JP2016053429A - Refrigeration machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a refrigeration machine that can improve accuracy of temperature control by quickly coping with temperature change of cold water (cooled fluid) and changing control output.SOLUTION: A refrigeration machine includes: an evaporator 3 that deprives cooled fluid of heat to evaporate refrigerant, and generates refrigerant gas; and a temperature control unit 14 that corrects output of the refrigeration machine with at least a value proportional to a value obtained by integrating the difference between a temperature of cooled fluid and a target temperature along a time axis. The temperature control unit 14 further uses a value proportional to the difference between a refrigerant pressure in the evaporator 3 and a reference pressure and/or a value proportional to a change rate of the refrigerant pressure in the evaporator 3, thereby correcting the output of the refrigeration machine.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a refrigerator (heat pump) that adjusts an output so that the temperature of a fluid to be cooled or a fluid to be heated becomes a target temperature, and particularly relates to a refrigerator that is excellent in temperature stability.

  A refrigerator that cools cold water or the like, or a refrigerator (heat pump) that heats hot water or the like generally has a temperature control function of cold water or hot water. Temperature control is a simple one that turns the refrigerator on and off with a thermostat, etc., detects the accurate temperature with a resistance temperature detector, etc., and saves energy by stepless control using an inverter based on the theory of control engineering It is a great variety. In a large refrigerator, a control method called PI control is common.

  In PI control, the control output is determined by the sum of a proportional term (P term) proportional to the difference between the target temperature and the current temperature and an integral term (I term) that integrates the temperature difference with time. is there.

JP 2006-234320 A

  PI control is excellent in stability and widely used. However, since the output is determined in proportion to the difference between the target temperature and the current temperature, a temperature deviation tends to occur in principle. That is, since the output is increased or decreased only when a temperature difference occurs, the control output does not change unless a certain temperature difference occurs. It is known that when the proportionality constant of the control output with respect to the temperature difference is increased, the temperature deviation is reduced but the control becomes unstable, and a temperature fluctuation called so-called hunting occurs.

As a method for reducing such deviation, a method called PID control is known. This is obtained by adding a differential term (D term) proportional to the rate of change of temperature to the aforementioned PI control. In general, PID control determines a control output according to the following equation.
Control output (operation amount) = proportional term (P term) + integral term (I term) + derivative term (D term)
= K _P × difference + K _I × integral value of difference + K _D × change rate (1)
Here, K _P , K _I , and K _D are predetermined constants (or coefficients).

  According to PID control, when the temperature of cold water or the like changes, the control output increases or decreases according to the rate of change, and the temperature change is suppressed. In this case, even if the temperature difference between the current temperature and the target temperature is small, the output (cooling capacity) of the refrigerator quickly increases / decreases if the temperature change is abrupt, so that the temperature difference can be kept small. .

In practice, however, PID control is not often used as temperature control for refrigerators.
The first reason is that the temperature detection accuracy of the refrigerator is limited. That is, the range of temperature change such as cold water is required to be suppressed to about ± 0.2 ° C. (more strictly, about ± 0.1) with respect to the target temperature, but in order to perform such temperature control, It is necessary to detect the temperature with an accuracy of about 0.01 ° C. However, the accuracy of temperature detectors such as resistance temperature detectors and thermistors generally used in refrigerators is about 0.1 ° C., and the detected temperature changes in a stepwise manner. Cannot be obtained correctly. For this reason, if PID control is used with a refrigerator, temperature control will be destabilized on the contrary. In addition, it is not realistic to equip the refrigerator with a temperature sensor with high detection accuracy, because it is a large cost burden.

  The second reason is that there is a slight time delay in the temperature change and its detection. That is, the temperature detector as described above is inserted into a pipe such as cold water in a short pipe called a protective pipe. Since cold water or the like flows in the pipe at a flow rate of about 1 m to 5 m, the protective tube is required to have a strength that can withstand the water flow. As a result, the heat transfer of the protective tube is not always good. For this reason, a time delay (so-called primary delay) occurs between the temperature change of the cold water and the detected temperature change. When a first-order lag occurs, PID control becomes unstable. For this reason, generally, as temperature control of a refrigerator, PI control or PID control with a very small D term is often used.

  The present invention has been made in view of the above circumstances, and provides a refrigerator capable of improving the accuracy of temperature control by changing a control output in response to a temperature change of cold water (a fluid to be cooled). The purpose is to do.

