Electric resistance wire structure of electric furnace and muffle furnace

The design procedure of electric furnace and muffle furnace is more complicated, and there are many related parameters. It is necessary to determine the comprehensive data of resistance wire by reasonable optimization. It is often necessary to calculate repeatedly, and the calculation of these parameters is a set of rings, so it is said that the program is good. Make sure you have to spend some energy and time. Therefore, how to quickly calculate and improve work efficiency is also the common aspiration of the vast number of technicians in the electric heating manufacturing industry.

1. The basic principle of resistance heating and related content Converting electrical energy into thermal energy and using it is an important effect in electricity.

There are also a variety of ways to convert electricity into heat, including plasma heating, electron beam heating, arc heating, induction heating, and resistance heating (not limited to resistance wire heating).

1.1 Resistance heating: using the resistance of the conductor to generate heat Heating the various substances with the aid of the relevant medium material is the basic principle and working principle of resistance heating. There are many kinds of materials for resistance heating, but the alloy resistance wire is used as a heating material, which is a widely used mainstream material, and the electric heating wire material as a metal tubular electric heating body is basically nickel-chromium Ni-Cr, iron-chromium-aluminum Fe-Cr- AL and chromium-aluminum Cr-AL-Mo with molybdenum are the main materials. The basic conditions and technical requirements for the alloy resistance wire are: resistivity, uniformity of resistance value, chemical stability, oxidation resistance, high temperature strength, and the like.

1.2 Resistance wire related parameters 1.2.1 Resistivity The resistance of the resistance wire is also called the resistivity or specific resistance. It is an electrical parameter indicating the resistance of the conductor through the current. The relationship between the resistivity of the conductor and the resistance is as follows:

R=ρC/S

R-conductor resistance Ω

L—the length of the conductor m

Cross-sectional area (cross-sectional area) of S-conductor mm2

ρ—The resistivity of the conductor μΩ.m

The resistivity is related to the chemical composition, metallographic structure and operating temperature of the alloy, and is an important data for calculating the resistance value of the resistance wire of different specifications. Therefore, we can use the above formula, as long as we know the resistivity of its material, we can calculate the resistance of the resistance wire of various specifications (resistance per meter length) at any time.

1.2.2 Surface load The surface load of the wire refers to the electric power value W/cm2 of the unit surface area of ​​the total length of the expanded wire. Under normal circumstances, the worse the working conditions of the components (poor heat dissipation conditions), the smaller the surface load of the wire should be selected, and the working conditions of the components are better (the heat dissipation conditions are good). The surface load of the wire can be relatively large, of course, the heat dissipation condition depends on the heating. The conditions of the material, the size of the object, the flow of the fluid, the wind speed of the air, etc., are not comprehensive if the heating medium is used to regulate the surface load and the surface load of the wire. For example, the heating element power is the same, but the heat dissipation conditions are different when it is cast in 1Kg aluminum and cast in 3Kg material. Another example is that the same wind air heating speed is not the same effect is not necessary. Therefore, it is reliable to consider the heating medium while considering other conditions to determine the surface load of the wire.

The surface load of the resistance wire is calculated as:

W/cm2=P/(D.л.L)

W/cm2—wire surface load P—electric power D—resistance wire diameter L—expanded length 1.2.3 temperature coefficient of resistance wire The resistance value (resistivity) of the alloy resistance wire changes with temperature, this change The value is called the temperature coefficient of resistance. The ratio of the resistivity Pt at the operating temperature to the resistivity P20 at 20 °C is called the resistivity correction factor, and the relationship is as follows:

Ct=Rt/R

Ct—When the temperature is °C, the temperature coefficient of resistance Rt—the resistance value when the temperature is t—the resistance value at the normal temperature state. If the resistance is Ct (resistance temperature coefficient) on the island, the above relationship can be calculated. The resistance value at different temperatures.

The temperature coefficient of the resistance wire is an important parameter in the design of the tubular electric heating element, which directly affects the power of the product. In the actual work, the general curve of the above figure should be referred to, and the simulation test should be carried out in combination with the actual working condition of the component, that is, at room temperature. The ratio of the resistance value in the state to the resistance value at the operating temperature, and the resistance value at the normal temperature state is determined based on this coefficient (measured). (here refers to the finished resistance value)

1.2.4 Winding diameter According to the rated voltage, rated power, heating medium condition and the selected wire surface load in the tubular electric heating element, the resistance wire specification is determined and assembled and fixed to the center of the electric heating element reasonably and scientifically. To achieve this, the wire must be spirally wound (specially few products do not need to be wound). Under the condition that other conditions are unchanged, it is also an important content in the design process to reasonably choose to wind the mandrel to achieve the ideal diameter. The details are as follows:

A. The winding diameter should not be too large or too large, which will reduce the distance between the resistance wire and the metal tube, which will reduce the insulation withstand voltage of the component. If the diameter of the ring is too large, the mechanical tension (elastic force) of the resistance wire will be reduced, which will easily cause the sagging phenomenon of the resistance wire during the powder adding process, thereby causing uneven heating of the finished product. Of course, sometimes the user's request for the diameter of the lead rod is larger, then we still consider the overall performance, what kind of ring diameter should be what kind of ring diameter, it is better to reduce the diameter of the end connecting the lead rod and the resistance wire ( See figure) for connection, and can not affect the rationality of the winding diameter.

B. The ring diameter should not be too small, and the winding diameter is too small, which will reduce the heat generated by the resistance wire and cannot be quickly transferred to the metal tube casing. Since the heat cannot be transferred quickly, it will inevitably lead to an increase in its own temperature, thereby shortening the life of the product. Secondly, for a few special products, the heating zone is still very long, but the power is not large. If the resistance wire is calculated according to the normal heat load, the winding length is often too short after winding, and the wire spacing is too large after opening. Forced to reduce the development of the core rod, in fact, there is no need, you can completely choose the resistance wire, or use double wire winding, so that this contradiction can be resolved.

From the current design of most manufacturers, the choice between the resistance wire and the mandrel is roughly:

4<D/d<8

D-core diameter d-resistance wire diameter 1.2.5 Winding distance The spiral resistance wire must have a certain distance between each turn. This distance is called the wire pitch (pitch). The wire pitch is an extremely important parameter considered in the design of the heating element, which has a great influence on the heat uniformity of the product, the packing density and the product life. Usually we are used to legalizing the wire pitch as a multiple of the diameter of the wire.

A. The wire pitch should not be too small. One of the dangers is that the heat generated by the resistance wire cannot be quickly transferred to the metal tube casing, resulting in an increase in the temperature of the resistance wire itself, which is easy to reduce electrical properties. The second hazard is that during the production process, the product is “sieving” when adding powder. That is, when the product is filled with powder, the magnesium oxide powder cannot be filled into the ring diameter sufficiently, because the dense electric resistance wire It became a layer of "sieve" to block the magnesium powder from entering the middle of the spiral. Due to the occurrence of this phenomenon, the temperature of the resistance wire itself is also increased, and the magnesium oxide around the resistance wire is burned to lower the electrical performance and shorten the life of the product.

B. The wire pitch is not as large as possible. In general, a larger wire pitch is beneficial to the performance of the product, but it is not infinitely large. If the wire distance is too large, it will cause wire breakage (small diameter resistance wire is easy to have this phenomenon). Or the resistance wire diameter is reduced. Therefore, it is necessary to properly select the wire pitch ratio. Usually the following data should be used as well.

2.5<S/d<5

S—wire distance d—resistance wire diameter Note: The hint here is <Φ0.2mm. The lower limit of the resistance wire must be considered to be greater than 2.5 times (or larger).

2, the skill of quickly calculating the comprehensive parameters of the resistance of the design of the tubular electric heating element in the design of the resistance wire has a close impact on its overall product performance and material consumption. Being able to master some of the techniques of rapid design will undoubtedly play a positive role in improving work efficiency and product quality.

One of the 2.1 techniques, quickly calculate the meter resistance of any specification. Everyone knows that the meter resistance of the resistance wire is the basic parameter of the heating element design. Whether it is the surface load of the wire or the wire pitch of the resistance wire, the resistance should be considered first. The wire resistance of the wire is the premise. For a few commonly used resistance wire meters, it should be remembered, but it may not be easy to remember all the resistance wires. As mentioned above, it is still possible to calculate the resistivity of a certain type of resistance wire, but it is still very complicated to calculate this value. The skill I summed up is that the meter resistance of a specification of Φ0.2mm is a reference number (actually this is also a resistivity). For example, the meter resistance of the Cr25AC5 grade Φ0.2mm is 45.2Ω, so just remember this value and ask for any A meter of meter resistance is easy. The specific calculation method is to calculate the square of the radius of the required specification, and then remove 45.2 to obtain the meter resistance of this specification.

Shanghai Island Han Industrial Co., Ltd. specializes in the research and development of scientific instruments and environmental equipment. It operates various types of drying ovens, incubators, test chambers, oscillators, and agents.