Heat recovery device, heat recovery method, and material manufacturing method

Abstract

Proposed is a heat recovery technique that manifests an energy-saving effect to the greatest extent possible. Provided is a device comprising: a drainage path through which flows post-cleaning wastewater that has been used in cleaning treatment of a material having increased temperature; a replenishment water path through which flows replenishment water for the cleaning treatment; a first circulation path through which a first heating medium circulates; a first heat exchanger that is provided to the drainage path and the replenishment water path, the first heat exchanger performing heat exchange between the wastewater and the replenishment water; a second heat exchanger that is provided downstream of the first heat exchanger in the drainage path, the second heat exchanger performing heat exchange between the wastewater and the first heating medium; a heat pump of which the low-temperature side is connected to the first circulation path and the high-temperature side is connected downstream of the first heat exchanger in the replenishment water path; and a control unit that controls the first heat exchanger so that the coefficient of performance of the first heat exchanger and the heat pump improves when the replenishment water that has been increased in temperature via the first heat exchanger is furthermore increased in temperature due to heat from the wastewater recovered in the first heating medium and electric power supplied to a compressor and an evaporator of the heat pump.

Description

Heat recovery device, heat recovery method, and material manufacturing method

[0001] The present invention relates to a heat recovery device and a heat recovery method for cleaning materials, and a method for manufacturing materials using the heat recovery method.

[0002] The supply of cleaning fluid to material washing equipment and heating media such as hot water to be replenished in the cleaning fluid tank is generally performed using well-known heaters that are heated electrically or by steam. However, heaters are used to heat the material to a desired temperature by resistance or induction heating, and therefore consume a large amount of power. Also, when steam is used to heat the material to a desired temperature, the amount of steam used is large.

[0003] To address these issues, the use of a heat pump as an energy-saving device has been disclosed. For example, Patent Document 1 proposes an apparatus that uses a heat pump to improve the heat utilization efficiency of a heat source.

[0004] Special Publication No. 2020-528128

[0005] The above-mentioned conventional technologies have the following problems. Simply combining a heat exchanger with a heat pump, as in the technology described in Patent Document 1, does not provide sufficient energy-saving effects. For example, simply improving the performance of a heat pump to increase the temperature of makeup water used in washing materials does not necessarily produce desirable results.

[0006] The present invention has been made in view of the above circumstances, and aims to provide a heat recovery device and a heat recovery method that maximize the energy-saving effect in the cleaning process of materials, and also to propose a method for manufacturing materials using the heat recovery method.

[0007] The inventors conducted extensive experiments and studies to solve the above problems. As a result, they discovered that by combining a heat pump and a heat exchanger, it is possible to maximize the energy saving effect by supplying heat at an appropriate temperature and flow rate to the heat pump. In addition, they discovered that by directly or indirectly using water used in the business, it is possible to purify the liquid fed into the heat exchanger of the heat pump.

[0008] The heat recovery device according to the present invention, which advantageously solves the above problems, is configured as follows. [1] A heat recovery device comprising: a drainage path through which wastewater used in the cleaning process of materials whose temperature has increased flows; a makeup water path through which makeup water for the cleaning process flows; a first circulation path through which a first heat medium circulates; a first heat exchanger provided in the drainage path and the makeup water path and performing heat exchange between the drainage and the makeup water; a second heat exchanger provided in the drainage path downstream of the first heat exchanger and performing heat exchange between the drainage and the first heat medium; a heat pump whose low-temperature side is connected to the first circulation path and whose high-temperature side is connected downstream of the first heat exchanger in the makeup water path; and a control unit that controls the first heat exchanger so that the coefficient of performance of the first heat exchanger and the heat pump is improved when the makeup water, whose temperature has been raised through the first heat exchanger, is further raised in temperature by the heat of the drainage recovered in the first heat medium and by electricity to a compressor and an evaporator of the heat pump. [2] In the heat recovery device in [1] above, the control unit controls the first heat exchanger to reduce the temperature of the wastewater to a temperature range where the wastewater will not freeze due to heat exchange in the second heat exchanger due to performance of the heat pump. [3] In the heat recovery device in [2] above, the control unit controls the first heat exchanger to reduce the temperature of the wastewater used in a cleaning process of materials whose temperature has increased to 10°C or higher after heat exchange. [4] In any one of [1] to [3] above, the heat recovery device further comprises a second circulation path through which a second heat medium circulates, and a third heat exchanger provided in the make-up water path downstream of the first heat exchanger and performing heat exchange between the make-up water and the second heat medium, and the make-up water path is connected to the high-temperature side of the heat pump instead of the high-temperature side of the heat pump.

