Refrigeration device

This application is a Continuation application of International Application No. PCT/JP2017/003835 filed on Feb. 2, 2017, which claims the benefit of Japanese Application No. 2016-028126 filed on Feb. 17, 2016, the entire contents of each are hereby incorporated by reference.

The present invention relates to a refrigeration apparatus provided with a cascade cycle.

For example, Patent Literature 1 describes a conventional refrigeration apparatus of this type. According to the conventional refrigeration apparatus, a controller starts a compressor (high-temperature side compressor) constituting a primary side refrigerant circuit based on an internal temperature of a freezer output from a temperature sensor during a normal operation, and then starts a compressor (low-temperature side compressor) of a secondary side refrigerant circuit after an elapse of a predetermined time. When the internal temperature reaches an upper limit of a target value, the controller initially starts the primary side refrigerant circuit including the high-temperature side compressor based on output from the temperature sensor which detects the internal temperature. Thereafter, the controller opens an electromagnetic valve provided between the compressor and an evaporator in the secondary side refrigerant circuit, and starts the low-temperature side compressor. Thereafter, the controller executes such control as to open an electromagnetic valve provided between a cascade condenser and an expander in the secondary side refrigerant circuit.

Patent Literature

PTL 1

Japanese Patent Application Laid-Open No. 2004-190917

Technical Problem

There has been a demand for a refrigeration apparatus of this type which is capable of rapidly returning an internal temperature (i.e., temperature of storage space for cooling target) to a target value at the time of a rise of the internal temperature.

Accordingly, an object of the present invention is to provide a refrigeration apparatus capable of rapidly returning a temperature of a storage space to a target value at the time of a rise of the temperature.

Solution to Problem

A refrigeration apparatus according to an aspect of the present invention includes: a first cooling section disposed such that a first compressor, a first condenser, a first expander, and a first evaporator achieve fluid communication with one another in a first fluid circuit through which first refrigerant circulates; a second cooling section disposed such that a second compressor, a second condenser that constitutes a cascade condenser in cooperation with the first evaporator, a second expander, and a second evaporator achieve fluid communication with one another in a second fluid circuit through which second refrigerant circulates; a storage section that includes a storage space for a cooling target cooled by the second evaporator, an internal temperature sensor that detects a temperature of the storage space; a control section that determines a second rotation speed of the second compressor based on a target temperature of the storage space and a detection result obtained by the internal temperature sensor, and determines a first rotation speed of the first compressor, the first rotation speed having a predetermined correspondence relation with the second rotation speed; and a first power supply section and a second power supply section that supply power to the first compressor and the second compressor, respectively, based on the first rotation speed and the second rotation speed determined by the control section, respectively.

Advantageous Effects of Invention

Provided according to the above aspect is a refrigeration apparatus capable of rapidly returning a temperature of a storage space to a target value at the time of a rise of the temperature.

FIG. 1 is a diagram illustrating a relationship between a cooling section and a block configuration of a control system included in a refrigeration apparatus according to a comparative example;

FIG. 2 is a schematic view illustrating a shift of heat in the cooling section in FIG. 1 ;

FIG. 3 is a front view of a refrigeration apparatus according to Embodiment 1 of the present invention;

FIG. 4 is a right side view of the present refrigeration apparatus;

FIG. 5 is a diagram illustrating an example of a cooling section included in the present refrigeration apparatus;

FIG. 6 is a diagram illustrating an example of a control system included in the present refrigeration apparatus;

FIG. 7A is a flowchart showing a part of internal temperature control (Example 1) of the present refrigeration apparatus;

FIG. 7B is a flowchart showing a remaining part of the internal temperature control (Example 1) of the present refrigeration apparatus;

FIG. 8 is a graph showing a change with time of an internal temperature of the present refrigeration apparatus, and a chart showing transitions of rotation speeds of a first compressor and a second compressor;

FIG. 9 is a diagram illustrating a relationship between the cooling section and a block configuration of the control system included in the present refrigeration apparatus;

FIG. 10A is a flowchart showing a part of internal temperature control (Example 2) of the present refrigeration apparatus;

FIG. 10B is a flowchart showing a remaining part of the internal temperature control (Example 2) of the present refrigeration apparatus;

FIG. 11A is a flowchart showing a part of internal temperature control (Example 3) of the present refrigeration apparatus;

FIG. 11B is a flowchart showing a remaining part of the internal temperature control (Example 3) of the present refrigeration apparatus;

FIG. 12 is a chart showing an example of changes of rotation speeds of a first compressor and a second compressor during execution of the internal temperature control (Example 3);

FIG. 13A is a flowchart showing a part of internal temperature control (Example 4) of the present refrigeration apparatus;

FIG. 13B is a flowchart showing a remaining part of the internal temperature control (Example 4) of the present refrigeration apparatus;

FIG. 14A is a flowchart showing a part of internal temperature control (Example 5) of the present refrigeration apparatus;

FIG. 14B is a flowchart showing a remaining part of the internal temperature control (Example 5) of the present refrigeration apparatus; and

FIG. 15 is a chart showing an optimum coefficient for each combination of an ambient temperature and an internal temperature.

1. Details of Technical Problem

A technical problem of internal temperature control performed by refrigeration apparatus 10 according to a comparative example is described in detail before touching upon refrigeration apparatus 1 according to the present invention.

1-1. General Configuration of Refrigeration Apparatus 10 of Comparative Example

Refrigeration apparatus 10 according to the comparative example in FIG. 1 includes cooling section 15 . Cooling section 15 includes first cooling section 16 H and second cooling section 17 L.

First compressor 161 H in first cooling section 16 H compresses first refrigerant, and discharges high-temperature and high-pressure gas refrigerant. First front stage condenser 162 H and first rear stage condenser 163 H cool discharge refrigerant received from first compressor 161 H, and discharge the cooled refrigerant. Fan 167 H is disposed herein in the vicinity of both condensers 162 H and 163 H. An airflow produced by fan 167 H promotes cooling of the first refrigerant passing through both condensers 162 H and 163 H. First expander 164 H depressurizes discharge refrigerant received from first rear stage condenser 163 H, and discharges the depressurized discharge refrigerant. First evaporator 165 H evaporates discharge refrigerant received from first expander 164 H, and discharges the evaporated discharge refrigerant.

Second compressor 171 L of second cooling section 17 L compresses second refrigerant which has a boiling point lower than a boiling point of the first refrigerant, and discharges high-temperature and high-pressure gas refrigerant. Both condensers 172 L and 173 L compress discharge refrigerant received from second compressor 171 L, and discharge the condensed discharge refrigerant. Second rear stage condenser 173 L constituting cascade condenser 18 in cooperation with first evaporator 165 H cools passing refrigerant which passes through second rear stage condenser 173 L by utilizing endothermic reaction of the first refrigerant at first evaporator 165 H, and discharges middle-temperature and high-pressure refrigerant. Second expander 174 L depressurizes discharge refrigerant received from second rear stage condenser 173 L, and discharges the depressurized discharge refrigerant. Second evaporator 175 L evaporates discharge refrigerant received from second expander 174 L, and discharges the evaporated discharge refrigerant.

Second evaporator 175 L is affixed onto an outer peripheral surface of an interior body which defines storage space S. Heat of storage space S shifts toward second evaporator 175 L by endothermic reaction caused during evaporation of the second refrigerant at second evaporator 175 L. As a result, a cooling target within storage space S is cooled.

Cooling section 15 includes at least first temperature sensor Se 11 which detects a temperature inside storage space S, and second temperature sensor Se 12 which detects a temperature inside cascade condenser 18 .

First control section 192 H included in refrigeration apparatus 10 constitutes a first feedback system in cooperation with second temperature sensor Se 12 , and controls a rotation speed of first compressor 161 H based on a deviation between a temperature detected by second temperature sensor Se 12 , and a target temperature of cascade condenser 18 . In addition, second control section 202 L constitutes a second feedback system in cooperation with first temperature sensor Se 11 , and controls a rotation speed of second compressor 171 L based on a deviation between a temperature detected by first temperature sensor Se 11 , and a target temperature inside storage space S.

1-2. Problem of Internal Temperature Control Arising from Comparative Example

For example, a warm load, such as opening of a door, may be applied to storage space S cooled to a target temperature (approximately −80° C.). In this case, there arises a first problem from the internal temperature control of the comparative example, i.e., a problem that the temperature inside storage space S is difficult to rapidly return to the target temperature. In addition to the first problem, there arises a second problem from the internal temperature control of the comparative example, i.e., a problem that unnecessary increase and decrease in the respective rotation speeds of the first compressor and the second compressor may cause a waste of energy. Both the foregoing problems are hereinafter detailed with reference to FIGS. 1 and 2 .

