BACKGROUND
1. TECHNICAL FIELD
[0001] The present invention relates to a freezing device, a freezing system, and a control
method of a freezing device.
2. DESCRIPTION OF RELATED ART
[0004] However, even when a higher pressure of the CO
2 refrigerant is set to be in the transcritical mode, when the outside air temperature
is lower than the critical point of CO
2, the high-pressure value of the CO
2 refrigerant may be greater than is required relative to an amount of exhaust heat
required to be removed by a gas cooler. Further, since the rotational rate of a compressor
is increased in order to have the transcritical mode, power loss is increased. Due
to the above, there is a drive state where the thermal efficiency is reduced in relation
to the outside air temperature.
[0005] On the other hand, a target evaporation temperature in an evaporator is determined
by a request from equipment that uses cold generated by the evaporator. Under the
determined target evaporation temperature, it is required to ensure a difference between
high and low pressures of a refrigerant required for obtaining predetermined freezing
performance.
BRIEF SUMMARY
[0006] The present disclosure has been made in view of such circumstances, and an object
is to provide a freezing device, a freezing system, and a control method of a freezing
device that can improve thermal efficiency even in a low outside air temperature and
obtain required freezing performance.
[0007] To achieve the above object, the freezing device, the freezing system, and the control
method of the freezing device of the present invention employ the following solutions.
[0008] The freezing device of the present invention as defined in claim 1 includes: a compressor
configured to compress a CO
2 refrigerant; a gas cooler configured to cool a CO
2 refrigerant discharged from the compressor; and a control unit configured to set
a high-pressure target value of the CO
2 refrigerant so that the CO
2 refrigerant in the gas cooler becomes subcritical and perform subcritical control
when an outside air temperature is less than a first predetermined temperature, which
is lower than the critical point of the CO
2 refrigerant, and a target evaporation temperature in an evaporator to which the CO
2 refrigerant cooled by the gas cooler is guided via an expansion valve is less than
or equal to a second predetermined temperature.
[0009] The freezing system of the present invention includes: the freezing device described
in any of the above; an expansion valve configured to expand a CO
2 refrigerant supplied from the gas cooler; and an evaporator configured to evaporate
the CO
2 refrigerant guided from the expansion valve.
[0010] The control method of a freezing device of the present invention as defined in claim
6 is a control method of a freezing device including a compressor configured to compress
a CO
2 refrigerant, and a gas cooler configured to cool a CO
2 refrigerant discharged from the compressor, and the control method includes: setting
a high-pressure target value of a CO
2 refrigerant so that a CO
2 refrigerant in the gas cooler becomes subcritical and performing subcritical control
when an outside air temperature is less than a first predetermined temperature, which
is lower than the critical point of the CO
2 refrigerant, and a target evaporation temperature in an evaporator to which the CO
2 refrigerant cooled by the gas cooler is guided via an expansion valve is less than
or equal to a second predetermined temperature.
[0011] It is possible to improve thermal efficiency even in a low outside air temperature
and obtain required freezing performance.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0012]
Fig. 1 is a schematic configuration diagram illustrating a refrigerant circuit of
a condensing unit of a freezing system of the present invention.
Fig. 2 is a schematic configuration diagram illustrating a load device of the freezing
system of the present invention.
Fig. 3 is a pressure-enthalpy (p-h) diagram of the freezing system of the present
invention.
Fig. 4 is a flowchart illustrating an operation of the freezing system of the present
invention.
DETAILED DESCRIPTION
[0013] The embodiments according to the present invention will be described below with reference
to the drawings. The freezing system of the present invention includes a condensing
unit (freezing device) 1 and a load device 2. As illustrated in Fig. 1, a refrigerant
circuit of the condensing unit 1 is illustrated. The condensing unit 1 supplies a
liquid refrigerant to the load device 2. As the refrigerant, carbon dioxide (CO
2) is used. The CO
2 refrigerant is not limited to a single refrigerant in which the content of CO
2 is 100% and may be a mixture refrigerant whose main component is CO
2 as long as the refrigerant has mainly properties of a CO
2 refrigerant.
