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Induction Heating Since 2000

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How to select and design and produce high quality induction forging furnace correctly?

  This of course must first meet the customer’s the production and process requirements. Mainly is productivity, its main is decided by the power of variable frequency power supply, then the energy saving, it depends on the power supply frequency and the rationality of the design of the sensor, the main is the heating temperature on the process requirement, temperature distribution, etc., want to rely on impedance matching with the power of the sensor design and mechanization and automation level, according to user requirements in consultation; The second is good product quality; Advanced technical and economic indicators of the equipment, such as low energy consumption, material saving, low operating costs, good working conditions, etc.; High reliability of equipment; Easy to use and maintain, safe and reliable to operate.

  The main disadvantage of induction diathermy furnace is the poor generality. If the size and specification of the burden vary greatly, several sensors should be designed in groups. Therefore, it is necessary to propose representative varieties for the multi-specification heating furnace as the main basis for design and product acceptance.

(1) Whether the heating frequency is reasonable or not is directly related to the electrical efficiency and processing quality of the heater. The selection of power frequency mainly considers two factors.

First, to ensure the electrical efficiency, and second, to improve the temperature uniformity of the cross-section.

It can be seen that a further reduction in frequency will not deepen the heating layer, but will affect the electrical efficiency. Further uniformization depends only on the heat conduction of the charge itself. So 0.4R2 is the maximum heating depth at this frequency.

Frequencies should be selected to the high end within this range. Of course, also should be flexible according to the specific situation, such as in the heating rate is slower (small unit power), can choose a higher frequency, heat transfer to make up for the lack of shallow heating layer, the high thermal conductivity of the material can also choose a higher frequency, etc. When the user’s economic investment allows, for the larger diathermy furnace, it is suggested to adopt more reasonable double-frequency or three-frequency heating in the technology, namely divided into low-temperature section (magnetic, low-frequency), high-temperature section (non-magnetic, high-frequency) even temperature section (or not).

(2) Determine the estimation of the average heating power of the power capacity inductor. In general, take the power Py > Pg, and try to use the value given in the standard series. In the case of periodic heating of magnetic materials, if there is no automatic control function, the power capacity should be increased to make Py≈ (1.5-1.7) Pg, and in the case of periodic heating of non-magnetic materials, Py≈ (1.05~1.10) Pg. In this way, we know the power and frequency of the power supply, we can combine the specific conditions and requirements of the user and the manufacturer to select the power supply reasonably.

(3) The determination of the scale of the inductor of the core components of the heating furnace to find out the geometry size of the inductor, you can roughly estimate the size of the furnace. First, find out the induction coil length A1. Forging heating furnace (including all diathermy furnace), of course, hope the heart table temperature delta T smaller the better. The minimum heating time tK is required to ensure △T in order to determine the total length of the coil a1 (continuous) or the number of furnace charge n (sequential) or the number of furnace table N (periodic).

Of course, the diathermy furnace prefers a smaller core temperature differential but from the above discussion, it is known that although induction heating is self-heating, its effective heating layer is only 0.4 ≤ 0.4r2, and the rest still needs to be uniform-temperature by heat transfer with an electrical efficiency d. The proper value of coil inner diameter ensures the efficiency and reliability of the furnace. Too large diameter, increase magnetic flux leakage, will reduce the electrical efficiency; And get too small, will make the lining is too thin, not only reduce its thermal efficiency but will also affect the lining strength, such as clearance is too small will hinder the operation of the burden. In principle, there is an optimal value of D1/D2.

From the above discussion, it can be seen that electrical efficiency is related to two factors: the relative frequency m2 and the air gap between the coil and the charge, i.e., their diameter ratio D1/D2.It can be clearly seen from FIG. 1 and FIG. 4 that the electrical efficiency increases with the rapid increase of frequency, and after the inflection point, the rising rate becomes slow and gradually approaches the limit value. As for the air gap, of course, the smaller the air gap is, the better the electromagnetic coupling is, the less magnetic flux leakage, and the higher the electrical efficiency is. As can be seen from FIG. 4, when D1/D2 increases from 1 to 2, the electrical efficiency decreases from 95% to 76%.

(4) Combining the above three points, its total efficiency can qualitatively make a curve.

The intersection point of the two curves is the optimal point for the selection of refractory and thermal insulation materials. For the steel forging heating of the main heating object, from the comprehensive consideration of the electro-thermal efficiency, it is suggested to take D1/D2=1.4 ~ 1.8, but D1/D2≈1.2 ~ 2.0 is also acceptable. When the diameter is large, the value is slightly small; when the diameter is small, the value is slightly large.If D2 is too thick or too thin, it may exceed this range. The final determination of D1 should be based on practicality, and the following factors should be considered

D1, D2 = + delta D1.1 + delta D1.2 + delta D1.3 + delta D1.4 + delta D1.5

Here △D1.1 — the gap (mm) necessary for the charge to run in the furnace;

D1.2 — Thickness of refractory lining (mm);

D1.3 — Thickness of furnace lining insulation layer (mm);

D1.4 — charge thermal expansion size (mm);

D1.5 — machining tolerance (mm).