When a warm load is applied to storage space S having a temperature of approximately −80° C. in FIG. 2 , heat quantity Q 1 is given to the second feedback system as disturbance. Second control section 202 L increases the rotation speed of second compressor 171 L and thereby raises a flow amount of the second refrigerant flowing from second compressor 171 L toward cascade condenser 18 to return the temperature inside storage space S to the target temperature. However, a time is required to shift the second refrigerant having absorbed heat quantity Q 1 to cascade condenser 18 after the absorption of heat quantity Q 1 by the second refrigerant. In other words, heat quantity Q 2 is generated at cascade condenser 18 after an elapse of a predetermined delay time from the increase in the rotation speed of second compressor 171 L. Heat quantity Q 2 contains a part of work W 1 of second compressor 171 L as well as heat quantity Q 1 generated by disturbance.

When the disturbance is transmitted to the first feedback system as a temperature rise of cascade condenser 18 , first control section 192 H increases the rotation speed of first compressor 161 H. As a result, a flow amount of the first refrigerant flowing between first compressor 161 H and first front stage condenser 162 H increases, wherefore heat quantity Q 3 is released from both condensers 162 H and 163 H to the outside of refrigeration apparatus 10 . Heat quantity Q 3 herein contains a part of work W 2 of first compressor 161 H as well as heat quantity Q 2 (=Q 1 +W 1 ). Accordingly, a time is required to release heat quantity Q 1 generated by disturbance to the outside of refrigeration apparatus 10 .

In addition, as apparent from the above description, the temperature of cascade condenser 18 does not immediately follow the increase or decrease in the rotation speed of second compressor 171 L. Accordingly, even after second control section 202 L returns the rotation speed of second compressor 171 L to the original speed based on a deviation of substantially zero between the detection temperature of first temperature sensor Se 11 and the target temperature, the temperature inside storage space S continues decreasing for a while. Moreover, the high-temperature and high-pressure second refrigerant also flows in both condensers 172 L and 173 L for a while. Accordingly, first compressor 161 H is also rotated under control by first control section 192 H to decrease the temperature of cascade condenser 18 . As can be understood, heat transmission delay, interference or others may be caused between the first feedback system and the second feedback system in the comparative example. Accordingly, a time is required to return the temperature of storage space S to the target temperature once the temperature of storage space S is raised.

Furthermore, unnecessary increase or decrease in the rotation speeds of both compressors 161 H and 171 L is made during the time for returning the temperature of storage space S to the target temperature. As a result, a waste of energy is produced in the internal temperature control according to the comparative example.

In consideration of the aforementioned problems, an object of the present embodiment is to provide refrigeration apparatus 1 capable of rapidly returning a temperature inside storage space S and thereby reducing a waste of energy.

2. Embodiments

Refrigeration apparatus 1 according to respective embodiments is hereinafter detailed with reference to FIG. 3 and following figures.

2-1. Definition

In FIGS. 3 and 4 , an X axis indicates a transverse direction of refrigeration apparatus 1 (more specifically, direction from right to left as viewed from user facing refrigeration apparatus 1 ). A Y axis indicates a front-rear direction of refrigeration apparatus 1 (more specifically, direction from depth side (rear side) to near side (front side) as viewed from foregoing facing position of user). In addition, a Z axis indicates an up-down direction of refrigeration apparatus 1 (more specifically, direction from lower side to upper side of refrigeration apparatus 1 ).

2-2. External Appearance of Refrigeration Apparatus 1

As illustrated in FIGS. 3 and 4 , refrigeration apparatus 1 generally includes heat insulation housing 2 , heat insulation door 3 , and mechanical chamber 4 .

Heat insulation housing 2 generally includes exterior body 21 , interior body 22 , and foam heat insulation material 23 . Each of exterior body 21 and interior body 22 is made of metal, for example, and opens on the front side. Exterior body 21 defines an external shape of heat insulation housing 2 . Interior body 22 is provided inside exterior body 21 , and defines space S for storing a cooling target (hereinafter referred to as storage space). Storage space S also opens on the front side. Foam heat insulation material 23 is made of synthetic resin foam body, vacuum heat insulation material or the like, and provided between exterior body 21 and interior body 22 . Note that configurations not visually recognizable from the outside, such as foam heat insulation material 23 , are represented by dotted lines and chain lines in FIGS. 3 and 4 .

Note that inner door 24 , which is made of resin, for example, is preferably attached to the front side of interior body 22 in a manner freely openable and closable. An opening of storage space S is closed by inner door 24 in a closed state. On the other hand, in an opened state of inner door 24 , the user is allowed to access storage space S. Inner door 24 thus provided increases a heat insulation effect inside storage space S.

For example, heat insulation door 3 includes interior body 31 and exterior body 32 each made of metal, and foam heat insulation material 33 filling a space between interior body 31 and exterior body 32 . Heat insulation door 3 rotates around a rotation center axis of two hinges 34 to open and close in accordance with an operation by the user. An opening of heat insulation housing 2 is closed by heat insulation door 3 in a closed state. On the other hand, in an opened state of heat insulation door 3 , the user is allowed to open or close inner door 24 described above.

Note that handle 35 is provided on heat insulation door 3 to be held by the user at the time of opening or closing. Handle 35 preferably has a lock mechanism (not shown). The lock mechanism locks heat insulation door 3 in the closed state, and cancels the locked state to open heat insulation door 3 .

Control panel 36 is further provided on a front face of exterior body 32 of heat insulation door 3 . Control panel 36 contains control circuit board 9 (see FIG. 6 ) inside, and further includes keyboard 36 a and display 36 b in a condition operable and visible by the user. Keyboard 36 a is a device through which the user inputs settings such as a target temperature of storage space S (i.e., target value SV of internal temperature), while display 36 b is a device which displays various types of information such as a current set temperature (target value SV of internal temperature).

For example, mechanical chamber 4 is provided in a lower part of heat insulation housing 2 . Mechanical chamber 4 stores cooling section 5 except for second evaporator 75 L (see FIG. 5 ) described below.

2-3. Configuration of Cooling Section 5

As illustrated in FIG. 5 , cooling section 5 includes first cooling section 6 H and second cooling section 7 L.

First cooling section 6 H is a high-temperature side cooling section in a what is known as cascade cycle. First compressor 61 H, first front stage condenser 62 H, first rear stage condenser 63 H, first expander 64 H, and first evaporator 65 H are connected in an annular shape in such a manner as to achieve fluid communication with one another in this order to cause a flow of first refrigerant in first fluid circuit 66 H of first cooling section 6 H.

First compressor 61 H compresses the sucked first refrigerant, and discharges high-temperature and high-pressure gas refrigerant.

Each of first front stage condenser 62 H and first rear stage condenser 63 H is a condenser produced by winding a tube member made of copper or aluminum, and configured to cool and condense the first refrigerant discharged from first compressor 61 H, and discharge middle-temperature and high-pressure refrigerant. First front stage condenser 62 H is also available as a heat source for achieving dew condensation prevention of respective sections of refrigeration apparatus 1 , as well for cooling the first refrigerant.

Fan 67 H is disposed herein in mechanical chamber 4 in the vicinity of first front stage condenser 62 H and first rear stage condenser 63 H. Fan 67 H is rotated by driving force generated by motor 68 H to apply air to first front stage condenser 62 H and first rear stage condenser 63 H. The air thus applied promotes heat release to the surrounding air from the first refrigerant flowing at first front stage condenser 62 H and first rear stage condenser 63 H. Note that two condensers 62 H and 63 H are presented by way of example of condensers included in first cooling section 6 H according to the present embodiment. However, the number of condensers to be provided is not limited to two, but may be one or three or more as long as the first refrigerant can be cooled by the condensers.

First expander 64 H is constituted by an expansion valve or a capillary tube, for example, and configured to expand and depressurize the middle-temperature and high-pressure first refrigerant discharged from first rear stage condenser 63 H, and discharge low-temperature and low-pressure first refrigerant.

First evaporator 65 H evaporates (vaporizes) the low-temperature and low-pressure first refrigerant discharged from first expander 64 H, and discharges the evaporated first refrigerant. Discharge refrigerant from first evaporator 65 H is sucked into first compressor 61 H via first fluid circuit 66 H. In this manner, the first refrigerant circulates in first fluid circuit 66 H.

First evaporator 65 H further constitutes cascade condenser 8 in cooperation with second rear stage condenser 73 L described below. More specifically, cascade condenser 8 is constituted by first evaporator 65 H and second rear stage condenser 73 L integrated with each other to achieve heat exchange, and cools the second refrigerant at second rear stage condenser 73 L by utilizing endothermic reaction caused during evaporation of the first refrigerant by first evaporator 65 H. Cascade condenser 8 configured as above is constituted by any one of a receiver, a double tube, and a plate type heat exchanger, for example.

Second cooling section 7 L is a low-temperature side cooling section in a what is known as cascade cycle. Second compressor 71 L, second front stage condenser 72 L, second rear stage condenser 73 L, second expander 74 L, and second evaporator 75 L are connected in an annular shape in such a manner as to achieve fluid communication with one another in this order to cause a flow of the second refrigerant, which has a lower boiling point than a boiling point of the first refrigerant, in second fluid circuit 76 L of second cooling section 7 L.

