Hot water curing process for special coating of warehouse of chemical tanker

Abstract

The invention discloses a hot water curing process for a special coating of a cargo hold of a chemical tanker, which comprises the following steps: S1, after the special coating construction of all cargo holds on a ship is finished and a seawater test is completed, checking and repairing a missed coating point, and then carrying out hot water curing on the special coating; s2, determining optimal curing parameters in advance according to a special coating construction process; s3, determining the minimum effective fresh water amount required for completing coating thermocuring of a single cargo hold, and injecting fresh water into a sump oil tank of the ship; s4, a thermocouple is installed on the back face of the bulkhead of the cargo hold Hi to be subjected to special coating hot water curing so as to monitor the temperature in the cargo hold Hi; s5, internal fresh water is preheated through an external heater, then the fresh water obtained after waste heat is subjected to secondary heating through a cabin washing water heater, and then hot water obtained after secondary heating is evenly sprayed to the cabin wall of the cargo cabin Hi through a cabin washing machine for hydrothermal curing; and S7, the steps S4-S6 are repeated, and curing of the next cargo hold is conducted till curing of all the cargo holds is completed.

Description

Technical Field

[0001] The invention relates to the technical field of coating curing, and in particular to a hot water curing process for a special coating in a cargo hold of a chemical ship. Background Art

[0002] Chemical tanker cargo holds are generally divided into stainless steel cargo holds and carbon steel cargo holds. In order to adapt to the characteristics of chemicals, stainless steel cargo holds need to be passivated, while carbon steel cargo holds need to be specially coated.

[0003] Conventional special coatings are naturally cured. The function of the coating is to form a tough solid film on the surface of the material. Generally speaking, the coating is first a flowing liquid and forms a solid film after coating. Therefore, it is a physical + chemical change process of vitrification, which is the so-called curing.

[0004] Often in order to accommodate special goods in warehouses, the coating needs to be formed into a film by hot melt, a process called thermal curing.

[0005] The special coating adopts heat curing and is used for loading strong alkaline products with too high pH, ​​strong acidic products with too low pH, and products with smaller molecular structure.

[0006] The main purpose of thermal curing is to solidify into a molecular structure with smaller molecular bonds to improve the chemical resistance and temperature resistance of the special layer.

[0007] Mechanism of thermal curing: When the coating is heated or in the presence of a catalyst (including an assimilation agent), a chemical cross-linking reaction occurs. The various film-forming components in the coating fuse together to form a coating with a cross-linked three-dimensional network structure. After curing, this type of coating will no longer be dissolved by solvents or softened by heat, and its physical and chemical properties are better.

[0008] There are two conventional methods of heat curing: one is to heat the indoor environment temperature of the cabin, usually by gas baking to achieve heat curing, and the other is to heat with hot water or hot shower.

[0009] Among them, hot water hot shower heating is highly safe, simple to operate, and widely used. For example, patent CN111097671A provides a coating thermal curing process for a special coating tank of a chemical bulk dual-purpose ship, which uses a tank cleaning machine to spray hot water to thermally cure the coating of the special coating tank.

[0010] Hot water curing generally has a certain temperature range to choose from, and different temperatures require different minimum curing times. Generally speaking, the lower the temperature, the longer the curing time. In traditional solutions such as the above-mentioned patent, the curing temperature is usually selected based on experience, and the energy consumption in the curing process is not fully considered. Then, because the area of ​​the coating to be cured is large, the energy consumed in this process is often huge. If the energy consumption can be considered according to the actual situation when choosing the curing temperature, and the curing parameters that can reduce energy consumption are selected, the economic benefits brought about will be very considerable. Unfortunately, there is no public and reliable solution in the prior art cases including the above-mentioned patents. Summary of the invention

[0011] The technical problem to be solved by the present invention is to provide a hot water curing process for special coatings in cargo tanks of chemical ships in view of the deficiencies in the above-mentioned prior art.

