What can be cooked from squid: quick and tasty

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 by heating the nichrome wire, the threads select the required number of turns.

Usually, such a procedure takes a long time, and nichrome loses its characteristics with multiple bends, which leads to rapid burnout in places of deformation. In the worst case, nichrome scrap is obtained from business nichrome.

With its help, you can accurately determine the length of 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 the nichrome spiral to a different voltage using a simple mathematical proportion.

The length of the nichrome spiral depending on the diameter of the nichrome and the diameter of the rod

For example, you need to determine the length of a nichrome spiral for a voltage of 380 V from a wire Ø 0.3 mm, a core for winding Ø 4 mm. It can be seen from the table that the length of such a spiral for 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

After winding the nichrome spiral, connect it without cutting it to a voltage source and make sure it is wound correctly. For closed spirals, the winding length is increased by 1/3 of the value given in the table.

Calculation of electric heating elements made of nichrome wire

The length of the nichrome wire for the manufacture of the spiral is determined based on the required power.

Example: Determine the length of the nichrome wire for a heating element of a tile with a capacity P= 600 W at U network = 220 V.

Solution:

1) I = P / U= 600/220 = 2.72 A

2) R = U / I= 220 / 2.72 = 81 ohms

3) Based on these data (see table 1), we choose d=0,45; S=0,159

then the length of nichrome

l = SR / ρ= 0.15981 / 1.1 = 11.6 m

where l- wire length (m)

S- wire section (mm 2)

R- wire resistance (Ohm)

ρ - resistivity(for nichrome ρ = 1.0 ÷ 1.2 Ohm mm 2 / m)

Some household heating devices still use nichrome wire. It possesses the high heat resistance characteristic of 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.

Mandatory input data are required to perform calculations. First of all, this is the value of the heater power 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 rated 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.

If the home craftsman needs a muffle furnace due to the nature of the work he performs, then, of course, he can purchase a ready-made device in a store or through advertisements. However, such factory-made equipment is very expensive. Therefore, many craftsmen undertake the manufacture of such ovens on their own.

The main "working unit" of an electric muffle furnace is a heater, which, under handicraft conditions, is usually made in the form of a spiral made of special wire with high resistance and thermal efficiency. Its characteristics must strictly correspond to the power of the equipment being created, the expected temperature conditions of operation, and also meet some other requirements. If planned self-production device, then we recommend using the algorithm and convenient calculators for calculating the muffle furnace heater proposed below.

The calculation requires certain explanations, which we will try to present as clearly as possible.

Algorithm and calculators for calculating the muffle furnace heater

What are heating coils made of?

To begin with, just a few words about the wire that is used to wind heating coils. Usually nichrome or fechral is used for such purposes.

• Nichrome(from the abbreviations nickel + chromium) is most often represented by alloys Kh20N80-N, Kh15N60 or Kh15N60-N.

Muffle furnace prices

muffle furnace

Her dignity :

- high safety margin at any heating temperatures;

- plastic, easy to process, weldable;

- durability, corrosion resistance, lack of magnetic properties.

disadvantages :

- high price;

- lower heating rates and heat resistance in comparison with fechraleva.

• Fekhralevaya(from the abbreviations ferrum, chrome, aluminum) - in our time, material from the X23Yu 5T alloy is more often used.

Advantages fechral:

- much cheaper than nichrome, due to which the material is mainly popular;

- has more significant indicators of resistance and resistive heating;

- high heat resistance.

disadvantages :

- low strength, and after even a single heating over 1000 degrees - pronounced fragility of the spiral;

- not outstanding durability;

- presence of magnetic properties, susceptibility to corrosion due to the presence of iron in the composition;

- unnecessary chemical activity - capable of reacting with the material of the fireclay lining of the furnace;

- excessive thermal linear expansion.

Each of the masters is free to choose any of the listed materials, analyzing their pros and cons. The calculation algorithm takes into account the peculiarities of such a choice.

Step 1 - determining the power of the furnace and the current flowing through the heater.

In order not to go into unnecessary given case of details, let's say right away that there are empirical conformity standardsvolumeworking chamber of a muffle furnace and its power. They are shown in the table below:

If there are design sketches of the future device, then the volume of the muffle chamber is easy to determine - the product of height, width and depth. Then the volume is converted into liters and multiplied by the recommended power rates indicated in the table. So we get the power of the furnace in watts.

The tabular values ​​are in some ranges, so either use interpolation or take an approximate average.

The found power, at a known mains voltage (220 volts), allows you to immediately determine the strength of the current that will pass through the heating element.

I = P / U.

I- current strength.

R- the power of the muffle furnace specified above;

U- supply voltage.

All this first step of calculation can be done very easily and quickly with the help of a calculator: all table values ​​have already been entered into the calculation program.

Calculator for muffle furnace power and heater current

In practice home master it is necessary to repair or design heating devices. These can be various furnaces, heaters, soldering irons and cutters. Most often, spirals or nichrome wire are used for this. The main task in this case is to determine the length and cross-section of the material. In this article, we will talk about how to calculate the length of a nichrome wire or spiral in terms of power, resistance and temperature.

Basic information and brands of nichrome

Nichrome is an alloy of nickel and chromium with additions of manganese, silicon, iron, aluminum. For this material, the parameters depend on the specific ratio of substances in the alloy, but on average lie within:

• specific electrical resistance - 1.05-1.4 Ohm * mm 2 / m (depending on the grade of the alloy);
• temperature coefficient of resistance - (0.1-0.25) · 10 −3 K −1;
• working temperature - 1100 ° C;
• melting point - 1400 ° C;

In tables, resistivity is often given in μOhm * m (or 10 -6 Ohm * m) - the numerical values ​​are the same, the difference is in dimension.

Currently, there are two most common brands of nichrome wire:

• Х20Н80. It consists of 74% nickel and 23% chromium, as well as 1% iron, silicon and manganese. Conductors of this brand can be used at temperatures up to 1250 ᵒ С, melting temperature - 1400 ᵒ С. It also has a high electrical resistance. The alloy is used for the manufacture of elements of heating devices. Specific resistance - 1.03-1.18 μOhm · m;
• Х15Н60. Composition: 60% Nickel, 25% Iron, 15% Chromium. Operating temperature no more than 1150 ᵒ С. Melting temperature - 1390 ᵒ С. Contains more iron, which increases the magnetic properties of the alloy and increases its anti-corrosion resistance.

You will learn more about the grades and properties of these alloys from GOST 10994-74, GOST 8803-89, GOST 12766.1-90 and others.

As already mentioned, nichrome wire is used everywhere where heating elements are needed. The high resistivity and melting point make it possible to use nichrome as a base for various heating elements, from a kettle or hair dryer to a muffle furnace.

Calculation methods

By resistance

Let's figure out how to calculate the length of the nichrome wire in terms of power and resistance. The calculation starts with determining the required power. Imagine that we need a nichrome thread for a small size soldering iron with a power of 10 watts, which will work from a 12V power supply. For this we have a wire with a diameter of 0.12 mm.

The simplest calculation of the length of nichrome by power without taking into account heating is performed as follows:

Determine the current strength:

I = P / U = 10/12 = 0.83 A

The calculation of the resistance of nichrome wire is carried out according to:

R = U / I = 12 / 0.83 = 14.5 Ohm

The length of the wire is:

l = SR / ρ ,

where S is the cross-sectional area, ρ – resistivity.

Or by this formula:

l = (Rπd2) / 4 ρ

L = (14.5 * 3.14 * 0.12 ^ 2) / 4 * 1.1 = 0.149m = 14.9cm

The same can be taken from GOST 12766.1-90 table. 8, where the value is indicated at 95.6 Ohm / m, if you recalculate it, you get almost the same:

L = R required / R tab = 14.4 / 95.6 = 0.151m = 15.1cm

For a 10 watt heater, which is powered by 12V, you need 15.1 cm.

If you need to calculate the number of turns of a spiral in order to twist it from a nichrome wire of this length, then use the following formulas:

Length of one turn:

Number of turns:

N = L / (π (D + d / 2)),

where L and d are the length and diameter of the wire, D is the diameter of the rod on which the spiral will be wound.

Suppose we wind nichrome wire on a rod with a diameter of 3 mm, then we carry out the calculations in millimeters:

N = 151 / (3.14 (3 + 0.12 / 2)) = 15.71 turns

But at the same time, it is necessary to take into account whether nichrome of such a cross section is generally capable of withstanding this current. Detailed tables for determining the maximum allowable current at a specific temperature for specific cross-sections are given below. In simple words- you determine how many degrees the wire should be heated and choose its cross-section for the calculated current.

Also note that if the heater is inside the liquid, then the current can be increased by 1.2-1.5 times, and if in a closed space, then on the contrary, it can be reduced.

By temperature

The problem with the above calculation is that we calculate the resistance of the cold spiral by the diameter of the nichrome thread and its length. But it depends on the temperature, while at the same time it is necessary to take into account under what conditions it will be possible to achieve it. If this calculation is still applicable for cutting foam or for a heater, then it will be too rough for a muffle furnace.

Let's give an example of calculating nichrome for a furnace.

First, determine its volume, say 50 liters, then determine the power, for this there is a rule of thumb:

• up to 50 liters - 100W / l;
• 100-500 liters - 50-70 W / l.

Then in our case:

I = 5000/220 = 22.7 Amperes

R = 220 / 22.7 = 9.7 Ohm

For 380V, when the spirals are connected with a star, the calculation will be as follows.

We divide the power into 3 phases:

Pf = 5/3 = 1.66 kW per phase

When connected with a star, 220V is applied to each branch (phase voltage may differ depending on your electrical installation), then the current:

I = 1660/220 = 7.54 A

Resistance:

R = 220 / 7.54 = 29.1 Ohm

To connect with a triangle, we calculate according to line voltage 380V:

I = 1660/380 = 4.36 A

R = 380 / 4.36 = 87.1 Ohm

To determine the diameter, the specific surface power of the heater is taken into account. We calculate the length, we take the resistivity from the table. 8. GOST 12766.1-90, but first let's determine the diameter.

To calculate the specific surface power of the furnace, use the formula.

Beff (depends on the heat-receiving surface) and a (coefficient of radiation efficiency) - are selected according to the following tables.

So, to heat the furnace to 1000 degrees, we take the temperature of the spiral at 1100 degrees, then according to the selection table B eff we choose the value of 4.3 W / cm 2, and according to the table for selecting the coefficient a - 0.2.

V add = V eff * a = 4.3 * 0.2 = 0.86 W / cm 2 = 0.86 * 10 ^ 4 W / m 2

The diameter is determined by the formula:

pt is the resistivity of the heater material at a given t, determined in accordance with GOST 12766.1, table 9 (given below).

For nichrome Х80Н20 - 1.025

p t = p 20 * p 1000 = 1.13 * 10 ^ 6 * 1.025 = 1.15 * 10 ^ 6 Ohm / mm

Then to connect to a three-phase network according to the "Star" scheme:

The length is calculated using the formula:

Let's check the values:

L = R / (p * k) = 29.1 / (0.82 * 1.033) = 34m

The values ​​differ due to high temperature spirals, the check does not take into account a number of factors. Therefore, we will take for the length of 1 spiral - 42m, then for three spirals we need 126 meters of 1.3 mm nichrome.

Conclusion

• environmental conditions;
• location of heating elements;
• coil temperature;
• the temperature to which the surface should heat up and other factors.

Even the above calculation, despite its complexity, cannot be called accurate enough. Because the calculation of heating elements is solid thermodynamics and a number of factors can be cited that affect its results, for example, the thermal insulation of the furnace, etc.

In practice, after the estimated calculations, the spirals are added or removed depending on the result obtained, or temperature sensors and devices are used to adjust it.

Materials (edit)

Electric heating elements are used in household and industrial appliances. The use of various heaters is known to everyone. it 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 vise 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 was 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 coil of the 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 indicates 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, accidentally 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 - 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 heat-resistant, 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 seawater desalination plants 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 boiling water in the usual 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 to boil 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 chemical agents, various devices are used to protect against scale. 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. These filters are available both in-line and overhead type.

Electronic water softeners

Recently, 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 lead to the water becoming 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 quite easy to determine. It all comes down to determining how much hard water flows 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 of 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 cross 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 capacity 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 just such a wire that is most often used to supply electricity to 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 quite simple 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.

This 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, measure the resistance between any of the terminals and the metal housing 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 quite suitable. 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 instantaneous water heater washing machine heating elements twisted into a spiral are used.

Some heating elements have protection elements. The simplest protection 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 again ready for operation.

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 is a metal container hidden in a “fur coat” made of a 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 a consumable item; it has to be changed periodically when servicing the boiler. But in some boilers, apparently of a 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. Here are the terminals for the supply 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, after which it 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 transmitted to environment(air under the containment) by convection and heat radiation. Naturally, such a heater in relation 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 heating elements, carbon heating elements, infrared heating elements, but that will be a topic for another article.