Liquid heating system

Abstract

This invention discloses a liquid heating system and its control method, aiming to solve the problems of unreliable connection between aluminum tubes and substrates, low heat transfer efficiency, and poor temperature control accuracy in existing liquid heating systems. The system includes a stainless steel substrate, the first surface of which has a thick-film resistance heating layer composed of an insulating dielectric layer, a resistance layer, and a capping layer stacked sequentially. The second surface is metallurgically bonded to a flattened aluminum tube through an Al-Si brazing layer. The brazing layer has a shear strength ≥60MPa, significantly improving the bonding strength and thermal conductivity of the dissimilar metal interface, overcoming the defects of easy delamination and high thermal resistance in traditional mechanical pressing or adhesive bonding. The system also includes a temperature controller, a temperature sensor, an overheat protection element, and a control circuit using an MCU+PID closed-loop algorithm. With the above-mentioned brazed metallurgical bonding structure, the heat conduction path is short, the thermal efficiency is ≥90%, the temperature rise rate is >70℃ / s, the temperature control accuracy is ±1℃, and it has multiple safety protections, expandable to 30kW.

Description

Technical Field

[0001] This invention relates to the field of liquid heating system technology, specifically to a liquid heating system that integrates an aluminum tube-stainless steel substrate brazed liquid heating device and an intelligent control circuit, suitable for applications requiring precise temperature control such as household water heaters, instant water dispensers, commercial coffee machines, and industrial fluid heating. Background Technology

[0002] Liquid heating systems are widely used in household appliances and industrial equipment. Traditional liquid heating solutions mainly include the following categories: (1) Nickel-chromium alloy heating wire: The metal heating wire is bent into a serpentine shape and embedded in the substrate or directly immersed in the liquid. It has a simple structure and low cost, but it has problems such as poor heating uniformity, limited service life (fatigue failure caused by thermal cycling), and difficulty in achieving precise temperature control. (2) PTC ceramic self-limiting heating: PTC ceramic has a positive temperature coefficient. When the temperature exceeds the Curie temperature, the resistance increases sharply and the current is automatically limited, which has good safety. However, the heating power decreases significantly with the increase of temperature, making it unsuitable for high-power continuous heating. In addition, its bonding force with the substrate is weak. (3) Quartz tube / glass tube electric heating tube heating: The electric heating element encapsulated in glass tube or quartz tube is used to heat the liquid. It has good corrosion resistance, but low mechanical strength. It is at risk of breaking when used in high pressure or flowing liquid. The thermal efficiency is also limited by the convective heat transfer between the fluid in the tube and the heating element. (4) Thick-film resistance heating: Functional pastes such as metal oxides (e.g., RuO2) or carbides (e.g., SiC) are screen-printed and then sintered onto a metal or ceramic substrate to form a thin-film resistance heating body. This method has high heating power density (up to 60W / cm² or more), fast response speed, and the fabrication process is compatible with mature processes in the microelectronics industry.

[0003] The liquid heating system involved in this invention adopts the following structure: a thick-film resistance heating layer is provided on the first surface (front) of a stainless steel substrate as a heating element, and an aluminum tube is brazed and fixed on the second surface (back) as a liquid flow channel, through which the heated liquid flows. This double-sided arrangement places the heating layer and the liquid flow channel on opposite sides of the substrate, and the heating heat is conducted to the liquid in the aluminum tube through the stainless steel substrate, resulting in a short heat conduction path and high efficiency.

[0004] However, existing liquid heating systems have the following main problems: Problem 1: Poor heating uniformity. Traditional heating elements (such as heating wires) have uneven power density in some areas, resulting in both localized overheating and underheating of the heated liquid, affecting the consistency of the outlet water temperature.

[0005] Problem 2: Low temperature control accuracy. Existing liquid heaters generally use bimetallic strip temperature controllers, whose operating temperature error is usually ±5℃ or even greater, with obvious response lag, making it difficult to meet the requirements of precise temperature control (such as the requirement of water temperature error ≤±1℃ for coffee extraction).

[0006] Question 3: Poor reliability of the connection between the aluminum tube and the stainless steel substrate. There is a significant difference in the coefficient of thermal expansion between aluminum tubes and stainless steel (aluminum approximately 23 × 10⁻ ... 6 / K, stainless steel approx. 16×10⁻ 6 During thermal cycling, stress accumulation can cause microcracks or even detachment at the contact interface, leading to a sharp increase in thermal resistance, a significant decrease in system thermal efficiency, and poor reliability over long-term use.

[0007] Issue 4: Inadequate safety protection. Most existing liquid heaters rely solely on a single hardware thermostat for protection, lacking a multi-layered protection mechanism linked to software. This poses a high safety risk when water flow is interrupted (dry burning) or the thermostat fails, and real-time fault diagnosis and recording are not possible.

