JP2001066003A - Refrigeration cycle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid malfunction such as cavitation phenomena or the like nearby a critical pressure in a refrigeration cycle controlling a high pressure along an optimum control line. SOLUTION: In the case where a target refrigerant pressure for obtaining an optimum coefficient of performance obtained from a refrigerant temperature nearby an upstream side of depressurizing means decreases or increases across a critical pressure, in order for the refrigerant pressure not to be equal to the critical pressure, in the case of decreasing, the target pressure is kept at one time to a first pressure higher than the critical pressure by a specified value, and in the case of increasing, the target pressure is kept at one time to a second pressure lower than the critical pressure by a specified value. Further in the case of decreasing, at a stage where the target pressure reaches the second pressure, the target pressure shifts to the second pressure from the first pressure, and in the case of increasing, at a stage where the target pressure reaches the first pressure, the target pressure shifts to the first pressure from the second pressure. As a result, fluctuations of a high pressure at the critical pressure or nearby the critical pressure are able to be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling a refrigeration cycle in which a refrigerant is compressed from a region below a critical point to a region above the critical point and a refrigeration cycle using this control method.

[0002]

2. Description of the Related Art Japanese Patent Application Laid-Open No. 9-264622 discloses a pressure for controlling a radiator outlet temperature and a radiator outlet pressure so as to efficiently operate a vapor compression refrigeration cycle operating in a supercritical region. A control valve is disclosed.

[0003] In this reference, the pressure control valve is formed in a refrigerant flow path from the radiator to the evaporator, and partitions the refrigerant flow path into an upstream space and a downstream space.
A valve port formed in the partition wall and communicating the upstream space and the downstream space, and a displacement member that forms a sealed space in the upstream space and is displaced according to a pressure difference between the inside and outside of the sealed space. A valve body connected to the displacement member to open and close the valve port. And, in the closed space, the refrigerant,
For the volume in the closed space with the valve port closed,
The temperature of the refrigerant is sealed at a density ranging from the saturated liquid density at 0 ° C. to the saturated density at the critical point of the refrigerant, whereby the outlet pressure of the radiator and the outlet temperature of the radiator are: It is controlled almost along the optimal control line.

[0004]

As described above, in a refrigeration cycle using a supercritical refrigerant, in particular, carbon dioxide,
It is desirable that the high pressure of the refrigerant is controlled along an optimal control line determined by the refrigerant temperature and the refrigerant pressure on the outlet side (high pressure side) of the radiator. When the pressure and the flow rate fluctuate in a state in which the heat exchanger is present, a cavitation phenomenon that repeats generation and disappearance of air bubbles occurs, which causes a problem that erosion and destruction of the heat exchanger and the like are accelerated.

Accordingly, an object of the present invention is to provide a refrigeration cycle for controlling high pressure along an optimal control line, which avoids problems such as cavitation near a critical pressure.

[0006]

SUMMARY OF THE INVENTION Accordingly, a refrigeration cycle according to the present invention comprises a compressor for compressing a refrigerant to a range above and below a critical pressure of the refrigerant, a radiator for cooling the compressed refrigerant, and a radiator for cooling the compressed refrigerant. The pressure control means comprises at least pressure reducing means for depressurizing the refrigerant cooled by the radiator, and an evaporator for evaporating the refrigerant depressurized by the pressure reducing means. The refrigerant pressure and the refrigerant temperature near the upstream side of the pressure reducing means are optimally controlled. In the refrigeration cycle controlled along the line, detection means for detecting a refrigerant temperature and a refrigerant pressure near the upstream side of the pressure reducing means, and a target refrigerant pressure set corresponding to the refrigerant temperature is directed toward the critical pressure. If the target refrigerant pressure decreases to the first pressure after decreasing the target refrigerant pressure to a first pressure higher than the critical pressure by a predetermined value, the target refrigerant pressure is maintained at the critical pressure. When the pressure becomes equal to or lower than a second pressure lower than a predetermined value, the target refrigerant pressure is shifted from the first pressure to the second pressure and lowered, and is set in accordance with the refrigerant temperature. When the target refrigerant pressure rises toward the critical pressure, the target refrigerant pressure is maintained at the second pressure after increasing the target refrigerant pressure to the second pressure, and the target refrigerant pressure is increased to the first pressure. If this is the case,
Target refrigerant pressure setting means for shifting from the second pressure to the first pressure and increasing the refrigerant pressure, and the refrigerant pressure detected by the detection means coincides with the target refrigerant pressure set by the target refrigerant pressure setting means And control means for performing the control.

