U.S. patent application number 14/890966 was filed with the patent office on 2016-03-24 for refrigeration cycle apparatus and method of operating the same.
The applicant listed for this patent is MITSUBISHI ELECTRIC CORPORATION. Invention is credited to Masanori AOKI, Yohei KATO.
Application Number | 20160084556 14/890966 |
Document ID | / |
Family ID | 52007721 |
Filed Date | 2016-03-24 |
United States Patent
Application |
20160084556 |
Kind Code |
A1 |
KATO; Yohei ; et
al. |
March 24, 2016 |
REFRIGERATION CYCLE APPARATUS AND METHOD OF OPERATING THE SAME
Abstract
A refrigeration cycle apparatus includes a compressor and sets a
heating-mode lower limit refrigerant volume flow rate through the
compressor. The heating-mode lower limit refrigerant volume flow
rate is a lower limit of a volume flow rate in a heating operation.
The apparatus controls a rotation speed of the compressor such that
the volume flow rate in the heating operation is greater than or
equal to the heating-mode lower limit refrigerant volume flow rate.
The heating-mode lower limit refrigerant volume flow rate includes
a first lower limit for use of an R410A refrigerant and a second
lower limit for use of an R32 refrigerant. When using the R32
refrigerant in the refrigeration cycle apparatus designed for the
R410A refrigerant, the apparatus performs correction such that the
lower limit is increased from the first lower limit to the second
lower limit to control the rotation speed of the compressor.
Inventors: |
KATO; Yohei; (Tokyo, JP)
; AOKI; Masanori; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MITSUBISHI ELECTRIC CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
52007721 |
Appl. No.: |
14/890966 |
Filed: |
June 6, 2013 |
PCT Filed: |
June 6, 2013 |
PCT NO: |
PCT/JP2013/065643 |
371 Date: |
November 13, 2015 |
Current U.S.
Class: |
62/115 ;
62/228.4; 62/468 |
Current CPC
Class: |
F25B 13/00 20130101;
F25B 2700/21152 20130101; Y02B 30/70 20130101; F25B 31/002
20130101; F25B 49/022 20130101; F25B 49/02 20130101; F25B 2500/31
20130101; F25B 49/025 20130101; F04B 53/18 20130101; F25B 2500/19
20130101; F25B 2600/2513 20130101; F25B 2600/0253 20130101; Y02B
30/741 20130101; F25B 2500/16 20130101 |
International
Class: |
F25B 49/02 20060101
F25B049/02; F04B 53/18 20060101 F04B053/18 |
Claims
1. A refrigeration cycle apparatus including a variable
displacement compressor and pipes designed for R410A refrigerant,
the apparatus setting a heating-mode lower limit refrigerant volume
flow rate in the compressor, the heating-mode lower limit
refrigerant volume flow rate being a lower limit of a volume flow
rate in a heating operation, the apparatus controlling a frequency
of the compressor such that the volume flow rate in the heating
operation is greater than or equal to the heating-mode lower limit
refrigerant volume flow rate, the heating-mode lower limit
refrigerant volume flow rate including a first lower limit for use
of R410A refrigerant and a second lower limit for use of R32
refrigerant in the refrigeration cycle apparatus designed for the
R410A refrigerant, the second lower limit being greater than the
first lower limit, and in the heating operation in the use of the
R32 refrigerant in the refrigeration cycle apparatus designed for
the R410A refrigerant, the apparatus performing correction such
that a lower limit is increased from the first lower limit to the
second lower limit to control the frequency of the compressor.
2. The refrigeration cycle apparatus of claim 1, wherein the
heating-mode lower limit refrigerant volume flow rate is set by
determining a lower limit frequency of the compressor.
3. The refrigeration cycle apparatus of claim 1, wherein the
heating-mode lower limit refrigerant volume flow rate is set in an
upward gas pipe.
4. The refrigeration cycle apparatus of claim 1, wherein the
apparatus uses refrigerating machine oil, the refrigerating machine
oil in the use of the R32 refrigerant has a density identical to
that in the use of the R410A refrigerant, and the second lower
limit is higher than the first lower limit by 17% or more.
