U.S. patent application number 14/401916 was filed with the patent office on 2015-06-04 for refrigeration apparatus.
This patent application is currently assigned to Mitsubishi Electric Corporation. The applicant listed for this patent is Takashi Ikeda, Tomotaka Ishikawa, So Nomoto, Takeshi Sugimoto, Keisuke Takayama. Invention is credited to Takashi Ikeda, Tomotaka Ishikawa, So Nomoto, Takeshi Sugimoto, Keisuke Takayama.
Application Number | 20150153076 14/401916 |
Document ID | / |
Family ID | 50149571 |
Filed Date | 2015-06-04 |
United States Patent
Application |
20150153076 |
Kind Code |
A1 |
Sugimoto; Takeshi ; et
al. |
June 4, 2015 |
REFRIGERATION APPARATUS
Abstract
A refrigeration apparatus includes a high-temperature side
circulation circuit and a low-temperature side circulation circuit.
The high-temperature side circulation circuit A is configured by
connecting a high-temperature side compressor, a high-temperature
side condenser, a high-temperature side expansion valve, and a
high-temperature side evaporator of a cascade heat exchanger to one
another. The low-temperature side circulation circuit is configured
by connecting a low-temperature side compressor, a low-temperature
side condenser of the cascade heat exchanger, a receiver that
stores a liquid refrigerant, a solenoid valve, a low-temperature
side expansion valve, and a low-temperature side evaporator to one
another. A refrigerant in the low-temperature side circulation
circuit includes a zeotropic refrigerant mixture containing at
least CO.sub.2 and R32. The content of R32 in the entire zeotropic
refrigerant mixture is 50% to 74% by mass, and the GWP of the
zeotropic refrigerant mixture is equal to or less than 500.
Inventors: |
Sugimoto; Takeshi; (Tokyo,
JP) ; Nomoto; So; (Tokyo, JP) ; Ishikawa;
Tomotaka; (Tokyo, JP) ; Takayama; Keisuke;
(Tokyo, JP) ; Ikeda; Takashi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sugimoto; Takeshi
Nomoto; So
Ishikawa; Tomotaka
Takayama; Keisuke
Ikeda; Takashi |
Tokyo
Tokyo
Tokyo
Tokyo
Tokyo |
|
JP
JP
JP
JP
JP |
|
|
Assignee: |
Mitsubishi Electric
Corporation
Tokyo
JP
|
Family ID: |
50149571 |
Appl. No.: |
14/401916 |
Filed: |
August 23, 2012 |
PCT Filed: |
August 23, 2012 |
PCT NO: |
PCT/JP2012/071263 |
371 Date: |
November 18, 2014 |
Current U.S.
Class: |
62/175 ;
62/335 |
Current CPC
Class: |
Y02B 30/741 20130101;
F25B 2313/0272 20130101; F25B 2600/2501 20130101; F25B 25/005
20130101; F25B 2600/2513 20130101; F25B 9/006 20130101; F25B 9/008
20130101; F25B 2600/0253 20130101; F25B 2400/08 20130101; F25B
2700/195 20130101; F25B 2700/2108 20130101; F25B 13/00 20130101;
Y02B 30/70 20130101; F25B 7/00 20130101; F25B 2313/02732 20130101;
F25B 2313/023 20130101; F25B 2313/02741 20130101; F25B 49/02
20130101; F25B 49/00 20130101 |
International
Class: |
F25B 7/00 20060101
F25B007/00; F25B 49/02 20060101 F25B049/02 |
Claims
1. A refrigeration apparatus comprising: a high-temperature side
circulation circuit configured by connecting a high-temperature
side compressor, a high-temperature side condenser, a
high-temperature side expansion valve, and a high-temperature side
evaporator of a cascade heat exchanger to one another; and a
low-temperature side circulation circuit configured by connecting a
low-temperature side compressor, a low-temperature side condenser
of the cascade heat exchanger, a receiver that stores a liquid
refrigerant, a solenoid valve, a low-temperature side expansion
valve, and a low-temperature side evaporator to one another,
wherein a refrigerant in the low-temperature side circulation
circuit includes a zeotropic refrigerant mixture containing at
least CO.sub.2 and R32, a content of R32 in the entire zeotropic
refrigerant mixture is 50% to 74% by mass, and a GWP of the
zeotropic refrigerant mixture is not more than 500.
