Everyone has probably heard about muffle furnaces, but rarely does anyone undertake to explain not only the structure, but also the purpose of this device. Meanwhile, a muffle furnace is a highly specialized design that is designed for smelting metals, firing clay or ceramic products, sterilizing instruments or growing certain crystals. In addition to industrial furnaces, sometimes there is a muffle furnace for the home, because the products of home craftsmen are widely known.
Compact factory-made ovens, which are intended for home use, are quite expensive, so more and more often people are talking about building the device themselves. To fully understand each stage of furnace manufacturing, you should first become familiar with general theoretical issues related to its features, structure, and classification.
Ready-made factory version
Classification
The first sign for division into subgroups is appearance. Based on orientation, furnaces are divided into vertical and horizontal. The material can be processed in normal air space, in an airless space, or in a capsule filled with an inert gas. It will be impossible to do the second and third processing methods yourself, which must be taken into account before starting work.
Firewood cannot act as a source of heat, since the temperature in the muffle can reach over 1000°C degrees, and wood does not have such a specific heat of combustion. Therefore, only two options for manufacturing the heater are used:
- The first option is a gas muffle furnace, which can only be found in production. It is known that any manipulations with gas equipment are immediately stopped by several regulatory authorities, and there can be no talk of making any devices using a homemade method.
- An electric muffle furnace allows you to use some creativity, provided that all necessary safety conditions are met.
Large furnace in production
Preparing for work
Any work must begin with a certain preparatory stage. Even if an action plan has been approved, it is necessary to prepare tools and materials, otherwise there may be long interruptions in the work that will negatively affect the performance of the craftsman and the quality of the constructed structure.
Before actual construction begins, you will have to immediately prepare a grinder for cutting sheet metal and processing fireclay bricks. The circles for the grinder must be appropriate. The list will be supplemented by electric welding with consumables and other plumbing tools for everyday use.
Materials include nichrome or fechral wire, basalt wool, fireclay brick and sheet iron with a thickness of at least 2 mm. Depending on how the structure is made, some tools or materials may not be needed, and additional ones will be acquired during the process.
Homemade stove
Some ready-made elements for making a stove
When planning work, you will have to show not only patience and the ability to use tools, but also ingenuity. After all, we are surrounded by so many unnecessary things that can become ready-made key elements of some structures. At the moment, we will use the ready-made experience and observations of some craftsmen to simplify the process of making a stove yourself.
You can use a metal oven as the body of the future oven. Surely you know where to get an old gas stove or electric oven. If the metal surface is not damaged by corrosion, then the find can serve as a housing, since it is structurally adapted to withstand high temperatures. All that remains is to dismantle the unnecessary parts and get rid of the plastic elements.
Old oven
You will have to make the heating element yourself, since in many electrical appliances it is filled with an insulating substance, and it is unlikely to be dismantled without damage. But in self-manufacturing there is one significant advantage - the ability to create an element of the desired geometry with the specified parameters.
It is most preferable to use fechral, since it can withstand higher temperatures and contact with air does not cause much harm to it, which cannot be said about nichrome.
The wire should have a diameter of 2 mm. The diameter of the coil and the length of the wire can be easily calculated based on the dimensions of the heating element using an elementary physical formula. It should be noted right away that the resulting oven consumes a lot of power. Its value reaches 4 kW, which means that you will have to draw a separate line from the panel with a circuit breaker rated at 25 A.
Finished wire
As thermal insulation, you need to use materials that not only have low thermal conductivity, but also withstand high temperatures. In order not to force the reader to rummage through physical tables, we immediately note that the suitable materials are basalt wool, heat-resistant glue, which is purchased in the store, and fireclay bricks or fireclay clay. If you do not provide the proper degree of insulation, then a large proportion of the heat will go away aimlessly, which will lead to unnecessary energy consumption.
Self-production
If it is not possible to find an old oven, then you will have to use sheet metal and electric welding. Using a grinder, the walls of our future product are cut out of a sheet of metal according to the required dimensions. To simplify the process, the oven is made in a cylindrical shape. Then the strip of metal is rolled into a cylinder and welded with one seam.
The metal circle will serve as one end, and a door will be installed on the other side a little later. The structure needs to be strengthened, and for this you will have to weld several corners at the junction of the walls of the cylinder and the circle.
Bend a sheet of metal into a cylinder
The inside walls of the resulting cylinder are lined with basalt wool. This material was not chosen by chance. The maximum temperature upon contact with an open fire is 1114°C degrees, the material has poor thermal conductivity, which is simply necessary for us in these conditions, and is also safe for human health even at critical temperatures.
The edges of the fireclay brick are processed with a grinder so that in cross-section it looks like a trapezoid. These elements can be used to form a kind of fire-resistant ring.
Creating a fireproof ring
Since the edges will be at different angles, and the structure will have to be disassembled, it is recommended to put a serial number on each brick. Having laid the bricks on a flat surface so that the inner edges “look” up, make shallow slots at a slight angle, a spiral will be inserted into these slots. The grooves should isolate the spiral turns from each other and ensure the distribution of the heating element throughout the active zone. Now you will again need to assemble the bricks into a ring and tighten them with wire or a clamp.
The prepared spiral is placed in the groove, and its ends are brought out, where the connecting terminals will be mounted. The spiral ring represents the heating element of the oven.
Spiral laying
The cylinder with basalt wool is installed with its end on a horizontal plane. Fireclay bricks are placed at the bottom to protect the round wall from exposure to high temperatures. A heating element is inserted inside, and all voids are filled with heat-resistant glue. It will take several days for the device to dry. During this time, you can design and make a door for the oven. The more tightly it covers the firebox, the longer the homemade spiral will last. A self-built muffle furnace is capable of melting precious metals, firing clay, and melting some metals.