[0009] The heat recovery method and material manufacturing method of the present invention, which advantageously solve the above-mentioned problems, are configured as follows: [5] A heat recovery method including: a first heat exchange step in which a first heat exchanger exchanges heat between post-wash wastewater used in the washing process of a material whose temperature has increased and makeup water for the washing process; a second heat exchange step in which a second heat exchanger exchanges heat between the wastewater whose temperature has been lowered through the first heat exchanger and a first heat medium circulated and connected to the low-temperature side of a heat pump; a makeup water heating step in which the heat pump heats the makeup water whose temperature has been increased through the first heat exchanger by using heat recovered in the first heat medium and electric power to a compressor and an evaporator; and a control step in which a control unit controls the first heat exchanger so as to improve the coefficient of performance of the first heat exchanger and the heat pump. [6] The heat recovery method according to the above item [5], further comprising a make-up water heating step in which a third heat exchanger further heats the make-up water heated through the first heat exchanger by heat exchange with a second heat medium circulated through the high-temperature side of the heat pump. [7] A material manufacturing method comprising a washing step in which make-up water to be used in a material washing treatment is heated using the heat recovery method according to the above item [5] or [6].

[0010] The present invention provides a heat recovery system and method using a heat pump and heat exchanger that can supply a heat medium energy-efficiently and at low cost. Furthermore, by using this heat recovery system to heat makeup water in the material washing process, it is possible to produce materials that are energy-efficient and environmentally friendly.

[0011] FIG. 1 is a system diagram showing a heat recovery device according to one embodiment of the present invention. FIG. 2 is a system diagram showing a heat recovery device according to another embodiment of the present invention. FIG. 3 is a system diagram showing a conventional method for heating makeup water for cleaning liquid. FIG. 4 is a system diagram showing an example of a heat recovery device according to the embodiment provided with a flow path changing unit. FIG. 5 is an enlarged system diagram showing an example of the configuration of the flow path changing unit according to the other embodiment. FIG. 6 is a system diagram showing another example of a heat recovery device according to the other embodiment provided with a flow path changing unit. FIG. 7 is an enlarged system diagram showing an example of the configuration of the flow path changing unit of the other example, where (a) represents the second flow path changing unit and (b) represents the first flow path changing unit. FIG. 8 is a graph showing heat distribution when the heat of wastewater is recovered to the maximum extent in the first heat exchanger using the heat recovery device according to the embodiment, and when heat is recovered so as to maximize the coefficient of performance of the heat pump.

[0012] The following describes in detail embodiments of the present invention. Note that the drawings are schematic and may differ from the actual embodiments. Furthermore, the following embodiments exemplify devices and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0013] In material washing equipment in the cold rolling mill of a steelworks, materials are washed with acid or alkali and then rinsed with rinse water. In the rinse washing process, clean hot water is replenished into a rinse circulation tank, and the rinse solution used for washing is simultaneously discharged from the rinse circulation tank. As shown in FIG. 3 , conventionally, to improve washing performance, the rinse solution 50 in the rinse circulation tank 5 is heated with steam 51 or a heater 52. Hot water 20 is supplied to the rinse circulation tank 5, and a portion of the solution in the tank is discharged outside the system as wastewater 10 for treatment.

[0014] <Heat Recovery Apparatus> As an example of this embodiment, Fig. 1 shows a heat recovery apparatus used in a cleaning process 100 for a steel sheet S. The heat recovery apparatus according to this embodiment includes a drainage path through which drainage water 10, 10A, and 10B flows, a make-up water path through which make-up water 20 and 20A flows, and a first circulation path through which a first heat medium 4A circulates. In order to effectively utilize heat, the heat recovery apparatus according to this embodiment includes a first heat exchanger 1 connected to the drainage path and the make-up water path. In the first heat exchanger 1, heat exchange occurs between the high-temperature drainage water 10 used in the cleaning treatment of the steel sheet S and the make-up water 20 for the cleaning treatment, thereby raising the temperature of the make-up water 20 and lowering the temperature of the high-temperature drainage water 10 after cleaning.

