Lightning is a spark discharge of an electrostatic charge of a cumulus cloud, accompanied by a blinding flash and a sharp sound (thunder). Thus, we should consider in detail the classification of discharges and understand why lightning flashes.
Types of discharges
dark (Townsend);
crown;
spark
Spark discharge
This discharge is characterized by an intermittent form (even when using sources direct current). It usually occurs in gases at pressures on the order of atmospheric pressure. In natural natural conditions spark discharge is observed in the form of lightning. Externally, a spark discharge is a bunch of bright zigzag branching thin strips that instantly penetrate the discharge gap, quickly extinguish and constantly replace each other. These strips are called spark channels. They start from both positive and negative, and from any point in between. The channels developing from the positive electrode have clear thread-like outlines, while those developing from the negative electrode have diffuse edges and finer branching.
Because Since a spark discharge occurs at high gas pressures, the ignition potential is very high. (For dry air, for example, at a pressure of 1 atm. and a distance between the electrodes of 10 mm, the breakdown voltage is 30 kV.) But after the discharge gap becomes a “spark” channel, the resistance of the gap becomes very small, a short-term current pulse passes through the channel great strength, during which there is only a small amount of resistance per discharge gap. If the source power is not very high, then after such a current pulse the discharge stops. The voltage between the electrodes begins to rise to its previous value, and the gas breakdown is repeated with the formation of a new spark channel.
An electric spark occurs if the electric field in a gas reaches a certain specific value Ek (critical field strength or breakdown strength), which depends on the type of gas and its state. For example, for air under normal conditions Ek3 * 106 V/m.
The value of Ek increases with increasing pressure. The ratio of the critical field strength to the gas pressure p for a given gas remains approximate over a wide range of pressure changes: Ek/pconst.
The greater the capacitance C between the electrodes, the longer the voltage rise time. Therefore, turning on a capacitor parallel to the discharge gap increases the time between two subsequent sparks, and the sparks themselves become more powerful. A large spark passes through the spark channel electric charge, and therefore the amplitude and duration of the current pulse increases. With a large capacitance C, the spark channel glows brightly and has the appearance of wide stripes. The same thing happens when the power of the current source increases. Then they talk about a condensed spark discharge, or a condensed spark. Maximum strength The current in the pulse during a spark discharge varies widely, depending on the parameters of the discharge circuit and the conditions in the discharge gap, reaching several hundred kiloamperes. With a further increase in source power, the spark discharge turns into an arc discharge.
As a result of the passage of a current pulse through the spark channel, a spark is released in the channel a large number of energy (about 0.1 - 1 J per centimeter of channel length). The release of energy is associated with an abrupt increase in pressure in the surrounding gas - the formation of a cylindrical shock wave, the temperature at the front of which is ~104 K. This occurs rapid expansion spark channel, with a speed on the order of the thermal speed of gas atoms. As the shock wave advances, the temperature at its front begins to drop, and the front itself moves away from the channel boundary. Emergence shock waves are explained sound effects, accompanying a spark discharge: characteristic crackling in weak discharges and powerful peals in the case of lightning.
At the time of the channel’s existence, especially when high pressures, a brighter glow of the spark discharge is observed. The brightness of the glow is nonuniform over the cross section of the channel and has a maximum in its center.
Let's consider the spark discharge mechanism.
Currently, the so-called streamer theory of spark discharge, confirmed by direct experiments, is generally accepted. Qualitatively, it explains the main features of a spark discharge, although quantitatively it cannot be considered complete. If an electron avalanche originates near the cathode, then along its path there is ionization and excitation of gas molecules and atoms. It is essential that light quanta, emitted by excited atoms and molecules, propagating towards the anode at the speed of light, themselves produce ionization of the gas, and give rise to the first electron avalanches. In this way, weakly glowing accumulations of ionized gas, called streamers, appear throughout the entire volume of gas. In the process of their development, individual electron avalanches catch up with each other and, merging together, form a well-conducting bridge of streamers. Therefore, at the next moment in time, a powerful flow of electrons rushes, forming a spark discharge channel. Since the conducting bridge is formed as a result of the merger of streamers that appear almost simultaneously, the time of its formation is much less than the time required for an individual electron avalanche to travel the distance from the cathode to the anode. Along with negative streamers, i.e. streamers propagating from the cathode to the anode, there are also positive streamers that propagate in the opposite direction.
