In a muffle furnace at a temperature of 820. Homemade electric muffle furnace (small). How to choose the right device

Master Kudelya © 2013 Copying of site materials is permitted only with indication of the author and a direct link to the source site

Homemade muffle electric furnace (small)

Here I will describe the design of a small budget electric muffle furnace. The oven power is 500 W, the theoretical temperature is up to 800 degrees, but I didn’t heat it up to there, because I have a more serious oven for that. The peculiarity of this design is its extreme simplicity and extremely low cost of components. Such a design can be made from scrap materials in just a few days, of which most of the time will be spent on drying the furnace muffle.
The upper body of the oven with the door open. The muffle itself is located in the center of the body. The door is thermally insulated, as can be seen in the photo, using asbestos cardboard on studs. The window is covered with two layers of mica with some gap between the layers.
Muffle furnace assembly. It consists of two bodies fastened together. The muffle itself is located in the upper housing, and the control unit is located in the lower housing.

I immediately advise you to make a stove like mine in different buildings. This will allow you not to worry about cooling the control unit with various fans. The upper housing will heat up and create a draft, which, combined with the perforations in the lower housing, will be sufficient to cool the temperature controller.

Making a muffle.

The muffle can be made in many ways different ways. You can take a ready-made ceramic pipe. It's best to use mullite-silica MKR, or a pipe from an old rheostat or a large fuse. If you prefer a rectangular chamber, it is better to sculpt it yourself. Since my site is focused on those practical designs that I managed to make myself, here is the recipe for my muffle.

Kaolin (kaolin clay) - 1 part. Can be found near the porcelain factory. They are brought by wagons for the production of porcelain, earthenware, and electrical ceramics. If not, any thicker clay will do.
Sand - 3 parts. Quarry sand is better than river sand.
Mix all this thoroughly, add water until the lump does not spread, but holds its shape, and leave it in a plastic bag for a couple of days. Then take it out and mix again until smooth. Then we sculpt the muffle.
Retreat.
There are a lot of things on sale now that weren’t available until recently. Now I use this binder for similar work. Mortar of Ekaterinburg Pechnik LLC and its characteristics. It is worth keeping in mind that this is a ready-made mortar, i.e. it already contains filler so that it does not lose volume during drying. Therefore, add to it a large fraction, such as sand, in a smaller volume.

So, modeling the muffle. The rectangular muffle is molded in a rectangular plywood or krags box. A muffle with a level bottom and an arched vault is molded in the same box. The size of the box is equal to the outer size of the muffle plus 3-6% shrinkage. It is always molded from the inside of the box, since the muffle shrinks during drying and when molding from the outside, cracks are inevitable. To prevent the mixture from sticking to the walls of the box, the inside walls are lined with polyethylene. If the mixture is semi-dry, then you can put paper. This way you can save drying time.
After the muffle is fashioned, it is left to dry for several days. When the walls of the muffle gain the necessary strength, turn the box over and remove it from the muffle. Further, if the muffle is not strong enough for spiral winding, it is dried for several days on a radiator or in an oven. Then it is slowly fired to 900 degrees. If you have problems with firing, as a last resort you can leave a dry, unfired muffle. But the strength will no longer be the same.
If the muffle is strong enough, then it is wrapped in a spiral, a coating is applied, and the entire assembly is dried and fired. It is preferable to do this when assembled, since the coating will stick better to the half-baked muffle. Make sure that there are no voids inside the spiral, everything is filled with coating. Otherwise there will be local overheating of the nichrome.

Heater calculation.

There are a lot of materials on the Internet about heater calculations. All of them have varying degrees of scientific knowledge of this issue. For example, you can not only read various considerations, but also calculate the heater using the built-in calculator. The input data is the furnace power, heater material, temperatures of the heater and the heated product, design and placement of the heaters. At the output we get the diameter and length of the heater wire. But upon closer examination, it turns out that the diameter was chosen for reasons of saving wire material and the operating conditions are close to ideal. In life, the opposite is usually true. Usually there is a skein of old nichrome in the bins and its owner is tormented by the question of whether it can be used for the benefit of a person. And there are also continuous questions with the power of the furnace.
Therefore, I will give my calculation method, although not so scientific, but based on my experience in manufacturing such devices.
So, the first thing you need to decide is the power of the furnace. The power directly depends on the size of the muffle and the lining used. You determine the size (volume) of the muffle yourself, depending on the size of the heated products.
For modern stoves using fiber heat insulators (MKRV, ShPV-350, etc.), the approximate power per liter of volume will be:
Furnace chamber volume (liters) Specific power (W/liter)
1-5 500-300
5-10 300-120
10-50 120-80
50-100 80-60
100-500 60-50
Let's say, for example, your chamber volume is 3 liters, so the oven power will be 1200 W. My muffle volume is a little more than a liter, so let’s take the heater power to 500 W.
Next, we calculate the current through the heater :
I = P/U= 500/220 = 2.27 A
And the heater resistance value
R = U/I = 220/2.27 = 97 Ohm
Next, we climb into the bins and look at the diameter of the existing nichrome. I found nichrome with a diameter of 0.65 mm. Next, using the table, we estimate whether our nichrome can withstand such a current.

