JP2009162392A - Sherbet ice producing device and its method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sherbet ice producing device capable of accurately producing the sherbet ice of a desired ice packing factor. <P>SOLUTION: This sherbet ice producing device comprises an ice making device 10 and a freezing machine 30 for cooling salt water 1 in an ice storage tank 3, a temperature gauge 6 for detecting a temperature of the salt water 1, and a control device 36 detecting an freezing start temperature of the salt water 1 on the basis of a lowering speed of the temperature of the salt water 1, and stopping the freezing machine 30 when the difference between the freezing start temperature detection value Tsa0 and the temperature detection value Ts of the salt water 1 reaches a predetermined temperature difference ΔT. Accordingly, the sherbet ice 2 of the desired ice packing factor (IPF) can be accurately made even when the detection value Ts of the temperature gauge 6 has a margin of an error. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a sherbet ice generating apparatus and a sherbet ice generating method, and more particularly to a sherbet ice generating apparatus and a sherbet ice generating method for generating a sherbet ice by cooling salt water in a tank.

  In recent years, saltwater fish has been cooled with sherbet ice generated from salt water to prevent marine fish from being scratched by massive ice for cooling and freshness of saltwater fish from falling due to the fresh water generated by melting the ice cubes. The method is being put into practical use.

Sherbet ice is generated by cooling salt water in an ice storage tank. That is, when the salt water in the ice storage tank is cooled, ice begins to be formed at the freezing start temperature corresponding to the salt concentration. Since the water contains salt, the ice crystals are not coarsened, and the salt water becomes sherbet. When the production concentration of sherbet ice, that is, the weight ratio (IPF: Ice Packing Factor) of the ice in the sherbet ice increases, the salinity concentration of the remaining salt water increases, and the freezing start temperature of the salt water also decreases. Therefore, if the cooling of the salt water is stopped when the salt water reaches a predetermined temperature, the desired IPF (%) sherbet ice can be generated. (For example, refer to Patent Document 1).
JP 2007-3064 A

  However, in the conventional sherbet ice production method, if there is an error in the thermometer that detects the temperature of salt water, a desired IPF could not be obtained. For example, when salt water having a salt concentration of 2.5% is cooled, it begins to freeze at −1.5 ° C., and the IPF becomes 25% at −2.0 ° C. Therefore, when there is no error in the thermometer, if cooling is stopped when the temperature of the salt water reaches −2.0 ° C., sherbet ice having an IPF of 25% can be obtained.

  However, when the thermometer has an error of -0.25 ° C, the actual temperature is -1.75 ° C even if the detected value of the temperature of the salt water is -2.0 ° C. If the cooling is stopped when the actual temperature of the salt water reaches −1.75 ° C., the IPF becomes 14.5%.

  Conversely, when the thermometer has an error of + 0.25 ° C., the actual temperature is −2.25 ° C. even if the detected value of the salt water temperature is −2.0 ° C. If the cooling is stopped when the actual temperature of the salt water reaches −2.25 ° C., the IPF becomes about 35%. When the IPF increases, the viscosity of the sherbet ice increases, and the pump for circulating or discharging the sherbet ice becomes overloaded.

  Therefore, a main object of the present invention is to provide a sherbet ice generating apparatus and a sherbet ice generating method capable of accurately generating a sherbet ice having a desired production concentration.

  The sherbet ice generating device according to the present invention is a sherbet ice generating device that cools salt water in a tank to generate sherbet ice, a cooling means that cools the salt water in the tank, and detects the temperature of the salt water in the tank The temperature on the ice and the temperature change of the salt water detected by the thermometer are detected, and the difference between the detected temperature and the temperature of the salt water reaches a predetermined temperature difference. And a control means for stopping the cooling of the salt water by the cooling means.

  The sherbet ice generating method according to the present invention is a sherbet ice generating method for cooling salt water in a tank to generate sherbet ice, cooling the salt water in the tank and detecting the temperature of the salt water in the tank. , Detecting the salt water freezing start temperature based on the time change of the salt water temperature, and cooling the salt water in response to the temperature difference between the detected freezing start temperature and the salt water reaching a predetermined temperature difference. It will stop.

