not limited thereto. For example, the buffer volume <NUM> may be disposed inside the compressor <NUM>.

Claim 1:
A method for operating a cryocooler (<NUM>) in which the cryocooler (<NUM>) includes a first compressor (<NUM>), an expander (<NUM>), and a high pressure line (<NUM>) and a low pressure line (<NUM>) connecting the first compressor (<NUM>) to the expander (<NUM>), the method comprising:
connecting a second compressor (<NUM>) in series with the first compressor (<NUM>) on the high pressure line (<NUM>) or the low pressure line (<NUM>);
connecting a buffer volume (<NUM>) to the low pressure line (<NUM>) via a supply valve (<NUM>);
executing initial cooling for cooling the expander (<NUM>) from an initial temperature to a cryogenic temperature in a state where the second compressor (<NUM>) and the buffer volume (<NUM>) are connected to the cryocooler (<NUM>); and
executing a steady operation of maintaining the expander (<NUM>) at the cryogenic temperature after the initial cooling, wherein
the execution of the initial cooling includes
supplying a working gas to the expander (<NUM>) by using the first compressor (<NUM>) and the second compressor (<NUM>), and
controlling the supply valve (<NUM>) to keep a pressure of the high pressure line (<NUM>) within a preset appropriate pressure range based on the measured pressure of the high pressure line (<NUM>).