[0015] The heat recovery system according to this embodiment includes a second heat exchanger 2, one of which is connected downstream of the first heat exchanger 1 in the wastewater path and the other of which is connected to the first circulation path. In the second heat exchanger, heat exchange occurs between the cooled post-cleaning wastewater 10A and the first heat medium 4A, raising the temperature of the first heat medium 4A and converting the wastewater 10A into the cooled post-cleaning wastewater 10B. The first circulation path is connected to the low-temperature evaporator 42 of the heat pump 4. The heated first heat medium 4A is cooled and circulated by transferring heat to the heat pump. By arranging the second heat exchanger 2 and the first circulation path through which the first heat medium 4A circulates, heat can be supplied indirectly without directly feeding the poor quality post-cleaning wastewater 10A into the heat pump 4, which cannot be decomposed and cleaned.

[0016] In the example of FIG. 1 , the make-up water path is connected to a condenser 41 on the high-temperature side of the heat pump 4 downstream of the first heat exchanger 1. As a result, make-up water 20 heated by the first heat exchanger 1 is input into the condenser 41 on the high-temperature side of the heat pump 4, and is further heated to become make-up water 20A by heat recovered by the evaporator 42 on the low-temperature side and power supplied to the compressor 43 and the evaporator 42. The make-up water 20A is used in the cleaning liquid temperature adjustment process 200. The heat pump 4 includes a general condenser (heat radiator) 41, an evaporator (heat absorber) 42, a compressor 43, and an expansion valve 44. Here, the condenser (heat radiator) 41 and the evaporator (heat absorber) 42 function as heat exchangers.

[0017] The temperature of the makeup water 20A heated by the heat pump 4 is preferably less than 100°C, and the temperature of the post-wash wastewater 10A sent to the heat pump for heat recovery is preferably less than 90°C.

[0018] FIG. 2 shows a system diagram of a heat recovery device according to another embodiment. This embodiment includes a second circulation path through which a second heat medium 4B circulates, and a third heat exchanger 3. In the example shown in FIG. 2, the second circulation path is connected to a condenser 41 on the high-temperature side of the heat pump 4, where the temperature of the second heat medium 4B is increased. The third heat exchanger is connected to the second circulation path and downstream of the first heat exchanger 1 in the makeup water path, and performs heat exchange between the makeup water heated by the first heat exchanger 1 and the second heat medium 4B. In the third heat exchanger 3, the makeup water 20 becomes makeup water 20A, which is further heated, and the second heat medium 4B is cooled and circulated. The makeup water 20 has better properties than the wastewater 10 after cleaning, but by arranging the third heat exchanger 3 and the second circulation path through which the second heat medium 4B circulates on the high-temperature side of the condenser 41 of the heat pump 4, it is possible to use liquid with even better properties in the heat pump 4.

[0019] To adjust the temperature of the cleaning liquid sent to the cleaning process 100, a heating means such as steam 51 or a heater 52 may be used in the cleaning liquid temperature adjustment process 200. The heating means further heats makeup water 20A obtained by directly or indirectly heating makeup water 20 at a temperature lower than the set temperature required for the cleaning treatment using a heat pump 4.

[0020] The control unit controls the first heat exchanger 1 so as to improve the coefficient of performance of the first heat exchanger 1 and the heat pump 4. Here, the coefficient of performance of the first heat exchanger 1 and the heat pump 4 refers to the sum of the power consumption of the heat pump 4, the heat recovery amount of the first heat exchanger 1, and the heat recovery amount of the heat pump 4, divided by the power consumption of the heat pump 4. Essentially, the first heat exchanger 1 is controlled to perform heat exchange at its maximum output. However, depending on the performance and output of the first heat exchanger 1, there is a risk that the temperature of the wastewater 10A may become lower than necessary, causing problems in the subsequent heat recovery by the heat pump 4 (second heat exchanger 2). Specifically, for example, if the temperature of the wastewater 10A is lowered to about 5°C by the first heat exchanger 1, the wastewater 10B may freeze during the subsequent heat recovery by the heat pump 4 (second heat exchanger 2), potentially clogging the piping.

[0021] Therefore, the control unit may control the first heat exchanger 1 to reduce the temperature of the wastewater 10A to a temperature range where the wastewater 10B does not freeze by heat recovery by the second heat exchanger 2 due to the performance of the heat pump 4. Specifically, the control unit may control the first heat exchanger 1 to raise the temperature of the post-wash wastewater 10 used in the washing process of the materials whose temperature has increased to 10°C or higher through heat exchange with the makeup water 20.