From the above estimates, we know the power and frequency of the equipment needed to power, has selected the power, we know the size of the induction coil, considering its working height installation method is given and the shell and the frame material is the basic idea of furnace body, water cooling water can be learned from table 1 the total efficiency of the stove and energy consumption, which need to take away the heated water, thus can also conduct a preliminary estimate.

Some of China’s existing major manufacturers of induction heating equipment have the ability to provide different technical levels of mechanical and electrical matching requirements according to the user’s process needs. According to the mechanization and automation requirements agreed with the user, the connection with the main machine, the mechanization of its own feeding and automation of the operating system, can put forward a technical and economic reasonable plan, and according to the elaborate design and production of inexpensive, durable products.

A few additional comments

(1) the main formula of rectangular section charge first, frequency selection formula, the second, power estimation, third, to ensure the shortest heating time △T.Carbon steel from room temperature heating to 1200 ~ 1300℃, the fourth, the determination of coil size. Line coil cavity height D1.When the rectangular blank (b2/D2 > 1) is heated, the height of the feed mouth has little influence on the electrical efficiency, so D1/D2=1.25 ~ 3.0. When the charge is large and the heating temperature is low, take a small value.On the other hand, it takes a larger value. Of course, also want via practical arrangement and compound just can. The other points are the same as the cylindrical charge. Line coil cavity width B1

When b2/D2 is less than or equal to 5, b1 is equal to B2 plus (d1-D2).

When b2/D2 > 5, b1=b2+(1.05 ~ 1.15)(d1-d2)

(2) the main formula of pipe heating so-called pipe, generally refers to the ratio of outside diameter and wall thickness, namely D2/ D2 > 5 and D2/ △2.First, the frequency selection formula K2≈ F (D2p/ A2), the corresponding curve can be consulted, and the value K2≈0.8 ~ 0.9 can be temporarily taken for estimation. There is an optimal frequency value for pipe heating: second, power estimation, other items can be referred to as cylindrical furnace charge heating.

(3) Dual-frequency heating With the development and improvement of semiconductor frequency conversion power supply and the popularity of electric energy, according to the difference of induction heating for magnetic and non-magnetic materials, the adoption of dual-frequency power supply for segmenting heating of steel parts from room temperature to forging temperature has been popularized.Low frequency power supply was used before the magnetic point, and relatively high frequency heating was used after the Curie point.Its main advantage is: to save electricity. Due to the reasonable power frequency configuration, the power can be fully used, generally can save 15%~20% of electricity. (2) to save time. In the case of single-frequency heating, the frequency selection is based on the hot state. For the cold state, the frequency is too high, the heating layer is shallow, and the unit power given is relatively small, which reduces the heating speed and extends the heating time. It increases energy consumption and reduces productivity.(3) Good product quality. As the heating time is shortened, energy consumption is reduced and oxidation is reduced. Meanwhile, the reasonable frequency also ensures the low temperature difference, so good heating quality can be obtained.

(4) Rapid heating (variable-turn heating) This is the reason that the heat transfer to the center is fast when the temperature difference is large, and the heating time is shortened when the same temperature difference is guaranteed.

For the periodic heating, the x-coordinate can be regarded as the heating time, and for the continuous and sequential heating, the length of the induction coil. In fact, when the blank surface rises to the final temperature, it accounts for 10%~30% of the total heating time (or 10%~30% of the total length at the sensor inlet), which can increase the heating speed and shorten the average temperature time. This heating specification is known as (impact) rapid heating.

(5) Comprehensive energy Heating Due to the different energy conditions in different regions, dual-energy comprehensive heating should be considered when necessary. For example, natural gas is abundant and cheap in a certain region. Can consider in 700~800℃ below with gas furnace heating, after entering the rapid induction heating furnace. In this way, it can not only make use of cheap air sources when oxidation is light in low temperature heating section but also adopt high quality rapid induction heating in high-temperature areas. It makes both economic and technological sense. To sum up, the design and production of induction heating equipment with high quality, low price, and economic durability should be based on local conditions and take full consideration of technological needs. In the end, although the problem is very small, it is often ignored by some people, resulting in big mistakes. That is when using the existing formula to calculate, be sure to pay attention to the physical quantity in the formula must use the integral average value of the heating temperature segment, the unit in the formula should be clear, put in the correct unit. In addition to forging reheating furnace, this information is also applicable to other kinds of induction diathermy furnace.

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