[0012] In order to solve the above technical problems, the technical solution adopted by the present invention is: the present invention provides a hot water curing process for a special coating in a cargo hold of a chemical ship, comprising the following steps:

[0013] S1. After the special coating construction of all cargo holds on the ship is completed and the seawater test is completed, the missing coating points are checked and repaired, and then the special coating is cured with hot water;

[0014] S2. Determine the optimal curing parameters in advance according to the special coating construction process. The optimal curing parameters include the curing temperature T y and curing time t y ;

[0015] S3. Determine the minimum effective fresh water volume Q required for thermal curing of coating in a single cargo hold min , the amount of water injected into the ship's dirty oil tank is Q min of fresh water;

[0016] S4. In the cargo hold H where the special coating is to be cured with hot water i Thermocouples are installed on the reverse side of the bulkhead to monitor the cargo hold H i The temperature in i = 1, 2, ..., N, where N is the total number of cargo holds that require hot water curing of the special coating;

[0017] S5. Preheat the fresh water inside through an external heater, and then reheat the fresh water with residual heat to a temperature of T through a tank washing water heater. y Then the hot water after secondary heating is sprayed evenly to the cargo hold H through the tank cleaning machine. i The bulkhead is hydrothermally cured for a curing time of t y ;

[0018] S6. Cargo hold H i All the water in the tank is pumped back into the dirty oil tank;

[0019] S7, repeat steps S4-S6, proceed to the next cargo hold H i+1 until all cargo holds have been cured.

[0020] Preferably, in step S2, the process of special coating construction includes: the type of coating used and the thickness of the special coating.

[0021] Preferably, step S2 specifically comprises:

[0022] S2-1. Pre-acquire the curing parameters corresponding to different special coating construction processes through experiments, and construct a process-curing parameter table. In the table, for each special coating construction process S, there are several allowable curing parameters P corresponding to each special coating construction process S. Each curing parameter P includes a curing temperature T and a curing time t required at the curing temperature T;

[0023] S2-2. Calculate the estimated unit energy consumption W required for each solidification parameter P according to the current temperature of the fresh water in the dirty oil tank, W = W 1 +W 2 , W 1 The energy required to heat the unit mass of fresh water in the current slop tank to temperature T, W 2 is the energy required to keep a unit mass of fresh water at temperature T for a period of time t;

[0024] S2-3, selecting the curing parameter corresponding to the minimum value of the unit energy consumption estimate W as the optimal curing parameter corresponding to the special coating construction process;

[0025] S2-4, according to the special coating construction S currently implemented in the cargo hold, select the optimal curing parameter Py corresponding to the special coating construction S, the optimal curing parameter includes the curing temperature T y and curing time t y .

[0026] Preferably, in step S2-3, when there are at least two unit energy consumption estimates that are tied for minimum, the curing parameter corresponding to the one with the shortest curing time is taken as the optimal curing parameter.

[0027] Preferably, the curing temperature T is in the range of 55-90°C.

[0028] Preferably, the curing time t is in the range of 1-24 h.

[0029] Preferably, in step S4, the temperature value detected by the thermocouple is analyzed by a temperature test error estimation model to obtain a test error ΔT, and then the temperature monitoring result detected by the thermocouple is corrected and output, and the temperature correction model is constructed by the following method:

[0030] 1) Construct training data set R:

[0031] 1-1) When the bulkhead thickness is D 0 The injection temperature in the cargo hold is T C0 The hot water is supplied by a thermocouple 1 and the actual temperature of the hot water is monitored in the cargo hold to maintain the hot water temperature at T C0 , until the temperature monitored by the thermocouple 2 installed on the reverse side of the bulkhead reaches T C0 ±ε T , ε T is the pre-set deviation threshold, ε T = 0.05-0.5℃; then change the temperature of the hot water in the cargo hold to T C1 , get the temperature value monitored by thermocouple 2 at this time, recorded as T' C1 ; Calculate the temperature test error value ΔT 1 , ΔT 1 =T C1 -T' C1 ;

[0032] D 0 , T' C1 , ΔT 1 Combine into a training data r;

[0033] 1-2) Follow the method in step 1-1) to change T C1 The value of , obtains several training data;

[0034] 1-3) Follow the method in step 1-1) to change D 0 The value of , and then obtain several training data; combine all the training data to obtain the training data set R;

[0035] 2) Using the training data set R, D 0 and T' C1 With ΔT as the target output and CNN as the input, the temperature test error estimation model is obtained after training.