Free electrons receive enormous accelerations in such a field. These accelerations are directed downward, since the lower part of the cloud is negatively charged, and the surface of the earth is positively charged. On the way from the first collision to the next, the electrons acquire significant kinetic energy. Therefore, when they collide with atoms or molecules, they ionize them. As a result, new (secondary) electrons are born, which, in turn, are accelerated in the field of the cloud and then ionize new atoms and molecules in collisions. Whole avalanches of fast electrons appear, forming clouds at the very “bottom”, plasma “threads” - a streamer.
Merging with each other, the streamers give rise to a plasma channel through which the main current pulse will subsequently pass. This plasma channel developing from the “bottom” of the cloud to the surface of the earth is filled with free electrons and ions, and therefore can conduct electric current well. He is called a leader, or more precisely a stepped leader. The fact is that the channel is not formed smoothly, but in jumps - “steps”.
Why there are pauses in the leader’s movement, and relatively regular ones at that, is not known for sure. There are several theories of stepped leaders.
In 1938, Schonland put forward two possible explanations for the delay that causes the step-like nature of the leader. According to one of them, electrons should move down the channel of the leading streamer (pilot). However, some electrons are captured by atoms and positively charged ions, so that it takes some time for new advancing electrons to arrive before there is a potential gradient sufficient for the current to continue. According to another point of view, time is required for positively charged ions to accumulate under the head of the leader channel and, thus, create a sufficient potential gradient across it. In 1944, Bruce proposed a different explanation, which was based on the development of a glow discharge into an arc discharge. He considered a "corona discharge", similar to a tip discharge, existing around the leader channel, not only at the head of the channel, but along its entire length. He explained that the conditions for the existence of an arc discharge will be established for some time after the channel has developed over a certain distance and, therefore, steps have arisen. This phenomenon has not yet been fully studied and there is no specific theory yet. And here physical processes, occurring near the leader's head are quite understandable. The field strength under the cloud is quite high - it is B/m; in the area of space directly in front of the leader's head it is even greater. The increase in field strength in this region is well explained by Fig. 4, where the dashed curves show sections of equipotential surfaces, and the solid curves show the field strength lines. In a strong electric field near the leader head, intense ionization of atoms and air molecules occurs. It occurs due to, firstly, the bombardment of atoms and molecules by fast electrons escaping from the leader (the so-called impact ionization), and, secondly, the absorption of photons by atoms and molecules ultraviolet radiation, emitted by the leader (photoionization). Due to the intense ionization of atoms and air molecules encountered on the path of the leader, the plasma channel grows, the leader moves towards the surface of the earth.
Taking into account stops along the way, it took the leader 10...20 ms to reach the ground at a distance of 1 km between the cloud and earth's surface. Now the cloud is connected to the ground by a plasma channel that perfectly conducts current. The channel of ionized gas seemed to short-circuit the cloud with the earth. This completes the first stage of development of the initial impulse.
The second stage proceeds quickly and powerfully. The main current flows along the path laid by the leader. The current pulse lasts approximately 0.1 ms. The current strength reaches values of the order of A. A significant amount of energy is released (up to J). The gas temperature in the channel reaches. It is at this moment that an extraordinary bright light, which we observe when lightning strikes, and thunder occurs, caused by the sudden expansion of suddenly heated gas.
It is significant that both the glow and the heating of the plasma channel develop in the direction from the ground to the cloud, i.e. down up. To explain this phenomenon, let us conditionally divide the entire channel into several parts. As soon as the channel has formed (the head of the leader has reached the ground), first of all the electrons that were in its lowest part jump down; therefore, the lower part of the channel first begins to glow and warm up. Then electrons from the next (higher part of the channel) rush to the ground; the glow and heating of this part begin. And so gradually - from bottom to top - more and more electrons are included in the movement towards the ground; As a result, the glow and heating of the channel propagate in the direction from bottom to top.
After the main current pulse has passed, there is a pause lasting from 10 to 50 ms. During this time, the channel practically goes out, its temperature drops, and the degree of ionization of the channel decreases significantly.
