Fused salt multi-band electromagnetic induction heater and heating method

Abstract

The invention discloses a fused salt multi-band electromagnetic induction heater and a heating method, and belongs to the technical field of high-temperature fused salt energy storage systems and electromagnetic induction heating. The electromagnetic induction heater comprises a fused salt inlet flange, a fused salt heating pipeline and a fused salt outlet flange which are fixedly connected in sequence; a spiral flow guide fin is arranged in the heating pipeline, the outer wall of the molten salt heating pipeline is coated with an insulation heat preservation layer, and an induction coil is arranged outside the insulation heat preservation layer; the induction coil comprises a plurality of sections of independently-driven spiral coils which are arranged at intervals in the axial direction. The problems that heat energy is wasted and temperature control is not accurate in a traditional heating mode are solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high-temperature molten salt energy storage systems and electromagnetic induction heating, and particularly relates to a molten salt multi-frequency electromagnetic induction heater and a heating method. BACKGROUND

[0002] Current energy storage technology plays a key role in promoting renewable energy utilization and energy structure transformation. As a high-efficiency, safe and environmentally friendly large-scale high-temperature heat storage technology, molten salt energy storage occupies an important position in the field of energy conversion and storage. Based on the good thermal properties of molten salt, molten salt is used as a heat transfer and storage medium to store and release heat energy, and is widely used in peak load regulation and frequency regulation of thermal power plants, solar thermal systems, and new energy fields such as green electricity consumption.

[0003] Current electromagnetic induction heaters mainly generate a magnetic field around the induction coil by applying an electric field to the induction coil wound around the heating pipe, thereby exciting an induced current in the metal pipe. The metal pipe is heated by the generated Joule heat, and the heat is transferred to the molten salt inside the pipe, so that the outlet temperature of the molten salt is increased.

[0004] The existing electromagnetic heating device is driven by a single frequency band, which cannot adapt to the changes in electrical conductivity, thermal conductivity and viscosity of molten salt under multiple working conditions, resulting in unbalanced skin effect distribution, high-frequency heat accumulation, low-frequency cold area residue, and high energy consumption.

[0005] Therefore, a new heating scheme is needed to solve these problems and improve the safety and efficiency of the system. SUMMARY

[0006] The application provides a molten salt multi-frequency electromagnetic induction heater and a heating method, which avoids the problems of heat waste and inaccurate temperature control of traditional heating methods.

[0007] In order to achieve the above-mentioned purpose, the application adopts the following technical scheme:

[0008] In a first aspect, the application provides a molten salt multi-frequency electromagnetic induction heater, which comprises a molten salt inlet flange, a molten salt heating pipe and a molten salt outlet flange connected in sequence. The molten salt heating pipe is provided with spiral flow guide fins inside, and the outer wall of the molten salt heating pipe is covered with an insulating heat preservation layer. An induction coil is arranged outside the insulating heat preservation layer. The induction coil comprises multiple segments of independently driven spiral coils arranged along the axial direction.

[0009] Further, the number of turns of the spiral coil is calculated by the following formula:

[0010]

[0011] Wherein,

[0012] wherein, N i is the number of turns of the i-th segment coil, respectively representing the number of the inlet segment coil, the middle segment coil and the outlet segment coil; k i is the power weight of the i-th segment coil; P total is the total heating power of the system; δ i is the penetration depth of the i-th segment coil; η is the comprehensive efficiency; I rmsi is the effective value of the i-th segment coil current; μ0 is the vacuum permeability; f i is the design value of the working frequency of the i-th segment coil; σ is the electrical conductivity of the tube material of the molten salt heating pipe; L i is the axial length of the i-th segment coil, which is a measured value; Γ end is the end compensation coefficient; ρ is the electrical resistivity of the tube material of the molten salt heating pipe; μ r is the relative permeability of the tube material of the molten salt heating pipe;

[0013] After calculating the number of turns of each segment coil, the middle segment coil is strengthened by the following formula:

[0014]

[0015] wherein, N2′ is the number of turns of the second segment coil after strengthening, f h is the high frequency setting value, f l is the low frequency setting value, and β is the power amplification ratio;

[0016] The constraint conditions are:

[0017] Further, the pitch of the spiral coil is determined by the following formula and constraint conditions:

[0018]

[0019] wherein, Pi is the pitch of the i-th segment coil; D is the outer diameter of the molten salt heating pipe in meters; ψ is the frequency response factor; λ is the end strengthening coefficient; f ref is the reference base frequency; f i is the design value of the working frequency of the i-th segment coil; v salt is the flow rate of the molten salt;

[0020] The constraint conditions include:

[0021] 1) Axial distance range constraint: 0.8D ≤ P i ≤ 1.2D;

[0022] 2) Eddy current suppression constraint:

[0023] 3) Power distribution constraint: and k3 = k1.

[0024] Further, the turns spacing gradient of the multi-section independently driven coil decreases, and the switchable working frequency of the multi-section independently driven coil is 1 kHz to 5 kHz or 10 kHz to 50 kHz.

[0025] Further, the induction coil comprises three-section independently driven spiral coils, the spacing between the coils is 0.8 to 1.2 times of the diameter of the molten salt heating pipe, and the power distribution ratio is (1:3:1) to (1:6:1) for the inlet section, the middle section and the outlet section.

[0026] Further, the cross section of the spiral flow guide fin is trapezoidal, and the width of the side facing the center of the pipe is greater than the side facing the pipe wall.

[0027] Further, the spiral flow guide fin and the pipe wall of the molten salt heating pipe are integrated.

[0028] Further, the insulation and heat preservation layer is fixed to the outer wall of the molten salt heating pipe by means of staggered stitching or prefabricated pipe sleeve.

[0029] In a second aspect, the application provides a molten salt heating method of the molten salt multi-frequency electromagnetic induction heater.

