Furnace lining. Lining or why don’t furnaces crumble due to high temperatures? Lining of tube furnaces

Our company offers a large selection of materials for furnace lining. WE offer products only from high-quality derivatives at the most affordable prices. Our specialists will advise you on any questions that arise. Remember that when you use a stove lining made from quality materials, you extend the life of the entire product. Having paid once, you will not have to spend money on furnace repairs in the future. We use only the most effective materials with the best characteristics in the manufacture of linings.

You can buy materials for furnace lining at Thermopech at affordable prices!

Furnace lining and its purpose

Let's figure out what lining is. It turns out that this is a specialized finish, the main purpose of which is to reliably protect the surface from exposure to high temperatures. The lining is also designed to protect against various mechanical influences. High-quality lining increases the fire-resistant qualities of the product many times over.

The lining is a protective structure that protects the furnace chamber from various harmful influences. The composition of the lining is usually as follows:

Thermal insulation (provides protection against temperature surges)

Fire resistant protective products

Acid-protective products

Cladding material

Requirements for furnace lining

The design of the lining must certainly be made hermetically, and must also ensure a certain specified density in the furnace itself. The service life of a furnace without a lining would be much shorter. When choosing such a design, it is very important to purchase it from high-quality raw materials. Otherwise, the lining will not provide the necessary protection for the furnace chamber.

As a rule, if you need to heat treat metal in the working chamber of the stove, the body and lining must provide the necessary conditions for strength and heat resistance. There are several materials that meet these requirements: cast iron, brick, iron.

However, the temperatures in the cooking chamber are so high that the listed materials cannot withstand them. That is why a special layer of materials is installed on the entire working surface, which will provide the necessary protection.

For the production of linings, mainly 3 types of materials are actively used:

Materials for lining class A - silica base;

Materials for lining class B – clay base (hydrosilicate of alumina);

Materials for lining class C – combined materials that cannot be classified as classes A and B;

Only accurate calculation of layers and technical characteristics will ensure maximum effect from the lining. Preventive maintenance on the furnace must be carried out regularly to prevent failure of the product. Poor-quality lining can lead to a fire in the furnace and a fire in the production area.

For the construction of furnaces, only fire-resistant materials are used. But even they are not able to withstand high temperatures on their own for a long time, which when burning fuel reach hundreds of degrees. To protect the outer walls near the firebox from radiation, thermal or chemical influences, a lining is installed inside.

The first question that an amateur stove maker asks when he hears the word lining is what it is and what its purpose is. This is the shell that covers the inner surface of the combustion chamber. Thanks to its resistance to constant exposure to high temperatures, it allows the structure to last for a long time.

In the chemical-thermal system “medium-lining-material” the lining is a kind of mediator. When starting to lay the central part, it cannot be considered as a separate component.

You can arrange the lining in one of the following ways:

Installation of protective screens that reflect thermal radiation, but some of the heat will escape through the chimney to the street
Treat the internal surfaces of the combustion chamber with materials that have a low thermal conductivity coefficient

The main purpose of the lining is thermal protection, but not thermal insulation. These two concepts are completely different from each other. If the inner surface is insulated from heat, the heat will be constantly retained inside the combustion chamber and removed outside along with the smoke, significantly reducing the efficiency of the stove.

It is necessary to begin laying the lining after carrying out engineering calculations and deciding on the type of foam installation. In this case, the characteristics of all building materials used are taken into account.

Introductory video

Features of lining for various types of furnaces

For each type of furnace or heating boiler, the lining has certain features. Let's look at the most common options:

Lining of metal furnaces
Lining
Steam boiler lining

Lining of metal furnaces

If the combustion furnace is made of metal, the different thermal expansion of the materials must be taken into account. A small gap must be left between the lining and the steel, otherwise the constant fluctuation in the size of the iron walls will destroy the masonry.

The cavity left should not be empty. It is filled with one of the non-flammable materials, for example, basalt wool. Sometimes asbestos is used, but the stove must be located in a non-residential area.

When making the lining of metal furnaces, bricks are sometimes laid on both sides. The answer to the question about the feasibility of such a solution is justified as follows: the heating of the furnace improves and the level of safety increases. However, this approach is wrong:

Brick has low thermal conductivity
Brick has a heat capacity

Analyzing the above, we can come to the conclusion that lining metal furnaces on both sides will lead not to thermal protection, but to thermal insulation. Moreover, cooled air will not flow inside the furnace, which can lead to burnout of surfaces.

Read also about how to make a Swedish oven

Brick kiln

The opinion that lining a brick kiln is much simpler than a metal kiln is erroneous. Such work also has some features, without taking into account which you cannot count on the long service life of the structure.

It is impossible to lay the lining close to the bricks that are elements of the outer walls. The reason for such consequences is the pushing out of the outer layers by the expanding inner ones. Cracking can be aggravated by the fact that clay has low adhesion rates and expands more than brick.

If the lining is of poor quality, this will lead to unpleasant consequences:

Overheating of external surfaces will result in cracking of seams and the formation of cracks
The latter, in turn, will cause carbon monoxide leaks into the room, but not into the chimney

In addition to the high-quality lining made from fireclay bricks, special attention is paid to the quality of the seams and the clay used. It is necessary to glue 2-3 bricks together and burn them at high temperatures. This action will allow you to clearly determine how the assembled structure will behave. If the results are not satisfactory, you will have to experiment with the composition and repeat the tests.