However, the cloud still retains a large charge, so new leader rushes from the cloud to the ground, preparing the way for a new impulse of current. The leaders of the second and subsequent strikes are not stepped, but arrow-shaped. Arrowhead leaders are similar to the steps of a stepped leader. However, since the ionized channel already exists, the need for a pilot and stages is eliminated. Since the ionization in the channel of the swept leader is “older” than that of the stepped leader, recombination and diffusion of charge carriers occurs more intensely, and therefore the degree of ionization in the channel of the swept leader is lower. As a result, the speed of the swept leader is less than the speed of the individual stages of the stepped leader, but greater than the speed of the pilot. The speed values of the swept leader range from to m/s.
If more time than usual elapses between subsequent lightning strikes, the degree of ionization may be so low, especially in the lower part of the channel, that a new pilot becomes necessary to re-ionize the air. This explains individual cases the formation of steps at the lower ends of the leaders, preceding not the first, but the subsequent main lightning strikes.
As stated above, the new leader follows the path that was blazed by the original leader. It runs all the way from top to bottom without stopping (1ms). And again a powerful pulse of the main current follows. After another pause, everything repeats. As a result, several powerful impulses are emitted, which we naturally perceive as a single lightning discharge, as a single bright flash.
Basic conditions for logging into the system
Consumption (Nm3/h) 140.544
Consumption (kg/h) 192,000
H2O in gas (% volume) 2.3
CO2 in gas (% volume) 12.4
O2 in gas (% volume) 3.7
Temperature (°C) 270
Hours of operation (hours per year) 8,760
Design working pressure Positive
Dust load at system inlet PM (mg/Nm3) 512
Guaranteed output dust level PM (mg/Nm3) 10
PM system dust removal efficiency (%) 98.05
Other
Source of pollution cat cracking
Expected energy consumption (kW) 136
Full load consumption (kW) 279
Total pressure loss (mm in st)
Scope of delivery
Electrostatic precipitator (electrostatic precipitator):
We offer you one modular electrostatic precipitator, Model 39R-1330-3712P, which includes all plates, discharge electrodes, roof sections, insulation compartments, access doors, all internal components and power supplies to create a complete air pollution control module.
The electrostatic precipitator will have the following design features:
Pressure drop (mm in st) 12.7
Design temperature of the structure (gr C) 371
Design pressure of the structure (mm in st) +/- 890
Hopper volume (m3) 152
Number of bunkers 3
Neck dimensions 457 x 864
Number of gas passages 39
Transformer output voltage (kV) 55
Transformer output current (mA) 1100
Number of transformers 3
New, heavier design style settling plates made from solid steel sheets with a minimum thickness of 18mm. The sheets have a more rigid rigidity relief in the form of a box reinforced with stiffening ribs, which form a smooth flow of gas on the surface of the plate to minimize its re-entrapment. Both upper and lower guides, stiffeners and fasteners will ensure the alignment of the plates, compensating thermal expansion. The plates will be designed for maximum temperature up to 371 ° C
The design provides electromagnetic lifts and shakers with gravitational influence. Shaking systems will be designed to operate automatically and will be designed to minimize particle recirculation. The operating parameters of the shaker will have adjustable frequency and intensity characteristics.
The design includes rigid electrodes, which will be made of a seamless tube with a wall thickness of 1.7 mm with evenly distributed corona pins welded to the tube. The electrodes are level stabilized for operation in all temperature ranges of the precipitator.
Each discharge electrode frame will vibrate individually, and the system will be designed so that both the duration and frequency of vibration can be varied.
The precipitator is equipped with step transformers/rectifiers. Each kit is installed externally, equipped with oil insulation, and the rectifier is air cooled. The transformer and rectifiers are located in a single tank.
The transformer will be equipped with a grounding switch and a key lock. Each kit will be rated for a temperature of max + 45 degrees C (at maximum temperature environment+50 degrees C).
Insulators high voltage cylindrical, under compressive load.
The insulators are porcelain, glazed inside and out and have grounding terminals. The insulators are located outside the gas processing area and are cleaned with purge air.
The precipitator is equipped with safety locks with sequential arrangement keys to prevent access to any high voltage equipment without locking the power supply and grounding the high voltage equipment. The following equipment will be locked: all precipitator quick access doors, transformer/rectifier and high voltage circuit breakers.
The scope of supply includes welded weather-resistant individual insulation compartments for insulators. Isolation compartments will be accessible by doors with safety interlocks to prevent access to all high voltage areas unless the precipitator is de-energized and grounded.