Second compressor 71 L compresses the sucked second refrigerant, and discharges high-temperature and high-pressure gas refrigerant.

Second front stage condenser 72 L, which has configuration and function similar to the configuration and function of the first front stage condenser, condenses discharge refrigerant received from second compressor 71 L, and discharges middle-temperature and high-pressure refrigerant. Note that second front stage condenser 72 L, which is a supplemental condenser for cooling the second refrigerant on the upstream side with respect to cascade condenser 8 , may be eliminated. In addition, two or more second front stage condensers 72 L may be provided.

As described above, second rear stage condenser 73 L constituting cascade condenser 8 in cooperation with first evaporator 65 H further cools discharge refrigerant from second front stage condenser 72 L by utilizing endothermic reaction of the first refrigerant caused at first evaporator 65 H, and discharges middle-temperature and high-pressure refrigerant.

Second expander 74 L having configuration and function similar to configuration and function of first expander 64 H expands and depressurizes the middle-temperature and high-pressure second refrigerant discharged from second rear stage condenser 73 L, and discharges the low-temperature and low-pressure second refrigerant.

Second evaporator 75 L evaporates (vaporizes) the low-temperature and low-pressure second refrigerant discharged from second expander 74 L, and discharges the evaporated second refrigerant. The discharge refrigerant from the second evaporator is sucked into second compressor 71 L via second fluid circuit 76 L. In this manner, the second refrigerant circulates in second fluid circuit 76 L.

Also refer to FIG. 4 hereinafter. As illustrated in dotted lines in FIG. 4 , second evaporator 75 L is disposed between exterior body 21 and interior body 22 , and affixed to an outer peripheral face of interior body 22 . Heat from storage space S shifts toward second evaporator 75 L by endothermic reaction caused during evaporation of the second refrigerant by second evaporator 75 L. A cooling target within storage space S is cooled by the shift of heat.

2-4. Control System (Hardware Configuration) of Cooling Section 5

Pressure sensors or temperature sensors are provided at various positions on cooling section 5 described above. An important feature of the present embodiment is associated with first temperature sensor Se 1 , second temperature sensor Se 2 , third temperature sensor Se 3 , fourth temperature sensor Se 4 , fifth temperature sensor Se 5 , and sixth temperature sensor Se 6 illustrated in FIGS. 5 and 6 .

As a typical example of an internal temperature sensor, first temperature sensor Se 1 is provided inside storage space S, and configured to detect a temperature of storage space S as an internal temperature, and output a signal indicating detection value PV of the internal temperature (hereinafter abbreviated as detection value PV of internal temperature) to control circuit board 9 .

As a typical example of an ambient temperature sensor, second temperature sensor Se 2 is provided at a position surrounding refrigeration apparatus 1 (e.g., vent hole (not shown) of fan 67 H), and configured to detect an ambient temperature, and output a signal indicating a detection value of the ambient temperature (hereinafter abbreviated as detection value ST of ambient temperature) to control circuit board 9 . Note that second temperature sensor Se 2 may be further attached to a surface of exterior body 21 in a manner not affected by first rear stage condenser 63 H (e.g., inside or surface of control panel 36 ) to detect an ambient temperature.

As an example of a second fluid circuit temperature sensor, third temperature sensor Se 3 is attached to an intermediate position between second rear stage condenser 73 L constituting cascade condenser 8 , and second expander 74 L in second fluid circuit 76 L. Third temperature sensor Se 3 disposed at this attachment position detects a temperature of the second refrigerant, and outputs a signal indicating a detection value of the second refrigerant temperature (hereinafter abbreviated as detection value of second refrigerant temperature) to control circuit board 9 . Note that third temperature sensor Se 3 is attached to the intermediate position between second rear stage condenser 73 L and second expander 74 L in second fluid circuit 76 L by thermal coupling in view of attachment easiness according to the embodiment described herein. More specifically, an area around piping constituting the intermediate position between second rear stage condenser 73 L and second expander 74 L in second fluid circuit 76 L is covered with a heat insulation material, such as glass wool. In this case, third temperature sensor Se 3 is easily attachable, and less affected by the outside temperature. This configuration therefore produces such an advantage that the temperature of the second refrigerant is accurately detectable.

As an example of a cascade temperature sensor, fourth temperature sensor Se 4 is attached to a refrigerant inlet side or refrigerant outlet side of first evaporator 65 H in first fluid circuit 66 H, for example, and is configured to detect a temperature of the first refrigerant, and output a signal indicating a detection value of the first refrigerant temperature (hereinafter abbreviated as detection value of first refrigerant temperature) to control circuit board 9 . Note that FIG. 5 illustrates an example of fourth temperature sensor Se 4 attached to the refrigerant inlet side.

Note that each of third temperature sensor Se 3 and fourth temperature sensor Se 4 may be attached to various positions other than the positions described above. Examples of other positions include following positions:

refrigerant inlet side and refrigerant outlet side of first evaporator 65 H in first fluid circuit 66 H, and intermediate position between the refrigerant inlet side and outlet side of first evaporator 65 H

refrigerant inlet side and refrigerant outlet side of second rear stage condenser 73 L in second fluid circuit 76 L, and intermediate position between the refrigerant inlet side and outlet side of second rear stage condenser 73 L

position close to refrigerant inlet of second expander 74 L in second fluid circuit 76 L

In the configuration of first fluid circuit 66 H and second fluid circuit 76 L covered with glass wool or the like for heat insulation, each of third temperature sensor Se 3 and fourth temperature sensor Se 4 is capable of detecting an accurate temperature of refrigerant. Moreover, in the configuration covered with glass wool for heat insulation, attachment of third temperature sensor Se 3 and fourth temperature sensor Se 4 is more easily achievable than in case of fixation with urethane foam.

Moreover, fifth temperature sensor Se 5 and sixth temperature sensor Se 6 are attached to shell surfaces of first compressor 61 H and second compressor 71 L, respectively, and configured to output signals indicating detection values correlated to temperatures of first compressor 61 H and second compressor 71 L (hereinafter abbreviated as detection value of first compressor temperature and detection value of second compressor temperature) to control circuit board 9 . Note that fifth temperature sensor Se 5 and sixth temperature sensor Se 6 may be attached to insides of the shells of first compressor 61 H and second compressor 71 L.

In addition, control circuit board 9 is built in control panel 36 illustrated in FIG. 1 . As illustrated in FIG. 6 , at least non-volatile memory 91 , at least control section 92 , and a static random access memory (SRAM) 93 are mounted on control circuit board 9 .

Non-volatile memory 91 is constituted by a flash memory, for example, and configured to store program P. Control section 92 is typically constituted by a microcomputer, and configured to execute program P by using SRAM 93 as a working area to perform controls of respective sections included in refrigeration apparatus 1 . An important feature in these controls according to the present embodiment is associated with control of respective rotation speeds of first compressor 61 H and second compressor 71 L to adjust detection value PV of the internal temperature to target value SV. This control is hereinafter referred to as internal temperature control. According to the present embodiment, five types of internal temperature control will be presented by way of example in column 2 - 5 and following columns described below. Note that the respective rotation speeds of first compressor 61 H and second compressor 71 L are specifically a rotation speed of first motor 611 H included inside first compressor 61 H, and a rotation speed of second motor 711 L included inside second compressor 71 L, respectively.

Further specifically, control section 92 determines such rotation speeds of first motor 611 H and second motor 711 L at which deviation e between detection value PV and a target temperature of the internal temperature becomes substantially zero. Thereafter, control section 92 generates first control signal CS 1 H and second control signal CS 2 L indicating frequencies correlated with the rotation speed of first motor 611 H and the rotation speed of second motor 711 L, respectively. Control section 92 outputs first control signal CS 1 H and second control signal CS 2 L thus generated to first power supply section 612 H and second power supply section 712 L, respectively.

Each of first power supply section 612 H and second power supply section 712 L is an inverter circuit. First power supply section 612 H changes a frequency of three-phase alternating current voltage based on input first control signal CS 1 H, and supplies the changed voltage to first motor 611 H. Accordingly, first motor 611 H rotates at a rotation speed proportional to the frequency of first control signal CS 1 (i.e., rotation speed determined by control section 92 ). Second power supply section 712 L changes a frequency of three-phase alternating current voltage based on input second control signal CS 2 L, and supplies the changed voltage to second motor 711 L. Accordingly, second motor 711 L rotates at a rotation speed proportional to the frequency of second control signal CS 2 (i.e., rotation speed determined by control section 92 ).

2-5. Internal Temperature Control (Example 1)

Example 1 of the internal temperature control is now described with reference to FIGS. 7A and 7B .