5. The refrigeration cycle apparatus of claim 1, wherein the
apparatus uses a refrigerating machine oil such that a density of a
mixture of the R32 refrigerant and a refrigerating machine oil is
greater than or equal to 90% and less than or equal to 110% of a
density of a mixture of the R410A refrigerant and a refrigerating
machine oil, and the second lower limit is higher than the first
lower limit by an amount in a range of 11% to 24%.
6. The refrigeration cycle apparatus of claim 1, wherein the
apparatus uses ether oil as a refrigerating machine oil in the use
of the R410A refrigerant and uses ester oil as a refrigerating
machine oil in the use of the R32 refrigerant.
7. A method of operating a refrigeration cycle apparatus that
includes a variable displacement compressor, a condenser, an
expansion valve, and an evaporator and that is designed for R410A
refrigerant, the method comprising: setting a heating-mode lower
limit refrigerant volume flow rate in the compressor, the
heating-mode lower limit refrigerant volume flow rate being a lower
limit of a volume flow rate in a heating operation; controlling
frequency of the compressor such that the volume flow rate in the
heating operation is greater than or equal to the heating-mode
lower limit refrigerant volume flow rate, the heating-mode lower
limit refrigerant volume flow rate including a first lower limit
for use of the R410A refrigerant and a second lower limit for use
of R32 refrigerant in the refrigeration cycle apparatus designed
for the R410A refrigerant, the second lower limit being greater
than the first lower limit; and in the heating operation in the use
of the R32 refrigerant in the refrigeration cycle apparatus
designed for the R410A refrigerant, performing correction such that
a lower limit is increased from the first lower limit to the second
lower limit to control the frequency of the compressor.
8. A refrigeration cycle apparatus including a variable
displacement compressor and pipes designed for R410A refrigerant,
the compressor being configured that frequency is controlled to
increase lower limit frequency in a heating operation to be higher
than lower limit frequency in a cooling operation, when R32
refrigerant is enclosed into the pipes.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a U.S. national stage application of
PCT/JP2013/065643 filed on Jun. 6, 2013, the content of which is
incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to a refrigeration cycle
apparatus and a method of operating the same in which an R32
refrigerant is enclosed and used instead of an R410A
refrigerant.
[0003] In a related-art refrigeration cycle apparatus in which
refrigerating machine oil that is immiscible or slightly miscible
with refrigerant is used as refrigerating machine oil, a rotation
speed of a compressor or an opening degree of an expansion valve is
controlled so that the flow velocity of refrigerant flowing through
a gas riser pipe, which is a gas pipe for upward flow of the
refrigerant, is higher than a flow velocity (zero penetration
velocity) at which the oil deposited on an inner wall of the gas
pipe is moved upward.
[0004] The above-described control of the rotation speed of the
compressor or the opening degree of the expansion valve can prevent
the refrigerating machine oil from building up inside the gas pipe,
thus reliably providing a necessary amount of oil for the
compressor. This can protect the compressor from poor lubrication
and failure (refer to Patent Literature 1, for example).
PATENT LITERATURE
[0005] Patent Literature 1: Japanese Unexamined Patent Application
Publication No. 2001-272117
[0006] Examples of refrigerants generally used currently include
R410A. This refrigerant has, as its features, an ozone depletion
potential of zero and a global warming potential of 2090, which is
still a high level. Attention is accordingly being given to R32
that has an ozone depletion potential of zero and a global warming
potential of 675, approximately one-third of that of R410A.
Furthermore, R32 has physical properties, such as the relationship
between the saturation temperature and the saturation pressure,
close to those of R410A. This enables R32 to be enclosed and used
in a refrigeration cycle apparatus designed for R410A, or enables
replacement of refrigerants.
[0007] If refrigerant is changed from R410A to R32, however, the
zero penetration velocity, serving as a gas refrigerant velocity
determined in consideration of oil returnability based on
refrigerant used, will also change. Disadvantageously, the
refrigeration cycle apparatus designed for R410A fails to ensure
good oil returnability in, particularly, an upward gas refrigerant
passage.
SUMMARY
[0008] The present invention has been made to overcome the
above-described disadvantage and aims to provide a refrigeration
cycle apparatus and a method of operating the same designed for
R410A configured such that in replacement of R410A by R32 as
refrigerant, a lower limit volume flow rate through a compressor is
changed to ensure good oil returnability in an upward gas
refrigerant passage.