Diameter (mm) 0.17 0.3 0.45 0.55 0.65 0.75 0.85
Allowable current (A) 1 2 3 4 5 6 7

As you can see, with a diameter of 0.65, the permissible current is 5 A, so it will withstand our 2.27 A with a large margin. In general, when making a heater, you need to take thicker wire, because the thicker the wire, the longer the temperature it can withstand and the service life.
Maximum operating temperatures of heating elements. Here:
GS 40 Nichrome
GS 23-5 Eurofechral
GS SY Superfehral
GS T Eurofehral

BUT! This is a double-edged sword. We cannot greatly thicken the diameter of the wire, because in order to obtain a calculated resistance of 97 Ohms, we will have to greatly increase the length of the wire, which may not be acceptable for design reasons.
Using the table, we determine the nominal resistance of 1 linear meter of wire. Here:
GS 40 Nichrome
GS 23-5 Eurofechral
GS SY Superfehral
GS T Eurofehral

So, from the table for a diameter of 0.65 mm we take (and confirm by subsequent measurement with the device), the nominal resistance is 3.2 ohm/meter. Therefore, the length of the wire will be:
L = R/3.2 = 97/3.2 = 30 Meters
This is the price to pay for the excess wire diameter in excess footage. But this is not a problem, because I will not wind this wire as it is, and there is a danger of not keeping track and allowing an interturn short circuit on our muffle. This wire needs to be wound onto the rod. The tip of the wire together with the rod is clamped into the chuck of a drilling machine, or at worst, the chuck of a hand drill. The wire is fed under slight tension.

When winding, the following recommendations must be observed. The diameter of the rod for winding wire with a diameter of up to 4.5 mm must be no less than:
- for nichrome, four times the diameter of the wire;
- for fechrals, five times the diameter of the wire.
For all alloys with a diameter greater than 4.5 mm, at least six times the diameter of the wire.
There is another ambush when working with fechral. Fechral, ​​unlike nichrome, becomes brittle after calcination, so it is no longer worth beating.
We evenly stretch the finished spiral to a length comfortable for winding the muffle. But no more, because it will be much more difficult to compress evenly. We wrap the muffle along the grooves and apply coating, as in Fig. 4.
Next, we place our muffle in a metal case.

The main lining is made of blocks of lightweight fireclay bricks ШЛ-0.4. Brick is easily processed with the tool previously described. Note the hole in the back block of the lightweight for the thermocouple and two holes for the nichrome leads.
During installation, the side wall of the muffle was damaged, but this is not a big deal, it will be restored with the same compound after installation.

I would like to warn you against some ambushes that may await you when making the lining.
First of all, I want to warn you if you are tempted to use asbestos. Yes, it melts at 1500 degrees, but at 800 degrees it loses chemically bound water and turns into powder. Therefore, products made from it, such as cardboard or cord, can work up to this temperature. In addition, fechral should not come into contact with asbestos. I used it because this stove is sharpened to this temperature and I have nichrome.
Next, regarding the use of liquid glass as a binder. It can be used for sculpting muffles operating up to 1088 degrees; when this temperature is exceeded, the muffle will float. In addition, fechral also does not like contact with liquid glass.
Regarding the use of fibrous materials on a mineral (basalt) basis, I will repeat what I wrote on one of the forums. It's almost the same thing. Produced by melt blowing. Holds temperature well. But they have a binder that will not withstand even 250 degrees. But on the Internet, cunning sellers cite the fire resistance of the fiber itself. Formally, they are right. But they don’t write that after the first calcination the binder will burn out and they will fall off in a heap. There are varieties with a refractory binder, but there is very little information. Only indirect signs - for example, intended for baths and fireplaces. And again the fire resistance of the fiber itself is tested. And needless to say, the fechral doesn’t like them either. So if you have the opportunity to fly, it is better to use already proven ones. And of the ones I tested, mullite-silica felts, for example, MKRVKh-250 (1300 g), are most suitable.
By the way, in Sukhoi Log they have launched the production of ceramic blankets Cerablanket, Cerachem Blanket, Cerachrom Blanket. I dealt with the first of them; it can withstand the direct flame of a burner. The last two are even more fireproof. But I haven't tried them myself.
There are descriptions of furnaces floating around the Internet, which are all being torn apart from each other, in which fireclay clay appears as a muffle material. Ordinary clay has a high shrinkage and is used as a binder. Chamotte is nothing more than baked clay. Fireclay is not molded, it is used as a filler and requires a binder, for example, ordinary unfired clay. Therefore, what is meant by the expression fireclay clay is completely unclear.