[0022] The heating means may be steam 51 or an electric heater 52. For example, when steam 51 is used as the heating means, the temperature and flow rate of the preheated makeup water 20 and the amount of steam used to heat the preheated makeup water 20A to the required supply temperature are controlled so that the net energy consumption is reduced based on the steam loss from the steam generation source to the supply destination in the business.

[0023] An inlet temperature sensor for measuring the temperature of the first heat medium 4A and a flow meter for measuring the flow rate of the first heat medium 4A can be provided inside the piping that forms the first circulation path connected to the evaporator 42 on the low-temperature side of the heat pump 4. The flow rate can be calculated from the differential pressure by measuring the pressure at the inlet and outlet of the first heat medium 4A to the evaporator (heat absorber) 42 and the structure of the evaporator 42. Pure water is preferably used as the first heat medium 4A.

[0024] The condenser (radiator) 41 on the high-temperature side of the heat pump 4 is connected to the piping downstream of the first heat exchanger 1 of the makeup water path ( FIG. 1 ), or to a second circulation path ( FIG. 2 ) through which a second heat transfer medium 4B circulates. In the latter case, makeup water 20 heated in the first heat exchanger 1 is heated by heat exchange in the third heat exchanger 3 with the second heat transfer medium 4B, which has been heated using the thermal energy obtained by heat exchange in the condenser (radiator) 41 of the heat pump 4. In the example of FIG. 1 , the piping of the makeup water path, or in the example of FIG. 2 , the piping of the second circulation path, is preferably equipped with temperature sensors on the inlet and outlet sides of the condenser (radiator) 41 to measure the fluid temperature. Air or an inert gas can be used as the second heat transfer medium, or a liquid such as water can be used.

[0025] The data obtained by the temperature sensor can be used to control the operation of the heat pump 4. Feedback control and feedforward control may be used alone or in combination. For example, feedback control may be performed in which a temperature gap is calculated from the temperature of the preheated makeup water 20A and the set temperature of the preheated water, and this result is used to calculate and output the amount of power consumed by the heat pump 4. Alternatively, feedforward control may be performed in which the amount of power consumed by the heat pump 4 is calculated from the inlet temperature, flow rate, and temperature of the first heat medium (chilled water) 4A connected to the low-temperature side of the heat pump 4. These types of control preferably use general PID control or inverter control. On the heat pump 4 side, these types of control may be performed by either the compressor 43 or the expansion valve 44, or both.

[0026] The temperature of the preheated make-up water 20A generated by the heat pump 4 is preferably set based on the flow rate and the output of a heating means such as steam 51, taking into consideration high overall energy efficiency. In the heat pump 4, downstream of the supply port of the preheated make-up water 20A, heating means such as steam 51 for heating the preheated make-up water 20A to a set temperature is preferably provided, and make-up water for washing materials is preferably supplied via the heating means. In addition, it is preferable to provide means for measuring the temperature and flow rate of the make-up water at the inlet and outlet of the heating means, and to control the amount of heat supplied by steam 51 or the like based on the measurement results. Feedback control and feedforward control can be used alone or in combination as a control method.

[0027] In this embodiment, the makeup water entering the rinse circulation tank 5 is obtained by heating preheated makeup water 20A with steam or the like. Therefore, compared to when room-temperature makeup water is heated to a set temperature with steam 51, this embodiment can reduce the amount of steam used to heat the makeup water, thereby saving energy. Furthermore, even if the set temperature of makeup water 20A is high enough that a heat pump cannot achieve it, the necessary makeup water can be obtained at low cost. Specifically, makeup water at the set temperature is generated by using a combination of preheated makeup water 20A and a heating means for steam 51. Initial controllability is also improved. The number of heat pumps 4 that generate makeup water to be preheated can be determined based on the capacity of the heat pumps 4 and the specifications (flow rate, temperature) of the heat recovery device.

[0028] To maximize the performance of the heat pump 4, a flow path changing unit may be provided in the first circulation path between the second heat exchanger 2 and the heat pump 4, or in the second circulation path between the third heat exchanger 3 and the heat pump 4, to control the temperatures of the first heat medium 4A and the second heat medium 4B. The flow path changing unit has a three-way valve or a bypass valve.

[0029] Fig. 4 is a system diagram showing a state in which a flow path changing unit 6 is provided in the first circulation path between the second heat exchanger 2 and the heat pump 4. Fig. 5 is a system diagram showing an enlarged configuration of the flow path changing unit 6. The flow path changing unit 6 is provided with a three-way valve 6B or a bypass valve.

