The most significant part of an electrothermal installation is the heating element. The main component of indirect heating devices is a resistor with a high resistivity. And one of the priority materials is a chromium-nickel alloy. Since the resistance of nichrome wire is high, this material occupies a leading place as a raw material for various types of electric thermal installations. The calculation of the nichrome wire heater is carried out in order to determine the dimensions of the heating element.
Content
- Basic concepts
- Calculation algorithm for single-phase installations
- Temperature classification of heaters
- Options contributing to problems
Basic concepts
In general, it is necessary to calculate a heating element made of nichrome according to four calculations: hydraulic, mechanical, thermal and electrical. But usually calculations are carried out in only two stages: by thermal and electrical indicators.
Thermal performance includes:
- thermal insulation;
- heat efficiency;
- required heat dissipating surface.
The main purpose of calculating nichrome is to determine the geometric dimensions of the heating resistance.
To electrical parameters of heatersare:
- supply voltage;
- method of power regulation;
- power factor and electrical efficiency.
When choosing a supply voltage for heating devices, preference is given to something that poses a minimum threat to animals and service personnel. The mains voltage in agricultural installations is 380/200 volts with a current frequency of 50 Hertz. In the case of using electrical installations in especially damp rooms, with 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 voltage;
- a change in the magnitude of resistance.
The most common way to change the power is to turn on a certain number of sections of a three-phase installation. In modern heating installations, the power is changed by voltage regulation using thyristors.
The calculation for the operating current is based on a tabular dependence, which connects the current load on the nichrome conductor, its cross-sectional area and temperature.
The tabulated data were compiled for nichrome wire, which was tensioned in air without taking into account vibrations and vibrations at a temperature of 20 ° C.
In order to go to real conditions, it is necessary to use correction factors in the calculations.
Calculation algorithm for single-phase installations
The calculation of the nichrome spiral should be carried out in stages, using the initial information about the heater: the required power and the brand of nichrome.
Power of one section:
Pc = P / (mn)
P is the power of the installation, 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:
Ic = P with / (Un)
U - mains voltage, for single-phase installations U = 220 V
Calculated wire temperature:
θр = θд / (Km Ks)
θд - permissible operating temperature, selected from table 1 depending on the material, ° C.
Table 1 - Parameters of materials for electric heaters.
| Material | Resistivity at 20 ° C, x10-6Ohm m | Temperature coefficient of resistance, x10— 6 ° C -1 | Permissible operating temperature, ° C | Melting point, ° C |
| Nichrome double (Х20Н80-Н) | 1,1 | 16,5 | 1200 | 1400 |
| Nichrome triple (Х15Н60-Н) | 1,1 | 16,3 | 1100 | 1390 |
Km - installation factor, selected from table 2, depending on the design.
table 2 - Installation factor for some types of heater designs in a calm air flow.
| Heater design | Km |
| Horizontal wire | 1,0 |
| Spiral made of wire without thermal insulation | 0,8 — 0,9 |
| Wire spiral on a fireproof frame | 0,7 |
| Refractory wire | 0,6 — 0,7 |
| Heating resistances between two layers of thermal insulation | 0,5 |
| Heating resistors with good thermal insulation | 0,3 — 0,4 |
The role of the installation factor is that it makes it possible to take into account the increase in heater temperature in real conditions compared to the data in the look-up table.
Кс - environmental factor, determined from table 3.
Table 3 - Correction factor for some environmental conditions.
| Environmental conditions | Ks |
| Spiral from a wire in an air stream with a speed of movement, m / s | |
| 3 | 1,8 |
| 5 | 2,1 |
| 10 | 3,1 |
| Heating element in still water | 2,5 |
| Heating element in water flow | 3,0−3,5 |
The environmental factor corrects for improved heat transfer due to environmental conditions. Therefore, the actual calculation results will slightly differ from the table values.
Diameter d, mm and cross-sectional area S, mm 2 selected by operating current and design temperature from table 4
Table 4 - Allowable load on nichrome wire at 20 ° C, suspended horizontally in calm air.
Wire length of one section:
L = (U f2S * 10-6) / (ρ 20 [1 + α (θ р -20)] Рс x103)
ρ 20 - resistivity at 20 ° C, selected from table 1;
α - temperature coefficient of resistance, is determined from the corresponding column in table 1.
Spiral diameter:
D = (6... 10) d, mm.
Determine the pitch of the spiral:
h = (2... 4) d, mm
The spiral pitch affects the work performance. With its large values, heat transfer increases.
Number of spiral turns
W = (lx103) / (√h2+ (πD)2)
Coil length:
L = h W x10-3
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 the case of a closed type heater, it is recommended to reduce the operating current by 1.2 times.
Temperature classification of heaters
According to the maximum permissible temperature, heaters are divided into five classes:
-
200 ° C. In this temperature range, the use of tubular electric heaters is most common. In order to maintain the optimal temperature in the working space, when installing TENs, attention must be paid to their correct location. - 200 to 400 ° C. Tape heaters are used. To create the required temperature in the working chamber, cover its entire perimeter.
- 400 to 600 ° C. The material for the heaters should only be a high resistance resistive element. The most common are constantan, fechral, nichrome. In order to ensure the required temperature, the heater must be open for air access. Therefore, it is located inside or outside the tube.
- 600 to 1250 ° C. Nichrome is used in old-style ovens. But in this temperature range, it is significantly inferior to an alloy of aluminum, iron and chromium (fechrali). Therefore, in more modern models of stoves, nichrome is replaced by fechral.
- 1250 to 1700 ° C. High-temperature heaters are made from molybdenum disilicide, silicon carbide. The main disadvantage of heaters is their shortage and high cost.
Options contributing to problems
The highest probability of failure of electric heaters due to oxidation of the surface of the heating resistance.
Factors that affect the rate of destruction of the heater:
-
working temperature; - environmental conditions in which the heater operates;
- switching frequency.
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.