  In the sherbet ice generating device and the sherbet ice generating method according to the present invention, the salt water in the tank is cooled, the temperature of the salt water in the tank is detected, and the temperature of salt water freezing is detected based on the time change of the temperature of the salt water. Then, the cooling of the salt water is stopped in response to the temperature difference between the detected freezing start temperature and the temperature of the salt water reaching a predetermined temperature difference. Therefore, even when there is an error in the detection value of the thermometer that detects the temperature of salt water, sherbet ice having a desired concentration can be accurately generated.

  Before describing the embodiment of the present invention, the principle of the present invention will be described first. FIG. 1 is a diagram showing the relationship between the salinity concentration C (%) of salt water and the freezing start temperature Ta (° C.) at which ice begins to form in the salt water. In FIG. 1, fresh water with a salinity concentration C of 0% begins to freeze at 0 ° C., and the ice temperature decreases in proportion to the salinity concentration C.

  For example, as shown in FIG. 2, when salt water having a salt concentration C of about 3% is cooled, ice begins to form at about −2 ° C. Since ice is formed only with fresh water, when ice is formed, the corresponding amount of fresh water is lost and the salinity concentration C of the remaining brine is increased. When the salinity concentration C increases, the freezing start temperature Ta decreases, and the temperature of the salt water decreases along a proportional straight line.

  FIG. 3 is a graph showing the relationship between the amount of decrease in salt water enthalpy (kcal / kg) and the actual temperature T (° C.) of salt water when the concentration C of salt water and the IPF of sherbet ice are changed. When cooling under constant conditions, the amount of enthalpy reduction on the horizontal axis corresponds to the cooling time of salt water. On the vertical axis, in addition to the actual temperature T, a temperature detection value Ts (° C.) by a thermometer is shown. FIG. 3 shows a case where the temperature detection value Ts is equal to the actual temperature T in the comparative example of the present application (Ts = T).

  For example, when salt water having a salt concentration C of 2.5% is cooled, the temperature T of the salt water decreases along a proportional line regardless of the salt concentration C until ice begins to form. The freezing temperature Ta at which ice begins to form decreases as the salinity concentration C increases. When the salinity is 2.5%, it begins to freeze at -1.5 ° C. When ice begins to form, the amount of enthalpy decrease changes to heat of solidification, and the rate of decrease in the temperature T of the salt water suddenly decreases. In addition, as shown in FIG. 2, when ice begins to form, the salinity concentration C increases, the icing start temperature Ta decreases, and the IPF increases. When the temperature T of salt water reaches −2 ° C., the IPF becomes 25%. In FIG. 3, since Ts = T, if the cooling of the salt water is stopped when the temperature detection value Ts reaches −2 ° C., sherbet ice having an IPF of 25% can be obtained.

  However, as shown in FIG. 4, when the detected value Ts of the thermometer is shifted to the positive side by 0.25 ° C. from the actual temperature T (Ts = T−0.25), the detected temperature value Ts is −. When the cooling of the salt water is stopped when the temperature reaches 2 ° C., the cooling is stopped although the actual temperature T is −1.75 ° C., and the IPF becomes 14.5%.

  FIG. 5 is a diagram showing the sherbet ice generation method of the present invention, and is a diagram contrasted with FIG. FIG. 5 shows a case where the detected value Ts of the thermometer is equal to the actual temperature T in the present invention (Ts = T).

  For example, when salt water having a salt concentration C of 2.5% is cooled, the temperature T of the salt water decreases along a proportional line regardless of the salt concentration C until ice begins to form. When the salt concentration C is 2.5%, it begins to freeze at -1.5 ° C. When ice begins to form, the amount of decrease in enthalpy changes to heat of solidification, and the rate of decrease in the actual temperature T of salt water suddenly decreases. In the present invention, when the ice begins to be formed, the freezing start temperature Tsa0 is detected by a thermometer utilizing the fact that the rate of decrease in the actual temperature T of the salt water suddenly decreases.

  When ice begins to form, the salinity concentration C increases, the freezing start temperature Tsa0 decreases, and the IPF increases. When the salt concentration C of the original brine is 2.5%, the IPF becomes 25% when the freezing start temperature Tsa0 is lowered by 0.5 ° C. In FIG. 5, since Ts = T, if the cooling of salt water is stopped when the temperature detection value Ts becomes −2 ° C. when the freezing start temperature Tsa0 = −1.5 ° C. decreases by 0.5 ° C., Sorbet ice with 25% IPF can be obtained.