[0014] The load device 2 may be, for example, a refrigerator or a freezer that cools or
refrigerates and stores products and refrigerating/freezing equipment such as a showcase
that cools or refrigerates and displays products. The load device 2 can be any device
that supplies cold and may be used for interior equipment of an air-conditioner, for
example.
[0015] As illustrated in Fig. 2, the load device 2 is supplied with a liquid refrigerant
from the condensing unit 1. The load device 2 includes an evaporator 6, a low-stage
expansion valve 8, a temperature sensor (not illustrated), and the like. A target
evaporation temperature set in the evaporator 6 is set by a user, for example, and
is stored in a control unit (not illustrated). A plurality of load devices 2 may be
provided in parallel to each other.
[0016] As illustrated in Fig. 1, the condensing unit 1 includes a compressor 3, gas coolers
5, a high-stage expansion valve 7, and intermediate-pressure receivers 9. The compressor
3, the gas coolers 5, the high-stage expansion valve 7, and the intermediate-pressure
receivers 9 are connected to each other by refrigerant pipes.
[0017] The compressor 3 compresses the refrigerant supplied from the load device 2 via an
accumulator 13 by an intake tube 11. The intake tube 11 includes a low-pressure sensor
12 on the inlet side of the accumulator 13 and includes an intake tube temperature
sensor 14 on the inlet side of the compressor 3. Measurement values of the low-pressure
sensor 12 and the intake tube temperature sensor 14 are transmitted to the control
unit.
[0018] The compressor 3 is a two-stage compressor and includes a low-stage compressor on
the first stage and a high-stage compressor on the second stage. The low-stage compressor
compresses a drawn refrigerant guided from the accumulator 13 to an intermediate pressure.
The high-stage compressor further compresses the refrigerant compressed to the intermediate
pressure by the low-stage compressor to a high pressure. As the low-stage compressor,
a rotary compressor is used, for example, and as the high-stage compressor, a scroll
compressor is used, for example. The displacement volume of the low-stage compressor
is 15 cc/rev, for example, and the displacement volume of the high-stage compressor
is 13 cc/rev, for example. The inside of a housing 3a of the compressor 3 is filled
with the intermediate-pressure refrigerant discharged from the low-stage compressor.
[0019] The compressor 3 includes an electric motor (not illustrated), and the rotational
rate is variable with inverter control. The rotational rate of the compressor 3 is
controlled by the control unit.
[0020] In the housing 3a of the compressor 3, an under-dome temperature sensor 4 is provided.
A measurement value of the under-dome temperature sensor 4 is transmitted to the control
unit.
[0021] After discharged from the compressor 3, a high-pressure and high-temperature refrigerant
is supplied to the gas cooler 5 via an oil separator 15. The gas cooler 5 is a fin
tube, for example, and performs heat exchange between the supplied high-pressure and
high-temperature refrigerant and the air supplied by a fan 17 to cool the refrigerant.
In the present embodiment, a plurality of (in the present embodiment, two) gas coolers
are provided in parallel to each other.
[0022] The high-stage expansion valve 7 is provided on the inlet side of the intermediate-pressure
receivers 9 and expands the refrigerant cooled in the gas coolers 5. The high-stage
expansion valve 7 is an electronic expansion valve, and the opening is controlled
by the control unit.
[0023] Each intermediate-pressure receiver 9 separates the refrigerant expanded in the high-stage
expansion valve 7 into a gas refrigerant and a liquid refrigerant. In the present
embodiment, the plurality of (in the present embodiment, two) intermediate-pressure
receivers 9 are provided in parallel to each other. A liquid feeding tube 19 and an
injection circuit 21 are connected to the intermediate-pressure receivers 9.
[0024] The liquid refrigerant separated in the intermediate-pressure receivers 9 is supplied
to the external load device 2 through the liquid feeding tube 19. In the liquid feeding
tube 19, a supercooling coil 23 is provided. In the supercooling coil 23, heat is
exchanged with a refrigerant expanded by a supercooling coil electronic expansion
valve 27 through a branch tube 25 branched from the liquid feeding tube 19. Accordingly,
supercooling is provided to the liquid refrigerant fed to the external load device
2. The downstream end of the branch tube 25 passing through the supercooling coil
23 is connected to the accumulator 13.