In order to fire small clay products at home, you can make a simpler version of the oven. It consists of an electric stove with an exposed heating element and a suitable sized ceramic pot. It is impossible to place the part directly on the spiral, so fireclay bricks are placed under it and covered with a pot on top.
Materials for creating a furnace
Disadvantages of homemade design
Each device is not without certain shortcomings, and a homemade device also multiplies them. Given the set goal, you can sacrifice some requirements for the sake of fulfilling others. However, everyone should know the list of negative consequences.
- A homemade design is deprived of all guarantees, including safety guarantees.
- Evaporation of metal from the heater coil can lead to it being contained in the form of impurities in the composition of the precious metal being processed.
- Homemade thermal insulation will not provide full concentration of heat in the firebox, so the body of a homemade stove is very hot and requires careful handling. By the way, this is also a disadvantage of some factory models.
- Failure to properly monitor and regulate temperature may result in the oven not being able to perform a particular heat treatment task.
Ready-made factory-made ovens are designed to perform a fairly narrow range of tasks, but this is more an indicator of professionalism than a disadvantage. The main parameters and scope of application of a particular device are indicated in its passport.
The leaders in the production of compact and stationary muffle furnaces are such companies as TSMP Ltd (England), SNOL-TERM (Russia), CZYLOK (Poland), Daihan (South Korea). The presented list reflects the top list of companies for evaluating suppliers of high-temperature equipment to the Russian market.
Gas heating furnaces differ from oil ones only in the way fuel is supplied to the furnace. In this case, gas supplied to the furnace by injection burners is used as fuel.
Gas heating furnaces include chamber furnaces with a fixed hearth and a retractable hearth, with a rotating retort, continuous muffle furnaces, etc.
The furnace frame is welded from sheet steel and lined with fireclay and dolomite bricks. To reduce heat loss, sheet asbestos is laid between the lining and the frame. The furnace is equipped with injection burners that run on natural gas.
Automatic temperature control, carried out by a diaphragm valve. The valve is controlled using a pulse coming from a thermocouple through a pyrometric device with a pneumatic attachment. The air pressure on the pyrometric device is set by a reducer.
1 - oven frame, 2 - sheet asbestos, 3 - refractory brick, 4 - folding desk, 5 - flap, 6 - injection burners, 7 - membrane valve, 8- under, 9 - chimney; V- side view: 10,11 - cast iron plates, 12 - chain, 13 - rollers, 14 - barbell, 15 - rod cylinder 16 - pneumatic system.
AChamber gas furnaces with a ball hearth. In the thermal departments of tool and stamping-mechanical shops, gas furnaces with a ball hearth are used for heat treatment of measuring and cutting tools made of carbon and alloy steels, as well as forging and sheet stamping dies and devices.
The furnace runs on natural gas burning in the working chamber, gas consumption is 35-40 m 3 / hour. Working hearth area 1150X1900 mm 2, working window height 520 mm. Four grooved guides are placed under the stove 4, which contains the balls 5, made of heat-resistant steel. Balls and rails in the working chamber of the furnace greatly facilitate the movement of pallets with parts when loading and unloading the furnace. On the loading table 2 The pallets are placed on balls and manually guided into the working chamber of the furnace using a steel hook.
The furnace is equipped with injection burners 7. The parts in the furnace are heated due to thermal radiation from the walls and roof of the furnace. flapper 1 raised and lowered by a pneumatic lift via a roller 3. The oven temperature is controlled and regulated automatically using a diaphragm valve . The thermocouple is installed in a special hole 6, located in the furnace masonry (top). Furnace productivity during hardening and normalization is about 250 kg/hour.
Chamber gas furnaces with a retractable hearth. The general view of an annealing furnace with a moving hearth is shown in the figure.
The pull-out for the furnace is made in the form of a trolley on wheels, lined with insulating and refractory fireclay bricks. This hearth device allows parts to be loaded and unloaded outside the furnace working space using an overhead crane. Bogie hearth annealing furnaces are used for annealing large and heavy frames, steel castings, rolling, artificial aging of cast iron castings and high tempering.
The furnace is designed for annealing steel castings, coil wire and artificial aging of cast iron beds of metal-cutting machines.
Under the stove is made in the form of a retractable trolley 7 . Oven from the outside 2 lined with building bricks, and from the inside 3 - fireclay. Vault 11 The furnace is made of special suspended fireclay bricks.
Moves under the stove on wheels 10 on rails 9, laid on the workshop floor. Movement is carried out using a steel cable connected to an electric motor.
The furnace is heated by natural gas supplied through 16 injection burners 6, which are located on both walls of the furnace. The bottom row of burners is at the level of the hearth, so combustion products enter below the hearth. The upper row of burners is located in such a way that combustion products can flow under the furnace roof. City gas consumption 11 m 3 /hour for one burner.
Flue gases are removed from the working chamber through channels 8, located at floor level, along the chimney pipe 1. The oven is equipped with a powerful fan 4, which ensures uniform circulation of the furnace atmosphere in its space. Temperature is measured by thermocouples through holes 5 And 12.
Furnaces with a retractable hearth can have not only oil and gas heating, but also electric.
Technical specificationsgas chamber furnace with retractable hearth
Maximum temperature, ° C 650-850
Retractable hearth area, m 2 27,6
Height from the floor level to the arch, m 4.5
Weight of metal charge, t...... 30
Mechanized ovens with a rotating retort. Such ovens (picture below)
are used for gas carburizing and hardening of small parts of simple shape and not requiring a large depth of the carburized layer.
The furnace is a lined metal cylinder 3, mounted on axles in a horizontal position. Inside the cylinder there is a heat-resistant cast retort 4, which is the working chamber of the furnace. The retort rotates on support rollers 8 using worm and chain drives 7 from an electric motor 6 power 0.85 kW The support rollers are mounted on the end plates of the metal frame furnace.