  Further, as shown in FIG. 6, when the temperature detection value Ts is shifted to the positive side by 0.25 ° C. from the actual temperature T (Ts = T−0.25), the freezing start temperature Tsa0 measured by the thermometer. Is −1.75 ° C., but when the temperature is lowered by 0.5 ° C. (when the temperature detection value Ts becomes −2.25 ° C.), the salt water cooling is stopped, so that the IPF is 25%. Of sherbet ice.

  Therefore, according to the present invention, even when the detected value Ts of the thermometer is shifted from the actual temperature T by an error, the desired IPF sherbet ice can be accurately generated. Hereinafter, the sherbet ice production | generation apparatus of this invention is demonstrated in detail using drawing.

  FIG. 7 is a block diagram showing a configuration of a sherbet ice generating device according to one embodiment of the present invention. In FIG. 7, the sherbet ice generating device is provided with an ice storage tank 3 for injecting a predetermined amount of salt water 1 (or seawater) and storing sherbet ice 2 generated from the salt water 1. The salt concentration C of the salt water 1 is set to 2.5%, for example.

  In the ice storage tank 3, a stirrer 4 for stirring the salt water 1 (or sherbet ice 2) is rotatably provided. On the water storage tank 3, a driving device 5 for driving the stirrer 4 is provided. Is provided. The reason why the sherbet ice 2 is stirred is to prevent the fine ice in the sherbet ice 2 from floating and separating the fine ice and the salt water 1 up and down.

  In addition, a thermometer 6 that detects the temperature of the salt water 1 in the tank 3 is provided below the ice storage tank 3. As the thermometer 6, for example, a thermometer using a resistance temperature detector is used. Further, a discharge pipe 7 and a discharge pump 8 for discharging the generated sherbet ice 2 from the tank 3 are connected to the bottom of the ice storage tank 3.

  The sherbet ice generating device includes an ice making device 10 that freezes a part of the salt water 1 in the ice storage tank 3 to generate fine ice. The ice making device 10 includes an outer cylinder 11, an inner cylinder 12 provided inside the outer cylinder 11, and a rotating cylinder 13 provided inside the inner cylinder 12. Between the rotating cylinder 13 and the inner cylinder 12, a salt water side passage 14 for flowing the salt water 1 is formed, and between the inner cylinder 12 and the outer cylinder 11, a refrigerant side passage 15 for flowing a refrigerant is formed. . On the outer peripheral surface of the rotating cylinder 13, a plurality of scrapers 16 are provided for scraping fine ice adhering to the inner peripheral surface of the inner cylinder 12 into the salt water side passage 14. In addition, a drive motor 17 for rotating the rotating cylinder 13 is provided at the upper part of the ice making device 10.

  The upper end portion of the salt water side passage 14 and the upper end portion of the ice storage tank 3 are connected by a forward salt water pipe 20, and the lower end portion of the ice storage tank 3 and the lower end portion of the salt water side passage 14 are connected by a reflux salt water pipe 21. The reflux saltwater pipe 21 is provided with a saltwater pipe valve 22, a circulation pump 23, and a pressure sensor 24 in order from the ice storage tank 3 side. When the valve 22 is opened and the pump 23 is driven, the salt water 1 (or sherbet ice) is circulated through the path of the ice storage tank 3, the reflux salt water pipe 21, the salt water side passage 14, the forward salt water pipe 20, and the ice storage tank 3. . When the pressure at the outlet of the pump 23 is detected by the pressure sensor 24 and the detected pressure exceeds a predetermined value, the pipes 20 and 21 and the passage 14 may be clogged with ice, so the pump 23 is stopped.

