Comparative analysis of medium frequency furnace selection

Energy saving and pollution reduction, anti-pollution and emission reduction are important means for sustainable social and economic development. Induction melting has fast heating speed, high efficiency, less burning loss, less heat loss, relatively low workshop temperature, reduced smoke generation, energy saving, The effects of improving productivity, improving labor conditions, reducing labor intensity, and quieting the workshop environment are remarkable. Especially for cast iron, the induction melting furnace is beneficial to obtain low-sulfur iron liquid, which is unmatched by the cupola. When selecting the medium frequency melting furnace, the foundry company should select the following according to the transformer capacity, production requirements, investment quota, etc., when purchasing equipment:

1, electricity conditions

1.1 Transformer capacity

For the SCR full-bridge parallel inverter intermediate frequency power supply, which is commonly used in the industry, the relationship between the capacity of the voltage regulator and the power supply is:

The number of transformer capacity = the number of power supply 値 * 1.2

For the series inverter intermediate frequency power supply (commonly known as one for two, one for melting one insulation, two for simultaneous operation), the capacity of the transformer and the power supply

The value of the transformer capacity = the value of the power supply *1.1

The transformer is a rectifier transformer. In order to reduce the interference of harmonics, it is used as far as possible for the special plane, that is, one intermediate frequency power supply is equipped with one rectifier transformer.

1.2 incoming voltage

For the intermediate frequency power supply below 1000Kw, three-phase five-wire 380V, 50Hz industrial power is used, and a 6-pulse single-rectifier intermediate frequency power supply is configured. For medium-frequency power supplies above 1000Kw, the focus is on using a 660v line voltage to configure a 12-pulse dual-rectifier IF power supply.

Some manufacturers use 575V or 750V, because 575V or 750V is a non-standard voltage level, accessories are not good to buy, it is recommended not to use

There are two reasons: one is to increase the rated working voltage by increasing the incoming line voltage; the other is that harmonics generated by high power can interfere with the power grid, and a relatively straight DC current can be obtained by double rectification, the load current is a rectangular wave, and the load voltage is close to sinusoidal. Waves reduce the impact of grid disturbances on other devices.

Some users blindly pursue high voltage (some 1000Kw use 900v line voltage), low current in order to achieve energy saving, I do not know that this is at the expense of the life of the electric furnace, it is not worth the loss, high voltage is easy to cause the life of electrical components Shortening, copper platoons, and cable fatigue make the life of the electric furnace greatly reduced. In addition, for high-voltage electric furnace manufacturers, the raw materials are reduced in terms of materials, and the cost is saved. Electric furnace manufacturers are certainly willing to do so (high-priced low-cost.) The ultimate loss is still the use of electric furnace manufacturers.

2. Capacity requirements

In general, the capacity of a medium frequency induction melting furnace can be determined by the weight of a single piece and the weight of molten iron required per working day. Then determine the power and frequency of the intermediate frequency power supply. Induction heating equipment is a non-standard product. At present, there is no standard in the country, and the general configuration of the industry is shown in Table 1.

It can be seen from Table 1 that the power density of domestic medium frequency induction melting furnace is about 500KW/ton, which is lower than the theoretical optimal value of 600-800Kw, mainly considering the life of the lining and production management. At high power density, electromagnetic stirring will produce a strong scouring of the lining, and the requirements for lining materials, furnace building methods, melting processes, materials, and auxiliary materials are high. According to the above configuration, the melting time per furnace is 75 minutes (including feeding, salvaging impurities, quenching and tempering time). If it is necessary to shorten the melting time per furnace, the power density of the power supply can be increased by 100 Kw/ton while the capacity of the furnace is constant.

3. Structural choice

According to industry practice, the industrial melting furnace of aluminum alloy structure with the reducer as the tilting method is commonly known as the aluminum crucible furnace. The induction melting furnace of the steel structure with the hydraulic cylinder as the tilting furnace is commonly called the steel shell furnace.

Compared with the aluminum crucible furnace, the advantages of the steel crucible furnace are five points:

1) Rugged and elegant, especially for large-capacity furnaces, which require a strong rigid structure. From the safety point of the tilting furnace, try to use steel shell furnace.

2) The yoke made of silicon steel sheet shields and emits the magnetic lines generated by the induction coil, reduces the magnetic flux leakage, improves the thermal efficiency, increases the output, and saves energy by about 5%-8%.

