Electric heating elements are used in household and industrial appliances. The use of various heaters is known to everyone. These are electric stoves, ovens and ovens, electric coffee makers, electric kettles and heating devices of all kinds.
Electric water heaters, more commonly referred to, also contain heating elements. Many heating elements are based on high electrical resistance wire. And most often this wire is made of nichrome.
Open nichrome spiral
The oldest heating element is, perhaps, an ordinary nichrome coil. Once upon a time, homemade electric stoves, water boilers and "goat" heaters were in use. Having at hand a nichrome wire, which could "get hold of" in production, making a spiral of the required power did not present any problems.
The end of the wire of the required length is inserted into the cut of the knob, the wire itself is passed between two wooden blocks. The vice must be clamped so that the entire structure is held, as shown in the figure. The clamping force should be such that the wire passes through the bars with some effort. If the clamping force is large, then the wire will simply break.
Figure 1. Winding nichrome spiral
By rotating the knob, the wire is pulled through the wooden bars, and neatly, turn to turn, is laid on a metal rod. In the arsenal of electricians there was a whole set of cranks of various diameters from 1.5 to 10 mm, which made it possible to wind spirals for all occasions.
It was known what diameter the wire and how long it took to wind a spiral of the required power. These magic numbers can still be found on the Internet. Figure 2 shows a table that shows data on spirals of various powers at a supply voltage of 220V.
Figure 2. Calculation of the electric spiral heating element(click on the picture to enlarge)
Everything here is simple and clear. Having set the required power and the diameter of the nichrome wire available at hand, it remains only to cut a piece of the required length and wind it on a mandrel of the corresponding diameter. In this case, the table shows the length of the resulting spiral. But what if there is a wire with a diameter not specified in the table? In this case, the spiral will simply have to be calculated.
If necessary, calculating the spiral is quite simple. As an example, the calculation of a spiral made of nichrome wire with a diameter of 0.45 mm (there is no such diameter in the table) with a power of 600W for a voltage of 220V is given. All calculations are performed according to Ohm's law.
On how to convert amperes to watts and, conversely, watts to amperes:
I = P / U = 600/220 = 2.72 A
To do this, it is enough to divide the given power by the voltage and get the value of the current passing through the spiral. Power in watts, voltage in volts, result in amperes. Everything is according to the SI system.
The formula for calculating the resistance of a conductor R = ρ * L / S,
where ρ is the specific resistance of the conductor (for nichrome 1.0 ÷ 1.2 Ohm.mm2 / m), L is the length of the conductor in meters, S is the cross-section of the conductor in square millimeters. For a conductor with a diameter of 0.45 mm, the cross section will be 0.159 mm2.
Hence L = S * R / ρ = 0.159 * 81 / 1.1 = 1170 mm, or 11.7 m.
In general, it turns out not so difficult calculation. Yes, in fact, the manufacture of a spiral is not so difficult, which, undoubtedly, is the advantage of ordinary nichrome spirals. But this advantage is overridden by many disadvantages inherent in open spirals.
First of all, it is a rather high heating temperature - 700… 800˚C. The heated coil has a faint red glow, accidental touching it can cause burns. In addition, electric shock is possible. A red-hot spiral burns out oxygen in the air, attracts dust particles, which, when burned out, give a very unpleasant aroma.
But the main disadvantage of open spirals is their high fire hazard. Therefore, the fire department simply prohibits the use of open coil heaters. These heaters, first of all, include the so-called "goat", the design of which is shown in Figure 3.
Figure 3. Homemade heater "goat"
Here is such a wild "goat": it was made deliberately carelessly, simply, even very badly. A fire with such a heater will not have to wait long. A more perfect design of such a heater is shown in Figure 4.
Figure 4. "Goat" home
It is easy to see that the spiral is closed by a metal casing, this is what prevents touching the heated live parts. The fire hazard of such a device is much less than that shown in the previous figure.
Once upon a time, reflector heaters were produced in the USSR. In the center of the nickel-plated reflector there was a ceramic cartridge, into which, like a light bulb with an E27 base, a 500W heater was screwed. The fire hazard of such a reflector is also very high. Well, they somehow did not think in those days what the use of such heaters could lead to.
Figure 5. Reflex heater
It is quite obvious that, contrary to the requirements of the fire inspectorate, it is possible to use various open coil heaters only under vigilant supervision: if you leave the room, turn off the heater! Better yet, simply refuse to use this type of heater.
Closed coil heating elements
To get rid of the open spiral, Tubular Electric Heaters - Tubular Heating Elements - were invented. The design of the heating element is shown in Figure 6.
Figure 6. The design of the heating element
Nichrome spiral 1 is hidden inside a thin-walled metal tube 2. The spiral is insulated from the tube by filler 3 with high thermal conductivity and high electrical resistance. Periclase is most often used as a filler (a crystalline mixture of magnesium oxide MgO, sometimes with impurities of other oxides).
After filling with an insulating compound, the tube is pressurized, and under high pressure the periclase turns into a monolith. After such an operation, the spiral is rigidly fixed, therefore, electrical contact with the body - the tube is completely excluded. The design is so strong that any heating element can be bent, if required by the design of the heater. Some heating elements have a very bizarre shape.
The spiral is connected to metal leads 4, which go out through insulators 5. Lead wires are connected to the threaded ends of the leads 4 using nuts and washers 7. The heating elements are fastened in the device case using nuts and washers 6, which ensure, if necessary, the tightness of the connection.
Subject to the operating conditions, such a design is quite reliable and durable. This is what led to the very widespread use of heating elements in devices for various purposes and designs.
According to the operating conditions, heating elements are divided into two large groups: air and water. But that's just the name. In fact, air heating elements are designed to work in various gas environments. Even ordinary atmospheric air is a mixture of several gases: oxygen, nitrogen, carbon dioxide, there are even impurities of argon, neon, krypton, etc.
The air environment is very diverse. It can be calm atmospheric air or a stream of air moving at a speed of up to several meters per second, as in fan heaters or heat guns.
Heating of the heating element shell can reach 450 ˚C and even more. Therefore, for the manufacture of the outer tubular shell are used various materials... It can be regular carbon steel, stainless steel, or high temperature, heat resistant steel. It all depends on the environment.