[0036] Preferably, in step S4, the method for correcting the temperature monitoring result detected by the thermocouple is:

[0037] The temperature value T' detected by the thermocouple C 、Current cargo hold H i Bulkhead thickness D i Input into the temperature test error estimation model to obtain the temperature test error value ΔT i , calculate the corrected temperature value T C , T C =T' C +ΔT i , TC Output as temperature monitoring result.

[0038] Preferably, in step S5, feedback adjustment is performed on the tank washing machine heater according to the temperature monitoring result output in step S4, so that the hot water is heated to the required temperature T y .

[0039] Preferably, in step S5, the temperature of the fresh water preheating is T x , satisfying: T y -T x =5~15℃.

[0040] The beneficial effects of the present invention are:

[0041] The present invention provides a hot water curing process for a special coating in a cargo hold of a chemical ship. In the present invention, according to the process of special coating construction: the type of coating and the thickness of the special coating, a curing parameter with relatively minimum energy consumption under the premise of meeting the curing requirements can be quickly analyzed. The hot water curing of the special coating is implemented according to the hot water curing process, which can reduce the energy consumption in the hot water curing process and reduce the cost, thereby bringing significant economic benefits.

[0042] In the present invention, the deviation and hysteresis of the temperature value monitored by the thermocouple 2 can be corrected through the temperature test error estimation model, so that the output result is closer to the actual temperature of the hot water in the cargo hold, thereby better ensuring the effect of the hot water curing process. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a flow chart of a hot water curing process for a special coating for a chemical tanker cargo hold of the present invention;

[0044] Figure 2 is a flow chart of step S2 of the present invention;

[0045] Figure 3 A flow chart for constructing a temperature correction model of the present invention;

[0046] Figure 4 The flowchart of constructing the training data set R in the present invention. DETAILED DESCRIPTION

[0047] The present invention is further described in detail below in conjunction with embodiments so that those skilled in the art can implement the invention with reference to the description.

[0048] It should be understood that the terms such as “having”, “including” and “comprising” used herein do not exclude the existence or addition of one or more other elements or combinations thereof.

[0049] Example 1

[0050] A hot water curing process for a special coating in a cargo hold of a chemical ship, characterized by comprising the following steps:

[0051] S1. After the special coating construction of all cargo holds on the ship is completed and the seawater test is completed, the missing coating points are checked and repaired, and then the special coating is cured with hot water;

[0052] S2. Determine the optimal curing parameters in advance according to the special coating construction process. The optimal curing parameters include the curing temperature T y and curing time t y ;

[0053] S3. Determine the minimum effective fresh water volume Q required for thermal curing of coating in a single cargo hold min , the amount of water injected into the ship's dirty oil tank is Q min of fresh water;

[0054] S4. In the cargo hold H where the special coating is to be cured with hot water i Thermocouples are installed on the reverse side of the bulkhead to monitor the cargo hold H i The temperature in i = 1, 2, ..., N, where N is the total number of cargo holds that require hot water curing of the special coating;

[0055] S5. Preheat the fresh water inside with the external heater, and then reheat the fresh water with residual heat to the temperature T through the tank washing water heater. y Then the hot water after secondary heating is sprayed evenly to the cargo hold H through the tank cleaning machine. i The bulkhead is hydrothermally cured for a curing time of t y ;

[0056] S6. Cargo hold H i All the water in the tank is pumped back into the dirty oil tank;

[0057] S7, repeat steps S4-S6, proceed to the next cargo hold H i+1 until all cargo holds have been cured.

[0058] In step S2, the process of special coating construction includes: the type of coating used and the thickness of the special coating. For example, the commonly used coating includes one of phenolic epoxy paint 930, 935, and 940; the thickness of the special coating can usually be between 75-125 μm.

[0059] Step S2 is specifically as follows:

[0060] S2-1. Pre-acquire the curing parameters corresponding to different special coating construction processes through experiments, and construct a process-curing parameter table. In the table, for each special coating construction process S, there are several allowable curing parameters P corresponding to each special coating construction process S. Each curing parameter P includes a curing temperature T and a curing time t required at the curing temperature T;

[0061] Generally speaking, the curing temperature T is selected to be in the range of 55-90°C, and the curing time t is in the range of 1-24h.