2. The refrigeration apparatus of claim 1, further comprising: a
composition calculation unit that obtains a composition of the
refrigerant circulating in the low-temperature side circulation
circuit; and a controller that controls the low-temperature side
circulation circuit based on the composition of the refrigerant
obtained by the composition calculation unit.
3. The refrigeration apparatus of claim 2, wherein the composition
calculation unit includes a pressure detector that detects a
pressure of a liquid refrigerant in the receiver, a temperature
detector that detects a temperature of the liquid refrigerant in
the receiver, and a composition calculator that obtains the
composition of the refrigerant circulating in the low-temperature
side circulation circuit, based on the pressure detected by the
pressure detector and the temperature detected by the temperature
detector.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a U.S. national stage application of
PCT/JP2012/071263 filed on Aug. 23, 2012, the contents of which are
incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to a refrigeration apparatus
that performs a binary refrigeration cycle.
BACKGROUND
[0003] In the conventional refrigeration apparatus, a
low-temperature side circulation circuit in which a low-temperature
refrigerant circulates and a high-temperature side circulation
circuit in which a high-temperature refrigerant circulates are
connected to one another by a cascade condenser. For such a
refrigeration apparatus, the use of carbon dioxide (CO.sub.2) as a
refrigerant in a low-temperature side circulation circuit has been
proposed (see, for example, Patent Literature 1). Since CO.sub.2
occurs in nature and has a low global warming potential (GWP), the
use of CO.sub.2 can avoid damaging the ozone layer even upon
leakage of CO.sub.2 from a refrigeration cycle apparatus due to an
unexpected accident. Thus, a refrigeration apparatus friendly to
the global environment can be provided.
PATENT LITERATURE
[0004] Patent Literature 1: Japanese Unexamined Patent Application
Publication No. 2004-190917 (page 14, FIG. 1)
[0005] To achieve a low-GWP refrigeration apparatus, CO.sub.2 is
preferably used as a refrigerant in a low-temperature side
circulation circuit. However, the use of CO.sub.2 poses the
following problems. Specifically, tuna, for example, requires
refrigeration in a freezer at temperatures of about -50 degrees C.,
and this can be done by setting the evaporating temperature in a
low-temperature side evaporator to about -80 degrees C. to -60
degrees C. However, when a single refrigerant of CO.sub.2 is used
in a low-temperature side circulation circuit, such an evaporating
temperature cannot be achieved because the triple point of CO.sub.2
is -56.6 degrees C. That is, in a refrigeration apparatus that
performs a binary refrigeration cycle using a single refrigerant of
CO.sub.2, it is difficult to achieve an evaporating temperature of
about -80 degrees C. The use of R404A for the low-temperature side
circulation circuit can reduce the evaporating temperature to about
-65 degrees C. However, R404A, which has a GWP of 3920, may
accelerate global warming upon leakage of a refrigerant due to an
accident.
[0006] In addition, in a refrigeration apparatus, an appropriate
type of zeotropic refrigerant mixture is generally known to be used
to obtain a low evaporating temperature that is unattainable when a
single refrigerant is used. Thus, the use of a zeotropic
refrigerant mixture containing CO.sub.2 as one mixture component is
expected to obtain both a low evaporating temperature and a low
GWP. However, a definite type of component to be contained in the
refrigerant mixture, in addition to CO.sub.2, that can obtain an
evaporating temperature of -80 degrees C. to -60 degrees C. has not
yet been proposed in the conventional techniques.
[0007] The inventors of the present invention have examined this
issue and reached the conclusion that the use of a zeotropic
refrigerant mixture containing at least CO.sub.2 and R32 in a
low-temperature side circulation circuit can obtain a composition
of a zeotropic refrigerant mixture with an evaporating temperature
of -80 degrees C. to -60 degrees C.
SUMMARY
[0008] The present invention has been made to solve the
above-mentioned problem, and has as its object to provide a
refrigeration apparatus that uses a zeotropic refrigerant mixture
containing at least CO.sub.2 and R32 in a low-temperature side
circulation circuit to obtain an evaporating temperature of -80
degrees C. to -60 degrees C. in a low-temperature side evaporator
of the low-temperature side circulation circuit with a low GWP.