Control section 92 and others start by power activation. Control section 92 initially generates first control signal CS 1 H indicating a frequency corresponding to target rotation speed A 0 to start first compressor 61 H after an elapse of a predetermined time from the power activation (step S 001 ). Target rotation speed A 0 herein is preferably a highest possible rotation speed to reduce a temperature of cascade condenser 8 to an appropriate temperature as rapidly as possible. However, target rotation speed A 0 may be a speed lower than the maximum rotation speed depending on a load or a capacity of the compressor. When first control signal CS 1 H is supplied to first power supply section 612 H, first motor 611 H (i.e., first compressor 61 H) starts (step S 002 ).

Subsequently, control section 92 acquires a detection value of the first refrigerant temperature from fourth temperature sensor Se 4 (step S 004 ) after an elapse of a predetermined time from the start of first motor 611 H (first compressor 61 H) (step S 003 ), and then determines whether or not the acquired detection value of the first refrigerant temperature is lower than or equal to a first target temperature determined beforehand (step S 005 ). When determination is NO in this step, control section 92 again executes step S 004 .

On the other hand, when determination is YES in step S 005 , control section 92 considers that first cooling section 6 H has come into a state capable of appropriately cooling the second refrigerant passing through second rear stage condenser 73 L with reduction of the temperature of cascade condenser 8 to an appropriate temperature. Accordingly, control section 92 generates second control signal CS 2 L indicating a frequency corresponding to target rotation speed B 0 to start second compressor 71 L. Target rotation speed B 0 designated herein is such an appropriate value at which the internal temperature can reach the target temperature as rapidly as possible. When second control signal CS 2 L is supplied to second power supply section 712 L, second motor 711 L (i.e., second compressor 71 L) starts (step S 006 ).

Meanwhile, present refrigeration apparatus 1 includes keyboard 36 a through which target value SV of the internal temperature is set. In response to an operation of keyboard 36 a by the user, control section 92 acquires target value SV of the internal temperature set by the user from keyboard 36 a , and writes the acquired target value SV to a storage area reserved in non-volatile memory 91 or others in parallel with the processes shown in FIGS. 7A and 7B .

Subsequently to step S 006 , control section 92 acquires target value SV of the internal temperature from non-volatile memory 91 or others, and acquires detection value PV of the internal temperature from first temperature sensor Se 1 . Thereafter, control section 92 adds a predetermined temperature to target value SV of the internal temperature to obtain first reference value Vref 1 as an example of a temperature reference value (step S 007 in FIG. 7B ). The predetermined temperature in step S 007 is a positive value of approximately +4° C., for example.

Subsequently, control section 92 determines whether or not detection value PV of the internal temperature is lower than or equal to first reference value Vref 1 obtained in step S 007 (step S 008 ). When determination is YES in this step, control section 92 performs feedback control based on target value SV and detection value PV of the internal temperature. More specifically, control section 92 initially obtains deviation e between detection value PV and target value SV of the internal temperature (step S 009 ). Subsequently, control section 92 performs PI control (proportional-integral control) which brings deviation e close to zero to calculate target rotation speed B of second compressor 71 L. However, maximum rotation speed Bmax has been set for second motor 711 L, wherefore an upper limit of target rotation speed B is set to maximum rotation speed Bmax (step S 010 ).

Note that target rotation speed B is calculated by PI control in a preferable mode according to the present embodiment. Target rotation speed B is thus calculated for following reasons. When only proportional control (P control) is performed, deviation e may remain for a long period depending on the outside air temperature. For eliminating remaining deviation e within a short period, PI control, which performs integral control (I control) using an integrated value of deviation e as well as P control, is executed.

In case of the foregoing PI control which uses an integrated value, there also arise problems such as a necessity of a time for adjusting detection value PV of the internal temperature to target value SV. For solving these problems, control section 92 preferably executes proportional-integral-derivative control (PID control) which performs derivative control (D control) using a change with time of deviation e as well as the PI control.

Meanwhile, as described in column 1 - 2 (see FIG. 2 ), with heat quantity Q 1 given to the feedback system as disturbance, heat quantity Q 2 (=Q 1 +W 1 ) is generated at cascade condenser 8 , while heat quantity Q 3 (=Q 1 +W 1 +W 2 ) is generated at both condensers 62 H and 63 H of first cooling section 6 H. Each of heat quantities Q 2 and Q 3 is determined by heat quantity Q 1 . Accordingly, once target rotation speed B of second compressor 71 L is determined based on heat quantity Q 1 (i.e., deviation e) generated by disturbance, target rotation speed A at which first compressor 61 H is operated to return the internal temperature to target value SV can be obtained beforehand based on determined target rotation speed B through experiments or the like carried out in a design development stage of present refrigeration apparatus 1 . Experiments and investigations conducted by the present inventor have revealed that target rotation speeds A and B exhibit a predetermined correspondence relation with each other. More specifically, it has been clarified that there is a positive correlation between target rotation speeds A and B (i.e., such relation that one increases as the other increases). The present inventor has found that first cooling section 6 H and second cooling section 7 L operate most efficiently when target rotation speed A is a value of target rotation speed B multiplied by predetermined coefficient k (k: value ranging from 0.25 to 4.00 (inclusive)) (i.e., when target rotation speed A is proportional to target rotation speed B) in a specific embodiment.

Considering the above point, control section 92 multiplies target rotation speed B determined in step S 010 by predetermined coefficient k after completion of step S 010 to calculate target rotation speed A proportional to target rotation speed B (step S 011 ).

Subsequently, control section 92 determines whether or not target rotation speed A calculated in step S 011 exceeds maximum rotation speed Amax of first motor 611 H (step S 012 ). When determination is YES in this step, control section 92 sets target rotation speed A to maximum rotation speed Amax (step S 013 ).

After step S 013 or when determination is NO in step S 012 , control section 92 determines whether or not either one of target rotation speeds A and B obtained in step S 013 or earlier is lower than minimum rotation speeds Amin and Bmin of motors 611 H and 711 L, respectively (steps S 014 A and S 014 B). When determination is YES in either one of steps S 014 A and S 014 B, one of motors 611 H and 711 L does not appropriately operates. In this case, control section 92 generates first control signal CS 1 H and second control signal CS 2 L to temporarily stop both motors 611 H and 711 L in consideration that the internal temperature is difficult to return to target value SV. When control signals CS 1 H and CS 2 L are supplied to power supply sections 612 H and 712 L, motors 611 H and 711 L stop (step S 015 ).

Subsequently, control section 92 acquires detection value PV of the internal temperature from first temperature sensor Se 1 (step S 016 in FIG. 7A ), and then determines whether or not detection value PV of the internal temperature acquired in step S 016 exceeds target value SV (step S 017 ). Steps S 016 and S 017 are repeated until determination of YES in step S 017 . When determination is YES in step S 017 , control section 92 starts first motor 611 H to operate first motor 611 H at target rotation speed A 0 by a method similar to the method in step S 002 (step S 018 ). Thereafter, control section 92 again executes step S 004 .

Again refer to steps S 014 A and S 014 B in FIG. 7B . When determination is NO in both the steps, control section 92 generates first control signal CS 1 H and second control signal CS 2 L for operating motors 611 H and 711 L at target rotation speeds A and B, respectively. When control signals CS 1 H and CS 2 L are supplied to power supply sections 612 H and 712 L, motors 611 H and 711 L soon come to operate at target rotation speeds A and B (step S 019 ). In addition, control section 92 preferably changes a rotation speed of motor 68 H for fan 67 H in accordance with target rotation speed A or target rotation speed B in this step. For example, when target rotation speed A is lower than 2500 min −1 , the rotation speed of motor 68 H is relatively lowered. When target rotation speed A is higher than or equal to 3000 min −1 , the rotation speed of motor 68 H is relatively raised. In this case, efficient cooling is achievable for the first refrigerant passing through the insides of both condensers 62 H and 63 H. Thereafter, control section 92 executes step S 020 .

Subsequently, control section 92 acquires detection value PV of the internal temperature from first temperature sensor Se 1 and updates acquired detection value PV (step S 020 ), and then again executes step S 008 .

On the other hand, when determination is NO in step S 008 , control section 92 generates first control signal CS 1 H and second control signal CS 2 L for operating first motor 611 H and second motor 711 L at maximum rotation speeds Amax and Bmax, respectively, based on a state that deviation e between detection value PV and target value SV of the internal temperature is still large. When first control signal CS 1 H and second control signal CS 2 L thus generated are supplied to first power supply section 612 H and second power supply section 712 L, respectively, first motor 611 H and second motor 711 L soon come to operate at maximum rotation speeds Amax and Bmax (step S 021 ). Thereafter, control section 92 again executes step S 008 after completion of step S 020 .