To improve heat transfer, some heating elements are equipped with ribs on tubes in the form of a wound metal tape. These heaters are called finned heaters. The use of such elements is most expedient in a moving air environment, for example, in fan heaters and heat guns.
Water heating elements are also not necessarily used in water, this is the general name for various liquid media. It can be oil, fuel oil and even various aggressive liquids. Liquid heating elements, distillers, electric desalination plants sea water and just titanium for boiling drinking water.
The thermal conductivity and heat capacity of water is much higher than that of air and other gaseous media, which provides, in comparison with the air environment, better, faster heat removal from the heating element. Therefore, with the same electrical power, the water heater has smaller geometric dimensions.
Here you can give a simple example: when water boils off in an ordinary electric kettle, the heating element can warm up red-hot, and then burn out to holes. The same picture can be observed with conventional boilers designed for boiling water in a glass or bucket.
The given example clearly shows that water heating elements should in no case be used to work in an air environment. Air heating elements can be used to heat water, but you will have to wait a long time for the water to boil.
The scale layer formed during operation will also not benefit the water heating elements. Scale, as a rule, has a porous structure and its thermal conductivity is low. Therefore, the heat released by the spiral goes into the liquid poorly, but the spiral itself inside the heater heats up to a very high temperature, which sooner or later will lead to its burnout.
To prevent this from happening, it is advisable to periodically clean the heating elements using various chemicals. For example, in television advertisements, Calgon is recommended to protect the heaters of washing machines. Although there are many very different opinions about this tool.
How to get rid of limescale
In addition to chemicals used to protect against scale various devices... First of all, these are magnetic water transducers. In a powerful magnetic field, crystals of "hard" salts change their structure, turn into flakes, and become smaller. Scale forms less actively from such flakes; most of the flakes are simply washed away with a stream of water. This is how the heaters and pipelines are protected from scale. Magnetic filter-converters are produced by many foreign firms, such firms also exist in Russia. Such filters are available both in-line and overhead type.
Electronic water softeners
V Lately electronic water softeners are becoming more and more popular. Outwardly, everything looks very simple. A small box is installed on the pipe, from which the antenna wires come out. The wires are wrapped around the pipe without even having to peel off the paint. The device can be installed in any accessible place, as shown in Figure 7.
Figure 7. Electronic water softener
The only thing that is required to connect the device is a 220V socket. The device is designed to be switched on for a long time; it does not need to be switched off periodically, since switching off will cause the water to become hard again, and scale will again form.
The principle of operation of the device is reduced to the emission of vibrations in the range of ultrasonic frequencies, which can reach up to 50 kHz. The oscillation frequency is regulated using the control panel of the device. Radiations are produced in packets several times per second, which is achieved using an integrated microcontroller. The power of vibrations is small, therefore, such devices do not pose any threat to human health.
The expediency of installing such devices is easy to determine. It all comes down to determining how hard the water is flowing from water pipe... Here you don't even need any "abstruse" devices: if after washing your skin becomes dry, from splashing water on tile white streaks appear, scale appears in the kettle, the washing machine washes more slowly than at the beginning of operation - clearly hard water flows from the tap. All this can lead to failure of the heating elements, and, consequently, of the kettles or washing machines themselves.
Hard water does not dissolve various detergents- from ordinary soaps to super-fashionable washing powders. As a result, you have to put more powders, but this does not help much, since crystals of hardness salts are retained in the fabrics, the washing quality leaves much to be desired. All the listed signs of water hardness speak volumes about the need to install water softeners.
Connecting and checking heating elements
When connecting the heating element, a wire of a suitable section must be used. It all depends on the current flowing through the heating element. There are two most commonly known parameters. These are the power of the heater itself and the supply voltage. In order to determine the current, it is enough to divide the power by the supply voltage.
A simple example. Let there be a heating element with a power of 1 kW (1000W) for a supply voltage of 220V. For such a heater, it turns out that the current will be
I = P / U = 1000/220 = 4.545A.
According to the tables in the PUE, such a current can provide a wire with a cross section of 0.5 mm2 (11A), but in order to ensure mechanical strength, it is better to use a wire with a cross section of at least 2.5 mm2. It is with such a wire that electricity is most often supplied to the outlets.
But before making the connection, you should make sure that even a new, just purchased heating element is in good working order. First of all, you need to measure its resistance and check the integrity of the insulation. The resistance of the heating element is easy enough to calculate. To do this, the supply voltage must be squared and divided by the power. For example, for a 1000W heater, this calculation looks like this:
220 * 220/1000 = 48.4 Ohm.
Such resistance should be shown by a multimeter when connected to the terminals of the heating element. If the spiral is broken, then, of course, the multimeter will show a break. If you take a heating element of a different power, then the resistance, of course, will be different.
To check the integrity of the insulation, you should measure the resistance between any of the terminals and the metal case of the heating element. The resistance of the insulating filler is such that at any measurement limit the multimeter should show an open circuit. If it turns out that the resistance is zero, then the coil has contact with the metal body of the heater. This can even happen with a new, just purchased heating element.
In general, it is used for checking insulation, but not always and not everyone has it at hand. So checking with an ordinary multimeter is fine. At least such a check must be done.
As already mentioned, heating elements can be bent even after filling with an insulator. Heaters are available in a wide variety of shapes: straight tube, U-shaped, coiled, snake or spiral. It all depends on the device of the heating device in which the heating element is supposed to be installed. For example, in an instantaneous water heater washing machine heating elements twisted into a spiral are used.
Some heating elements have protection elements. The most simple protection this is a thermal fuse. If it burns out, then you have to change the entire heating element, but it will not come to a fire. There is also a more complex protection system that allows you to use the heating element after it is triggered.
One of these protections is a protection based on a bimetallic plate: the heat from an overheated heating element bends the bimetallic plate, which opens the contact and de-energizes the heating element. After the temperature drops to the permissible value, the bimetallic plate unbends, the contact closes and the heating element is ready for operation again.
Heating elements with thermostat
In the absence of hot water supply, you have to use boilers. The design of the boilers is quite simple. This metal container, hidden in a "fur coat" made of heat insulator, on top of which there is a decorative metal case. A thermometer is embedded in the body, showing the temperature of the water. The boiler design is shown in Figure 8.