[0009] The present invention provides a refrigeration cycle
apparatus including a variable displacement compressor and pipes
designed for R410A. The apparatus sets a heating-mode lower limit
refrigerant volume flow rate through the compressor. The
heating-mode lower limit refrigerant volume flow rate is a lower
limit of a volume flow rate in a heating operation. The apparatus
controls a frequency of the compressor such that the volume flow
rate in the heating operation is greater than or equal to the
heating-mode lower limit refrigerant volume flow rate. The
heating-mode lower limit refrigerant volume flow rate includes a
first lower limit for use of an R410A refrigerant and a second
lower limit for use of an R32 refrigerant in the refrigeration
cycle apparatus designed for the R410A refrigerant. The second
lower limit is greater than the first lower limit. In the heating
operation in the use of the R32 refrigerant in the refrigeration
cycle apparatus designed for the R410A refrigerant, the apparatus
performs correction such that the lower limit is increased from the
first lower limit to the second lower limit to control the
frequency of the compressor.
[0010] According to the present invention, in replacement of R410A
by R32 as refrigerant in the refrigeration cycle apparatus designed
for R410A, the lower limit volume flow rate in the heating
operation is changed, so that good oil returnability in an upward
gas refrigerant passage can be ensured. This can protect the
compressor from poor lubrication and failure.
BRIEF DESCRIPTION OF DRAWINGS
[0011] FIG. 1 is a diagram illustrating the configuration of a
refrigeration cycle apparatus 100 designed for R410A according to
the present invention.
[0012] FIG. 2 is a comparison graph illustrating the difference in
physical properties between R410A and R32 related to the present
invention.
[0013] FIG. 3 is a comparison graph illustrating the relationship
between the saturation temperature and a compressor lower limit
frequency associated with the zero penetration velocity of each of
R410A and R32 in Embodiment 1 of the present invention.
[0014] FIG. 4 is a graph illustrating the ratio of a gas
refrigerant density of R32 to that of R410A in the present
invention.
[0015] FIG. 5 is a diagram illustrating the relationship between
the zero penetration velocity and the diameter of each of pipes in
Embodiment 1 of the present invention.
[0016] FIG. 6 is a comparison graph illustrating the relationship
between the saturation temperature and a compressor lower limit
volume flow rate associated with the zero penetration velocity of
each of R410A and R32 in Embodiment 1 of the present invention.
[0017] FIG. 7 is a comparison graph illustrating the relationship
between the ratio of the density of a mixture of R32 and a
refrigerating machine oil to the density of a mixture of R410A and
a refrigerating machine oil and the ratio of an increase in lower
limit frequency in use of R32 to that in use of R410A in Embodiment
2 of the present invention.
[0018] FIG. 8 is a graph illustrating the ratio of the densities of
oils at which the zero penetration velocity is achieved in the use
of R32 in Embodiment 3 of the present invention.
[0019] FIG. 9 is a comparison graph illustrating the relationship
between the saturation temperature and the compressor lower limit
volume flow rate associated with the zero penetration velocity of
each of R410A and R32 in Embodiment 4 of the present invention.
DETAILED DESCRIPTION
[0020] Embodiments of the present invention will be described with
reference to the drawings. Embodiments which will be described
below should not be construed as limiting the present
invention.
Embodiment 1
[0021] FIG. 1 is a diagram illustrating the configuration of a
refrigeration cycle apparatus 100 designed for R410A. Referring to
FIG. 1, an outdoor unit 61 is connected to an indoor unit 62 by a
liquid pipe 5 and a gas pipe 7. The outdoor unit 61 includes a
compressor 1, an outdoor heat exchanger 2, an expansion valve 3
having a variable opening degree, an accumulator 9, serving as a
refrigerant container, and a four-way valve 8 which are connected
by pipes. The outdoor unit 61 further includes a discharge
temperature sensor 41, serving as a compressor outlet temperature
unit, and a control board 50 that controls the compressor 1, the
expansion valve 3, and an outdoor fan 31. The indoor unit 62
includes an indoor heat exchanger 6, an indoor fan 32, and a
control board (not illustrated) that controls the indoor fan. The
control board 50 includes a storage unit for storing a discharge
temperature detected by the discharge temperature sensor 41, the
opening degree of the expansion valve 3, and a rotation speed of
the compressor 1, a calculation unit for calculating the opening
degree of the expansion valve 3 and the rotation speed of the
compressor 1, and a control unit for controlling the opening degree
of the expansion valve 3 and the rotation speed of the compressor
1.