2-6. Operation and Effect of Internal Temperature Control (Example 1)

According to the present internal temperature control (Example 1) described above, control section 92 operates compressors 61 H and 71 L at maximum rotation speeds Amax and Bmax, respectively, while detection value PV of the internal temperature exceeds first reference value Vref 1 (i.e., while determination is NO in step S 008 ) (step S 021 in FIG. 7B ). Accordingly, even when the temperature inside storage space S is raised in response to opening of inner door 24 or for other reasons, the internal temperature can rapidly reach a temperature slightly higher than target value SV of the internal temperature (first reference value Vref 1 ) (see period from time t 14 to time t 2 in FIG. 8 ).

In addition, after detection value PV of the internal temperature reaches first reference value Vref 1 , control section 92 determines target rotation speeds A and B (steps S 010 and S 011 in FIG. 7B ). In principle (basically), compressors 61 H and 71 L are operated at target rotation speeds A and B, respectively (step S 019 ). At this time, target rotation speed B is determined by PI control based on deviation e between detection value PV and target value SV of the internal temperature, while target rotation speed A is set to a value corresponding to target rotation speed B determined by PI control. According to the present embodiment, target rotation speed A is calculated by multiplying target rotation speed B by predetermined coefficient k.

By elimination of deviation e under the internal temperature control described above, the internal temperature returns to target value SV. According to the internal temperature control performed as above, target rotation speeds A and B are determined by the single feedback system as illustrated in FIG. 9 after detection value PV of the internal temperature reaches first reference value Vref 1 . Particularly, target rotation speed A is determined based on target rotation speed B calculated by PI control regardless of the temperature of cascade condenser 8 . In this case, first compressor 61 H is operated without effects such as heat transmission delay and interference described in column 1 - 2 . Detection value PV of the internal temperature therefore is not excessively reduced to a temperature lower than target value SV (see time t 2 and later in FIG. 8 ). Accordingly, efficient operations of compressors 61 H and 71 L are achievable without producing a waste of energy by present refrigeration apparatus 1 .

In addition to the above description, before detection value PV of the internal temperature reaches first reference value Vref 1 (see period from t 1 to t 2 in FIG. 8 ), the rotation speeds of compressors 61 H and 71 L are set to maximum rotation speeds Amax and Bmax. After detection value PV reaches first reference value Vref 1 (see time t 2 and later in FIG. 8 ), the rotation speeds of compressors 61 H and 71 L are changed to target rotation speeds A and B determined by the single feedback system. In this case, both compressors 6 H and 7 L are operated without effects such as heat transmission delay and interference as described in column 1 - 2 . Accordingly, refrigeration apparatus 1 provided herein is capable of rapidly returning the temperature inside storage space S to target value SV even at the time of a rise of the temperature of storage space S.

2-7. (Other) Operation and Effect of Internal Temperature Control (Example 1)

Moreover, according to the present internal temperature control, only first cooling section 6 H is operated immediately after power activation. Accordingly, high-load operation of second cooling section 7 L is avoidable.

2-8. Note to Internal Temperature Control (Example 1)

According to the description of the internal temperature control (Example 1), target rotation speed A is obtained by multiplying target rotation speed B by coefficient k. However, in a mode where non-volatile memory 91 stores beforehand a table which describes a correspondence relation between target rotation speeds A and B (positive correlation) (i.e., appropriate target rotation speed A for each target rotation speed B), control section 92 may read target rotation speed A corresponding to target rotation speed B obtained in step S 010 from the table, and designate read target rotation speed A as target rotation speed A to be set in step S 011 .

Note that the description in column 2 - 8 is applicable to internal temperature control (Example 2 to Example 5) described below.

2-9. Internal Temperature Control (Example 2)

Example 2 of the internal temperature control is now described with reference to FIGS. 10A and 10B . Flows in FIGS. 10A and 10B are different from flows in FIGS. 7A and 7B in that steps S 101 to S 105 are included instead of steps S 004 to S 006 . No further difference is present between both the flowcharts. Accordingly, steps in FIGS. 10A and 10B similar to corresponding steps in FIGS. 7A and 7B are given identical step numbers, and the same detailed description is not repeated herein.

After completion of step S 003 in FIG. 10A , control section 92 starts second motor 711 L (i.e., second compressor 71 L) to rotate second motor 711 L at target rotation speed B 0 in a manner similar to the manner of step S 006 described above (step S 101 ).

Note that a time elapsed for a predetermined length from the start of first compressor 61 H (delay time) in step S 003 in FIG. 10A , i.e., a time until a start of second compressor 71 L in step S 101 after the start of first compressor 61 H may be determined in accordance with the second refrigerant temperature or an elapsed time from a stop of second compressor 71 L.

More specifically, the delay time is reduced in accordance with a detection value acquired by third temperature sensor Se 3 as the second refrigerant temperature when the acquired detection value is lower than or equal to −20° C. with an elapsed time shorter than two hours from the stop of second compressor 71 L, for example. In a specific example, the delay time is set to one minute.

On the other hand, when the value acquired by third temperature sensor Se 3 exceeds −20° C. with an elapsed time exceeding two hours from the stop of second compressor 71 L, the delay time is prolonged. In a specific example, the delay time is set to eight minutes.

As described above, when the second refrigerant temperature is a temperature lower than a predetermined value, or when an elapsed time from the stop of second compressor 71 L is not longer than a predetermined time, the time until the start of second compressor 71 L is reduced in step S 101 .

In this case, the time until the start of second compressor 71 L from the start of first compressor 61 H does not become excessively long in the state that the second refrigerant temperature is lower than the predetermined value, and that a long time is not elapsed yet after the stop of second compressor 71 L. Accordingly, the internal temperature control is more efficiently achievable.

Subsequently, control section 92 acquires a detection value of the second refrigerant temperature from third temperature sensor Se 3 (step S 102 ), and determines whether or not the acquired detection value of the second refrigerant temperature is lower than or equal to a second target temperature determined beforehand (step S 103 ). When determination is YES in this step, control section 92 executes step S 007 and following steps (described above) in FIG. 10B .

On the other hand, when determination is NO in step S 103 , control section 92 stops operation of second compressor 71 L for a fixed period to stop high-load operation of second cooling section 7 L, and then restarts second compressor 71 L (steps S 104 and S 105 ). Thereafter, control section 92 again executes step S 102 to recheck the second refrigerant temperature.

2-10. Operation and Effect of Internal Temperature Control (Example 2)

According to the internal temperature control (Example 2) described above, the rotation speeds of both compressors 61 H and 71 L are controlled similarly to the internal temperature control (Example 1). Accordingly, operation and effect similar to those described in column 2 - 6 are offered. Moreover, high-load operation of second cooling section 7 L immediately after power activation is avoidable by steps S 101 to S 105 . A heat quantity generated by opening of heat insulation door 3 or inner door 24 during operation of present refrigeration apparatus 1 , or by loading of a warm cooling target is sequentially transmitted from second cooling section 7 L to first cooling section 6 H. In this case, the foregoing heat quantity change status is detectable earlier than a temperature change of the first refrigerant based on detection of the temperature of the second refrigerant, as in the manner of the internal temperature control (Example 2). Accordingly, the internal temperature control is more accurately achievable.

2-11. Note to Internal Temperature Control (Example 2)

According to the above description of the internal temperature control (Example 2), control section 92 stops second compressor 71 L for the fixed period, and then restarts second compressor 71 L in steps S 104 and S 105 . However, when target rotation speed A 0 of first compressor 61 H set in step S 002 is lower than or equal to maximum rotation speed Amax, control section 92 may set the rotation speed of first compressor 61 H to a speed higher than target rotation speed A 0 set in step S 002 , rather than performing steps S 104 and S 105 . In this manner, a high-load state of second cooling section 7 L immediately after power activation is avoidable.

In addition, control section 92 may start second compressor 71 L in step S 101 in FIG. 10A , acquire a detection value of the second refrigerant temperature from third temperature sensor Se 3 in step S 102 , and then stop second compressor 71 L in accordance with the second refrigerant temperature after an elapse of a predetermined time from the start of second compressor 71 L.

For example, second compressor 71 L may be stopped in accordance with a detection value acquired as the second refrigerant temperature by third temperature sensor Se 3 , such as a value higher than or equal to −10° C., after an elapse of 30 seconds as the predetermined time from the start of second compressor 71 L.

In this case, second compressor 71 L is not operated in a state that the second refrigerant temperature is higher than the predetermined temperature. Accordingly, a high-load state of second cooling section 7 L immediately after power activation is avoidable.

Moreover, the predetermined time from the start of second compressor 71 L in this case may be changed in accordance with conditions.

These conditions include such a condition that the second refrigerant temperature higher than a predetermined value is lowering. In this condition, the predetermined time after the start of second compressor 71 L is set to a period longer than the corresponding predetermined time in other situations.

More specifically, when the second refrigerant temperature higher than −10° C. is lowering, for example, the predetermined time from the start of second compressor 71 L is set to 120 seconds.