Control block.

Since I promised a description of the cheapest, simplest oven, the temperature controller will be appropriate. A good inexpensive regulator Sh-4501, which can be purchased at a price of 1 to 2 thousand rubles. The cheapest and most cheerful regulator. Available with temperature measurement and control ranges from 0-200 to 0-1600 degrees. As a measuring element, thermocouples XK, XA and PP.
Technical description and operating instructions for the Sh4501 regulating millivoltmeter. Read at your leisure.
Front panel of the control unit. This version of the regulator is for the range from 0 to 800 degrees, thermocouple XA.
Below, from right to left, there is a control unit switch, a TLO neon lamp (orange) indicating the voltage supply to the load, a TLZ lamp (green) indicating load disconnection, and a red lamp indicating a broken thermocouple.

Connections on the back side of Ш4501. For those who don't understand, the plastic cover once again shows the wiring diagram. Please pay attention - the compensation wire must go all the way to the terminal block with the compensation coil.
Such fittings for indicator lamps are no longer produced, so I recommend using modern types XB2-EV161. They come in red, yellow, green, white and blue. Electrical diagram of the control unit. If you do not find a sufficiently powerful toggle switch for turning on the control unit, then place it after the contacts of the PE23 relay. The relay comes complete with the Sh4501 device. The power of the relay contacts is 500 VA in the alternating current circuit. The diagram does not show - I have 3 groups of contacts in parallel, so the switching power is up to 1500 VA. The diagram has been corrected - the TLZ lamp is suitable for normally closed contacts, TLO for normally open ones.

Implementation of installation of the control unit in this box. The regulator is tucked into the front of the skis. The connector is connected (on the right). The relay is mounted on the back cover from the inside.

Furnace assembly. Back view. As you can see, the thermocouple wires and heater leads are simply cooled in air, without any frills. The heater wires are connected through a terminal block, preferably with a ceramic base. I recommend using a ceramic socket from a socket or a ceramic lamp socket.
The thermocouple leads are also through the terminal block. A piece of compensation wire corresponding to the graduation is connected to the same terminal block contacts. If this is an ordinary wire, then the device will lie on the value of the temperature difference between this terminal block and the rear panel of the Sh4501 with the measuring coil. An overhead socket for connecting the load is mounted on the outside of the back cover, and a terminal block for connecting a thermocouple is mounted on the back cover of the muffle box. This allows you to use this control unit not only with this muffle, but also for temperature control in your other devices. It is enough to screw a thermocouple of this calibration to the terminal block and insert the plug into the socket.

A little about a homemade thermocouple. For the final budget of our furnace, I used a homemade thermocouple with XA calibration. I prefer homemade thermocouples not out of greed, but simply because they have less inertia compared to factory ones. Although there is a risk of burning the regulator input circuits. I will not dwell in detail on the manufacture of such a thermocouple, because this process is well covered in the literature (Bastanov. 300 practical advice) and on the Internet.

The material was cores from compensation wire of HA calibration. The ends are welded with a tungsten electrode in an argon atmosphere. If you weld it this way, it’s weak, whereas it’s described in books in graphite with borax using a powerful transformer. Then the thermocouple is inserted into a ceramic two-channel MCR tube. At this point, sorry, you'll have to fork out the cash.

Heating chamber assembly. The wall has been finished, the cracks have been sealed. Then some excess putty is applied around the mouth of the muffle. Then it is covered with polyethylene and the lid is closed. The relief of the lid is imprinted on the putty. The polyethylene is removed and the whole thing is dried. The gaps between the cover and the chamber are minimal.

Muffle assembled. After laying the spiral, it is coated with the same composition that the muffle is made of. The ends of the spiral are secured with a loop made of glass tape with mica. Don't forget to put a mounting rod under the spiral. When the muffle dries, the rod is removed and a hole remains for the thermocouple.

Muffle without strapping. Pay attention to the grooves on the corners of the muffle. They are designed to ensure that the spiral does not move during coating. At the bottom there is a groove for a thermocouple. The thermocouple should be in close proximity to the coil.