[0030] The temperature of the first heat medium 4A at the inlet that passes through the first circulation path and enters the evaporator 42 of the heat pump 4 is defined as TCi, and the upper limit of the target temperature of the first heat medium 4A at the inlet is defined as TC. Based on TCi measured by a measuring unit 7 such as a thermocouple, the second control unit 8 performs feedback or feedforward control on the flow path changing unit 6 so that TCi≦TC. Under the control of the second control unit 8, the flow path changing unit 6 changes the flow path of the heat medium in the first circulation path between the second heat exchanger 2 and the evaporator 42 of the heat pump 4. The flow path changing unit 6 adjusts the flow path and flow rate based on the actual measurement value, thereby enabling accurate temperature control of the fluid.

[0031] Note that when the flow path changing unit 6 bypasses the first circulation path so as not to pass through the second heat exchanger 2, the pressure loss in the bypass path is smaller than the pressure loss in the path through the second heat exchanger 2, so the flow rate is not necessarily constant in both paths. For this reason, a flow rate adjustment valve 6A may be provided to maintain a constant flow rate in both paths. The valve opening of the flow rate adjustment valve 6A may be set in advance, or may be feedback-controlled or feedforward-controlled by the second control unit 8. This reduces flow rate fluctuations even when the flow path and flow rate are adjusted by the flow path changing unit 6, enabling more accurate temperature control of the first heat medium 4A.

[0032] Fig. 6 is a system diagram showing another embodiment of the heat recovery system shown in Fig. 2, in which flow path changing units 61 and 62 are provided in the first circulation path and the second circulation path, respectively. Fig. 7(a) is a system diagram showing an enlarged configuration of the flow path changing unit 62 provided in the second circulation path. Fig. 7(b) is a system diagram showing an enlarged configuration of the flow path changing unit 61 provided in the first circulation path. The flow path changing units 61 and 62 are provided with a three-way valve 6B or a bypass valve.

[0033] The temperature of the second heat medium 4B at the outlet of the condenser 41 of the heat pump 4 after passing through the second circulation path is defined as THo, and the upper limit of the target temperature of the second heat medium 4B at the outlet is defined as TH. Based on THo measured by a measuring unit 7 such as a thermocouple, the third control unit 82 performs feedback control or feedforward control on the flow path changing unit 62 so that THo≦TH. Under the control of the third control unit 82, the flow path changing unit 62 changes the flow path of the second heat medium 4B in the second circulation path between the third heat exchanger 3 and the condenser 41 of the heat pump 4. The flow path changing unit 62 adjusts the flow path and flow rate based on actual measurements, thereby enabling accurate temperature control of the second heat medium 4B.

[0034] A flow rate adjusting valve 6A may also be provided on the side of the flow path changing unit 62 provided in the second circulation path, taking into consideration the difference in pressure loss between the bypass path and the heat exchanger path. This reduces flow rate fluctuations regardless of whether the flow path and flow rate are adjusted by the flow path changing unit 62, and enables more accurate temperature control of the second heat medium 4B.

[0035] The function of the flow path changing unit 61 provided in the first circulation path and the operation of the second control unit 81 are similar to the function of the flow path changing unit 6 and the operation of the second control unit 8. The control unit, the second control unit, and the third control unit may be configured as physically different units or may be configured as physically the same unit.

[0036] <Heat Recovery Method> A heat recovery method of this embodiment will be described using the heat recovery device described above.

[0037] The heat recovery method of this embodiment effectively utilizes the heat of the wastewater from the rinse circulation tank, which has not been utilized in the past. This heat recovery method increases the temperature of the hot water supplied to the rinse circulation tank and reduces the amount of steam used to heat the cleaning liquid in the rinse circulation tank, thereby achieving energy savings.

[0038] In the first heat exchange step, heat is exchanged between the post-wash wastewater 10 used in the washing process of the materials, the temperature of which has risen in the first heat exchanger 1, and the make-up water for the washing process 20. This is to increase the temperature of the make-up water for the washing process by using the heat of the post-wash wastewater, the temperature of which has risen.

