Plasma generator - plasma torch
If a solid is heated strongly, it will turn into a liquid. If you raise the temperature even higher, the liquid will evaporate and turn into a gas.
But what happens if you continue to increase the temperature? Atoms of matter will begin to lose their electrons, turning into positive ions. Instead of gas, a gaseous mixture is formed, consisting of freely moving electrons, ions, and neutral atoms. It's called plasma.
Nowadays, plasma is widely used in various fields of science and technology: for the heat treatment of metals, the application of various coatings on them, melting and other metallurgical operations. In recent years, plasma has been widely used by chemists. They found that the speed and efficiency of many chemical reactions greatly increases in a plasma jet. For example, by introducing methane into a hydrogen plasma jet, it can be converted into very valuable acetylene. Or arrange oil vapors into a number of organic compounds - ethylene, propylene and others, which later serve as an important raw material for the production of various polymeric materials.
Scheme of a plasma generator - plasma torch
1 - plasma jet;
3 - arc discharge;
4 - gas "spin" channels;
5 - refractory metal cathode;
6 - plasma gas;
7 - electrode holder;
8 - bit camera;
9 - solenoid;
10 - copper anode.
How to create plasma? For this purpose, a plasma torch, or a plasma generator, serves.
If you place metal electrodes in a vessel with gas and apply a high voltage to them, an electric discharge will occur. There are always free electrons in a gas. Under the action of an electric current, they accelerate and, colliding with neutral gas atoms, knock out electrons from them and form electrically charged particles - ions, i.e. ionize atoms. The released electrons are also accelerated by the electric field and ionize new atoms, further increasing the number of free electrons and ions. The process develops like an avalanche, the atoms of the substance are very quickly ionized and the substance turns into plasma.
This process takes place in an arc plasma torch. A high voltage is created in it between the cathode and the anode, which can be, for example, a metal that needs to be processed using plasma. In the space of the discharge chamber, a plasma-forming substance is most often supplied with gas - air, nitrogen, argon, hydrogen, methane, oxygen, etc. Under the action of high voltage, a discharge occurs in the gas, and a plasma arc is formed between the cathode and anode. To avoid overheating of the walls of the discharge chamber, they are cooled with water. Devices of this type are called plasma torches with an external plasma arc. They are used for cutting, welding, melting metals, etc.
The plasma torch for creating a plasma jet is arranged somewhat differently. The plasma-forming gas is blown through a system of spiral channels at high speed and “ignited” in the space between the cathode and the walls of the discharge chamber, which are the anode. Plasma, swirling into a dense jet due to spiral channels, is ejected from the nozzle, and its speed can reach from 1 to 10,000 m/s. The magnetic field, which is created by the inductor, helps to “squeeze” the plasma from the walls of the chamber and make its jet denser. The temperature of the plasma jet at the outlet of the nozzle is from 3000 to 25000 K.
Look again at this drawing. Does it remind you of something well known?
Of course, it's a jet engine. The thrust force in a jet engine is created by a jet of hot gases ejected at high speed from a nozzle. The greater the speed, the greater the traction force. What's wrong with plasma? The speed of the jet is quite suitable - up to 10 km / s. And with the help of special electric fields, plasma can be accelerated even more - up to 100 km / s. This is about 100 times the speed of gases in existing jet engines. This means that the thrust of plasma or electric jet engines can be greater, and fuel consumption can be significantly reduced. The first samples of plasma engines have already been tested in space.
To cut a thick metal workpiece, you can use three tools: a grinder, an oxygen gas torch and a plasma welding machine. With the help of the first one, an even and neat cut is obtained, but only in a straight line, the second one can cut patterns, but the cut is obtained with metal influxes and torn. But the third option is smooth cut edges that do not need additional processing. In addition, metal can be cut in this way along any curved line. True, the plasma torch is not cheap, so many home craftsmen have a question, is it possible to make this device on their own. Of course, you can, the main thing is to understand the principle of operation of the plasma torch.
And the principle is quite simple. An electrode made of durable and heat-resistant material is installed inside the cutter. In fact, it is a wire to which an electric current is applied. An arc is ignited between it and the cutter nozzle, which heats the space inside the nozzle up to 7000C. After that, compressed air is supplied into the nozzle. It heats up and ionizes, that is, it becomes a conductor of electric current. Its electrical conductivity becomes the same as that of a metal.
It turns out that the air itself is a conductor, which, when in contact with the metal, forms a short circuit. Since compressed air has a high pressure, it tries to exit the nozzle at high speed. This ionized air with high speed is plasma, the temperature of which is more than 20,000C.