The electrostatic precipitator body will be made of 4.8 mm thick ASTM A-36 steel with external structural elements stiffness ASTM A-36, which strengthen the structure to withstand internal pressure, wind, and other loads. The body is sealed by welding to form a completely gas-tight structure.
The precipitator is equipped with bins with a transverse tray. Each hopper is constructed from 3.8mm thick ASTM A-36 steel, which is reinforced with ASTM A-36 ribs. Each hopper is designed to support its weight when filled with particles. Particle density is 1041 kg/m3 for structural screening and 320 kg/m3 for hopper size. In addition, the bins will have sufficient capacity to store particles collected during a minimum period of 12 hours of operation. The side will be sloped to provide a minimum hopper wall angle of 60 degrees from horizontal. The end angle will be adjusted to provide a minimum hopper angle of 55 degrees.
Precipitator Supports: The precipitator will include all steel structures with self-lubricating sliding plates between the precipitator and the support structure. The structure will be designed to provide a clearance of 2438mm - 0mm between the hopper discharge and the ground.
Connections: The precipitator is equipped with flanged inlet and outlet connections. The pipes are made of ASTM A-36 steel with external stiffeners.
Inlet pipe: the inlet pipe is a horizontal pyramid type inlet with the lower angle of the pipe being 45 degrees from the horizontal. The inlet nozzle includes three distribution devices to ensure uniform flow through the precipitator. Organization of external access to the pipe is not required.
Outlet: The outlet is a horizontal pyramid type with the lower angle of the outlet 60° from the horizontal. The outlet pipe includes a flow distribution device to ensure uniform flow through the electrostatic precipitator. No access required.
Thermal Insulation and External Covering: The manufacturer will provide factory thermal insulation for the electrostatic precipitator (including housing, hopper, inlet and outlet pipes). Insulation will consist of 76mm thick 128kg/m3 density mineral wool on all surfaces except the electrostatic precipitator roof. The precipitator roof will be insulated with 152mm of 128kg/m3 density mineral wool plus 51mm fiberglass insulation over the stiffeners and then covered with a 6.4mm thick 'checkered plate' casing.
The insulation on the inlet, outlet and sides of the electrostatic precipitator will be covered with unpainted 0.8mm thick aluminum sheet type 3003, 1 x 4 box ribbed aluminum sheet or painted corrugated steel. The sheets will be installed vertically and will cover all seams in one section. The thermal insulation of the bins will be covered with unpainted 0.8mm thick aluminum sheet type 3003, 1 x 4 box ribbed aluminum sheet or painted corrugated steel. All roof joints will also be covered with flat materials.
The cover material will be secured using TEK No. 4.5 12-24 x 1¼" Weather Mounting Screws with Neoprene Washers. All sheet to sheet joints will be made using ¼ - 14 x 7/8" Pins with Neoprene Washers. All roof seams will be sealed with clear silicone sealant.
Painting: The factory will paint the structural supports, access hatches, insulation compartments, handrails and exterior roof surfaces with one coat of red primer and one coat of industrial enamel paint. All hot metal surfaces that will be exposed after thermal insulation is completed will be painted with high temperature black paint. All stairs, platforms (including supports) and railings will be painted yellow for safety.
ELECTRICAL CONTROL: The following electrical control equipment will be provided in the project.
Protection class of equipment on the roof: Protection class 4 is established in accordance with EEMAC for the equipment on the roof of the precipitator, namely the control panel of the deposition plate shaker and the control panel of the electrode vibrator.
Blower Control Panel: The EEMAC Class 4 roof mounted blower control panel will be equipped with a built-in starter and start/stop controls.