In this manner, more efficient internal temperature control is achievable by changing the predetermined time from the start of second compressor 71 L to the stop of second compressor 71 L in accordance with conditions.

Second compressor 71 L is restarted in response to confirmation that a predetermined time (first predetermined time) has elapsed after the stop of the second refrigerant by the foregoing steps, and that the temperature of the second refrigerant is lower than a predetermined value, or in response to an elapse of a maximum predetermined time longer than the foregoing first predetermined time.

More specifically, second compressor 71 L is restarted in response to confirmation that a period of four minutes as the first predetermined time has elapsed after the stop of the second refrigerant, and that the temperature of the second refrigerant is lower than or equal to −34° C., or in response to an elapse of 15 minutes as the maximum predetermined time from the stop of the second refrigerant.

In addition, control section 92 may temporarily raise the number of rotations of first compressor 61 H in accordance with the outside temperature only for a predetermined time after an elapse of a predetermined time from the start of second compressor 71 L in step S 101 . For example, when an ambient temperature detected by second temperature sensor Se 2 is higher than or equal to 32° C., the rotation speed of first compressor 61 H is raised from 3600 rpm (one example of target rotation speed A 0 ) to 4000 rpm. In other words, the number of rotations of first compressor 61 H is temporarily raised to a speed higher than target rotation speed A 0 when the outside temperature is higher than a predetermined value.

More specifically, the number of rotations of first compressor 61 H is raised after an elapse of 20 seconds, for example, from the start of second compressor 71 L in step S 101 . Thereafter, the rotation speed of first compressor 61 H is returned to target rotation speed A 0 when the second refrigerant temperature has become lower than or equal to a predetermined value with detection of a high-temperature peek value of the second refrigerant temperature after operation of first compressor 61 H having the raised number of rotations. For example, the rotation speed of first compressor 61 H is returned to target rotation speed A 0 when the second refrigerant temperature becomes lower than or equal to −30° C.

More specifically, when the outside temperature is higher than a predetermined value, the number of rotations of first compressor 61 H is temporarily raised after an elapse of a predetermined time from the start of second compressor 71 L. First compressor 61 H is operated in the state of the raised number of rotations, whereafter the rotation speed of first compressor 61 H is returned to target rotation speed A 0 in accordance with the second refrigerant temperature.

In this manner, performance of first cooling section 6 H increases by the rise of the rotation speed of first compressor 61 H in a state of the excessively high outside temperature. Accordingly, a high-load state of second cooling section 7 L immediately after power activation is avoidable.

2-12. Internal Temperature Control (Example 3)

Example 3 of the internal temperature control is now described with reference to FIGS. 11A and 11B .

After power activation, control section 92 initially acquires target value SV of the internal temperature from non-volatile memory 91 or others, and also acquires detection value PV of the internal temperature from first temperature sensor Se 1 (see step S 201 in FIG. 11A ). Subsequently, control section 92 obtains deviation e between detection value PV and target value SV of the internal temperature thus acquired, and stores deviation e in SRAM 93 or others (step S 202 ).

Thereafter, control section 92 determines whether or not current deviation e is larger than or equal to second reference value Vref 2 determined beforehand (step S 203 ). Second reference value Vref 2 , as an example of a second deviation reference value, corresponds to a reference temperature for starting both compressors 61 H and 71 L, and is set to approximately 50° C., for example. When determination is NO in this step, control section 92 again executes step S 201 .

On the other hand, when determination is YES in step S 203 , control section 92 generates control signals CS 1 H and CS 2 L for operating motors 611 H and 711 L (i.e., compressors 61 H and 71 L) at target rotation speeds A 0 and B 0 , respectively, and supplies generated control signals CS 1 H and CS 2 L to power supply sections 612 H and 712 L, respectively. In this manner, control section 92 simultaneously starts both motors 611 H and 711 L (step S 204 ). Target rotation speeds A 0 and B 0 at this time are set to appropriate speeds at which cascade condenser 8 and others can reach an appropriate temperature as rapidly as possible.

Subsequently, control section 92 acquires a detection value of the second refrigerant temperature from third temperature sensor Se 3 (step S 205 ), and determines whether or not the acquired detection value of the second refrigerant temperature is lower than or equal to a second target temperature determined beforehand (step S 206 ).

When determination is NO in step S 206 , control section 92 stops second compressor 71 L for a predetermined time in consideration that second cooling section 7 L is in a high-load state, and then restarts second compressor 71 L (steps S 207 and S 208 ). Thereafter, control section 92 again executes step S 205 to recheck the second refrigerant temperature.

On the other hand, when determination is YES in step S 206 , control section 92 acquires detection value PV of the internal temperature from first temperature sensor Se 1 , and also acquires target value SV stored in non-volatile memory 91 or others (step S 209 in FIG. 11B ). Subsequently, control section 92 obtains deviation e between detection value PV and target value SV acquired in step S 209 , and then adds a predetermined temperature to deviation e to obtain third reference value Vref 3 as an example of a first deviation reference value (step S 210 ). The predetermined temperature in step S 210 is a positive value of approximately +4° C., for example.

Subsequently, control section 92 determines whether or not current deviation e is lower than or equal to third reference value Vref 3 obtained in step S 210 (step S 211 ).

When determination is NO in step S 211 , control section 92 operates first motor 611 H and second motor 711 L at maximum rotation speeds Amax and Bmax, respectively, in a manner similar to the manner of step S 021 described above (step 3212 ).

Subsequently, control section 92 acquires detection value PV of the internal temperature from first temperature sensor Se 1 , obtains and updates deviation e based on acquired detection value PV and target value SV (step S 213 ), and then again executes step S 211 .

On the other hand, when determination is YES in step S 211 , control section 92 calculates target rotation speed B of second compressor 71 L in a manner similar to the manners of steps S 010 and S 011 described above, and then calculates target rotation speed A having a correspondence relation with target rotation speed B (steps S 214 and S 215 ).

Subsequently, control section 92 sets target rotation speed A to maximum rotation speed Amax when calculated target rotation speed A exceeds maximum rotation speed Amax in a manner similar to the manner of steps S 012 and S 013 described above (steps S 216 and S 217 ). When determination is NO in step S 216 or after step S 217 , control section 92 determines whether or not obtained target rotation speed A is lower than minimum rotation speed Amin, and whether or not obtained target rotation speed B is lower than minimum rotation speed Bin, in a manner similar to the manner of steps S 014 A and S 014 B after completion of step S 217 (steps S 218 A and S 218 B). When determination is YES in either one of steps S 218 A and S 218 B, motors 611 H and 711 L are stopped in a manner similar to the manner of step S 015 (step S 219 ). Thereafter, control section 92 again executes step S 201 in FIG. 11A .

On the other hand, when determination is NO in steps S 218 A and S 218 B in FIG. 11B , control section 92 operates motors 611 H and 711 L at target rotation speeds A and B, respectively, in a manner similar to the manner of step S 019 described above (step S 220 ). In addition, control section 92 in this step preferably changes a rotation speed of motor 68 H for fan 67 H in accordance with target rotation speed A in a manner similar to the manner of step S 019 described above. Thereafter, control section 92 again executes step S 213 .

2-13. Specific Example of Internal Temperature Control (Example 3)

Described now with reference to FIG. 12 is an example of changes of rotation speeds of first compressor 61 H and second compressor 71 L produced when the internal temperature control (Example 3) described above are executed under predetermined conditions.

Suppose that detection value PV of the internal temperature is 20° C., and that target value SV is −80° C. when power is activated for storage space S not cooled at all. Deviation e at this time is 100° C.

In addition, assuming that second reference value Vref 2 is 5° C., it is determined that current deviation e (100° C.) is higher than or equal to second reference value Vref 2 in step S 203 . In this case, compressors 61 H and 71 L are started to operate at target rotation speeds A 0 and B 0 , respectively, in step S 204 . As a result, the first refrigerant and the second refrigerant start circulating within first fluid circuit 66 H and second fluid circuit 76 L.

When it is determined in step S 206 that the second refrigerant temperature is higher than the second target temperature, second compressor 71 L is stopped for a predetermined time and restarted in steps S 207 and S 208 as described above. Alternatively, second compressor 71 L may be operated at a low rotation speed for the predetermined time.

When it is determined in step S 206 that the second refrigerant temperature is lower than or equal to the second target temperature, compressors 61 H and 71 L are operated at maximum rotation speeds Amax and Bmax, respectively, until determination becomes YES in step S 211 as indicated from time t 1 to time t 2 in FIG. 8 . When determination is YES in step S 211 , target rotation speed B of second compressor 71 L is calculated by PI control shown in step S 214 and following steps. Besides, target rotation speed A is calculated by multiplying target rotation speed B by coefficient k.

When it is determined that calculated target rotation speed A exceeds maximum rotation speed Amax, target rotation speed A is limited within maximum rotation speed Amax (e.g., 4500 min −1 ) by processing in steps S 216 and S 217 as indicated by broken line segments connecting ( 4 ) and ( 5 ) in FIG. 12 .