[0022] FIG. 2 is a comparison graph illustrating the difference in
physical properties between R410A and R32 in Embodiment 1 of the
present invention. In comparison between the saturation pressure
plotted against the refrigerant saturation temperature of R410A and
that of R32, an increase in pressure of R32 relative to the
pressure of R410A at each saturation temperature is small
(approximately +0.1 Mpa over the entire range). In replacement of
R410A by R32 as refrigerant, the same specification as that of the
refrigeration cycle apparatus designed for R410A can advantageously
be used.
[0023] The zero penetration velocity, serving as a refrigerant flow
velocity limit for oil returnability, will now be described.
[0024] In gas pipes, refrigerant and oil flow in a two-phase
gas-liquid state. In particular, in upward flow, the flowing state
of liquid (oil) changes depending on the flow velocity of gas. A
phenomenon occurs in such a manner that when the flow velocity of
the gas is high, the liquid is entrained into the gas flow and is
accordingly moved upward, and when the flow velocity of the gas
decreases, the liquid moves downward along an inner wall of a pipe.
A state in which an increased flow velocity of the gas results in a
reduction in downward-moving liquid membrane is called "zero
penetration". A flow velocity at the time of zero penetration is
called a "zero penetration velocity" (hereinafter, "ZP
velocity").
[0025] If an actual refrigerant flow velocity is higher than the ZP
velocity, refrigerating machine oil can be smoothly circulated
through a refrigerant circuit and be returned to the compressor 1
without building up in an upward gas refrigerant pipe.
[0026] The ZP velocity is typically represented by Expression
(1).
u G = C ( g D h ( .rho. L - .rho. G ) .rho. G ) 1 2 [ Math . 1 ]
##EQU00001##
where C: correction factor specific to an actual apparatus;
[0027] uG: ZP velocity [m/s];
[0028] g: gravitational acceleration [m/s.sup.2];
[0029] Dh: pipe inside diameter [m];
[0030] .rho.L: liquid density [kg/m.sup.3]; and
[0031] .rho.G: gas density [kg/m.sup.3].
[0032] In this expression, C is the correction factor for the ZP
velocity. Since the expression representing the ZP velocity is
based on experiments in a water or air system, this factor is
obtained in consideration of fluids (the refrigerant and the oil in
Embodiment 1) and the shape of an inner surface of a pipe in the
actual apparatus.
[0033] If the actual refrigerant flow velocity is higher than the
ZP velocity given by the above-described expression, therefore, the
refrigerating machine oil will be smoothly circulated through the
refrigerant circuit and be returned to the compressor without
building up in a vertical gas pipe.
[0034] The relationship between the ZP velocity and a lower limit
frequency of the compressor will now be described.
[0035] A method of calculating a compressor frequency at which the
actual flow velocity is at or above the ZP velocity will first be
described.
[0036] A flow rate Qcomp [kg/s] of refrigerant discharged from the
compressor is represented by Expression (2) including a stroke
volume Vst [cc], the number of revolutions F [r/s], a volumetric
efficiency .eta.v, and a compressor suction density .rho.s
[kg/m.sup.3].
Q.sub.comp=V.sub.st.times.10.sup.-6.times.F.times..eta..sub.v.times..rho-
..sub.s [Math. 2]
[0037] The flow velocity, ur [m/s], of gas refrigerant passing
through a gas pipe having a cross-sectional area Ai
(=.pi./4.times.di.sup.2) [m.sup.2] in the refrigerant circuit can
be given by Expression (3) including the density, pr, of the gas
refrigerant in the pipe.
u r = Q comp A i .times. .rho. r = Q comp .pi. 4 .times. d i 2
.times. .rho. r [ Math . 3 ] ##EQU00002##
[0038] Expressions (2) and (3) give Expression (4) that represents
the relationship between the gas refrigerant flow velocity ur and
the number of revolutions F.
F = u r .times. A i V st .times. 10 - 6 .times. .eta. v .times.