Figure 8. Storage type boiler
Some boilers contain a magnesium anode. Its purpose is to protect the heater and the internal tank of the boiler against corrosion. The magnesium anode is consumable, it has to be changed periodically when servicing the boiler. But in some boilers, apparently cheap price category, such protection is not provided.
A heating element with a thermostat is used as a heating element in boilers, the design of one of them is shown in Figure 9.
Figure 9. Heating element with thermostat
The plastic box contains a microswitch that is triggered by a liquid temperature sensor (straight tube next to the heating element). The form of the heating element itself can be very diverse, the figure shows the simplest one. It all depends on the power and design of the boiler. The degree of heating is regulated by the position of a mechanical contact controlled by a white round handle located at the bottom of the box. There are also terminals for supplying electric current. The heater is fastened with a thread.
Wet and dry heating elements
Such a heater is in direct contact with water, therefore such a heating element is called "wet". The service life of the "wet" heating element is within 2 ... 5 years, after which it has to be changed. In general, the service life is short.
To increase the service life of the heating element and the entire boiler as a whole, the French company Atlantic in the 90s of the last century developed the design of a "dry" heating element. To put it simply, the heater was hidden in a metal protective flask, which excludes direct contact with water: the heating element is heated inside the flask, which transfers heat to the water.
Naturally, the temperature of the flask is much lower than that of the heating element itself, therefore, the formation of scale at the same hardness of water is not so intense, more heat is transferred to the water. The service life of such heaters reaches 10 ... 15 years. The above is true for good operating conditions, especially the stability of the supply voltage. But even in good conditions"Dry" heating elements also deplete their resource, and they have to be changed.
Here is another advantage of the "dry" heating element technology: when replacing the heater, there is no need to drain the water from the boiler, for which it should be disconnected from the pipeline. Simply unscrew the heater and replace it with a new one.
Atlantic, of course, patented its invention, and then began to sell the license to other firms. Currently, boilers with a "dry" heating element are produced by other companies, for example, Electrolux and Gorenje. The design of a boiler with a "dry" heating element is shown in Figure 10.
Figure 10. Boiler with "dry" heater
By the way, the picture shows a boiler with a ceramic steatite heater. The design of such a heater is shown in Figure 11.
Figure 11. Ceramic heater
On the ceramic base there is an ordinary open spiral made of high resistance wire. The heating temperature of the coil reaches 800 degrees and is transferred to the environment (air under the protective shell) by convection and heat radiation. Naturally, such a heater as applied to boilers can only work in a protective shell, in an air environment, direct contact with water is simply excluded.
The spiral can be wound in several sections, as evidenced by the presence of several terminals for connection. This allows the power of the heater to be changed. The maximum specific power of such heaters does not exceed 9W / cm 2.
The condition for the normal operation of such a heater is the absence of mechanical stress, bends and vibrations. The surface must be free from rust and oil stains. And, of course, the more stable the supply voltage, without surges and surges, the more durable the heater will work.
But electrical engineering does not stand still. Technologies are developing, improving, therefore, in addition to heating elements, a wide variety of heating elements have now been developed and successfully used. These are ceramic heaters, carbon heaters, infrared heaters, but that will be a topic for another article.
Calculation of a wire heater for an electric furnace.
This article reveals the biggest secrets of electric furnace design - the secrets of calculating heaters.
How the volume, power and heating rate of the furnace are related.
As discussed elsewhere, there are no conventional ovens. Likewise, there are no stoves for firing earthenware or toys, red clay or beads. It happens just a furnace (and here we are talking exclusively about electric furnaces) with a certain volume of useful space, made of some refractories. You can put one large or small vase for firing into this furnace, or you can put a whole stack of slabs on which thick fireclay tiles will lie. You need to burn a vase or tiles, maybe at 1000 o C, and maybe at 1300 o C. For many industrial or household reasons, the firing should take 5-6 hours or 10-12 hours.
Nobody knows what you need from the stove better than yourself. Therefore, before proceeding with the calculation, you need to clarify all these questions for yourself. If the furnace already exists, but it is necessary to install heaters in it or replace the old ones with new ones, there is no need for construction. If the oven is being built from scratch, one must start by finding out the dimensions of the chamber, that is, from the length, depth, width.
Let's assume you already know these values. Let's say you want a camera that is 490 mm high, 350 mm wide and deep. Further in the text, we will call a stove with such a chamber a 60-liter one. At the same time, we will design a second, larger furnace with a height of H = 800 mm, a width of D = 500 mm and a depth of L = 500 mm. We will call this oven a 200 liter oven.
Furnace volume in liters = H x D x L,
where H, D, L are expressed in decimetres.
If you have correctly converted millimeters to decimeters, the volume of the first furnace should be 60 liters, the volume of the second - really 200! Do not think that the author is sneering: the most common errors in calculations are errors in dimensions!
We proceed to the next question - what are the walls of the furnace made of? Almost all modern furnaces are made of lightweight refractories with low thermal conductivity and low heat capacity. Very old stoves are made of heavy fireclay. Such furnaces are easily recognizable by their massive lining, the thickness of which is almost equal to the width of the chamber. If you have this case, you are out of luck: during firing, 99% of the energy will be spent on heating the walls, not the products. We assume that the walls are made of modern materials (MKRL-08, ShVP-350). Then only 50-80% of the energy will be spent on heating the walls.
The bulk of the download remains very uncertain. Although it is generally less than the mass of the refractory walls (plus the hearth and roof) of the furnace, this mass will of course contribute to the heating rate.
Now about the power. Power is how much heat the heater generates in 1 second. The unit of measure for power is watt (abbreviated as W). A bright incandescent light bulb is 100 W, an electric kettle is 1000 W, or 1 kilowatt (abbreviated as 1 kW). If you turn on a 1 kW heater, it will emit heat every second, which, according to the law of conservation of energy, will go to heating the walls, products, and fly away with air through the slots. Theoretically, if there are no losses through the slots and walls, 1 kW is able to heat anything up to an infinite temperature in an infinite time. In practice, real (approximate average) heat losses are known for furnaces, therefore there is the following recommendation rule:
For a normal heating rate of a furnace of 10-50 liters, power is needed
100 watts per liter of volume.
For a normal heating rate of a furnace of 100-500 liters, power is needed
50-70 W for each liter of volume.