3) The presence of the furnace cover reduces heat loss and increases the safety of the equipment.

4) Long service life, aluminum oxidation is more serious at high temperatures, resulting in metal toughness fatigue. At the foundry enterprise site, it is often seen that the aluminum crucible furnace used for one year or so is ruined, and the steel crucible furnace has less magnetic leakage, and the service life of the equipment greatly exceeds that of the aluminum shell furnace.

5) Safety performance The steel shell furnace should be greatly improved by the aluminum shell furnace. When the aluminum shell furnace is smelted, it is heated by high temperature and heavy pressure. The aluminum shell is easily deformed and the safety is poor. The steel shell furnace uses a hydraulic tilting furnace and is safe and reliable.

The most important question when purchasing: Why are the prices of the same model different? How to choose the medium frequency melting furnace?

The price of the same type of medium frequency induction furnace is very different. Take the 1 ton furnace that is commonly used as an example. The market price sometimes differs several times, which is related to the structure of the furnace, component selection, technical content, after-sales service and quality. A multifaceted factor is relevant.

1. Different materials

1) Furnace and yoke

As for the choice of the outer shell of the aluminum shell furnace, the standard 1 ton aluminum crucible furnace has a furnace shell weight of 400 kg and a thickness of 40 mm. Some manufacturers often have insufficient furnace and weight. The most important thing for the steel shell furnace is the choice of the yoke. The choice of the same type of steel furnace yoke is different, and the price difference is very large. Generally, a new high-permeability cold-rolled silicon steel sheet with z11 should be used. The thickness of the silicon steel sheet is 0.3mm, and the square structure is adopted. The inner arc surface and the outer arc of the induction coil are the same, so that the yoke can be closely attached to the outer side of the induction coil, and the maximum restraint coil is outwardly radiated, and the yoke is bilaterally The stainless steel plate and stainless steel are clamped, welded and fixed, and cooled by water.

Some manufacturers use waste, non-oriented, or even silicon steel sheets removed from used transformers to make yokes. Looks like the price is cheap, but it is useless.

2) Copper tube and copper row:

The core of the melting furnace is the induction coil. The effect and price of cold extruded copper tubes and cast copper tubes are quite different. The T2 cold extrusion copper tube with rectangular cross section should be used. The surface insulation treatment of the copper tube is treated with TSC-L ultra-high temperature insulating varnish or APC-H+ high temperature arc-resistant insulating enamel, and the temperature resistance exceeds 200 °C.

Why choose high temperature insulation paint?

The induced current through the coil has a skin effect, that is, the current is mainly concentrated on the surface of the copper tube, and the higher the frequency of the induced current, the larger the surface current density. Therefore, the heat generated by the copper coil of the induction coil is concentrated on the surface, and the surface temperature in contact with the insulating varnish is much higher than the temperature in the contact portion of the copper tube with the cooling water. Even under normal circulating water cooling conditions, the outlet water temperature is controlled at 50-60 ° C, and the temperature of the copper tube surface will exceed 80 ° C.

The heat transfer of molten steel in the furnace. The new furnace lining is thicker, which can effectively prevent the heat of the molten steel in the furnace from being transmitted to the surface of the coil. However, with the rapid erosion of the lining in the later stage, the lining of the lining becomes thinner in the later stage, and the heat transferred from the molten steel to the surface of the coil is much higher than that of the new lining. Actual measurement surface, when the new furnace lining (the thickness of the lining is about 15cm), the temperature of the coil slurry layer is above 80°, to the later stage of the lining (thickness is about 5cm), the temperature of the coil slurry layer has risen to nearly 200 °C. Conventional insulating varnishes have completely failed to carbonize.

Cooling water cooling capacity is reduced, mainly due to the influence of water quality. Cooling water is easy to scale at high temperature, especially in the north and west areas where the water quality is hard. The cooling water fouling is prominent, the copper pipe is blocked, the water pressure is reduced, the cooling capacity is obviously reduced, and the temperature rise in turn accelerates the scaling. . Once this happens, the temperature of the copper tube surface will rise rapidly and the conventional insulating varnish will be destroyed by carbonization in a short time.

If ordinary insulating varnish is used, in order to protect its insulation strength, the mica tape and the alkali-free glass ribbon may be wrapped and wrapped once on the surface, and then the moisture-proof insulating enamel may be applied. However, the construction of this method is complicated, and the material cost and labor cost are high. And the effect is not as obvious as the direct application of high temperature insulating varnish.