[0025] The gas refrigerant separated in the intermediate-pressure receiver 9 is guided to
the inside of the housing 3a of the compressor 3 via the injection circuit 21. In
the injection circuit 21, an intermediate-pressure sensor 31 and an intermediate-pressure
intake electromagnetic valve 33 are provided. A measurement value of the intermediate-pressure
sensor 31 is transmitted to the control unit. Opening and closing of the intermediate-pressure
intake electromagnetic valve 33 is controlled by the control unit.
[0026] The oil separator 15 is provided in a discharge tube 35 connected to the discharge
side of the high-stage compressor of the compressor 3. In the discharge tube 35, a
discharge temperature sensor 37 is provided, and a measurement value of the discharge
temperature sensor 37 is transmitted to the control unit. The oil separator 15 collects
a lubricating oil included in a discharge refrigerant. The upstream end of an oil
return tube 39 is connected to the bottom of the oil separator 15. The downstream
end that is the other end of the oil return tube 39 is connected to the compressor
3. In the oil return tube 39, an oil return electromagnetic valve 41 and a capillary
43 as a regulation unit are provided. Opening and closing of the oil return electromagnetic
valve 41 is controlled by the control unit.
[0027] A high-pressure refrigerant from which the lubricating oil has been removed by the
oil separator 15 is guided to the gas cooler 5 via a high-pressure gas pipe 45. In
the high-pressure gas pipe 45, a check valve 47, and a high-pressure sensor 49 are
provided in this order from the upstream. Further, a gas cooler inlet temperature
sensor 51 is provided on the gas cooler 5 side of the high-pressure gas pipe 45. Measurement
values of the high-pressure sensor 49 and the gas cooler inlet temperature sensor
51 are transmitted to the control unit.
[0028] The condensing unit 1 includes an outside air temperature sensor 53 that measures
an outside air temperature. A measurement value of the outside air temperature sensor
53 is transmitted to the control unit.
[0029] The control unit is formed of a central processing unit (CPU), a random access memory
(RAM), a read only memory (ROM), a computer readable storage medium, and the like,
for example. Further, a series of processes for implementing respective functions
is stored in a storage medium or the like in a form of a program as one example, and
when the CPU loads the program into the RAM or the like and performs a processing
and calculation process on information, respective functions are implemented. Note
that an applicable form of the program may be a form in which a program is installed
in advance in a ROM or another storage medium, a form in which a program is provided
in a state of being stored in a computer readable storage medium, a form in which
a program is delivered via a wired or wireless communication scheme, or the like.
The computer readable storage medium may be a magnetic disk, a magneto-optical disk,
a CD-ROM, a DVD-ROM, a semiconductor memory, or the like.
[0030] The operation of the freezing device described above will be described with reference
to Fig. 3.
[Supercritical control]
[0031] First, as a basic operation mode of the freezing device, the transcritical control
will be described. Fig. 3 is a pressure-enthalpy (p-h) diagram in which the horizontal
axis corresponds to a specific enthalpy, and the vertical axis corresponds to a pressure,
and illustrates the operation of the freezing device.
[0032] The transcritical control is illustrated by the dashed line in Fig. 3, which means
that the refrigerant CO
2 refrigerant) is in the transcritical state in the gas cooler 5. In Fig. 3, the dash-dot
line represents a saturation vapor line and a saturation liquid line of CO
2. When the pressure is higher than or equal to the critical point CP, CO
2 is in the transcritical state in the gas cooler 5.
[0033] The refrigerant guided via the intake tube 11 is drawn into the low-stage compressor
of the compressor 3 under intake pressure P1 (state S1). The pressure of the drawn
refrigerant is increased to intermediate pressure P2 in the low-stage compressor (state
S2). The refrigerant whose pressure has been increased to intermediate pressure P2
is discharged into the housing 3a. On the other hand, the intermediate-pressure refrigerant
guided from the intermediate-pressure receiver 9 via the injection circuit 21 is guided
into the housing 3a.
