Resistance wire basic knowledge and calculation formula

Time：2022-10-21 13:56:07 Page view：

The design procedure of the metal tubular electric heating element is more complex, and there are more relevant parameters. To reasonably optimize the comprehensive data of the resistance wire, it is often necessary to calculate repeatedly, and the calculation of these parameters is a loop, so the determination of the best scheme must spend some energy and time. Therefore, how to calculate quickly and improve work efficiency is also the common desire of the majority of technicians in the electric heating manufacturing industry.

Basic principle of resistance heating and related content

The conversion of electrical energy into heat and its utilization is an important effect in electricity. There are many ways to convert electricity into heat, including plasma heating, electron beam heating, arc heating, induction heating and resistance heating (not simply limited to resistance wire heating).
Resistance heating: The use of the resistance of the conductor to generate heat in the aid of related media materials heating various substances is the basic principle and working principle of resistance heating. Resistance heating depends on a variety of materials, but the alloy resistance wire as a heating material is the most widely used mainstream material, and as a metal tubular heating body of the heating wire material is basically nickel chromium Ni-Cr, iron chromium aluminum Fe-Cr-AL and chromium aluminum Cr-AL-Mo with molybdenum as the main material. The most basic conditions and technical requirements for alloy resistance wire are: resistivity, uniformity of resistance value, chemical stability, oxidation resistance, high temperature strength and so on.

Related parameters of resistance wire

The resistivity of the resistance wire is also known as the resistivity or specific resistance, which represents an electrical parameter of the conductor's resistance to current passing characteristics. The relationship between the resistivity of the conductor and the resistance is as follows:
R = rho C/S
R - Resistance Ω of the conductor
L - Length of conductor m
S - Cross-sectional area (cross-sectional area) mm2 of the conductor
ρ - Resistivity of conductor μω.m
The resistivity is related to the chemical composition, metallographic structure and working temperature of the alloy, and is an important data for calculating the resistance value of different specifications of resistance wires. Therefore, we can use the above formula, as long as we know the resistivity of its material, we can calculate the meter resistance of various specifications of the resistance wire at any time (resistance value per meter length).

电阻丝的温度系数

The resistance value (resistivity) of the alloy resistance wire also changes with the change of temperature, and the value of this change is called the resistance temperature coefficient. The ratio of the resistivity Pt at the operating temperature to the resistivity P20 at 20℃ is called the resistivity correction coefficient, and the relationship is as follows:
Ct=Rt/R
Ct - Temperature coefficient of resistance at ° C
Rt - Resistance value at temperature t
R - Temperature Resistance at room temperature
If a certain type of resistance wire Ct (resistance temperature coefficient) is known, then the above relationship can be used to calculate 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 actual work, the approximate change curve of the figure above should be referred to, and the actual working condition of the element should be combined with the simulation test, that is, the ratio of the resistance value at normal temperature and the resistance value at working temperature. According to this coefficient (measured) determine the resistance value at normal temperature. (Here refers to the resistance value of the finished product)

surface load

Wire surface load refers to the electric power W/cm2 borne by the unit surface area of the total length of the electric heating wire. Under normal circumstances, the worse the working conditions of the components (poor heat dissipation conditions), the smaller the wire surface load should be selected, on the contrary, the working conditions of the components are better (good heat dissipation conditions), the relatively large wire surface load can be selected, of course, the heat dissipation conditions depend on the heating material, the size of the object, the flow of fluid, the wind speed of the air and so on. It is not comprehensive to say that simply using the heating medium to determine the pipe surface load and the wire surface load.
For example, the heating element power is the same, but the heat dissipation conditions are not the same when cast in 1Kg aluminum and in 3Kg material. Another example is that the same flowing air heating wind speed is not the same, the effect is not the same. Therefore, it is reliable to consider other conditions while considering the heating medium to determine the surface load of the wire.
The surface load of the resistance wire is calculated as follows:
W/cm2 = P/ (d.le.l)
Where W/cm2 is the wire surface load
P - Electrical power
D - Diameter of resistance wire
L - Expansion length

Winding diameter

According to the rated voltage, rated power, heating medium conditions and selected wire surface load, the specification of the resistance wire is determined and reasonably and scientifically assembled and fixed to the center of the heating element. To achieve this purpose, it is necessary to spiral the resistance wire (special few products do not need to be wound). In the case of other conditions unchanged, a reasonable choice of wound mandrel, to achieve the ideal coil diameter of the heating wire is also an important part of the design process, as follows:
■ The diameter of the ring should not be too large, too large will reduce the distance between the resistance wire and the metal tube, which will reduce the insulation and voltage resistance of the component. Secondly, too large ring diameter reduces the mechanical tension (elasticity) of the resistance wire, which is easy to bring the sag phenomenon of the resistance wire during the powder adding process, resulting in uneven heating of the finished product. Of course, sometimes the user's drawing of the diameter of the rod is larger, then we still consider the overall performance, what kind of ring diameter should be what kind of ring diameter, rather reduce the diameter of the end of the rod and the resistance wire connection (see figure) to connect, can not affect the rationality of the ring diameter.
■ The ring diameter should not be too small, too small the ring diameter will reduce the heat generated by the resistance wire can not be quickly transferred to the metal tube shell. Because it cannot transfer heat quickly, it will inevitably bring about an increase in its own temperature, thereby shortening the life of the product. Secondly, for a few special products, the heating area 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 distance between the wires is too large after opening, as if it is forced to reduce the development of the winding mandrel, in fact, it is unnecessary to choose a larger resistance wire, or use double wire winding. This would solve the contradiction.
From the current design of most manufacturers, the choice between resistance wire and mandrel is roughly:
4 < D/d < 8
D- mandrel diameter
d- Diameter of resistance wire