[0030] S1, obtaining the temperature difference between the current molten salt temperature and the target temperature;

[0031] S2, selecting a heating control mode according to the temperature difference:

[0032] If the temperature difference is greater than or equal to the first threshold value: apply low-frequency current to the multi-section heating coil, and use segmented excitation timing control;

[0033] If the temperature difference is less than the first threshold value and greater than the second threshold value, execute the flow rate adaptive mode:

[0034] (a) obtaining the real-time flow rate of the molten salt;

[0035] (b) dynamically distributing the current amplitude and frequency of each section coil according to the interval to which the real-time flow rate of the molten salt belongs:

[0036] When the real-time flow rate of the molten salt is less than the set first flow rate, the current frequency of the middle section coil is less than the current frequency of the outlet section coil and the current frequency of the inlet section coil;

[0037] When the real-time flow rate of the molten salt is greater than the set first flow rate and less than or equal to the set second flow rate, the current frequency of the outlet section coil and the inlet section coil is reduced, and the current frequency of the middle section coil is increased;

[0038] When the real-time flow rate of the molten salt is greater than the set second flow rate, the current frequency of the inlet section coil, the middle section coil and the outlet section coil is increased in turn;

[0039] The current amplitude applied by each segment coil increases with the increase of the flow rate;

[0040] If the temperature difference is less than or equal to the second threshold value: reduce the current amplitude of each segment coil, and dynamically adjust the current frequency of each segment coil based on the current frequency of the previous control mode and the temperature difference;

[0041] S3, repeating S1-S2 until the molten salt temperature is stable at the target temperature.

[0042] Further, in step S2, when the molten salt flow rate is less than the set first flow rate, the outlet segment coil is excited for a longer time than the inlet segment coil and the middle segment coil; the inlet segment coil and the outlet segment coil are heated with a current of 10-50 kHz; the middle segment coil is heated with a current of 1-5 kHz;

[0043] When the molten salt flow rate is greater than the set first flow rate and less than or equal to the set second flow rate, the current amplitude of each segment is adjusted according to the real-time heat load, the inlet segment coil and the outlet segment coil are heated with a current of 5-20 kHz, and the middle segment coil is heated with a current of 10-20 kHz;

[0044] When the molten salt flow rate is greater than the set second flow rate, the current of each segment coil increases linearly, the inlet segment coil is heated with a current of 5-15 kHz, the middle segment coil is heated with a current of 15-35 kHz, and the outlet segment coil is heated with a high-frequency current of 35-50 kHz.

[0045] Compared with the prior art, the present application has at least the following beneficial technical effects:

[0046] The present application comprises a molten salt heating pipe and an induction coil, which can heat the molten salt by using the Faraday electromagnetic induction principle, solving the problems of low heating efficiency, inaccurate temperature control and waste of heat energy in traditional heating methods. Electromagnetic induction heating can directly heat the molten salt heating pipe through eddy current effect, and then heat the molten salt through the pipe, avoiding the traditional heat conduction method, reducing heat loss and improving overall heating efficiency. At the same time, the setting of multiple independent induction coils can effectively suppress the skin effect and promote the efficient and uniform heating of the molten salt. At the same time, the present application uses the spiral flow guide fins added inside the molten salt heating pipe to increase the flow and heat exchange inside the pipe, promoting uniform heating of the molten salt. In addition, through the insulating and heat insulating layer between the induction coil and the molten salt heating pipe, the insulating and heat insulating layer can effectively prevent heat leakage, and also effectively prevent the short circuit problem caused by the energized coil, ensuring the safe operation and service life of the electromagnetic induction heater.

[0047] Further, the application adopts multi-section induction coils to arrange coils with different pitches and turns for different temperature characteristics and heating requirements of the inlet section, the middle section and the outlet section, to change and adjust the temperature and the heating depth of the pipe section by adjusting the frequency and the current voltage of the coil of the corresponding section, so that the temperature is accurately controlled, and the heating effect and efficiency are significantly improved.

[0048] In addition, the different temperature zones of the pipeline can be adjusted by the size, spacing and power distribution of the outer three-section coil, temperature self-balancing is realized, dynamic algorithms are saved, the system response time is shortened, the precision control of the outlet temperature of the molten salt is realized, and the stability and efficiency of the system operation are ensured.

[0049] Further, the cross section of the spiral flow guide fin is a trapezoidal cross section with a narrow outside and a wide inside, which effectively strengthens the heat exchange of the medium flow in the pipeline and promotes the uniformity and stability of the molten salt temperature.

[0050] Further, the spiral flow guide fin and the pipe wall of the molten salt heating pipeline are integrated, the contact thermal resistance between the fin and the pipe wall is eliminated, the molten salt turbulent heat exchange efficiency is maximized, and the problems of gap corrosion, thermal fatigue cracking and loose falling caused by welding or mechanical connection are avoided, and the structural reliability is significantly improved.

[0051] Further, the insulating layer is made of glass material, which does not melt, shrink or deform at high temperature, has stable heat preservation performance, and avoids the risk of heat loss or over-temperature of the outer wall caused by failure of the insulating layer.

[0052] The heating method provided by the application generates eddy current heating in the molten salt heating pipeline by applying current voltage to the three-section independently variable frequency electromagnetic induction coil, and transmits the heat to the internal spiral flow channel to realize the temperature rise of the molten salt. The method can dynamically adjust the excitation frequency, output power and heating time of each section of the coil according to the real-time flow rate or temperature requirement of the molten salt, so as to realize precise temperature control, rapid response and high-efficiency and energy-saving heating effect, and significantly improve the heat transfer efficiency and operation stability of the system. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 is a whole structure schematic diagram of a multi-frequency section spiral electromagnetic induction heater for heating molten salt provided by the application;

[0054] Figure 2 is an internal structure diagram of a multi-frequency section spiral electromagnetic induction heater for heating molten salt provided by the application;

[0055] Figure 3It is a kind of structure diagram of variable frequency coil for heating molten salt multi-frequency band spiral electromagnetic induction heater outer section provided by the present application;

[0056] Figure 4 It is an inlet and outlet flange diagram of the multi-frequency band spiral electromagnetic induction heater for heating molten salt provided by the present application;

[0057] Figure 5 It is a circumferential section view of the multi-frequency band spiral electromagnetic induction heater for heating molten salt provided by the present application;

[0058] Figure 6 It is a local enlarged view of the spiral fin provided by the present application;

[0059] Figure 7 It is a heating method flow chart provided by the present application.