[0008] Question 5: Low level of intelligence. Traditional heating systems lack precise power regulation capabilities and data communication functions, making it impossible to dynamically adjust heating power according to actual water consumption, and they do not support remote monitoring and smart home integration.

[0009] This invention systematically solves the above problems by integrating a high-efficiency brazing heating substrate with an intelligent control circuit, and provides a liquid heating system with high thermal efficiency, precise temperature control, multiple safety protections, and intelligent functions. Summary of the Invention

[0010] In view of the above-mentioned problems in the prior art, the technical problem to be solved by the present invention is to provide a liquid heating system and its control method, which overcomes the defects of existing liquid heating devices such as long heat conduction path, low thermal efficiency, interface connection failure caused by thermal expansion mismatch between aluminum tube and stainless steel substrate, insufficient temperature control accuracy, and single safety protection mechanism, and achieves the organic unity of efficient heat transfer, precise temperature control, highly reliable interface combination and multiple active safety protection.

[0011] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: The first aspect of this invention provides a liquid heating system, comprising a stainless steel substrate, an aluminum tube, a thermostat, a temperature sensor, an overheat protection element, and a control circuit; a thick-film resistance heating layer is provided on a first surface of the stainless steel substrate, and an aluminum tube is fixedly connected to a second surface of the stainless steel substrate; at least one section of the aluminum tube is flattened into a flat cross-section, and the flat cross-section is metallurgically bonded to the second surface of the stainless steel substrate through an Al-Si brazing layer, the shear strength of which is not less than 60 MPa; the thermostat is electrically connected to the electrode terminals of the thick-film resistance heating layer and is used to control the on / off state of the thick-film resistance heating layer; the temperature sensor is used to collect the temperature signal of the water outlet connector and output it to the control circuit; the overheat protection element is connected in series in the heating circuit and cuts off the heating circuit when the surface temperature of the stainless steel substrate exceeds a set threshold; the rated power of the liquid heating system under 220V AC power is 500W to 5000W, the liquid temperature rise rate is greater than 70℃ / s (at a water flow rate of 6.5mL / s), and the system thermal efficiency is not less than 90%.

[0012] The microstructure of the brazing layer, from the aluminum tube side to the stainless steel substrate side, sequentially includes an Al-Si eutectic layer with a thickness of 80 to 200 μm and an Al-Fe-Si intermetallic compound transition layer with a thickness of 5 to 15 μm, wherein the main phase of the transition layer is Al. 13 The Fe4 phase is followed by Al3Fe, with the Fe content exhibiting a continuous gradient distribution from the aluminum tube side to the stainless steel side. This gradient transition layer structure effectively mitigates the thermal expansion of the aluminum tube (coefficient of thermal expansion approximately 23 × 10⁻⁻⁻). 6 / K) and stainless steel substrate (thermal expansion coefficient approximately 16×10⁻) 6 The interfacial thermal stress caused by the mismatch in the coefficients of thermal expansion between / K) ensures that the shear strength of the connection interface is not lower than the rated value after 5000 hours of thermal cycling.

[0013] The thick-film resistance heating layer is formed on the first surface of the stainless steel substrate through screen printing and sintering processes, with a surface resistivity of 0.05 to 5 Ω / □. The thick-film resistance heating layer comprises, from bottom to top, an insulating dielectric layer, a resistive layer, and a capping layer. The functional material of the resistive layer is RuO2-based paste or SiC-based paste. The aluminum tube is made of one of 1060 pure aluminum, 3003 aluminum alloy, or 5052 aluminum alloy. The stainless steel substrate is made of 304 or 316L stainless steel with a thickness of 0.3 to 5.0 mm. The thick-film resistance heating layer and the liquid flow channel of the aluminum tube are positioned on opposite sides of the stainless steel substrate, ensuring a uniform distribution of heating power density and eliminating the localized overheating problem of traditional electric heating wires. Simultaneously, heat is directly conducted through the stainless steel substrate to the flowing liquid in the aluminum tube, resulting in an extremely short heat conduction path and achieving a system thermal efficiency of no less than 90%.

[0014] The control circuit includes a microcontroller, a solid-state relay, or a bidirectional thyristor. The microcontroller, based on the real-time temperature signal from a temperature sensor, adjusts the heating power of the thick-film resistor heating layer using pulse width modulation or phase control to achieve constant temperature control. The temperature control accuracy is ±1℃, and the response time is no more than 5 seconds. Compared to traditional bimetallic strip temperature controllers, MCU+PID closed-loop control combined with an NTC temperature sensor (thermal time constant no more than 5 seconds) can improve the temperature control accuracy from ±5℃ to ±1℃ and shorten the response time to less than 5 seconds, meeting the stringent temperature consistency requirements of applications such as specialty coffee extraction.