[0009] A refrigeration apparatus according to the present invention
includes: a high-temperature side circulation circuit configured by
connecting a high-temperature side compressor, a high-temperature
side condenser, a high-temperature side expansion valve, and a
high-temperature side evaporator of a cascade heat exchanger to one
another; and a low-temperature side circulation circuit configured
by connecting a low-temperature side compressor, a low-temperature
side condenser of the cascade heat exchanger, a receiver that
stores a liquid refrigerant, a solenoid valve, a low-temperature
side expansion valve, and a low-temperature side evaporator to one
another. A refrigerant in the low-temperature side circulation
circuit includes a zeotropic refrigerant mixture containing at
least CO.sub.2 and R32. The content of R32 in the entire zeotropic
refrigerant mixture is 50% to 74% by mass. The GWP of the zeotropic
refrigerant mixture is equal to or less than 500.
[0010] The present invention can provide a refrigeration apparatus
which has a low GWP and includes a low-temperature side evaporator
that achieves an evaporating temperature of -80 degrees C. to -60
degrees C.
BRIEF DESCRIPTION OF DRAWINGS
[0011] FIG. 1 is a refrigerant circuit diagram of a refrigeration
apparatus according to Embodiment of the present invention.
[0012] FIG. 2 is a gas-liquid equilibrium diagram showing the
characteristics of a zeotropic refrigerant mixture containing two
types of refrigerants.
[0013] FIG. 3 shows graphs of the relationships between the mole
fraction of CO.sub.2 and the freezing point in a CO.sub.2
refrigerant mixture.
[0014] FIG. 4 shows graphs for comparisons of the refrigeration
capacity and the COP as functions of the mixing ratio of CO.sub.2
and R32 in Embodiment of the present invention.
[0015] FIG. 5 illustrates a configuration in which a configuration
for detecting a circulation composition is added to the refrigerant
circuit of FIG. 1.
[0016] FIG. 6 is a graph for explaining principle 1 of detecting a
circulation composition in a composition calculator illustrated in
FIG. 5.
[0017] FIG. 7 is a graph for explaining principle 2 of detecting a
circulation composition in the composition calculator illustrated
in FIG. 5.
DETAILED DESCRIPTION
[0018] FIG. 1 is a refrigerant circuit diagram of a refrigeration
apparatus according to Embodiment of the present invention.
[0019] The refrigeration apparatus includes a high-temperature side
circulation circuit A and a low-temperature side circulation
circuit B. The high-temperature side circulation circuit A forms a
series connection of a high-temperature side compressor 1, a
high-temperature side condenser 2, a high-temperature side
expansion valve 3, and a high-temperature side evaporator 4. The
high-temperature side circulation circuit A uses R410A, R134a, R32,
or a HFO refrigerant.
[0020] The low-temperature side circulation circuit B forms a
series connection of a low-temperature side compressor 5, a
low-temperature side condenser 6, a low-temperature side expansion
valve 9, and a low-temperature side evaporator 10, and includes a
receiver 7 for storing an excess refrigerant and a solenoid valve 8
both at the outlet of the low-temperature side condenser 6. The
low-temperature side circulation circuit B uses a zeotropic
refrigerant mixture containing at least CO.sub.2 and R32. The
zeotropic refrigerant mixture refers to a refrigerant mixture
containing two or more types of refrigerants having different
boiling points. Note that CO.sub.2 is a low-boiling-point
refrigerant and R32 is a high-boiling-point refrigerant.
[0021] The high-temperature side circulation circuit A and the
low-temperature side circulation circuit B share a cascade
condenser 14. The cascade condenser 14 includes the
high-temperature side evaporator 4 and the low-temperature side
condenser 6. The cascade condenser 14 is implemented using, for
example, a plate heat exchanger and exchanges heat with a
high-temperature refrigerant circulating in the high-temperature
side circulation circuit A and a low-temperature refrigerant (a
zeotropic refrigerant mixture) circulating in the low-temperature
side circulation circuit B.
[0022] Features of the composition of a refrigerant circulating in
the refrigeration cycle of the refrigeration apparatus using a
zeotropic refrigerant mixture specified in the foregoing way for
the low-temperature side circulation circuit B will now be
described. The zeotropic refrigerant mixture exhibits
characteristics as shown in FIG. 2.