Therefore, according to the present invention, when the target refrigerant pressure for obtaining the optimum coefficient of performance obtained from the refrigerant temperature near the upstream side of the pressure reducing means falls or rises across the critical pressure, When the pressure does not become the critical pressure, the target pressure is temporarily held at the first pressure higher than the critical pressure by a predetermined value when the pressure decreases, and the target pressure is temporarily maintained at the second pressure lower than the critical pressure when the pressure rises, When the target pressure reaches the second pressure, the pressure changes from the first pressure to the second pressure when the pressure decreases, and when the target pressure reaches the first pressure, the pressure decreases when the target pressure reaches the first pressure. From the pressure of the first
Since the pressure shifts to the critical pressure, fluctuation of the high pressure near the critical pressure or the critical pressure can be prevented, so that the problem due to the cavitation phenomenon can be prevented.

[0008]

Embodiments of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, a refrigeration cycle 1 using a supercritical refrigerant, for example, carbon dioxide, as a refrigerant includes a compressor 2 for compressing the refrigerant, and a radiator 3 for cooling the refrigerant compressed by the compressor 2. A high-pressure side heat exchanger 5 through which the refrigerant cooled by the radiator 3 passes and a low-pressure side heat exchanger 9 through which the refrigerant sucked into the compressor 2 passes. And an expansion valve 6 as expansion means for expanding the supercooled refrigerant that has passed through the high-pressure side heat exchanger 5 of the internal heat exchanger 4 and the expansion valve 6. Evaporator 7 for evaporating the refrigerant expanded by the evaporator 7, an accumulator 8 for adjusting the amount of refrigerant into which the refrigerant evaporated by the evaporator 7 flows and circulating through the refrigeration cycle 1, and a position between the accumulator 8 and the compressor 2. I At least constituted by a low-pressure side heat exchanger 9 for heating the refrigerant to be sucked into the compressor 2.

In the refrigeration cycle 1 having the above structure, the Mollier diagram abcd shown in FIG. 3 or FIG.
This shows a case where the high pressure is increased to a supercritical range (about 10 MPa) by the expansion valve 12 being throttled. In this case, the refrigerant passes through the compression stroke (ab) of the compressor 2.
Compressed from about 4 MPa to about 10 MPa, and is cooled in the cooling process (bc) by the radiator 3 and the high-pressure side heat exchanger 5 of the internal heat exchanger 4, and its enthalpy (i) is reduced. When compressed to the supercritical range,
Since the refrigerant does not liquefy, there is a pressure difference between the pressure at the point b and the pressure at the point c. Therefore, the high pressure control by the expansion valve 6 is desirably executed by the pressure near the upstream side of the expansion valve. .

In the expansion stroke (cd) of the expansion valve 6, the pressure of the refrigerant is reduced to the gas-liquid mixing region, evaporates in the evaporator 7, and further flows to the low-pressure heat exchanger 9 of the internal heat exchanger 4. Overheated and sucked into the compressor 2 (d-
a). Thus, the heat absorbed by the evaporator 7 is transferred to the radiator 3.
A heat exchange cycle for radiating heat is configured.