When either one of calculated target rotation speeds A and B is lower than minimum rotation speeds Amin and Bmin (e.g., 1500 min −1 ), respectively, refrigeration apparatus 1 does not appropriately operate by operations of compressors 61 H and 71 L at speeds lower than minimum rotation speeds Amin and Bmin. Accordingly, both compressors 61 H and 71 L are stopped at any calculated value of rotation speed B under PI control by processing in steps S 218 A, S 218 B, and S 219 as indicated by a solid line segment connecting ( 1 ) and ( 2 ) in FIG. 12 .

When calculated target rotation speeds A and B are higher than or equal to minimum rotation speeds Amin and Bmin, respectively, first compressor 61 H is operated at target rotation speed A (=k×B) as indicated by a broken line segment connecting ( 3 ) and ( 4 ) in FIG. 12 . In addition, second compressor 71 L is operated at target rotation speed B as indicated by a solid straight line connecting ( 6 ) and ( 5 ) in FIG. 12 . Note that FIG. 12 shows an example when k is 1.2.

Detection value PV of the internal temperature becomes substantially equivalent to target value SV by cyclically repeating the foregoing processing in steps S 214 to S 218 B, S 220 , and S 213 (see time t 2 and later in FIG. 8 ).

2-14. Operation and Effect of Internal Temperature Control (Example 3)

According to the internal temperature control (Example 3) described above, the rotation speeds of both compressors 61 H and 71 L are controlled similarly to the internal temperature control (Example 1). Accordingly, operation and effect similar to those described in column 2 - 6 are offered. Moreover, high-load operation of second cooling section 7 L immediately after power activation is avoidable by steps S 205 to S 208 .

2-15. Note to Internal Temperature Control (Example 3)

In the above description, a detection value of the second refrigerant temperature is acquired from third temperature sensor Se 3 , and whether or not the acquired second refrigerant temperature is lower than or equal to the second target temperature is determined in steps S 205 and S 206 in FIG. 11A . However, a detection value of the first refrigerant temperature may be acquired from fourth temperature sensor Se 4 , and whether or not the acquired first refrigerant temperature is lower than or equal to the first target temperature may be determined as shown in parentheses in steps S 205 and S 206 in FIG. 11A . In this manner, whether or not second cooling section 7 L is in a high-load operation can be similarly determined.

In addition, according to the above description, both compressors 61 H and 71 L are stopped in step S 219 in FIG. 11B . However, only second compressor 71 L may be stopped, while first compressor 61 H may be operated at minimum rotation speed Amin. In other words, first compressor 61 H may be only required to operate at a speed lower than a previous speed.

Moreover, according to the above description, target rotation speed A is calculated by multiplying target rotation speed B by coefficient k in step S 215 in FIG. 11B . Alternatively, target rotation speed A may be calculated by an equation A=k×B+c. In this case, target rotation speed A (A=k×B+c) may be higher or lower than target rotation speed B (indicated by solid line segment connecting ( 6 ) and ( 5 )) obtained by calculation of PI control as indicated by a broken line segment connecting ( 7 ) and ( 8 ) in FIG. 12 .

In addition, when present refrigeration apparatus 1 is in a normal condition, the second refrigerant temperature becomes a temperature lower than or equal to the second target temperature after repeating a loop from step S 205 to step S 208 in FIG. 11A at most several times by control section 92 . In this case, the process shifts from step S 206 to step S 209 . Accordingly, by utilizing this feature, control section 92 may display message information indicating abnormality of refrigeration apparatus 1 on display 36 b when the loop is executed a predetermined number of times or more, in consideration that refrigeration apparatus 1 is in an abnormal state.

Note that the description in present column 2 - 15 is applicable to the internal temperature control (Example 4 and Example 5) described below, and further to the internal temperature control (Example 1 and Example 2) described above.

2-16. Internal Temperature Control (Example 4)

Example 4 of the internal temperature control is now described with reference to FIGS. 13A and 13B .

Flows in FIGS. 13A and 13B are different from flows in FIGS. 11A and 11B in that steps S 301 to S 304 are included instead of steps S 204 to S 208 . No further difference is present between both the flowcharts. Accordingly, steps in FIGS. 13A and 13B similar to corresponding steps in FIGS. 11A and 11B are given identical step numbers, and the same detailed description is not repeated herein.

When determination is YES in step S 203 , control section 92 generates first control signal CS 1 H for operating first compressor 61 H at target rotation speed A 0 determined beforehand, and supplies generated first control signal CS 1 H to first power supply section 612 H. At this time, target rotation speed A 0 is set to 4000 min −1 , for example, to adjust the temperature of cascade condenser 8 to the target temperature as rapidly as possible. In addition, control section 92 generates first control signal CS 1 H for raising the rotation speed of first motor 611 H to target rotation speed A 0 based on an acceleration sequence determined beforehand (step S 301 ).

Note that an actual rotation speed of first compressor 61 H is preferably detected by first power supply section 612 H constituted by an inverter circuit, and output to control section 92 such that control section 92 can output first control signal CS 1 H with reference to the actual rotation speed. However, control section 92 is not necessarily required to generate first control signal CS 1 H based on the actual rotation speed, but may adjust the rotation speed of first compressor 61 H to target rotation speed A 0 in accordance with the predetermined acceleration sequence as long as a rotation abnormal signal or the like of first compressor 61 H is not issued.

When the rotation speed of first compressor 61 H reaches target rotation speed A 1 (A 1 : numerical value meeting A 1 ≤A 0 ) (i.e., after elapse of predetermined time) (step S 302 ), control section 92 subsequently generates second control signal CS 2 L for operating second compressor 71 L at target rotation speed B 0 determined beforehand based on an acceleration sequence determined beforehand, and supplies generated second control signal CS 2 L to second power supply section 712 L. In this case, target rotation speed B 0 is set to 2000 min −1 , for example, to adjust the temperature of cascade condenser 8 to an appropriate temperature as rapidly as possible (step S 303 ).

When the rotation speed of second compressor 71 L reaches target rotation speed B 0 (i.e., after elapse of predetermined time) (step S 304 ), control section 92 executes step S 209 and following steps (described above) in FIG. 13B .

2-17. Operation and Effect of Internal Temperature Control (Example 4)

According to the internal temperature control (Example 4) described above, the rotation speeds of both compressors 61 H and 71 L are controlled similarly to the internal temperature control (Example 1). Accordingly, operation and effect similar to those described in column 2 - 6 are offered. Moreover, high-load operation of second cooling section 7 L immediately after power activation is avoidable by steps S 301 to S 304 .

2-18. Internal Temperature Control (Example 5)

Example 5 of internal temperature control is now described with reference to FIGS. 14A and 14B .

Flows in FIGS. 14A and 14B are different from flows in FIGS. 11A and 11B in that steps S 401 to S 404 are included instead of steps S 204 to S 208 . No further difference is present between both the flowcharts. Accordingly, steps in FIGS. 14A and 14B similar to corresponding steps in FIGS. 11A and 11B are given identical step numbers, and the same detailed description is not repeated herein.

When determination is YES in step S 203 , control section 92 starts first compressor 61 H to operate first compressor 61 H at target rotation speed A 0 in a manner similar to the manner of step S 301 described above (step S 401 ).

Subsequently, control section 92 acquires a detection value of the second refrigerant temperature from third temperature sensor Se 3 (step S 402 ), and determines whether or not the acquired detection value of the second refrigerant temperature is lower than or equal to a second target temperature determined beforehand (step S 403 ). Note that a detection value of the first refrigerant temperature may be acquired from fourth temperature sensor Se 4 , and whether or not the acquired first refrigerant temperature is lower than or equal to the first target temperature may be determined as shown in parentheses in steps S 402 and S 403 in FIG. 14A , similarly to the description in column 2 - 15 . When determination is NO in step S 403 , control section 92 again executes step S 402 .

On the other hand, when determination is YES in step S 403 , control section 92 generates second control signal CS 2 L indicating a frequency corresponding to target rotation speed B 0 to start second compressor 71 L in consideration that a sufficient condition has been achieved for appropriate cooling of the second refrigerant passing through second rear stage condenser 73 L, and supplies generated second control signal CS 2 L to second power supply section 712 L. As a result, second motor 711 L (i.e., second compressor 71 L) starts (step S 404 ).

Thereafter, control section 92 executes step S 209 and following steps (described above) in FIG. 14B .

2-19. Operation and Effect of Internal Temperature Control (Example 5)

Operation and effect described in column 2 - 6 are similarly offered by the internal temperature control (Example 5) described above. Moreover, high-load operation of second cooling section 7 L immediately after power activation is avoidable by steps S 401 to S 404 .