Steam boiler

Sometimes the lining needs to be laid not for classic fireplaces and stoves, but for combustion steam boilers. In this case, choose one of its 3 varieties:

Heavy
Lightweight
Natrubnaya

Heavy lining and what it means is one of the exciting questions. They resort to it when boilers have weak shielding. Sometimes the temperature inside the furnace can exceed 1200 degrees, causing fires in external materials. It is necessary to lay out heavy lining from 2-3 layers of fireclay bricks. Such protection allows you to reduce the temperature outside to a safe 60-80 degrees.

If the brick is laid in just one layer, the lining is called lightweight. However, each of the belts must be supported by an intermediate beam or boiler frame. Then the surface of the heating installation is wrapped in non-flammable materials and covered with a metal sheet. Thanks to this combination, it is possible to reduce the outside temperature and increase the efficiency of the boiler room.

The simplest is the pipe lining. To arrange it, it is necessary to prepare a viscous non-flammable mass and apply it to the pipes in 2-4 layers. To make the structure better resist thermal expansion, a reinforcing mesh is laid.

Household lining: answers to common questions

More often, the lining is a laying of fireclay bricks, which will be in direct contact with the burning surface. However, a novice builder, faced with such a task for the first time, will ask himself a number of questions:

What is the best way to lay bricks?
How thick should the furnace lining be?
What should be the gap between the bricks and is it necessary at all?

When constructing classic stoves and fireplaces, it is enough to lay only one layer of lining. In this case, products measuring 25x15x6.5 cm are used - such a brick is considered standard.

Laying is usually done vertically. Nobody forbids making it horizontal, but most of the internal usable area will be lost.

Unlike classic brick, the thermal expansion of fireclay products is minimal. Therefore, there is no need to leave a large gap between them. A clay solution is used as a connecting link and a metal spike is installed. The use of cement-sand mortar is prohibited.

If the lining comes into contact with metal products, for example, a grate, a gap should be provided between them. Metal under the influence of heat will constantly change in size and can be pushed out, bend over time and even burst.

Materials used

Having learned what a lining is, it is necessary to take a closer look at the materials for its construction. The materials used for lining furnaces and boilers can be divided into three main classes:

Specific examples

You can protect the inner surface of the combustion chamber using a variety of methods:

Fireclay brick masonry
Application of non-flammable mixtures
Installation of protective screens

When choosing brickwork, it is not recommended to use classic brick - it quickly collapses and requires replacement. Before the advent of fireclay materials, products of the M-300 brand were used. They are relatively reliable and durable, but still inferior in efficiency.

How to prepare the solution

It was previously mentioned that it is necessary to experiment with the mortar for laying the lining. To minimize the number of tests, it is recommended to prepare several options at once and fire them under the same conditions. Don't forget to label.

A solution prepared on the basis of brick dust and fireclay clay is considered widely popular. However, if only wood is burned in the stove, you can use a classic clay mixture with a small sand content.

Lining for stucco stoves

Classic stone or metal heating structures in baths are gradually being replaced by more modern and beautiful stucco stoves. They are built using the adobe method. For them, the lining can be made of fireclay bricks, but craftsmen recommend resorting to special mixtures:

Mullite
Fireclay
Corundum

To use these compounds, it is enough to dilute them in warm water on site. The solution is applied to fire-resistant surfaces. After drying, the monolith will not crack.

About repairs

Nothing lasts forever, including the furnace lining. Before the start of the next heating season, it is recommended to inspect surfaces and restore damaged areas.

It is recommended to use a mixture based on aluminous cement and fireclay powder as patches. First, the mass is applied to the damaged areas, and then dried with an open flame. The last stage should begin after a week. If you rush, the renewed surface can quickly collapse again.

A little about industrial lining

The main difference between industrial and household linings is the structure of the fireclay bricks used. In the first case, it will be exposed to high temperatures, and therefore requires additional pressing during the production process. They are much more expensive, but can withstand thermal shock.

Let's consider the most optimal ratios of such characteristics as material-density-temperature:

In industry, linings perform much more practical functions than in everyday life:

Reduces the temperature of external surfaces
Protects furnace walls from thermal destruction
Reduces heat loss
Does not allow furnace materials to come into contact with chemical compounds or molten metals

Let's sum it up

Having become familiar with the various features of arranging linings for various types ovens, you can safely get to work. However, neglect of the considered rules can significantly affect the reliability and durability of heating structures.

Furnace lining - description, basic technical solutions, methods of arranging a protective layer and a description of the most common materials that can effectively protect the product from the effects of negative factors and the influence of extreme temperatures.

The interior finishing of the furnace firebox is exposed to high temperatures generated during combustion fuel materials. Particularly extreme conditions exist when using high-calorie fuel. And, despite the fact that the furnace is usually made of refractory materials, additional treatment of the internal surfaces - furnace lining - will be very useful for extending its service life.

Lining is a specialized procedure for arranging the internal surfaces of the furnace, ensuring high degree protection from thermal, mechanical and chemical influences to which the product is constantly exposed throughout its entire service life. In general, a lining is a protective and insulating layer of heat-resistant materials that serves to protect parts from the extreme temperatures created by the combustion process. It also prevents excessive heat loss from the external surfaces of the oven, making the process more energy efficient.

Under conditions of regular use, the materials from which the stove is made are exposed to extremely high temperatures, varying depending on the type of fuel used. The internal surfaces of the firebox are subject to cracking, and, in the case of metal products, excessive permanent heating and constant “expansion-compression” cycles lead to its gradual destruction. High temperatures typically occur when coal is used, although pallets often cause excessive heat.
Extreme temperatures are not the only factor that negatively affects the surfaces of metal or brick stoves. The lining is also used for:

As thermal insulation, protecting against temperature fluctuations.
As protection against aggressive environments. (acid fuel reagents).
Protection against mechanical damage to the firebox.
As a facing material.

In what cases is it used?