.rho. r .rho. s [ Math . 4 ] ##EQU00003##
[0039] Expressions (1) and (4) give the number of revolutions F of
the compressor 1 at the ZP velocity uG. The obtained number of
revolutions is a "lower limit frequency F*" of the compressor 1.
The lower limit frequency F* or higher allows for good oil
returnability in an upward gas pipe.
[0040] Expression (4) may be expressed as Expression (5). The left
side of Expression (5) represents a volume flow rate [m.sup.3/s]
through the compressor 1 at the lower limit frequency F* of the
compressor 1 and the right side thereof represents a volume flow
rate [m.sup.3/s] through the pipe.
F * .times. V st .times. 10 - 6 .times. .eta. v = u G .times. A i
.times. .rho. G .rho. s [ Math . 5 ] ##EQU00004##
[0041] The lower limit frequency F* of the compressor 1 is
proportional to the ZP velocity uG and the pipe cross-sectional
area Ai and is inversely proportional to the stroke volume Vst and
the volumetric efficiency .eta.v and is proportional to the ratio
of the pipe gas density .rho.G to the compressor suction density
.rho.s.
[0042] FIG. 3 illustrates the relationship between the refrigerant
saturation temperature and the lower limit frequency of the
compressor associated with the ZP velocity of each of R410A and
R32.
[0043] FIG. 3 illustrates the results of calculation of the lower
limit frequency of the compressor 1 associated with the ZP velocity
relative to the refrigerant saturation temperature. The calculation
was performed by using Expressions (1) to (5) described above. FIG.
3 also depicts the ratio of an increase in lower limit frequency of
the compressor 1 in the use of the R32 refrigerant to that in the
use of the R410A refrigerant at each saturation temperature.
[0044] Calculation conditions are as follows.
[0045] Actual apparatus used: Apparatus designed for R410A
[0046] Refrigerants compared: R410A and R32
[0047] Compressor stroke volume: 33 cc
[0048] Compressor volumetric efficiency: 0.9
[0049] Target pipe: Compressor suction-side pipe having a diameter
.phi. of 19.05
[0050] Refrigerating machine oil: Common to R410A and R32
[0051] Pressure: Saturation temperatures -40 degrees C. to 63.6
degrees C.
[0052] (R410A: 0.40 to 4.15 Mpa/R32: 0.41 to 4.25 MPa)
[0053] Compressor suction temperature: Saturation temperature+0.0
K
[0054] As is evident from the relationship between the refrigerant
saturation temperature and the lower limit frequency of the
compressor 1 associated with the ZP velocity of each of R410A and
R32 in FIG. 3, the lower limit frequency of the compressor 1
associated with the ZP velocity in the use of R32 is higher than
that in the use of R410A over the entire range of refrigerant
saturation temperatures. This is because R32 has a lower gas
refrigerant density than R410A as illustrated in FIG. 4 and
accordingly requires a higher ZP velocity than R410A. The above
results demonstrate that it is necessary to set a high lower limit
frequency of the compressor 1 in order to ensure good oil
returnability if the refrigerant saturation temperature of R32 is
the same as that of R410A.
[0055] Operation conditions that allow an increase in ZP velocity
as well as an increase in lower limit frequency of the compressor 1
will now be described.
[0056] As understood from FIG. 3 illustrating the relationship
between the refrigerant saturation temperature and the lower limit
frequency of the compressor 1 associated with the ZP velocity of
each of R410A and R32, the lower the refrigerant saturation
temperature, the higher the lower limit rotation speed of the
compressor 1. The reason is that as the refrigerant temperature is
lower, the refrigerant gas density decreases and the ZP velocity
increases (refer to Expression (1)). The lower limit rotation speed
of the compressor 1 tends to increase as a low-pressure side
refrigerant saturation temperature, that is, an evaporating
pressure of an evaporator is lower.
[0057] Considering operations throughout the year, these conditions
that allow an increase in lower limit rotation speed of the
compressor 1 can be true in a heating operation during the winter
season during which outdoor air temperature related to an
evaporating temperature of the refrigerant decreases. As regards
pipes in a refrigerant cycle, as understood from Expression (4),
the conditions that allow an increase in lower limit rotation speed
include a pipe having a large cross-sectional area Ai.