In this case, in contact with the metal being cut, an arc is formed between the plasma and the workpiece, as is the case with electrode welding. The heating of the metal occurs instantly, the heating area is equal to the cross section of the hole in the nozzle. The metal of the part being cut immediately passes into a liquid state and is blown out of the cut by plasma. This is how cutting happens.
From the principle of operation of the plasma cutting machine, it becomes clear that this process will require an electrical power source, a source of compressed air, a torch, which includes a nozzle made of heat-resistant material, cables for supplying electricity and hoses for supplying compressed air.
Since we are talking about a plasma torch that will be assembled by hand, it is necessary to take into account the moment that the equipment should be inexpensive. Therefore, a welding inverter is selected as a power source. This is an inexpensive device with a good stable arc, with its help you can save a lot on the consumption of electric current. True, they can cut metal blanks with a thickness of not more than 25 mm. If there is a need to increase this indicator, then you will have to use a welding transformer instead of an inverter.
As for the source of compressed air, then there should not be any problems. A conventional compressor with a pressure of 2-2.5 atmospheres will perfectly maintain a stable arc for cutting. The only thing you need to pay attention to is the volume of air released. If the process of cutting metals is long, then the compressor may not withstand such intensive work. Therefore, it is recommended to install a receiver after it. In fact, this is a container in which air will accumulate under the required pressure. Here it is important to make adjustments so that the pressure drop in the receiver immediately causes the compressor to turn on to fill the tank with compressed air. It should be noted that compressors complete with a receiver are now sold as a single complex.
The most difficult element of the plasma torch to manufacture is a burner with a nozzle. The easiest option is to buy a ready-made nozzle, or rather several of its types with different diameters of its opening. Thus, by changing the nozzle, cutting of different widths can be carried out. The standard diameter is 3 mm. Some of the home craftsmen make do-it-yourself nozzles from heat-resistant metals, which are not so easy to get. So it's easier to buy.
The nozzle is installed on the cutter, it is simply screwed onto the end of the burner. If an inverter is used in a home-made plasma torch, then its kit includes a handle on which you can put the purchased nozzle.
Mandatory elements of a plasma torch are a welding cable and a hose. They are usually combined in one set, which creates the convenience of their use. It is recommended to insulate the double element, for example, install it inside a rubber hose.
And one more element of a homemade plasma torch is an oscillator. Its purpose is to ignite the arc at the very beginning of work, that is, this device creates a primary spark to ignite a non-consumable electrode. At the same time, there is no need to touch the metal surface with the end of the consumable. Oscillators operate on both AC and DC. If in factory devices this device is installed inside the equipment case, then in home-made devices it can be installed next to the inverter by connecting it with wires.
It must be understood that the oscillator is intended only for ignition of the arc. That is, after its stabilization, the device must be turned off. The connection scheme is based on the use of a relay, which controls the stabilization process. After switching off the device, the arc works directly from the inverter.
As you can see, no drawings are needed to assemble a plasma torch with your own hands. The whole assembly is quite simple, the main thing is to follow the safety rules. For example, a welding cable is bolted together, compressed air hoses are connected with factory crimps and clamps.
How a homemade plasma torch works
In principle, a homemade plasma torch works in exactly the same way as a factory one. True, he has his own resource, which depends mainly on the material from which the nozzle is made.
- First, the oscillator and inverter are turned on, through which the current is supplied to the electrode. It is ignited. The ignition is controlled by a button located on the burner handle.
- 10-15 seconds, during this time the duty arc will fill the entire space between the electrode and the nozzle. Now you can supply compressed air, because during this time the temperature inside the nozzle will reach 7000C.
- As soon as the plasma escapes from the nozzle, you can proceed to the metal cutting process.
- It is very important to correctly guide the torch along the intended cutting contour. For example, if the speed of the cutter advance is not very high, then this is a guarantee that the width of the cut will be large, plus the edges will be exactly uneven with sagging and clumsy. If the speed of the cutter, on the contrary, is high, then the molten metal will be poorly blown out of the cutting zone, which will lead to the formation of a torn cut, its continuity will be lost. Therefore, it is necessary to select the cutting speed experimentally.
It is very important to choose the right material for the manufacture of the electrode. Most often, hafnium, beryllium, thorium or zirconium are used for this. In the process of high temperatures acting on them, refractory oxides of these metals are formed on the surface, so that the electrode from them is destroyed slowly. True, heated beryllium becomes radioactive, and thorium begins to release toxic substances. Therefore, the best option is an electrode made of hafnium.
Stabilization of the pressure at the outlet of the receiver is provided by the installed reducer. It is inexpensive, but it solves the problem of uniform supply of compressed air to the torch nozzle.
All work on the operation of a home-made plasma cutting machine should be carried out only in protective clothing and shoes. Be sure to wear gloves and goggles.