It is worth noting that there is no need for lining if the stove is needed for intermittent heating of a summer house or cooking over an open fire (barbecue, kebabs, etc.). In these cases, there is no need for additional protection, since the destruction that occurs during use is minimal.
The lining is recommended for products used for long-term and constant heating of premises or cooking in various kinds of cafes and restaurants, where the required temperatures are continuously maintained in the firebox - for example, “Russian” stoves and the like. Lining is especially necessary when using high-calorie fuel, during combustion, which releases a large amount of energy, as a result of which an extremely high temperature regime is observed in the firebox.

Basic lining methods

Important! High-quality lining can be carried out after engineering surveys and the correct selection of insulating material.

Work on lining the inner chamber of the furnace made of metal, natural stone or brick is carried out taking into account the effect that is desired in the end. For example, heat insulating screens affect the percentage of heating of gases, removing most of the thermal energy through the exit of air flows and combustion products through the chimney. Fire-resistant materials with low thermal conductivity effectively protect the furnace materials from thermal effects, taking most of it on themselves, which eliminates the interaction of the internal walls of the furnace with an open fire. One of the main methods of lining is laying an additional layer of special brick, which also helps to effectively protect the firebox from excessive heat.

Attention! The installation of special heat-protective screens significantly reduces the heat transfer of the stove, which is not recommended if the product is used to heat a room. The basis of the lining is the use of “fireclay” materials, specially prepared mixtures in which, through prolonged firing at high temperatures, the plastic properties of the substances are destroyed. In addition, effective lining materials are:
A natural sandstone or quartz stone that is highly resistant to extreme temperatures.
Special “fireclay” bricks – manufactured using a special technology, with the addition of various admixtures to improve the final properties of the material, for example, baked clay. The use of such bricks is a fairly popular lining method, since they can withstand very high temperatures and are relatively inexpensive.
Roll protection: basalt-containing materials, vermiculite boards, kaolin paper, cotton wool or cardboard.
Special solutions: fireclay refractory concrete, corundum, silicate-mullite liquid glass mixtures of various compositions.

Fire brick
Materials for lining are selected individually, based on the individual characteristics of the products and the possibilities of arranging protection from one or another material. In industrial furnaces, fireclay bricks or natural stone are often used. But, in a private home, where the size of the firebox does not allow laying an additional layer of these materials, alternative methods of thermal protection are more suitable.
Comparative table of properties of lining materials
Attention! To carry out effective lining yourself, you need to have the appropriate skills or contact specialists. Lining using bricks

Lining using bricks
Lining made of fireclay bricks is one of the effective ways to protect the firebox, characterized by low cost. To arrange such protection with your own hands, you should adhere to certain rules:
Brick laying is carried out using the “edge-on-edge” method along the entire length of the inner walls of the furnace, without moving the elements relative to each other.
The joints are filled using special “fireclay” and clay-containing solutions.
If the furnace is made of brick, the lining layer is joined to the main vertical seam, without bandaging.
If the firebox is metal, a prerequisite is masonry with a gap between the metal and the brick. This is a critical aspect for maintaining the structural integrity of the masonry, which, due to the expansion-contraction cycles of the metal when heated, can be subject to constant mechanical stress.

Important! Brick lining is susceptible to cracking due to inhomogeneous heating of the outer and inner layers. For a higher-quality finish, you should be especially careful when choosing a solution for joints. To check its quality, several bricks should be held together and fired in a firebox, and based on the results obtained, decide on the use of one or another solution. Also, it is not recommended to use fireclay and ordinary refractory bricks at the same time. This can lead to damage to the integrity of the masonry due to uneven expansion of materials when heated.
Lining of metal furnaces

Lining of metal furnaces
The main nuance when lining metal furnaces is the expansion of metal when heated, which is much higher than that of stone or brick. This property should be taken into account when arranging the protective layer. A prerequisite is the need to leave a gap that will cover the metal backlash and protect the lining protection from mechanical damage and cracking.
Advice! To seal gaps, you can use sheets or strands of basalt wool, and asbestos is also suitable in non-residential premises.
It is worth noting that the most common mistake when installing metal stoves is laying bricks, both in the firebox and outside the product. Brick is a fairly heat-intensive material, but at the same time, it has low thermal conductivity. As a result, most of the heat is removed along with combustion products through the chimney.
Attention! A dangerous consequence of laying a metal product externally with bricks can also be carbon monoxide entering the room due to the burning of the walls of the firebox. Lining with roll materials

Lining with roll materials
The optimal way to lining small furnaces in order to save the internal space of the firebox may be to arrange protection from rolled materials, plates or mats. The nominal thickness of most of these materials does not exceed 1 centimeter, and often even less: kaolin cardboard, for example, is made with a maximum thickness of 7 mm. This method will effectively preserve the internal space for storing the required amount of fuel and ensure the unhindered passage of combustion products through the chimney.
Important! It should be borne in mind that the amount of materials is calculated taking into account their possible linear expansion under the influence of high temperatures.
Individual plates of material are attached using reinforcing elements, metal pins inserted into pre-prepared grooves. The order of arranging the firebox with rolled materials should also be taken into account: the plates are placed first on the bottom, then on the side walls and finally on the top surface of the firebox.
Important! It should be remembered that most modern metal stoves have a factory lining layer, for example, finished with vermiculite. Additional arrangement can be made with basalt cardboard.
Coating with solutions

Coating with solutions
Recently, a fairly popular lining method has become the treatment of the firebox surface with special solutions based on fireclay, mullite or corundum mixtures. This protection is characterized by an ultra-thin coating that is not difficult to apply. To prepare the solution, the mixture is diluted with the required amount of water and brought to the desired consistency. After application, the lining layer must be subjected to heat treatment or, in other words, firing. You can burn the solution with a blowtorch until a hardened crust forms.
Service life of the protective layer

Service life of the protective layer
The lining is not an eternal protection of the furnace firebox and, depending on the intensity of work, temperature conditions and the selected material, the protective layer wears out over time, as a result of which the initial tightness is lost.
Important! A high-quality lining, equipped by specialists, where the materials will be applied correctly, in an even layer, without damage, gaps or cracks, will last the longest.
You can extend the service life of the lining layer yourself; to do this, you should check the inner surface of the firebox for structural integrity, possible mechanical damage and, if necessary, repair them in a timely manner.

To find out what a lining is and why it is needed, you need to understand that in any furnace, when fuel is burned, a high temperature is generated. Exposure to high temperatures leads to changes in the structure and properties of the material. The materials from which the stove is made, although most are fireproof, also need protection.

Protecting the internal walls of the firebox can be done in several ways:

Firstly, you can use protective screens that can absorb most of the radiant fluxes. The screens installed inside the firebox affect the heating of the flue gases, so most of the heat will be removed through the chimney.
Secondly, the inside of the firebox can be lined with fire-resistant, low-thermal conductivity material.

Thermal protection and thermal insulation - there is a difference

There is a huge difference between thermal protection and thermal insulation principles. At thermal insulation the heat generated in the work area is not removed anywhere. This is what thermal insulation is for.

Unlike her, thermal protection designed to protect a certain area from the thermal radiation of the working area. From all that has been said, it becomes clear that thermal protection is not at all intended to preserve heat.

Brick for lining: chamotte - the ideal solution

If you are installing an external screen protection in heating boilers, the use of this method will be very wasteful, since most of the heat will be spent completely in vain. But this method works great for installation.

Previously, under the USSR, most private owners were familiar with simple building bricks GOST 530-95. This brick is intended for the construction of industrial and residential buildings and in no way for use in furnaces and heating boilers. By the way, many still use it to build furnaces and boilers.

Many craftsmen at that time managed to get a brick of this GOST, but of a higher grade M – 300, in the popular expression “oven”. This brick lined the furnace part of the furnaces to protect against high temperatures.

Later, fireclay bricks became available for mass use. GOST 390 – 96.

Our information: fireclay materials are made from clay mixed with fired clay powder and then fired at high temperatures.

The use of fireclay is the lining

Due to the fact that all fractions have a homogeneous chemical composition, all fireclay products have a high degree of heat resistance and do not crack when exposed to high temperatures.

Lining the combustion part with fireclay material to protect the heating unit from the effects of high temperature in the combustion part is called lining.

Manufacturing lining and material properties

In production, in metallurgy, the lining of an induction furnace is made using special, pressed fireclay bricks. They are very wear-resistant, do not shrink and withstand thermal shocks well. The use of fireclay bricks made it possible to achieve great cost savings and greatly reduce the cost of the metal production process.

The table shows the properties of some refractory materials

Household problems - lining as a solution

In everyday life, people usually have to solve simpler problems. At the very last stage, the burning of the coals, the bottom of the metal stove overheats, sometimes red hot. A man decides to cover the overheating wall from the inside with his own hands.

But even in this case, you have to decide to what level to lining, whether to leave a gap and how to secure the bricks?

Specifically in this situation, the issue can be resolved as follows:

If the width of the firebox allows, fireclay bricks of standard size are laid around its perimeter - 250Х150Х65 mm. In this case, the brick is laid flat against the wall. Can be laid vertically, with height 250 mm or horizontally on 120 mm.
In order to securely fix the bricks together, a simple method is used. Using a grinder, a cut is made in part of the joint, and the bricks are held together using an inserted metal spike of a suitable size.

Lining device: temperature and metal

Particular attention should be paid to the fact that the thermal linear expansion of metal is much greater than that of brick. Therefore, all metal parts exposed to heat must have free space for expansion.

Our information: the absence of a gap between the metal wall of the furnace and the lining leads to destruction of the lining.

When installing this lining, the gap is sealed with basalt or kaolin wool, twisted into bundles, cords or in the form of a compressed sheet. Asbestos can be used for this purpose.

Wrong defense. Consequences

This picture occurs very often. it is covered with bricks using clay mortar, and even without a gap. This is motivated by ensuring safety and improving the heating of the furnace.

This is completely misleading!

Firstly, brick is very heat-intensive.
Secondly, at the brick low thermal conductivity. What happens during operation of such a furnace?

The brick simply plays the role of thermal protection. The main heat generated by the stove simply flies out into the chimney due to excess in the firebox.
In addition, the lack of a gap prevents the penetration of cooling air to the metal walls of the furnace, which clearly leads to their burnout, and metal expansion will lead to cracking of the brick protection.

As a result, carbon monoxide will begin to enter the room.

Not always brick: lining mixtures

Recently, “modeled” sauna stoves of various shapes have become very popular. Basically, such stoves are installed in bathhouses made in a stylized “fairytale style”. Such stoves are made using an ancient method called adobe.

After the clay has dried naturally, the oven is fired from the inside using firewood. The outer part is burned using blowtorches until a crust forms. It is very convenient to line such furnaces using special compounds.

These can be dry mixtures:

Fireclay.
Mullite.
Corundum.

These compositions can be perfectly used for the on-site production of various refractory products, including linings. A solid, monolithic lining made from these materials far exceeds in quality seam furnace linings made with fireclay bricks.

On-the-job lining

Currently, there are a number of technologies in which lining or restoration of the lining is carried out without stopping the furnace. The compositions are applied by “spraying” directly onto a hot surface or by gunning.

Our information: shotcrete is a method in which concrete or other mixture is applied layer by layer to a structure using compressed air and a special shotcrete installation.

This method is mainly used in production, which allows lining work to be carried out, if necessary, without stopping the production process.

Brick kiln: problems with overheating

We have already considered the need for lining the combustion part of metal stoves, but what about brick stoves? What happens during “overtoppings”?

There is a concept - thermal cracking. This phenomenon applies both to the material of the stove itself and to its seams.

The inner walls of the furnaces, heating up from the hot gases produced by the burned fuel, expand and begin to expand the outer “cold” walls.
The outer walls of the furnace act as a “bandage”, maintaining the dimensions of the furnace and maintaining its integrity.
The more the oven heats up, the thinner the “bandage” becomes, which is already under heavy loads, and therefore becomes even less durable.
At "overheating" of furnaces the outer walls, under internal pressure, begin to “move apart” to the sides.
Cracks form and even individual bricks break.

Local solution: main disadvantages

It is worth noting that in a brick kiln clay is used to connect the bricks together. Therefore, either the seams themselves or the contact with the bricks rupture due to low adhesion. In any case, a seam in which a mortar with a high sand content was used will expand significantly more than a brick.

This situation is further complicated by the fact that the thermal coefficients of bricks and joints generally differ greatly from each other. This is understandable, because the bulk of the furnace solution is prepared directly on site. Stove makers primarily strive for the plasticity of the solution to the touch and the absence of cracking when drying and compressing.

Our reference: the correct assessment of the mortar for compliance with the bricks used can be checked as follows. Two bricks are held together with a solution; after drying, the test block is calcined in an oven, followed by an assessment for adhesion, cracking and peeling.

Methods and options for preparing the right solutions

Of course, this method requires a certain amount of time, because it is not a fact that you will get the required solution the first time.

Therefore, if you still decide to use this testing method, prepare several “blocks” for testing with different solutions. Then all you have to do is choose the most optimal one. The main thing is not to confuse and forget which solution is which.

Another good option is to use the most common clays for the solution, mixed with powder made from ground bricks.

If you use only firewood when burning, the use of special fireclay clays with a high content of kaolinite is not entirely advisable, since the temperature of the firewood is not sufficient for kaolin to sinter into mullite. But if you use coal, then this option will be completely justified.

Lining: no more questions

Having considered this issue, we can conclude that the lining of stoves used for domestic purposes mainly represents protection of the stove structure from the destructive effects of high temperatures. This is a kind of screen that prevents the heat flow of an open fire from directly affecting the walls of the stove.

Unlike “household” linings, industrial linings, for example, the lining of an induction furnace, are intended to significantly reduce heat losses, protect the furnace casing from direct exposure to high temperatures and protect against direct contact with molten metal.

Having considered this issue, we hope you will draw the right conclusion, which will help you when building stoves with your own hands.

The term lining comes from the German futter - lining. In technology and construction, lining means a finish that carries certain protective and, possibly, technological functions. There are no requirements for the appearance of the lining. For example, sections of newly laid pipelines, already covered with insulation, are additionally lined from the outside before being pulled under the existing ones. The furnace is lined from the inside, i.e. from the side of the greatest destructive influence.

According to the technology of work, the internal refractory lining of furnaces for various purposes is divided into:

Overlay (overlay) - lining with piece or sheet fire-resistant materials.
Pressed - by manually coating the masonry with viscous heat-resistant compounds.
Shotcrete - spraying under fairly high pressure a fluid heat-resistant mixture from a special apparatus; As a rule, the lined surfaces are shotcrete with heat-resistant concrete.

Shotcrete is used to line furnaces, casting ladles, pipelines, chimneys, etc. a volume large enough so that a worker-operator with equipment can fit inside. Shotcrete is applicable for mobile equipment (casting ladles, thermoses for transporting molten metal, etc.), but requires solid professional training and special equipment, which is not applicable outside production conditions, so it is not discussed further in this article. Small furnaces are lined almost exclusively with overlay; Sometimes a rammed lining is used for repairs.

Note: The external lining of heating devices is called coating, because very different from internal fire resistant. For example, steam boilers are often coated on the outside to reduce heat loss and the risk of superheated steam escaping during an accident.

Purpose of the furnace lining

There are many methods for producing specific types of furnace lining only. There are no less types of lining materials, and new ones are being developed all the time. However, all this diversity can be divided generally into 4 classes according to the purpose of the lining, the composition of the materials and the characteristics of the work:

Lining of large industrial furnaces for various purposes;
Lining of fireboxes and other hot parts of household brick heating and cooking stoves;
Lining of small metal furnaces, domestic and technological, applicable outside production conditions;
Lining of induction furnaces, both industrial and small.

Large industrial

In all large industrial furnaces, the purpose of the lining is to reduce heat loss and the effect of high temperatures on the furnace body (casing). In firing kilns, the lining also serves as a heat accumulator, providing an optimal firing mode (smooth increase in temperature), as well as an infrared (IR) emitter that uniformly heats the firing chamber.

The task of lining metal smelting furnaces is much more complex. Here, too, it should emit IR well in order to heat the slag more strongly. High-quality metal is smelted only when the layer of molten slag is heated significantly higher than the liquid metal. But at the same time, the lining of industrial furnaces for smelting metal should not be wetted by molten slag and the metal itself, be chemically inert towards them and not release harmful impurities from the melt. Additionally, be strong enough to withstand your own enormous weight, because... The economic efficiency of metal smelting furnaces increases with increasing size. Therefore, for example, at least 7 types of lining materials are used for lining various parts of blast furnaces and steel-smelting furnaces (see figure).

Lining an industrial furnace is a complex and responsible job. Even with 3-4 years of vocational education in this specialty, you need to work in a brigade for several more years as a student or (informally) journeyman, i.e. at lower grades before being allowed to work independently. Therefore, if you are interested in what and how industrial metal smelting furnaces are lined, you can start with Wikipedia ( wikipedia.org/wiki/Lining), there it is stated quite intelligibly, or Metalopedia. As for small kilns, they are lined like metal technological ones (see below), and we will go further - to the lining, which you can make yourself.

Note: in ultra-small metal smelting furnaces, e.g. jewelry, the lining can make up the entire structure (body) of the furnace or most of it, see video:

Video: jewelry furnace made of a slab, which is the body and lining

Brick lining

The lining of the firebox of a brick heating and cooking furnace serves 4 purposes. The first is common: to reduce thermal stress on a brick structure and thereby increase its service life and reliability. The second is to increase the thermal efficiency (efficiency) of the furnace. Fireclay, the best material for lining a brick kiln, has a lower heat capacity but greater thermal conductivity than working kiln brick. In the furnace, the fireclay lining will serve as an intermediate thermal buffer, quickly absorbing the first heat of the fuel and gradually releasing it to the furnace body; this will reduce fuel consumption and increase the heat transfer time of the heated stove.

The third goal is to make the stove suitable for burning not only wood, but also coal. The heat from the coal at the beginning of the fire is intense for a short time; If you constantly heat a wood-burning stove, it will soon begin to crack. The fireclay lining “spreads” the thermal shock from burning coal over time so much that the brick oven can withstand it for many years.

And finally, the last goal: repairing the dilapidated firebox. Both piece and sheet lining materials are applicable here, as well as plastic compositions for printed lining, see below.

Brick for lining

The lining of the heating and cooking furnace can be made with either ceramic refractory bricks (high-alumina high-burnt, pos. 1 in the figure) or fireclay bricks, pos. 2. Wedge-shaped bricks are used for laying the firebox floor to create a slope on it towards the grate.

Ceramic refractory bricks are also more resistant to external influences, like red worker. Its heat capacity, thermal conductivity and modulus of the coefficient of thermal expansion (TCR) are approximately the same. So lining with ceramic bricks will not save fuel, but it will make the stove more resistant to long breaks in the firebox. Therefore, you need to line a country stove, a decorative fireplace, a stove with ceramic bricks. country house and so on. It is impossible to force the firebox of such a furnace (to overheat it) - the heat resistance limit of ceramic refractory brick is 1200-1300 degrees.

Fireclay bricks grades ША and ШБ (GOST 390-96) maintain temperatures up to 1500-1700 degrees. However, in a cold oven, the more porous fireclay quickly becomes damp and begins to crumble. The density of fireclay bricks is greater than that of ceramic bricks (1.8-2 g/cubic cm versus 1.6 g/cubic cm), and the weight of a stove with a fireclay firebox will be greater than that of a solid brick one. But fireclay increases the efficiency of the furnace (see above), so it is advisable to make fireclay lining in a furnace that is regularly fired during the cold season. To repair fireboxes and convert them to firewood and coal, it is better to use the so-called. fireclay plinth (brick of reduced height, item 3 in the figure). It is also convenient to use plywood to line metal furnaces together with sheet materials, see below.

Note: Ceramic refractory and fireclay bricks in the same lining are incompatible. Either one or the other.

Masonry mortars

The ceramic brick lining is placed on a heat-resistant furnace mortar with a heat resistance limit of at least 1300 degrees. It is not recommended to use homemade solutions, because... the mechanical connectivity of the lining masonry is extremely low, see below. For fireclay masonry, the so-called. fireclay mortar (not marl!) - a ready-made dry heat-resistant masonry mortar to which water is added; You can also lay ceramic lining bricks on it.

You can prepare mortar for lining masonry yourself, but this is a labor-intensive task. Composition by volume of the dry mixture – 92-95% fireclay chips, fraction 0.6-1.5 mm (can be from waste fireclay bricks); the rest is heat-resistant clay. Kaolin is best; For a wood-coal stove, gray will also work. A colored tint is unacceptable: the impurities that give it (especially iron and nitrogen) make the lining extremely short-lived.

The content of the binder (clay), depending on its quality, is determined experimentally. In general, the thinner the solution (with less clay content), the longer the lining will last. But the mechanical connectivity (adhesion) of the mortar to the brick is no less important. Therefore, prepare several portions of a dry solution with a clay content of 0.5-1%. A little bit of water is added, just so that the finished solution is “smeared”, so flaws in the masonry are irreparable.

Test compounds are used to connect pairs of bricks end to end, i.e. short narrow sides. Bricks are “blinded” on a sheet of iron, because they need to be connected “in one click”, without shifting. Marks are knocked out on the bricks in advance, because The samples will then be fired. Seam thickness – 0.5-1.5 mm; the thinner the better.

Next, the samples are dried for 3-4 days in the shade at a temperature not lower than +17 degrees. Then, carefully so as not to fall apart, they are transferred to the running oven (a properly functioning oven); it is best directly with a sheet of iron. You cannot burn it in a fire - the required temperature will not be reached. Heating with a blowtorch, gas or liquid burner will also not result in uniform heating.

In the oven, fuel (preferably coal) is carefully placed on the samples, and the oven is heated properly. When it has completely cooled down, the samples are taken out and with both hands, so that some of the bricks do not end up floating, they are turned vertically, grabbed by the top brick and shaken slightly. From the samples that have not disintegrated, they select the thinnest solution (with the least clay content) - this is what will be used.

Sheet materials

Sheet refractory materials are most suitable for lining metal furnaces. But they can also find application in the lining of a brick kiln. Especially in a repaired oven without disassembling it. It’s hard to get both hands into the firebox of the house stove; It is sometimes simply impossible to operate in it with bricks. In addition, brick repair lining seriously reduces the volume of the firebox; therefore, the thermal power of the furnace. But the entire furnace structure (its dimensions, weight, configuration, channels, caps, etc.) is designed for a firebox of a certain power. If its power drops below a given limit, the stove will lose efficiency: it will consume more fuel and heat worse.

The best thermal insulation is vermiculite (a type of mica). The insulating properties of vermiculite are phenomenal: a 30-mm slab of it “does not penetrate” and an autogen flame - on the other hand, you can hold it with your hand. Vermiculite boards for lining are produced incl. decorative (an expensive pleasure, I must say): with such a special glue (see below) the metal firebox is glued from the inside (item 1 in the figure) and it turns out “like a brick” both in appearance and functionality.

There are also cheaper non-decorative vermiculite boards on sale (item 2); They are convenient for lining metal technological furnaces, because The useful volume of the working space is practically not reduced, and the installation of vermiculite slabs is quite possible to do with your own hands, see video:

Video: installation of vermiculite lining slabs

However, vermiculite is unsuitable for repairing and lining brick kilns - it conducts heat too poorly. The firebox of a brick stove lined with vermiculite will not transfer it to the body of the stove: it will either overheat and deteriorate prematurely, or it will waste money for fuel down the chimney. But inexpensive basalt slabs (item 3) are a poor substitute for vermiculite: their fire resistance is low. In brick linings, non-piercing basalt mats (item 4) have proven themselves to be good as thermal damper gaskets.

But the best sheet lining material for a brick kiln is kaolin cardboard, pos. 5. It is heat-resistant, chemically neutral, its TCR is average between those of fireclay and red brick. Disadvantage: it is not very suitable for metal furnaces, because does not adhere well to metal using oven adhesives.

Furnace lining using sheet materials is often done dry - a box made of basalt or kaolin cardboard is inserted into the opening under the firebox (on the left in the figure, and a fireclay firebox is laid out in it (in the center). It is useful to make the box with wings overlapping with 100% overlap (on the right ).

After the stove is erected to the top of the firebox, the wings of the box are wrapped, supported with a flat circle (a wooden shield on supports) and glued together with stove glue. The circle is removed when the furnace is completely built. For more information on how furnace lining is made using sheet lining materials, see e.g. plot:

Video: lining a two-bell furnace

Glue instead of mortar

The lining of large furnaces is laid on mortars or, sometimes dry, with shaped bricks with hooks - other options are too expensive for large volumes of work. But it is better to line household or small technological non-industrial stoves with stove adhesives - they are more mobile (fluid), which allows you to correct flaws in the masonry along the way, and allow you to get a more durable, extremely thin seam.

There are many types of oven adhesives on sale. For lining non-iron and steel-smelting furnaces, you need to choose aluminosilicate-based adhesive with a heat resistance limit of at least 1300 degrees. In practice, the German ScannMix and the Russian K-170 have proven themselves well: they are chemically neutral up to the limit of heat resistance; do not release any iron or nitrogen. ScannMix adheres vermiculite lining more firmly to steel and cast iron; K-170 is no worse for other cases.

Work procedure

The brick furnace lining is usually developed in the furnace design procedure. The standard lining is built integrally with the stove, see, for example, a video about the construction of a compact 2-bell stove (3.5x3.5 bricks) with a hob:

Video: detailed masonry and lining of a brick oven

It is not recommended to develop the furnace firebox lining yourself without careful calculations: due to the difference in TKR materials, the structure and furnace lining can mutually destroy each other. If there is such a need, you need to adhere to certain rules. The main thing is: the lining, the structure of the furnace and/or metal embedded parts should not touch each other anywhere, at any point. Everywhere you need a deformation gap of 6-12 mm; The easiest way is to provide it with heat-resistant cardboard gaskets, see above.

The lined firebox itself can be of any complex design, pos. 1 in Fig. But in the structure of the furnace it should be a floating core that has no mechanical connections with it, pos. 2. If there are several fireboxes in the stove (for example, a fireplace stove), they are lined separately, pos. 3. Fireclay jumpers between individual fireboxes can become focal points (places of concentration) of large thermal stresses, from which the entire stove will malfunction.

For the same reason, it is completely unacceptable to release “whiskers”, etc. from the lining. hooks, pos. 4, and also make its opposite walls of different thicknesses, pos. 5. A firebox with a lining of uneven thickness is acceptable if its walls of the same thickness are located symmetrically relative to the thermal focus of the firebox (which in fireboxes of a conventional configuration is located on their longitudinal axis of symmetry).

Next, piece materials (bricks) must be placed in beds (wide sides) inside the firebox and against its walls. It is better to mount the bricks upright (vertically): the fewer horizontal seams there are in relation to the height of the firebox, the more reliable and longer the lining will last. And the last rule is to make the masonry seams of the lining as thin as possible. In the lining of industrial furnaces, the thickness of horizontal seams is allowed no more than 1.5 mm; vertical up to 0.5 mm. It is highly advisable to adhere to these same standards in house stoves; It is possible for a home craftsman to fit them only using oven glue.

Repair lining

The easiest way to line a not very dilapidated firebox (small nicks, potholes, thin cracks that are not through) in order to repair it is by printing using the adhesives indicated above. The immutable rule here is that the glue is the same everywhere. Covering defects partially with one glue and partially with another is unacceptable! After correcting the defects, the inside of the firebox is coated with a layer of the same glue 1-2 mm thick, carefully leveling it with a smooth spatula. If the firebox is more dilapidated, then it will have to be lined inside with fireclay plinth (see figure on the right) in order to reduce thermal stress in the damaged structure of the furnace. The firebox roof can be corrected in this way, again, only with heat-resistant glue (see above), but it is often the underside of the hob, which simplifies the work.

