[0001] This invention relates to a cooling cycle which employs a supercritical fluid such
as carbon dioxide (CO
2) as the coolant.
[0002] As the temperature of the coolant discharged from the compressor becomes extremely
high at times of high load, there is a danger with this type of cooling cycle that
the durability of the components which make up the cooling cycle can deteriorate significantly.
In particular the temperature range within which the durability of rubber or resin
parts can be maintained is low compared to other components, and during times of high
load the discharge temperature of the coolant can exceed this temperature range leading
to potential damage.
[0003] In a supercritical vapour compression cooling cycle which uses carbon dioxide or
the like as the coolant, an internal heat exchanger which exchanges heat between the
high-pressure coolant passing through the radiator and the low-pressure coolant introduced
into the compressor is provided with the purpose of increasing cooling performance,
and also to prevent compression of the liquid by the compressor.
[0004] For this reason proposals have been made in prior art, as shown in Japanese laid-open
patent application 2002-228282, to recover the liquid coolant or oil separated out
by the accumulator at the inlet side of the internal heat exchanger, which is designed
to prevent overheating with the very high temperatures of the coolant introduced into
the compressor, or as shown in Japanese laid-open patent applications 11-193967 or
11-201568, to recover gas coolant within the accumulator between the outlet side of
the internal heat exchanger and the inlet side of the compressor.
[0005] However, with the structure cited in Patent application N°2002-228282, whilst it
is possible to reduce the temperature of the coolant introduced into the compressor
and even to reduce the temperature of the coolant discharged from the compressor,
there is the disadvantage that there will be more pressure lost within the internal
heat exchanger as the liquid coolant and oil pass through the internal heat exchanger,
due to the fact that liquid coolant and oil cooled within the accumulator become mixed
with coolant from which heat has been absorbed in the evaporator.
[0006] Moreover, with the structure cited in Japanese laid-open patent applications 11-193967
or 11-201568, as the gas coolant within the accumulator is supplied after bypassing
the internal heat exchanger, being mixed with the gas coolant flowing out from the
internal heat exchanger, effective recovery of the gas coolant requires a large regulator
valve to regulate the diameter of the tubing of the diversion path and the amount
recovered, which negates the requirement for reduced size, weight and cost.
[0007] Thus the main purpose of the invention is to provide a cooling cycle which, with
a structure which prevents a rise of temperature in the coolant discharged from the
compressor, avoids increased pressure loss in the internal heat exchanger and enables
a reduction in the size, weight and cost of the necessary components which make up
the structure in question.
[0008] In order to achieve this aim, in a cooling cycle having a compressor which raises
the pressure of a coolant, a radiator which cools the coolant compressed in said compressor,
an expansion device which reduces the pressure of the coolant cooled by said radiator,
an evaporator which evaporates the coolant whose pressure has been reduced by said
expansion device, an accumulator which in addition to separating out the gas and liquid
in the coolant which has passed through said evaporator, separates out the oil in
the coolant, and an internal heat exchanger which exchanges heat between the low-pressure
coolant conducted from said accumulator to said compressor and the high-pressure coolant
conducted from said radiator to said expansion device, the cooling cycle of the invention
is characterized in that a recovery path is provided which bypasses said internal
heat exchanger, enabling recovery of the liquid coolant or oil inside said accumulator
between said internal heat exchanger and compressor, and a regulator valve which enables
regulation of the quantity of said liquid coolant or oil recovered via this recovery
path.
[0009] Thus as it is arranged that the liquid coolant or oil separated in the accumulator
is recovered between the internal heat exchanger and the compressor and mixed with
the gas coolant that has passed through the internal heat exchanger, it is possible
to reduce the temperature of the coolant introduced into the compressor, and even
to reduce the temperature of the coolant discharged from the compressor. Moreover,
as it is arranged that the liquid coolant or oil separated in the accumulator is made
to bypass the internal heat exchanger and is recovered between the internal heat exchanger
and the compressor, it is possible to avoid pressure losses within the internal heat
exchanger, and as the liquid coolant or oil are recovered via the recovery path, it
is possible to make the diameter of the tubing of the recovery path smaller than with
gas coolant passing through, thus enabling reductions in size and weight.
[0010] The above structure may also be realized by providing a coolant discharge temperature
sensor which detects the temperature of the coolant discharged from said compressor,
and a control means which controls said regulator valve in accordance with the coolant
discharge temperature detected by said coolant discharge temperature sensor, or a
control means which controls said regulator valve where it is expected that the discharge
temperature will rise with a high cycle load due to the outside temperature, pressure
within the cycle, room temperature or the like. The internal heat exchanger and regulator
valve may also be built into the accumulator to enable reductions in the size, weight
and cost of the cooling cycle.
[0011] The structure of the above cycle is suited to a supercritical vapour compression
cooling cycle which uses carbon dioxide as a coolant, in which the coolant discharge
temperature of the compressor becomes extremely high at times of high load.
[0012] As has been described above, according to the invention, in a cooling cycle provided
with an internal heat exchanger which exchanges heat between the low-pressure coolant
conducted from the accumulator to the compressor and the high-pressure coolant conducted
from the radiator to the expansion device, the liquid coolant or oil within the accumulator
bypasses the internal heat exchanger via the recovery path, enabling recovery between
the internal heat exchanger and the compressor, the quantity of liquid coolant or
oil recovered via this recovery path being regulated by a regulator valve, thus enabling
the temperature of the coolant introduced to the compressor to be reduced, and even
enabling a reduction in the coolant discharge temperature. Moreover, in addition to
preventing increases in pressure loss in the internal heat exchanger, it is possible
to have a smaller diameter for the tubing of the recovery path than is possible with
gas coolant passing through, thus permitting a smaller size of regulator valve, and
allowing the cooling cycle to be of smaller size, weight and cost.
[0013] An optimal configuration of an embodiment of the invention will now be described
with reference to the attached drawings :
Figure 1 is a diagram showing the overall structure of an example of the cooling cycle
of the invention.
Figure 2 is an expanded diagram showing the vicinity of the accumulator and internal
heat exchanger in Figure 1.
Figure 3 is a flow chart showing an example of the operation of the control unit from
Figure 1.
Figure 4 is a Mollier diagram illustrating the operating mechanism of the cooling
cycle.
Figure 5 is a graph showing the relationship between the quantity of oil recovered
from the accumulator and the coolant discharge temperature of the compressor.
Figure 6 is a graph showing the relationship between the quantity of oil recovered
from the accumulator and cooling performance.
[0014] In Figure 1, cooling cycle 1 has a structure comprising compressor 2 which increases
the pressure of the coolant, radiator 3 which cools the coolant compressed by compressor
2, expansion device 4 which reduces the pressure of the coolant cooled by radiator
3, evaporator 5 which evaporates coolant whose pressure is reduced in expansion device
4, accumulator 6 which in addition to separating out the gas and liquid in the coolant
passing through evaporator 5 separates the oil (lubricating oil) mixed in with the
coolant, and internal heat exchanger 7 which exchanges heat between the low-pressure
coolant conducted from accumulator 6 to compressor 2 and the high-pressure coolant
conducted from radiator 3 to expansion device 4.
[0015] In other words, with cooling cycle 1 the discharge side of compressor 2 is connected
to high-pressure duct 7a of internal heat exchanger 7 via radiator 3, the outlet side
of this high-pressure duct 7a being connected to expansion device 4. Moreover, the
outlet side of expansion device 4 is connected to the inlet port of accumulator 6
via evaporator 5, the outlet port of accumulator 6 being connected to the inlet side
of compressor 2 via low-pressure duct 7b of internal heat exchanger 7. Thus high-pressure
line 8 is comprised of the path which reaches expansion device 4 via radiator 3 and
high-pressure duct 7a from the discharge side of compressor 2, and low-pressure line
9 is comprised of the path which reaches from expansion device 4 to compressor 2 via
evaporator 5, accumulator 6 and low-pressure duct 7b.
[0016] The above described cooling cycle 1 uses carbon dioxide (CO
2) as the coolant, and the coolant whose pressure is increased in compressor 2 is introduced
into radiator 3 as a high-temperature and high-pressure supercritical coolant, being
cooled here by radiation. Thereafter it enters high-pressure duct 7a of internal heat
exchanger 7 where it is further cooled by heat exchange with the low-temperature gas
coolant flowing out from accumulator 6, and sent on without being liquefied to expansion
device 4. In this expansion device 4 the pressure is then reduced, creating a low-temperature
low-pressure wet vapour, which is vaporized by heat exchange with the air passing
through evaporator 5, before flowing into accumulator 6 as a two-phase coolant in
which gas and liquid are mixed.
[0017] Not only is the coolant which flows into accumulator 6 here separated out into gas
and liquid, the oil which has become mixed into the coolant is also separated, the
separated liquid coolant and oil remaining within the accumulator, and the gas coolant
being sent to low-pressure duct 7b of internal heat exchanger 7, being returned to
compressor 2 after complete conversion to a gas after absorbing more heat through
heat exchange with the high-temperature coolant flowing out from radiator 3.
[0018] However, with this structure, as is shown in Figure 2, abovementioned cooling cycle
1 is provided with recovery path 10, one end of which has an opening at the bottom
of accumulator 6, the other end being connected between low-pressure duct 7b of internal
heat exchanger 7 and the inlet side of compressor 2, the degree of opening of this
recovery path 10 being regulated by regulator valve 11, which comprises an electromagnetic
valve.
[0019] Coming back to Figure 1, the reference 12 is a temperature sensor for the coolant
discharge which detects temperature Td of the coolant discharged from compressor 2,
and the temperature signal output from this coolant discharge temperature sensor 12
is input to control unit 13, said control unit 13 being part of the control means.
This control unit 13 has a structure comprising a central processing unit (CPU), a
read-only memory (ROM), a random access memory (RAM), an input/output port and the
like, and in addition a drive circuit which controls regulator valve 11, it being
arranged that regulator valve 11 is controlled using the control routine shown in
Figure 3 which is executed at prescribed intervals. In other words, the coolant discharge
temperature Td is input at prescribed intervals (step 50), it is determined whether
the coolant discharge temperature Td thus input is above a fixed temperature (step
52) and where determined that it is above the prescribed temperature, the degree of
opening of regulator valve 11 is adjusted in accordance with the coolant discharge
temperature Td (step 54) so that regulator valve 11 opens up more with rises in coolant
discharge temperature Td (and a greater quantity of oil or liquid coolant is recovered
via recovery path 10).
[0020] Thus oil 20a separated within accumulator 6, as shown in Figure 2, sinks to a lower
level than liquid coolant 20b as it has a higher specific gravity than liquid coolant
20b, and when recovery path 10 is opened up using regulator valve 11, recovery takes
place between internal heat exchanger 7 and compressor 2 via recovery path 10 starting
with the liquid coolant. After oil 20a has been recovered from inside accumulator
6, liquid coolant 20b is recovered between internal heat exchanger 7 and compressor
2 via recovery path 10, mixed in with the gas coolant passing through internal heat
exchanger 7 and recovered to compressor 2. For this reason the temperature of the
low-pressure gas coolant which has passed through internal heat exchanger 7 rises
due to heat exchange with the high-temperature high-pressure gas coolant, but is then
cooled again due to mixing with oil 20a or liquid coolant 20b cooled within accumulator
6, thus enabling the inlet temperature of compressor 2 to be reduced.
[0021] To describe the above mechanism using the Mollier diagram in Figure 4, in conventional
structures which do not have recovery path 10 the changes proceed as shown in the
Mollier diagram as A1 > B1 > C > D > E > A1, but with this structure since low temperature
oil 20a or liquid coolant 20b recovered via recovery path 10 is mixed with the gas
coolant passing through internal heat exchanger 7, the enthalpy momentarily increased
by internal heat exchanger 7 is reduced from A1 to A2. Thus while in this cooling
cycle the presence of oil 10a or liquid coolant 10b recovered via recovery path 10
has almost no influence on the cooling effect, the enthalpy on the discharge side
of the compressor is reduced from B1 to B2. As a result in the above structure it
is possible to reduce the temperature of the coolant introduced to compressor 2, and
even to reduce the temperature of the coolant discharged from compressor 2.
[0022] In fact, where coolant discharge temperature Td is high the oil from accumulator
6 is made to bypass internal heat exchanger 7 and is recovered between the internal
heat exchanger and the compressor, so that when considering changes in coolant discharge
temperature, the greater the quantity of oil recovered the more it is possible to
lower the coolant discharge temperature, as shown in Figure 5. Furthermore, changes
in the cooling performance at this time, as shown in Figure 6 have been shown to remain
almost the same regardless of the quantity of oil recovered.
[0023] Thus with the above structure, it is possible to prevent damage to components due
to rises in the discharge coolant temperature Td of the compressor, and because of
reductions in the coolant discharge temperature of compressor 2, oil 20a and liquid
coolant 20b in accumulator 6 can be recovered while being made to bypass internal
heat exchanger 7, thus preventing increases in pressure loss caused by flows of oil
or liquid coolant into internal heat exchanger 7. Further, the liquid flowing through
recovery path 10 is oil or liquid coolant, enabling the diameter of the tubing which
makes up recovery path 10 to be made smaller than in the case of tubing through which
a gas coolant flows, and also enabling the diameter of regulator valve 11 provided
in recovery path 10 to be made smaller. For this reason a reduction in the size, weight
and cost of components is feasible.
[0024] In the above described structure, it is arranged such that oil 20a or liquid coolant
20b within accumulator 6 is recovered with recovery path 10 connected to the bottom
of accumulator 6, but as a structure for the recovery of oil or liquid coolant within
accumulator 6 not limited to the above structure, the tubing which forms recovery
path 10 may be inserted within the accumulator, and the regulator valve provided for
recovering the oil or liquid coolant may be positioned within this inserted portion.
[0025] Furthermore, internal heat exchanger 7 may be built into accumulator 6, and the regulator
valve, which regulates the quantity recovered and the recovery path for oil or liquid
coolant, may also be provided within accumulator 6. With this kind of structure, there
is no need to provide new tubing external to accumulator 6, thus making it easier
to realize a reduction in size, weight and cost for cooling cycle 1.
[0026] Furthermore, with the above structure, it is arranged that the quantity recovered
is regulated where the coolant discharge temperature is above a prescribed value,
but where it is expected that a high coolant discharge temperature will result from
a high cycle load due to external temperature, internal cycle pressure, room temperature
or the like, said regulator valve may be controlled to allow the quantity recovered
to be regulated. In practical terms, the quantity recovered may be regulated where
the room temperature, the external temperature or the air temperature at the inlet
of radiator 3 exceeds a prescribed temperature, where the air temperature at the exit
of the evaporator 5 or the pressure of the low-pressure line is above a prescribed
value, or where the flow of air blown from the air-conditioning unit is above a prescribed
level.
1. Cooling cycle (1) having a compressor (2) which raises the pressure of a coolant,
a radiator (3) which cools the coolant compressed in said compressor, an expansion
device (4) which reduces the pressure of the coolant cooled by said radiator, an evaporator
(5) which evaporates the coolant whose pressure has been reduced by said expansion
device, an accumulator (6) which in addition to separating out the gas and liquid
in the coolant which has passed through said evaporator, separates out the oil in
the coolant, and an internal heat exchanger (7) which exchanges heat between the low-pressure
coolant conducted from said accumulator to said compressor and the high-pressure coolant
conducted from said radiator to said expansion device, characterized in that a recovery path (10) is provided which bypasses said internal heat exchanger, enabling
recovery of the liquid coolant or oil inside said accumulator between said internal
heat exchanger and the compressor, and a regulator valve (11) is provided which enables
regulation of the quantity of said liquid coolant or oil recovered via this recovery
path (10).
2. Cooling cycle according to Claim 1 characterized in that it is provided with a coolant discharge temperature sensor (12) which detects the
temperature of the coolant discharged from said compressor, and a control means which
controls said regulator valve in accordance with the coolant discharge temperature
detected by said coolant discharge temperature sensor.
3. Cooling cycle according to Claim 1 characterized in that it is provided with a control means which controls said regulator valve where it
is expected that the discharge temperature will rise with a high cycle load due to
the outside temperature, pressure within the cycle, room temperature or the like.
4. Cooling cycle according to Claim 1 characterized in that said internal heat exchanger (7) and said regulator valve (11) are built into said
accumulator (6) .
5. Cooling cycle according to Claim 1 in which said cooling cycle is a supercritical
vapour compression cooling cycle which uses carbon dioxide as the coolant.