  The sherbet ice generating device includes a refrigerator 30 that supplies a refrigerant to the refrigerant side passage 15 of the ice making device 10, and the refrigerator 30 includes a compressor 31 and a condenser 32. The upper end of the refrigerant side passage 15 is connected to the inlet of the compressor 31 via the refrigerant pipe 33, the outlet of the compressor 31 is connected to the inlet of the condenser 32, and the outlet of the condenser 32 is the refrigerant pipe 34 and the expansion valve. It is connected to the lower end of the refrigerant side passage 15 via 35. The refrigerant is compressed by the compressor 31, liquefied by the condenser 32, and ejected into the refrigerant side passage 15 via the expansion valve 35, and the inner cylinder 12 is brought to −12 ° C., for example, by removing heat of vaporization from the inner cylinder 12. Cooling. On the inner peripheral surface of the cooled inner cylinder 12, fresh water in the salt water 1 is frozen and fine ice is generated.

  The sherbet ice generating device further includes a control device 36 for controlling the entire device and an operation unit 37 for operating the device via the control device 36. The operator uses the operation unit 37 to set the salinity concentration C of the salt water 1 and the desired IPF of the sherbet ice 2. The control device 36 stores, for example, the state diagram of the salt water shown in FIG. 5, and is set based on the state diagram, the set salt concentration C and IPF, and the detection value Ts of the thermometer 5. The entire sherbet ice generator is controlled so that the IPF sherbet ice 2 is generated.

  FIG. 8 is a flowchart showing the operation of the sherbet ice generating device. The operator first puts a predetermined amount of fresh water and a predetermined amount of salt in the ice storage tank 3 and mixes them with the stirrer 3 to generate salt water 1 having a predetermined salt concentration C (for example, 2.5%). Next, in step S <b> 1, the operator uses the operation unit 37 to set the salt concentration C of the salt water 1 and the IPF (for example, 25%) of the sherbet ice 2.

  In step S2, the controller 36 determines the icing start temperature Ta0 (for example, −1.5 ° C.) and the ice making completion temperature based on the stored salt water state diagram (see FIG. 5) and the set salinity concentration C and IPF. Te (for example, −2 ° C.) is calculated, and the temperature difference ΔT = Ta0−Te (for example, 0.5 ° C.) is calculated and set.

  As shown in FIG. 9, if a diagram showing the relationship between the salinity concentration C and the temperature difference ΔT for various IPFs is stored, if the salinity concentration C and the IPF are known, the temperature difference ΔT (° C.) can be known immediately. Therefore, the temperature difference ΔT (° C.) can be set easily and quickly.

  Returning to FIG. 8, in step S3, the control device 36 starts ice making. That is, the control device 36 opens the salt water piping valve 22 and drives the circulation pump 23 to circulate the salt water 1 through the ice storage tank 3 and the ice making device 10. In addition, the control device 36 drives the refrigerator 30 to supply the ice making device 10 with a low-temperature refrigerant and rotates the rotating cylinder 13.

  Thereby, the inner cylinder 12 is cooled by the refrigerant, the salt water 1 flowing along the inner peripheral surface of the inner cylinder 12 is cooled, and a part of the fresh water in the salt water 1 becomes fine ice to form the inner cylinder 12. It adheres to the entire inner surface. The fine ice adhering to the inner peripheral surface of the inner cylinder 12 is scraped off by a plurality of scrapers 16 provided on the outer peripheral surface of the rotating cylinder 13. The fine ice scraped off is mixed with the salt water 1 flowing through the salt water side passage 14 and then flows into the ice storage tank 3 through the forward salt water pipe 20. In this manner, the temperature of the salt water 1 in the ice storage tank 3 gradually decreases, and the IPF of the sherbet ice 2 gradually increases.

  In step S4, the control device 36 determines whether or not icing has started based on the detection value Ts of the thermometer 6. The control device 36 determines that icing has started when the rate of decrease in the temperature detection value Ts decreases stepwise. This determination method will be described in detail later. If it is determined that icing has started, the process proceeds to step S5.

  In step S5, the control device 36 determines the temperature detection value Tsa0 (° C.) when ice formation starts, and subtracts the temperature difference ΔT from the ice formation start temperature detection value Tsa0 to obtain the ice making completion temperature detection value Tse. To do. In step S6, the control device 36 determines whether or not the detection value Ts of the thermometer 6 has reached the ice making completion temperature detection value Tse. If so, the drive of the refrigerator 30 is stopped in step S7, and ice making is performed. Complete. When the ice making is completed, the discharge pump 8 is driven to discharge the sherbet ice.