Metal lining

The lining of a metal furnace is done only from the inside. Additional external cladding or coating of the stove will speed up its warming up, but, contrary to popular belief, it will have almost no effect on the thermal efficiency of the stove: the rule of insulating rooms “from the cold side” arose because the dew point cannot be allowed into the room; for a lined furnace it is not relevant. But the outer lining/coating of the furnace will greatly increase the thermal stresses in the metal of its body and its sensitivity to chemical influences.

It is best to line a metal stove with vermiculite using ScannMix glue or its analogues: the packaging or specification of the glue should indicate that it glues both metal and/or ceramics to metal (if it glues ceramics to metal, it will also glue slab materials). For good thermal insulation of a muffle furnace, a vermiculite plate 20-30 mm thick is sufficient. If a metal stove is lined with brick, then it must be installed according to the same rules as in the firebox of a brick stove. For example, how a gas forge furnace is lined with fireclay, see video:

Video: lining a forging gas furnace

A special case is a metal stove with stiffening ribs inside. Line it with bricks as shown in Fig. on the right there is little use: the metal ridges protruding inward are excellent thermal bridges and favorite locations for thermal focal points. Such a furnace must first be covered inside with a basalt mat with a thickness of 6 mm using any glue suitable for this, and only then assemble the piece lining. And the bricks need to be placed upright, hemmed if necessary. The above condition - as few horizontal seams as possible - arose from the large wearing effect on the lining of the vertical temperature gradient in the firebox (temperature difference per unit height), and in metal stoves it is much greater than in brick stoves.

Induction furnace lining

The lining of an induction metal-smelting furnace is no simpler than that of a blast furnace. In fact, manufacturers of such furnaces develop and patent their own lining compositions and methods, which alone guarantee the operability of the furnace. “Alternative” suppliers sometimes inform buyers (but without guarantee) how to line their furnaces, see e.g. video:

Video: induction furnace lining

The main reason for this is that the slag in the induction furnace is heated by the metal, i.e. turns out to be colder than him. Thermoconvection occurs in the melt, the metal becomes slagged and its output turns out to be defective. In a blast furnace, for comparison, a steam shaft directs the heat flow downward, and the first melt on its way is slag. Try heating the blank so that it is hotter than the flame! In an induction melting furnace, this is exactly what needs to be achieved, and the crucible lining plays an important role here.

An additional condition is that the worn lining must be ejectable, because it is made shotcrete (in small furnaces it is solidly stuffed), and dismantling an unusable crucible is hard, harmful and dangerous work. The molten metal in an induction furnace is heated throughout the entire mass, it is very fluid and will seep through the piece lining, and it is poured by tilting the entire furnace. As a result, the lining of an induction furnace turns into a complex multi-component structure (on the left in the figure), parts of which are made of different compositions. Table in the center in Fig. contains information only about a small fraction of them (MP-2 is a brand of one of the mullite masses); (more, and even then with reservations, can be found here: studfiles.net/preview/1864729/page:4/).

But if you line the crucible of an ultra-small furnace with several kg or hundreds of grams of metal in this way, you will get a situation similar to the one on the right in the figure: the crucible gets very hot, releasing impurities into the melt and spoiling the metal. Here it turns out to be a matter of the frequency of the supply current.

Metal in an induction furnace, as is known, is heated due to its electrical conductivity by eddy currents (Foucault currents). It is often written that metal in such a furnace is heated by high frequency currents (HFC). But the greater the mass of the metal, the lower the optimal frequency of the supply current turns out to be - the lower the loss of its power. For tons and tens of tons of metal, “HDTV” is needed with frequencies of tens to hundreds of Hz; Inductors of large furnaces of this type are often powered directly from a 50 Hz industrial network.

But for small masses of metal, high-frequency frequencies are needed already “real” ones - from tens of kHz to several MHz. In this case, HF energy losses become of great importance. electromagnetic field(EMF) in the non-conductive material of the crucible. They are characterized by the so-called. dielectric loss tangent tgδ. For frequencies up to 200-300 kHz (up to 5 kg of metal), its value should be tgδ<0,02, а для частот до 1-5 МГЦ (граммы-сотни грамм металла) нужен tgδ<0,006, причем с повышением температуры диэлектрика его tgδ довольно быстро растет. Фактически из материалов, пригодных для футеровки печи, в заданные значения tgδ при ее рабочих температурах укладываются только каолин, чисто белый (без железа и азота) кварцевый песок, дробленый плавленый кварц (кварцевая крошка марки А) и вермикулит. Последний отпадает: непрочен и не позволяет получить прочные набивочные и намазочные составы. Итого остается всего одно пригодное сочетание наполнителя со связующим; и то, как говорится, слава богу.

The lining of the crucible of a small and ultra-small induction furnace is made from them in generally the same way as fireclay mortar. Fraction of quartz chips 0.3-0.6 mm; kaolin 5-8% by volume. It is necessary to make 4 test crucibles with 1% volume of binder, burn each one empty 3-4 times in the inductor, and carry out test melts in the remaining ones intact. Is it dangerous! The oven must have a remote switch (can be wired). During the melting, leave the room and watch its progress from afar through the protective glass!

Finally

Refractory lining of a furnace for any purpose is a very difficult task. The maintainability of furnace linings is low, and lining a furnace must be undertaken with full responsibility and knowledge of the matter. Let's hope that the materials in this article will help you master the specialty of a liner, at least for yourself.