T/R Controller: Each high voltage transformer/rectifier will be equipped with a microprocessor control panel in an EEMAC class 12 panel and the panel shall be installed in the customer's control room. All components of the panel will be accessible for maintenance through the hinged front door. Voltage control will be fully automatic with additional manual control. Both manual and automatic systems will provide complete control. Arc suppression will be provided by a current limiting device to reduce the voltage when a spark condition exists in the precipitator. The controllers are rated for a maximum ambient temperature of 40°C. All panel enclosures are made of 2.8mm steel and painted with ASA 61 gray enamel. We will provide you with a remote Graphics Voltage Controller (GVC) for each transformer/rectifier. Each GVC controller will be mounted on the front panel of a free-standing high voltage control box. The graphical controller provides bar graph and digital readouts of primary and secondary voltages and currents, as well as kW power, spark generation, SCR (Silicon Controlled Rectifier) conduction angle and T/R status. This controller must be installed in a safe area of the customer's control room. Alarms will be provided on the GVC control unit for overcurrent alternating current, overheating T/ R, high temperature SCR, SCR imbalance, memory loss, DC undervoltage and DC overvoltage. The main menu is provided for selecting operating functions and troubleshooting. The graphics controller display is 16 lines of 40 characters. The device can produce voltage/current curves, 24-hour trend graphs and 30-minute trend graphs. The operator can remotely set all precipitator parameters such as rollback, lift speed, current limit, etc. Text is available in the help line to make all the settings. Each controller will also have three indicators next to each GVC. These indicators are designed to indicate control on, HV on and alarm.
Current limiting reactor: For each transformer/rectifier there will be a current limiting reactor, protection class 3R according to EEMAC, which will be placed near the transformer/rectifier.
Factory-installed electrical equipment: We will install transformers/rectifiers at the manufacturer's factory and install high-voltage bus ducts and bus trays. We will provide conduit and cable management from the rooftop control panel/distribution panel (PCDP) for shakers, vibrators and blowers. We will install all high-voltage insulators, vibration isolators and power supply insulators. We will supply and install terminal boxes for all roof connections (customer's responsibility for initial connection conditions).
Wired harness
We use following types wiring for the connections below (we reserve the right to replace the XLPE wire below):
Cable cable channels
This cable is used between panels and junction boxes on the roof, and between these junction boxes and the terminals of shakers, blowers and vibrators. Channels will have a nominal 40% capacity in accordance with N.E.C.
THHN/MTW/THWN-2/T90 copper conductor
Underwriters Laboratories Standards UL-83, UL-1063, UL-758
AWM Specification 1316, 1317, 1318, 1319, 1320, 1321
ASTM twist class B3, B8, B787
Federal Specification A-A-59544
Canadian Association Standard C22.2 No. 75
NEMA WC70/ICEA S-95-658
Institute of Electrical and Electronics Engineers ARRA 2009; Section 1605
Conductor: Stranded bare copper conductors per ASTM-B3, ASTM-B787 and ASTM-B8
Insulation: Colored polyvinyl chloride (PVC), heat and moisture resistant, fire retardant compound to UL-1063 and UL-83
Sheath: Rigid polyamide, nylon to UL-1063 and UL-83. Slippery, nylon outer shell for easy draw. VW-1 is rated 14 AWG - 8 AWG. All sizes are petrol and oil resistant.
Applications: Typical THHN/THWN-2 construction wire is intended for general purpose applications as defined by the National Electrical Code (NEC). Type THHN/THWN-2 is approved for new construction or reinstallation for 600 volt applications. Applications requiring Type THHN or THWN-2: The conductor is suitable for use in wet or dry locations at temperatures not exceeding 90°C or not exceeding 75°C in oil or refrigerants. Applications requiring MTW type: The conductor is suitable for use in dry locations at 90°C or should not exceed 60°C in wet locations or when exposed to oils or coolants. Applications requiring Type AWM: The conductor is suitable for use at temperatures not exceeding 105°C in dry locations.
Vibration isolation wire
This wire is used between duct junction boxes and shakers, blowers and vibrators.
SOOW/SJOOW 90ºC Black ROHS
Engineering Specification/Standards:
UL Standard 62
NEC Article 501.140 Class I Div. 2
NEC Article 400
CSA C22.2 No. 49
CSA FT2 flame test
EPA 40 CFR Part 26 Subpart C heavy metals according to Table 1, TCLP method
Conductor: 18 AWG - 10 AWG Class K stranded bare copper per ASTM B-174
Insulation: EPDM
Shell: CPE
Legend: SOOW E54864 (UL) 600V -40C TO 90C -- CSA LL39753 SOOW 600V -40C TO 90C FT2 Waterproof P-07-KA070018-1-MSHA
Applications: Manufactured using advanced synthetic rubber compounds to perform in temperatures ranging from -40°C to 90°C with excellent resistance to flame, deformation, ozone, oils, acids and chemicals. SOOW has wear-resistant and oil-resistant insulation and casing. SOOW is flexible at low temperatures and exceptionally flexible under normal conditions for electric motors, portable lamps, chargers for battery, portable lighting and portable equipment. National Electrical Code Section 400 Appendix.