Winding distance

There must be a certain distance between each coil of spiral resistance wire, and this distance is called wire distance (pitch). Wire distance is an extremely important parameter to be considered in the design of electric heating elements, which has a great impact on product heating uniformity, packing compactness and product life. Under normal circumstances, we are used to saying that the wire distance is a multiple of the diameter of the resistance wire.
The wire distance should not be too small, too small one of the dangers is that the heat generated by the resistance wire can not be quickly transferred to the metal tube housing, resulting in the resistance wire itself temperature rise, which is easy to reduce the electrical performance. The second hazard is that in the production process of the product, the "screening" phenomenon is brought when the powder is added, that is, when the product is added with powder, the magnesium oxide powder cannot be fully filled into the diameter of the ring, because the dense resistance wire has become a layer of "sieve" to block the magnesium powder into the middle of the spiral ring. Due to the emergence of this phenomenon, it will also bring the temperature of the resistance wire itself to rise, burn the magnesium oxide around the resistance wire to reduce the electrical performance and shorten the product life.
■ The wire distance is not the larger the better, in general, the larger the wire distance is beneficial to the performance of the product, but it is not infinite. If the wire distance is large to a certain range, it will bring the phenomenon of broken wire (small diameter resistance wire is most prone to this phenomenon). Or resistance wire diameter reduction phenomenon. Therefore, it is necessary to choose the wire distance ratio appropriately. In general, the following data should be used.
2.5 < S/d < 5
S - Wire pitch
d - Diameter of resistance wire

Quick calculation of resistance synthesis parameters

In the design of tubular electric heating element, the quality of resistance wire design has a close influence on its overall product performance and material consumption. Can master some quick design skills will undoubtedly play a positive role in improving work efficiency and product quality.
One of the tips is to quickly calculate the meter resistance of any specification
We all know that the meter resistance of the resistance wire is the most basic parameter in the comprehensive parameters of the design of the electric heating wire, whether it is the wire surface load, or the distance between the resistance wire are inseparable from the first consideration of the meter resistance of the resistance wire. For a few commonly used specifications of resistance wire meter resistance should be able to remember, but if all specifications of resistance wire can remember may not be easy. As described earlier, as long as you remember the resistivity of a certain grade of resistance wire can also be calculated, but to calculate this value is still very tedious.
Lin summarized the technique of using a meter resistance value of Φ0.2mm as a reference number (in fact, this is also a resistivity). For example, the meter resistance of Cr25AC5 grade Φ0.2mm is 45.2Ω, so long as you remember this value, it is easy to find any specification of the meter resistance. The specific calculation method is to calculate the square of the radius of this specification, and then 45.2 remove 100 times of this number to obtain the meter resistance of this specification.
Example 1:
Obtain the meter resistance value of Φ0.76mm
1) 0.382=0.1444
2) 45.2/14.44=3.13Ω
Example 2: Find the meter resistance value of Φ0.11mm
1) 0.0552=0.003025
2) 45.2/0.3025=149Ω
For nichrome alloy or other alloy materials resistance as long as Φ0.2mm meter resistance can also be calculated.
Tip two: List the comprehensive parameters of resistance wire commonly used in tubular electric heating elements through calculation.
This table is based on the Cr25AC5 material as the benchmark calculation, as long as the use of the product specifications (rated voltage, rated power) and pipe diameter, the length of the heating zone quickly find the relevant data you need, and then a little calculation can be obtained comprehensive parameters.
Example 1:220V, 1000W, pipe diameter Φ8, length of heating area 900mm, the use of mobile air (air conditioning).
First of all, it should be known that the Φ8 pipe diameter of the element is usually less than Φ3mm
If the winding resistance value is 47.4Ω after 52 shrinkage, then the Φ0.3 ~ Φ0.4 can be taken between the Φ3.6mm specification, corresponding to the diameter of the watch core rod is Φ2mm, the resistance value is 28.6Ω when winding 100mm, and the wire surface area is 27.2cm2.
Calculation:
52/28.6 = 1.818
Known: close winding length =181.8mm
Known: Length of heat zone is 900
The winding length is 181.8mm
The wire distance is 900/181.8=4.95 times
It is known that the surface area of 100mm tightly wound length is 27.2cm2, then the surface area of the branch element wire is 27.2×1.818=49.45
Unit load: 1000/49.45=20.2W/cm2
If you feel that the load of 20.2W/cm2 is too large, you can check the table again. In short, it is easy to quickly calculate the comprehensive parameters of the resistance wire through this table.

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