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 ordinary transformer, no pulse block 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 onto the junction with different temperatures from a hot air gun ( Fig.3), 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 work with unipolar power supply(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 circuit is introduced into it feedback– 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 printed circuit board made of one-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. Packet switch S2 – PV 3-16 (version 1) or similar from the PV or PP series under required quantity 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 radiator was taken from an old one computer unit nutrition.

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

A laboratory muffle furnace is a special high temperature heating equipment, intended for use in laboratory conditions. This device is a furnace with a special design.

It provides complete lack of interaction heated objects with various components released into the air as a result of fuel combustion ( soot, gaseous substances, and soot).

To create such heating conditions, it is used muffle- a fireproof chamber, which is a kind of barrier between the heated product and the fuel used.

What is a laboratory muffle furnace?

Most of these muffles are made from fire-resistant brick, heat-resistant steel or high strength ceramic fiber. It is thanks to this device that manufacturers have the opportunity to prevent contamination of various expensive metals, as well as chemically pure samples, by foreign substances.

Due to the fact that the equipment has special technical characteristics, it Suitable for use in many areas industry:

• V chemical laboratories;
• at enterprises engaged in production jewelry;
• V geophysical laboratories;
• at enterprises that produce wax objects;
• V food industry;
• at enterprises performing cupellation of various precious metals;
• V dental centers;
• to perform various analytical work(heating and drying, burning or growing crystals);
• For firing various forms for casting;
• for the manufacture of porcelain or ceramic products;
• For swimming trunks, and hardening of various metals and their alloys;
• for cremation.

Modern equipment must have the following characteristics:

1. Sufficient inner space so that the objects being processed fit freely inside the device.
2. Large temperature range, allowing you to perform different types works
3. Thermostat.
4. System hoods.
5. Opportunity connecting to a computer(the requirement applies to some models of devices).

Design Features

The equipment has a special structure, which is adapted to create special conditions for processing various products. Main difference from other types of ovens is the availability fireproof chamber or the so-called muffle. This creates a barrier that prevents the surface of the materials from interacting with gaseous substances released from the fuel used.

For making a muffle- the main part of the device - and other elements, manufacturers usually use heat-resistant steel, refractory brick, and also ceramic fiber, which has high strength.

Photo 1. Schematic representation of the structure of a laboratory muffle furnace. Only the main parts are indicated.

How to choose the right device?

To operate the equipment as efficiently as possible, you need to pay attention to the following: characteristics:

• options;
• maximum possible loads;
• power;
• maximum firing temperature;
• operating voltage;
• supply voltage;
• uniform heating;
• safety of equipment operation;
• price.

First of all, you need to decide volume working chamber, as well as temperature range . In addition, it is imperative to pay attention to heating difficulty.

Types of laboratory furnaces

No less important indicators when choosing equipment are speed and uniform heating muffle chamber.

Depending on individual requirements, you can choose horizontal or vertical oven: the first has a fairly large capacity, and the second heats up in a short period of time.

Laboratory muffle furnaces are equipped with open or closed heating elements. Devices of the first type are ideal for use in conditions where it is necessary to warm up the chamber to high temperature in a short time. However, such equipment is more susceptible to the negative effects of various aggressive substances released during the processing of objects.

Ovens that use a closed heating element differ longer service life, uniform heating working chamber, but it takes much longer to warm up to maximum. A significant disadvantage of devices of this type is that if the heating element breaks down, the entire chamber will have to be replaced.

The simplest design is equipment that has single stage thermostat. Its main peculiarity— from the very beginning, the chamber is heated to a certain temperature, and then it is maintained throughout the working process. Most often, these furnaces are used to perform such simple tasks like drying or firing.

For more complex analytical work, muffle furnaces are intended, which operate due to special program control.

They allow you to adjust the heating process to several different levels. Control occurs using a microprocessor with a digital indicator and an audible alarm.

If necessary, the program can be launched automatically.

To choose a working oven, need to check equipment for the absence of any mechanical damage(chips, abrasions, scratches and others) on all components.

Useful video

Watch a video showing what a large volume muffle furnace for metal processing looks like.

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:

1. 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.
2. The electric muffle furnace allows for some creativity, as long as all necessary conditions security.

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. On this 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 body, 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-production 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 as suitable material basalt wool, heat-resistant glue, which is purchased in the store, and fireclay brick 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 sheathed 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 companies such 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.

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 maximum temperature 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. Foam glass. - Minsk, 1975). 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 a method for 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 from siliceous rocks. - 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 goal 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 Sahaptinskoe deposit (Krasnoyarsk Territory) of the following was used as a siliceous raw material 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. Technological process does not require high temperatures, which makes production cost-effective.

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.