[0023] FIG. 2 is a gas-liquid equilibrium diagram showing the
characteristics of a zeotropic refrigerant mixture containing two
types of refrigerants. Referring to FIG. 2, the ordinate represents
the temperature, the abscissa represents the circulation
composition (that is, the composition ratio of a low-boiling-point
component where CO.sub.2 has a low boiling point and R32 has a high
boiling point), and the pressure is used as a parameter. FIG. 2
shows the characteristics of the zeotropic refrigerant mixture at
each of a high pressure P.sub.H and a low pressure P.sub.L in the
low-temperature side circulation circuit B. As shown in FIG. 2, in
a two-component zeotropic refrigerant mixture, a saturated vapor
line and a saturated liquid line are determined depending on the
pressure. Note that the circulation composition is "0" when only a
high-boiling-point component is used, and is "1" when only a
low-boiling-point component is used. In a refrigerant mixture, a
saturated liquid line and a saturated vapor line are determined
depending on the composition, as shown in FIG. 2.
[0024] A region above the saturated vapor line represents a
superheated vapor state, a region below the saturated liquid line
represents a subcooling state, and a region enclosed by the
saturated vapor line and the saturated liquid line represents a
gas-liquid two-phase state. Referring to FIG. 2, Z indicates the
composition of a refrigerant enclosed in the refrigeration cycle,
and points 1 to 4 indicate representative points of the
refrigeration cycle. Specifically, point 1 indicates a compressor
outlet, point 2 indicates a condenser outlet, point 3 indicates an
evaporator inlet, and point 4 indicates a compressor inlet.
[0025] In the refrigeration cycle using a zeotropic refrigerant
mixture, the composition of a refrigerant circulating in the
refrigeration cycle (circulation composition) is not necessarily
the same as the composition of a refrigerant charged in the
refrigeration cycle (charging composition). This is because in a
gas-liquid two-phase portion of the refrigeration cycle indicated
by point A in FIG. 2, the liquid composition is X, which is lower
than the circulation composition Z, and the vapor composition is Y,
which is higher than the circulation composition. In other words,
in the gas-liquid two-phase portion indicated by point A, a liquid
rich in a high-boiling-point component and a vapor rich in a
low-boiling-point component are present.
[0026] An excess liquid refrigerant generated depending on the
operating conditions and the load conditions in the refrigeration
apparatus is accumulated in the receiver 7. The refrigerant in the
receiver 7 is separated into a liquid refrigerant rich in a
high-boiling-point component and a gas refrigerant rich in a
low-boiling-point component, and the liquid refrigerant rich in a
high-boiling-point component is stored in the receiver 7. Thus, in
the presence of the liquid refrigerant in the receiver 7, the
circulation composition of the zeotropic refrigerant mixture
circulating in the low-temperature side circulation circuit B is
likely to have a larger amount of low-boiling-point component than
the charging composition. That is, the circulation composition
(that is, the composition ratio of a low-boiling-point component)
is likely to increase.
[0027] When the operating conditions and the load conditions change
so that the amount of refrigerant stored in the receiver 7 changes,
the circulation composition in the refrigeration cycle changes.
With a change in circulation composition in the refrigeration
cycle, the relationship between the pressure and the saturation
temperature of the refrigerant changes, as can be seen from FIG. 2,
and the cooling capacity also changes. Thus, to make the
refrigeration cycle operate stably with a predetermined
performance, it is necessary to detect the circulation composition
in the refrigeration cycle accurately, and optimally control, for
example, the rotation speed of the low-temperature side compressor
5 and/or the opening degree of the low-temperature side expansion
valve 9 to adjust the refrigerant flow rate, in accordance with the
detected circulation composition.
[0028] Table 1 below shows the values of the physical
characteristics of CO.sub.2 refrigerant mixtures each containing
CO.sub.2 as a refrigerant mixture component and one of three types
of refrigerants to be combined with CO.sub.2. Table 1 also shows
the GWP and the flame resistance of each type of refrigerant to be
contained in the refrigerant mixture, the mixing ratio (the mole
ratio and the mass ratio) between a refrigerant to be contained in
the refrigerant mixture and CO.sub.2, the GWP of a refrigerant
mixture with the mixing ratio, the freezing point of the
refrigerant mixture, and the -70 degrees C. temperature gradient of
the refrigerant mixture. FIG. 3 shows the mole fraction of CO.sub.2
and the freezing point in the CO.sub.2 refrigerant mixture. FIG. 3
shows in (a) a refrigerant mixture containing R32, and in (b) a
refrigerant mixture containing R125.
