Graphite is the main component in an electric arc heater. The quality of its electrodes is critical to the performance and efficiency of an electric arc furnace, particularly when it comes down to electrical conductivity and heat resistance. It is important to test the graphite before they are installed in a furnace. This will ensure that you get maximum performance. The True Density Test is used to test the density of the graphite. This test is intended to calculate the actual density of a material made from graphite, by excluding both open and sealed pores. The true density is higher when a graphite has been graphitized, which results in better conductivity.
As part of the manufacturing process graphite electrodes undergo various steps in order to achieve the proper dimensions and tolerances that will allow them to be used inside furnaces. In order to get rid of any inconsistencies or imperfections, the electrode is rough turned. The thread on the ends of the electrode is then engaged with ease in a furnace to prevent any connection problems. Moreover, the internal thread is also tested to make sure that it is not uneven or has any burrs.
The electrodes will also be calcined and then soaked in pitch tar to help fill in any gaps on the graphite surface. The electrodes become stronger through this calcination and soaking process. This is also called the "second and third pigging". Moreover, this heat treatment also helps to reduce the porosity of the graphite and makes it more conductive.
Analyzing the plasma arc behavior when an electrode is subjected to different conditions is another way of determining the quality of the electrode. To do this, the electrodes are tested under different temperatures and current levels. These electrodes are then examined to find out how they perform under various conditions, and whether they contain any flaws or defects. These tests are a great way to identify issues which could pose a risk to safety.
The shape of the tip, as well as the size of the electrode gaps affect the stability. Even with small electrode gaps, the cylindrical standard tip can lead to unsteady plasma arc behaviour. A machined step is more stable, and can allow for higher current densities at lower voltage levels.
The comparison was made between eight different experiment series for the electrode type, tip shape, and electrode spacing. Each graphite arc was recorded, and then classified according to its behavior. This data has been used to develop stability maps that are specific for graphite and tip geometries.
Results showed that GE-quality, with a grain maximum of 2 mm was less stable than CGS-quality. CGS has a grain maximum larger. This is due to the fact that more grain boundaries are created in the GE-quality and this results in more unstable conditions.
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