  Next, a method for determining whether or not icing has started will be described. FIG. 10A is a time chart showing the time change of the detection value Ts of the thermometer 6 during the sherbet ice generation period. In FIG. 10A, when the salt water 1 is cooled under a constant condition, the temperature detection value Ts of the salt water 1 decreases at a constant speed, and when the temperature detection value Ts reaches the icing start temperature detection value Tsa0, temperature detection is performed. The decrease rate of the value Ts suddenly decreases. The control device 36 monitors the temperature detection value Ts, and sets the temperature detection value Ts at the time ta at which the rate of decrease of the temperature detection value Ts has suddenly decreased as the icing start temperature detection value Tsa0.

  Specifically, the control device 36 samples the temperature detection value Ts at a predetermined cycle, and 1 of two consecutive sampling values Ts (i−1) and Ts (i) (where i is a natural number). Next difference value ΔTs (i) = Ts (i) −Ts (i−1) is calculated, and as shown in FIG. 10B, the primary difference value ΔTs (i) is in the range in FIG. The temperature detection value Ts at the time ta moved from A to the range B is set as the icing start temperature detection value Tsa0.

  Alternatively, as shown in FIG. 10C, the secondary difference value ΔΔTs (i) = [Ts (i) −Ts (i−1)] − [Ts (i−1) −Ts (i−2)] is obtained. The temperature detection value Ts at time ta when the secondary difference value ΔΔTs (i) exceeds the predetermined threshold value TH may be calculated as the icing start temperature detection value Tsa0.

  FIG. 11 is a flowchart showing a method of detecting the start of icing using the primary difference value ΔTs (i). In step S <b> 11, the operator uses the operation unit 37 to input the salinity concentration C and the raw water amount of the salt water 1.

  In step S12, the control device 36 determines the range A of the primary difference value ΔTs (i) before the start of freezing and the primary difference value ΔTs (i) after the start of freezing based on the set salinity concentration C and the amount of raw water. A range B is set. Further, the control device 36 resets the count value CT of the built-in counter to 0 and sets the upper limit value N of the count value. In step S13, the control device 36 starts measurement.

  The control device 36 samples the temperature detection value Ts (i) in step S14, and in step S15, the primary difference value ΔTs (i) = Ts (i) −Ts (from the previous sampling value Ts (i−1). i-1) is calculated.

  In step S16, the control device 36 determines whether or not the primary difference value ΔTs (i) is continuously within the range A. If it is determined that the primary difference value ΔTs (i) is within the range A, icing starts in step S17. If it is determined to be before, the process returns to step S14, and if it is determined that it is not within the range A, the process proceeds to step S18.

  In step S18, the control device 36 determines whether or not the primary difference value ΔTs (i) is continuously within the range B. If it is determined that the primary difference value ΔTs (i) is not within the range B, the process returns to step S14. If it is determined that it falls within the range B, the process proceeds to step S19. At this time, the control device 36 counts the number of times the primary difference value ΔTs (i) has entered the range B with the built-in counter, and if the count value does not exceed the upper limit value N, the primary difference value ΔTs (i ) Is not continuously within the range B, and if the count value exceeds the upper limit N, it is determined that the primary difference value ΔTs (i) is continuously within the range B. To do.

  In step S19, the controller 36 determines that icing has started, and sets the detected temperature value Ts (i−N + 1) sampled i−N + 1 times before as the icing start temperature detected value Tsa0.

  FIG. 12 is a flowchart showing a method of detecting the start of icing using the secondary difference value ΔΔTs (i). In step S <b> 21, the operator inputs the salinity concentration C and the raw water amount of the salt water 1 using the operation unit 37.

  In step S22, the control device 36 sets a threshold value TH of the secondary difference value ΔΔTs (i) for determining the start of icing based on the set salinity concentration C and the amount of raw water. In step S23, the control device 36 starts measurement. The controller 36 samples the temperature detection value Ts (i) in step S24, and in step S25, the secondary difference value ΔΔTs (i) from the previous and previous sampling values Ts (i−1) and Ts (i−2). ) = [Ts (i) -Ts (i-1)]-[Ts (i-1) -Ts (i-2)] = Ts (i) -2Ts (i-1) -Ts (i-2) Calculate