During the carburization process, the retort, loaded with parts, rotates continuously, due to which the parts are flown around the carburizing gas or gas mixture. To ensure that when the retort rotates, parts do not accumulate in one place,
the loading side of the retort is hermetically sealed with a lid 2 lined inside the retort there are small longitudinal ribs. With a screen. The cementing gas is supplied to the kiln through a tube 5, located in the rear wall of the retort. Exhaust gas is discharged through a tube 1 through the retort lid, where it is set on fire.
The stove is heated with city gas using And injection burners. Natural gas is used as a carburizer for cementation. Carburizer gas consumption - 3.0- 3.5 m 3 /hour; gas required for heating - 60 m 3 / hour.
To speed up the carburization of parts and protect them from nicks during rotation of the furnace, 1.5-2.0 is poured into the retort kg small pieces of charcoal. Furnace productivity at a cemented layer depth of 0.6-0.8 mm- 50 kg/hour. The cementation rate on average does not exceed 0.15-0.20 mm/hour, Cemented parts are hardened only after they have cooled to a temperature of 830-840°C.
When unloading cemented parts, the furnace is easily tilted using a flywheel and the parts are poured into a quenching tank, at the bottom of which there is an iron mesh basket. In the absence of cementing gases or gas mixtures in factories, parts in retort furnaces are cemented using a solid carburizer. Disadvantages of furnaces with a rotating retort are uneven cementation and the possible formation of small nicks on the parts.
Technical characteristics of a mechanized furnace with a rotating retort
Maximum temperature, °C............ 930
Dimensions of the retort working space, mm:
diameter........................................ 360
length........................................ 1324
Continuous gas muffle furnaces. In continuous mass production, when it is necessary to produce a large number of parts with the same depth of the cemented layer, continuous gas muffle furnaces are used
with periodic loading of parts onto pallets
Pallets with parts move along rails using a mechanical pusher 1, mounted at the loading end of the furnace, which is equipped with a quenching tank with a mechanized table for direct quenching with preliminary cooling of parts after carburization. A loading chamber is installed on the inlet side of the furnace 2, made of sheet steel without lining. There are two gas burners inside the chamber 3, during combustion of which oxygen from the air is absorbed and the force of the flash of gases escaping from the cementation working chamber is reduced 6 when opening the damper 4 muffle.
Camera 6 is a muffle 5 assembled from cast sections with flanges, which are secured with bolts and welded with a gas-tight seam. The muffle sections and fastening bolts are made of heat-resistant steel X18N25S2. The muffle dimensions are as follows: length 7-8 m, width 0.82 m, height 0.43 m. The muffle can accommodate 24 pallets at a time; Only 22 subjects are practically exploited. This is done to ensure that there is always free space at the discharge end of the furnace.
Each pallet is loaded with parts weighing from 100 to 120 kg. The number of parts loaded and the order in which they are stacked depend on the shape of the parts and their weight.
Unloading chamber 9 (cooling chamber) has two independent burners and a damper 11. It is separated from the cementation chamber by a hermetic valve 10 with hydraulic lock. The presence of such a chamber allows you to cool parts from the carburization temperature (930°C) to the hardening temperature (820-840°C).
As soon as the subject with the parts reaches the hardening chamber, he is pulled out of the chamber using iron hooks 12,. installed on a mechanized table, after which, together with the table, they are immersed in a quenching tank with oil. The table with parts is lowered using a pneumatic lift. The muffle furnace is typically heated with city natural gas using 28 burners staggered in two rows on either side of the muffle. The stove can also operate on fuel oil.
The temperature inside the cementation chamber is controlled by 7 thermocouples installed in three zones. The temperature in quiet zones is maintained at 920-940°C. The fourth zone is the cooling chamber. Carburizing gas is introduced into the muffle through three holes (inputs) 8, located above, and one located below the muffle.
From the muffle, exhaust gases are directed into a hydraulic seal so that outside air, which oxidizes the cemented parts, cannot enter the muffle. Then the exhaust gases are removed outside and ignited at the entrance of the tube. Carburizer gas consumption 5-6 m 3 / hour, and the gas required for heating is 60-70 m 3 / hour. Productivity of a carburizing furnace with a cemented layer depth of 1.0-1.2 mm 200-250 kg/hour, The duration of the process is 7-8 hours.
One of the main disadvantages of muffle furnaces is the presence in them of expensive heat-resistant cast muffles, the operational durability of which is low - no more than 10-12 months. When the carburization temperature increases to 1000 °C, the resistance of the muffle becomes even lower. Changing the muffle when repairing a furnace also causes significant difficulties. To remove the old, burnt-out muffle and replace it with a new one, you have to almost completely dismantle the furnace masonry. These disadvantages are completely eliminated when using modern muffle-free units.
Muffleless cementation units. Such units are used for gas carburizing and nitrocarburizing of automobile and tractor gears, shafts, steering worms, parts of metal-cutting machines and agricultural machines. Muffleless units can be single-row or double-row.
A single row muffleless unit is shown in the figure below.
In such a unit, all processes are nitrocarburization, i.e. saturation of the surface of steel parts with carbon And nitrogen; hardening; washing and tempering are fully automated.
The unit is a tightly welded frame, inside of which there is a carburizing muffle-free furnace 5, loading chamber 2, hardening chamber with a mechanized tank for isothermal hardening, washing machine 9, tempering furnace 10 and roller table system 11. The cementation kiln is lined with silica refractory bricks. To protect the furnace masonry from carburization, the working chamber of the furnace is lined with blast furnace bricks. The carburization furnace is heated using radiant heating pipes 17, made of heat-resistant chromium-nickel steel, vertically located in the working chamber. This allows you to receive the greatest amount of heat when burning natural gas in them and remove them for replacement in case of burnout. There is a sand seal at the top and bottom of the pipe 16. Burners 18 are placed in the lower part of the pipe, and so that the combustion products in the pipe move upward due to the draft of the pipe itself. When leaving the pipe, combustion products (gases) enter the box 15. They are sucked out of this box by a fan located on the roof of the furnace. Such a fan promotes mixing of the carburizing atmosphere in the furnace.
Laying the furnace, sealing the steel frame and sand seals near the pipes ensure constant pressure in the furnace - 20 mm water Art. Compaction on the loading side is achieved by continuous hydraulic pressing of the walls of the loading trolley through an asbestos gasket to the frame of the loading chamber, and on the unloading side - by an oil seal at the outlet of the trunk into the oil quenching tank. The temperature in the furnace along the length of the 2/3 chamber for carburization should be 930 ± 10 ° C, for nitrocarburization - 850-860 ° C. At the unloading end of the furnace it drops to 800-820 ° C. Before unloading, the furnace is cooled to increase the carbon concentration to normal and obtain the smallest amount of austenite in the cemented layer after direct quenching in hot oil.
The carburizer is a gas mixture consisting of endothermic gas, ammonia and natural gas.
Nitrocarburization is carried out on heat-resistant pallets 13, Moreover, there are 17 pallets in the oven at the same time. The unit works as follows. The loading platform feeds the pallet of parts onto the loading table 12. The table, moving upward, directs the pallet with parts into the unloading chamber. After this, the front and rear flaps rise 4. From the discharge side, a shovel pusher is fed into the furnace and at the same time the table in the quenching tank is raised. Pusher 1 moves the pallet with “raw” parts, and the shovel pusher with the pallet of cemented parts moves back and places them on the oil tank table. Then the dampers are lowered and the table with the parts is immersed in an oil tank with quenching oil having a temperature of 170°C, where their isothermal hardening occurs. Next, the hardened parts are fed into a tank with cold oil and from it into the washing chamber, and the loading platform returns to its original position. After this, the dampers of the washing machine and the tempering furnace are raised, the pallet with the released parts goes onto the roller table, and a pallet with the washed parts is installed in its place. Then the dampers of the washer and tempering furnace are lowered, and a new carburizing cycle begins.
For automatic control of the technological process, the unit is equipped with two systems of sequential action of hydraulic mechanisms and temperature control. The system of sequential operation of hydraulic mechanisms consists of an electric clock with a bell to maintain the time interval, a pusher of pallets with parts, limit switches and relays that guarantee a given sequence of operation of the mechanisms. After a sound signal received from the contact clock, the mechanisms operate without the intervention of a thermist. Pallets with parts are loaded every 12-15 minutes.
The productivity of muffle-free units for the nitrocarburization of automotive gears is 350 kg/hour.
The invention relates to the field of technology of foam silicate materials. The technical result of the invention is to create a method for producing granulates for the production of glass-crystalline foam materials without carrying out the glass melting process. A fraction of high-silica raw materials with a SiO 2 content of more than 60 wt.% is prepared by heating at a temperature of 200-450°C. Then soda ash is added in an amount of 12-16 wt.%, the resulting mixture is compacted in a heat-resistant steel mold. The mold is placed in a continuous oven and heat-treated at a maximum temperature of 10-20 minutes, and the resulting cake is crushed. 1 table
The invention relates to the field of technology of foam silicate materials obtained by foaming at temperatures above 800°C - foam glass, expanded clay, petrosites, including penozeolites, and can be used for the manufacture of thermal insulation materials with a density of 150-350 kg/m 3. Before foaming the initial mixture, granules or granules are obtained, which in some cases are crushed to a powder with a specific surface of 6000-7000 m 2 /g.
There is a known method for producing granulates for foaming by molding plastic masses on screw or roller presses, followed by drying at a temperature of 100-120°C, while foaming of the material occurs at temperatures of 1180-1200°C. The disadvantage of this method is its limited applicability - only for clay-containing charges when producing granular porous material (Onatsky S.P. Expanded clay production. - M.: Stroyizdat, 1987). It is impossible to obtain the initial mixture for foaming, for example, from cullet, using this method.
There is a known method for producing glass granulate by mixing the components of the charge of the required composition and melting the glass melt at temperatures above 1400°C, cooling the glass melt, followed by crushing and grinding to a specific surface of 6000-7000 m 2 /g (Kitaygorodsky I.I., Keshishyan T.N. Foam glass . - M., 1958; Demidovich V.K. Foamglass. The disadvantage of this method is the need to organize the process at high temperatures with high energy consumption.
The closest to the proposed solution in terms of technical essence is the method of producing granulates, which includes preparing a fraction of high-silica raw materials, adding soda ash, mixing powders and firing in continuous ovens at a temperature of 750-850 ° C (Ivanenko V.N. Construction materials and products made from siliceous breeds - Kyiv: Budivelnik, 1978, pp. 22-25). The disadvantage of this method is its limited applicability - thermolites are obtained that are used as porous aggregates for concrete, which are made only from siliceous opal rocks (diatomite, tripolite, opoka).
The objective of the invention is to prepare granulate based on heat treatment of a mixture of components: a) raw materials with SiO 2 more than 60 wt.%, for example zeolite tuffs, marshallites, diatomites, tripoli, etc. and b) technological additives that ensure silicate formation processes without glass melting.
The task is achieved as follows:
1. Siliceous rock containing SiO 2 more than 60 wt.% is crushed, crushed, sifted (fraction less than 0.3 mm);
2. Siliceous rock powder is activated by heating at a temperature of 200-450°C to remove the so-called. "molecular water";
3. To prepare the raw material mixture, add soda ash in an amount of 12-16 wt.%;
4. The resulting mixture is compacted in a mold made of heat-resistant steel and heat-treated in continuous ovens at a temperature of 750-850°C with exposure at a maximum temperature of 10-20 minutes;
5. The resulting cake is crushed to a fraction of less than 0.15 mm and used to prepare a charge with a blowing agent and other additives for the production of foam glass and foam glass-crystalline materials using known technological processes.
The proposed method for producing granulate is illustrated by an example:
1. Zeolitized tuff from the Sakhaptinskoe deposit (Krasnoyarsk Territory) of the following chemical composition, wt.%: SiO 2 - 66.1; Al 2 O 3 - 12.51; Fe 2 O 3 - 2.36; CaO - 2.27; MgO - 1.66; Na 2 O - 1.04; K 2 O - 3.24; TiO 2 - 0.34; loss on ignition - 10.28.
2. The prepared sample - crushed, sifted with a fraction of less than 0.3 mm - is activated by heating in an oven at 400°C for 10 minutes.
3. The calculation of the amount of soda ash is carried out based on the prerequisites for the maximum formation of Na 2 SiO 3 during the solid-phase interaction of SiO 2 and Na 2 CO 3 - i.e. per 100 g of activated sample, 18.62 g of soda ash is added.
4. For sintering, molds made of heat-resistant steel are used. The inner surface of the mold is coated with a kaolin suspension to prevent the coating from sticking to the metal.
5. The prepared powder mixture is compacted in a mold, placed in a muffle furnace and heated to a temperature of 800°C and held for 15 minutes.
6. The resulting cake with a glass phase content of 65-85% is cooled, crushed and is a semi-finished product for preparing a charge for the production of foam glass.
The granulate obtained by this method has been tested in the technological process of foam glass production:
The granulate was crushed to a fraction of less than 0.15 mm;
A gas-forming agent - coke, anthracite, liquid hydrocarbons in an amount of 1% by weight - was introduced into the resulting powdery mixture;
The charge was compacted in molds and thermally treated in a muffle furnace at a temperature of 820°C for 15 minutes. After curing, the molds were removed from the oven to cool and stabilize the cellular structure.
A glass-crystalline foam material with the characteristics given in the table was obtained.
Thus, the authors propose a method for producing granules for the production of glass-crystalline foam material, which allows the use of natural raw materials instead of scarce cullet. The technological process does not require high temperatures, which makes production cost-effective.
| Main characteristics of the method and properties of glass-crystalline foam material | |||||||
| Type of granulate | Processing mode, parameter | Properties of foam glass crystallite | |||||
| Processing temperature, °C | Granulate particle size for batch preparation | Temperature for producing foam glass and foam glass crystallite, °C | Holding temperature, min | Amount of glass phase, wt.% | Density kg/m3 | Compressive strength, MPa | |
| Glass granulate (melt zeolite + soda mixture) | 1480-1500 | 6000 cm 2 /g | 820 | 15 | 100 | 300 | 08,-1,5 |
| Solid-phase sintering of zeolite + soda mixture | 750 | 0.15 mm | 820 | 15 | 65 | 350 | 3-4 |
| Same | 800 | 0.15 mm | 820 | 15 | 70 | 300 | 2,5-3,5 |
| Same | 850 | 0.15 mm | 820 | 15 | 80 | 300 | 2,5-3,5 |
| Cullet | 1500 | 6000 cm 2 /g | 750-850 | 15 | 100 | 150-200 | 0,8-2,0 |
CLAIM
A method for producing granulate for the production of foam glass and foam glass-crystalline materials, including preparing a fraction of high-silica raw materials with a SiO 2 content of more than 60 wt.%, adding soda ash, mixing powders and firing in continuous furnaces at a temperature of 750-850 ° C, characterized in that the resulting the fraction of high-silica raw materials is activated by heating at a temperature of 200-450°C, then soda ash is added in an amount of 12-16 wt.%, the resulting mixture is compacted in a mold made of heat-resistant steel, the mold is placed in a continuous furnace, heat-treated with exposure at a maximum temperature of 10 -20 min and the resulting cake is crushed.
Start
This venture began, as many similar ventures usually begin - I accidentally went into a friend’s workshop, and he showed me a new “toy” - a half-disassembled MP-2UM muffle furnace ( Fig.1). The stove is old, the original control unit is missing, there is no thermocouple, but the heater is intact and the chamber is in good condition. Naturally, the owner has a question: is it possible to attach some kind of homemade control to it? Even if it’s simple, even with little precision in maintaining the temperature, but for the oven to work? Hmm, it’s probably possible... But first it would be nice to look at the documentation for it, and then clarify the technical specifications and evaluate the possibilities of its implementation.
So, first, the documentation is online and can be easily found by searching for “MP-2UM” (also included in the appendix to the article). From the list of main characteristics it follows that the furnace power supply is single-phase 220 V, power consumption is approximately 2.6 kW, the upper temperature threshold is 1000 ° C.
Secondly, you need to assemble an electronic unit that could control the power supply to the heater with a current consumption of 12-13 A, and could also show the set and actual temperatures in the chamber. When designing a control unit, you should not forget that there is no normal grounding in the workshop and it is not known when there will be one.
Taking into account the above conditions and the available electronic database, it was decided to assemble a circuit that measures the thermocouple potential and compares it with the set “set” value. The comparison is carried out with a comparator, the output signal of which will control the relay, which in turn will open and close a powerful triac, through which the 220 V mains voltage will be supplied to the heating element. Refusal of phase-pulse control of a triac is associated with high currents in the load and lack of grounding. We decided that if with “discrete” control it turns out that the temperature in the chamber fluctuates within wide limits, then we will convert the circuit into a “phase” one. A dial gauge can be used to indicate temperature. The power supply of the circuit is an ordinary transformer; the refusal of a switching power supply is also due to the lack of grounding.
The hardest part was finding the thermocouple. In our little town, stores don’t sell this kind of stuff, but, as usual, radio amateurs came to the rescue with their desire to forever store all sorts of radio-electronic junk in their garages. About a week after notifying my closest friends about the “thermocouple need,” one of the oldest radio amateurs in the city called and said that there was some kind that had been lying around since Soviet times. But it will need to be checked - it may turn out that it is a low-temperature chromel-copel. Yes, of course we’ll check it, thank you, but any one will be suitable for experiments.
A short “trip to the net” to look at what has already been done by others on this topic, showed that basically according to this principle, home-made people construct them - “thermocouple - amplifier - comparator - power control” ( Fig.2). Therefore, we will not be original - we will try to repeat what has already been proven.
Experiments
First, let's decide on the thermocouple - there is only one and it is single-junction, so there will be no change in room temperature in the compensation circuit. By connecting a voltmeter to the thermocouple terminals and blowing air at the junction at different temperatures from a hot air gun ( Fig.3), we compile a table of potentials ( Fig.4) from which it can be seen that the voltage increases with a gradation of approximately 5 mV for every 100 degrees. Taking into account the appearance of the conductors and comparing the readings obtained with the characteristics of different junctions according to tables taken from the network ( Fig.5), it can be assumed with high probability that the thermocouple used is chromel-alumel (TCA) and that it can be used for a long time at a temperature of 900-1000 °C.
After determining the characteristics of the thermocouple, we experiment with circuit design ( Fig.6). The circuit was tested without a power section, in the first versions an LM358 operational amplifier was used, and in the final version an LMV722 was installed. It is also two-channel and is also designed to operate with single-supply power (5 V), but, judging by the description, it has better temperature stability. Although, it may very well be that this was excessive reinsurance, since with the circuitry used, the error in setting and maintaining the set temperature is already quite large.
results
The final control diagram is shown in Fig.7. Here, the potential from the terminals of thermocouple T1 is supplied to the direct and inverse inputs of the operational amplifier OP1.1, which has a gain of approximately 34 dB (50 times). The amplified signal is then passed through a low-pass filter R5C2R6C3, where the 50-THz noise is attenuated to -26 dB from the level coming from the thermocouple (this circuit was previously simulated in the program, the calculated result is shown in Fig.8). Next, the filtered voltage is supplied to the inverse input of the operational amplifier OP1.2, which acts as a comparator. The comparator threshold level can be selected using variable resistor R12 (approximately from 0.1 V to 2.5 V). The maximum value depends on the connection circuit of the adjustable zener diode VR2, on which the reference voltage source is assembled.
To ensure that the comparator does not have switching “bounce” at input voltages that are close in level, a positive feedback circuit is introduced into it - a high-resistance resistor R14 is installed. This allows each time the comparator is triggered to shift the reference voltage level by several millivolts, which leads to a trigger mode and eliminates “bouncing”. The output voltage of the comparator through the current-limiting resistor R17 is supplied to the base of the transistor VT1, which controls the operation of relay K1, the contacts of which open or close the triac VS1, through which a voltage of 220 V is supplied to the heater of the muffle furnace.
The power supply for the electronic part is based on transformer Tr1. The mains voltage is supplied to the primary winding through a low-pass filter C8L1L2C9. The alternating voltage from the secondary winding is rectified by a bridge on diodes VD2...VD5 and, smoothed out on capacitor C7 at a level of about +15 V, is supplied to the input of the stabilizer microcircuit VR1, from the output of which we obtain stabilized +5 V to power OP1. To operate relay K1, an unstabilized voltage of +15 V is taken, the excess voltage is “extinguished” by resistor R19.
The appearance of voltage in the power supply is indicated by the green LED HL1. The operating mode of relay K1, and therefore the heating process of the furnace, is shown by the HL2 LED with a red glow.
Pointer device P1 serves to indicate the temperature in the furnace chamber in the left position of the push-button switch S1 and the required temperature in the right position of S1.
Details and design
The parts in the circuit are used both ordinary output ones and those designed for surface mounting. Almost all of them are installed on a printed circuit board made of single-sided foil PCB measuring 100x145 mm. A power transformer, surge protector elements and a radiator with a triac are also attached to it. On Fig.9 shows a view of the board from the printing side (the file in the program format is in the appendix to the article; the drawing for LUT must be “mirrored”). An option for installing the board into the case is shown in rice. 10. Here you can also see the pointer P1, LEDs HL1 and HL2, button S1, resistor R12 and packet switch S2 mounted on the front wall.
The ferrite ring cores for the surge protector are taken from an old computer power supply and then wrapped until filled with insulated wire. You can use other types of chokes, but then you will need to make the necessary changes to the printed circuit board.
Just before installing the control unit on the stove, a break resistor was soldered into the gap of one of the conductors going from the filter to the transformer. Its purpose is not so much to protect the power supply as to reduce the quality factor of the resonant circuit obtained by shunting the primary winding of the transformer with capacitor C9.
Fuse F1 is soldered at the 220 V input to the board (installed vertically).
Any power transformer is suitable, with a power of more than 3...5 W and with a voltage on the secondary winding in the range of 10...17 V. It is possible with less, then you will need to install the relay at a lower operating voltage (for example, five-volt).
Operational amplifier OP1 can be replaced with LM358, transistor VT1 with similar parameters, having a static current transfer coefficient of more than 50 and an operating collector current of more than 50...100 mA (KT3102, KT3117). There is also space on the printed circuit board for installing an SMD transistor (BC817, BC846, BC847).
Resistors R3 and R4 with a resistance of 50 kOhm are 4 resistors with a nominal value of 100 kOhm, two in parallel.
R15 and R16 are soldered to the terminals of the LEDs HL1, HL2.
Relay K1 – OSA-SS-212DM5. Resistor R19 is made up of several connected in series so as not to overheat.
Variable resistor R12 – RK-1111N.
Push-button switch S1 – KM1-I. Package switch S2 – PV 3-16 (version 1) or similar from the PV or PP series for the required number of poles.
Triac VS1 – TC132-40-10 or another from the TC122…142 series, suitable for current and voltage. Elements R20, R21, R22 and C10 are wired to the terminals of the triac. The heatsink was taken from an old computer power supply.
Any suitable size and sensitivity up to 1 mA can be used as a pointer electrical measuring device P1.
The conductors going from the thermocouple to the control unit are made as short as possible and are made in the form of a symmetrical four-wire line (as described).
The power input cable has a core cross-section of about 1.5 sq. mm.
Setup and configuration
It is better to debug the circuit step by step. Those. solder the rectifier elements with voltage stabilizers - check the voltages. Solder the electronic part, connect the thermocouple - check the relay response thresholds (at this stage you will need either some kind of heating element connected to an external additional power supply ( Fig.11), or at least a candle or lighter). Then unsolder the entire power section and connect the load (for example, a light bulb ( Fig.12 And Fig.13)) make sure that the control unit maintains the set temperature by turning the light bulb on and off.
Adjustment may only be necessary in the amplification part - the main thing here is that the voltage at the output of OP1.1 at maximum heating of the thermocouple does not exceed the level of 2.5 V. Therefore, if the output voltage is high, then it should be lowered by changing the gain of the cascade (by reducing the resistance of resistors R3 and R4). If a thermocouple with a low output EMF value is used and the voltage at the output of OP1.1 is small, then in this case it is necessary to increase the cascade gain.
The value of the tuning resistor R7 depends on the sensitivity of the device P1 used.
It is possible to assemble a version of the control unit without voltage indication and, accordingly, without a mode for pre-setting the desired temperature threshold - i.e. remove S1, P1 and R7 from the circuit and then to select the temperature you should make a mark on the handle of the resistor R12 and draw a scale with temperature marks on the block body.
It is not difficult to calibrate the scale - at the lower limits this can be done using a soldering iron hot air gun (but you need to warm up the thermocouple as much as possible so that its long and relatively cold leads do not cool down the thermal junction). And higher temperatures can be determined by the melting of various metals in the furnace chamber ( Fig.14) – this is a relatively long process, since it is necessary to change the settings in small steps and give the furnace sufficient time to warm up.
Photo shown on rice. 15, done during the first starts in the workshop. Temperature calibration has not yet been done, so the scale of the device is clean - in the future, many multi-colored marks will appear on it, applied with a marker directly to the glass.
After some time, the owner of the stove called and complained that the red LED stopped lighting up. Upon inspection, it turned out that it was out of order. Most likely, this happened due to the fact that the last time it was turned on, the capabilities of the oven were checked and the chamber, according to the owner, heated up to white. The LED was replaced, but the control unit was not moved - firstly, perhaps it was not a matter of overheating of the control unit, and secondly, there will be no more such extreme modes, since there is no need for such temperatures.
Andrey Goltsov, r9o-11, Iskitim, summer 2017
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| OP1 | Operational amplifier | LMV722 | 1 | Can be replaced with LM358 | To notepad | |
| VR1 | Linear regulator | LM78L05 | 1 | To notepad | ||
| VR2 | Voltage reference IC | TL431 | 1 | To notepad | ||
| VT1 | Bipolar transistor | KT315V | 1 | To notepad | ||
| HL1 | Light-emitting diode | AL307VM | 1 | To notepad | ||
| HL2 | Light-emitting diode | AL307AM | 1 | To notepad | ||
| VD1...VD5 | Rectifier diode | 1N4003 | 5 | To notepad | ||
| VS1 | Thyristor & Triac | TS132-40-12 | 1 | To notepad | ||
| R1, R2, R5, R6, R9, R17 | Resistor | 1 kOhm | 6 | smd 0805 | To notepad | |
| R3, R4 | Resistor | 100 kOhm | 4 | see text | To notepad | |
| R8, R10, R11 | Resistor | 15 kOhm | 3 | smd 0805 | To notepad | |
| R13 | Resistor | 51 Ohm | 1 | smd 0805 | To notepad | |
| R14 | Resistor | 1.5 MOhm | 1 | smd or MLT-0.125 | To notepad | |
| R15, R16 | Resistor | 1.2 kOhm | 2 | MLT-0.125 | To notepad | |
| R18 | Resistor | 510 Ohm | 1 | smd 0805 | To notepad | |
| R19 | Resistor | 160 Ohm | 1 | smd 0805, see text | To notepad | |
| R20 | Resistor | 300 Ohm | 1 | MLT-2 | To notepad | |
| R21 | Resistor |
A muffle furnace is designed to uniformly heat substances to different temperatures. The muffle present in it protects the heated object from direct exposure to combustion products.
Navigation:
Muffle furnaces are distinguished according to several criteria.
- By heating source.
- According to processing mode.
- According to design data.
The heating source of a muffle furnace can be gas or electricity.
The processing mode is:
- in normal (air) atmosphere;
- in a special gas environment - hydrogen, argon, nitrogen and other gases;
- at vacuum pressure.
Structurally, muffle furnaces are divided into furnaces:
- top loading;
- horizontal filling;
- bell-shaped - the oven will be separated from the hearth;
- tube furnaces.
In addition, there are several types of furnaces according to thermal indicators:
- ovens with low temperatures: 100 - 500 degrees;
- ovens with average temperature: 400 - 900 degrees;
- high temperature ovens: 400 - 1400 degrees;
- ovens with very high temperatures: up to 1700 - 2000 degrees.
Note. The temperature of the muffle furnace directly determines its cost, i.e., the higher the maximum temperature, the more expensive the furnace will be.
The advantages of muffle furnaces include protection of the heated substance from fuel combustion products or evaporation of heating elements and its uniform heating throughout the chamber.
In the event of a muffle failure, the furnace design allows it to be quickly replaced, which greatly facilitates repairs.
The disadvantage is the slow heating rate (although this is not always necessary). It is impossible to produce high-speed heating modes in a muffle furnace. This is due to the fact that it takes time for the muffle to heat up. Which entails another drawback - additional energy costs for heating.
The main component of a muffle furnace is the muffle, which is most often made of ceramic. This material is universal for making various types of ovens. There are also corundum muffles, but they are used only in chemical environments.
A heating element in the form of a wire is wound around the muffle and covered with ceramic coating.
There is thermal insulation material around the muffle and the whole thing is sheathed with a metal casing made of a sheet of metal 1.5-2 mm thick.
Since heating of the furnace begins around the muffle, it is not possible to reach high temperatures (above 1150 degrees). In this regard, manufacturers have developed a special fibrous material for the manufacture of the muffle, which allows the heating elements to be located from the inside. This makes it possible to increase the temperature limit of muffle furnaces. But the disadvantage of fibrous material is its fragility: under the influence of gas fumes, salts and oils from the heated material, the fiber is destroyed.
Today, for high-temperature muffle furnaces, Japanese very high-quality heating elements are used, which make it possible to reach temperatures in the furnace of up to 1750 degrees.
Furnaces operating on gaseous fuel initially have higher temperatures.
To heat the working chamber more evenly, some manufacturers build in ventilation. And to remove combustion products, there is an exhaust mechanism that removes smoke and steam from the furnace through a pipe.
To control and regulate the temperature in the furnace, an electronic thermostat is used, which is connected to a heater and a thermocouple. The thermostat allows you to control not only the temperature, but also the holding time of the product in the oven. Moreover, these indicators have very high accuracy, especially in a laboratory muffle furnace, because the accuracy of the research depends on their value and the result obtained.
Application of muffle furnaces
The muffle furnace is widely used, primarily as equipment for the heat treatment of metals. But, thanks to its advantages, the muffle furnace (which can be purchased in any region of Russia) has greatly expanded its scope of application, and this is:
- heat treatment of metals (hardening, tempering, annealing, aging);
- firing of ceramic materials is the final stage of ceramic processing;
- ashing - transformation of the test substance into ash without combustion for examination;
- cremation;
- Assay analysis is a method of identifying and separating precious metals (gold, silver, platinum) from ores, alloys, and finished products;
- drying – separation of moisture in the form of water or other liquid substance from materials;
- sterilization of instruments in medicine (dentistry).
Heat treatment of metals can be done at home, in a laboratory or on an industrial scale. Based on this, there is a whole range of muffle furnaces with different working chamber volumes, capacities and maximum heating temperatures. For personal use, you can buy a muffle furnace for hardening knives; for research, a laboratory muffle furnace is suitable.
For heat treatment of metals and alloys, a muffle furnace must have special characteristics.
First of all, a muffle furnace for metal hardening, tempering, etc. must have very good insulating characteristics. They are usually provided with several layers: fire brick, fiber ceramic material and a sheet metal protective casing. The bottom of the furnace must be equipped with special silicon carbide plates and an additional tray to protect it from impacts of heating elements during loading and unloading. And most importantly, an electric muffle furnace must have special heating coils made of high-quality alloy to ensure a sufficiently high heating temperature - up to 1400 degrees.
A laboratory muffle furnace (price depends on power and design features) can be used to heat materials of different compositions.
A muffle kiln for firing ceramics is used in art and pottery workshops. In addition to firing, it heats the flasks and melts the glass. The muffle furnace for ceramics has a temperature range of up to 1300 degrees and is equipped with an automatic regulator that allows you to slowly heat and cool products without temperature jumps. Such a smooth transition is also necessary when clay is fired in a muffle furnace.
You can buy a muffle furnace for ceramics directly from the manufacturer, which significantly reduces its cost.
Note. A muffle furnace is often equipped with removable heating elements that can be easily replaced if they fail.
A muffle furnace for firing ceramics (the price depends on the size, power, loading method and configuration) can have an internal chamber volume from 1 liter to 200 liters and even more. The design of the furnace can be round with loading from above, chamber with loading in front, there are bell-type furnaces. Therefore, a muffle furnace for firing ceramics, which you can even buy for home use, is available to a wide range of activities of any craftsman.
For working with precious metals, as well as in dentistry, a small muffle furnace or even a mini muffle furnace with a working chamber volume of about two liters is perfect.
When thinking about how much a muffle furnace costs, you should take into account the required characteristics that should be present in it and choose a good manufacturer. Russian-made muffle furnaces have received good reviews among consumers and have a good pricing policy.
A wide range of models allows you to choose RF muffle furnaces of different designs: horizontal and vertical muffle furnaces with the required loading location, laboratory muffle furnaces (the production base is located in Samara).
Nacal muffle furnaces are known for their quality. This muffle furnace (you can buy it in Moscow immediately with delivery) has received many positive reviews from leading enterprises in various fields.
The muffle furnace (you can buy different models in St. Petersburg) from the Elektropribor company has also proven itself well among buyers.
The Belarusian muffle furnace is of good quality (buying it in Minsk will not be a problem, since there are many online stores that stock such furnaces).
Some craftsmen take on the task of making a muffle furnace with their own hands, since a factory muffle furnace (the price of which is still quite high) is beyond their means. When making a furnace yourself, you need to pay great attention to making the muffle. For home use, the muffle can be made of refractory clay, forming a working chamber around a cardboard frame. When the clay dries, the cardboard is removed. Just before further assembly, be sure to burn the clay muffle so that it hardens and acquires the necessary hardness. Further assembly is no different from the factory one.
But there are not many such home-made specialists; most consumers still prefer to buy a muffle furnace; the price is chosen according to their capabilities.
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