As for the size of the nozzle, it is not recommended to make it very long. This leads to its rapid destruction. In addition, it is very important to correctly set the cutting mode. The thing is that sometimes in home-made plasma cutters not one arc appears, but two. This negatively affects the operation of the device itself. And of course, this reduces its lifespan. It's just that the nozzle starts to break down faster. Yes, and the inverter may not withstand such a load, so there is a possibility of its failure.
And the last. A characteristic feature of this type of metal cutting is its melting only in the place that is affected by the plasma flow. Therefore, it is necessary to ensure that the cut spot is in the center of the end of the electrode. Even a minimal displacement of the spot will lead to a deflection of the arc, which will create conditions for the formation of an incorrect cut, and, accordingly, a decrease in the quality of the process itself.
As you can see, the drawing of the cutting process depends on many factors, therefore, when assembling a plasma torch without the help of specialists with your own hands, it is necessary to strictly comply with all the requirements for each element and device. Even small deviations will reduce the quality of the cut.
Almost everyone who was interested in energy has heard about the prospects of MHD generators. But the fact that these generators have been in the status of promising for more than 50 years is known to few. The problems associated with plasma MHD generators are described in the article.
History with plasma, or magnetohydrodynamic (MHD) generators remarkably similar to the situation with . It seems that it takes only one step or a little effort, and the direct conversion of heat into electrical energy will become a familiar reality. But another problem postpones this reality indefinitely.
First of all, about terminology. Plasma generators are one of the varieties of MHD generators. And those, in turn, got their name from the effect of the appearance of an electric current during the movement of electrically conductive liquids (electrolytes) in a magnetic field. These phenomena are described and studied in one of the sections of physics - magnetohydrodynamics. This is where generators got their name from.
Historically, the first experiments to create generators were carried out with electrolytes. But the results showed that it is very difficult to accelerate electrolyte flows to supersonic speeds, and without this, the efficiency (efficiency factor) of generators is extremely low.
Further research was carried out with high-speed ionized gas flows, or plasmas. Therefore, today, speaking about the prospects for using MHD generators, it must be borne in mind that we are talking exclusively about their plasma variety.
Physically, the effect of the appearance of a potential difference and an electric current when charges move in a magnetic field is similar. Those who have worked with Hall sensors know that when current passes through a semiconductor placed in a magnetic field, a potential difference appears on the crystal plates perpendicular to the lines of the magnetic field. Only in MHD generators, a conducting working fluid is passed instead of a current.
The power of MHD generators directly depends on the conductivity of the substance passing through its channel, the square of its velocity and the square of the magnetic field strength. From these relationships it is clear that the greater the conductivity, temperature and field strength, the higher the power taken.
All theoretical studies on the practical conversion of heat into electricity were carried out back in the 50s of the last century. And a decade later, pilot plants "Mark-V" in the USA with a capacity of 32 MW and "U-25" in the USSR with a capacity of 25 MW appeared. Since then, various designs and effective operating modes of generators have been tested, various types of working bodies and structural materials have been tested. But plasma generators have not reached wide industrial use.
What do we have today? On the one hand, a combined power unit with an MHD generator with a capacity of 300 MW is already operating at Ryazanskaya GRES. The efficiency of the generator itself exceeds 45%, while the efficiency of conventional thermal power plants rarely reaches 35%. The generator uses plasma with a temperature of 2800 degrees, obtained by burning natural gas, and.
It would seem that plasma energy has become a reality. But similar MHD generators in the world can be counted on the fingers, and they were created in the second half of the last century.
The first reason is obvious: generators require high-temperature structural materials to operate. Some of the materials were developed within the framework of thermonuclear fusion programs. Others are used in rocket science and are classified. In any case, these materials are extremely expensive.
Another reason lies in the peculiarities of the operation of MHD generators: they produce exclusively direct current. Therefore, powerful and economical inverters are required. Even today, despite the advances in semiconductor technology, this problem has not been fully resolved. And without this, it is impossible to transfer huge capacities to consumers.
The problem of creating superstrong magnetic fields has not been completely solved either. Even the use of superconducting magnets does not solve the problem. All known superconducting materials have a critical magnetic field strength, above which superconductivity simply disappears.
One can only guess what can happen with a sudden transition to the normal state of conductors in which the current density exceeds 1000 A/mm2. An explosion of windings in close proximity to a plasma heated to almost 3000 degrees will not cause a global catastrophe, but an expensive MHD generator will certainly disable it.
The problems of plasma heating to higher temperatures remain: at 2500 degrees and additions of alkali metals (potassium), the plasma conductivity, however, remains very low, incommensurable with the conductivity of copper. But an increase in temperature will again require new heat-resistant materials. The circle closes.