[0015] The overheat protection element is a temperature fuse or thermal switch with an operating temperature of 100 to 150°C, mounted on a stainless steel substrate near the heating core area. The control circuit also has a software overheat protection function: when the temperature sensor detects that the temperature exceeds a first threshold, the power is reduced to 30%; when it exceeds a second threshold, the heating circuit is cut off. By combining the hardware overheat protection element (independent of the MCU and not bypassable by software) with the software-based graded power reduction protection, a dual independent redundant protection architecture is constructed to ensure that if any single protection element fails, the other element can still maintain system safety.

[0016] The system also includes a dry-burn protection device, which detects dry-burning status by monitoring the water flow rate at the aluminum pipe outlet or the heating rate of the stainless steel substrate. When the heating rate exceeds 15°C / s and lasts for more than 2 seconds, dry-burning is determined, and the control circuit cuts off the heating circuit within 3 seconds. This dry-burning protection provides early warning by actively monitoring the substrate heating rate. Compared to protection methods that rely solely on temperature thresholds, it can advance the protection response time to within 3 seconds after the water flow is interrupted, effectively preventing damage to the heating element due to prolonged dry-burning.

[0017] The system also includes a thermal insulation layer disposed on the first surface side of the stainless steel substrate, with a thermal conductivity of not more than 0.05 W / (m·K) and a thickness of 5 to 20 mm; and a waterproof sealed shell encapsulating the stainless steel substrate, the thick film resistance heating layer and the temperature controller therein, with a protection level of not less than IPX4.

[0018] In some embodiments, the main component of the insulating dielectric layer is borosilicate glass, sintered at a temperature of 750 to 850°C, with a thickness of 20 to 60 μm, a dielectric strength of not less than 15 kV / mm, and an insulation resistance of not less than 100 MΩ (under 500V DC test conditions); the covering layer is a glaze protective layer with a thickness of 10 to 30 μm, which has waterproof and moisture-proof functions.

[0019] In some embodiments, the contact area between the flattened cross-sectional width W of the aluminum tube and the stainless steel substrate in the flattened region satisfies the following conditions: the contact width W is 0.3-1.1 times the original outer diameter D of the aluminum tube, and the flattening height H is 0.3-0.8 times the original outer diameter D of the aluminum tube; the spacing between adjacent aluminum tubes is 8 to 30 mm. These geometric constraints ensure that the contact area between the flattened aluminum tube and the stainless steel substrate is sufficiently large, minimizing the interfacial thermal resistance, thereby supporting a system-level thermal efficiency of no less than 90%.

[0020] The temperature sensor is an NTC thermistor with a resistance of 10kΩ (25℃) and a thermal time constant of no more than 5 seconds, and is attached to the first surface of a stainless steel substrate; the control circuit acquires the temperature signal with a sampling period of 500ms and uses a moving average filtering algorithm to eliminate noise interference.

[0021] The microcontroller implements a PID closed-loop control algorithm with a proportional coefficient Kp of 0.5 to 2.0, an integral time Ti of 10 to 30 s, and a derivative time Td of 1 to 5 s. The control circuit also integrates an energy metering module to count the cumulative power consumption in real time. When the cold resistance of the thick film resistor heating layer deviates from the rated value by ±20%, a fault alarm is triggered.

[0022] The system also includes a flow sensor installed at the inlet of the aluminum pipe, with a range of 0.5 to 20 mL / s and a resolution of no more than 0.1 mL / s. The control circuit dynamically adjusts the heating power based on the real-time signal from the flow sensor to maintain a constant outlet water temperature, with a power adjustment response time of no more than 1 second. The introduction of the flow sensor enables the system to actively adjust the heating power when the flow rate fluctuates, significantly improving the consistency of the outlet water temperature compared to a fixed power output method, and serving as the first criterion for dry-burn protection when the flow rate returns to zero.

[0023] The aluminum tube is connected to an inlet connector at the inlet end and an outlet connector at the outlet end. Both the inlet and outlet connectors are made of food-grade materials. The inner wall of the aluminum tube is anodized, and the oxide layer thickness is 10 to 50 μm.

[0024] The system also has a communication interface, selected from at least one of Wi-Fi, Bluetooth BLE 5.0 or Zigbee, for communicating with an external control terminal to realize remote temperature setting, operation status monitoring and fault diagnosis; the control circuit stores at least 7 days of operation logs, including temperature curves, heating power and fault records.

[0025] The liquid heating system supports series or parallel expansion through modular design. Multiple liquid heating units are uniformly scheduled through the main controller, and the total rated power of the system can be expanded to 30kW. The heating units communicate with each other through RS-485 bus, and the main controller realizes load balancing control, with the power deviation of each unit not exceeding 5%.