[0034] The pressure of the intermediate-pressure refrigerant in the housing 3a is increased
to high pressure P3 by the high-stage compressor (state S3). The high-pressure refrigerant
whose pressure has been increased is guided to the oil separator 15 via the discharge
tube 35 and a lubricating oil in the refrigerant is separated therefrom. The refrigerant
from which the oil has been removed by the oil separator 15 passes through the high-pressure
gas pipe 45 and is guided to the gas cooler 5.
[0035] In the gas cooler 5, the refrigerant is cooled by being subjected to heat exchange
with air (outside air) supplied by the fan 17 (state S4). At this time, the refrigerant
is in the transcritical state.
[0036] The refrigerant that has left from the gas cooler 5 is expanded to intermediate pressure
P2 by the high-stage expansion valve 7 and guided to the intermediate-pressure receiver
9. The liquid refrigerant taken out from the intermediate-pressure receiver 9 passes
through the liquid feeding tube 19 and is then supercooled by the supercooling coil
23 (state S5). The liquid refrigerant is then fed to the external load device 2.
[0037] In the load device 2, the liquid refrigerant is expanded to intake pressure P1 by
the low-stage expansion valve 8 (state S6) and fed to the evaporator 6. In the evaporator
6, the refrigerant removes heat from the load and evaporates into a gas refrigerant.
Evaporation latent heat generated in the evaporator 6 is taken out as a freezing load.
The gas refrigerant that has left from the evaporator 6 is returned to the condensing
unit 1 and again fed to the compressor 3 via the intake tube 11.
[Subcritical Control]
[0038] The subcritical control is illustrated by the solid line in Fig. 3, which means that
the refrigerant (CO
2 refrigerant) is in a subcritical state in the gas cooler 5. That is, in the subcritical
control, high pressure P3' is a pressure lower than that at the critical point CP,
and a gas-liquid two-phase state is in the gas cooler 5.
[0039] Since the operation on the pressure-enthalpy (p-h) diagram in the subcritical control
is the same as the transcritical control except for a difference in high pressure
P3', the description thereof is omitted.
[Switching control]
[0040] Switching control for performing switching from the transcritical control to the
subcritical control will be described with reference to Fig. 4.
[0041] If the temperature measured by the outside air temperature sensor 53 is less than
a first predetermined temperature (step 102) and a target evaporation temperature
is less than or equal to a second predetermined temperature (step 103) in the operation
under the transcritical control (step 101), the control is switched to the subcritical
control after step 104 by an instruction from the control unit.
[0042] The first predetermined temperature is 18 °C, for example. Note that the first predetermined
temperature is variable in accordance with the capacity of the freezing system or
the like and is set to a temperature that is less than the critical temperature (31.1
°C) of CO
2 and at which the thermal efficiency of the freezing system is greater than that in
the transcritical control. The first predetermined temperature is determined by a
test operation or a simulation in advance, for example. When the outside air temperature
is greater than or equal to the first predetermined temperature, the process returns
to step 101 from step 102, and the transcritical control is maintained.
[0043] The second predetermined temperature is -10 °C, for example. The second predetermined
temperature is a temperature set by the user and is set as a target value for the
control unit. When the target evaporation temperature is greater than the second predetermined
temperature, a difference between high and low pressures is small, the bore diameter
of an expansion valve is insufficient, and a differential pressure required for the
operation of the compressor is not ensured. In such a case, the process returns to
step 101 from step 103, and the transcritical control is maintained.
[0044] If the target evaporation temperature is less than or equal to the second predetermined
temperature in step 103 and the control is switched to the subcritical control, the
process proceeds to step 104, and the control unit sets a high-pressure target value
to the value for the subcritical state. Specifically, as illustrated in Fig. 3, the
control unit sets a high-pressure target value from high pressure P3 to high pressure
P3'. The high-pressure target value that is a set value is stored in the control unit.