[0039] In the second heat exchange step, heat is exchanged between the post-cleaning wastewater 10A whose temperature has been lowered in the first heat exchange step and the first heat medium 4A circulatingly connected to the low-temperature side of the heat pump in the second heat exchanger 2. This is to further transfer the heat of the post-cleaning wastewater 10A whose temperature has been lowered in the first heat exchange step to the first heat medium 4A circulatingly connected to the low-temperature side of the heat pump.

[0040] In the makeup water heating process, the makeup water 20, which has been heated through the first heat exchanger 1, is further heated using the heat pump 4 by the heat recovered in the first heat medium 4A and the power supplied to the compressor 43 and the evaporator 42. The makeup water is then supplied to the rinse circulation tank as high-temperature makeup water 20A. The heat recovered from the wastewater and the power required to operate the heat pump contribute to raising the temperature of the makeup water.

[0041] In the control process, the temperature of the first heat medium 4A supplied to the low-temperature side of the heat pump 4 decreases, and even if the coefficient of performance of the heat pump 4 itself decreases, the first heat exchanger 1 is controlled so that the coefficient of performance of the entire heat recovery apparatus system is improved. Here, the coefficient of performance of the heat pump 4 refers to the sum of the power consumption of the heat pump 4 and the heat recovery amount of the heat pump 4 divided by the power consumption of the heat pump 4. In addition, the coefficient of performance of the entire heat recovery apparatus system refers to the sum of the power consumption of the heat pump 4, the heat recovery amount of the first heat exchanger 1, and the heat recovery amount of the heat pump 4 divided by the power consumption of the heat pump 4.

[0042] In the control step, the first heat exchanger 1 may be controlled so that the temperature of the wastewater 10A is reduced to a temperature range in which the wastewater 10B does not freeze due to heat recovery by the second heat exchanger 2 due to the performance of the heat pump 4. Specifically, the first heat exchanger may be controlled so that the temperature of the post-wash wastewater 10, which has been used in the washing process of the materials and has been increased, is raised to 10°C or higher through heat exchange with the makeup water 20.

[0043] Therefore, in the heat recovery method according to this embodiment, the temperature of the makeup water sent to the rinse circulation tank is higher than in the heat recovery method using a conventional heat recovery device, so the amount of heat generated by the steam 51 or the heater 52 in the rinse circulation tank can be reduced compared to the conventional method. In other words, the amount of electricity required for heating can be reduced, thereby achieving energy savings.

[0044] The temperature of makeup water 20A heated by the heat pump is set to less than 100°C so as not to exceed the set temperature of the cleaning liquid in the rinse circulation tank. The temperature of wastewater 10A sent to the heat pump for heat recovery is set to less than 90°C, taking into account the specified temperature of the heat source of the heat pump.

[0045] Furthermore, it is preferable to set up a third heat exchange process in which heat is exchanged between the makeup water and the high-temperature side heat source of the heat pump in the third heat exchanger 3. This is to avoid the problem that if makeup water is drawn directly into the heat pump, the properties of the makeup water directly affect the maintenance of the heat pump.

[0046] In a heat recovery method according to another embodiment, makeup water 20A preheated by a heat pump is heated in a rinse circulation tank in the heating step, where the makeup water is heated to a temperature lower than the set temperature required for the cleaning process.

[0047] The heat recovery method according to this embodiment can be applied to a material manufacturing method that includes a cleaning process for heating makeup water used in the cleaning process of the material. As an example, the material is a metal material, more specifically, a steel material. The steel material includes steel plates, thick plates, shaped steel, steel bars, wire rods, etc.

[0048] According to the heat recovery device and heat recovery method of this embodiment, the coefficient of performance can be improved by the specific control method of the control unit that controls the first heat exchanger described above. This makes it possible to achieve heat recovery with high energy efficiency in terms of the total energy consumption of the heat exchanger and heat pump at low cost. Furthermore, in this embodiment, the specific control method described above reduces the temperature of the wastewater to a temperature range where the wastewater will not freeze, ensuring system safety and reducing maintenance costs. Furthermore, by controlling the temperature of the wastewater after cleaning to be 10°C or higher, system efficiency can be optimized and energy consumption can be reduced. Additionally, by connecting the second circulation path to the high-temperature side of the heat pump using the specific connection method described above, heat recovery efficiency can be improved.