[0026] The second aspect of this invention provides a control method for a liquid heating system, applied to the aforementioned liquid heating system, comprising the following steps: a power-on self-test step, detecting whether the temperature sensor is normal, whether the cold resistance of the thick-film resistor heating layer is within the rated range, and whether the overheat protection element is in normal condition; if any abnormality occurs, a fault protection state is entered and a fault code is reported; an initial heating step, heating at full power and monitoring the heating rate in real time to determine the dry-burning state; a constant temperature control step, switching to PID closed-loop control when the detected temperature reaches the target temperature minus 5°C, adjusting the power duty cycle to maintain the target temperature, with a control accuracy of ±1°C; a safety monitoring step, monitoring the temperature throughout the process, reducing the power to 30% if it exceeds a first threshold, cutting off heating if it exceeds a second threshold, and cutting off the heating circuit within 3 seconds when dry-burning is detected; and a normal shutdown step, maintaining residual heat utilization operation after shutdown until the substrate temperature is below 50°C, then completely shutting down and recording the operating data. The above control method integrates power-on self-test, graded power reduction protection, and dry-burning rate monitoring into the same control flow, constructing a complete active safety closed loop at the software level, and achieving multiple redundant protections together with the hardware overheat protection element.

[0027] Compared with the prior art, the present invention has the following beneficial effects: (1) High thermal efficiency: The thick film resistance heating layer and the aluminum tube liquid flow channel are placed on both sides of the stainless steel substrate. The heat is directly conducted to the liquid through the substrate. The heat conduction path is extremely short, eliminating the heat loss in the traditional scheme through the air gap or low thermal conductivity adhesive layer. The system thermal efficiency is not less than 90%, and the temperature rise rate is greater than 70℃ / s at a water flow rate of 6.5mL / s. (2) Precise temperature control: MCU+PID closed-loop control combined with fast-response NTC temperature sensor, temperature control accuracy ±1℃, response time not exceeding 5 seconds, which is a significant improvement over the traditional bimetallic strip temperature controller with an action error of ±5℃, and can meet the application occasions with strict requirements for water temperature consistency, such as specialty coffee extraction and infant formula preparation. (3) Reliable connection: The aluminum tube and the stainless steel substrate form a metallurgical bonding interface with an Al-Fe-Si intermetallic compound gradient transition layer through Al-Si brazing. The shear strength is not less than 60MPa. The gradient transition layer effectively alleviates the interface thermal stress caused by the mismatch of the thermal expansion coefficients of the two materials, ensuring that the connection performance does not degrade after 5000 hours of thermal cycling. This fundamentally solves the problem of interface desoldering caused by stress accumulation during thermal cycling in traditional physical contact methods. (4) High safety: It adopts a triple independent safety mechanism of hardware temperature fuse (independent of the control circuit and cannot be bypassed by software), software graded power reduction protection and active anti-dry burning protection based on the heating rate to form a redundant protection system; if any link fails, the other links can still ensure the safety of the system, overcoming the safety blind spot caused by existing products relying on only a single hardware temperature controller. (5) High level of intelligence: The integrated flow sensor realizes dynamic power adjustment (response time not more than 1 second), supports Wi-Fi / Bluetooth remote communication and fault diagnosis log recording, and the communication interface is selected from at least one of Wi-Fi, Bluetooth BLE 5.0 or Zigbee. It can be linked with the smart home system to realize precise heating on demand, and significantly reduce standby power consumption compared with traditional systems. (6) High scalability: The modular design supports the expansion of multiple units in series and parallel via RS-485 bus. The main controller implements load balancing control, and the power deviation of each unit does not exceed 5%. The total power of the system can be expanded to 30kW, meeting the wide power range application requirements from domestic to industrial fluid heating. Attached Figure Description

[0028] Figure 1 The diagram shows the overall architecture of the liquid heating system of the present invention, illustrating the connection relationships between the thick film resistance heating layer (4) on both sides of the stainless steel substrate (1), the aluminum tube (2), the control circuit (13), the temperature sensor (8), the overheat protection element (9), and the external interface. Figure 2 The electrical schematic diagram of the control circuit (13) shows the electrical connections of the MCU, solid-state relay (SSR) or triac, temperature sensor (8), overheat protection element (9) and power module; Figure 3 The flowchart of the control method shows five steps: S1 power-on self-test, S2 initial heating, S3 constant temperature control, S4 safety monitoring, and S5 normal shutdown, as well as the judgment logic between each step. Figure 4 The schematic diagram of the cross-sectional structure of the liquid heating device clearly shows the double-sided arrangement of the stainless steel substrate (1), the first surface thick film resistance heating layer (4), the three-layer structure (insulating dielectric layer 41, resistance layer 42, and cover layer 43), and the second surface flattened aluminum tube (2) connected by a brazing layer (3). Figure 5 The curve showing the relationship between system thermal efficiency and heating power demonstrates that the system thermal efficiency remains above 90% within the rated power range of 500W to 5000W. Figure 6 The temperature control accuracy test curve shows that the temperature fluctuation is always controlled within ±1℃ of the target temperature under PID closed-loop control. Figure 7The microstructure of the brazing layer (3) is shown in the SEM image (schematic diagram), and the location and thickness range of the Al-Si eutectic layer (thickness 80-200 μm) and the Al-Fe-Si intermetallic compound transition layer (thickness 5-15 μm) are marked.