The Mollier diagram effgh shown in FIG. 3 or 4 shows a heat exchange cycle executed when the heat load is low, and the expansion valve 12 is opened. This shows a case where the high pressure has been reduced to a subcritical region (about 6.5 MPa). In this case, the refrigerant is
About 3.5 MPa depending on the compression stroke (ef) of the compressor 2
To about 6.5 MPa, and the cooling process (fg) by the high-pressure side heat exchanger 5 of the radiator 3 and the internal heat exchanger 4
And the enthalpy (i) thereof is reduced, and the refrigerant is liquefied. Then, in the expansion stroke (gh) of the expansion valve 6, the pressure of the refrigerant is reduced from the liquid phase region to the gas-liquid mixing region, the refrigerant evaporates in the evaporator 7, and the internal heat exchanger 4
Is superheated in the low-pressure side heat exchanger 9 and is sucked into the compressor 2 (he). This constitutes a heat exchange cycle in which the heat absorbed by the evaporator 7 is radiated by the radiator 3.

In the refrigeration cycle 1 described above, the expansion valve 6
Coefficient of performance (C) corresponding to the refrigerant temperature near the inlet side of
It is known that there is a refrigerant pressure with OP)
A line connecting points of the refrigerant temperature and the refrigerant pressure at which the optimum COP is obtained is an optimum control line η in FIGS. 2, 3 and 4.
Shown as max.

Therefore, the refrigerant temperature on the high pressure side (Texpin
) Obtains the target high-pressure pressure Phm from the optimum control line ηmax, and controls the opening of the expansion valve 13 so that the actual high-pressure pressure coincides with the target high-pressure pressure Phm. It is.

As described above, the pressure sensor 10 for detecting the pressure of the refrigerant and the temperature sensor 11 for detecting the temperature of the refrigerant are arranged near the upstream side of the expansion valve 6.
The refrigerant pressure P and the refrigerant temperature Texpin in the vicinity of the upstream side are detected. As a result, the refrigerant temperature Texpin detected by the temperature sensor 11 is input to the control unit (C / U) 12, and a target high-pressure refrigerant pressure Phm is obtained.
An output signal is output to drive means 13 such as an electromagnetic coil or an actuator for adjusting the valve opening of the expansion valve 13 so that the difference ΔP from the refrigerant pressure P detected by 0 becomes 0.

However, when the high pressure of the refrigerant is equal to the critical pressure Pc of the refrigerant (in the case of carbon dioxide, it is approximately 7.34MPa).
a) When stable in the vicinity, when pressure fluctuations and refrigerant amounts fluctuate due to rapid heat load fluctuations and the like, cavitation phenomena occur in which bubbles are generated or disappear in the high-pressure side refrigerant, so that heat is released. There is a possibility that erosion and destruction of the vessel may be accelerated.

For this reason, in the present application, as shown in FIG. 2, a hysteresis for avoiding the critical pressure Pc near the critical pressure Pc is formed on the optimum characteristic line ηmax for obtaining the target high pressure Phm, This is to prevent the pressure from becoming the critical pressure Pc.

Specifically, when the target high pressure Phm set by the refrigerant temperature Texpin decreases toward the critical pressure Pc along the optimal control line ηmax, the first higher pressure Phm is higher than the critical pressure Pc by a predetermined value. The target high pressure Phm is maintained so as not to drop any more at the pressure Pa, and when the refrigerant temperature Texpin further decreases, the refrigerant temperature Texpin
At a stage where the target high pressure Phm corresponding to expin (Tb) coincides with the second pressure Pb lower than the critical pressure Pc by a predetermined value, the target high pressure Phm shifts from the first pressure Pa to the second pressure Pb. And, on the contrary, the refrigerant temperature T
When the target high pressure Phm set by expin rises toward the critical pressure Pc along the optimal control line ηmax, the target high pressure Phm is maintained so as not to rise any more at the second pressure Pb. When the refrigerant temperature Texpin further increases, the refrigerant temperature Texpin
When the target high-pressure pressure Phm corresponding to (Ta) coincides with the first pressure Pa, the target high-pressure pressure Phm shifts from the second pressure Pb to the first pressure Pa and is set. It is.

Thus, the optimal control line ηmax is
As shown in FIG. 3, hysteresis is formed in which the pressure changes from the pressure Pb to the pressure Pa along the isotherm Ta, and the pressure changes from the pressure Pa to the pressure Pb along the isotherm Tb.