[0060] In the drawings, 1 is a molten salt inlet flange; 2 is a molten salt inlet temperature measuring probe; 3 is a spiral flow guide fin; 4 is a molten salt heating pipeline; 5 is an insulation layer; 6 is an induction coil; 61 is an inlet section coil; 62 is a middle section coil; 63 is an outlet section coil; 7 is a molten salt outlet temperature measuring probe; 8 is a molten salt inlet mixing chamber orifice plate; 9 is a molten salt outlet mixing chamber orifice plate; 10 is a molten salt outlet flange; 11 is an inlet annular distributor; 12 is an outlet annular current collector; 13 is a liquid level probe; 14 is a flow meter. DETAILED DESCRIPTION

[0061] The present application will be described in detail below in conjunction with the drawings and specific embodiments.

[0062] In order to make the person in the art better understand the technical solutions in the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings of the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by the person in the art without creative labor should belong to the scope of protection of the present application.

[0063] It should be noted that when an element is referred to as being "on" another element, it can be directly on the other element or intervening elements can also be present. When an element is referred to as being "connected" to another element, it can be directly connected to the other element or intervening elements can also be present. The terms "on", "under", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", and the like as used herein refer to the orientation or position shown in the drawings, and are used only for convenience in describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore should not be construed as limiting the present application.

[0064] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0065] The molten salt heating generally adopts an electric heating tube heating mode, and mainly has the following problems: ① Electrical contact with molten salt: the traditional heating tube is easy to cause insulation aging under long-term high temperature of 400-600 DEG C and molten salt corrosion, thereby existing the risk of electric leakage, affecting safety. ② Low heating efficiency: the power limitation of the electric heating tube and the problem of watt density of the heating surface cause uneven heat distribution, and low thermal efficiency. ③ Easy to damage and difficult to maintain: the electric heating tube is a consumable, and the heating tube needs to be frequently replaced after long-time operation of the equipment, and the maintenance is difficult. The main electromagnetic induction heater design has complex pipe bundle structure processing, low thermal coupling degree of the preheating zone and the main heating zone, and significant temperature stratification; the coil magnetic field is unevenly distributed, causing local overheating of the molten salt.

[0066] The application provides a molten salt electromagnetic induction heater and a heating method thereof, which can reduce heat loss and improve service life, and solves the problems of low efficiency, inaccurate temperature control, short equipment life and safety hazards of the existing electric heater in high-temperature molten salt heating, so that safe, reliable, efficient and energy-saving heating of the molten salt is realized; specifically, spiral flow guiding and segmented induction collaborative design are adopted to realize the dual breakthroughs of magnetic-heat flow coupling strengthening and partition intelligent control, significantly improve the molten salt heating efficiency and safety, and achieve efficient and uniform heating.

[0067] Example 1

[0068] Reference Figures 1 to 5A molten salt multi-frequency electromagnetic induction heater, comprising a molten salt inlet flange 1, a molten salt inlet temperature measuring probe 2, a spiral flow guide fin 3, a molten salt heating pipeline 4, an insulation layer 5, an induction coil 6, a molten salt outlet temperature measuring probe 7, a molten salt inlet mixing chamber orifice plate 8, a molten salt outlet mixing chamber orifice plate 9, a molten salt outlet flange 10, an inlet annular distributor 11, an outlet annular current collector 12 and a liquid level probe 13.

[0069] The molten salt inlet flange 1 is connected to the molten salt inlet mixing orifice plate 8 by welding. The molten salt inlet mixing chamber orifice plate 8 is used to mix the molten salt uniformly. The tail end of the molten salt inlet mixing chamber orifice plate 8 is provided with an opening for setting the molten salt inlet temperature measuring probe 2. The tail end of the molten salt inlet mixing orifice plate 8 is fixedly connected to the head end of the inlet annular distributor 11 by welding. The tail end of the inlet annular distributor 11 is fixedly connected to the head end of the molten salt heating pipeline 4. The tail end of the molten salt heating pipeline 4 is fixedly connected to the head end of the outlet annular current collector 12 by welding. The tail end of the outlet annular current collector 12 is fixedly connected to the head end of the molten salt outlet mixing chamber orifice plate 9. The tail end of the molten salt outlet mixing chamber orifice plate 9 is fixedly connected to the molten salt outlet flange 10. The tail end of the molten salt outlet mixing chamber orifice plate 9 is provided with an opening for setting the molten salt outlet temperature measuring probe 7. The molten salt heating pipeline 4 serves as a conductor for the heating medium. The molten salt heating pipeline 4 is uniformly provided with the spiral flow guide fin 3. The outer wall of the molten salt heating pipeline 4 is covered with the insulation layer 5. The insulation layer 5 is provided with the induction coil 6.

[0070] The molten salt inlet temperature measuring probe 2 is a thermocouple, and the molten salt outlet temperature measuring probe 7 is a platinum resistance.

[0071] Referring to Figure 6 The cross section of the spiral flow guide fin 3 is a trapezoidal section with a narrow outer part and a wide inner part, which is used to strengthen the heat exchange inside the pipeline and ensure the uniformity and stability of the molten salt temperature. The top width of the fin cross section of the spiral flow guide fin 3 is 3 mm, the bottom width is 8 mm, the height is 10 mm, and the pitch is 68 mm. The fin height is 10% to 15% of the pipe diameter, and the surface is sprayed with a boron nitride anti-adhesion coating. The internal spiral trapezoidal fin 3 and the molten salt heating pipeline 4 are made of stainless steel 316 and are laser welded to form an integrated structure of the internal spiral flow guide fin 3 and the wall of the molten salt heating pipeline 4, forming a spiral fin heating pipe to ensure the stability of the structure when the molten salt flows.

[0072] The outer surface of the molten salt heating pipeline wall 4 is sandblasted and electroplated with a high-temperature resistant epoxy coating to maintain the stability of the steel material.

[0073] The insulation layer 5 is tightly wrapped around the outer wall of the molten salt pipeline 4. The insulation layer 5 is wrapped around the outer layer of the molten salt heating pipeline 4 using glass fiber staggered bandaging, or wrapped around the outer side of the molten salt heating pipeline 4 using a glass fiber tube and fixed with a metal mesh. At the same time, glass fiber serves as a good insulating material, providing electrical isolation and thermal insulation effect.