[0049] Example 1: Steel plates were washed at a steel mill using the heat recovery system, washing process, and washing liquid temperature adjustment process shown in FIG. 1 . The heat recovery system included a first heat exchanger, a second heat exchanger, and a heat pump. In this example, the output of the first heat exchanger was controlled to maximize the amount of heat recovered by the first heat exchanger, while preventing the wastewater from freezing during heat exchange by the second heat exchanger. The temperature of the makeup water supplied to the first heat exchanger was 10°C, and the temperature of the post-wash wastewater was 70°C. The post-wash wastewater, whose temperature had been lowered through the first heat exchanger, was utilized as a low-temperature heat source for the heat pump to recover heat. The makeup water, which had been heated through the first heat exchanger, was then further heated using the heat and electricity recovered by the heat pump. Subsequently, makeup water heated to a high temperature by a heat pump was supplied to a rinse circulation tank, where it was heated by steam to produce a cleaning liquid in the rinse circulation tank.

[0050] The inlet and outlet temperatures of the first heat medium to the low-temperature side evaporator of the heat pump were 20°C and 10°C, and the inlet and outlet temperatures to the high-temperature side condenser were 65°C and 75°C. The temperature of the makeup water heated by the heat pump was 67°C. Furthermore, the temperature of the wastewater sent to the second heat exchanger for heat recovery, i.e., the post-wash wastewater cooled through the first heat exchanger, was 30°C, and the temperature after the second heat exchange was 26°C.

[0051] The results are shown in Figure 8 (a), an example of the invention with high total performance. In this example, the steam consumption (ST) was reduced by approximately 83% compared to the conventional steam heating SH, to 16.7% of the total heating amount. The power input (EP) to the heat pump 4 was 3.9% of the total heating amount, and together with the heat recovery (HR) of 9.3%, 13.2% of the total heating amount was obtained. The first heat exchanger 1 obtained heat recovery (HR) of 70.2% of the total heating amount. In addition, the coefficient of performance (COP) of the heat pump 4 was 3.4, but the total COP including heat recovery was 17.7, including auxiliary equipment such as the cooling water circulation pump, and a good result was obtained.

[0052] [Example 2] As shown in Figure 2, in addition to the heat recovery system configuration of Example 1, a second circulation path was connected to the high-temperature side of the heat pump, and a third heat exchanger was arranged to perform heat exchange between the second heat medium and the heated makeup water.

[0053] By providing the third heat exchanger, makeup water does not flow directly to the heat pump, the heat exchanger inside the heat pump is kept clean, and the frequency of cleaning maintenance can be reduced.

[0054] Comparative Example: As shown in Figure 3, the temperature of the rinse circulation tank was increased using conventional steam heating, and steel sheets were cleaned. A large amount of steam was used in the rinse circulation tank to generate a high-temperature cleaning solution for cleaning the steel sheets.

[0055] Steel plates were washed at a steel mill using the heat recovery system, cleaning process, and cleaning solution temperature adjustment process configured as shown in FIG. 1 . In this comparative example, the output of the first heat exchanger was controlled to maximize the amount of heat recovered by the heat pump. The heat recovery system included a first heat exchanger, a second heat exchanger, and a heat pump. The temperature of makeup water supplied to the first heat exchanger was 10°C, and the temperature of the post-cleaning wastewater was increased to 70°C. Heat was recovered by using the post-cleaning wastewater, whose temperature had been reduced through the first heat exchanger, as a low-temperature heat source for the heat pump. The makeup water, which had been heated through the first heat exchanger, was then further heated using the heat and electricity recovered in the heat pump. Next, the makeup water, heated to a high temperature by the heat pump, was supplied to a rinse circulation tank and heated with steam, and a cleaning solution was produced in the rinse circulation tank.

[0056] The inlet and outlet temperatures of the first heat medium to the low-temperature side evaporator of the heat pump were 40°C and 30°C, and the inlet and outlet temperatures to the high-temperature side condenser were 65°C and 75°C. The temperature of the makeup water heated by the heat pump was 47°C. Furthermore, the temperature of the wastewater sent to the heat pump for heat recovery (i.e., the post-wash wastewater cooled through the first heat exchanger) was 50°C, and the temperature after the second heat exchanger was 43°C. The results are shown in Figure 8 (b) as an example of high heat pump performance. In this example, the steam consumption ST was reduced by approximately 54% compared to the conventional steam heating SH. The input power EP to the heat pump 4 was 4.2% of the total heating amount, and together with the heat recovery HR of 14.3%, 18.5% of the total heating amount was obtained. The first heat exchanger 1 obtained heat recovery HR of 35.1% of the total heating amount. Furthermore, the coefficient of performance of the heat pump 4 was 4.4, which was better than that of Example 1. The total coefficient of performance including heat recovery was 10.6, including auxiliary equipment such as the cooling water circulation pump, which was good, but the result was lower than that of Example 1. This is because the temperature after the second heat exchange was 43°C, which was good as a heat source water condition for the heat pump, and although the coefficient of performance of the heat pump could be made large, waste heat of 43°C remained.