[0058] On the basis of the above-described conditions, it is
assumed that the outside air temperature is low and the evaporating
temperature in the heating operation during the winter season is
-20 degrees C. FIG. 5 illustrates ZP velocities in pipes in a
refrigeration cycle calculated on the above-described
assumption.
[0059] FIG. 5 demonstrates that a suction gas pipe connecting to
the compressor 1 and having the largest inside diameter among the
pipes provides the highest ZP velocity.
[0060] In other words, the ZP velocity, or the oil returnability
may be evaluated in the suction gas pipe that is a gas pipe through
which upward flow is provided in the heating operation, that has
the largest inside diameter, and that connects to the compressor
1.
[0061] On the above-described assumption, the lower limit frequency
of the compressor 1 in the refrigeration cycle apparatus 100
designed for R410A can accordingly be set on the basis of an
evaporating temperature of -20 degrees C. in the heating operation
at which the ZP velocity is high.
[0062] A lower limit frequency of the compressor 1 in the use of
R32 in the refrigeration cycle apparatus 100 designed for R410A in
the heating operation will now be evaluated. As described above,
the lower limit frequency of the compressor 1 in the refrigeration
cycle apparatus designed for R410A is set on the basis of an
evaporating temperature of -20 degrees C. in the heating operation
at which the ZP velocity is high.
[0063] In evaluation based on the relationship between the
refrigerant saturation temperature and the lower limit frequency of
the compressor 1 associated with the ZP velocity of each of R410A
and R32 in FIG. 3, the lower limit frequency in the use of R32 is
higher than that in the use of R410A by approximately 17% at an
evaporating temperature of -20 degrees C. in the heating operation,
as is obvious from FIG. 3.
[0064] Furthermore, as is clear from FIG. 3, when the evaporating
temperature in a cooling operation is 0 degrees C., the lower limit
frequency of the compressor 1 in the cooling operation in the use
of R32 is higher than the lower limit frequency (lower limit
frequency based on an evaporating temperature of -20 degrees C.) in
the use of R410A.
[0065] Consequently, the lower limit frequency of the compressor 1
in the use of R32 in the refrigeration cycle apparatus designed for
R410A may be corrected such that the lower limit frequency is
increased only in the heating operation.
[0066] Thus, the lower limit frequency of the compressor 1 in the
heating operation in the refrigeration cycle apparatus 100 in the
use of R32 can be corrected, or increased to be higher than that in
the use of R410A by approximately 17% or more so that the ZP
velocity is achieved, as is evident from FIG. 3.
[0067] In addition, as described above, the lower limit frequency
of the compressor 1 in the cooling operation in the refrigeration
cycle apparatus 100 in the use of R32 does not have to be corrected
relative to that in the use of R410A.
[0068] As defined in Expression (5), the lower limit volume flow
rate through the gas pipe increases in proportion to the increase
of the lower limit rotation speed of the compressor 1 associated
with the ZP velocity.
[0069] FIG. 6 illustrates the results of calculation of the lower
limit volume flow rate in the compressor 1 associated with the ZP
velocity relative to the refrigerant saturation temperature. The
calculation was performed by using Expressions (1) to (5) described
above. FIG. 6 also depicts the ratio of an increase in lower limit
volume flow rate in the compressor 1 in the use of R32 to that in
the use of R410A at each refrigerant saturation temperature.
[0070] In FIG. 6, the lower limit volume flow rates through the
compressor 1 are obtained by simply converting the lower limit
frequencies of the compressor 1 associated with the ZP velocities
and represented by the axis of ordinates in FIG. 3.
[0071] As is evident from the left side of Expression (5) described
above, the volume flow rate in the compressor 1 is proportional to
the rotation speed of the compressor 1. The rate of correction for
the increase of the compressor lower limit frequency accordingly
agrees with that for the increase of the lower limit volume flow
rate in the compressor 1. In evaluation based on the relationship
between the refrigerant saturation temperature and the lower limit
volume flow rate in the compressor 1 associated with the ZP
velocity of each of R410A and R32 in FIG. 6, as is obvious from
FIG. 6, the lower limit volume flow rate in the use of R32 is
higher than that in the use of R410A by approximately 17% at an
evaporating temperature of -20 degrees C. in the heating operation
in a manner similar to FIG. 3.