[0045] The process proceeds to step 105, and the rotational rate of the fan 17 is adjusted
by an instruction from the control unit. When the CO
2 refrigerant is operated under the subcritical state, since the work of the compressor
decreases compared to a case of being operated under the transcritical state, the
amount of exhaust heat from the gas cooler also decreases. Since electric power consumption
is determined by the work of the compressor and the fan, the fan rotational rate is
adjusted so that the electric power consumption decreases.
[0046] In the subcritical control compared to the transcritical control, the pressure in
the gas cooler 5 decreases from high pressure P3 to high pressure P3', and intermediate
pressure P2 also tends to decrease. Since the temperature in the compressor 3 decreases
in response to a decrease in intermediate pressure P2, an oil temperature in the compressor
3 decreases. When the oil temperature decreases, the refrigerant is easily dissolved,
and the dilution ratio increases. When the dilution ratio increases, the viscosity
of the oil decreases, and an oil film is not ensured, and this may lead to damage
of a sliding portion. In the present embodiment, to ensure the oil temperature, a
threshold is set to the degree of under-dome superheat (= under-dome temperature -
intermediate-pressure saturation temperature) obtained by the measurement value of
the under-dome temperature sensor 4, and protection control to stop the compressor
3 is performed when the degree of under-dome superheat is out of the threshold. This
protection control makes it possible not only to protect the compressor 3 but also
to prevent liquid back where a liquid refrigerant is drawn into the intake side of
the compressor 3.
[0047] The effects and advantages of the present embodiment described above are as following.
[0048] When the outside air temperature is less than the first predetermined temperature
(for example, -18 °C) that is lower than the critical point CP of CO
2, even if the operation is performed so that the CO
2 refrigerant in the gas cooler 5 becomes transcritical, the difference between high
and low pressures of the CO
2 refrigerant may be greater than is required and the thermal efficiency may decrease.
Accordingly, when the target evaporation temperature in the evaporator is less than
or equal to the second predetermined temperature (for example, -10 °C) and the difference
between high and low pressures of the CO
2 refrigerant can be ensured, a thermally efficient operation can be realized by setting
the high-pressure target value of the CO
2 refrigerant to be lower so that the CO
2 refrigerant in the gas cooler 5 becomes subcritical and reducing the load on the
compressor 3.
[0049] When the CO
2 refrigerant is operated under the subcritical state, since the amount of heat absorption
in the evaporator becomes relatively small compared to a case of the operation under
the transcritical state, the amount of exhaust heat in the gas cooler 5 also decreases.
Accordingly, it is possible to reduce the amount of exhaust heat to maintain heat
balance by adjusting the rotational rate of the fan 17.
[0050] The freezing device, the freezing system, and the control method of a freezing device
according to respective embodiments described above are recognized as follows, for
example.
[0051] The freezing device of the present invention includes: a compressor (3) configured
to compress a CO
2 refrigerant; a gas cooler (5) configured to cool a CO
2 refrigerant discharged from the compressor (3); and a control unit configured to
set a high-pressure target value of the CO
2 refrigerant so that the CO
2 refrigerant in the gas cooler becomes subcritical and perform subcritical control
when an outside air temperature is less than a first predetermined temperature, which
is lower than the critical point of the CO
2 refrigerant, and a target evaporation temperature in an evaporator (6) to which the
CO
2 refrigerant cooled by the gas cooler is guided via an expansion valve (7, 8) is less
than or equal to a second predetermined temperature.
[0052] When the outside air temperature is less than the first predetermined temperature
that is lower than the critical point of CO
2, if the operation under a high pressure is performed so that the CO
2 refrigerant becomes transcritical in the gas cooler, the difference between high
and low pressures of the CO
2 refrigerant may be greater than is required and the thermal efficiency may decrease.
Accordingly, when the target evaporation temperature in the evaporator is less than
or equal to the second predetermined temperature and the difference between high and
low pressures of the CO
2 refrigerant can be ensured, a thermally efficient operation is realized by setting
the high-pressure target value of the CO
2 refrigerant to be lower so that the CO
2 refrigerant in the gas cooler becomes subcritical, performing the subcritical control,
and reducing the load on the compressor.
