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Influence of heat accumulation on performance of electric heating elements and exploration of new product development direction

Author: Source: Date: 2019/6/18 17:54:32 Popularity: 144

Influence of heat accumulation on performance of electric heating elements and exploration of new product development direction

Sheng Jianyin

Guide:

1. Analyze how the heat accumulation phenomenon occurs in the electric heating pipe, and understand the fact that the thermal safety of the electric heating pipe decreases due to heat accumulation and the fact that the life or electric heating efficiency is low. Find a suitable way to make up for it.

2. Since the industry created conductive thermal elements in the direction of high power and small volume fifteen years ago, the industry has almost stagnated in innovation projects in recent years, because there is no theoretical understanding of the current technology problems, that is, Not to mention the foothold of innovation, to solve the problem of heat accumulation from the demand for energy saving to induce the research ideas and focus of development of new products.

3. The relevant theories of new product development have achieved unprecedented success in practice, but the relevant principles and structural features have obtained relevant intellectual property rights, warning not to be easily imitated to avoid infringement.

4. In the process of introducing the relevant content in the full text, we have omitted its formula calculation process and product structure diagram. The reason is that new technical features are not introduced in the scope of infringement. Please forgive me for this.

The technical status of electric heating pipes has only been the popularization and small changes of the existing technology for more than five years. There has been no significant improvement and discovery. Therefore, the current industry as a whole is in the fault phase of innovative products, and related The long-standing defects in the electrothermal technology industry have not been able to break through, which has hindered the development of the application industry. For example: We want to realize high-power and small-volume electric heating elements, but urgently need a filler material with super thermal conductivity and good electrical insulation cannot provide the market in the short term, so its plan cannot be achieved; In fact, there are many unsatisfactory defects in the structure and application of electric heating pipes. For example: the electric heating efficiency of electric heating tubes can only be determined by the quality of the magnesium powder provided by the supplier (the purity of magnesium oxide is difficult to self-check); the thermal stability of the filling material limits the further volume optimization of high-load products Design schemes, etc., it is necessary to conduct a comprehensive qualitative study of the dead zone content of the current electrothermal technology, and then seek structural breakthroughs from principle breakthroughs, so that the overall level will naturally improve, otherwise only waiting for materials It is too late to re-introduce industry breakthroughs. The first part of this article is to analyze the problem, and the second is to explore the solution of the problem. Since I am busy with project development and testing and other engineering issues, there is no systematic summary of the theory. The arguments or arguments put forward in the article, if there are principle defects or errors, please peers to criticize and correct them.

I. Characteristics of heat accumulation and the main factors

In the structure of the electric heating tube, the filling material plays a role of heat conduction, and it must have corresponding electrical safety, and its characteristics must have corresponding thermal stability. Expressed in the professional language of the industry, it is resistant to high temperature and low leakage and must be moisture-proof. Based on the analysis of the characteristics of the current filling materials, the fact that fused modified magnesium powder is the leading material used in the industry has a relatively economical and practical fact, but the thermal conductivity of this material itself is not too large Advantage, that is, its thermal resistance characteristics still exist, even if its purity and density are ideal, they have certain limitations on the ability to conduct heat. In popular terms, the existence of the material's thermal resistance characteristics can not be sufficient Moreover, in order to ensure a certain electrical safety strength from the surface of the heating wire to the inner surface of the tube, it must exist at a certain distance, so the total thermal resistance at this time is directly proportional to the creepage distance. The characteristic of zero thermal resistance of the transmission path is a reality that is almost impossible to solve in the current materials industry. During the work of the heating tube, the heating wire is the main component for converting electrical energy into thermal energy, and the heat generated by it can theoretically be considered to have a continuous function. If the thermal conductivity of the filling material is less than the heat generated by the heating wire, Then there must be a residual heat in the tube or in the filling material. The temporal accumulation of this phenomenon is the heat accumulation we have defined;

For the content introduced above, we can generally summarize the heat accumulation phenomenon. The conclusion is that the heat remaining on the heat transfer path due to the insufficient heat transfer capacity of the heat transfer material is the heat accumulation, so the thermal resistance characteristics of the material determine the heat accumulation. The magnitude of the degree (the mathematical model corresponding to the above conclusions and conditions does not attempt to build it by itself).

Comprehensive evaluation of the effects of thermal accumulation of electric heating elements on the performance of electric heating elements

In the past, we have introduced the thermal inertia characteristics of electric heating pipes in related articles. The meaning of the expressions is similar to the phenomenon of heat accumulation. There is no such professional systematization in the theoretical structure, but this phenomenon has four effects on the performance of electric heating pipes. The deadly effects are summarized as follows:

1. Impact on life: It is well known that the higher the ambient temperature of the heating wire, its two major characteristics of online working state are that the CT coefficient increases significantly, and the second is that the resistance of the heating wire to the hot corrosion of the high temperature environment directly determines its Service life; due to the existence of heat accumulation, the ambient temperature in the tube must be much higher than the temperature of the heat absorbing medium, and the PH value of the filling material in the hot state has shown that it has a strong corrosive force on the heating wire. Therefore, thermal corrosion has sufficient theoretical support and experimental data to support the conclusion that the life of the heating wire or electric tube is decisive. We have published this conclusion in public.

2. Decrease of electrothermal efficiency: The thermal resistance characteristics of filling materials are basically similar to other materials, that is, their thermal resistance values at different temperature states are different, which means that the consistency of non-constant characteristics of thermal resistance values of materials is similar. Moreover, the positive temperature of this thermal resistance value indicates that the higher the temperature of the filler material, the larger the thermal resistance value, so the heat accumulation phenomenon becomes more serious in the higher temperature environment. The lower the heat conduction capacity or the electric heating efficiency, the theory and the actual detection conclusions correspond completely.

3. Decrease in moisture and moisture resistance: The moisture resistance of electric heating pipes is determined by the moisture-proofing agent material in sealing materials and filling materials, and whether the moisture-proofing agent is currently used, whether it is methyl silicone oil or dimethyl silicone oil or even hydrogen-containing silicone oil, Although their temperature resistance is different, the endurance required to reach a high temperature of 600 ° C or higher does not meet this requirement from the analysis of current technical characteristics. Therefore, there are quite a lot of electric heating pipes that are resistant to moisture after being used for a long time. It is not surprising that there is a decline or failure of the capacity. The reason is very clear, that is, the silicone oil at this time has been partially or completely carbonized by the high temperature in the tube caused by the heat accumulation. This is the final conclusion.

4. The effect of thermal inertia on control: The description of thermal inertia is divided into two different characteristics: startup and shutdown. If there is no thermal resistance in the electric heating tube, the temperature of the tube surface should quickly reach the temperature when it starts. The temperature of the silk watch, in fact, the temperature of the silk watch is never reached by the pipe watch, and it takes a considerable time to reach the equilibrium temperature of the filling material in the pipe. Expressed in technical terms of electrical engineering, this response is a lag. After the power is turned off, the residual heat of the electric heating pipe meter will still be released for a long time. If the electric heating pipe is in the liquid during the qualitative inspection, the heat accumulation energy in the electric heating pipe will continue to increase the temperature of the liquid after the power failure. . One of these features is to further explain the fact that there is a heat accumulation phenomenon in the tube. On the other hand, it is told that this heat accumulation feature must be aware of it in advance. For example, the special case of the residual temperature that will further increase the temperature of the liquid, In instantaneous electric water heaters, there is a burn accident at this time, and electrical control can hardly achieve synchronous control of this feature. Please pay special attention.

Third, circumvention measures

At the same time that we analyze this phenomenon, finding the corresponding solution is the concern of our peers. Before introducing the theme, we first guide how to improve the existing products. We hope that the quality will be greatly improved in the short term. The following content states that its role is to improve rather than eradicate the heat accumulation phenomenon, because we cannot provide superconducting materials for the manufacture of electric heating pipes, and we can only minimize the harmfulness of this feature from an engineering perspective.

1. Density method: The number of thermally conductive materials per unit volume determines the overall thermal conductivity. This is a common theoretical basis. In order to reduce the phenomenon of heat accumulation due to insufficient thermal conductivity, the density value of the filling material in the electric heating tube is The focus of attention is on the analysis of the current filling materials and the functions of the machinery. The basic guarantee of the density is between 3.05 and 3.10 g / cm 3 , and the process tooling level of some enterprises is far below that due to the lack of corresponding theoretical knowledge. This level, so the comprehensive performance of its products is far from reaching the target. This is only an overall evaluation. As the application of electric heating pipes, it is inevitable that different degrees of bending are required during the design process. For products with R structure, due to the extent of the extension of the pipe wall, the filling rate density in the pipe will definitely decrease. In this case, the response of the project is to use an area-reforming plastic treatment of these parts to restore the design value. For more details, please refer to the relevant chapters of Electrothermal Technology and Engineering Practice.

2. Purity method: Magnesia is the main filling material. There are five main components of magnesium oxide, calcium oxide, aluminum oxide, silicon dioxide, and iron oxide in its composition. The thermal conductivity of these materials From its thermal conductivity, it can be judged that the thermal conductivity of the three components in the middle is smaller than that of magnesium oxide, and iron oxide is not included in hazardous impurities because of its electrical unsuitability for basic safety requirements. Material as a filler is another cost problem to reduce heat accumulation, but as the main factors of safety and energy saving, its optionality should be taken into consideration.

3. Design for controlling power density: The size of the power density is the determining factor for the heat accumulation under the environmental conditions where the thermal conductivity has been determined. Therefore, before the key indicator of the area design load of the electric heating tube designed with the finished product is determined, First of all, a corresponding physical model must be made for quantitative analysis, and a qualitative calculation is performed from the perspective of heat conduction to determine whether it is reasonable or not, instead of looking for a reliable basis through a blind proofing test. Of course, the specific value of the pipe surface load The determination method also depends on the working environment of the component and the nature of the pipe and the needs of the site.

4. Strict control of creepage distance: The length of the heat transfer path directly determines the size of the thermal resistance during the heat transfer process, which determines the amount of heat accumulation. However, it should be said that many companies choose how to choose the creepage distance. There is a conceptual difference, because the too small creepage distance solves the problem of heat accumulation (of course, thermal efficiency and other issues have also been improved), but its defective rate will also increase. If you want to achieve the desired quality effect It is also worthwhile to choose local sacrifice, and in fact the practice of focusing on the surface pass rate will actually have greater hidden dangers and risks, which is not desirable.

From the above content, we introduced the generation of heat accumulation and the corresponding engineering remedy methods. To achieve the above content, there should be no difficulties for engineering and technical personnel, especially in design and process. Even if it is based on this book, Its products are only partial improvements, and they are limited, because you can't change the way of heat conduction, and you can't change the characteristics of the material, but after the comprehensive analysis above, we already know the cause of the problem, so we have The corresponding method is to deepen the discussion again from the heat transfer path.

Reanalyze the root cause of heat accumulation from the heat transfer path

In the current electric heating tube structure, we can think of the heat conduction process of the electric heating tube as a concentric circle heat radiation field with the spiral heating coil as the particle. In theory, the unit of the particle tends to be infinitesimal. Why do we use the spiral heating coil as the particle? How about it? This is where the description of the problem stands.

First of all, we conduct a longitudinal analysis of any used electric heating tube. You can directly see that the filling material inside the core, especially the core, has a gray feature. The degree of this gray is determined by factors such as the length of the tube surface load. For the time being, I will not expand on this, and now I will focus on the physical phenomena related to the heat emission field;

1. Model of one-way or one-dimensional heat conduction field: When the heating wire is wound into a spiral shape, the space in the core rod area does not participate in any heat conduction in the working state, and a high temperature characteristic will be formed in its interval. The constant temperature field, due to the thermal resistance characteristics of its filling material, has only the function of storing heat. The real mass point involved in heat conduction is the nearly half area of the silk surface near the surface of the tube, and its theoretical support is that the heat is always Diffusion in the direction of low energy. From this we can clearly know that the outer side of the spiral heating wire is the effective heat conduction and heat dissipation area of the electric heating tube, and the core is the high temperature dead zone of the electric heating tube. It will only be when the temperature in the dead zone is higher than the outside temperature. There is heat overflow. This thermal conduction method is theoretically flat or unidirectional. The reason for using the entire heating coil as a mass point is that its internal temperature is almost the same as the temperature outside the silk surface, so its hypothesis Has credibility on quantitative indicators (unless this description is flawed when the wire pitch is sufficiently large).

2. Overview of the working characteristics of the core: The fact that the core of the heating wire is the high temperature region of the entire electric heating tube can be recognized, but it is also the area where the heat accumulation of the entire electric heating tube is the most serious, and there is no heat emission. There is only heat accumulation in the space. Whether it is a moisture-proof material or a thermally unstable material such as silicon dioxide, which is similar to low melting point in magnesia, the process of being carbonized in this area first need no further explanation. Yes: The factors related to the heat accumulation of the core filling material must be explained clearly: ①The smaller the distance between the wires, the more the accumulation amount (the smaller the wire distance is equivalent to the formation of a closed loop area ); ② the lower the density of the core filler, the faster the core heating rate (confirmed by the temperature imager), and the lower the current-carrying capacity of the wire. Facts can confirm its consistency; ③ The characteristics of the degree of thermal corrosion inside and outside the wire surface under the microscope have fully demonstrated the correctness of the inference.

After the above series of expressions, we found a new understanding, that is, the concept of silk surface load is only a mathematical average, but in fact, there are different surface areas of the surface surface load. While the existence of heat accumulation in the tube has been confirmed, we also found that there is a dead zone of heat conduction in the core, and the electric heating tube is a kind of unidirectional thermal characteristic element, so we take this as a breakthrough point, that is, The concept of two-way heat conduction is used to introduce the idea of new product development (the products of the surface expansion type and heat sink structure previously published have certain functions, but they are not in the category of principle breakthrough).

For all analysis contents, we give a conclusion that the heat accumulation phenomenon is caused by the insufficient thermal conductivity of the filling material, which is determined by the current structure and material characteristics.

Fifth, the structural characteristics of two-way thermal conductive electric components

If the one-way heating electric heating tube is a solid core, the core's heat cannot participate in heat conduction (or the heating wire is actually a single-sided or surface area heat radiation), so we have developed a kind of heating wire core An electric heating tube that can induce and participate in heat exchange with internal heat, referred to as a two-way heat conduction electric tube, which is characterized by an air-core electric heating tube, which is used in instant electric water heaters or electric heating appliances with circulating cooling devices, such as instant heat Instant water boiler, steam generator and electric heating tube similar to coffee maker; what is different from traditional products in structure is the "empty core", and the core directly participates in heat conduction. If it is said that it has solved the one-way principle in principle It is better to evaluate the thermal defect from its performance. Please analyze it from the following test data:

1. The design load of the tube is 3300W / 220V, and the load of the tube surface on one side is 23W / cm 2 and 34W / cm 2 respectively . Working under the environment of water supply pressure of 0.03MPa, the core is the water inlet, and its The effluent is still cyclically heated by the electric outer tube, the flow rate is 0.8L / min, the original water temperature is 18.7 ℃, and the output temperature is constant from the moment of power on. It only takes 7.2s in time, and the water temperature reaches 77.6 ℃. Measured by the thermal efficiency value, it has reached 99%. The contents of these data include: ① the thermal response speed has been accelerated; ② the electrical heating efficiency has increased, and it has more energy-saving features; ③ the volume of the entire electric heating pipe is larger than that of traditionally designed products Small space of nearly 40%, so high power and small volume hope to be truly reflected in this type of product.

2. Overload test: The purpose is to demonstrate the change from the bearing capacity of the heating wire in the tube of this type of product to the thermal stability test of the tube. Before the introduction, I will explain the relevant materials and parameters of the electric heating tube during manufacture: the heating wire is made of 0Cr25Al5 material with a domestic wire diameter of 0.5mm, and the filling material is a magnesium tube plus ordinary medium-temperature fused modified magnesia. cm 3 ; the experimental detection method is: using a voltage regulator to continuously change the input power to the electric heating tube, in this process, the leakage current is separately detected, and the current heating wire material is truly analyzed under this structural feature The carrying capacity is compared with traditional products. The principle of binning of overload is that each increment is 15% of the rated value. (The purpose of this setting is to prevent overload caused by the grid voltage during peak fluctuations. In addition, it is in line with European standards. The electricity environment is approximate, and the value is 60% of the rated value. Therefore, the whole process is carried out in four steps. Because the relevant test parameters are quite cumbersome, the conclusions are now informed as follows: The thermal stability is very ideal. Without any thermal breakdown (1500V / 0.5mA), the bearing capacity of the heating wire can reach 150% of the current design parameters, and the time of thermal inertia is only 3.3s (time for the bubble disappearance of the tube surface after 150% of rated load is powered off); based on the above, we can give full confidence that the possibility of designing a new generation of immersion type electric heating pipe using the existing material conditions is already available It is theoretically feasible. The principle and characteristics of similar products have been certified on related products, and its popular application time is not too long.


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