Wire for connecting panels
This wire is used to connect various components inside panels (switches, lights, plc, blocks, fuses, terminals, etc.).
MIL-W-16878/2 Type C wire (M16878/2 wire) / Mil-DTL-16878/2
Engineering Specification/Standards:
UL VW-1 flame test
RoHS Hook-up Wire RoHS compliance
MIL-W-16878/2 Type C wire (M16878/2 wire)
Description:
Conductor: Tinned copper, solid and stranded
Insulation: Polyvinyl chloride (PVC), colored
Application: The connecting wire complies with UL VW-1 flame test and is used in a wide range of industries requiring high temperature wire that can also withstand harsh conditions. Due to its size, non-flammable materials and resistance to chemicals Typical applications for MIL-Spec wire include complex applications for the military or aerospace industry. The wire can also be used for internal wiring of electronic equipment. The wire has a temperature range of -55°C to +105°C (M16878/2 Type C) and 1000 volts. All MIL Spec cable types have excellent temperature range and voltage ratings. M16878E connects to wired applications: military equipment, power wire, electrical appliance wiring and medical electronics. M16878EE can be used for electronic use in rugged applications where high temperatures are encountered and is a highly reliable OEM product. The M16878ET is used in aerospace, industrial, military and many other commercial markets.
Targets and guarantees
DEFINITION: The equipment we offer here under design conditions and an input dust load of 512 mg/Nm3 guarantees a dust content at the outlet of the precipitator of no more than 10 mg/Nm3, which is 98.05% of the input load. If the input specific load exceeds the design one, the efficiency of 98.05% is also guaranteed; if the specific load is equal to or less than the calculated one, a residual dust content of 10 mg/nm3 is guaranteed.
OPACITY: The plant guarantees an average flue gas opacity of less than 10% for one hour when operating at design conditions. Transparency must be determined by a certified smoke reading device or certified opacity monitor.
Particle Testing Qualification: The particulate sampling method will be EPA Method No. 5 as specified in the Federal Register. Particles are defined as solids under the operating conditions of the precipitant that can be collected. Condensates are not included here.
Spark discharge. At a sufficiently high field strength of about 3 MVm, an electric spark appears between the electrodes, which has the appearance of a brightly glowing winding channel connecting both electrodes.
The gas near the spark heats up to a high temperature and suddenly expands, causing sound waves, and we hear a characteristic crack. The described form of gas discharge is called a spark discharge or gas spark breakdown. When a spark discharge occurs, the gas suddenly loses its dielectric properties and becomes a good guide.
The field strength at which gas spark breakdown occurs has different meaning for different gases and depends on their state of pressure and temperature. The greater the distance between the electrodes, the greater the voltage between them is necessary for spark breakdown of the gas to occur. This voltage is called breakdown voltage.
Knowing how the breakdown voltage depends on the distance between the electrodes of any a certain shape, you can measure the unknown voltage by maximum length sparks. The device of a spark voltmeter for rough high voltages is based on this. It consists of two metal balls mounted on stands 1 and 2, the 2nd stand with the ball can move closer or further from the first using a screw. The balls are connected to a current source, the voltage of which needs to be measured, and brought together until a spark appears.
By measuring the distance using a scale on the stand, you can give a rough estimate of the voltage along the length of the spark; for example, with a ball diameter of 5 cm and a distance of 0.5 cm, the breakdown voltage is 17.5 kV, and at a distance of 5 cm 100 kV. The occurrence of a breakdown is explained as follows: in a gas there is always a certain number of ions and electrons arising from random reasons. However, their number is so small that the gas practically does not conduct electricity. At a sufficiently high field strength, the kinetic energy accumulated by the ion in the interval between two collisions can become sufficient to ionize a neutral molecule upon collision.
As a result, a new negative electron and a positively charged ion residue is formed. Free electron 1, when colliding with a neutral molecule, splits it into electron 2 and a free positive ion. Electrons 1 and 2, upon further collision with neutral molecules, again split them into electrons 3 and 4 and free positive ions, etc. This ionization process is called impact ionization, and the work that needs to be expended to remove an electron from an atom is called ionization work.