[0062] In this embodiment, for the coating being any one of phenolic epoxy paints 930, 935, and 940, when the special coating thickness is 75μm, 80μm, 85μm, 90μm, 95μm, 100μm, 105μm, 110μm, 115μm, 120μm, and 125μm, several curing parameters allowed within the curing temperature T range of 55-90°C are determined in advance through experiments.

[0063] For example, when the coating is phenolic epoxy paint 930 and the coating thickness is 85μm, the required curing time at several different curing temperatures is shown in Table 1 below:

[0064] Table 1

[0065]

[0066] S2-2. Calculate the estimated unit energy consumption W required for each solidification parameter P according to the current temperature of the fresh water in the dirty oil tank, W = W 1 +W 2 , W 1 The energy required to heat the unit mass of fresh water in the current slop tank to temperature T, W 2 is the energy required to keep a unit mass of fresh water at temperature T for a period of time t;

[0067] S2-3, selecting the curing parameter corresponding to the minimum value of the unit energy consumption estimate W as the optimal curing parameter corresponding to the special coating construction process;

[0068] For example, among the several curing parameters corresponding to the special coating construction process in Table 1, the optimal curing parameters obtained by analysis are: curing temperature 85°C and curing time 2.5h.

[0069] S2-4, according to the special coating construction S currently implemented in the cargo hold, select the optimal curing parameter Py corresponding to the special coating construction S, the optimal curing parameter includes the curing temperature T y and curing time t y .

[0070] Among them, in step S2-3, when there are at least two unit energy consumption estimates that are tied for the minimum, the curing parameter corresponding to the one with the shortest curing time is taken as the optimal curing parameter. For example, if the unit energy consumption estimates for the two cases of curing temperature 85°C and curing time 2.5h and curing temperature 75°C and curing time 6h are tied for the minimum, then the curing temperature 85°C and curing time 2.5h are selected as the optimal curing parameters to reduce the construction time.

[0071] By adopting the above method, according to the different thicknesses of the paint and special coating in the specific process of special coating construction, the pre-constructed process-curing parameter table can be used to quickly analyze and obtain the curing parameters with relatively minimal energy consumption while meeting the curing requirements. The special coating can be cured with hot water based on the process-curing parameter table to reduce energy consumption and costs.

[0072] In step S4 of this embodiment, the temperature value detected by the thermocouple is analyzed by the temperature test error estimation model to obtain the test error ΔT, and then the temperature monitoring result detected by the thermocouple is corrected and output. The temperature correction model is constructed by the following method:

[0073] 1) Construct training data set R:

[0074] 1-1) When the bulkhead thickness is D 0 The injection temperature in the cargo hold is T C0 The hot water is supplied by a thermocouple 1 and the actual temperature of the hot water is monitored in the cargo hold to maintain the hot water temperature at T C0 , until the temperature monitored by the thermocouple 2 installed on the reverse side of the bulkhead reaches T C0 ±ε T , ε T is the pre-set deviation threshold, ε T =0.05-0.5°C, for example, in this embodiment, ε T =0.2℃; when the temperature monitored by thermocouple 2 reaches T C0 ±ε T When , it is considered that the heat transfer of the bulkhead has reached stability;

[0075] Then change the temperature of the hot water in the cargo hold to T C1 , get the temperature value monitored by thermocouple 2 at this time, recorded as T' C1 ; Calculate the temperature test error value ΔT 1 , ΔT 1 =T C1 -T' C1 ;

[0076] D 0 , T' C1 , ΔT 1 Combine into a training data r;

[0077] 1-2) Follow the method in step 1-1) to change T C1 The value of , obtains several training data;

[0078] 1-3) Follow the method in step 1-1) to change D 0 The value of , and then obtain several training data; combine all the training data to obtain the training data set R;

[0079] 2) Using the training data set R, D 0 and T' C1 With ΔT as the target output and CNN as the input, the temperature test error estimation model is obtained after training.

[0080] In step S4, the method for correcting the temperature monitoring result detected by the thermocouple is:

[0081] The temperature value T' detected by the thermocouple C 、Current cargo hold H i Bulkhead thickness D i Input into the temperature test error estimation model to obtain the temperature test error value ΔT i , calculate the corrected temperature value T C , T C =T' C +ΔT i , T C Output as temperature monitoring result.

[0082] In step S5, feedback adjustment is performed on the tank washing machine heater according to the temperature monitoring result output in step S4, so that the hot water is heated to the required temperature T y .