[0072] As described above, the lower limit frequency of the
compressor 1, that is, the lower limit volume flow rate in the
compressor 1 is set so as to increase in the heating operation,
thereby controlling the displacement of the compressor 1. If the
density of gas refrigerant is changed from the density of gas
refrigerant of R410A to that of R32, this control allows the gas
refrigerant velocity in the suction gas pipe of the compressor to
be maintained at or above the ZP velocity so that the volume flow
rate through the gas pipe is increased, thus ensuring good oil
returnability of the refrigerating machine oil.
[0073] The lower limit frequency of the compressor 1 increases with
decreasing outside air temperature. If a frequency required for the
heating operation is lower than the lower limit frequency at a low
load in the heating operation, the capacity will be excessive, so
that the units will enter an intermittent operation to repeat
thermo-ON and thermo-OFF. Typically, an operation frequency lower
than or equal to the lower limit frequency may be permitted for a
predetermined period of time. It is assumed that the refrigerating
machine oil is not returned from the cycle during this period of
time. After the predetermined period of time during which the
operation at a frequency lower than or equal to the lower limit
frequency is permitted, the operation at a frequency higher than or
equal to the lower limit frequency is continued for a certain
period of time to return the oil built up in the refrigeration
cycle. Such control can reliably ensure good oil returnability at
the low load.
Embodiment 2
[0074] In Embodiment 1, when R32 is used in the refrigeration cycle
apparatus 100 designed for R410A, the same refrigerating machine
oil is used and the lower limit rotation speed of the compressor 1
and the lower limit volume flow rate in the compressor 1 are
determined. According to Embodiment 2, different refrigerating
machine oils are used for R410A and R32 such that the ratio of the
density of a mixture of R32 and a refrigerating machine oil is
greater than or equal to 90% and less than or equal to 110% of the
density of a mixture of R410A and a refrigerating machine oil.
Other calculation conditions are the same as those in Embodiment
1.
[0075] In Embodiment 2, the lower limit rotation speed of the
compressor 1 and the lower limit volume flow rate in the compressor
1 are determined in consideration of the range of densities of
refrigerating machine oils typically used for R32.
[0076] FIG. 7 illustrates the relationship between the ratio of the
density of a mixture of R32 and a refrigerating machine oil to the
density of a mixture of R410A and a refrigerating machine oil given
by Expressions (1) to (5) and the ratio of an increase in lower
limit frequency of the compressor in the use of R32 to that in the
use of R410A at each saturation temperature.
[0077] In FIG. 7, the axis of abscissas represents the ratio of the
densities of the mixtures of the refrigerants and refrigerating
machine oils and the axis of ordinates represents the refrigerant
saturation temperature. In FIG. 7, a thick line indicates a range
of 90% to 110% as the ratio of the density of the mixture of R32
and a refrigerating machine oil to the density of the mixture of
R410A and a refrigerating machine oil.
[0078] A refrigerant saturation temperature of -20 degrees C.,
serving as an evaporating temperature in the heating operation, is
included in conditions for correcting an increase in lower limit
frequency of the compressor 1, as in Embodiment 1. As can be seen
from the plots in FIG. 7, the lower limit frequency of the
compressor 1 in the use of R32 is set to be higher than that in the
use of R410A by an amount in a range of approximately 11% to
approximately 24%.
[0079] As described in Embodiment 1, it is unnecessary to correct
the lower limit frequency in the cooling operation in the use of
R32 relative to that in the use of R410A.
[0080] As defined by Expression (5), the lower limit volume flow
rate in the compressor 1 increases in proportion to the increase of
the lower limit rotation speed of the compressor 1 associated with
the ZP velocity.
[0081] As described above, the lower limit frequency of the
compressor 1, that is, the lower limit volume flow rate in the
compressor 1 is set so as to increase in the heating operation,
thereby controlling the displacement of the compressor 1. If the
density of the refrigerating machine oil is changed from a value
for R410A to a value for R32, this control allows the gas
refrigerant velocity in the suction gas pipe of the compressor to
be maintained at or above the ZP velocity so that the volume flow
rate through the gas pipe is increased, thus ensuring good oil
returnability of the refrigerating machine oil.