[0074] The molten salt inlet mixing type orifice plate 8 and the molten salt outlet mixing chamber orifice plate 9 are of the same structure, and the orifice diameter is selected The sintered silicon carbide porous plate with a porosity of 40% is used to uniformly converge the molten salt and reduce the flow resistance.

[0075] In order to ensure smooth flow of the molten salt, the electromagnetic induction coil 6 is wound outside the insulating and heat-insulating layer 5 outside the molten salt heating pipe 4. The present application separates the molten salt heating pipe and the electromagnetic induction coil with the insulating and heat-insulating layer 5, avoids direct contact between the electromagnetic induction coil and the pipe, reduces heat dissipation to the outside, and ensures safe operation. In order to improve stability and safety, the entire heater device is fixed to the bottom insulating support through bolts.

[0076] The induction coil 6 is a three-section independently driven spiral electromagnetic induction coil, including an inlet section coil 61, an intermediate section coil 62 and an outlet section coil 63 arranged in sequence, the inlet section coil 61, the intermediate section coil 62 and the outlet section coil 63 are distributed along the pipe axis with equal length, the turn spacing gradient of each section decreases, the inlet section coil 61, the intermediate section coil 62 and the outlet section coil 63 can switch the working frequency of 1-5 / 10-50 kHz, the induction coil 6 is wound outside the insulating and heat-insulating layer 5 and is fixed outside the glass fiber insulating and heat-insulating layer 5 through buckles. The three-section coil is connected with three groups of variable frequency power supply modules, and the frequency can be independently switched to 1-5 kHz and 10-50 kHz, realizing multi-frequency partition driving. An outer insulation shell is arranged outside the induction coil 6, covering the induction coil 6 and the insulating and heat-insulating layer 5, aerogel composite material is filled in the interlayer between the induction coil 6 and the outer insulation shell, a heat dissipation air duct and a fan assembly are integrated outside the outer insulation shell, the width of the heat dissipation air duct is 8 mm, which is used for forced air convection cooling. The heat dissipation air duct and the fan assembly form a cooling system.

[0077] The intermediate section coil 62 has stronger magnetic field penetration ability due to the application of low-frequency power supply, and has the highest heating performance, the three-section coil is stably installed outside the insulating and heat-insulating layer 5 through the buckle structure, forming a stable axial segmented arrangement. The spacing between the segmented coils is 0.8-1.2 times the pipe diameter, and the power distribution ratio is 1:3:1 to 1:6:1. The power difference is realized by the comprehensive setting of power parameters including voltage, current, frequency and coil structure parameters such as number of turns, wire diameter and spacing, so as to construct a multi-section induction heating system with axial controllability and adjustable heating distribution.

[0078] When the three-section independently driven electromagnetic induction heating coils apply alternating currents with different frequencies and amplitudes to different heating sections, eddy current heating is generated to realize segmented heating of the pipe, and the molten salt heating pipe 4 transfers heat to the molten salt inside to realize heating of the molten salt.

[0079] The induction coil is a solid spiral induction coil made of red copper, wound outside the insulation layer 5 according to different turns and axial distances, and adopts three-stage power supply, corresponding to the inlet section, the middle section and the outlet section induction heating area. Different frequencies of power supply such as high frequency 10-50 kHz and low frequency 1-5 kHz are applied to each section coil according to the heating requirement, for regulating the local electromagnetic field intensity, realizing the thermal field distribution adjustment, skin effect control and heating uniformity optimization.

[0080] Wherein, the turns and axial distances of the coil are determined according to the following principles:

[0081] The turns are electromagnetically designed according to the required heating power density per unit length, current size and magnetic flux requirement, and the axial distance is comprehensively optimized according to the magnetic field distribution uniformity, heating area length, eddy current superposition effect and coil thermal management requirement and other factors.

[0082] The turns of each section coil are calculated by the following formula:

[0083]

[0084] Wherein, N i is the number of turns of the i-th section coil, i=1, 2, 3, respectively representing the numbers of the inlet section coil, the middle section coil and the outlet section coil;

[0085] k i is the power weight of the i-th section coil, k1:k2:k3=1:β:1, β∈[3, 6]; k i The axial power gradient is controllable (β is flexibly adjustable);

[0086] P total is the total heating power of the system, equal to the molten salt flow rate x the specific heat capacity of the molten salt x the temperature rise;

[0087] δi is the penetration depth of the i-th section coil;

[0088] η is the comprehensive efficiency, including electromagnetic loss and heat loss, and the value is 0.65-0.75;

[0089] I rmsi is the effective value of the i-th section coil current, set by the power supply;

[0090] μ0 is the magnetic permeability of vacuum;

[0091] f i is the working frequency design value of the i-th section coil, and the middle section coil is low frequency to enhance the penetration;

[0092] σ is the electrical conductivity of the molten salt heating pipe, and the electrical conductivity of 304 stainless steel is: 1.45×10 6 S / m;

[0093] Li L is the axial length of the i-th coil, measured value;

[0094] Γ end L is the axial length of the i-th coil, measured value;

[0095] ρ is the electrical resistivity of the tube material of the molten salt heating pipe;

[0096] μ r μ is the relative permeability of the tube material of the molten salt heating pipe;

[0097] After calculating the number of turns of each coil, the intermediate coil is reinforced by the following formula:

[0098]

[0099] where N2' is the number of turns of the reinforced intermediate coil, f h is the high-frequency set value, f l is the low-frequency set value, and β is the power amplification ratio;

[0100] Constraints:

[0101] Engineering significance: the low-frequency power supply (f l ) reduces the number of turns required, and the power density is inversely proportional to the increase;

[0102] The coil axial distance P i is determined by the following formula and constraints:

[0103]

[0104] where P i is the axial distance (pitch) of the i-th coil, with a value range of 35.7-36.4 mm; D is the outer diameter of the molten salt heating pipe, which can be taken as 0.045 m, and is the design set value; ψ is the frequency response factor, which is dimensionless, and in the low-frequency range, it is taken as 0.9, and in the high-frequency range, it is taken as 1.1, which can adapt to frequency changes; λ is the end reinforcement coefficient, which is dimensionless, and the value of the inlet coil / outlet coil is 0.25, and the value of the intermediate coil is 0, which is used to enhance the end magnetic field strength; f ref is the reference frequency, which can be taken as 10000 Hz, and usually f ref = max (fi); f i is the design value of the working frequency of the i-th coil, which is optimized in coordination with the number of turns formula; v salt is the flow rate of the molten salt, which can be taken as 0.3, and is the design working condition value.

[0105] Constraints include:

[0106] 1) Axial distance range constraint, the axial distance range constraint is used to ensure coil winding feasibility and cooling space, and is specifically as follows:

[0107] 0.8D≤P i ≤1.2D

[0108] 2) Eddy current suppression constraint, the eddy current suppression constraint can avoid flow molten salt causing eddy current phase cancellation, and the eddy current suppression constraint formula is:

[0109] 3) Power distribution constraint, the power distribution constraint is used to ensure that the intermediate section is dominated by heating:

[0110] And k3=k1.

[0111] In order to realize real-time monitoring of the molten salt level, a liquid level probe 13 is arranged in the inlet mixing chamber orifice plate 8, which is used to detect the liquid level of the molten salt in the pipeline to prevent dry burning, empty load and other safety hazards. The liquid level probe 13 can be a float ball type, a capacitive type or a conductive electrode type, and its signal is connected to the control system to realize automatic protection, alarm or output power limitation adjustment when the molten salt level exceeds the limit.

[0112] When only a single-point flow rate monitoring is arranged in the high-temperature molten salt system, the measurement accuracy, anti-interference and maintenance convenience three core requirements need to be met at the same time. Therefore, the flow meter 14 is arranged downstream of the middle section of the heating zone; the flow meter 14 adopts an ultrasonic time difference method flow meter, which is suitable for high-temperature molten salt (400-550℃) strong corrosion and strong electromagnetic interference working conditions.

[0113] The downstream of the middle section of the heating zone has the following advantages:

[0114] 1) The fluid reaches a stable turbulent flow state (Reynolds number Re>10000) after being heated by the intermediate section, and the velocity distribution is more uniform;

[0115] 2) The distance from the outlet section is more than 5D, which avoids high-frequency magnetic field interference;

[0116] 3) The molten salt temperature at this place is usually 20-30℃ lower than that at the outlet, which is beneficial to prolong the service life of the flow meter.

[0117] The specific position of the flow meter 14 is determined by the following formula:

[0118] L=L coil62 +3D+0.5m

[0119] Wherein, L is the installation position of the liquid level probe, L coil62 is the coordinate of the end of the intermediate section, and D is the inner diameter of the pipeline; for example, if D=0.2m, the installation point is 3x0.2+0.5=1.1m away from the end of the intermediate section.

[0120] At the beginning of the heating process, low-power low-frequency alternating current 1-5 kHz is applied to the three-section induction coil 6 to preheat the molten salt heating pipe 4, ensuring that the pipe temperature is suitable for molten salt flow. When the molten salt inlet temperature probe 2 installed at the position of the molten salt inlet flange 1 detects that the molten salt heating pipe 4 has reached the preset preheating temperature, the low-temperature molten salt passes through the molten salt inlet flange 1, is uniformly mixed by the molten salt inlet mixing chamber orifice plate 8, and then flows into the molten salt heating pipe 4 along the flow channel formed by the spiral guide fins 3 until the liquid level probe detects that the liquid level reaches the set point. As the power of the power supply gradually increases, the eddy current generated by the electromagnetic induction coil 6 increases, thereby inducing a large amount of heat energy in the molten salt heating pipe 4. At this time, the insulating and heat-insulating layer 5 is wrapped around the outer wall of the pipe, effectively reducing heat loss and improving heating efficiency. The molten salt flows axially in the pipe, and the heat is transferred to the salt body through the heat-conducting wall, achieving efficient heat transfer. At the same time, the heat generated by electromagnetic induction is transmitted through the molten salt heating pipe 4, and the molten salt is uniformly heated in the pipe. The device uses three independently arranged distributed induction coils, namely the inlet section coil 61, the middle section coil 62, and the outlet section coil 63. The three coils are independently powered and applied with different frequency currents to match the heating requirements. The inlet section coil 61 and the outlet section coil 63 apply high-frequency current 10-50 kHz to strengthen the rapid heating near the wall surface; the middle section coil 62 applies low-frequency current 1-5 kHz to enhance the electromagnetic field penetration depth, thereby improving the heating uniformity of the center of the molten salt volume. The three coils can be excited with different frequencies in parallel, i.e., different frequencies are applied in the same time period; or they can be selected to be periodically alternated or superimposed with a composite frequency, forming a spatial and temporal multi-frequency coordination. The key of this multi-frequency excitation strategy is to overcome the problem of "skin effect" in high-frequency induction heating, which causes energy to concentrate on the surface of the wall. The skin effect determines that the distribution depth of the induced current in the conductor becomes shallower as the frequency increases. By increasing the magnetic field penetration with low-frequency current, it helps to conduct heat to the inside of the pipe, complementing the high-frequency rapid heating, and ultimately achieving a more uniform temperature distribution in the radial direction, thereby improving the overall heating efficiency and stability of the molten salt.

[0121] The present application greatly improves the safety and reliability of the system. The power supply of the present application is completely isolated from the heating medium, avoiding direct contact and reducing safety risks. The heating device has a simple and compact structure, reducing the volume of the equipment and saving space. Due to the high efficiency of the electromagnetic induction heating system, the service life of the equipment is prolonged and the maintenance requirements are reduced. Therefore, the present application not only improves the overall system efficiency, but also greatly reduces the maintenance cost, and is suitable for high-efficiency heating requirements in modern photothermal power generation and other fields.

[0122] The above-mentioned molten salt multi-frequency electromagnetic induction heater is assembled by the following method:

[0123] Step 1: Molten salt heating pipe pretreatment and inner fin welding

[0124] Put the molten salt heating pipe 4 on the roller stand, and remove the oxide on the inner wall by laser cleaning with a laser power of 2 kW and a speed of 0.3 m / min.

[0125] Use a numerical control spiral milling machine to process a trapezoidal groove on the inner wall of the molten salt heating pipe 4, with a groove depth of 4.5 mm, a pitch of 68 mm, and a groove surface roughness Ra≤6.3 μm.

[0126] Customize a fin strip made of stainless steel 316 material compatible with the base pipe and insert it into the trapezoidal groove, and use high-frequency induction brazing flux: BAg-8, temperature 720℃, pressure 0.6 MPa x 15s; Use an endoscope to detect the quality of the weld, and require 100% penetration flaw detection qualification.

[0127] Install a liquid level probe vertically at the centerline position of the pipe top between the molten salt inlet flange 1 and the inlet mixing chamber orifice plate 8, fixed through a G1 / 2 threaded interface, and the liquid level probe extends to a depth of 1 / 3 of the pipe diameter, to monitor the molten salt liquid level in real time.

[0128] Step 2: Pipe exterior insulation layer coating

[0129] Wrap 5 layers of glass fiber blanket 10mm thick with a density of 128kg / m horizontally outside the heater pipe 3 , with a 10% offset for each layer to avoid thermal bridges; the outer layer is fastened with a 1.0mm stainless steel mesh 306L through a V-shaped clamp, with a V-shaped clamp spacing of 300mm and a tension of 250N; apply high-temperature silicone glue at the joint.

[0130] Step 3: Induction coil installation and segmented control

[0131] Use a numerical control winding machine to segmentally wind the copper bar along the pipe axis, winding three coils: the winding position of the inlet section coil is 0-0.5m of the molten salt heating 4: 11 turns, working frequency 5kHz; the winding position of the middle section coil is 0.5m-1.3m of the molten salt heating pipe 4: 27 turns, working frequency 10kHz; the winding position of the outlet section coil is 1.3m-1.8m of the molten salt heating pipe 4: 12 turns, working frequency 5kHz.

[0132] Coat the copper bar with ceramicized silicone rubber, with a thickness of 0.5mm and a breakdown voltage ≥15kV; cover the outer layer of the three coils with glass fiber tape resistant to 500℃, and the insulation shell is located outside the coil and glass fiber tape, used to reduce heat loss; install an air cooling channel on the outermost layer of the insulation shell, with a gap of 8mm, used for effective heat dissipation and temperature control.

[0133] Weld ultrasonic flowmeter saddle at 3D+0.5m downstream of the end of the middle section coil.

[0134] Step 4: Integration of mixing chamber orifice plate and temperature probe First, weld or flange-connect the integrated molten salt inlet flange 1 with short tube section, inlet annular distributor 11, and molten salt heating pipe 4, ensuring coaxiality and sealing performance. Then, install the molten salt inlet mixing chamber orifice plate 8 inside the short tube section. The orifice plate is fixed to the inside of the flange by bolt clamping, and the recommended bolt pre-tightening force is 45 N·m. To prevent high-temperature molten salt leakage, the contact surface between the orifice plate and the flange is coated with high-temperature resistant graphite-based sealing paste to enhance sealing effectiveness. The molten salt inlet temperature probe 2 is installed on the inner wall of the pipe about 100 mm downstream of the mixing chamber orifice plate 8, fixed with a threaded interface, and equipped with a ceramic sleeve for high-temperature electrical insulation and mechanical protection, ensuring real-time and accurate temperature measurement under molten salt flow conditions. Install the flowmeter at the flowmeter saddle (3D+0.5m from the end).

[0135] Install the outlet annular current collector 12 at the end of the molten salt heating pipe 4, and weld the molten salt outlet mixing chamber orifice plate 9 at the end of the outlet annular current collector 12, spaced 200 mm axially from the molten salt outlet flange 10, for uniformizing the outlet flow field and reducing local temperature difference fluctuations. The molten salt outlet temperature probe 7 is located on the inner wall of the molten salt heating pipe 4 outlet section 50 mm upstream of the molten salt outlet mixing chamber orifice plate 9, with an insertion depth of 1 / 3 of the molten salt heating pipe 4 inner diameter; to ensure sensitive temperature measurement response and accurate numerical value.

[0136] Example 2

[0137] Reference Figure 7 The present embodiment provides a multi-frequency electromagnetic induction heating method for high-temperature molten salt, suitable for molten salt flow heating systems, especially for complex working conditions requiring high power, precise temperature control, and multi-frequency collaborative regulation. This method combines the control of three independent induction coil inlet section, middle section, and outlet section with independent variable frequency power supply, dynamically adjusts the frequency, current, and excitation time of each section by real-time monitoring of molten salt temperature and flow rate, and realizes efficient, safe, and stable molten salt heating control. The method includes the following steps:

[0138] S1, measure the current molten salt temperature with the molten salt outlet temperature probe 7, and determine the size relationship between the target temperature and the current temperature:

[0139] If the current molten salt temperature is less than the target temperature, perform step S2;

[0140] If the current molten salt temperature is greater than the target temperature, perform step S3;

[0141] S2, dynamically adjust the frequency and current applied to the induction coil based on the temperature difference and flow rate to heat the molten salt;

[0142] S2.1, initial preheating stage

[0143] Continuously monitor the current molten salt temperature T current and calculate the temperature difference ΔT with the target temperature Ttarget:

[0144] ΔT = T target - Ttarget current

[0145] According to the temperature difference ΔT, automatically adjust the heating or cooling intensity;

[0146] When the molten salt temperature difference is greater than or equal to the set threshold, for example, ΔT≥30℃: all three sections of the induction coil are preheated at a low frequency of 5-10 kHz to increase the magnetic field penetration depth and achieve overall temperature rise of the molten salt. The three sections of the coil can be activated in turn or opened in the default order, and the current amplitude and activation time are set to high values, for example, the current amplitude applied to the inlet section coil 61 is 300-350A, the current amplitude applied to the middle section coil 62 is 550-600A, and the current amplitude applied to the outlet section coil 63 is 300-350A.

[0147] When the molten salt temperature difference is less than or equal to the threshold value: execute step S2.2;

[0148] S2.2, flow rate response adjustment stage

[0149] 1) When the molten salt temperature difference is 30℃>ΔT>20℃:

[0150] Real-time monitoring of molten salt flow, and calculating the molten salt flow rate v according to the molten salt flow, dynamically adjusting the excitation frequency band, current and time weight of the three sections of the coil according to the following strategy:

[0151] Based on the real-time flow rate of the molten salt, the coil section excitation time is dynamically adjusted, when the molten salt flow rate v salt ≤0.5m / s, the current is dynamically adjusted by the frequency conversion power supply according to the real-time heat demand, and the outlet section excitation time is increased by 50%(compensation for local heat transfer), for example, v salt =0.2m / s, the current inlet / outlet 735A, middle section 840A; the inlet section coil 61 and the outlet section coil 63 apply a current of 10-50 kHz for rapid heating near the wall surface; the middle section coil 62 applies a low-frequency current of 1-5 kHz to enhance the electromagnetic field penetration depth, thereby improving the heating uniformity of the center of the molten salt volume.

[0152] When the molten salt flow rate is 0.5m / s<v salt ≤1m / s, the segmented collaborative excitation and medium frequency compensation strategy is adopted. At this time, all three sections of the induction coil are involved in heating, and the frequency conversion power supply adjusts the current amplitude of each section according to the real-time heat load to ensure the overall temperature rise and the outlet temperature balance. Typical working conditions (such as vsalt = 0.8 m / s), the inlet section coil 61 and the outlet section coil 63 apply a medium-low frequency current of 5-20 kHz, and the current amplitude is appropriately increased according to the heat demand (inlet / outlet 1200 A / 1500 A) to intensify the wall surface area and the local heating at the outlet end; the intermediate section coil 62 uses a medium frequency current of 10-20 kHz, and the current amplitude is 1600-2400 A to enhance the penetration of the electromagnetic field and improve the heating uniformity of the central volume of the flow field.

[0153] When the molten salt flow rate v salt > 1 m / s, the full frequency range synchronous excitation is activated, and the current linearly increases with the power demand, for example, when v salt = 1.2 m / s, the current amplitudes of the inlet section coil, the intermediate section coil 62 and the outlet section coil 63 are 1920 A, 3600 A and 1200 A, respectively. The three section induction coils work at different frequencies at the same time: the inlet section coil 61 uses a low frequency of 5-15 kHz for deep preheating, the intermediate section coil 62 uses a medium frequency of 15-35 kHz for main heating, and the outlet section coil 63 uses a high frequency of 35-50 kHz for fine temperature control. The three groups of coils are driven by independent variable frequency power sources, and the frequency, current amplitude and excitation time can be adjusted respectively to realize the synergistic superposition of multi-frequency magnetic field and effectively improve the heating uniformity and efficiency.

[0154] 2) When the molten salt temperature difference ΔT satisfies 10℃ < ΔT≤20℃: the current amplitude applied to the inlet section coil 61 is 1200-1800 A, the current amplitude applied to the intermediate section coil 62 is 1000-1400 A, and the current amplitude applied to the outlet section coil 63 is 480-560 A; the applied frequency is calculated by the following formula:

[0155]

[0156] Wherein, i = 1, 2, 3, represents the number of the inlet section coil, the intermediate section coil and the outlet section coil, f i ′ represents the frequency applied to the i-th section coil, f base is the reference frequency; for example: ΔT = 10℃→f 入口 = 5000×(10 / 40) 0.6 ≈2.38 kHz.

[0157] 3) When the molten salt temperature difference ΔT≤10℃: the current amplitude applied to the inlet section coil 61 is 300-900 A, the current amplitude applied to the intermediate section coil 62 is 200-600 A, and the current amplitude applied to the outlet section coil 63 is 80-240 A; the frequency calculation formula is the same as that when the molten salt temperature difference ΔT satisfies 10℃ < ΔT≤20℃.

[0158] S3, starting the cooling system;

[0159] When the molten salt temperature exceeds the set safety upper limit or the heating is terminated, the cooling system is started. The system automatically turns on the fan to send air into the air cooling channel to take out the heat and prevent the equipment from overheating; the cooling system can use a segmented air control strategy as needed, only turning on the cooling air duct in a specific area to reduce energy consumption and improve response speed.

[0160] S4, judge whether the molten salt temperature is stable:

[0161] If the molten salt temperature continues to meet, the temperature fluctuation is not more than ±1℃, and the fluctuation is not more than the above range for 5 minutes continuously, it is considered that the temperature is stable, and the control process is ended;

[0162] If the temperature fluctuation exceeds ±1℃ during this process, the stability timer will be reset, the process jumps back to step S1, and if it is stable, the process is ended; otherwise, it jumps to step S1 and reenters the temperature control closed loop.

[0163] The term "consisting of" to describe a combination is intended to include the specified elements, ingredients, components, or steps and any other elements, ingredients, components, or steps that do not materially affect the basic and novel characteristics of the combination. The use of the term "comprising" or "including" to describe combinations of elements, ingredients, components, or steps herein is also intended to cover embodiments in which the use of "comprising" or "including", the combination of elements, ingredients, components, or steps is optional. Here, the use of the term "may" is intended to convey that "any" of the described attributes are optional.

[0164] Multiple elements, ingredients, components, or steps can be provided by a single integrated element, ingredient, component, or step. Alternatively, a single integrated element, ingredient, component, or step can be divided into separate multiple elements, ingredients, components, or steps. The disclosure of an element, ingredient, component, or step using the article "a" or "an" does not exclude the use of "more than one" of that element, ingredient, component, or step.

[0165] It is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments and many applications besides the examples provided herein will be apparent to those of skill in the art upon reading the above description. The scope of the technology should, therefore, be determined not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. The disclosures of all articles and references, including patent applications and publications, are incorporated by reference for the purpose of the present disclosure. The omission of any aspect of the subject matter disclosed herein does not, therefore, create a patentable prior art restriction against the recapture of that subject matter as a part of the application.