[0083] In step S5, the temperature of fresh water preheating is T x , satisfying: T y -T x =5~15℃, for example, in this embodiment, T y -T x =10℃.

[0084] Affected by the heat transfer efficiency of the bulkhead, the temperature value monitored by thermocouple 2 cannot accurately reflect the actual temperature of the hot water in the cargo hold in real time. Specifically, the temperature value monitored by thermocouple 2 will have a certain deviation and lag. For example, when the temperature of the hot water in the cargo hold reaches 85°C, affected by the heat transfer efficiency of the bulkhead, the temperature value detected by thermocouple 2 on the back of the bulkhead may be 84.6°C. If the temperature value detected by thermocouple 2 is used as the output result to feedback control the temperature of the hot water in the cargo hold, the temperature of the hot water in the cargo hold will be controlled at 84.6°C, which is lower than the actual temperature control target of 85°C, which will eventually increase the temperature error of the hot water curing and adversely affect the curing process effect.

[0085] The heat transfer efficiency of the bulkhead is mainly affected by the bulkhead material, thickness and the temperature of the hot water in the cargo hold. The bulkhead material is assumed to be the same, so the thickness and the temperature of the hot water in the cargo hold are mainly considered. In this embodiment, the temperature monitored by the thermocouple 2 and the actual temperature of the hot water in the cargo hold are obtained through experiments at different bulkhead thicknesses and different hot water temperatures in advance, and the difference between the two is calculated. The obtained experimental data is constructed into a training data set R, and the CNN convolutional neural network is used as the basic framework for training. The CNN network is used to analyze the influence of the bulkhead thickness and the hot water temperature in the cargo hold on the temperature test error value, and finally a temperature test error estimation model is obtained. In this embodiment, the temperature test error estimation model can be used to correct the deviation and lag of the temperature value monitored by the thermocouple 2, so that the output result is closer to the actual temperature of the hot water in the cargo hold, thereby better ensuring the effect of the hot water curing process.

[0086] Effect test

[0087] Test example: the initial temperature of fresh water is 17°C, the process of Example 1 is adopted, the coating is phenolic epoxy paint 930, the thickness of the special coating is 85 μm, and the optimal curing parameters obtained by step S2 are: curing temperature 85°C, curing time 2.5h, and the special coating is cured by hot water according to this.

[0088] As a control example, the only difference from the test example is that the optimal curing parameters are not determined in step S2, but the previously specified curing parameters are used: curing temperature 75° C., curing time 6 h.

[0089] The total energy Wc consumed in the entire curing process in the test example and the total energy Wd consumed in the entire curing process in the control example were calculated. The result showed that Wc / Wd=0.924, indicating that the total energy consumed for heating fresh water in the test example was lower than that in the control example, that is, determining the optimal curing parameters through step S2 in Example 1 can reduce energy consumption.

[0090] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and the implementation modes. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily implemented. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to specific details.

Claims

1. A hot water curing process for special coatings in cargo holds of chemical ships, characterized in that: The following steps are involved: S1. After the special coating construction of all cargo holds on the ship is completed and the seawater test is completed, the missing coating points are checked and repaired, and then the special coating is cured with hot water; S2. Determine the optimal curing parameters in advance according to the special coating construction process. The optimal curing parameters include the curing temperature T y and curing time t y ; S3. Determine the minimum effective fresh water volume Q required for thermal curing of coating in a single cargo hold min , the amount of water injected into the ship's dirty oil tank is Q min of fresh water; S4. In the cargo hold H where the special coating is to be cured with hot water i Thermocouples are installed on the reverse side of the bulkhead to monitor the cargo hold H i The temperature in i = 1, 2, ..., N, where N is the total number of cargo holds that require hot water curing of the special coating; S5. Preheat the fresh water inside through an external heater, and then reheat the fresh water with residual heat to a temperature of T through a tank washing water heater. y Then the hot water after secondary heating is sprayed evenly to the cargo hold H through the tank cleaning machine. i The bulkhead is hydrothermally cured for a curing time of t y ; S6. Cargo hold H i All the water in the tank is pumped back into the dirty oil tank; S7, repeat steps S4-S6, proceed to the next cargo hold H i+1 until all cargo holds have been cured.