The work of ionization depends on the structure of the atom and is therefore different for different gases. Electrons and ions formed under the influence of impact ionization increase the number of charges in the gas, and in turn they come into motion under the influence of an electric field and can produce impact ionization of new atoms.
Thus, the process reinforces itself, and ionization in the gas quickly reaches a very large value. The phenomenon is similar to a snow avalanche, so this process was called an ion avalanche. The formation of an ion avalanche is the process of spark breakdown, and the minimum voltage at which an ion avalanche occurs is the breakdown voltage. Thus, during a spark breakdown, the reason for gas ionization is the destruction of atoms and molecules during collisions with ions - impact ionization. 2.2.3. Electric arc If, after ignition of the spark discharge, the resistance of the circuit is gradually reduced, the current strength in the spark will increase.
When the circuit resistance becomes low enough, a new form of gas discharge occurs, called an arc discharge. In this case, the current increases sharply, and the voltage across the discharge gap decreases to several tens of volts. This shows that new processes arise in the discharge, imparting a very high conductivity to the gas.
Currently, an electric arc is most often produced between special carbon electrodes. The hottest point of the arc is the depression formed on the positive electrode and is called the arc crater. Its temperature is 4000 K, and at a pressure of 20 atm it exceeds 7000 K. An arc discharge occurs in all cases when, due to heating of the cathode, thermionic emission becomes the main cause of gas ionization. For example, in a glow discharge, positive ions bombarding the cathode not only cause secondary electron emission, but also heat the cathode.
Therefore, if you increase the current in a glow discharge, the temperature of the cathode increases, and when it reaches such a value that noticeable thermionic emission begins, the glow discharge turns into an arc. In this case, the cathode potential drop also disappears. The electric arc is a powerful light source and is widely used in projection, floodlight and other installations. The specific power consumed by it is less than that of incandescent lamps.
High-pressure arc lamps are also used as light sources. The arc is ignited by a discharge from a high voltage source using a third electrode. Due to the high temperature of the arc, it is used for welding and cutting metals. Field-electronic arcs with a mercury cathode are used to rectify alternating electric current. 2.2.4. Corona discharge The discharge, which received this name, is observed at relatively high gas pressures in a highly inhomogeneous field. To obtain significant field inhomogeneity, the electrodes must have a very unequal surface, that is, one very large, the other very small.
The electric field strength lines become denser as they approach the wire, and, therefore, the field strength near the wire has highest value. When it reaches approximately 3106 Vm, a discharge is ignited between the wire and the cylinder and a current appears in the circuit. In this case, a glow appears near the wire, which has the form of a shell or crown surrounding the wire, which is where the name of the discharge comes from.
Corona discharge occurs as if negative potential there is a negative corona on the wire, and with a positive corona there is a positive corona, as well as with an alternating voltage between the wire and the cylinder. As the voltage between the wire and the cylinder increases, the current in the corona discharge also increases. At the same time, the thickness of the luminous layer of the corona increases. The processes inside the corona boil down to the following: if the wire is negatively charged, then upon reaching the breakdown voltage, electron avalanches are generated at the surface of the wire, which spread from the wire to the cylinder.
In the case of a positive corona, electron avalanches originate at outer surface crowns and move towards the wire. Corona discharge occurs not only near wires, but also near any conductors with a small surface area. The crown also appears in nature under the influence of the atmospheric electric field and appears on the tops of trees, ship masts, etc. 3.
End of work -
This topic belongs to the section:
Electric current in nonmetals
Electrolytes include, for example, solutions of salts, acids and alkalis. In some cases, electrolytes are also melts of any substances or... Electrolysis is the release of a substance on the electrodes when an electric current passes through the electrolyte solution. Laws..
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Depending on the gas pressure, electrode configuration and external circuit parameters, there are four types of independent discharges:
- glow discharge;
- spark discharge;
- arc discharge;
- corona discharge.
1. Glow discharge occurs when low pressures. It can be observed in a glass tube with flat metal electrodes soldered at the ends (Fig. 8.5). Near the cathode there is a thin luminous layer called cathode luminous film 2.