The value of the specific power must be determined not only taking into account the volume of the furnace, but also taking into account the massiveness of the lining and loading. The larger the load, the more greater importance you have to choose. Otherwise, the oven will heat up, but for a longer time. Let's choose a specific power of 100 W / l for our 60-liter, and 60 W / l for a 200-liter. Accordingly, we get that the power of the 60-liter heaters should be 60 x 100 = 6000 W = 6 kW, and 200-liter heaters - 200 x 60 = 12000 W = 12 kW. Look how interesting it is: the volume has increased more than 3 times, and the capacity - only 2. Why? (Question for independent work).
It happens that there is no 6 kW socket in the apartment, but there is only 4 kW. But you need exactly a 60-liter! Well, you can count the heater for 4 kilowatts, but come to terms with the fact that the heating stage during firing will last 10-12 hours. It happens that, on the contrary, heating is required for 5-6 hours of a very massive load. Then you will have to invest 8 kW in a 60-liter stove and not pay attention to the red-hot wiring ... For further reasoning, we will restrict ourselves to the classic powers - 6 and 12 kW, respectively.
Power, amperes, volts, phases.
Knowing the power, we know the heat demand for heating. According to the inexorable law of conservation of energy, we must take the same power from the electrical network. We remind you of the formula:
Heater power (W) = Heater voltage (V) x Current (A)
or P = U x I
There are two tricks to this formula. First: the voltage must be taken at the ends of the heater, and not in general at the outlet. Voltage is measured in volts (abbreviated V). Second: I mean the current that flows precisely through this heater, and not in general through the machine. Current is measured in amperes (abbreviated A).
We are always given the voltage in the network. If the substation is operating normally and it is not rush hour, the voltage in an ordinary household outlet will be 220 V. The voltage in an industrial three-phase network between any phase and neutral wire is also equal to 220V, and the voltage between any two phases- 380 V. Thus, in the case of a household, single-phase, network, we have no choice in voltage - only 220 V. In the case of a three-phase network, there is a choice, but a small one - either 220 or 380 V. But what about amperes? They will be obtained automatically from the voltage and resistance of the heater according to the great law of the great Ohm:
Ohm's law for a section of an electrical circuit:
Current (A) = Line Voltage (V) / Line Resistance (Ohm)
or I = U / R
In order to get 6 kW from a single-phase network, current is needed I = P / U= 6000/220 = 27.3 amperes. This is a large, but real current of a good household network. For example, such a current flows in an electric stove, in which all the burners are turned on at full power and the oven too. To get 12 kW in a single-phase network for a 200-liter, you will need twice as much current - 12000/220 = 54.5 amperes! This is unacceptable for any household network. Better to use three phases, i.e. distribute power to three lines. In each phase, 12000/3/220 = 18.2 amperes will flow.
Pay attention to the last calculation. At the moment, we DO NOT KNOW what kind of heaters will be in the oven, we DO NOT KNOW what voltage (220 or 380 V) will be applied to the heaters. But we KNOW for sure that 12 kW must be taken from the three-phase network, the load must be distributed evenly, i.e. 4 kW in each phase of our network, i.e. 18.2A will flow through each phase wire of the input (common) automatic machine of the furnace, and it is not at all necessary that such a current will flow through the heater. By the way, 18.2 A will also pass through the electricity meter. (And by the way: there will be no current through the zero wire due to the peculiarities of the three-phase power supply. These features are ignored here, since we are only interested in the thermal work of the current). If you have questions at this point in the presentation, read it all over again. And think: if 12 kilowatts are released in the volume of the furnace, then according to the law of conservation of energy, the same 12 kilowatts pass through three phases, each - 4 kW ...
Let's go back to the single phase 60 liter stove. It is easy to find that the resistance of the furnace heater should be R = U / I= 220 V / 27.3 A = 8.06 Ohm. Therefore, in the very general view the electrical diagram of the furnace will look like this:
A heater with a resistance of 8.06 Ohm should flow a current of 27.3 A
For a three-phase furnace, three identical heating circuits are required: in the figure - the most general electrical circuit of a 200-liter.
The power of a 200 liter oven must be evenly distributed over 3 circuits - A, B and C.
But each heater can be turned on either between a phase and zero, or between two phases. In the first case, there will be 220 volts at the ends of each heating circuit, and its resistance will be R = U / I= 220 V / 18.2 A = 12.08 Ohm. In the second case, there will be 380 volts at the ends of each heating circuit. To obtain a power of 4 kW, the current must be I = P / U= 4000/380 = 10.5 amperes, i.e. resistance should be R = U / I= 380 V / 10.5 A = 36.19 Ohm. These connection options are called "star" and "delta". As can be seen from the values of the required resistance, simply changing the power supply circuit from a star (heaters of 12.08 Ohm) to a triangle (heaters of 36.19 Ohm) will not work - in each case, you need your own heaters.
In a star circuit, each heating circuit
connected between phase and zero for a voltage of 220 volts. A current of 18.2 A flows through each heater with a resistance of 12.08 Ohm. No current flows through the N wire.
In a delta circuit, each heating circuit
switched on between two phases for a voltage of 380 volts. A current of 10.5 A flows through each heater with a resistance of 36.19 Ohm. A current of 18.2 A flows through the wire connecting point A1 with the power switch (point A), so that 380 x 10.5 = 220 x 18.2 = 4 kilowatt! Likewise with lines B1 - B and C1 - C.
Homework. There was a star in the 200-liter. The resistance of each circuit is 12.08 Ohm. What will be the power of the furnace if these heaters are turned on with a triangle?
Limiting loads of wire heaters (Х23Ю5Т).
Complete victory! We know the resistance of the heater! All that remains is to rewind a piece of wire of the required length. Let's not get tired of calculations with resistivity - everything has long been calculated with an accuracy sufficient for practical needs.
| Diameter, mm | Meters in 1 kg | Resistance of 1 meter, Ohm |
| 1,5 | 72 | 0.815 |
| 2,0 | 40 | 0.459 |
| 2,5 | 25 | 0.294 |
| 3,0 | 18 | 0.204 |
| 3,5 | 13 | 0.150 |
| 4,0 | 10 | 0.115 |
For a 60-liter stove, 8.06 Ohm is needed, choose a one and a half and get that the required resistance will be given by only 10 meters of wire, which will weigh only 140 grams! An amazing result! Let's check again: 10 meters of 1.5 mm wire have a resistance of 10 x 0.815 = 8.15 Ohm. The current at 220 volts will be 220 / 8.15 = 27 amperes. The power will be 220 x 27 = 5940 W = 5.9 kW. We wanted 6 kW. We were not mistaken anywhere, the only alarming thing is that there are no such ovens ...