[0029] Explanation of reference numerals in the attached diagram: 1-Stainless steel substrate; 2-Aluminum tube; 3-Bratter layer (Al-Si system); 4-Thick film resistance heating layer; 41-Insulating dielectric layer; 42-Resistant layer; 43-Covering layer; 5-Heat insulation layer; 6-Sealing clamp; 7-Thermostat; 8-Temperature sensor (NTC); 9-Overheat protection element (temperature fuse / thermal switch); 10-Waterproof sealed housing; 13-Control circuit; 14-Water inlet connector; 15-Water outlet connector; 16-Wire assembly; 27-Flow sensor; 28-Communication module; W-Contact surface width; D-Original outer diameter of aluminum tube; H-Flattening height; L-Live wire; N-Neutral wire; PE-Grounding wire. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The following embodiments are only used to explain and illustrate the present invention, and should not be construed as limiting the scope of protection of the present invention.

[0031] Reference Figure 4 The core of the liquid heating device is a double-sided arrangement structure of a stainless steel substrate (1) and an aluminum tube (2): The stainless steel substrate (1) is the supporting base of the entire heating device. On the first surface (front) of the stainless steel substrate (1), an insulating dielectric layer (41), a resistive layer (42) and a cover layer (43) are sequentially prepared to form a thick film resistive heating layer (4). The insulating dielectric layer (41) is mainly composed of borosilicate glass, and its thickness is 20-60 μm after sintering. Its dielectric strength is ≥15 kV / mm, which insulates the resistive layer (42) from the stainless steel substrate (1). The resistive layer (42) is formed by screen printing RuO2-based or SiC-based conductive paste and sintering at 750-850℃. Its surface resistivity is 0.05-5 Ω / □. The cover layer (43) is a glaze protective layer with a thickness of 10-30 μm to prevent the resistive layer (42) from getting damp or oxidized.

[0032] On the second surface (back side) of the stainless steel substrate (1), aluminum tubes (2) are flattened and arranged in a serpentine pattern with a spacing of 8 to 30 mm between adjacent rows. The flattened cross-section is connected to the second surface of the substrate by an atmosphere-protected brazing process using Al-Si solder (typical composition Al-12Si) to form an Al-Fe-Si intermetallic compound transition layer, achieving metallurgical bonding. The joint shear strength is ≥60 MPa. The flattening process makes the contact width W between the aluminum tube and the stainless steel substrate 0.3 to 1.1 times the original outer diameter D of the aluminum tube, which greatly increases the thermally conductive contact area and reduces the interfacial thermal resistance.

[0033] The aluminum tube (2) is connected to an inlet connector (14) and an outlet connector (15) at both ends. The heated liquid (such as tap water) enters the aluminum tube (2) through the inlet connector (14), is heated in the serpentine flow channel, and then flows out through the outlet connector (15).

[0034] Reference Figure 1 and Figure 2 The control circuit (13) includes the following main modules: (1) Microcontroller (MCU): responsible for acquiring temperature signals, executing PID algorithms, controlling power output, and interacting with external communication module (28). The MCU acquires the signal from temperature sensor (8) with a period of 500ms, and calculates the control quantity after passing the moving average filter.

[0035] (2) Power execution unit: Solid-state relays (SSRs) or triacs (TRIACs) are used to perform PWM or phase control on the thick film resistor heating layer (4) to achieve continuous power regulation within the range of 0% to 100%. The rated current of the SSR should not be less than 1.5 times the rated operating current.

[0036] (3) Temperature sensor (8): NTC thermistor, resistance 10kΩ at 25℃, B value 3950K, thermal time constant ≤5 seconds, attached to the outer surface of the water outlet connector (15).

[0037] (4) Overheat protection element (9): temperature fuse or thermal switch, connected in series in the main heating circuit, with an operating temperature of 100-150℃, independent of MCU control, and is a hardware protection that cannot be reset (or manually reset).

[0038] (5) Flow sensor (27, optional): installed at the inlet end of aluminum tube (2), with a range of 0.5 to 20 mL / s, used for dry burning detection and dynamic power adjustment.

[0039] (6) Communication module (28, optional): integrates Wi-Fi, Bluetooth BLE 5.0 or Zigbee, and supports remote temperature setting and status monitoring.