[0053] The first predetermined temperature may be, for example, 18 °C, and the second predetermined
temperature may be, for example, -10 °C.
[0054] Note that the CO
2 refrigerant is not limited to a single refrigerant in which the content of CO
2 is 100% and may be a mixture refrigerant whose main component is CO
2 as long as the refrigerant has mainly properties of a CO
2 refrigerant.
[0055] In the freezing device of the present invention, when the outside air temperature
is greater than or equal to the first predetermined temperature, the control unit
sets the high-pressure target value so that the CO
2 refrigerant in the gas cooler becomes transcritical and performs transcritical control.
[0056] When the outside air temperature is greater than or equal to the first predetermined
temperature, the high-pressure target value is increased to obtain a difference between
the high and low pressures corresponding to the amount of exhaust heat required for
the gas cooler, and the CO
2 refrigerant is operated under the transcritical state.
[0057] The freezing device of the present disclosure includes a fan (17) configured to supply
cooling air to the gas cooler, and when the subcritical control is performed, the
control unit adjusts rotational rate of the fan.
[0058] When the CO
2 refrigerant is operated under the subcritical state, since the work of the compressor
decreases compared to a case of an operation under the transcritical state, the amount
of exhaust heat from the gas cooler also decreases. Since electric power consumption
is determined by the work of the compressor and the fan, the fan rotational rate is
adjusted so that the electric power consumption decreases.
[0059] In the freezing device of the present invention, the compressor includes a low-stage
compressor configured to compress a refrigerant guided from the evaporator to an intermediate
pressure and a high-stage compressor configured to draw in and compress the refrigerant,
which was compressed to the intermediate pressure by the low-stage compressor, to
a high pressure, and the control unit performs control so that discharge pressure
of the high-stage compressor becomes the high-pressure target value.
[0060] When the compressor is a two-stage compressor including a low-stage compressor and
a high-stage compressor, the discharge pressure of the high-stage compressor is controlled
to become a high-pressure target value.
[0061] The freezing system of the present invention includes: the freezing device described
in any of the above; an expansion valve configured to expand a CO
2 refrigerant supplied from the gas cooler; and an evaporator configured to evaporate
the CO
2 refrigerant guided from the expansion valve.
[0062] The control method of a freezing device of the present invention is a control method
of a freezing device including a compressor configured to compress a CO
2 refrigerant, and a gas cooler configured to cool a CO
2 refrigerant discharged from the compressor, and the control method includes: setting
a high-pressure target value of a CO
2 refrigerant so that a CO
2 refrigerant in the gas cooler becomes subcritical and performing subcritical control
when an outside air temperature is less than a first predetermined temperature, which
is lower than the critical point of the CO
2 refrigerant, and a target evaporation temperature in an evaporator to which the CO
2 refrigerant cooled by the gas cooler is guided via an expansion valve is less than
or equal to a second predetermined temperature.
[List of Reference Symbols]
[0063]
- 1
- condensing unit (freezing device)
- 2
- load device
- 3
- compressor
- 3a
- housing
- 4
- under-dome temperature sensor
- 5
- gas cooler
- 6
- evaporator
- 7
- high-stage expansion valve
- 8
- low-stage expansion valve
- 9
- intermediate-pressure receiver
- 11
- intake tube
- 12
- low-pressure sensor
- 13
- accumulator
- 14
- intake tube temperature sensor
- 15
- oil separator
- 17
- fan
- 19
- liquid feeding tube
- 21
- injection circuit
- 23
- supercooling coil
- 25
- branch tube
- 27
- supercooling coil electronic expansion valve
- 31
- intermediate-pressure sensor
- 33
- intermediate-pressure intake electromagnetic valve
- 35
- discharge tube
- 37
- discharge temperature sensor
- 39
- oil return tube
- 41
- oil return electromagnetic valve
- 43
- capillary
- 45
- high-pressure gas pipe
- 47
- check valve
- 49
- high-pressure sensor
- 51
- gas cooler inlet temperature sensor
- 53
- outside air temperature sensor
- CP
- critical point
- P1
- intake pressure
- P2
- intermediate pressure
- P3
- high pressure (transcritical)
- P3'
- high pressure (subcritical)
1. Gefriervorrichtung (1), umfassend:
einen Kompressor (3), der dazu ausgestaltet ist, ein CO2-Kältemittel zu komprimieren,
einen Gaskühler (5), der dazu ausgestaltet ist, ein von dem Kompressor abgelassenes
CO2-Kältemittel zu kühlen, und
eine Steuereinheit, die dazu ausgestaltet ist, einen Hochdrucksollwert des CO2-Kältemittels so einzustellen, dass das CO2-Kältemittel in dem Gaskühler unterkritisch wird, und eine unterkritische Steuerung
durchzuführen, wenn eine Außenlufttemperatur unter einer ersten vorgegebenen Temperatur
liegt, die unter einem kritischen Punkt des CO2-Kältemittels liegt, und eine Soll-Verdampfungstemperatur in einem Verdampfer (6),
zu dem das von dem Gaskühler gekühlte CO2-Kältemittel über ein Expansionsventil (7, 8) geleitet wird, kleiner oder gleich einer
zweiten vorgegebenen Temperatur ist.