A lonely red-hot heater in a 60-liter oven.
The heater is very small or something. This is the feeling when looking at the above picture. But we are doing calculations, not philosophy, so let's move on from feelings to numbers. The numbers say the following: 10 running meters of wire with a diameter of 1.5 mm have an area S = L x d x pi = 1000 x 0.15 x 3.14 = 471 sq. cm. From this area (where else?), 5.9 kW is radiated into the furnace volume, i.e. for 1 sq. cm of area has a radiated power of 12.5 watts. Omitting the details, we point out that the heater needs to be heated to an enormous temperature before the temperature in the oven rises significantly.
The overheating of the heater is determined by the value of the so-called surface load p, which we calculated above. In practice, there are limit values for each type of heater p depending on heater material, diameter and temperature. With a good approximation for a wire made of the domestic alloy Kh23Yu5T of any diameter (1.5-4 mm), a value of 1.4-1.6 W / cm 2 can be used for a temperature of 1200-1250 o C.
Physically, overheating can be associated with the temperature difference on the surface of the wire and inside it. Heat is released throughout the entire volume, therefore, the higher the surface load, the more these temperatures will differ. At surface temperatures close to the operating temperature limit, the temperature in the wire core may approach the melting temperature.
If the furnace is designed for low temperatures, the surface load can be selected more, for example, 2 - 2.5 W / cm 2 for 1000 o C. Here you can make a sad remark: real canthal (this is an original alloy, the analogue of which is the Russian fechral Kh23Yu5T) allows p up to 2.5 at 1250 o C. Such canthal is made by the Swedish company Kantal.
Let's go back to our 60-liter and choose a thicker wire from the table - two. It is clear that deuces will have to take 8.06 Ohm / 0.459 Ohm / m = 17.6 meters, and they will already weigh 440 grams. We consider the surface load: p= 6000 W / (1760 x 0.2 x 3.14) cm 2 = 5.43 W / cm 2. Lot. For a wire with a diameter of 2.5 mm, you get 27.5 meters and p= 2.78. For the troika - 39 meters, 2.2 kilograms and p= 1.66. Finally.
Now we have to wind 39 meters of the troika (if it bursts, start winding over again). But you can use TWO heaters connected in parallel. Naturally, the resistance of each should no longer be 8.06 Ohm, but twice as much. Therefore, for a two, two heaters will turn out, 17.6 x 2 = 35.2 m each, each will have 3 kW of power, and the surface load will be 3000 W / (3520 x 0.2 x 3.14) cm 2 = 1, 36 W / cm 2. And the weight is 1.7 kg. Have saved half a kilo. A total of many turns were obtained, which can be evenly distributed over all the walls of the furnace.
Well distributed heaters in a 60 liter oven.
| Diameter, mm | Limiting current for p= 2 W / cm 2 at 1000 o C | Limiting current for p= 1.6 W / cm 2 at 1200 o C |
| 1,5 | 10,8 | 9,6 |
| 2,0 | 16,5 | 14,8 |
| 2,5 | 23,4 | 20,7 |
| 3,0 | 30,8 | 27,3 |
| 3,5 | 38,5 | 34,3 |
| 4,0 | 46,8 | 41,9 |
An example of calculating a 200 liter oven.
Now that the basic principles are known, we will show how they are used to calculate a real 200 liter furnace. All stages of the calculation, of course, can be formalized and written into a simple program that will do almost everything by itself.
Let's draw our oven "in a sweep". We seem to be looking at it from above, in the center - under, on the sides of the wall. We will calculate the area of all the walls, so that then correctly, in proportion to the area, to organize the supply of heat.
"Sweep" of a 200-liter oven.
We already know that when connected by a star, a current of 18.2A must flow in each phase. From the above table for the limiting currents it follows that for a wire with a diameter of 2.5 mm, one heating element can be used (limiting current 20.7A), and for a wire of 2.0 mm, two elements connected in parallel must be used (since the limiting current 14.8A), in total there will be 3 x 2 = 6 in the oven.
According to Ohm's law, we calculate the required resistance of the heaters. For wire with a diameter of 2.5 mm R= 220 / 18.2 = 12.09 ohms, or 12.09 / 0.294 = 41.1 meters. You will need 3 such heaters, about 480 turns each, if wound on a 25 mm mandrel. The total weight of the wire will be (41.1 x 3) / 25 = 4.9 kg.
For a wire of 2.0 mm, there are two parallel elements in each phase, so the resistance of each should be twice as much - 24.18 Ohm. The length of each will be 24.18 / 0.459 = 52.7 meters. Each element will have 610 turns with the same winding. Total weight of all 6 heating elements (52.7 x 6) / 40 = 7.9 kg.
Nothing prevents us from dividing any spiral into several pieces, which are then connected in series. What for? First, for ease of installation. Secondly, if a quarter of the heater fails, only this quarter will need to be changed. In the same way, no one bothers to shove a whole spiral into the oven. Then a separate spiral will be required for the door, but we have, in the case of a diameter of 2.5 mm, there are only three of them ...
We put one phase of 2.5 mm wire. The heater was divided into 8 independent short coils, all of them connected in series.
When we put all three phases in the same way (see the figure below), the following becomes clear. We forgot about the pod! And it occupies 13.5% of the area. In addition, the spirals are in dangerous electrical proximity to each other. Particularly dangerous is the proximity of the spirals on the left wall, where there is a voltage of 220 Volts between them (phase - zero - phase - zero ...). If, due to something, the neighboring spirals of the left wall touch each other, a large short circuit cannot be avoided. We propose to independently optimize the arrangement and connection of the spirals.
We put all the phases.
For the case if we decided to use two, the diagram is shown below. Each element, 52.7 meters long, is divided into 4 successive spirals of 610/4 = 152 turns (winding on a 25 mm mandrel).
Heater arrangement option for 2.0 mm wire.