Therefore, all power units with MHD generators created to date demonstrate the level of achieved technologies rather than economic feasibility. The country's prestige is an important factor, but building expensive and capricious MHD generators on a massive scale today is very expensive. Therefore, even the most powerful MHD generators remain in the status of pilot plants. On them, engineers and scientists work out future designs, test new materials.
It is difficult to say when this work will end. The abundance of various designs of MHD generators suggests that the optimal solution is still far away. And the information that thermonuclear fusion plasma is the ideal working medium for MHD generators postpones their widespread use until the middle of our century.
Science knows for sure: the conversion of heat into work is the more profitable, the stronger the steam is heated. If the steam temperature is raised to 1000-1500°C at a conventional modern power plant, its efficiency will automatically increase one and a half times. But the trouble is that this cannot be done in any way, because such a terrible heat will very quickly destroy any turbine.
So, the scientists reasoned, we should try to do without a turbine at all. It is necessary to build a generator that would itself convert the energy of a jet of hot gas into an electric current! And they built it. A rapidly developing science, magnetohydrodynamics, which studies the movement of fluids that conduct electric current in a magnetic field, helped build a plasma power generator.
It was found that a liquid conductor placed in a magnetic field does not differ in behavior from a solid conductor, such as a metal. But we know well what happens in a metal conductor if it is moved between the poles of a magnet: an electric current is induced (induced) in it. This means that the current will also appear in the liquid jet if this jet crosses the magnetic field.
However, it was still not possible to build a generator with a liquid conductor. The jet of liquid had to be accelerated to a very high speed, and this required an enormous amount of energy, most of which was lost in the jet itself in turbulence. It was then that the thought came up: why not replace the liquid with a gas? After all, we have long been able to communicate huge speeds to gas jets - remember at least a jet engine. But this thought had to be discarded immediately: not a single gas conducts current.
It turned out like a complete dead end. Solid conductors do not withstand high temperatures; liquid ones do not accelerate to high speeds; gaseous are not conductors at all. But…
We are accustomed to thinking that matter can exist only in three states - solid, liquid and gaseous. And it, after all, also happens in the fourth state - plasma. Plasma, as is known, consists of the Sun and most stars. Here it is - a plasma power generator!
Plasma is a gas, but ionized
In it, among the molecules, charged ions come across, that is, "fragments" of atoms with disturbed electronic orbits. There are also free electrons. Ions and electrons are carriers of electric charges, which means that the plasma is electrically conductive.
But in order to get a plasma, it is necessary to heat the gas harder. As the temperature rises, the gas molecules move faster and faster, they often and strongly collide with each other. There comes a moment when the molecules gradually disintegrate into atoms. But the gas does not conduct current. Let's keep heating it up!
Here the thermometer showed 4000 °. Atoms have acquired high energy. Their speeds are enormous, and individual collisions end "catastrophically": the electron shells of atoms are broken. This is what we need - now there are ions and electrons in the gas - plasma has appeared.
Heating a gas to 4000° is no easy task. The best grades of coal, oil and natural gases give a much lower temperature when burned. How to be?
Scientists have coped with this difficulty. Rescued potassium - a cheap and common alkali metal. It turned out that in the presence of potassium, the ionization of many gases begins much earlier. It is worth adding only one percent of potassium to ordinary flue gases - the products of combustion of coal and oil, as ionization in them begins at 3000 ° and even a little lower.
From the furnace, where hot gases are born, they are diverted into a branch pipe, where potash - potassium carbonate - is continuously fed in a thin stream. There is a weak, but still sufficient ionization. The nozzle then expands smoothly to form a nozzle.
The properties of the expanding nozzle are such that when moving through it, the gas gains high speed, losing pressure. The speed of gases escaping from the nozzle can compete with the speeds of modern aircraft - it reaches 3200 km / h.
An incandescent plasma stream bursts into the main channel of the generator
Its walls are not made of metal, but of quartz or refractory ceramics. Outside, the poles of the strongest magnet are brought to the walls. Under the action of a magnetic field in a plasma, as in any conductor, an electromotive force is induced.
Now it is necessary, as electricians say, to "remove" the current, to take it to the consumer. To do this, two electrodes are introduced into the channel of the plasma generator - also, of course, non-metallic, most often graphite. If they are closed by an external circuit, then a direct current will appear in the circuit.
For small plasma power generators, already built in different countries, the efficiency reached 50% (the efficiency of a thermal power plant is not more than 35-37%). Theoretically, you can get 65%, and even more. Scientists working on a plasma generator face many challenges related to the choice of materials, with the increase in the life of the generator (current designs so far only work for minutes).