2. Gefriervorrichtung nach Anspruch 1, wobei die Steuereinheit den Hochdrucksollwert
so einstellt, dass das CO2-Kältemittel in dem Gaskühler transkritisch wird und eine transkritische Steuerung
durchführt, wenn die Außenlufttemperatur größer oder gleich der ersten vorgegebenen
Temperatur ist.
3. Gefriervorrichtung nach Anspruch 1 oder 2, ferner umfassend einen Lüfter (17), der
dazu ausgestaltet ist, dem Gaskühler (5) Kühlluft zuzuführen,
wobei die Steuereinheit die Drehzahl des Lüfters anpasst, wenn die unterkritische
Steuerung durchgeführt wird.
4. Gefriervorrichtung nach einem der Ansprüche 1 bis 3,
wobei der Kompressor (3) einen Niederstufenkompressor, der dazu ausgestaltet ist,
ein von dem Verdampfer geleitetes Kältemittel auf einen Zwischendruck zu komprimieren,
und einen Hochstufenkompressor umfasst, der dazu ausgestaltet ist, das Kältemittel,
das durch den Niederstufenkompressor auf den Zwischendruck komprimiert wurde, anzusaugen
und auf einen Hochdruck zu komprimieren, und
wobei die Steuereinheit eine Steuerung so durchführt, dass ein Auslassdruck des Hochstufenkompressors
zu dem Hochdrucksollwert wird.
5. Gefriersystem, umfassend:
die Gefriervorrichtung (1) nach einem der Ansprüche 1 bis 4,
ein Expansionsventil (7, 8), das dazu ausgestaltet ist, ein von dem Gaskühler (5)
zugeführtes CO2-Kältemittel zu expandieren, und
einen Verdampfer (6), der dazu ausgestaltet ist, das von dem Expansionsventil geleitete
CO2-Kältemittel zu verdampfen.
6. Steuerungsverfahren für eine Gefriervorrichtung, umfassend
einen Kompressor (3), der dazu ausgestaltet ist, ein CO2-Kältemittel zu komprimieren, und
einen Gaskühler (5), der dazu ausgestaltet ist, ein von dem Kompressor abgelassenes
CO2-Kältemittel zu kühlen,
wobei das Steuerungsverfahren umfasst:
Einstellen eines Hochdrucksollwerts des CO2-Kältemittels so, dass das CO2-Kältemittel in dem Gaskühler unterkritisch wird, und Durchführen einer unterkritischen
Steuerung, wenn eine Außenlufttemperatur unter einer ersten vorgegebenen Temperatur
liegt, die unter einem kritischen Punkt des CO2-Kältemittels liegt, und eine Soll-Verdampfungstemperatur in einem Verdampfer, zu
dem das von dem Gaskühler gekühlte CO2-Kältemittel über ein Expansionsventil geleitet wird, kleiner oder gleich einer zweiten
vorgegebenen Temperatur ist.