Features of winding, installation, operation.
The wire is convenient because it can be wound into a spiral, and then the spiral can be stretched as it is convenient. It is believed that the winding diameter should be more than 6-8 wire diameters. The optimal pitch between turns is 2-2.5 wire diameters. But it is necessary to wind the coil to the coil: it is very easy to stretch the spiral, to squeeze it is much more difficult.
Thick wire may break during winding. It is especially disappointing if 5 of 200 turns are left to wind. It is ideal to wind on a lathe at a very slow rotation speed of the mandrel. Alloy Kh23Yu5T is produced as tempered and not dispensed. The latter bursts especially often, therefore, if you have a choice, be sure to purchase the wire that was released for winding.
How many turns do you need? Despite the simplicity of the question, the answer is not obvious. First, the diameter of the mandrel is not known exactly, and hence the diameter of one turn. Secondly, it is known for sure that the diameter of the wire walks slightly along the length, so the resistance of the spiral will also walk. Thirdly, the specific resistance of an alloy of a particular weld may differ from the reference. In practice, the spiral is wound 5-10 turns more than by calculation, then its resistance is measured - with a VERY ACCURATE device that you can trust, and not with a soap dish. In particular, you need to make sure that with short-circuited probes, the device shows zero, or a number of the order of 0.02 Ohm, which will need to be subtracted from the measured value. When measuring resistance, the spiral is slightly stretched to exclude the influence of turn-to-turn short circuits. The extra turns are bitten off.
It is best to place the coil in a furnace on a mullite-silica tube (MCR). For a winding diameter of 25 mm, a tube with an outer diameter of 20 mm is suitable, for a winding diameter of 35 mm - 30 - 32 mm.
It is good if the stove is heated evenly from five sides (four walls + under). It is necessary to concentrate significant power on the hearth, for example, 20 -25% of the entire design capacity of the furnace. This compensates for the intake of cold air from the outside.
Unfortunately, it is still impossible to achieve absolute uniformity of heating. You can get closer to it using ventilation systems with LOWER air intake from the oven.
During the first heating, or even the first two or three heatings, dross forms on the surface of the wire. We must not forget to remove it both from the heaters (with a brush) and from the surface of slabs, bricks, etc. Scale is especially dangerous if the coil simply lies on bricks: iron oxides with aluminosilicates at high temperatures (a heater in one millimeter!) Form fusible compounds, due to which the heater can burn out.
You will need
- Spiral, caliper, ruler. It is necessary to know the material of the spiral, the value of the current I and the voltage U at which the spiral will work, and what material it is made of.
Instructions
Find out how much resistance R your coil should have. To do this, use Ohm's law and substitute the value of the current I in the circuit and the voltage U at the ends of the spiral into the formula R = U / I.
Using the reference book, determine the specific electrical resistance of the material ρ, from which the spiral will be made. ρ should be expressed in Ohm m. If the value of ρ in the reference book is given in Ohm mm² / m, then multiply it by 0.000001. For example: the resistivity of copper ρ = 0.0175 Ohm mm² / m, when translated into SI we have ρ = 0 , 0175 0.000001 = 0.0000000175 Ohm m.
Find the length of the wire by the formula: Lₒ = R S / ρ.
Measure an arbitrary length l with a ruler on the spiral (for example: l = 10cm = 0.1m). Count the number of loops n coming to this length. Determine the helix pitch H = l / n or measure it with a caliper.
Find how many turns N can be made from a wire of length Lₒ: N = Lₒ / (πD + H).
Find the length of the spiral itself by the formula: L = Lₒ / N.
A spiral scarf is also called a boa scarf, a wave scarf. The main thing here is not the type of yarn, not the knitting pattern and not the color of the finished product, but the technique of execution and the originality of the model. The spiral scarf personifies festivity, splendor, solemnity. It looks like an elegant lace frill, an exotic boa, and an ordinary, but very original scarf.
How to knit a spiral scarf with knitting needles
To knit a spiral scarf, cast on 24 loops on the needles and knit the 1st row:
- 1 edge loop;
- 11 facial;
- 12 purl loops.
The quality and color of the yarn for this spiral scarf is up to you.
1st row: first 1 edge loop, then 1 yarn over, then 1 front loop, then 1 yarn over and 8 front loops. Remove one on the right knitting needle as purl, pull the thread between the knitting needles forward. Return the removed loop to the left knitting needle, pull the thread between the knitting needles back (in this case, the loop will turn out to be a wrapped thread). Turn the work over and knit 12 purl stitches.
2nd row: first knit 1 edge loop, then 1 yarn over, then knit 3 front loops, 1 yarn over and 6 front loops. Remove one on the right knitting needle as purl, pull the thread between the knitting needles forward. Next, return the loop to the left knitting needle, pull the thread back between the knitting needles, then turn the work and knit 12 purl loops.
3rd row: knit 1 edge stitch, then knit 2 loops together, then knit 1, then knit 2 and 4 knit loops together. Remove one on the right knitting needle as purl, pull the thread between the needles forward, return the loop to the left knitting needle, then pull the thread between the needles back. After that, turn the work and knit 8 purl loops.
4th row: knit 1 hem, then 3 loops together with the front one, then 4 front loops, * get the wrapped loop from below and knit together with the next front one, 1 front * (repeat knitting from * to * 3 times). Without turning the work over, tie the purl loops.
Thus, knit the coiled scarf to the required length in blocks of these 4 rows.
Almost all women are faced with the issue of contraception. One of the reliable and proven methods is the intrauterine device, which is still in demand today.
Types of spirals
Intrauterine devices are made of plastic and are of two types: spirals containing copper (silver) and spirals containing hormones. Their size is 3X4 cm. The choice of the method of contraception and the coil itself takes place at the reception of the gynecologist. You should not do this on your own. The intrauterine device is installed by a gynecologist during menstruation. It is small in size and resembles the shape of the letter T.
Copper spiral is made from copper wire. Its feature is the ability to act on the uterus in such a way that the egg cannot attach to it. This is facilitated by two copper tendrils.
The hormone coil has a container that contains a progestin. This hormone prevents the onset of ovulation. In the case of using hormonal intrauterine device sperm cannot fertilize an egg. As women note, when using such a spiral, menstruation becomes leaner and less painful. However, this does not bring harm, because it is associated with the action of hormones inside the spiral. Gynecologists recommend that women suffering from painful periods install a hormonal coil.