  In step S26, the control device 36 determines whether or not the secondary difference value ΔΔTs (i) exceeds the threshold value TH. If it is determined that the secondary difference value ΔΔTs (i) does not exceed the threshold value TH, in step S27, before the start of freezing. If it is determined that the threshold value TH has been exceeded, the process proceeds to step S28. In step S28, the control device 36 determines that icing has started, and sets the temperature detection value Ts (i) as the icing start temperature detection value Tsa0.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a figure which shows the relationship between the salt concentration of salt water, and the freezing start temperature. It is a figure which shows the change of salt concentration and the freezing start temperature at the time of cooling salt water. It is a figure which shows the comparative example of this invention. It is another figure which shows the comparative example of this invention. It is a figure which shows the principle of this invention. It is another figure which shows the principle of this invention. It is a block diagram which shows the structure of the sherbet ice production | generation apparatus by one Embodiment of this invention. It is a flowchart which shows operation | movement of the sherbet ice production | generation apparatus shown in FIG. It is a figure which shows the other method of calculating | requiring the temperature difference shown in FIG. It is a time chart which illustrates the icing start detection method shown in FIG. It is a flowchart which shows the method of detecting the start of icing using the primary difference value shown in FIG. It is a flowchart which shows the method of detecting the start of icing using the secondary difference value shown in FIG.

Explanation of symbols

  1 salt water, 2 sherbet ice, 3 ice storage tank, 4 stirrer, 5 drive device, 6 thermometer, 7 discharge pipe, 8 discharge pump, 10 ice making device, 11 outer cylinder, 12 inner cylinder, 13 rotating cylinder, 14 Salt water side passage, 15 Refrigerant side passage, 16 Scraper, 17 Drive motor, 20 Outward salt water piping, 21 Reflux salt water piping, 22 Salt water piping valve, 23 Circulation pump, 24 Pressure sensor, 30 Refrigerator, 31 Compressor, 32 Condensation , 33, 34 Refrigerant piping, 35 expansion valve, 36 control device, 37 operation unit.

Claims (7)

A sherbet ice generating device that cools salt water in a tank to generate sherbet ice,
Cooling means for cooling the salt water in the tank;
A thermometer for detecting the temperature of salt water in the tank;
Based on the time change of the temperature of the salt water detected by the thermometer, the freezing start temperature is detected, and the difference between the detected freezing start temperature and the temperature of the salt water has reached a predetermined temperature difference. And a control means for stopping cooling of the salt water by the cooling means.   The control means stores information indicating the relationship between the salt concentration of the salt water, the generation concentration of the sherbet ice, and the predetermined temperature difference, and the stored salt information and the salt concentration of the set salt water The sherbet ice generating device according to claim 1, wherein the predetermined temperature difference is obtained based on the generation concentration of the sherbet ice.   3. The sherbet ice generating device according to claim 1, wherein the control means sets the temperature of the salt water when the rate of decrease in the temperature of the salt water decreases stepwise as the icing start temperature. 4.   The control means samples the temperature of the salt water at a predetermined cycle, obtains a primary difference value of the plurality of sampled temperatures, and calculates the temperature of the salt water when the obtained primary difference value changes in a stepped manner. The sherbet ice production | generation apparatus of Claim 3 made into icing start temperature.   The control means samples the temperature of the salt water at a predetermined cycle, obtains secondary difference values of a plurality of sampled temperatures, and the obtained secondary difference value exceeds a predetermined threshold value. The sherbet ice production | generation apparatus of Claim 3 which makes temperature of salt water the said freezing start temperature. Furthermore, a stirring means for stirring the salt water in the tank is provided,
The cooling means is
A water channel provided outside the tank for flowing the salt water;
A pump for circulating salt water in the tank through the water channel;
6. The sherbet ice production according to claim 1, further comprising: an ice maker that freezes a part of the salt water flowing through the water channel to generate fine ice and flows the generated fine ice into the water channel. apparatus. A sherbet ice generating method for cooling salt water in a tank to generate sherbet ice,
The salt water in the tank is cooled and the temperature of the salt water in the tank is detected, and the freezing start temperature of the salt water is detected based on the time change of the salt water temperature, and the detected freezing start temperature and the temperature of the salt water are detected. A sherbet ice generating method in which the cooling of the salt water is stopped in response to a temperature difference between and a predetermined temperature difference being reached.

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