[0057] Although the above examples have been described as embodiments of the present invention, the present invention is not limited to these.

[0058] The heat recovery device and the heat recovery method used in the cleaning treatment of materials of the present invention can be applied not only to a specific process but also to any process in which heat is utilized.

[0059] 100 Cleaning process 200 (Cleaning liquid) temperature adjustment process 1 First heat exchanger 2 Second heat exchanger 3 Third heat exchanger 4 Heat pump 4A First heat medium 4B Second heat medium 41 Condenser 42 Evaporator 43 Compressor 44 Expansion valve 5 Rinse circulation tank 50 Cleaning liquid 51 Steam 52 Heater 6, 61, 62 Flow path change unit 6A Flow rate control valve 6B Three-way valve 7 Measuring unit 8, 81 Second control unit 82 Third control unit 10 Drainage water (after cleaning) 10A Drainage water (after passing through the first heat exchanger) (after cleaning) 10B Drainage water (after passing through the second heat exchanger) (after cleaning) 20 Make-up water (hot water) 20A Make-up water (after passing through the heat pump) S Steel plate SH Steam heating ST Steam consumption HR Heat recovery EP Input power

Claims

1. A heat recovery device comprising: a drainage path through which post-washing wastewater used in the washing process of materials whose temperature has increased flows; a make-up water path through which make-up water for the washing process flows; a first circulation path through which a first heat medium circulates; a first heat exchanger provided in the drainage path and the make-up water path, for exchanging heat between the drainage water and the make-up water; a second heat exchanger provided in the drainage path downstream of the first heat exchanger, for exchanging heat between the drainage water and the first heat medium; a heat pump having a low-temperature side connected to the first circulation path and a high-temperature side connected to the make-up water path downstream of the first heat exchanger; and a control unit that controls the first heat exchanger so that the coefficient of performance of the first heat exchanger and the heat pump is improved when the make-up water, whose temperature has been increased through the first heat exchanger, is further increased in temperature by the heat of the drainage water recovered in the first heat medium and by electric power to a compressor and an evaporator of the heat pump.

2. The heat recovery device described in claim 1, wherein the control unit controls the first heat exchanger to reduce the temperature of the wastewater to a temperature range in which the wastewater does not freeze due to heat exchange in the second heat exchanger due to the performance of the heat pump.

3. The heat recovery device described in claim 2, wherein the control unit controls the first heat exchanger so that the temperature of the wastewater used in the cleaning process of materials with elevated temperatures is 10°C or higher after heat exchange.

4. A heat recovery device as described in any one of claims 1 to 3, comprising: a second circulation path through which a second heat medium circulates; and a third heat exchanger located downstream of the first heat exchanger in the makeup water path, which exchanges heat between the makeup water and the second heat medium; wherein, instead of the makeup water path being connected to the high-temperature side of the heat pump, the second circulation path is connected to the high-temperature side of the heat pump.

5. A heat recovery method comprising: a first heat exchange step in which a first heat exchanger exchanges heat between post-wash wastewater used in the washing process of materials whose temperature has increased and makeup water for the washing process; a second heat exchange step in which a second heat exchanger exchanges heat between the wastewater whose temperature has been lowered through the first heat exchanger and a first heat medium that is circulated and connected to the low-temperature side of a heat pump; a makeup water heating step in which the heat pump heats the makeup water that has been heated through the first heat exchanger by using heat recovered in the first heat medium and electric power to a compressor and an evaporator; and a control step in which a control unit controls the first heat exchanger so as to improve the coefficient of performance of the first heat exchanger and the heat pump.

6. A heat recovery method as described in claim 5, further comprising a make-up water heating step in which, instead of the make-up water heating step, a third heat exchanger further heats the make-up water by heat exchange between the make-up water heated through the first heat exchanger and a second heat medium circulated and connected to the high-temperature side of the heat pump.

7. A method for manufacturing materials, comprising a washing step in which make-up water used in the washing treatment of materials is heated using the heat recovery method according to claim 5 or 6.

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