  In order to achieve the above-described object, one embodiment of the present invention is a refrigerator that operates to maintain a temperature of a fluid to be cooled or a fluid to be heated at a target temperature, and takes heat from the fluid to be cooled. The evaporator that evaporates the refrigerant and generates the refrigerant gas, the condenser that cools and condenses the refrigerant gas with the heated fluid, and the output of the refrigerator, the difference between the temperature of the cooled fluid and the target temperature A temperature control unit that corrects using at least a value proportional to the value integrated along the time axis, the temperature control unit is a value proportional to the difference between the pressure of the refrigerant in the evaporator and a reference pressure, Further, the output of the refrigerator is corrected by further using a value proportional to the change rate of the pressure of the refrigerant in the evaporator.

In a preferred aspect of the present invention, the temperature control unit further corrects the output of the refrigerator using a value proportional to a value obtained by integrating the rate of change of the pressure in the evaporator along the time axis. Features.
A preferable aspect of the present invention further includes a compressor that compresses the refrigerant gas generated in the evaporator and sends the refrigerant gas to the condenser, and the correction of the output of the refrigerator is correction of the rotation speed of the compressor. It is characterized by being.

  As a first method, a pressure differential term, which is a value proportional to the pressure fluctuation rate, is added to the control output. If it does in this way, the rate of change of pressure will serve as D term of PID control, and the temperature of cold water etc. can be stabilized.

  As a second method, a pressure fluctuation correction value that is a value proportional to the difference between the set reference pressure and the current pressure is added to the control output. If it does in this way, the output according to the pressure change by the temperature change of cold water, ie, the value substantially proportional to the fluctuation | variation of temperature, will be added to a control output. This pressure fluctuation correction value corresponds to the P term of PI control or PID control, and is substituted or supplemented. The reference pressure is only necessary for calculation, and the value itself is not significant, and it is sufficient to set the rated pressure of the evaporator during operation, or even if the pressure itself is handled as a pressure difference There is no inconvenience. In principle, when this correction is performed, the control based on the cold water temperature may be only the I control without the P term.

  As a third method, a pressure fluctuation correction value obtained by integrating (integrating) a value proportional to the pressure fluctuation rate with time is added to the control output. In this way, since the value obtained by integrating the fluctuation rate is the amount of change, the same effect as the second method can be obtained.

  In the present invention, the control output is basically based on PI control based on the cold water temperature as in the past, but the control output is corrected by the pressure in the heat exchanger and the pressure change. The pressure in the heat exchanger of the refrigerator follows very quickly to changes in the cold water temperature. For example, when the chilled water temperature decreases, the evaporation pressure of the refrigerant in contact with the heat transfer tube immediately decreases. Moreover, since the pressure sensor is generally based on a mechanical principle such as a diaphragm deflection, minute pressure fluctuations can be detected with high sensitivity. Therefore, it is possible to improve the temperature control accuracy by substituting the change in temperature for the change in pressure and correcting the control output in accordance with the change in pressure.

It is a mimetic diagram showing a refrigerator concerning one embodiment of the present invention. It is a schematic diagram (block diagram) of a temperature control part. It is a graph explaining the temperature control by the conventional PI control. It is a graph explaining temperature control using the sum of PI operation value and pressure differential value as a control output value. It is a block diagram which shows the temperature control part which concerns on another Example of this invention. It is a block diagram which shows the temperature control part which concerns on another Example of this invention. It is a graph which shows the temperature fluctuation in another Example of this invention.

The present invention will be described in detail by taking cold water temperature control of a refrigerator as an example.
FIG. 1 is a schematic diagram showing a refrigerator according to an embodiment of the present invention. As shown in FIG. 1, the refrigerator includes a compressor 1 that compresses refrigerant gas, a condenser 2 that cools and compresses the compressed refrigerant gas with hot water (a fluid to be heated), and decompresses the condensed refrigerant. And an evaporator 3 that takes heat from cold water (a fluid to be cooled) and evaporates the refrigerant to exert a refrigeration effect. The expansion valve 4 is disposed between the condenser 2 and the evaporator 3. The refrigerator shown in FIG. 1 is a compression refrigerator provided with a compressor 1. As the compressor 1, a turbo compressor having a multistage impeller can be used.

  The compressor 1, the condenser 2, the expansion valve 4, and the evaporator 3 are connected by refrigerant pipes 5A, 5B, 5C, and 5D through which the refrigerant circulates. More specifically, the compressor 1 and the condenser 2 are connected by a refrigerant pipe 5A, the condenser 2 and the expansion valve 4 are connected by a refrigerant pipe 5B, and the expansion valve 4 and the evaporator 3 are connected by a refrigerant pipe 5C. The evaporator 3 and the compressor 1 are connected by a refrigerant pipe 5D.