Why is the method of wrapping the insulation tape not as good as directly spraying the high temperature insulation paint?

Currently commonly used insulation with mica tape, alkali-free glass ribbon or polyimide tape, these materials have good insulation properties, but the adhesion on the copper tube is not ideal, mainly by applying a layer of insulating paint. Or resin to bond. However, the intermediate frequency furnace has strong vibration during the production process. Over time, the wrapped insulation tape is easy to fall off the copper tube or form many gaps. The moisture or other water vapor lining the new furnace has already penetrated into these gaps, forming a short circuit and completely destroying the insulation effect. The high-temperature insulating varnish has good adhesion on the surface of the copper tube, and a dense and hard insulating protective film is formed on the surface, and the above situation does not occur.

At the same time, glass ribbons or mica tapes are easily brittle in long-term high temperature environments, and their effective use time is not long.

There is a certain gap between the turns of the coil. When the refractory mortar is coated in the coil, the refractory clay should be infiltrated into the gap to strengthen the adhesion of the glue on the coil on the coil. After the refractory cement is built, the inner surface is smoothed, which is convenient for removing the lining. Protect the coil, the adhesion of the mud on the coil, after the refractory cement is built, the inner surface is smooth, easy to remove the lining to protect the coil, and the upper and lower ends of the coil are added with several stainless steel water cooling rings to increase the overall rigidity and facilitate heat dissipation.

Some manufacturers use copper or T3 copper tubes, which have poor electrical conductivity and are easy to break and leak.

3) SCR

The quality of the thyristors used by various manufacturers is generally uneven. The quality of the thyristors is good, the reaction is fast, and the failure rate is low. Therefore, the thyristors of well-known manufacturers are selected, and the quality is reliable and stable.

4) Power cabinet

Regular manufacturers use standard spray panels. Not a tin spray booth. And the size specifications of the power cabinet are standard.

Inconsistent manufacturers of power cabinets have also shrunk, height, width and thickness are not enough, and even some of the reactors are placed outside the power cabinet. The regular manufacturer's IF power supply is equipped with a low-voltage switch inside, and no additional voltage switch cabinet is required for the user. Some irregular manufacturers do not have a low-voltage switch installed inside the power supply. Invisible increase in user fees

5) Capacitor:

The most important thing to do with reactive power compensation is that it must be equipped with a sufficient amount. Generally, the compensation value of the capacitor is 18-20 times the power of the power supply, ie: Capacitance compensation amount (Kvar) = (20-18)x Power supply

And use the capacitors of regular manufacturers.

6) Reactor

The main material of the reactor is silicon steel sheet. New products produced by regular manufacturers should be used, and recycled second-hand silicon steel sheets cannot be used.

7) Water pipe clamp

In the complete set of medium-frequency melting furnaces, there are a large number of water pipes connected. Strictly speaking, stainless steel clamps should be used, and it is better to use copper slip knots. The slip knot is easy to install and disassemble, and it is not suitable for maintenance. It is especially suitable for application on water-cooled cables. It is advantageous for sub-current transmission and no water leakage, which is safe and reliable.

In addition to the above points, there are other components to choose from, such as inverter capacitors, water-cooled cables connected to copper drains, etc., which will affect the quality and price of the equipment, we will not elaborate here, I hope that when you choose to buy, you can pay attention to the details of the main components required by the melting furnace manufacturer. You can't just ignore the structure and quality of the equipment.

Since the intermediate frequency melting furnace is a non-standard product, it is ordered to be remanufactured, and the quality is closely related to the price.

2. Technical strength

The regular manufacturers have invested a lot of manpower and material resources, advanced equipment and exquisite technology for the advanced technology of research and experiment. It reflects differences in melting speed, power consumption, operational complexity and failure.

Many manufacturers do not have the conditions for on-site commissioning, and the cost is naturally lower. The impact of assembly and commissioning processes on quality is very large. Different manufacturers and different processes have different prices and different quality.

3. After sales service

Good after-sales service is the guarantee of equipment quality. It is inevitable that the electromechanical products will fail. This requires good after-sales service. The regular manufacturers have enough technical personnel and ability to guarantee after-sales service. Before the factory, the intermediate frequency induction melting furnace has a one-year warranty period after repeated static and dynamic debugging. During this period, any failure caused by non-human responsibility will be the responsibility of the manufacturer.