Embodiment 3
[0082] In Embodiments 1 and 2, the lower limit frequency of the
compressor 1 is corrected so as to increase in the heating
operation, thereby ensuring good oil returnability. According to
Embodiment 3, the refrigerating machine oil is replaced by a
refrigerating machine oil having a lower density in order to ensure
good oil returnability.
[0083] FIG. 8 illustrates the ratio, at which the ZP velocity is
achieved relative to the lower limit frequency of the compressor,
of the density of oil in the use of R32 in the refrigeration cycle
apparatus 100 designed for R410A to that in the use of R410A. In
FIG. 8, the axis of abscissas represents the refrigerant saturation
temperature and the axis of ordinates represents the ratio of the
density of a refrigerating machine oil for R32 to the density of a
refrigerating machine oil for R410A.
[0084] A refrigerant saturation temperature of -20 degrees C.,
serving as an evaporating temperature in the heating operation, is
a condition where the ZP velocity is high and good oil
returnability is difficult to achieve, as in Embodiments 1 and 2.
The ratio of the density of a refrigerating machine oil for R32 to
the density of a refrigerating machine oil for R410A is accordingly
set to substantially 72% or lower, thus ensuring good oil
returnability.
[0085] As described above, a low-density refrigerating machine oil
is used as a refrigerating machine oil for R32 in the use of R32 in
the refrigeration cycle apparatus 100 designed for R410A, thereby
setting the density of the refrigerating machine oil at which the
ZP velocity is achieved. This can ensure good oil returnability of
the refrigerating machine oil.
Embodiment 4
[0086] In Embodiment 1, in the use of R32 in the refrigeration
cycle apparatus 100 designed for the R410A refrigerant, the same
refrigerating machine oil is used and the lower limit rotation
speed of the compressor 1 is determined. According to Embodiment 4,
ether oil is used as a refrigerating machine oil in the use of
R410A, ester oil is used as a refrigerating machine oil in the use
of R32, and the lower limit frequency of the compressor 1 and the
lower limit volume flow rate in the compressor 1 are determined in
the use of R32. Other calculation conditions are the same as those
in Embodiment 1.
[0087] For example, ether oil having a density of 936.9
[kg/m.sup.3] at one atmosphere pressure and a temperature of 15
degrees Celsius is used for R410A and ester oil having a density of
947 [kg/m.sup.3] at one atmosphere pressure and a temperature of 15
degrees Celsius is used for R32.
[0088] The reason why the refrigerating machine oil used for R32 is
ester oil is because ester oil has a viscosity and density close to
those of ether oil but has a higher two-phase separation
temperature than ether oil and accordingly reduces a likelihood
that two-phase separation may occur in a refrigerant container,
such as an accumulator.
[0089] In other words, the oil returnability of ester oil is higher
than that of ether oil, thus improving the reliability of the
refrigeration cycle.
[0090] FIG. 9 illustrates the results of calculation of the lower
limit volume flow rate in the compressor 1 associated with the ZP
velocity relative to the refrigerant saturation temperature. The
calculation was performed by using Expressions (1) to (5) described
above. FIG. 9 also depicts the ratio of an increase in lower limit
volume flow rate in the compressor 1 in the use of R32 to that in
the use of R410A at each refrigerant saturation temperature.
[0091] In evaluation based on the relationship between the
refrigerant saturation temperature and the lower limit volume flow
rate in the compressor 1 associated with the ZP velocity of each of
R410A and R32 in FIG. 9, the lower limit volume flow rate in the
use of R32 is higher than that in the use of R410A by approximately
18% at an evaporating temperature of -20 degrees C. in the heating
operation, as is obvious from FIG. 9.
[0092] As described above, the lower limit frequency of the
compressor 1, that is, the lower limit volume flow rate in the
compressor 1 is set so as to increase in the heating operation,
thereby controlling the displacement of the compressor 1. If the
density of gas refrigerant is changed from the density of gas
refrigerant of R410A to that of R32, this control allows the gas
refrigerant velocity in the suction gas pipe of the compressor to
be maintained at or above the ZP velocity so that the volume flow
rate through the gas pipe is increased, thus ensuring good oil
returnability of the refrigerating machine oil.
* * * * *