Spiral selection
Gynecological intrauterine devices are of different brands, both domestic and foreign. In addition, their cost can vary from 250 rubles to several thousand. This is influenced by many factors.
The Juno Bio spiral is quite popular among Russian women. It attracts, first of all, by its low cost. However, the low efficiency of the action of this coil entails a high risk of pregnancy.
The Mirena intrauterine device has proven itself well, but it is one of the most expensive in its series. At the same time, the use of an intrauterine device is considered the cheapest and most affordable form of contraception.
This hormonal coil... Its manufacturers promise that the Mirena spiral is less likely to shift in the uterus or fall out. Namely, this leads to the onset of pregnancy, therefore, patients are advised to regularly check for the presence of an intrauterine contraceptive in the right place.
The standard voltage in the household electrical network is U = 220V. The current strength is limited by the fuses in the switchboard and is, as a rule, equal to I = 16A.
Sources:
- Tables of physical quantities, I.K. Kikoin, 1976
- spiral length formula
An electric soldering iron is a hand tool designed to fasten parts together by means of soft solders, by heating the solder to a liquid state and filling the gap between the parts to be soldered with it.
Electric soldering irons are available for 12, 24, 36, 42 and 220 V supply voltages, and there are reasons for this. The main thing is human safety, the second is the mains voltage in the place where the soldering work was performed. In production where all equipment is grounded and there is high humidity, it is allowed to use soldering irons with a voltage of no more than 36 V, while the body of the soldering iron must be grounded. The on-board network of a motorcycle has a DC voltage of 6 V, a car - 12 V, a truck - 24 V. Aviation uses a network with a frequency of 400 Hz and a voltage of 27 V. There are also design restrictions, for example, a 12 W soldering iron is difficult to make for the supply voltage 220 V, since the spiral will need to be wound from a very thin wire and therefore many layers are wound, the soldering iron will turn out to be large, not convenient for small work. Since the winding of the soldering iron is wound from nichrome wire, it can be powered by both alternating and direct voltage. The main thing is that the supply voltage matches the voltage for which the soldering iron is designed.
The power of electric soldering irons is 12, 20, 40, 60, 100 W and more. And this is also no coincidence. In order for the solder to spread well during soldering over the surfaces of the parts to be soldered, they must be heated to a temperature slightly higher than the melting temperature of the solder. On contact with the workpiece, heat is transferred from the tip to the workpiece and the temperature of the tip drops. If the diameter of the soldering iron tip is not sufficient or the power of the heating element is small, then after giving off heat, the tip will not be able to heat up to the set temperature, and it will be impossible to solder. In the best case, you will get a loose and not strong solder. A more powerful soldering iron can be used to solder small parts, but there is a problem of inaccessibility to the soldering point. How, for example, can a microcircuit be soldered into a printed circuit board with a foot pitch of 1.25 mm with a soldering iron tip 5 mm in size? True, there is a way out, several turns of copper wire with a diameter of 1 mm are wound onto such a sting and the end of this wire is already soldered. But the cumbersomeness of the soldering iron makes the job almost impossible. There is one more limitation. At high power, the soldering iron will quickly warm up the element, and many radio components do not allow heating above 70˚С, and therefore, the permissible time for their soldering is no more than 3 seconds. These are diodes, transistors, microcircuits.
Soldering iron device
The soldering iron is a red copper rod that is heated by a nichrome spiral to the melting point of the solder. The soldering iron rod is made of copper due to its high thermal conductivity. After all, when soldering, you need to quickly transfer the soldering iron tip from the heating element to the heat. The end of the rod has a wedge-shaped shape, is the working part of the soldering iron and is called a tip. The rod is inserted into a steel tube wrapped in mica or fiberglass. A nichrome wire is wound on mica, which serves as a heating element.
A layer of mica or asbestos is wound on top of nichrome, which serves to reduce heat loss and electrical insulation of the nichrome spiral from the metal body of the soldering iron.
The ends of the nichrome spiral are connected to the copper conductors of the electrical cord with a plug at the end. To ensure the reliability of this connection, the ends of the nichrome spiral are bent and folded in half, which reduces heating at the junction with copper wire... In addition, the joint is crimped with a metal plate, it is best to make the crimp from an aluminum plate, which has a high thermal conductivity and will more efficiently remove heat from the joint. For electrical insulation, tubes made of heat-resistant insulating material, fiberglass or mica are put on the junction.
A copper rod and a nichrome spiral is closed by a metal case, consisting of two halves or a solid tube, as in the photo. The soldering iron body is fixed on the tube with cap rings. To protect a person's hand from burns, a handle is placed on the tube made of a material that does not provide heat well, such as wood or heat-resistant plastic.
When you insert the plug of the soldering iron into the outlet, the electric current flows to the nichrome heating element, which heats up and transfers heat to the copper rod. The soldering iron is ready for soldering.
Low-power transistors, diodes, resistors, capacitors, microcircuits and thin wires soldered with a 12 W soldering iron. Soldering irons 40 and 60 W are used for soldering powerful and large-sized radio components, thick wires and small parts. For soldering large parts, for example, gas column heat exchangers, you will need a soldering iron with a power of one hundred or more watts.
As you can see in the drawing, the electrical circuit of the soldering iron is very simple, and consists of only three elements: a plug, a flexible electric wire and a nichrome spiral.
As you can see from the diagram, the soldering iron does not have the ability to adjust the heating temperature of the tip. And even if the power of the soldering iron is chosen correctly, it is still not a fact that the temperature of the tip will be required for soldering, since the length of the tip decreases over time due to its constant refueling, solders also have different melting temperatures. Therefore, to maintain the optimum temperature of the soldering iron tip, it is necessary to connect it through thyristor power regulators with manual adjustment and automatic maintenance of the set temperature of the soldering iron tip.
Calculation and repair of the heating winding of a soldering iron
When repairing or making an electric soldering iron or any other heating device on your own, you have to wind a heating winding made of nichrome wire. The initial data for calculating and choosing a wire is the winding resistance of a soldering iron or a heating device, which is determined based on its power and supply voltage. You can use the table to calculate what the resistance of the winding of a soldering iron or heating device should be.
