Graphite rods are often used as electric heaters in high-temperature vacuum furnaces. The maximum working temperature can reach 3000℃, and it is easy to oxidize at high temperatures. They can only be used in neutral or reducing atmospheres except for vacuum. So, how to use the vacuum furnace graphite rod? Let's introduce it below.
1. The heating part uses graphite rods with good red heat uniformity. Poor red heat uniformity of the bar will affect the uniformity of the furnace temperature and shorten the service life of the bar. During use, the red heat uniformity of the bar will gradually deteriorate, and in severe cases, the bar will break.
2. The life of graphite rods will become shorter as the temperature rises, especially when the surface temperature of the rod exceeds 1500°C, the oxidation rate will be accelerated and the life will be shortened. During use, try not to let the surface temperature of the graphite rod be too high.
3. After the graphite rod is heated in the air, a dense silicon oxide film is formed on the surface, which becomes an anti-oxidation protective film and prolongs the service life. Intermittent use, with the rise and fall of the kiln temperature, will cause the protective film on the surface of the rod to rupture, the protection effect will be weakened, and the resistance value of the rod will increase. To ensure the temperature stability of the kiln and meet the needs of rapid temperature rise, the supporting electric control system should leave enough voltage adjustment margin-that is when the new rod is low, it can meet the kiln design and use power. ; With the use of time Extended, the resistance of the rod becomes greater. At this time, the operating voltage needs to be increased accordingly to meet the furnace design and operating power.
Voltage margin value: the voltage of graphite rods in the later period of use is generally 1.5--1.7 times the voltage of new rods. Depending on the voltage adjustment method and wiring method, the upper voltage limit in the later period is generally calculated as 220V or 380V.
The way to adjust the power of the graphite rod is to adjust the power by adjusting the voltage. It is recommended that the voltage stabilization method of the graphite rod be controlled by a silicon-controlled rectifier or a voltage stabilizer. It is generally adjusted without the aid of a power regulator that changes the frequency. 4. In general, the surface load density of graphite rods is obtained according to the relationship between the furnace temperature and the surface temperature of graphite rods. It is recommended to use the power of 1/2-1/3 of the maximum surface load density of graphite rods. The greater the current applied to the graphite rod, the higher the surface temperature of the graphite rod. It is recommended to use as little surface loading density (power) as possible. Please note that the value recorded at the cold end of the graphite rod is the current and voltage measured in the air at 1050°C+-50°C, which may not be consistent with the actual use.
5. When the graphite rod is used continuously, it is hoped to increase the voltage slowly to maintain a longer life.
6. Graphite rods should be connected in parallel as much as possible. If the resistance values of the graphite rods are different, the graphite rods with high resistance will be concentrated in series, resulting in a rapid increase in the resistance of a certain graphite rod and a shortened life. At the same time, the matching of resistance values should be strengthened, that is, the resistance values of the same group of rods should be as close as possible. Under normal circumstances, the resistance value deviation of the same group of rods in a parallel connection is within 10%-15%, and the resistance value deviation of the same group of rods in a series connection is within 5%-10%. The higher the furnace temperature, the smaller the required resistance deviation should be.