Claims

1. A molten salt multi-band electromagnetic induction heater characterized by, The molten salt inlet flange (1), the molten salt heating pipeline (4) and the molten salt outlet flange (10) are sequentially fixedly connected. The spiral flow guide fin (3) is arranged in the molten salt heating pipeline (4), the outer wall of the molten salt heating pipeline (4) is covered with the insulation layer (5), and the outer wall of the insulation layer (5) is provided with the induction coil (6). The induction coil (6) comprises multiple segments of independently driven spiral coils arranged at intervals along an axial direction.

2. A fused salt multi-frequency electromagnetic induction heater according to claim 1, wherein The number of turns of the spiral coil is calculated by the following formula: wherein Wherein, N i is the number of turns of the i-th segment coil, respectively representing the number of the inlet segment coil, the middle segment coil and the outlet segment coil; k i is the i-th segment coil power weight; P total is the total heating power of the system; δ i is the penetration depth of the i-th segment coil; η is the comprehensive efficiency; I rmsi is the effective value of the i-th segment coil current; μ0 is the vacuum permeability; f i is the design value of the working frequency of the i-th segment coil; σ is the electrical conductivity of the tube material of the molten salt heating pipeline; L i is the axial length of the i-th segment coil, which is a measured value; Γ end is the end compensation coefficient; ρ is the electrical resistivity of the tube material of the molten salt heating pipeline; μ r is the relative permeability of the tube material of the molten salt heating pipeline; After the number of turns of each segment of coil is calculated, the intermediate segment of coil is strengthened by the following formula: Wherein, N2' is the second segment of the coil after reinforcement number of turns, f h is the high frequency set value, f l is the low frequency set value, β is the power amplification ratio; Constraints:

3. The multi-band electromagnetic induction heater of claim 1 or 2, wherein, The pitch of the spiral coil is determined by the following formula and constraint condition: Wherein, Pi is the i-th segment coil pitch; D is the outer diameter of the molten salt heating pipe m; ψ is the frequency response factor; λ is the end reinforcement coefficient; f ref is the reference frequency; f i is the working frequency design value of the i-th segment coil; v salt is the molten salt flow rate; The constraint condition comprises: 1) Wheelbase range constraint: 0.8D < P i ≤ 1.2D; 2) Vortex suppression constraints: 3) power allocation constraints: and k3= k1.

4. The molten salt multi-band electromagnetic induction heater of claim 1, wherein, The turn spacing gradient of the multiple segments of independently driven coils decreases, and the switchable working frequency of the multiple segments of independently driven coils is 1 kHz-5 kHz or 10 kHz-50 kHz.

5. The molten salt multi-band electromagnetic induction heater of claim 1, wherein, The induction coil (6) comprises three segments of independently driven spiral coils, the interval between each segment of coil is 0.8-1.2 times the pipe diameter of the molten salt heating pipeline (4), and the power distribution ratio is inlet segment: intermediate segment: outlet segment = 1: (3-6):

1.

6. A fused salt multi-band electromagnetic induction heater according to claim 1, wherein, The cross section of the spiral flow guide fin (3) is trapezoidal, and the width of the side facing the center of the pipeline is greater than the side facing the wall.

7. The molten salt multi-band electromagnetic induction heater of claim 1, wherein, The spiral flow guide fin (3) and the wall of the molten salt heating pipeline (4) are in an integrated structure.

8. The molten salt multi-band electromagnetic induction heater of claim 1, wherein, The insulation layer (5) is fixed to the outer wall of the molten salt heating pipeline (4) by glass fiber through staggered wrapping or prefabricated pipe sleeve.

9. A molten salt heating method of a molten salt multi-band electromagnetic induction heater according to any one of claims 1 to 8, characterized by, The method comprises the following steps: S1, obtaining the temperature difference between the current molten salt temperature and the target temperature; S2, selecting a heating control mode according to the temperature difference: If the temperature difference is greater than or equal to the first threshold value: apply low-frequency current to the multiple segments of heating coils, and use segmented excitation timing control; If the temperature difference is less than the first threshold value and greater than the second threshold value, execute the flow rate self-adaptive mode: (a) obtaining the real-time flow rate of the molten salt; (b) dynamically distributing the current amplitude and frequency of each segment of coil according to the interval to which the real-time flow rate of the molten salt belongs: When the real-time flow rate of the molten salt is less than the set first flow rate, the current frequency of the intermediate segment of coil is less than the current frequency of the outlet segment of coil and the current frequency of the inlet segment of coil; When the real-time flow rate of the molten salt is greater than the set first flow rate and less than or equal to the set second flow rate, the current frequency of the outlet segment of coil and the current frequency of the inlet segment of coil are reduced, and the current frequency of the intermediate segment of coil is increased; When the real-time flow rate of the molten salt is greater than the set second flow rate, the current frequency of the inlet segment of coil, the intermediate segment of coil and the outlet segment of coil is increased in turn; The current amplitude applied to each segment of coil increases with the increase of the flow rate; If the temperature difference is less than or equal to the second threshold value: reduce the current amplitude of each segment of coil, and dynamically adjust the current frequency of each segment of coil based on the current frequency and the temperature difference of the previous control mode; S3, repeatedly execute S1-S2 until the temperature of the molten salt is stable at the target temperature.

10. A molten salt heating method according to claim 9, wherein In the step S2, When the flow rate of the molten salt is less than the set first flow rate, the excitation duration of the outlet segment of coil (63) is greater than that of the inlet segment of coil (61) and the intermediate segment of coil (62); the inlet segment of coil (61) and the outlet segment of coil (63) are heated by current with a frequency of 10-50 kHz; and the intermediate segment of coil (62) is heated by current with a frequency of 1-5 kHz. When the flow rate of the molten salt is greater than the first set flow rate and less than or equal to the second set flow rate, the current amplitude of each section is adjusted according to the real-time heat load, the inlet section coil (61) and the outlet section coil (63) are heated with a current of 5-20 kHz, and the middle section coil (62) is heated with a current of 10-20 kHz; When the flow rate of the molten salt is greater than the second set flow rate, the current of each section coil increases linearly, the inlet section coil (61) is heated with a current of 5-15 kHz, the middle section coil (62) is heated with a current of 15-35 kHz, and the outlet section coil (63) is heated with a high-frequency current of 35-50 kHz.

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