When repairing or self-production an electric soldering iron or any other heating device, you have to wind a heating winding of nichrome wire. The initial data for calculating and choosing a wire is the winding resistance of a soldering iron or a heating device, which is determined based on its power and supply voltage. You can use the table to calculate what the resistance of the winding of a soldering iron or heating device should be.
Knowing the supply voltage and measuring resistance any heating appliance, such as a soldering iron, or an electric iron, you can find out the power consumed by this household electrical appliance. b. For example, the resistance of a 1.5 kW electric kettle will be 32.2 ohms.
| Table for determining the resistance of the nichrome spiral, depending on the power and supply voltage electrical appliances, Ohm | |||||
|---|---|---|---|---|---|
| Power consumption soldering iron, W | Soldering iron supply voltage, V | ||||
| 12 | 24 | 36 | 127 | 220 | |
| 12 | 12 | 48,0 | 108 | 1344 | 4033 |
| 24 | 6,0 | 24,0 | 54 | 672 | 2016 |
| 36 | 4,0 | 16,0 | 36 | 448 | 1344 |
| 42 | 3,4 | 13,7 | 31 | 384 | 1152 |
| 60 | 2,4 | 9,6 | 22 | 269 | 806 |
| 75 | 1.9 | 7.7 | 17 | 215 | 645 |
| 100 | 1,4 | 5,7 | 13 | 161 | 484 |
| 150 | 0,96 | 3,84 | 8,6 | 107 | 332 |
| 200 | 0,72 | 2,88 | 6,5 | 80,6 | 242 |
| 300 | 0,48 | 1,92 | 4,3 | 53,8 | 161 |
| 400 | 0,36 | 1,44 | 3,2 | 40,3 | 121 |
| 500 | 0,29 | 1,15 | 2,6 | 32,3 | 96,8 |
| 700 | 0,21 | 0,83 | 1,85 | 23,0 | 69,1 |
| 900 | 0,16 | 0,64 | 1,44 | 17,9 | 53,8 |
| 1000 | 0,14 | 0,57 | 1,30 | 16,1 | 48,4 |
| 1500 | 0,10 | 0,38 | 0,86 | 10,8 | 32,3 |
| 2000 | 0,07 | 0,29 | 0,65 | 8,06 | 24,2 |
| 2500 | 0,06 | 0,23 | 0,52 | 6,45 | 19,4 |
| 3000 | 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 220 V supply voltage. Select 60 W from the leftmost column of the table. On the upper horizontal line, choose 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, to be powered from a 36 V network, then the resistance of the new winding should already be 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 winding of the soldering iron, the 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 with Х20Н80. 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 spiral of a soldering iron, the turns are stacked close to each other. When heated, the red-hot 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 the second is wound.
For electrical and thermal insulation of the heating element winding the best materials is mica, fiberglass cloth and asbestos. Asbestos possesses 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 should be taken into account that wet asbestos conducts an eclectic current well and it will be possible to turn on the soldering iron into the power grid only after the asbestos has completely dried.
The nichrome coil is a wire-wound heating element for compact placement. The wire is made from nichrome- a precision alloy, the main components of which 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, which designates the group of chromium-nickel alloys - "nichrome".
The most famous nichrome grades - Х20Н80 and Х15Н60... The first one is close to the "classics". It contains 72-73% nickel and 20-23% chromium. The second is designed to reduce the cost and improve the machinability of the wire. The content of nickel and chromium in it is reduced - up to 61% and up to 18%, respectively. But the amount of iron increased - 17-29% versus 1.5 for X20H80.
On the basis of these alloys, their modifications were obtained with a higher survivability and resistance to oxidation at high temperature... These are brands X20N80-N (-N-VI) and Kh15N60 (-N-VI). They are used for heating elements in contact with air. Recommended maximum operating temperature - from 1100 to 1220 ° С
Application of nichrome wire
The main quality of nichrome is its high resistance to electric current. It defines the areas of application of the alloy. Nichrome spiral it is used in two qualities - as a heating element or as a material for electrical resistance electrical circuits.
Used for heaters electric spiral from alloys Х20Н80-Н and Х15Н60-Н. 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, obtained in vacuum induction furnaces used in industrial equipment increased reliability.
Nichrome spiral grades Х15Н60, Х20Н80, Kh20N80-VI differs in that its electrical resistance changes little with temperature. Resistors, connectors for electronic circuits, critical parts of vacuum devices are made from it.
How to wind a nichrome spiral
Resistive or heating coil can be made at home. To do this, you need a nichrome wire of a suitable brand and the correct calculation of the required length.
Some household heating devices still use nichrome wire. It possesses the high heat resistance characteristic of a nickel-chromium alloy. This material has good plasticity, high electrical resistivity and low temperature coefficient of resistance. Therefore, when calculating a nichrome wire for a heater, these parameters must be taken into account. Otherwise, the calculation results will be inaccurate and will not give the desired result.
Using the online calculator in calculations
Quick calculations can be done using an online calculator. With its help, you can calculate and approximately set the required length of nichrome wire. As a rule, the brands that are most widely used in heating devices are considered - Х20Н80, Х20Н80-Н, Х15Н60.
To carry out the calculations, the required input data are required. First of all, this is the amount of power of the heater that is planned to be obtained, the diameter of the nichrome wire and the value of the supply voltage of the network.
The calculations are carried out as follows. First of all, you need to set in accordance with the specified parameters, according to the formula: I = P / U. After that, the resistance for the entire heating element is calculated. Next, you need specific electrical resistance for a specific brand of nichrome wire. This value will be needed in order to establish the most optimal length of the heating element using a different formula: l = SR / ρ. Right choice length will bring the heater resistance R to the desired value.
After performing the calculations, it is recommended to check the obtained data using the table and make sure that the rated current corresponds to the permissible value. If the calculated current exceeds the permissible limits, recalculations should be performed by increasing the diameter of the nichrome wire or reducing the power of the heating element itself. It is necessary to take into account the fact that all the parameters given in the tables are calculated for heaters located in a horizontal position and operating in an air environment.
If the nichrome spiral is planned to be used placed in a liquid, the value of the permissible current should be multiplied by a factor of 1.1-1.5. When the spiral is closed, on the contrary, it must be reduced by 1.2-1.5 times.