[0040] PID control is the core of this system's ±1℃ temperature control accuracy. The discretized form of the control algorithm is as follows: u(k) = Kp × e(k) + Ki × Σe(i) + Kd × [e(k) - e(k-1)] Where: u(k) is the current control quantity (PWM duty cycle, range 0 to 100%); e(k) is the current error (target temperature - measured temperature); Kp is the proportional coefficient (typical value 0.5 to 2.0); Ki = Kp / Ti is the integral coefficient; Kd = Kp × Td is the derivative coefficient; Ti is the integral time (typical value 10 to 30 s); Td is the derivative time (typical value 1 to 5 s).

[0041] To prevent integral saturation, integral accumulation stops when the control quantity u(k) reaches its upper limit (100%) or lower limit (0%). The PID parameters are fixed in the MCU program after experimental tuning, or they can be adjusted online via the communication interface.

[0042] This embodiment applies to instant hot water dispensers with a rated power of 1800W and a target water temperature of 95℃.

[0043] Heating substrate ( Figure 4 ): 304 stainless steel substrate, size 150×80mm, thickness 1.0mm.

[0044] First surface (front): RuO2-based thick film resistance heating layer (4) is prepared by screen printing process. The insulating dielectric layer (41) is made of borosilicate glass paste, sintered at 820℃, and has a thickness of 40μm. The resistance layer (42) is made of RuO2-based conductive paste, with a resistivity of 1.2Ω / □ after sintering, a total resistance of about 26Ω, and a rated power of 1800W (220V AC). The cover layer (43) is a glaze protective layer with a thickness of 20μm.

[0045] Second surface (back side): 5052 aluminum alloy tube (outer diameter 8mm, wall thickness 0.8mm), flattened (flattening height H=4mm, contact width W=8.8mm) and arranged in 6 rows in a serpentine pattern with a row spacing of 12mm. It is brazed with Al-12Si solder in a mesh belt nitrogen protective atmosphere furnace (brazing temperature 595℃, holding time 8 minutes, see APP-002 application for details). The shear strength of the brazed joint is 64MPa, the Al-Si eutectic layer thickness is about 130μm, and the Al-Fe-Si transition layer thickness is about 9μm.

[0046] Control circuit (13): STM32F030 MCU + SSR solid-state relay (rated current 25A) + NTC temperature sensor (8) (10kΩ, B=3950K, attached to the outer surface of the water outlet connector) + temperature fuse (9) (operating temperature 130℃) + Wi-Fi communication module (28).

[0047] PID parameters: Kp=1.2, Ti=15s, Td=3s.

[0048] Performance verification: With a rated power of 1800W, it can heat tap water from 25℃ to 95℃ at a water flow rate of 5.7mL / s, with a liquid temperature rise rate of approximately 81℃ / s and a thermal efficiency of 92.3%; the temperature control accuracy is ±0.8℃; after 5000 hours of continuous operation, the brazed joint showed no abnormalities in appearance, the shear strength remained at 62MPa, and the performance did not degrade.

[0049] This embodiment is applicable to industrial precision temperature-controlled liquid circulation systems with a rated power of 5000W and a target temperature of 60℃ (precision control).

[0050] Heating substrate: 316L stainless steel substrate, dimensions 300×200mm, thickness 1.5mm. First surface: SiC-based thick film heating layer, surface resistivity 0.8Ω / □, insulating dielectric layer sintering temperature 800℃, thickness 55μm. Second surface: 3003 aluminum alloy tube (outer diameter 12mm, wall thickness 1.2mm, 6 parallel lines, row spacing 25mm), brazed with Al-12Si solder, joint shear strength 67MPa.

[0051] Control circuit: Industrial-grade ARM Cortex-M4 MCU + bidirectional thyristor (TRIAC, 50A) + platinum resistance PT100 temperature sensor (accuracy ±0.1℃) + multi-level overheat protection (hardware fuse operating temperature 120℃ + software protection dual threshold) + RS-485 communication interface.

[0052] PID parameters: Kp=0.8, Ti=25s, Td=5s (self-tuned via relay method).

[0053] Performance verification: Rated power 5000W, temperature control accuracy ±0.3℃, response time 2 seconds, thermal efficiency 94.1%, no performance degradation after 6000 hours of continuous operation, meeting the requirements of industrial precision liquid temperature control.

[0054] This embodiment is applicable to commercial semi-automatic coffee machines with a rated power of 2500W and dual-channel control for target water temperatures of 92℃ (extraction water) and 125℃ (steam water).

[0055] Heating substrate: 316L stainless steel substrate, dimensions 200×120mm, thickness 1.2mm. First surface: RuO2-based thick film heating layer, surface resistivity 0.6Ω / □, total resistance approximately 19Ω (220V rated power 2500W). Second surface: 3003 aluminum alloy tube (outer diameter 10mm, wall thickness 1.0mm, 8 rows of serpentine wire), brazed with Al-12Si solder, joint shear strength 65MPa.