2. The hot water curing process for special coatings in cargo holds of chemical tankers according to claim 1, characterized in that: In step S2, the process of special coating construction includes: the type of coating used and the thickness of the special coating.

3. The hot water curing process for special coatings in cargo holds of chemical tankers according to claim 2 is characterized in that: Step S2 is specifically as follows: S2-1. Pre-acquire the curing parameters corresponding to different special coating construction processes through experiments, and construct a process-curing parameter table. In the table, for each special coating construction process S, there are several allowable curing parameters P corresponding to each special coating construction process S. Each curing parameter P includes a curing temperature T and a curing time t required at the curing temperature T; S2-2. Calculate the estimated unit energy consumption W required under each solidification parameter P according to the current temperature of the fresh water in the slop oil tank, where W=W1+W2, W1 is the energy required to heat the unit mass of fresh water in the current slop oil tank to temperature T, and W2 is the energy required to keep the unit mass of fresh water at temperature T for time t; S2-3, selecting the curing parameter corresponding to the minimum value of the unit energy consumption estimate W as the optimal curing parameter corresponding to the special coating construction process; S2-4, according to the special coating construction S currently implemented in the cargo hold, select the optimal curing parameter Py corresponding to the special coating construction S, the optimal curing parameter includes the curing temperature T y and curing time t y .

4. The hot water curing process for special coatings in cargo holds of chemical tankers according to claim 3 is characterized in that: In step S2-3, when there are at least two unit energy consumption estimates that are tied for minimum, the curing parameter corresponding to the one with the shortest curing time is taken as the optimal curing parameter.

5. The hot water curing process for special coatings in cargo holds of chemical tankers according to claim 3, characterized in that: The curing temperature T ranges from 55 to 90°C.

6. The hot water curing process for special coatings in cargo holds of chemical tankers according to claim 5, characterized in that: The curing time t ranges from 1 to 24 hours.

7. The hot water curing process for special coatings in cargo holds of chemical tankers according to claim 1, characterized in that: In step S4, the temperature value detected by the thermocouple is analyzed by a temperature test error estimation model to obtain a test error ΔT, and then the temperature monitoring result detected by the thermocouple is corrected and output. The temperature correction model is constructed by the following method: 1) Construct training data set R: 1-1) Inject a temperature of T into a cargo hold with a bulkhead thickness of D0 C0 The hot water is supplied by a thermocouple 1 and the actual temperature of the hot water is monitored in the cargo hold to maintain the hot water temperature at T C0 , until the temperature monitored by the thermocouple 2 installed on the reverse side of the bulkhead reaches T C0 ±ε T , ε T is the pre-set deviation threshold, ε T = 0.05-0.5℃; then change the temperature of the hot water in the cargo hold to T C1 , get the temperature value monitored by thermocouple 2 at this time, recorded as T' C1 ; Calculate the temperature test error value ΔT1, ΔT1 = T C1 -T' C1 ; D0、T' C1 , ΔT1 are combined into a training data r; 1-2) Follow the method in step 1-1) to change T C1 The value of , obtains several training data; 1-3) According to the method of step 1-1), change the value of D0 and obtain several training data; combine all the training data to obtain the training data set R; 2) Using the training data set R, with D0 and T' C1 With ΔT as the target output and CNN as the input, the temperature test error estimation model is obtained after training.

8. The hot water curing process for special coatings in cargo holds of chemical tankers according to claim 7, characterized in that: In step S4, the method for correcting the temperature monitoring result detected by the thermocouple is: The temperature value T' detected by the thermocouple C 、Current cargo hold H i Bulkhead thickness D i Input into the temperature test error estimation model to obtain the temperature test error value ΔT i , calculate the corrected temperature value T C , T C =T' C +ΔT i , T C Output as temperature monitoring result.

9. The hot water curing process for special coatings in cargo holds of chemical tankers according to claim 8, characterized in that: In step S5, feedback adjustment is performed on the tank washing machine heater according to the temperature monitoring result output in step S4, so that the hot water is heated to the required temperature T y .

10. The hot water curing process for special coatings in cargo holds of chemical tankers according to claim 1, characterized in that: In step S5, the temperature of fresh water preheating is T x , satisfying: T y -T x =5~15℃.

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