TABLE-US-00001 TABLE 1 Freezing GWP after Point Refrigerant to
Mixing Ratio Mixture after -70 Degrees be Mixed
(CO.sub.2:Refrigerant (Calculated Mixture C. GWP Flame Mole Mass in
Mass (Degrees Temperature Name (alone) Resistance Ratio Ratio
Ratio) C.) Gradient (K) R290 3 Poor 71:29 71:29 1.58 -72 Unclear
R32 675 Fair 54:46 50:50 338 -81 11.9 R125 3500 Good 50:50 27:73
2555 -83 11.2
[0029] Possible examples of a combination that can keep the
freezing point below -70 degrees C. include R290, R32, and R125, as
shown in Table 1. However, R290 has a poor flame resistance, that
is, is flammable, is required in a large amount in, for example, a
display case or a unit cooler, and thus is difficult to use due to
concerns for safety.
[0030] Regarding R125, the GWP is 3500 in itself, and is more than
2000 even when it is mixed with CO.sub.2. Thus, the use of R125 is
difficult in terms of preventing global warming.
[0031] Regarding R32, the GWP is 675 in itself, which is
sufficiently lower than that of R125. A mixture of R32 with
CO.sub.2 can keep the GWP lower than that in the case without a
mixture. Thus, R32 is suitable as a refrigerant that can have no
adverse effect in preventing global warming. As shown in (a) of
FIG. 3, with a reduction in mole fraction of CO.sub.2, that is, an
increase in proportion of R32 in the refrigerant mixture, the
freezing point can reduce.
[0032] FIG. 4 shows in (a) a graph of the refrigeration capacity
[W] corresponding to the refrigerant mixing ratio of CO.sub.2 and
R32. FIG. 4 shows in (b) a graph of the COP [-] corresponding to
the refrigerant mixing ratio of CO.sub.2 and R32. FIG. 4 shows on
the abscissa the mixing ratio range in which the freezing point is
-81 degrees C. or less. Referring to FIG. 4, the condensing
temperature of the refrigerant on the low-temperature side
(low-side) is -25 degrees C., and the evaporating temperature of
the refrigerant on the low-temperature side (low-side) is -60
degrees C., where the displacement volume of the low-temperature
side (low-side) compressor 5 is calculated to be 30 cc. FIG. 4 also
shows in (a) and (b) the refrigeration capacity (5400 W) and the
COP (0.701) of a single-stage cycle
(compressor.fwdarw.condenser.fwdarw.expansion
valve.fwdarw.evaporator) of 15 HP using conventional R404A as a
comparative example. FIG. 4 reveals that a displacement volume of
about 60 cc is needed in order to obtain a refrigeration capacity
corresponding to that of 15 HP of an R404A single-stage cycle.
[0033] To achieve the object of the present invention by setting
the freezing point low to keep the evaporating temperature of the
low-temperature side evaporator 10 to fall within the range of -80
degrees C. to -60 degrees C., the proportion of R32 need only be
increased, as described above. As is obvious from FIG. 4, with an
increase in proportion of R32, the refrigeration capacity decreases
but the COP increases. However, with an increase in proportion of
R32, the GWP increases. The GWP is 1 (one) in CO.sub.2 and is 675
in R32 alone as shown in Table 1. Thus, when the content of R32 in
the entire zeotropic refrigerant is 50% by mass, the GWP is about
340. When the proportion of R32 is increased to 65% by mass, the
GWP is about 440. When the proportion of R32 is further increased
to 74% by mass, the GWP is about 500. That is, the freezing point
and the GWP have a trade-off relationship. When the proportion of
R32 is increased, the freezing point decreases, but the GWP
increases.
[0034] Thus, in Embodiment, the GWP is kept as low as 500 or less,
which is lower than a GWP of 3920 for R404A, and the refrigerant
mixing ratio is determined in order to increase the COP to 80% or
more of that in the case of using R404A. The line representing 80%
of the COP in the case of using R404A is shown in (b) of FIG. 4.
This requirement is satisfied when the proportion of R32 is equal
to or greater than 50% by mass.
[0035] As described above, in the zeotropic refrigerant mixture,
R32 is preferably mixed with CO.sub.2 such that the proportion of
R32 is 50% to 74% by mass. In this case, the freezing point of the
zeotropic refrigerant mixture can be reduced to -81 degrees C. or
less, which is lower than the triple point of CO.sub.2. Thus, an
evaporating temperature of -80 degrees C. to -60 degrees C. can be
obtained, and the refrigeration apparatus has a low GWP.
[0036] The refrigerant mixture containing CO.sub.2 and R32 includes
a zeotropic refrigerant mixture, and disadvantageously has a high
temperature gradient (11 to 12 K) as shown in Table 1. Thus, in an
appropriate operation, the circulation composition needs to be
determined accurately. A configuration for detecting a circulation
composition will now be described with reference to FIG. 5.
[0037] FIG. 5 illustrates a configuration in which a configuration
for detecting a circulation composition is added to the refrigerant
circuit of FIG. 1.
[0038] As illustrated in FIG. 5, the refrigerant circuit of FIG. 1
further includes a pressure detector 15, a temperature detector 16,
and a composition calculator 17. The pressure detector 15 detects
the pressure of a liquid refrigerant in the receiver 7. The
temperature detector 16 detects the temperature of the liquid
refrigerant in the receiver 7. The pressure and temperature of the
liquid refrigerant detected by the pressure detector 15 and the
temperature detector 16, respectively, are input to the composition
calculator 17. Based on the pressure and temperature of the liquid
refrigerant input to the composition calculator 17, the composition
calculator 17 calculates a circulation composition of a zeotropic
refrigerant mixture in a refrigeration cycle. The circulation
composition calculated by the composition calculator 17 is output
to the controller 20 that controls the entire refrigeration
apparatus and is used to control the low-temperature side
circulation circuit B. The composition calculation unit of the
present invention includes the pressure detector 15, the
temperature detector 16, and the composition calculator 17.
[0039] The operation of the refrigeration apparatus will now be
described. In the high-temperature side circulation circuit A, a
refrigerant discharged from the high-temperature side compressor 1
is condensed in the high-temperature side condenser 2 into a liquid
refrigerant. The liquid refrigerant is decompressed by the
high-temperature side expansion valve 3, and evaporates in the
high-temperature side evaporator 4 of a cascade heat exchanger into
a gas refrigerant. This gas refrigerant is drawn by suction into
the high-temperature side compressor 1 again, and this cycle is
repeated.
[0040] On the other hand, in the low-temperature side circulation
circuit B, a vapor of a high-temperature high-pressure zeotropic
refrigerant mixture compressed by the low-temperature side
compressor 5 is liquefied into a condensate by the low-temperature
side condenser 6, and the resulting refrigerant enters the receiver
7. The liquid refrigerant that has flowed out of the receiver 7
passes through an open solenoid valve 8 and is decompressed by the
low-temperature side expansion valve 9. This refrigerant becomes a
low-temperature, low-pressure gas-liquid two-phase refrigerant and
flows into the low-temperature side evaporator (a display case or a
unit cooler) 10. The refrigerant that has flowed into the
low-temperature side evaporator 10 exchanges heat with the air in
the display case, evaporates, and returns to the low-temperature
side compressor 5. By repeating this cycle, cooled air is generated
in the low-temperature side evaporator 10 and cools the inside of
the display case.
[0041] In the above-described operation, an excess zeotropic
refrigerant mixture generated in accordance with the operating
conditions and load conditions is accumulated in the receiver 7. As
described above, when the amount of refrigerant stored in the
receiver 7 changes, the circulation composition in the
refrigeration cycle changes. The principle of detecting the
circulation composition in the refrigeration cycle by the
composition calculator 17 will be described with reference to FIG.
6.
[0042] FIG. 6 is a gas-liquid equilibrium diagram of a zeotropic
refrigerant mixture.