  In the refrigeration cycle of the compression refrigerator configured as shown in FIG. 1, the refrigerant circulates through the compressor 1, the condenser 2, the expansion valve 4, and the evaporator 3, and cold water is generated by a cold heat source obtained by the evaporator 3. Is produced and corresponds to the load, and the amount of heat from the evaporator 3 taken into the refrigeration cycle and the amount of heat corresponding to the work of the compressor 1 are released to the hot water supplied to the condenser 2.

  The evaporator 3 is provided with a pressure sensor 10 for measuring the pressure of the refrigerant gas inside. Moreover, the temperature sensor 11 which measures the temperature (outlet temperature) of the cold water which came out of the evaporator 3 is attached to the cold water piping (cooled fluid piping). The temperature sensor 11 is disposed in the cold water pipe while being accommodated in the protective tube. The refrigerator further includes a temperature control unit 14 that controls its output. The temperature control unit 14 is configured to correct the output of the refrigerator using at least a value proportional to a value obtained by integrating the difference between the temperature of the cold water (cooled fluid) and the target temperature along the time axis. . The pressure sensor 10 and the temperature sensor 11 are connected to the temperature control unit 14, and the vapor pressure and the outlet temperature of the cold water measured by the pressure sensor 10 and the temperature sensor 11 are sent to the temperature control unit 14. It is supposed to be.

  FIG. 2 is a schematic diagram (block diagram) of the temperature control unit 14 and uses the first method described above. In the present invention, the chilled water temperature is detected as in the conventional method, and PI calculation is performed to obtain the PI value (cold water PI calculated value) based on the chilled water temperature. In general, since a commercially available microcontroller has a built-in PID calculator, it is easy to obtain a PI calculation value. On the other hand, the pressure in the evaporator 3 is detected by the pressure sensor 10, and the temperature control unit 14 calculates the rate of change of the pressure to obtain a pressure differential value. The change rate may be calculated by an electrical differentiating circuit, or the amount of change in the pressure in the evaporator 3 may be calculated every preset time and used as the change rate. The temperature control unit 14 adjusts the output of the refrigerator using the sum of the cold water PI calculation value and the pressure differential value as a control output value.

  The output of the refrigerator is the cooling capacity of the refrigerator and is expressed as the amount of heat. The refrigeration capacity of the refrigerator is obtained by (cold water inlet temperature−cold water outlet temperature) × cold water mass flow rate × specific heat. Changing the rotation speed of the compressor 1 changes the output of the refrigerator.

  A graph for explaining temperature control by conventional PI control is shown in FIG. In this PI control, the sum of the P value proportional to the difference between the target temperature of the cold water and the current temperature and the I value obtained by integrating the P value is the control output. In FIG. 3, the solid line represents the actual cold water temperature, and the dotted line represents the temperature indicated by the temperature sensor 11. As described above, since the temperature sensor 11 is housed in a strong protective tube, the detected change in the chilled water temperature (dotted line) is slightly delayed as compared to the actual temperature (solid line).

  In PI control, even if the load increases, the control output does not change at that time. When the chilled water temperature gradually increases due to an increase in load, the P value gradually increases according to the difference between the chilled water temperature and the target temperature. However, the P value starts to rise slightly later than the actual rise of the cold water temperature due to the delay of the temperature detection operation of the temperature sensor 11. Since the control output is the sum of the P term and the I term, the control output gradually increases. When the control output increases sufficiently, the chilled water temperature changes from increasing to decreasing, the P value gradually decreases, and the increase in the I term becomes gradual, and the control output gradually begins to gradually decrease. When the chilled water temperature returns to the target temperature, the P value also returns to the original value, the I value does not increase, and the load and the control output are balanced to stabilize the chilled water temperature.

  However, the detection of the change in the chilled water temperature is delayed from the actual temperature change, and the basic operation of the PI control in which the chilled water temperature once deviates from the target temperature and then gradually corrects this, so the maximum amount of deviation of the chilled water temperature Inevitably grows.

  Next, FIG. 4 shows temperature control in the case of the present invention using the sum of the PI calculation value and the pressure differential value as the control output value. In the case of the present invention, when the load increases and the cold water temperature starts to rise, the pressure in the evaporator 3 immediately rises as compared with the temperature change, and the pressure differential value also rises immediately. In this embodiment, since this pressure differential value is added to the chilled water PI value, the control output rises quickly, and the chilled water temperature rises more slowly than in the conventional method. Eventually, as the chilled water temperature rises, the I value rises and the control output also rises, the chilled water temperature turns from rising to falling, and the chilled water temperature returns to the target temperature.