2-20. Determination of Optimum Coefficient k

According to the internal temperature control described above, coefficient k may be a fixed value in a preferable range from 0.25 to 4.00 (inclusive). However, based on extensive investigations repeated by the present inventor, it has clarified that a ratio of target rotation speeds A to B (i.e., k) is preferably varied in accordance with target value SV of the internal temperature and an ambient temperature to minimize total power consumption of both compressors 61 H and 71 L (hereinafter abbreviated as total power consumption).

Curve C 1 in FIG. 15 represents a correspondence relation between rotation speeds A and B appropriate for obtaining an internal temperature of −70° C. under an ambient temperature condition of 15° C. A point at which the total power consumption becomes the minimum on curve C 1 is point ( 1 ) (i.e., target rotation speeds A and B are approximately 1900 min −1 and 2000 min −1 ).

FIG. 15 also shows curves C 2 , C 3 , and C 4 . Curve C 2 represents a correspondence relation between rotation speeds A and B for obtaining an internal temperature of −70° C. at an ambient temperature of 30°. Curves C 3 and C 4 represent a correspondence relation between rotation speeds A and B for obtaining an internal temperature of −80° C. at ambient temperatures of 15° C. and 30° C., respectively. Points at which the total power consumption becomes the minimum on curves C 2 , C 3 , and C 4 are points ( 2 ), ( 3 ), and ( 4 ), respectively.

Points ( 1 ) to ( 4 ) can be obtained by measuring the total power consumption while appropriately changing the ambient temperature and a value of the internal temperature, and changing the rotation speeds of both compressors 61 H and 71 L in a design development stage of present refrigeration apparatus 1 . According to actual measurements thus obtained, target rotation speeds A and B at point ( 2 ) were 2300 min −1 and 2000 min −1 , target rotation speeds A and B at point ( 3 ) were 2400 min −1 and 2250 min −1 , and target rotation speeds A and B at point ( 4 ) were 3000 min −1 and 2250 min −1 , respectively.

Coefficient k is a value A/B, wherefore k is preferably 1900/2000=0.95 for obtaining an internal temperature of −70° C. under an ambient temperature condition of 15° C. Similarly, coefficient k is preferably 2300/2000=1.15 for obtaining an internal temperature of −70° C. under an ambient temperature condition of 30° C. Coefficient k is preferably 2400/2250=1.06 for obtaining an internal temperature of −80° C. under an ambient temperature condition of 15° C. Coefficient k is preferably 3000/2250=1.33 for obtaining an internal temperature of −80° C. under an ambient temperature condition of 30° C.

The present inventor calculated optimum coefficient k for each of several combinations of the ambient temperature and the target value of the internal temperature in the foregoing manner. Results thus obtained are shown in following Table 1. Note that coefficient k for an ambient temperature of 23° C. corresponds to an interpolated value of respective coefficients k for ambient temperatures of 15° C. and 30° C. in following Table 1.

[Table 1]

Control section 92 retains beforehand a table (Table 1 shown above) which describes optimum coefficient k for each combination of the ambient temperature and the internal temperature (target value). In steps S 011 in FIG. 7B and FIG. 10B , or in step S 215 in FIGS. 11B, 13B, and 14B , before calculation of target rotation speed A, control section 92 acquires detection value ST of the ambient temperature from second temperature sensor Se 2 , and target value SV of the internal temperature stored in non-volatile memory 91 or others, and acquires, from the table, and determines coefficient k corresponding to a combination of detection value ST of the ambient temperature and target value SV of the internal temperature thus acquired (in other words, correspondence relation between rotation speeds A and B). Note that control section 92 obtains coefficient k by the interpolation process described above when coefficient k corresponding to detection value ST of the ambient temperature and target value SV of the internal temperature thus acquired is not described in the table.

Control section 92 calculates target rotation speed A in step S 011 in FIGS. 7B and 10B , or in step S 215 in FIGS. 11B, 13B, and 14B by utilizing coefficient k obtained as described above. In this manner, the total power consumption in the internal temperature control can be minimized.

2-21. Various Safety Functions of Present Refrigeration Apparatus 1 (Example 1)

As described above, each of first power supply section 612 H and second power supply section 712 L includes an inverter circuit. This inverter circuit has a built-in protection function in some cases. For example, first power supply section 612 H and second power supply section 712 L output a first abnormal signal and a second abnormal signal, respectively, to control section 92 at the time of a failure of start of first motor 61 H and second motor 72 L, an abnormal operation such as an excessive load, a flow of large current, or on other occasions.

Control section 92 executes interruption processing or the like in consideration that second cooling section 7 L is operating in a high-load state when receiving the first abnormal signal from first power supply section 612 H during output of control signals CS 1 H and CS 2 L to power supply sections 612 H and 712 L, respectively, to instruct operations of both compressors 61 H and 71 L. In the interruption processing or the like, control section 92 reduces the operation speed of second compressor 71 L causing a high load to a speed lower than the previous speed, confirms an elapse of a fixed time or cancellation of the abnormal signal, and returns the operation speed to the original speed. Alternatively, control section 92 temporarily stops operation of second compressor 71 L, and restarts second compressor 71 L after an elapse of a fixed time.

On the other hand, control section 92 having received the second abnormal signal reduces the operation speed of second compressor 71 L to a speed lower than the previous speed, confirms an elapse of a fixed time or cancellation of the abnormal signal, and returns the operation speed to the original speed. Alternatively, control section 92 temporarily stops operation of second compressor 71 L, and restarts second compressor 71 L after an elapse of a fixed time.

2-22. Various Safety Functions of Present Refrigeration Apparatus 1 (Example 2)

Moreover, in Example 1 through Example 5 of the internal temperature control, control section 92 regularly acquires a detection value of the temperature of cascade condenser 8 from at least either one of third temperature sensor Se 3 and fourth temperature sensor Se 4 , and performs such control as to return the acquired detection value to a temperature within a predetermined temperature range when determining that the acquired detection value is a value out of the predetermined temperature range. The predetermined temperature range herein is a temperature range intended to be maintained by cascade condenser 8 for protection of the compressors. In a specific example, control section 92 changes or adjusts to zero at least either one of the rotation speed of first compressor 61 H and the rotation speed of second compressor 71 L when the detection value of the temperature of cascade condenser 8 is lower than a lower limit value in the predetermined temperature range.

2-23. Various Safety Functions of Present Refrigeration Apparatus 1 (Example 3)

In addition to the above description, in Example 1 through Example 5 of the internal temperature control, control section 92 regularly acquires a detection value of the second refrigerant temperature from third temperature sensor Se 3 during operations of first compressor 61 H and second compressor 71 L, and performs such control as to return the acquired detection value to a temperature within a predetermined temperature range when determining that the acquired detection value is a value out of the predetermined temperature range. In a specific example, control section 92 changes or adjusts to zero at least either one of the rotation speed of first compressor 61 H and the rotation speed of second compressor 71 L when the detection value of the second refrigerant temperature is lower than a lower limit value in the predetermined temperature range.

2-24. Various Safety Functions of Present Refrigeration Apparatus 1 (Example 4)

In addition to the above description, in Example 1 through Example 5 of the internal temperature control, control section 92 regularly acquires a detection value of the first compressor temperature and a detection value of the second compressor temperature from fifth temperature sensor Se 5 and sixth temperature sensor Se 6 , respectively, during operations of first compressor 61 H and second compressor 71 L. When determining that at least one of the acquired detection values is higher than or equal to a predetermined temperature, control section 92 reduces or sets to zero the rotation speed or speeds of at least one or both of first compressor 61 H and second compressor 71 L, in consideration that at least either first compressor 61 H or second compressor 71 L is an abnormal overheat state. When determining that at least one of the acquired detection values is higher than or equal to the predetermined temperature, control section 92 may further change the rotation speed of motor 68 H for fan 67 H to a speed higher than the previous speed.

The disclosure of Japanese Patent Application No. 2016-028126, filed on Feb. 17, 2016, including the specification, drawings, and abstract, is incorporated herein by reference in its entirety.

A refrigeration apparatus according to the present invention is capable of rapidly returning a temperature of a storage space, and is suitable for an ultra-deep freezer or the like.

1 Refrigeration apparatus

S Storage space

6 H First cooling section

61 H First compressor

611 H First motor

612 H First power supply section

62 H First front stage condenser

63 H First rear stage condenser

64 H First expander

65 H First evaporator

66 H First fluid circuit

67 H Fan

68 H Motor

7 L Second cooling section

71 L Second compressor

711 L Second motor

712 L Second power supply section

72 L Second front stage condenser

73 L Second rear stage condenser

74 L Second expander

75 L Second evaporator

76 L Second fluid circuit

8 Cascade condenser

92 Control section

Se 1 First temperature sensor (internal temperature sensor)

Se 2 Second temperature sensor (ambient temperature sensor)

Se 3 Third temperature sensor (second fluid circuit temperature sensor)

Se 4 Fourth temperature sensor (cascade temperature sensor)