[0043] The composition calculator 17 receives the pressure P and
the temperature T of the liquid refrigerant in the receiver 7 from
the pressure detector 15 and the temperature detector 16. The
saturated liquid temperature of the zeotropic refrigerant mixture
at the pressure P changes according to the circulation composition
in the refrigeration cycle, as shown in FIG. 6. Thus, supposing
that a refrigerant in the receiver 7 is in a saturated liquid
state, a circulation composition Z in the refrigeration cycle can
be determined from the temperature T and the pressure P detected by
the temperature detector 16 and the pressure detector 15,
respectively, with reference to the relationship shown in FIG. 6.
Therefore, when the relationship among the pressure, the
temperature, and the circulation composition is stored in the
composition calculator 17 in advance, the circulation composition Z
can be calculated based on the pressure P and the temperature
T.
[0044] The detection of the circulation composition is not limited
to the above-described method and may be performed in the following
manner. In a two-component zeotropic refrigerant mixture, the
circulation composition can be detected on the basis of a quality X
(=mass flow rate of refrigerant vapor/flow rate of entire
refrigerant) of a refrigerant and the temperature and the pressure
of the refrigerant having the quality X. Specifically, in the
two-component zeotropic refrigerant mixture, under a constant
pressure P, the temperature of a refrigerant and the circulation
composition Z with the quality X have a relationship as indicated
by an alternate long and short dashed line in FIG. 7, illustrating
the principle of detecting the circulation composition of the
two-refrigerant mixture shown in FIG. 7, including a saturated
vapor line at a quality X of 1 (one) and a saturated liquid line at
zero quality X.
[0045] That is, the temperature of the zeotropic refrigerant
mixture in a gas-liquid two-phase state having the quality X at the
pressure P changes depending on the circulation composition in the
refrigeration cycle as shown in FIG. 7. Therefore, with this
relationship, the circulation composition in the refrigeration
cycle can be detected by determining the pressure, the temperature,
and the quality of a refrigerant in a gas-liquid two-phase state
including a saturated vapor and a saturated liquid. That is, the
circulation composition in the refrigeration cycle can be detected
by using the temperature T detected by the temperature detector 16,
the pressure P detected by the pressure detector 15, and the
quality in the receiver 7.
[0046] In Embodiment, the pressure detector 15 is provided to the
receiver 7. Alternatively, the pressure detector 15 may be provided
on the discharge side of the low-temperature side compressor 5,
detect the discharge pressure of the low-temperature side
compressor 5, and determine, as the pressure of a liquid
refrigerant that has flowed into the receiver 7, the pressure
obtained by converting a pressure loss of the low-temperature side
condenser 6. In Embodiment, the temperature detector 16 is provided
to the receiver 7. Alternatively, the temperature detector 16 may
be provided at the outlet of the low-temperature side condenser 6,
and determine, as the temperature of a liquid refrigerant that has
flowed into the receiver 7, the outlet liquid temperature of the
low-temperature side condenser
[0047] In this manner, the circulation composition in the case of
using the two-component zeotropic refrigerant mixture of CO.sub.2
and R32 for the low-temperature side circulation circuit B can
accurately be detected. Thus, the rotation speed of the
low-temperature side compressor 5 of the low-temperature side
circulation circuit B and/or the opening degree of the
low-temperature side expansion valve 9 can be optimally controlled
in accordance with the circulation composition, and an ultra-low
temperature freezer (for example, for storing tuna) with a low GWP
and an ultra-low evaporating temperature of about -80 degrees C. to
-60 degrees C. can be obtained.
[0048] As described above, in Embodiment, a refrigerant in the
low-temperature side circulation circuit B includes a zeotropic
refrigerant mixture containing at least CO.sub.2 and R32 and having
a GWP of 500 or less, and the proportion of R32 in the mixture is
50% to 74% by mass. Thus, a refrigeration apparatus which has a low
GWP and includes a low-temperature side evaporator 10 that achieves
an evaporating temperature of -80 degrees C. to -60 degrees C. can
be obtained. This refrigeration apparatus implements an ultra-low
temperature freezer (for example, for storing tuna) having a
freezer internal temperature of about -50 degrees C.
[0049] Accurate detection of the circulation composition allows
control of the rotation speed of the low-temperature side
compressor 5 and/or the opening degree of the low-temperature side
expansion valve 9 in consideration of a change in circulation
composition in the low-temperature side circulation circuit B.
Thus, a refrigeration apparatus that can perform optimum control in
accordance with the circulation composition and operate stably can
be obtained.
* * * * *