  The temperature control unit 14 changes the rotation speed of the compressor 1 in accordance with the control output (that is, the operation amount of the compressor 1) obtained in this way, thereby changing the output of the refrigerator, that is, the cooling capacity of the refrigerator. Change. Instead of changing the rotation speed of the compressor 1, the refrigeration capacity of the refrigerator may be changed by operating a flow rate adjusting mechanism that adjusts the flow rate of the refrigerant gas. For example, in the case of the turbo compressor 1, a suction vane arranged on the suction side of the impeller may be operated. When a screw compressor is used instead of the turbo compressor 1, the refrigerant gas is compressed. A slide valve that adjusts the volume may be operated.

  Comparing the conventional method shown in FIG. 3 with the method according to the present invention shown in FIG. 4, the conventional method does not increase the control output until the chilled water temperature deviates from the target temperature and the difference becomes large. In the present invention, the control output rapidly increases when the cold water temperature starts to increase. Therefore, the rise in the chilled water temperature is moderate as compared with the conventional method, and the temperature deviation can be suppressed. Thereby, the fluctuation | variation of cold water temperature can be suppressed and cold water temperature can be stabilized.

  FIG. 5 is another embodiment of the present invention. In this embodiment, instead of the PI calculation of the previous embodiment, only the I value is calculated based on the temperature, the pressure differential value is calculated as in the previous embodiment, and the pressure in the evaporator 3 and the reference The difference between the pressure and the pressure fluctuation correction value is obtained, and the sum of these is used as the control output.

  As described above, the P value calculator may be used to calculate the I value, but it can be obtained by a relatively easy method such as integrating the difference between the target temperature and the current chilled water temperature for each preset time. You can also. The sum of the pressure differential value and the pressure fluctuation correction value can also be obtained by PD calculation with the reference pressure as the target temperature for the pressure in the evaporator 3. In this case, the PD output value plus the I value is used as the control output. In practice, this method is easier as an operation. A block diagram in this case is shown in FIG.

  Note that the reference pressure is a pressure provided for convenience in order to obtain a pressure fluctuation correction value, and may be close to the pressure during actual operation. This is because the magnitude of the pressure fluctuation correction value is compensated by the I value.

  In addition to using the difference between the reference pressure and the current pressure, the pressure fluctuation correction value may be a correction value obtained by integrating the pressure differential value (pressure change rate) with time as shown in the third method. FIG. 7 is a graph showing temperature fluctuations in this example. As in the first embodiment, in this embodiment, the change in the chilled water temperature is immediately reflected in the control output, and the chilled water temperature can be stabilized.

  Although no specific examples are shown, there are many variations such as temperature P value, I value, pressure P value, D value constant (refer to the above formula (1)), or use or not use each value. is there. The essence of the present invention is that at least the I value of the temperature that is the object of control is used for the temperature control of the refrigerator, and at least one of the P value or D value of the pressure is used. By doing in this way, this invention can control the output of a refrigerator precisely and stably as mentioned above.

  In the embodiment described above, an example in which the present invention is applied to a compression refrigerator has been described. However, the present invention can also be applied to an absorption refrigerator.

  Although the embodiment of the present invention has been described so far, the present invention is not limited to the above-described embodiment, and it is needless to say that the present invention may be implemented in various different forms within the scope of the technical idea.

DESCRIPTION OF SYMBOLS 1 Compressor 2 Condenser 3 Evaporator 4 Expansion valve 5 Refrigerant piping 10 Pressure sensor 11 Temperature sensor 14 Temperature control part

Claims (3)

A refrigerator that operates to maintain a temperature of a fluid to be cooled or a fluid to be heated at a target temperature,
An evaporator that draws heat from the fluid to be cooled, evaporates the refrigerant, and generates refrigerant gas;
A condenser that cools and condenses the refrigerant gas with a heated fluid;
A temperature control unit that corrects the output of the refrigerator using at least a value proportional to a value obtained by integrating the difference between the temperature of the fluid to be cooled and the target temperature along the time axis;
The temperature control unit further uses the value proportional to the difference between the refrigerant pressure in the evaporator and a reference pressure and / or the value proportional to the rate of change of the refrigerant pressure in the evaporator to further reduce the refrigeration. A refrigerator that corrects the output of the machine.   2. The refrigerator according to claim 1, wherein the temperature control unit further uses a value proportional to a value obtained by integrating a rate of change in pressure in the evaporator along a time axis to output the refrigerator. A refrigeration machine characterized by correcting the above. The refrigerator according to claim 1 or 2,
A compressor for compressing the refrigerant gas generated in the evaporator and sending it to the condenser;
Correction of the output of the said refrigerator is correction | amendment of the rotational speed of the said compressor, The refrigerator characterized by the above-mentioned.

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