In the second embodiment shown in FIG. 4, the enthalpy i is calculated from the refrigerant temperature Texpin, and the target high pressure is controlled based on the enthalpy i. When the target high-pressure pressure Phm set by the calculated enthalpy i decreases toward the critical pressure Pc along the optimal control line ηmax, the target high-pressure pressure Phm becomes higher at the first pressure Pa higher than the critical pressure Pc by a predetermined value. When the target high pressure Phm is maintained so as not to drop, and when the enthalpy i calculated from the refrigerant temperature Texpin decreases, the target high pressure Phm corresponding to the enthalpy ib calculated from the refrigerant temperature Texpin is increased to the critical pressure. At the stage when the target pressure coincides with the second pressure Pb lower than the predetermined pressure Pc, the target high pressure Phm shifts from the first pressure Pa to the second pressure Pb and is set. Conversely, when the target high pressure Phm set by the enthalpy i calculated from the refrigerant temperature Texpin rises toward the critical pressure Pc along the optimal control line ηmax, the second pressure Pb , The enthalpy i calculated from the refrigerant temperature Texpin rises, and the target high-pressure pressure Phm corresponding to the enthalpy ia calculated from the refrigerant temperature Texpin rises. At the stage when the pressure matches the first pressure Pa,
The target high pressure Phm is changed from the second pressure Pb to the first pressure Pa.
To be set.

Thus, the optimal control line ηmax is
As shown in FIG. 4, hysteresis is formed in which the pressure Pb shifts to the pressure Pa along the isenthalpy line ia, and the pressure Pa shifts to the pressure Pb along the isenthalpy line ib.

[0022]

As described above, according to the present invention, the target high pressure is set to be different from the critical pressure so that the actual high pressure does not become the critical pressure, so that the problem due to the cavitation phenomenon can be prevented. In addition, since the erosion and destruction of the heat exchanger and the like can be prevented, the life of the air conditioner can be extended.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an example of a refrigeration cycle according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing an optimal control line having hysteresis according to the first embodiment of the present invention.

FIG. 3 is a diagram showing an optimal control line having hysteresis and a Mollier diagram of a refrigeration cycle controlled along the optimal control line according to the first embodiment of the present invention.

FIG. 4 is a diagram showing an optimal control line having hysteresis and a Mollier diagram of a refrigeration cycle controlled along the optimal control line according to a second embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Refrigeration cycle 2 Compressor 3 Radiator 4 Internal heat exchanger 5 High pressure side heat exchanger 6 Expansion valve 7 Evaporator 8 Accumulator 9 Low pressure side heat exchanger 10 Pressure sensor 11 Temperature sensor 12 Control unit 13 Driving means

Claims (1)

[Claims] A compressor for compressing the refrigerant to a range above and below a critical pressure of the refrigerant; a radiator for cooling the compressed refrigerant; a pressure reducing means for depressurizing the refrigerant cooled by the radiator; A refrigeration cycle configured at least by an evaporator for evaporating the refrigerant decompressed by the decompression means, wherein the refrigerant pressure and the refrigerant temperature near the upstream side of the decompression means are controlled along an optimal control characteristic line. Detecting means for detecting a refrigerant temperature and a refrigerant pressure in the vicinity of the upstream side of the refrigerant, and when the target refrigerant pressure set in accordance with the refrigerant temperature decreases toward the critical pressure, the target refrigerant pressure is set higher than the critical pressure. After the pressure is lowered to the first pressure higher than the predetermined pressure, the pressure is maintained at the first pressure, and when the target refrigerant pressure becomes equal to or lower than the second pressure lower than the critical pressure by a predetermined value, When the target refrigerant pressure is shifted from the first pressure to the second pressure and lowered, and the target refrigerant pressure set corresponding to the refrigerant temperature increases toward the critical pressure, the target refrigerant pressure Is maintained at the second pressure after the pressure is increased to the second pressure, and when the target refrigerant pressure is equal to or higher than the first pressure, the first pressure is reduced from the second pressure to the first pressure. And a control means for controlling the refrigerant pressure detected by the detection means to coincide with the target refrigerant pressure set by the target refrigerant pressure setting means. A refrigeration cycle characterized in that: