The most significant part of an electrothermal installation is the heating element. The main component of the devices indirect heating- resistor with high specific resistance. And one of the priority materials is chromium-nickel alloy. Since the resistance of nichrome wire is high, this material takes a leading place as a raw material for various types electrothermal installations. Calculation of a heater made of nichrome wire is carried out in order to determine the size of the heating element.
Basic Concepts
In general, it is necessary to calculate a heating element made of nichrome using four calculations: hydraulic, mechanical, thermal and electrical. But usually calculations are carried out only in two stages: based on thermal and electrical indicators.
Thermal characteristics include:
- thermal insulation;
- heat efficiency;
- required heat transfer surface.
The main purpose of calculating nichrome is to determine the geometric dimensions of the heating resistance.
To the electrical parameters of heaters are:
- supply voltage;
- power control method;
- power factor and electrical efficiency.
When choosing the supply voltage for heating devices, preference is given to what poses minimal threat to animals and service personnel. Mains voltage in installations Agriculture is 380/200 volts with a current frequency of 50 Hertz. If electrical installations are used in particularly damp areas, or if there is an increased electrical hazard, the voltage should be reduced. Its value should not exceed 12, 24, 36 volts.
Adjust the temperature and power of the heater can be done in two ways:
- changing the voltage;
- changing the resistance value.
The most common way to change power is to turn on a certain number of sections of a three-phase installation. In modern heating installations, power is changed by adjusting the voltage using thyristors.
The calculation of the operating current is based on a tabular relationship that relates the current load on a nichrome conductor, its cross-sectional area and temperature.
Tabular data were compiled for nichrome wire, which was stretched in air without taking into account oscillations and vibrations at a temperature of 20 °C.
In order to move to real conditions, it is necessary to use correction factors in the calculations.
Calculation of a nichrome spiral should be carried out in stages, using initial information about the heater: the required power and brand of nichrome.
Power of one section:
P - installation power, W;
m - number of phases, for single-phase m = 1;
n is the number of sections in one phase, for installations with a capacity of about 1 kW n = 1.
Operating current of one heater section:
U - mains voltage, for single-phase installations U = 220 V
Design wire temperature:
θр = θd/(Km Ks)
θd - permissible operating temperature, selected from Table 1 depending on the material, °C.
Table 1- Parameters of materials for electric heaters.
Km is the installation coefficient, selected from Table 2 depending on the design.
table 2- Installation coefficient for some types of heater designs in a quiet air flow.
The role of the installation coefficient is that it makes it possible to take into account the increase in heater temperature in real conditions compared to the data in the reference table.
Ks - coefficient environment, is determined from Table 3.
Table 3- Correction factor for certain environmental conditions.
The environmental coefficient corrects for improved heat transfer due to environmental conditions. That's why real results calculations will differ slightly from the table values.
Diameter d, mm and cross-sectional area S, mm 2 are selected according to operating current and design temperature from Table 4
Table 4- Permissible load on nichrome wire at 20 °C suspended horizontally in calm air.
Wire length of one section:
L = (U f 2 S*10 -6)/(ρ 20 Рс x10 3)
ρ 20 - resistivity at a temperature of 20 °C, selected from table 1;
α - temperature coefficient resistance is determined from the corresponding column in Table 1.
Spiral diameter:
D = (6…10) d, mm.
Determine the spiral pitch:
h = (2…4) d, mm
The spiral pitch affects work performance. At higher values, heat transfer increases.
Number of spiral turns
W = (lx10 3)/ (√h 2 +(πD) 2)
Spiral length:
If the purpose of the wire heater is to increase the temperature of the liquid, the operating current is increased by 1.5 times the calculated value. In case of calculating the heater with closed type It is recommended to reduce the operating current by 1.2 times.
Classification of heaters by temperature
Heaters to the limit permissible temperature are divided into five classes:
Troubleshooting Settings
The highest probability of failure of electric heaters is due to oxidation of the surface of the heating resistance.
Factors that influence the rate of heater destruction:
Due to the fact that electric heating installations operate in excess of the permissible values of these parameters, the most frequent breakdowns occur: burning of contacts, violation of the mechanical strength of nichrome wire.
Repair of a nichrome heating element is carried out by soldering or twisting.
On this page we will look at background information about the materials that are used to make electric heaters, and also provide examples of calculations for nichrome heaters for electric furnaces.
Heater materials
Heaters are the most important element of the furnace, and they must meet many requirements.
- Heat resistance and heat resistance. Wire heaters must have good heat resistance (the resistance of a metal or alloy at high temperatures to gas corrosion), as well as high-temperature resistance.
- Low temperature coefficient of resistance. This factor is important when choosing a material. A low coefficient means that even when the material is heated, its electrical resistance changes very little. For example, if this temperature coefficient is large, then in order to turn on the furnace in a cold state, it is necessary to use reduced voltage transformers at the initial moment.
- High electrical resistivity. The heater in an electric furnace must have this characteristic. The higher the resistance value, the more the material can heat up, and the shorter it is needed. The larger the diameter of the heating wire, the longer its service life. Materials with very high electrical resistance are chromium-nickel precision alloys and, and.
- Good technological properties. The materials must have good ductility and weldability, since they are used to make: wires, tapes, complex shape heating elements.
- Permanent physical properties. Neither should change at high temperatures, for long periods of time.
Nichrome and fechral, which have high electrical resistance, are best suited for the production of electric heaters for electric furnaces. For more information about the grades and their properties, see GOST 10994-74.
Grades of nichrome suitable for the manufacture of heaters:
Fechral grades suitable for the manufacture of heaters: .
Also iron - chromium-nickel alloys: Kh27N70YuZ, Kh15N60Yu3.
All these alloys have the characteristics described above. For example, high heat resistance is ensured due to the formation of a chromium oxide film on the surface.
Compare nichrome and fechral
Advantages of nichrome:
- Excellent mechanical properties at any temperature;
- creep resistance;
- Plastic and well processed;
- Has excellent weldability;
- does not age;
- non-magnetic.
Advantages of fehrali:
- has more low price than nichrome, since it does not contain expensive nickel;
- Fechral X23Yu5T has better heat resistance than nichrome. Fechral wire 6 mm thick can operate at 1400 °C.
Disadvantages of nichrome:
- More expensive than fechral, since the main component nickel has a high cost;
- The operating temperature is lower than that of fechral.
Disadvantages of fehrali:
- the alloy is more brittle, especially at temperatures of about 1000 °C and more;
- Low creep resistance;
- the alloy is magnetic because it contains iron. Fechral also rusts in humid environments.
- Interacts with iron oxides and fireclay lining;
- During operation, fechral heaters elongate.
There are also alloys Kh27N70YuZ and Kh15N60Yu3 which contain 3% aluminum. This element makes it possible to improve the heat resistance of alloys. These alloys do not react with iron oxides or fireclay. They are non-fragile, durable and well processed. The maximum operating temperature is 1200 °C.
Heaters are also made from refractory metals or non-metals (coal, molybdenum disilicide, graphite, carborundum). Molybdenum disilicide and carborundum are used for heaters in high-temperature furnaces. Graphite and carbon heaters are used in furnaces with a protective atmosphere.
Refractory metals that are often used are tantalum, molybdenum, niobium, and tungsten. Tungsten and molybdenum are used in furnaces with a protective atmosphere, as well as high-temperature vacuum furnaces. Molybdenum heaters are used in vacuum up to 1700 °C and in a protective atmosphere at temperatures up to 2200 °C. This feature is that molybdenum begins to evaporate at a temperature of 1700 ° C (vacuum). Tungsten heaters are capable of operating at... up to 3000 °C. Niobium and tantalum are very rarely used to produce heaters.
Calculation of heaters for electric furnaces
When calculating heaters for electric furnaces, the following initial data are taken into account:
- volume of furnace working space;
- heaters power;
- Maximum temperature(required for implementation technological process: hardening, tempering, sintering).
Important: If there is no data on the power of the furnace, it is calculated according to an empirical rule. You need to know: the length and diameter of the wire, or the length and cross-sectional area of the tape, heater.
We will look at one of the most popular alloys for the production of heaters - nichrome X20N80.
Simple calculation of the length and diameter of the heater wire for a certain furnace power. With one small feature.
Example. Nichrome wire X20N80.
Initial data:
- Device power P = 1.5 kW = 1500 W.
- The maximum temperature to which the heater will heat up is 900 °C.
- Voltage U = 220 V.
- The current strength is determined as follows:
- The heater resistance is determined as follows:
- Current strength plays key moment when choosing the wire diameter of a nichrome heater. Using the table below, we select the required diameter. In our example, current = 6.8181 A, and heater temperature = 900 °C, then the wire diameter will be d = 0.55 mm, and accordingly the cross section will be S = 0.238 mm2.
We obtained such values because the wire is selected such that it has an allowable current strength. Which in turn is less than the calculated current strength. That is, we select nichrome wire with the nearest higher permissible current value.
Note:
Provided that the nichrome heater is located inside the heating liquid, the permissible current strength is increased by 10-50%.
If the heater is in a closed position, then the permissible current is reduced by 20% for thick wire, and by 50% for thin wire.
- Determination of wire length.
R - electrical resistance, Ohm,
p—electrical resistivity of the material, Ohm mm2/m,
l – heater length, m,
S—cross-sectional area, mm2.
Based on the formula above, we find that the length of the heater is calculated as follows:
In the example, a wire diameter of d = 0.55 mm was used.
Nominal value of specific electrical resistance wire X20N80 is taken from Table 2, in accordance with GOST 12766.1-90 and has a value of ρ = 1.1 Ohm mm2/m.
Calculation summary showed that under the conditions:
device power P = 1.5 kW = 1500 W;
heater temperature 900 °C;
you need nichrome wire with a valley of 6.91 m and a diameter of 0.55 mm.
table 2
Detailed calculation of the length and diameter of nichrome wire for heaters of a particular furnace.
Here is a complex calculation that takes into account: additional parameters of the heaters, various options their connection to a three-phase network.
The calculation is carried out based on the internal volume of the furnace.
- The chamber volume is calculated using the well-known formula:
For example, let's take:
- height h = 490 mm,
- chamber width d = 350 mm,
- chamber depth l = 350 mm.
The volume will be:
- The power of the furnace is calculated according to the rule of thumb: electric furnaces with a volume of 10 to 50 liters have a specific power of about 100 W/l, furnaces with a volume of 100 - 500 liters - respectively, a power of 50 to 70 W/l.
In our example, the specific power of the furnace will be 100 W/l.
Based on this, the power of the nichrome heater should be:
Important!
Heaters with a power of 5-10 kW are made single-phase. With a power above 10 kW, the heaters are made three-phase.
- The current strength that passes through the heater is calculated by:
P is the power of the nichrome heater,
U is voltage.
The heater resistance is calculated using the formula:
If the heater is connected to one phase, then U = 220 V, if to a three-phase, then U = 220 V will be between zero and any other phase, or U = 380 V will be between two phases.
Single-phase current (household network)
– current strength on the heater wire.
— furnace heater resistance.
With a three-phase connection, the load is distributed evenly across three phases, that is, 6 divided by 3 and you get 2 kW for each phase. It follows from this that we need 3 heaters of 2 kW each.
There are two ways to connect three heaters at once. “TRIANGLE” and “STAR”.
The “STAR” connection means connecting each heater between zero and its phase (Fig. 2). In this case, the voltage is U = 220 V.
Current strength:
Resistance:
Rice. 1 “STAR” connection in a three-phase network
The “TRIANGLE” connection implies the location of the heater between two phases (Fig. 3). From this it follows that the voltage U = 380 V.
Current strength:
Resistance:
Rice. 2 “TRIANGLE” connection in a three-phase network
- Having determined the resistance of the nichrome heater, you need to calculate its diameter and length.
It is also necessary to analyze the specific surface power of the wire (the power that is released from 1 cm2 of surface area). This power depends on the design of the heater itself and the temperature of the heated material.
At single-phase connection, for 60 l. furnace resistance: R = 8.06 Ohm.
We take wire X20N80 with a diameter of d=1 mm.
To get our resistance, we need to calculate the length:
ρ is the nominal value of the electrical resistance of a wire 1 meter long according to GOST 12766.1-90, (Ohm/m).
The required piece of nichrome wire will have the following mass:
μ is the mass of 1 meter of nichrome wire.
The surface area of a wire length l=5.7 meters is calculated by the formula:
l – length in centimeters.
d – diameter in centimeters.
According to calculations, we found that the wire surface area of 179 cm2 emits 6 kW. Thus, 1 cm2 of wire area releases power:
β is the surface power of the heating wire.
IN in this example we have received too much surface power of the wire, which is why the heater will simply melt when heated to the temperature required to obtain the surface power. This temperature will definitely be higher than the melting point of nichrome. This calculation example shows the incorrect choice of the diameter of the heating wire for the manufacture of the heater.
Each material has its own permissible surface power value depending on temperature. The values are taken from the tables.
High-temperature furnaces (700 - 800 °C) have an allowable surface power, (W/m2), which is calculated by the formula:
βeff – surface power depending on the temperature of the heat-receiving medium, (W/m2).
α is the radiation efficiency coefficient.
Table 4 
A low-temperature furnace (200 – 300 °C) has a permissible surface power of (4 – 6) × 104 W/m2.
Let's assume that the temperature of our heater is 1000 °C, and we need to heat the conditional workpiece to 700 °C. Then from the table. 3 is taken
βeff = 8.05 W/cm2,
and calculate:
- Next, you need to calculate the diameter of the wire heater or the thickness and width of the tape heater, and of course the length of the heater.
The diameter is determined by the formula:
d—diameter, m;
U is the voltage at the ends of the heater, V;
P—power, W;
βadd — permissible surface power, W/m2.
ρt is the resistivity of the material at a certain temperature, Ohm m;
ρ20 is the electrical resistivity of the material at a temperature of 20 °C, Ohm m.
k - Correction factor that is used to calculate the change in electrical resistance depending on temperature.
The length of the nichrome wire is determined as follows:
l — length, m.
Electrical resistivity X20N80 –
Single-phase current (household network)
Looking at the previous calculations, it became clear that for a 60 liter stove connected to a single-phase network:
U = 220 V, P = 6000 W, permissible surface power βadd = 1.6 × 104 W/m2. Substituting these numbers into the formula we get the thickness of the wire.
This thickness is rounded to the closest standard size, which is located in Table 8 according to GOST 12766.1-90.
Appendix 2, Table. 8.
In our example, the wire diameter from the formula is rounded to d = 2.8 mm.
The heater will have this length
Our example requires a wire length l = 43 m.
Sometimes you also need to find out the mass of all the wire you need.
There is a formula for this:
m is the mass of the piece of wire we need, kg;
l — length, m.
μ—specific gravity (1 m of wire), kg/m;
The calculation showed that our nichrome wire will have a mass m = 43 × 0.052 = 2.3 kg.
Our calculation example allows us to determine the minimum wire diameter required for the heater under certain conditions. This method is the most economical and optimal. Of course, you can use wire with a larger diameter, but its quantity will of course increase then.
Examination
The calculation of nichrome wire can be checked.
We obtained a wire diameter d = 2.8 mm. The length is calculated as follows:
l—length, m;
ρ is the nominal value of the electrical resistance of a wire 1 m long, Ohm/m.
R—resistance, Ohm;
k — correction factor electrical resistance depending on temperature;
The calculation showed that the obtained length of the wire coincides with the length obtained in another calculation.
To check the surface power and compare with the permissible power. In accordance with paragraph 4.
and does not exceed the permissible βadd = 1.6 W/cm2.
Bottom line
In our example, we need 43 meters of nichrome wire grade X20N80 with a diameter d = 2.8 mm. Wire weight - 2.3 kg.
Three-phase current (industrial network)
We find the length and diameter of the wire that is necessary for the production of heaters.
Connection to three-phase current according to the “STAR” type.
We have 3 heaters, each of which requires 2 kW of power.
We find the length, diameter and mass of only one heater.
Nearest standard larger size d = 1.4 mm.
Length, l = 30 meters.
Heater weight
Checking
With a nichrome wire diameter d = 1.4 mm, calculate the length
The length is almost the same as the calculation above.
The surface power of the wire is
Total count
We have three identical heaters connected according to the “STAR” type, and for them we need:
l = 30×3 = 90 meters of wire weighing m = 0.39×3 = 1.2 kg.
Connection to three-phase current according to the “TRIANGLE” type. (Fig. 3)
Comparing our obtained value, the nearest large standard size is d = 0.95 mm.
One heater will have a length of l = 43 meters.
Heater weight
Checking the calculation
With a wire diameter d = 0.95 mm, we calculate the wire length:
The values for the wire length are almost the same for both calculations.
Surface power will be:
and does not exceed the permissible limit.
Summarize
Connecting three heaters according to the “TRIANGLE” scheme, you need:
l = 43×3 = 129 meters of wire, weighing
m = 0.258×3 = 0.8 kg.
Summing up the results for both types of connection “STAR” and “DELTA” to three phases, we get interesting data.
For a “STAR” you need a wire with a diameter of d=1.4 mm, and for a “TRIANGLE” a diameter of d=0.95 mm,
The length of the wire for the “STAR” scheme will be 90 meters with a mass of 1.2 kg, and for the “TRIANGLE” scheme 129 meters with a mass of 0.8 kg, that is, 800 g.
To use nichrome wire, it is wound into a spiral. The diameter of the spiral is assumed to be:
for chromium-nickel alloys.
- for chrome-aluminium.
D—spiral diameter, mm.
d—wire diameter, mm.
To eliminate overheating, the spiral is stretched to such an extent that the distance between the turns is 1.5-2 times greater than the diameter of the nichrome wire itself.
We reviewed information about electric heaters, examples of calculating wire heaters for electric furnaces.
It is also worth remembering that in addition to wire, tape can also be used as heaters. In addition to choosing the wire size, it is worth considering the heater material, type, and location.
Very often, if you want to make or repair heater When making electric furnaces with your own hands, a person has many questions. For example, what diameter should the wire be taken, what should its length be, or what power can be obtained using a wire or tape with the given parameters, etc. At the right approach To solve this issue, it is necessary to take into account quite a lot of parameters, for example, the strength of the current passing through heater, operating temperature, type of electrical network and others.This article provides background information on the materials most common in the manufacture of heaters electric ovens, as well as methods and examples of their calculation (calculation of heaters for electric furnaces).
Heaters. Materials for the manufacture of heaters
Directly heater- one of the most important elements of the furnace, it is it that carries out heating, has the highest temperature and determines the performance of the heating installation as a whole. Therefore, heaters must meet a number of requirements, which are given below.Requirements for heaters
Basic requirements for heaters (heater materials):- Heaters must have sufficient heat resistance (scale resistance) and heat resistance. Heat resistance - mechanical strength at high temperatures. Heat resistance - resistance of metals and alloys to gas corrosion at high temperatures (the properties of heat resistance and heat resistance are described in more detail on the page).
- Heater in an electric furnace must be made of a material with high electrical resistivity. Speaking in simple language, the higher the electrical resistance of the material, the more it heats up. Therefore, if you take a material with lower resistance, you will need a heater of greater length and with a smaller cross-sectional area. It is not always possible to place a sufficiently long heater in the oven. It is also worth considering that, the larger the diameter of the wire from which the heater is made, the longer its service life . Examples of materials with high electrical resistance are chromium-nickel alloy, iron-chromium-aluminum alloy, which are precision alloys with high electrical resistance.
- A low temperature coefficient of resistance is an essential factor when choosing a material for a heater. This means that when the temperature changes, the electrical resistance of the material heater doesn't change much. If the temperature coefficient of electrical resistance is high, to turn on the furnace in a cold state it is necessary to use transformers that initially provide a reduced voltage.
- The physical properties of heater materials must be constant. Some materials, for example carborundum, which is a non-metallic heater, can change their physical properties over time, in particular electrical resistance, which complicates their operating conditions. To stabilize electrical resistance, transformers with a large number of steps and a voltage range are used.
- Metal materials must have good technological properties, namely: ductility and weldability, so that they can be used to make wire, tape, and from the tape - heating elements of complex configuration. Also heaters can be made from non-metals. Non-metallic heaters are pressed or molded into a finished product.
Materials for the manufacture of heaters
The most suitable and most used in the production of heaters for electric furnaces are precision alloys with high electrical resistance. These include alloys based on chromium and nickel ( chromium-nickel), iron, chromium and aluminum ( iron-chromium-aluminum). The grades and properties of these alloys are discussed in “Precision alloys. Stamps". Representatives of chromium-nickel alloys are grades X20N80, X20N80-N (950-1200 °C), X15N60, X15N60-N (900-1125 °C), iron-chromium-aluminum alloys - grades X23Yu5T (950-1400 °C), X27Yu5T (950-1350 °C ), X23Yu5 (950-1200 °C), X15YU5 (750-1000 °C). There are also iron-chromium-nickel alloys - Kh15N60Yu3, Kh27N70YUZ.The alloys listed above have good heat resistance and heat resistance properties, so they can operate at high temperatures. good heat resistance provides a protective film of chromium oxide that forms on the surface of the material. The melting point of the film is higher than the melting point of the alloy itself; it does not crack when heated and cooled.
Let's give comparative characteristics nichrome and fechral.
Advantages of nichrome:
- good mechanical properties at both low and high temperatures;
- the alloy is creep-resistant;
- has good technological properties - ductility and weldability;
- well processed;
- does not age, non-magnetic.
- high cost of nickel - one of the main components of the alloy;
- lower operating temperatures compared to fechral.
- a cheaper alloy compared to nichrome, because does not contain ;
- has better heat resistance compared to nichrome, for example, fechral X23Yu5T can operate at temperatures up to 1400 °C (1400 °C is the maximum operating temperature for a heater made of wire Ø 6.0 mm or more; Ø 3.0 - 1350 °C; Ø 1.0 - 1225 °C; Ø 0.2 - 950 °C).
- a brittle and weak alloy, these negative properties are especially pronounced after the alloy has been at temperatures above 1000 °C;
- because Since fechral contains iron, this alloy is magnetic and can rust in a humid atmosphere at normal temperatures;
- has low creep resistance;
- interacts with fireclay lining and iron oxides;
- During operation, fechral heaters elongate significantly.
IN Lately alloys of the type Kh15N60Yu3 and Kh27N70YuZ have been developed, i.e. with the addition of 3% aluminum, which significantly improved the heat resistance of the alloys, and the presence of nickel practically eliminated the disadvantages of iron-chromium-aluminum alloys. Alloys Kh15N60YUZ, Kh27N60YUZ do not interact with fireclay and iron oxides, are fairly well processed, mechanically strong, and non-fragile. The maximum operating temperature of the X15N60YUZ alloy is 1200 °C.
In addition to the above-mentioned alloys based on nickel, chromium, iron, and aluminum, other materials are used for the manufacture of heaters: refractory metals, as well as non-metals.
Among non-metals for the manufacture of heaters, carborundum, molybdenum disilicide, coal, and graphite are used. Carborundum and molybdenum disilicide heaters are used in high-temperature furnaces. In furnaces with a protective atmosphere, coal and graphite heaters are used.
Among the refractory materials, tantalum and niobium can be used as heaters. In high-temperature vacuum furnaces and furnaces with a protective atmosphere, they are used molybdenum heaters And tungsten. Molybdenum heaters can operate up to temperatures of 1700 °C in a vacuum and up to 2200 °C in a protective atmosphere. This temperature difference is due to the evaporation of molybdenum at temperatures above 1700 °C in a vacuum. Tungsten heaters can operate up to 3000 °C. In special cases, heaters made of tantalum and niobium are used.
Calculation of electric furnace heaters
Typically, the initial data for this are the power that the heaters must provide, the maximum temperature that is required to carry out the corresponding technological process (tempering, hardening, sintering, etc.) and the dimensions of the working space of the electric furnace. If the furnace power is not specified, it can be determined using a rule of thumb. When calculating heaters, it is necessary to obtain the diameter and length (for wire) or cross-sectional area and length (for tape), which are necessary for manufacturing of heaters.It is also necessary to determine the material from which to make heaters(this point is not discussed in the article). In this article, a chromium-nickel precision alloy with high electrical resistance, which is one of the most popular in the manufacture of heating elements, is considered as a material for heaters.
Determination of the diameter and length of the heater (nichrome wire) for a given furnace power (simple calculation)
Perhaps the simplest option heater calculations from nichrome is the choice of diameter and length for a given heater power, supply voltage, as well as the temperature that the heater will have. Despite the simplicity of the calculation, it has one feature, which we will pay attention to below.An example of calculating the diameter and length of a heating element
Initial data:
Device power P
= 800 W; mains voltage U
= 220 V; heater temperature 800 °C. Nichrome wire X20N80 is used as a heating element.
1. First you need to determine the current strength that will pass through the heating element:
I=P/U
= 800 / 220 = 3.63 A.
2. Now you need to find the heater resistance:
R=U/I
= 220 / 3.63 = 61 Ohm;
3. Based on the value of the current strength obtained in step 1 passing through nichrome heater, you need to select the wire diameter. And this point is important. If, for example, with a current of 6 A you use nichrome wire with a diameter of 0.4 mm, it will burn. Therefore, having calculated the current strength, it is necessary to select the appropriate wire diameter value from the table. In our case, for a current of 3.63 A and a heater temperature of 800 °C, we select nichrome wire with a diameter d = 0.35 mm and cross-sectional area S = 0.096 mm 2.
General rule selection of wire diameter can be formulated as follows: it is necessary to select a wire whose permissible current strength is not less than the calculated current strength passing through the heater. In order to save heater material, you should choose a wire with the nearest higher (than calculated) permissible current strength.
Table 1
| Diameter, mm | Cross-sectional area of nichrome wire, mm 2 | Nichrome wire heating temperature, °C | ||||||
|---|---|---|---|---|---|---|---|---|
| 200 | 400 | 600 | 700 | 800 | 900 | 1000 | ||
| Maximum permissible current, A | ||||||||
| 5 | 19,6 | 52 | 83 | 105 | 124 | 146 | 173 | 206 |
| 4 | 12,6 | 37,0 | 60,0 | 80,0 | 93,0 | 110,0 | 129,0 | 151,0 |
| 3 | 7,07 | 22,3 | 37,5 | 54,5 | 64,0 | 77,0 | 88,0 | 102,0 |
| 2,5 | 4,91 | 16,6 | 27,5 | 40,0 | 46,6 | 57,5 | 66,5 | 73,0 |
| 2 | 3,14 | 11,7 | 19,6 | 28,7 | 33,8 | 39,5 | 47,0 | 51,0 |
| 1,8 | 2,54 | 10,0 | 16,9 | 24,9 | 29,0 | 33,1 | 39,0 | 43,2 |
| 1,6 | 2,01 | 8,6 | 14,4 | 21,0 | 24,5 | 28,0 | 32,9 | 36,0 |
| 1,5 | 1,77 | 7,9 | 13,2 | 19,2 | 22,4 | 25,7 | 30,0 | 33,0 |
| 1,4 | 1,54 | 7,25 | 12,0 | 17,4 | 20,0 | 23,3 | 27,0 | 30,0 |
| 1,3 | 1,33 | 6,6 | 10,9 | 15,6 | 17,8 | 21,0 | 24,4 | 27,0 |
| 1,2 | 1,13 | 6,0 | 9,8 | 14,0 | 15,8 | 18,7 | 21,6 | 24,3 |
| 1,1 | 0,95 | 5,4 | 8,7 | 12,4 | 13,9 | 16,5 | 19,1 | 21,5 |
| 1,0 | 0,785 | 4,85 | 7,7 | 10,8 | 12,1 | 14,3 | 16,8 | 19,2 |
| 0,9 | 0,636 | 4,25 | 6,7 | 9,35 | 10,45 | 12,3 | 14,5 | 16,5 |
| 0,8 | 0,503 | 3,7 | 5,7 | 8,15 | 9,15 | 10,8 | 12,3 | 14,0 |
| 0,75 | 0,442 | 3,4 | 5,3 | 7,55 | 8,4 | 9,95 | 11,25 | 12,85 |
| 0,7 | 0,385 | 3,1 | 4,8 | 6,95 | 7,8 | 9,1 | 10,3 | 11,8 |
| 0,65 | 0,342 | 2,82 | 4,4 | 6,3 | 7,15 | 8,25 | 9,3 | 10,75 |
| 0,6 | 0,283 | 2,52 | 4 | 5,7 | 6,5 | 7,5 | 8,5 | 9,7 |
| 0,55 | 0,238 | 2,25 | 3,55 | 5,1 | 5,8 | 6,75 | 7,6 | 8,7 |
| 0,5 | 0,196 | 2 | 3,15 | 4,5 | 5,2 | 5,9 | 6,75 | 7,7 |
| 0,45 | 0,159 | 1,74 | 2,75 | 3,9 | 4,45 | 5,2 | 5,85 | 6,75 |
| 0,4 | 0,126 | 1,5 | 2,34 | 3,3 | 3,85 | 4,4 | 5,0 | 5,7 |
| 0,35 | 0,096 | 1,27 | 1,95 | 2,76 | 3,3 | 3,75 | 4,15 | 4,75 |
| 0,3 | 0,085 | 1,05 | 1,63 | 2,27 | 2,7 | 3,05 | 3,4 | 3,85 |
| 0,25 | 0,049 | 0,84 | 1,33 | 1,83 | 2,15 | 2,4 | 2,7 | 3,1 |
| 0,2 | 0,0314 | 0,65 | 1,03 | 1,4 | 1,65 | 1,82 | 2,0 | 2,3 |
| 0,15 | 0,0177 | 0,46 | 0,74 | 0,99 | 1,15 | 1,28 | 1,4 | 1,62 |
| 0,1 | 0,00785 | 0,1 | 0,47 | 0,63 | 0,72 | 0,8 | 0,9 | 1,0 |
Note :
- if the heaters are located inside the heated liquid, then the load (permissible current) can be increased by 1.1 - 1.5 times;
- with a closed arrangement of heaters (for example, in chamber electric furnaces), it is necessary to reduce the load by 1.2 - 1.5 times (a smaller coefficient is taken for thicker wire, a larger one for thinner wire).
R = ρ l/S ,
Where R - electrical resistance of the conductor (heater) [Ohm], ρ - specific electrical resistance of the heater material [Ohm mm 2 / m], l - length of conductor (heater) [mm], S - cross-sectional area of the conductor (heater) [mm 2 ].
Thus, we obtain the length of the heater:
l = R S / ρ
= 61 · 0.096 / 1.11 = 5.3 m.
In this example, nichrome wire Ø 0.35 mm is used as a heater. In accordance with "Wire made of precision alloys with high electrical resistance. Technical specifications" the nominal value of the electrical resistivity of nichrome wire grade X20N80 is 1.1 Ohm mm 2 / m ( ρ = 1.1 Ohm mm 2 / m), see table. 2.
The result of the calculations is the required length of nichrome wire, which is 5.3 m, diameter - 0.35 mm.
table 2
Determination of the diameter and length of the heater (nichrome wire) for a given furnace (detailed calculation)
The calculation presented in at this point, is more complex than the above. Here we will take into account the additional parameters of the heaters and try to understand the options for connecting the heaters to a three-phase current network. We will calculate the heater using an electric furnace as an example. Let the initial data be inner dimensions ovens.1. The first thing you need to do is calculate the volume of the chamber inside the oven. In this case let's take h = 490 mm, d = 350 mm and l = 350 mm (height, width and depth respectively). Thus, we get the volume V = h d l = 490 · 350 · 350 = 60 · 10 6 mm 3 = 60 l (measure of volume).
2. Next, you need to determine the power that the oven should produce. Power is measured in Watts (W) and is determined by rule of thumb: for an electric oven with a volume of 10 - 50 liters, the specific power is 100 W/l (Watt per liter of volume), for a volume of 100 - 500 liters - 50 - 70 W/l. Let us take the specific power of 100 W/l for the furnace in question. Thus, the heater power of the electric furnace should be P = 100 · 60 = 6000 W = 6 kW.
It is worth noting that with a power of 5-10 kW heaters are usually manufactured single-phase. At high powers, to ensure even loading of the network, the heaters are made three-phase.
3. Then you need to find the current passing through the heater I=P/U , Where P - heater power, U - voltage across the heater (between its ends), and heater resistance R=U/I .
There may be two options for connecting to the electrical network:
- to a single-phase household network - then U = 220 V;
- to an industrial three-phase current network - U = 220 V (between neutral wire and phase) or U = 380 V (between any two phases).
I=P/U
= 6000 / 220 = 27.3 A - current passing through the heater.
Next, you need to determine the resistance of the furnace heater.
R=U/I
= 220 / 27.3 = 8.06 Ohm.
Figure 1 Wire heater in a single-phase current network
The required values of the wire diameter and its length will be determined in paragraph 5 of this paragraph.
At this type connection, the load is distributed evenly into three phases, i.e. 6 / 3 = 2 kW per phase. So we need 3 heaters. Next, you need to select a method for connecting the heaters (load) directly. There can be 2 ways: “STAR” or “TRIANGLE”.
It is worth noting that in this article the formulas for calculating the current strength ( I ) and resistance ( R ) for a three-phase network are not written in classic form. This is done in order not to complicate the presentation of the material on calculating heaters with electrical terms and definitions (for example, phase and linear voltages and currents and the relationships between them are not mentioned). The classical approach and formulas for calculating three-phase circuits can be found in specialized literature. In this article, some mathematical transformations carried out on classical formulas are hidden from the reader, and this does not have any effect on the final result.
When connecting “STAR” type the heater is connected between phase and zero (see Fig. 2). Accordingly, the voltage at the ends of the heater will be U
= 220 V.
I=P/U
= 2000 / 220 = 9.10 A.
R=U/I
= 220 / 9.10 = 24.2 Ohms.
Figure 2 Wire heater in a three-phase current network. STAR connection
When connecting the “TRIANGLE” type the heater is connected between two phases (see Fig. 3). Accordingly, the voltage at the ends of the heater will be U
= 380 V.
Current passing through the heater -
I=P/U
= 2000 / 380 = 5.26 A.
Resistance of one heater -
R=U/I
= 380/ 5.26 = 72.2 Ohm.
Figure 3 Wire heater in a three-phase current network. Connection according to the "TRIANGLE" scheme
4. After determining the resistance of the heater with appropriate connection to the electrical network it is necessary to select the diameter and length of the wire.
When determining the above parameters, it is necessary to analyze specific surface power of the heater, i.e. power that is released per unit area. The surface power of the heater depends on the temperature of the material being heated and on the design of the heaters.
Example
From the previous calculation points (see paragraph 3 of this paragraph), we know the resistance of the heater. For a 60 liter stove with a single-phase connection it is R
= 8.06 Ohm. Let's take 1mm diameter as an example. Then, to obtain the required resistance, it is necessary l = R / ρ
= 8.06 / 1.4 = 5.7 m nichrome wire, where ρ
- nominal value of electrical resistance of 1 m of wire, [Ohm/m]. The mass of this piece of nichrome wire will be m = l μ
= 5.7 · 0.007 = 0.0399 kg = 40 g, where μ
- mass of 1 m of wire. Now you need to determine the surface area of a piece of wire 5.7 m long. S = l π d
= 570 · 3.14 · 0.1 = 179 cm 2, where l
– wire length [cm], d
– wire diameter [cm]. Thus, 6 kW should be released from an area of 179 cm2. Solving a simple proportion, we find that power is released from 1 cm 2 β = P/S
= 6000 / 179 = 33.5 W, where β
- surface power of the heater.
The resulting surface power is too high. Heater will melt if heated to a temperature that would provide the resulting surface power value. This temperature will be higher than the melting point of the heater material.
The given example is a demonstration of the incorrect choice of the diameter of the wire that will be used to make the heater. In paragraph 5 of this paragraph an example will be given with correct selection diameter
For each material, depending on the required heating temperature, the permissible value of surface power is determined. It can be determined using special tables or graphs. These calculations use tables.
For high temperature furnaces(at temperatures above 700 - 800 °C) permissible surface power, W/m2, is equal to β additional = β eff · α , Where β eff – surface power of heaters depending on the temperature of the heat-receiving medium [W/m2], α – radiation efficiency coefficient. β eff selected according to table 3, α - according to table 4.
If low temperature oven(temperature less than 200 - 300 °C), then the permissible surface power can be considered equal to (4 - 6) · 10 4 W/m2.
Table 3
| Temperature of the heat-receiving surface, °C | β eff, W/cm 2 at heater temperature, °C | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 800 | 850 | 900 | 950 | 1000 | 1050 | 1100 | 1150 | 1200 | 1250 | 1300 | 1350 | ||
| 100 | 6,1 | 7,3 | 8,7 | 10,3 | 12,5 | 14,15 | 16,4 | 19,0 | 21,8 | 24,9 | 28,4 | 36,3 | |
| 200 | 5,9 | 7,15 | 8,55 | 10,15 | 12,0 | 14,0 | 16,25 | 18,85 | 21,65 | 24,75 | 28,2 | 36,1 | |
| 300 | 5,65 | 6,85 | 8,3 | 9,9 | 11,7 | 13,75 | 16,0 | 18,6 | 21,35 | 24,5 | 27,9 | 35,8 | |
| 400 | 5,2 | 6,45 | 7,85 | 9,45 | 11,25 | 13,3 | 15,55 | 18,1 | 20,9 | 24,0 | 27,45 | 35,4 | |
| 500 | 4,5 | 5,7 | 7,15 | 8,8 | 10,55 | 12,6 | 14,85 | 17,4 | 20,2 | 23,3 | 26,8 | 34,6 | |
| 600 | 3,5 | 4,7 | 6,1 | 7,7 | 9,5 | 11,5 | 13,8 | 16,4 | 19,3 | 22,3 | 25,7 | 33,7 | |
| 700 | 2 | 3,2 | 4,6 | 6,25 | 8,05 | 10,0 | 12,4 | 14,9 | 17,7 | 20,8 | 24,3 | 32,2 | |
| 800 | - | 1,25 | 2,65 | 4,2 | 6,05 | 8,1 | 10,4 | 12,9 | 15,7 | 18,8 | 22,3 | 30,2 | |
| 850 | - | - | 1,4 | 3,0 | 4,8 | 6,85 | 9,1 | 11,7 | 14,5 | 17,6 | 21,0 | 29,0 | |
| 900 | - | - | - | 1,55 | 3,4 | 5,45 | 7,75 | 10,3 | 13 | 16,2 | 19,6 | 27,6 | |
| 950 | - | - | - | - | 1,8 | 3,85 | 6,15 | 8,65 | 11,5 | 14,5 | 18,1 | 26,0 | |
| 1000 | - | - | - | - | - | 2,05 | 4,3 | 6,85 | 9,7 | 12,75 | 16,25 | 24,2 | |
| 1050 | - | - | - | - | - | - | 2,3 | 4,8 | 7,65 | 10,75 | 14,25 | 22,2 | |
| 1100 | - | - | - | - | - | - | - | 2,55 | 5,35 | 8,5 | 12,0 | 19,8 | |
| 1150 | - | - | - | - | - | - | - | - | 2,85 | 5,95 | 9,4 | 17,55 | |
| 1200 | - | - | - | - | - | - | - | - | - | 3,15 | 6,55 | 14,55 | |
| 1300 | - | - | - | - | - | - | - | - | - | - | - | 7,95 | |
Table 4
Wire spirals, semi-closed in lining grooves
Wire spirals on shelves in tubes
Wire zigzag (rod) heaters
Let's assume that the heater temperature is 1000 °C, and we want to heat the workpiece to a temperature of 700 °C. Then, according to Table 3, we select β eff = 8.05 W/cm2, α = 0,2, β additional = β eff · α = 8.05 · 0.2 = 1.61 W/cm2 = 1.61 · 10 4 W/m2.
5. After determining the permissible surface power of the heater, it is necessary find its diameter(for wire heaters) or width and thickness(for tape heaters), as well as length.
The wire diameter can be determined by the following formula:
d
- wire diameter, [m]; P
- heater power, [W]; U
- voltage at the ends of the heater, [V]; β additional
- permissible surface power of the heater, [W/m 2 ]; ρ t
- specific resistance of the heater material at a given temperature, [Ohm m].
ρ t = ρ 20 k
, Where ρ 20
- specific electrical resistance of the heater material at 20 °C, [Ohm m] k
- correction factor for calculating changes in electrical resistance depending on temperature (by ).
The length of the wire can be determined using the following formula:
l
- wire length, [m].
Select the diameter and length of the wire from nichrome X20N80. The specific electrical resistance of the heater material is
ρ t = ρ 20 k
= 1.13 · 10 -6 · 1.025 = 1.15 · 10 -6 Ohm m.
Single-phase household network
For a 60 liter stove connected to a single-phase household network, it is known from the previous stages of calculation that the stove power is P
= 6000 W, voltage at the ends of the heater - U
= 220 V, permissible surface heater power β additional
= 1.6 · 10 4 W/m2. Then we get
The resulting size must be rounded to the nearest larger standard. Standard sizes for nichrome and fechral wire can be found in, Appendix 2, Table 8. In this case, the nearest large standard size is Ø 2.8 mm. Heater diameter d = 2.8 mm.
Heater length l = 43 m.
It is also sometimes necessary to determine the mass required quantity wire.
m = l μ
, Where m
- weight of a piece of wire, [kg]; l
- wire length, [m]; μ
- specific gravity (weight of 1 meter of wire), [kg/m].
In our case, the heater mass m = l μ = 43 · 0.052 = 2.3 kg.
This calculation gives the minimum wire diameter at which it can be used as a heater under given conditions. From the point of view of saving material, this calculation is optimal. In this case, wire of a larger diameter can also be used, but then its quantity will increase.
Examination
Calculation results can be checked in the following way. A wire diameter of 2.8 mm was obtained. Then the length we need will be
l = R / (ρ k)
= 8.06 / (0.179 1.025) = 43 m, where l
- wire length, [m]; R
- heater resistance, [Ohm]; ρ
- nominal value of electrical resistance of 1 m of wire, [Ohm/m]; k
- correction factor for calculating changes in electrical resistance depending on temperature.
This value coincides with the value obtained as a result of another calculation.
Now we need to check whether the surface power of the heater we have chosen will not exceed the permissible surface power, which was found in step 4. β = P/S = 6000 / (3.14 · 4300 · 0.28) = 1.59 W/cm2. Received value β = 1.59 W/cm 2 does not exceed β additional = 1.6 W/cm2.
Results
Thus, the heater will require 43 meters of X20N80 nichrome wire with a diameter of 2.8 mm, which is 2.3 kg.
Three-phase industrial network
You can also find the diameter and length of the wire required for the manufacture of furnace heaters connected to a three-phase current network.
As described in paragraph 3, each of the three heaters accounts for 2 kW of power. Let's find the diameter, length and mass of one heater.
STAR connection(see Fig. 2)
In this case, the nearest larger standard size is Ø 1.4 mm. Heater diameter d = 1.4 mm.
Single heater length l
= 30 m.
Weight of one heater m = l μ
= 30 · 0.013 = 0.39 kg.
Examination
A wire diameter of 1.4 mm was obtained. Then the length we need will be
l = R / (ρ k)
= 24.2 / (0.714 · 1.025) = 33 m.
β = P/S = 2000 / (3.14 · 3000 · 0.14) = 1.52 W/cm2, it does not exceed the permissible limit.
Results
For three heaters connected in a “STAR” configuration, you will need
l
= 3 30 = 90 m of wire, which is
m
= 3 · 0.39 = 1.2 kg.
TRIANGLE connection(see Fig. 3)
In this case, the nearest larger standard size is Ø 0.95 mm. Heater diameter d = 0.95 mm.
Single heater length l
= 43 m.
Weight of one heater m = l μ
= 43 · 0.006 = 0.258 kg.
Examination
A wire diameter of 0.95 mm was obtained. Then the length we need will be
l = R / (ρ k)
= 72.2 / (1.55 · 1.025) = 45 m.
This value practically coincides with the value obtained as a result of another calculation.
The surface thickness will be β = P/S = 2000 / (3.14 · 4300 · 0.095) = 1.56 W/cm2, it does not exceed the permissible limit.
Results
For three heaters connected in a “TRIANGLE” pattern, you will need
l
= 3 43 = 129 m of wire, which is
m
= 3 · 0.258 = 0.8 kg.
If you compare the 2 options for connecting heaters to a three-phase current network discussed above, you will notice that “STAR” requires a larger diameter wire than “TRIANGLE” (1.4 mm versus 0.95 mm) to ensure a given oven power of 6 kW. Wherein the required length of nichrome wire when connecting according to the “STAR” scheme is less than the length of the wire when connecting according to the “TRIANGLE” type(90 m versus 129 m), and the required mass, on the contrary, is greater (1.2 kg vs. 0.8 kg).
Spiral calculation
During operation, the main task is to place the heater of the calculated length in limited space ovens. Nichrome and fechral wire are wound in the form of spirals or bent in the form of zigzags, the tape is bent in the form of zigzags, which allows you to fit a larger amount of material (along the length) into the working chamber. The most common option is the spiral.The relationship between the pitch of the spiral and its diameter and the diameter of the wire is chosen in such a way as to facilitate the placement of heaters in the furnace, ensure their sufficient rigidity, eliminate local overheating of the turns of the spiral itself to the maximum extent possible, and at the same time do not impede the heat transfer from them to the products.
The larger the diameter of the spiral and the smaller its pitch, the easier it is to place heaters in the furnace, but as the diameter increases, the strength of the spiral decreases and the tendency of its turns to lie on top of each other increases. On the other hand, with an increase in the winding frequency, the shielding effect of the part of its turns facing the products on the rest increases and, consequently, the use of its surface deteriorates, and local overheating may also occur.
Practice has established well-defined, recommended relationships between the diameter of the wire ( d ), step ( t ) and the diameter of the spiral ( D ) for wire Ø from 3 to 7 mm. These ratios are as follows: t ≥ 2d And D = (7÷10) d for nichrome and D = (4÷6) d - for less durable iron-chromium-aluminum alloys, such as fechral, etc. For thinner wires the ratio D And d , and t usually take more.
Conclusion
The article discussed various aspects related to calculation of electric furnace heaters- materials, calculation examples with the necessary reference data, links to standards, illustrations.In the examples, calculation methods were considered only wire heaters. In addition to wire made of precision alloys, tape can also be used to make heaters.
The calculation of heaters is not limited to the choice of their sizes. Also it is necessary to determine the material from which the heater should be made, the type of heater (wire or tape), the type of location of the heaters and other features. If the heater is made in the form of a spiral, then it is necessary to determine the number of turns and the pitch between them.
We hope that the article was useful to you. We allow its free distribution provided that a link to our website is maintained http://www.site
If you find any inaccuracies, please notify us by email at info@site or using the Orfus system by highlighting the text with the error and pressing Ctrl+Enter.
Bibliography
- Dyakov V.I. "Typical calculations for electrical equipment".
- Zhukov L.L., Plemyannikova I.M., Mironova M.N., Barkaya D.S., Shumkov Yu.V. "Alloys for heaters".
- Sokunov B.A., Grobova L.S. "Electrothermal installations (electric resistance furnaces)".
- Feldman I.A., Gutman M.B., Rubin G.K., Shadrich N.I. "Calculation and design of resistance electric furnace heaters".
- http://www.horss.ru/h6.php?p=45
- http://www.electromonter.info/advice/nichrom.html
Calculation of a nichrome spiral. We are ready to make a nichrome spiral for you. Nichrome length for 220 volts
Calculation of a nichrome spiral. We are ready to make a nichrome spiral for you
When winding a nichrome spiral for heating elements, the operation is often performed by trial and error, and then voltage is applied to the spiral and after heating the nichrome wire, the threads select the required number of turns.
Typically, such a procedure takes a lot of time, and nichrome loses its characteristics with multiple bends, which leads to rapid burning in places of deformation. In the worst case, business nichrome turns into nichrome scrap.
With its help, you can accurately determine the length of the winding turn to turn. Depending on the Ø of the nichrome wire and the Ø of the rod on which the nichrome spiral is wound. It is not difficult to recalculate the length of a nichrome spiral to a different voltage using a simple mathematical proportion.
|
Ø nichrome 0.2 mm |
Ø nichrome 0.3 mm | nichrome 0.4 mm | Ø nichrome 0.5 mm | Ø nichrome 0.6 mm | Ø nichrome 0.7 mm | ||||||
| Rod Ø, mm | spiral length, cm |
rod, mm |
spiral length, cm |
rod, mm |
spiral length, cm |
rod, mm |
spiral length, cm |
rod, mm |
spiral length, cm |
rod, mm |
spiral length, cm |
| 1,5 | 49 | 1,5 | 59 | 1,5 | 77 | 2 | 64 | 2 | 76 | 2 | 84 |
| 2 | 30 | 2 | 43 | 2 | 68 | 3 | 46 | 3 | 53 | 3 | 64 |
| 3 | 21 | 3 | 30 | 3 | 40 | 4 | 36 | 4 | 40 | 4 | 49 |
| 4 | 16 | 4 | 22 | 4 | 28 | 5 | 30 | 5 | 33 | 5 | 40 |
| 5 | 13 | 5 | 18 | 5 | 24 | 6 | 26 | 6 | 30 | 6 | 34 |
| 6 | 20 | 8 | 22 | 8 | 26 | ||||||
For example, it is necessary to determine the length of a nichrome spiral for a voltage of 380 V from a wire Ø 0.3 mm, a winding rod Ø 4 mm. The table shows that the length of such a spiral at a voltage of 220 V will be equal to 22 cm. Let’s make a simple ratio:
220 V - 22 cm
380 V - X cm
X = 380 22 / 220 = 38 cm
Calculation of electric heating elements made of nichrome wire
The length of the nichrome wire for making a spiral is determined based on the required power.
Example: Determine the length of nichrome wire for a tile heating element with a power of P = 600 W at Unetwork = 220 V.
1) I = P/U = 600/220 = 2.72 A
2) R = U/I = 220/2.72 = 81 Ohm
3) Based on these data (see Table 1), we select d=0.45; S=0.159
then the length of nichrome
l = SR / ρ = 0.159 81 /1.1 = 11.6 m
where l is wire length (m)
S - wire cross-section (mm2)
R - wire resistance (Ohm)
ρ - resistivity (for nichrome ρ=1.0÷1.2 Ohm mm2/m)
Our Company PARTAL is ready to produce a nichrome spiral according to the customer’s specifications and sketches
It is profitable to buy a nichrome spiral from PARTAL
Nichrome for spiral High Quality only Russian production. Strict compliance in quality and brand
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Calculation of nichrome spiral | Useful
Calculating a nichrome spiral is, in fact, a very important process. Very often, in plants, factories, factories, this is neglected and calculations are made “by eye”, after which they connect the spiral to the network, and then select required quantity turns depending on the heating of the nichrome wire. This procedure may be very simple, but it takes a long period of time and part of the nichrome is simply wasted.
However, this procedure can be performed much more accurately, easier and faster. In order to rationalize your work, to calculate a nichrome spiral for a voltage of 220 Volts, you can use the table below. From the calculation that the resistivity of nichrome is equal to (Ohm mm2 / m)C, you can quickly calculate the length of winding turn to turn depending on the diameter of the rod on which the nichrome thread is wound, and directly on the thickness of the nichrome wire itself. And using a simple mathematical proportion, you can easily calculate the length of the spiral for a different voltage.
For example, you need to determine the length of a nichrome spiral for a voltage of 127 Volts from a wire whose thickness is 0.3 mm, and the winding rod is 4 mm in diameter. Looking at the table, you can see that the length of this spiral at a voltage of 220 Volts will be 22 cm. We make a simple ratio: 220 V - 22 cm 127 V - X cm then: X = 127 22 / 220 = 12.7 cm
Having wound the nichrome spiral, carefully connect it, without cutting it, to the voltage source and make sure of your calculations, or rather, the calculations of the correctness of winding. And it is worth remembering that for closed spirals the winding length is increased by a third of the value given in this table.
Nichrome wire, calculation of nichrome weight, use of nichrome
www.olakis.ru
We produce electrical spirals from NICHROMA according to the customer’s specifications and sketches
Nichrome spiral
Everyone knows what a nichrome spiral is. This is a heating element in the form of a wire, rolled up with a screw for compact placement.
This wire is made from nichrome, a precision alloy whose main components are nickel and chromium.
The “classic” composition of this alloy is 80% nickel, 20% chromium.
The composition of the names of these metals formed the name that denotes the group of chromium-nickel alloys - “nichrome”.
The most famous brands nichrome – Х20Н80 and Х15Н60. The first of them is close to the “classics”. It contains 72-73% nickel and 20-23% chromium.
The second is designed to reduce the cost and increase the machinability of the wire.
Based on these alloys, modifications with higher survivability and resistance to oxidation at high temperatures were obtained.
These are brands X20N80-N (-N-VI) and X15N60 (-N-VI). They are used for heating elements in contact with air. Recommended maximum operating temperature – from 1100 to 1220 °C
Application of nichrome wire
The main quality of nichrome is high resistance electric current. It determines the applications of the alloy.
Nichrome spiral is used in two qualities - as a heating element or as a material for electrical resistances electrical diagrams.
For heaters, an electric spiral made of X20N80-N and X15N60-N alloys is used.
Application examples:
- household thermoreflectors and fan heaters;
- Heating elements for household heating devices And electric heating;
- heaters for industrial furnaces and thermal equipment.
Alloys Kh15N60-N-VI and Kh20N80-N-VI, produced in vacuum induction furnaces, used in industrial equipment of increased reliability.
A spiral made of nichrome grades X15N60, X20N80, X20N80-VI, N80HYUD-VI is distinguished by the fact that its electrical resistance changes little with temperature changes.
Resistors and connectors are made from it electronic circuits, critical parts of vacuum devices.
How to wind a spiral from nichrome
A resistive or heating coil can be made at home. To do this, you need nichrome wire of a suitable grade and the correct calculation of the required length.
The calculation of a nichrome spiral is based on the resistivity of the wire and the required power or resistance, depending on the purpose of the spiral. When calculating power, you need to take into account the maximum permissible current at which the coil heats up to a certain temperature.
Temperature accounting
For example, a wire with a diameter of 0.3 mm at a current of 2.7 A will heat up to 700 °C, and a current of 3.4 A will heat it to 900 °C.
There are reference tables for calculating temperature and current. But you still need to take into account the operating conditions of the heater.
When immersed in water, heat transfer increases, then the maximum current can be increased by up to 50% of the calculated one.
A closed tubular heater, on the contrary, impairs heat dissipation. In this case, the permissible current must be reduced by 10-50%.
The intensity of heat removal, and therefore the temperature of the heater, is affected by the pitch of the spiral winding.
Densely spaced coils generate more heat, larger step enhances cooling.
It should be taken into account that all tabular calculations are given for a heater located horizontally. When the angle to the horizon changes, the heat removal conditions worsen.
Calculation of the resistance of a nichrome spiral and its length
Having decided on the power, we proceed to calculate the required resistance.
If the determining parameter is power, then first we find the required current using the formula I=P/U.
Having the current strength, we determine the required resistance. To do this, we use Ohm's law: R=U/I.
The notations here are generally accepted:
- P – allocated power;
- U is the voltage at the ends of the spiral;
- R – spiral resistance;
- I – current strength.
The calculation of the resistance of nichrome wire is ready.
Now let's determine the length we need. It depends on the resistivity and wire diameter.
You can make a calculation based on the resistivity of nichrome: L=(Rπd2)/4ρ.
- L – required length;
- R – wire resistance;
- d – wire diameter;
- ρ – resistivity of nichrome;
- π – constant 3.14.
But it’s easier to take ready-made linear resistance from the tables of GOST 12766.1-90. You can also take temperature corrections there if you need to take into account changes in resistance when heated.
In this case, the calculation will look like this: L=R/ρld, where ρld is the resistance of one meter of wire having a diameter d.
Spiral winding
Now we will make a geometric calculation of the nichrome spiral. We have selected the wire diameter d, the required length L has been determined, and we have a rod with diameter D for winding. How many turns do you need to make? The length of one turn is: π(D+d/2). Number of turns – N=L/(π(D+d/2)). The calculation is completed.
In practice, rarely does anyone wind their own wire for a resistor or heater.
It’s easier to buy a nichrome spiral with the required parameters and, if necessary, separate the required number of turns from it.
To do this, you should contact the PARTAL company, which since 1995 has been a major supplier of precision alloys, including nichrome wire, tape and spirals for heaters.
Our company is able to completely remove the question of where to buy a nichrome spiral, since we are ready to make it to order according to sketches and technical specifications customer.
partalstalina.ru
Calculation and repair of the heating winding of a soldering iron
For repairs or self-production electric soldering iron or any other heating device, you have to wind a heating winding made of nichrome wire. The initial data for calculating and selecting wire is the winding resistance of a soldering iron or heating device, which is determined based on its power and supply voltage. You can calculate what the winding resistance of a soldering iron or heating device should be using the table.
Knowing the supply voltage and measuring the resistance of any heating electrical appliance, such as a soldering iron, electric kettle, electric heater or an electric iron, you can find out the power consumed by this household electrical appliance. For example, the resistance of a 1.5 kW electric kettle will be 32.2 Ohms.
| 12 | 48,0 | 108 | 1344 | 4033 |
| 6,0 | 24,0 | 54 | 672 | 2016 |
| 4,0 | 16,0 | 36 | 448 | 1344 |
| 3,4 | 13,7 | 31 | 384 | 1152 |
| 2,4 | 9,6 | 22 | 269 | 806 |
| 1.9 | 7.7 | 17 | 215 | 645 |
| 1,4 | 5,7 | 13 | 161 | 484 |
| 0,96 | 3,84 | 8,6 | 107 | 332 |
| 0,72 | 2,88 | 6,5 | 80,6 | 242 |
| 0,48 | 1,92 | 4,3 | 53,8 | 161 |
| 0,36 | 1,44 | 3,2 | 40,3 | 121 |
| 0,29 | 1,15 | 2,6 | 32,3 | 96,8 |
| 0,21 | 0,83 | 1,85 | 23,0 | 69,1 |
| 0,16 | 0,64 | 1,44 | 17,9 | 53,8 |
| 0,14 | 0,57 | 1,30 | 16,1 | 48,4 |
| 0,10 | 0,38 | 0,86 | 10,8 | 32,3 |
| 0,07 | 0,29 | 0,65 | 8,06 | 24,2 |
| 0,06 | 0,23 | 0,52 | 6,45 | 19,4 |
| 0,05 | 0,19 | 0,43 | 5,38 | 16,1 |
Let's look at an example of how to use the table. Let's say you need to rewind a 60 W soldering iron designed for a supply voltage of 220 V. In the leftmost column of the table, select 60 W. From the top horizontal line, select 220 V. As a result of the calculation, it turns out that the resistance of the soldering iron winding, regardless of the winding material, should be equal to 806 Ohms.
If you needed to make a soldering iron from a 60 W soldering iron, designed for a voltage of 220 V, for power supply from a 36 V network, then the resistance of the new winding should already be equal to 22 Ohms. You can independently calculate the winding resistance of any electric heating device using an online calculator.
After determining the required resistance value of the soldering iron winding, the appropriate diameter of the nichrome wire is selected from the table below, based on the geometric dimensions of the winding. Nichrome wire is a chromium-nickel alloy that can withstand heating temperatures up to 1000˚C and is marked X20N80. This means that the alloy contains 20% chromium and 80% nickel.
To wind a soldering iron spiral with a resistance of 806 Ohms from the example above, you will need 5.75 meters of nichrome wire with a diameter of 0.1 mm (you need to divide 806 by 140), or 25.4 m of wire with a diameter of 0.2 mm, and so on.
When winding a soldering iron spiral, the turns are laid close to each other. When heated red-hot, the surface of the nichrome wire oxidizes and forms an insulating surface. If the entire length of the wire does not fit on the sleeve in one layer, then the wound layer is covered with mica and a second one is wound.
For electrical and thermal insulation of heating element windings the best materials is mica, fiberglass cloth and asbestos. Asbestos has an interesting property: it can be soaked with water and it becomes soft, allows you to give it any shape, and after drying it has sufficient mechanical strength. When insulating the winding of a soldering iron with wet asbestos, it must be taken into account that wet asbestos conducts electric current well and it will be possible to turn on the soldering iron to the electrical network only after completely dry asbestos.
felstar.mypage.ru
HOW TO CALCULATE A NICHROME SPIRAL?
Post written by admin at 01/18/2015 23:23Categories: 3. Home electrical, Electrical workshop
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Winding a nichrome spiral for heating devices is often done “by eye”, and then, including the spiral in the network, the required number of turns is selected based on the heating of the nichrome wire. Usually this procedure takes a lot of time, and nichrome is wasted.
When using a 220 V spiral, you can use the data given in the table, based on the calculation that the resistivity of nichrome ρ=(Ohm mm2/m). Using this formula, you can quickly determine the length of winding turn to turn depending on the thickness of the nichrome wire and the diameter of the rod on which the spiral is wound.
For example, if you need to determine the length of a spiral for a voltage of 127 V from nichrome wire with a thickness of 0.3 mm, the winding rod dia. 4 mm. The table shows that the length of such a spiral at a voltage of 220 V will be equal to 22 cm.
Let's make a simple ratio:
220 V - 22 cm
X = 127 * 22 / 220 = 12.7 cm.
After winding the spiral, connect it, without cutting it, to a voltage source and make sure that the winding is correct. For closed spirals, the winding length is increased by 1/3 of the value given in the table.
Legend in the table: D - diameter of the rod, mm; L - length of the spiral, cm.
| dia. nichrome0.2 mm | dia. nichrome0.3 mm | dia. nichrome0.4 mm | dia. nichrome0.5 mm | dia. nichrome0.6 mm | dia. nichrome0.7 mm | dia. nichrome0.8 mm | dia. nichrome0.9 mm | dia. nichrome1.0 mm | |||||||||
| D | L | D | L | D | L | D | L | D | L | D | L | D | L | D | L | D | L |
| 1,5 | 49 | 1,5 | 59 | 1,5 | 77 | 2 | 64 | 2 | 76 | 2 | 84 | 3 | 68 | 3 | 78 | 3 | 75 |
| 2 | 30 | 2 | 43 | 2 | 68 | 3 | 46 | 3 | 53 | 3 | 62 | 4 | 54 | 4 | 72 | 4 | 63 |
| 3 | 21 | 3 | 30 | 3 | 40 | 4 | 36 | 4 | 40 | 4 | 49 | 5 | 46 | 6 | 68 | 5 | 54 |
| 4 | 16 | 4 | 22 | 4 | 28 | 5 | 30 | 5 | 33 | 5 | 40 | 6 | 40 | 8 | 52 | 6 | 48 |
| 5 | 13 | 5 | 18 | 5 | 24 | 6 | 26 | 6 | 30 | 6 | 34 | 8 | 31 | 8 | 33 | ||
| 6 | 20 | 8 | 22 | 8 | 26 | 10 | 24 | 10 | 30 | ||||||||
| 10 | 22 | ||||||||||||||||
elctricvs.ru
Electrical resistance is one of the most important characteristics nichrome. It is determined by many factors, in particular the electrical resistance of nichrome depends on the size of the wire or tape, and the grade of the alloy. General formula for active resistance has the form: R = ρ l / S R - active electrical resistance (Ohm), ρ - specific electrical resistance (Ohm mm), l - conductor length (m), S - cross-sectional area (mm2)
To rationalize this work when using a nichrome spiral for a voltage of 220 V, I propose to use the data given in the table, based on the calculation that the resistivity of nichrome = (Ohm mm2 / m)C. With its help, you can quickly determine the length of winding turn to turn depending on the thickness of the nichrome wire and the diameter of the rod on which the nichrome spiral is wound. It is not difficult to recalculate the length of a nichrome spiral to a different voltage using a simple mathematical proportion.
For example, you need to determine the length of a nichrome spiral for a voltage of 380 V from a wire 0.3 mm thick, a winding rod Ø 4 mm. The table shows that the length of such a spiral at a voltage of 220 V will be equal to 22 cm. Let’s make a simple ratio: 220 V - 22 cm 380 V - X cm then: X = 380 22 / 220 = 38 cm Having wound the nichrome spiral, connect it, without cutting it, to a voltage source and make sure that the winding is correct. For closed spirals, the winding length is increased by 1/3 of the value given in the table. This table shows the theoretical weight of 1 meter of nichrome wire and tape. It varies depending on the size of the product.
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www.metotech.ru
| Calculation of the heating element Calculation example. Given: U=220V,t=700°C, type X20H80, d=0.5mm-----------L,P-?Solution: From Table 1 we find that the diameter d=0.5mm corresponds to S = 0.196 mm², and the current at 700 ° C I = 5.2 A. The type of alloy X20N80 is nichrome, the resistivity of which is ρ = 1.11 μOhm m. We determine the resistance R = U/I = 220/5.2 = 42.3 Ohm. From here we calculate the length of the wire: L = RS/ρ = 42.3 0.196/1.11 = 7.47 m. Determine the power of the heating element: P = U I = 220 5.2 = 1.15 kW .When winding the spiral, the following relationship is adhered to: D = (7÷10)d, where D is the diameter of the spiral, mm, d is the diameter of the wire, mm. Note: - if the heaters are inside the heated liquid, then the load (current) can be increased by 1 ,1-1.5 times; - in a closed version of the heater, the current should be reduced by 1.2-1.5 times. A smaller coefficient is taken for thicker wire, a larger one for thinner wire. For the first case, the coefficient is chosen exactly the opposite. I’ll make a reservation: we're talking about about simplified calculation of the heating element. Perhaps someone will need a table of electrical resistance values for 1 m of nichrome wire, as well as its weight Table 1. Permissible current strength of nichrome wire at normal temperature
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www.elektrikii.ru
There are several types of tandoor heating. Today it is becoming increasingly common electric method, since it does not require the purchase of fuel, does not emit combustion products, and makes it easier to use behind the stove.
Collapse
The device is heated by heating the spirals and subsequent uniform heat transfer. The article discusses in detail the features of the tandoor spiral. This information will help you select and install the heating element on the stove correctly.
What is a tandoor spiral?
The spiral is an important element of the tandoor; without it, the device will not work. Warms up quite quickly. Allows you to maintain the required temperature for a long time, which is especially important if you have to cook on the stove all day.
This is what a spiral looks like
The heating element is made from wire with high electrical resistance. The length of the wire is quite long, so it is twisted in turns for convenience. Spirals can have the shape of cylinders or flat coils and be equipped with contact leads. Heaters are attached to the furnace on ceramic or metal bases with special heat-resistant inserts or insulators.
Purpose of the spiral
The main function of a tandoor coil is incandescence and subsequent uniform distribution of heat. To do this, the element must have the following qualities:
- Heat resistance (does not collapse at high temperatures in tandoors).
- High current resistance (the heating rate, the resulting temperature, and the service life of the element depend on this).
- Constancy of properties (does not change depending on environmental conditions, duration of operation).
Kinds
The most practical materials For heating parts, nichrome and fechral compounds are used. Let's briefly consider their features.
Nichrome
Nichrome spirals are made from Cr+Ni. This alloy allows the device to heat up to 1200 degrees. It is characterized by creep resistance and oxidation resistance. Minus - smaller temperature regime in comparison with fechral alloys.
The price of nichrome products is affordable. For example, brand Х20Н80(20% chromium, 80% nickel), suitable for a standard voltage of 220 volts will cost 150-170 rubles. per meter
Fechral
Fechral is a combination chromium, iron, aluminum and titanium. The material is different good performance current resistance. It has increased heat resistance: the maximum melting temperature of spirals made of this material reaches 1500 degrees.
Fechral spiral
Types
When selecting a heating device, it is important to pay attention not only to the material, but also to the type of product: a 220 or 380 volt tandoor coil has some differences.
220 V is the standard voltage for home electrical networks (that is, for connecting to regular sockets in apartments and country cottages). Can also be used in small restaurants with low productivity. According to safety rules, spirals with a power of 3.5-7 kilowatts are connected to 220 volts.
A powerful tandoor is not connected to a standard consumer electrical network. This will cause the heater to burn out and short out. Requires connection to an industrial three-phase power supply of 380 volts. The power of each spiral in the tandoor in this case increases to 12 kilowatts. Special requirements for wires used in heating elements: they must have a cross-section of at least 4 mm.
How to choose the right spiral?
The dimensions of the wire used to create heaters are determined by the power of the tandoor, the voltage in the electrical network and the heat that should be produced by the stove. First, you need to determine the current strength using the formula: I = P:U
- P is the technical power of the furnace.
- U is the voltage in the electrical network.
For example, for an 800-watt stove and a mains voltage of 220 volts, the electric current will be 3.6 amperes. Afterwards, using the specified parameters (temperature and electric current), suitable wire dimensions are searched in a special table.
The length of the wire for the spiral is calculated by the formula l=RхS:ρ. For example, with a resistance of 61 Ohms, a cross-sectional size of 0.2 square meters. mm and a resistance of 1.1 requires a spiral made of wire 5.3 meters long.
Installation work
Specialists charge about 2300-3000 rubles for installing heating elements in a furnace. If you want to save money and install the spiral in the tandoor yourself, here are some important tips:
- You should not place the heating element vertically. The hot wire is soft and may bend due to gravity. It is better to lay it horizontally.
- It is not recommended to install the heater close to the insulating brick - the risk of overheating increases. A small “air cushion” is created between the walls of the furnace and the wire.
- When installing, you need to stretch the spiral so that all the turns are at a small distance from each other (experts recommend the distance between the rings is 1.5-2 times greater than the diameter of the wire).
An alternative option: a heating element (tubular electric heater with a wire spiral inside) is installed at the bottom of the tandoor. This is a convenient and safe option. But as practice shows, heating from the heating element will slower than in the case of an open spiral.
The photographs below show several types of spiral installation:
Example of spiral installation
Another way
heating element instead of a spiral
Conclusion
Correct and safe work tandoor depends on this important element like a spiral. At the time of buying finished oven or making a device with your own hands, it is important to choose suitable material, type, size of heaters. If you do not have confidence in your abilities and knowledge, it is better to entrust the selection and installation of foam spirals to specialists.
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