Instructions for the use of graphite rods in vacuum furnaces

1. Select graphite rods with good uniformity of red heat in the heating section. Poor uniformity of red heat in the rods can affect the uniformity of furnace temperature and shorten the service life of the rods. During use, the uniformity of red heat in the rods will gradually deteriorate, and in severe cases, it may lead to rod breakage.

2. The lifespan of graphite rods decreases as the operating temperature increases. Especially when the surface temperature of the rod exceeds 1500°C, the oxidation rate accelerates and the lifespan is shortened. During use, care should be taken to avoid excessively high surface temperature of the graphite rod.

3. When heated in air, the surface of the graphite rod forms a dense silicon dioxide film, which becomes an antioxidant protective layer, thereby extending its lifespan. Intermittent use, coupled with temperature fluctuations in the kiln, can lead to the rupture of the protective layer on the rod surface, weakening its protective effect and accelerating the increase in the rod's resistance value.

To ensure stable kiln temperature and meet the demand for rapid temperature rise, the supporting electrical control system should have sufficient voltage adjustment margin. Specifically, when using new rods, the kiln's design and operational power can be met at a lower voltage. However, as the rods age and their resistance increases, the operating voltage needs to be correspondingly increased to maintain the kiln's design and operational power.

Voltage margin value: The voltage of a graphite rod in its later stage of use is generally 1.5 to 1.7 times the voltage of a new rod. Depending on the voltage regulation method and wiring method, the upper limit of the later-stage voltage is generally calculated as 220V or 380V.

Regarding the adjustment method of graphite rod power, it is recommended to adjust the power by adjusting the voltage. It is recommended to choose thyristor voltage regulation or voltage regulator for graphite rod voltage regulation. Generally, power regulators that change the number of cycles are not used for adjustment.

4. Generally, the surface load density of the graphite rod is determined by the relationship between the temperature inside the furnace and the surface temperature of the graphite rod. It is recommended to use a power that corresponds to 1/2 to 1/3 of the surface load density of the graphite rod. The greater the current applied to the graphite rod, the higher its surface temperature. It is recommended to use the smallest possible surface load density (power).

Please note that the values recorded at the cold end of the graphite rod represent the current and voltage measured in air within the range of 1050℃+-50℃, which may not necessarily correspond to actual usage.

5. When using graphite rods continuously, it is advisable to gradually increase the voltage to maintain a long lifespan.

6. Graphite rods should be connected in parallel as much as possible. If the graphite rods have different resistance values, when connected in series, the graphite rod with higher resistance will bear a concentrated load, leading to a rapid increase in resistance of a certain graphite rod and a shortened lifespan.

At the same time, it is necessary to strengthen the grouping of resistance values, meaning that the resistance values of the same group of rods should be as close as possible. Generally, the deviation of resistance values within the same group of rods should be within 10% to 15% when connected in parallel, and within 5% to 10% when connected in series. The higher the furnace temperature, the smaller the required deviation of resistance values should be.