Between the cathode and the film there is Aston's dark space 1. To the right of the luminous film is placed a weakly luminous layer called cathode dark space 3. This layer goes into a luminous area, which is called smoldering glow 4, the smoldering space is bordered by a dark gap - Faraday dark space 5. All of the above layers form cathode part glow discharge. The rest of the tube is filled with glowing gas. This part is called positive column 6.
As the pressure decreases, the cathode part of the discharge and the Faraday dark space increase, and the positive column shortens.
Measurements showed that almost all potential drops occur in the first three sections of the discharge (Aston's dark space, cathode luminous film and cathode dark spot). This portion of the voltage applied to the tube is called cathode potential drop.
In the region of the smoldering glow, the potential does not change - here the field strength is zero. Finally, in the Faraday dark space and positive column the potential slowly increases.
This potential distribution is caused by the formation of a positive space charge in the cathode dark space due to increased concentration positive ions.
Positive ions, accelerated by the cathode potential drop, bombard the cathode and knock electrons out of it. In the Aston dark space, these electrons, flying without collisions into the region of the cathode dark space, have high energy, as a result of which they more often ionize molecules than excite them. Those. The intensity of the gas glow decreases, but many electrons and positive ions are formed. The resulting ions initially have a very low speed and therefore a positive space charge is created in the cathode dark space, which leads to a redistribution of potential along the tube and the occurrence of a cathode potential drop.
Electrons generated in the cathode dark space penetrate into the region of smoldering glow, which is characterized by a high concentration of electrons and positive ions and a polar space charge close to zero (plasma). Therefore, the field strength here is very low. In the region of the smoldering glow, an intense recombination process takes place, accompanied by the emission of energy released during this process. Thus, the smoldering glow is mainly a recombination glow.
From the region of smoldering glow into Faraday dark space, electrons and ions penetrate due to diffusion. The probability of recombination drops greatly here, because the concentration of charged particles is low. Therefore, there is a field in Faraday dark space. The electrons entrained by this field accumulate energy and often eventually create the conditions necessary for the existence of a plasma. The positive column represents gas-discharge plasma. It acts as a conductor connecting the anode to the cathode parts of the discharge. The glow of the positive column is caused mainly by transitions of excited molecules to the ground state.
2. Spark discharge occurs in gas usually at pressures on the order of atmospheric pressure. It is characterized by an intermittent form. By appearance a spark discharge is a bunch of bright zigzag branching thin stripes that instantly penetrate the discharge gap, quickly extinguish and constantly replace each other (Fig. 8.6). These strips are called spark channels.
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T gas = 10,000 K ~ 40 cm I= 100 kA t= 10 –4 s l~ 10 km |
After the discharge gap is “broken” by the spark channel, its resistance becomes small, a short-term pulse of high current passes through the channel, during which only a small voltage falls on the discharge gap. If the source power is not very high, then after this current pulse the discharge stops. The voltage between the electrodes begins to increase to its previous value, and the gas breakdown is repeated with the formation of a new spark channel.
Under natural conditions, a spark discharge is observed in the form of lightning. Figure 8.7 shows an example of a spark discharge - lightning, duration 0.2 ÷ 0.3 with a current strength of 10 4 - 10 5 A, length 20 km (Fig. 8.7).
3. Arc discharge . If, after receiving a spark discharge from a powerful source, the distance between the electrodes is gradually reduced, then the discharge from intermittent becomes continuous, and a new form of gas discharge arises, called arc discharge(Fig. 8.8).
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| ~ 10 3 A | ||
| Rice. 8.8 | ||
In this case, the current increases sharply, reaching tens and hundreds of amperes, and the voltage across the discharge gap drops to several tens of volts. According to V.F. Litkevich (1872 - 1951), the arc discharge is maintained mainly due to thermionic emission from the cathode surface. In practice, this means welding, powerful arc furnaces.
4. Corona discharge (Fig. 8.9).occurs in a strong non-uniform electric field at relatively high gas pressures (on the order of atmospheric). Such a field can be obtained between two electrodes, the surface of one of which has a large curvature (thin wire, tip).
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The presence of a second electrode is not necessary, but its role can be played by nearby, surrounding grounded metal objects. When the electric field near an electrode with a large curvature reaches approximately 3∙10 6 V/m, a glow appears around it, looking like a shell or crown, which is where the name of the charge comes from.