[0056] Control circuit: STM32F103 MCU + dual SSR solid-state relays + dual NTC temperature sensors (one each for extraction water circuit and steam water circuit) + dual temperature fuses (operating temperatures of 145℃ and 160℃ respectively) + Bluetooth BLE 5.0 communication module.

[0057] Performance verification: The temperature control accuracy of the extraction water circuit is ±0.5℃, the temperature control accuracy of the steam water circuit is ±1.0℃, and the system thermal efficiency is 91.8%, meeting the stringent requirements for water temperature stability in specialty coffee extraction.

[0058] Without departing from the spirit of this invention, those skilled in the art can make various modifications and combinations to the above embodiments, all of which fall within the protection scope of this invention.

Claims

1. A liquid heating system, characterized in that, include: A substrate (1) has a thick film resistance heating layer (4) on its first surface and a liquid flow channel (2) fixedly connected to its second surface. At least one section of the liquid flow channel (2) is flattened into a flat cross section, and the flat cross section is metallurgically bonded to the second surface of the substrate (1) through a brazing layer (3). A control circuit (13) is electrically connected to the thick film resistance heating layer (4) and is used to control the heating power of the thick film resistance heating layer (4). A temperature sensor (8) is used to collect the temperature signal of the liquid flow channel (2) or the substrate (1) and output it to the control circuit (13). An overheat protection element (9) is connected in series in the heating circuit and cuts off the heating circuit when the temperature exceeds a set threshold. The thick film resistance heating layer (4) and the liquid flow channel (2) are placed on opposite sides of the substrate (1), and heat is conducted from the first surface of the substrate (1) to the liquid in the liquid flow channel (2) on the second surface.

2. The liquid heating system according to claim 1, characterized in that: The microstructure of the brazing layer (3) from the aluminum tube (2) side to the stainless steel substrate (1) side includes an Al-Si eutectic layer with a thickness of 80-200 μm and an Al-Fe-Si intermetallic compound transition layer with a thickness of 5-15 μm, wherein the main phase of the transition layer is Al. 13 Fe4, with Al3Fe as the secondary phase, and the Fe content shows a continuous gradient distribution from the aluminum tube side to the stainless steel side.

3. The liquid heating system according to claim 1, characterized in that: The thick film resistance heating layer (4) is formed on the first surface of the stainless steel substrate (1) by screen printing and sintering process, and the surface resistivity is 0.05~5Ω / □; the thick film resistance heating layer (4) includes an insulating dielectric layer (41), a resistance layer (42) and a cover layer (43) from bottom to top; the functional material of the resistance layer (42) is RuO2-based paste or SiC-based paste; the material of the aluminum tube (2) is one of 1060 pure aluminum, 3003 aluminum alloy or 5052 aluminum alloy; the material of the stainless steel substrate (1) is 304 or 316L stainless steel, and the thickness is 0.3~5.0mm.

4. The liquid heating system according to claim 1, characterized in that: The control circuit (13) includes a microcontroller (MCU), a solid-state relay (SSR) or a bidirectional thyristor (TRIAC). The microcontroller adjusts the heating power of the thick film resistor heating layer (4) based on the real-time temperature signal from the temperature sensor (8) through pulse width modulation (PWM) or phase control to achieve constant temperature control. The temperature control accuracy is ±1℃ and the response time is ≤5 seconds.

5. The liquid heating system according to claim 1, characterized in that: The overheat protection element (9) is a temperature fuse or thermal switch with an operating temperature of 100-150°C, and is installed on a stainless steel substrate (1) near the heating core area. The control circuit (13) also has a software overheat protection function. When the temperature sensor (8) detects that the temperature exceeds the first threshold, the power is reduced to 30% and the heating circuit is cut off when the temperature exceeds the second threshold.

6. The liquid heating system according to claim 1, characterized in that: The system also includes a dry-burn protection device, which detects the dry-burning state by monitoring the water flow rate at the outlet of the aluminum pipe (2) or the heating rate of the stainless steel substrate (1). When the heating rate exceeds 15℃ / s and the duration exceeds 2 seconds, it is determined to be dry-burning, and the control circuit (13) cuts off the heating circuit within 3 seconds.

7. The liquid heating system according to claim 1, characterized in that: It also includes a heat insulation layer (5), which is disposed on the first surface side of the stainless steel substrate (1), with a thermal conductivity of ≤0.05W / (m·K) and a thickness of 5 to 20mm; and a waterproof sealing shell (10), which encapsulates the stainless steel substrate (1), the thick film resistance heating layer (4) and the temperature controller (7) therein, with a protection level of not less than IPX4.

8. The liquid heating system according to claim 3, characterized in that: The main component of the insulating dielectric layer (41) is borosilicate glass, with a sintering temperature of 750-850℃, a thickness of 20-60μm, a dielectric strength ≥15kV / mm, and an insulation resistance ≥100MΩ (500VDC test conditions); the covering layer (43) is a glaze protective layer with a thickness of 10-30μm, which has waterproof and moisture-proof functions.

9. The liquid heating system according to claim 1, characterized in that: The contact area between the flat cross-section width W of the aluminum tube (2) in the flattened area and the stainless steel substrate (1) satisfies the following: the contact width W is 0.3 to 1.1 times the original outer diameter D of the aluminum tube, the flattening height H is 0.3 to 0.8 times the original outer diameter D of the aluminum tube, and the spacing between adjacent aluminum tubes is 8 to 30 mm.

10. The liquid heating system according to claim 1, characterized in that: The temperature sensor (8) is an NTC thermistor with a resistance of 10kΩ (25℃) and a thermal time constant of ≤5 seconds, and is attached to the surface of the water outlet connector (15); the control circuit (13) collects the temperature signal with a sampling period of 500ms and uses a moving average filtering algorithm to eliminate noise interference.

11. The liquid heating system according to claim 4, characterized in that: The microcontroller (MCU) implements a PID closed-loop control algorithm with a proportional coefficient Kp of 0.5 to 2.0, an integral time Ti of 10 to 30 s, and a derivative time Td of 1 to 5 s. The control circuit (13) also integrates an energy metering module to count the cumulative power consumption in real time. When the cold resistance of the thick film resistor heating layer (4) is detected to deviate from the rated value by ±20%, a fault alarm is triggered.

12. The liquid heating system according to claim 1, characterized in that: The system also includes a flow sensor installed at the inlet of the aluminum tube (2), with a range of 0.5 to 20 mL / s and a resolution of ≤0.1 mL / s; the control circuit (13) dynamically adjusts the heating power according to the real-time signal of the flow sensor to maintain a constant outlet water temperature, with a power adjustment response time of ≤1 second.

13. The liquid heating system according to claim 1, characterized in that: The aluminum tube (2) is connected to an inlet connector (14) at the inlet end and an outlet connector (15) at the outlet end. Both the inlet connector (14) and the outlet connector (15) are made of food-grade materials. The inner wall of the aluminum tube (2) is anodized, and the oxide layer thickness is 10-50 μm.

14. The liquid heating system according to claim 1, characterized in that: The system also has a communication interface, selected from at least one of Wi-Fi, Bluetooth BLE 5.0 or Zigbee, for communicating with an external control terminal to realize remote temperature setting, operation status monitoring and fault diagnosis; the control circuit (13) stores at least 7 days of operation logs, including temperature curves, heating power and fault records.

15. The liquid heating system according to claim 1, characterized in that: The liquid heating system supports series or parallel expansion through modular design. Multiple liquid heating units are uniformly scheduled through the main controller, and the total rated power of the system can be expanded to 30kW. The heating units communicate with each other through RS-485 bus, and the main controller realizes load balancing control, with the power deviation of each unit ≤5%.

16. A control method for a liquid heating system, characterized in that, The liquid heating system described in claim 1 includes the following steps: S1, power-on self-test: Detects whether the temperature sensor (8) is normal, detects whether the cold resistance of the thick film resistance heating layer (4) is within ±20% of the rated range, and detects whether the overheat protection element (9) has not activated; if any abnormality occurs, it enters the fault protection state and reports the fault code through the indicator light or communication interface; S2, initial heating: Full power heating, the temperature sensor (8) collects the temperature in real time (sampling period 500ms), and monitors the heating rate to determine the dry burning state; S3, constant temperature control When the detected temperature reaches the target temperature of -5℃, switch to PID closed-loop control, adjust the PWM duty cycle to maintain the target temperature, and control accuracy ±1℃; S4, Safety monitoring: Monitor the temperature throughout the process; if it exceeds the first threshold (target temperature +20℃), reduce the power to 30%; if it exceeds the second threshold (target temperature +40℃), cut off the heating; if dry burning is detected, cut off the heating circuit within 3 seconds; S5, Normal shutdown: After shutdown, maintain residual heat utilization until the substrate temperature is below 50℃, then shut down completely, and record the cumulative power consumption and highest temperature of this operation.

17. The control method according to claim 16, characterized in that: The S2 step also includes a flow detection sub-step: before powering on for heating, the control circuit (13) first detects the output of the flow sensor. If the flow rate is lower than 0.3 mL / s, it is determined to be in a waterless state, heating is prohibited and an alarm is issued. The target temperature of the S3 step can be set in the range of 40℃~98℃ through the human-machine interface or communication interface, in 1℃ increments.

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