Hot zones are the hearth of every vacuum furnace. When purchasing a new furnace, you might face with the Hamletic doubt about hot zones: graphite-based or all-metal design hot zone? That’s the question!
This is the first of two articles where I’m going to help shed light on this issue, so you will be able to choose between the different types of insulation materials, with a focus on energy efficiency and durability. We will see, then, strengths and weaknesses of both hot zone designs, with an eye on heat losses during processing and the need to minimize these losses. But before delving into the main differences in heat shield insulation, let’s take a look at the elements that must be taken into account when selecting the hot zone.
How to select the right thermal shield
The hot zone is by far the most critical component of your vacuum furnace as it is the functional core. Hot zone designs and internal supports and hardware need to be designed with consideration to low mass and specific heat wherever possible to avoid hot zone energy absorption. Considerations in selecting a particular type of hot zone include maximum temperature range of operation, types of cycles to be performed, expected holding times at elevated temperatures, peak power concerns and overall operating power costs. All of these factors relate to hot zone construction and its related power losses.
Vacuum furnace heating chambers are designed to heat the load and insulate the hot zone. They consist of a frame that forms the load-bearing structure containing the insulation system, the resistor and the zone where the load to be treated is housed. Now let us look at the insulation system, namely two main solutions to prevent dissipation of the energy transmitted by the resistor in radiation form onto the load. The insulation materials most commonly used in vacuum furnaces are:
- graphite wafer or ceramic fibre (Al and/or Si base) panel;
- multiple reflecting shields in Molybdenum (Mo) or Tungsten (W).
Graphite-based hot zones: a high level of thermal efficiency
The graphite wafer is an excellent material. It allows operation at very high temperatures (up to 3000°C based on the vacuum level), has low density, reduced weight and modest thermal capacity. It creates the ideal black body conditions (emissivity about 1) inside the heating chamber for obtaining high uniformity.
Graphite hot zones are easily repairable and suitable expedients can be applied to guarantee a long life. These include applying a sacrificial layer made of CFC or with a graphite laminate, protecting the wafer both against the effects of the high temperature (deformation – expansion), and gas erosion during the load cooling phase. Wafer thickness depends on the maximum operating temperature.
A furnace with this type of heating chamber is the best solution at least for the more conventional vacuum treatments. On the downside, there are also less positive aspects regarding graphite’s capacity to adsorb vapors. A graphite’s property which we should take into account when it is time to decide on the right technology for the vacuum furnace hot zone design. So, I will now provide some more tips for selecting the proper vacuum insulation panel according to your process.
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Vacuum insulation panel: the critical aspects of graphite
As I was saying, there are also less positive aspects regarding the graphite’s capacity to adsorb vapors and the risk of graphite micro-particles being released. The manufacturer must be able to select those wafer suppliers that have a suitable product. The choice must be geared towards low bond wafer panels (phenolic resin), where possible with a three-dimensional structure, rejecting panels made up of bonded layers.
Ceramic fiber panels are used whenever the process excludes the presence of graphite. Alumina behaves like a dielectric material at least up to temperatures close to 1800°C and so may impede electrical short circuits with the resistor. It does not have the mechanical strength of carbon fibers.
For some years MPG (Max Plank Inst.) has been certifying graphite wafer compatibility for aeronautic uses as well, and so the companies in this sector find it expedient to use vacuum furnaces with this type of heating chamber.
Graphite’s vapor-adsorbing property may increase the installation’s evacuation times when the prescribed pressures (vacuum levels) are reached. It is also for this reason that every precaution must be taken when making the choice. Not all the materials marketed are the same.
If your process cannot tolerate incidental dust or dirt, an all-metal hot zone is the best choice for your needs. These sensitive processes include diffusion bonding and aluminum brazing. Specially engineered to handle materials such as super alloys (e.g., Titanium, Rene-80 nickel-based super alloy, Hastealloy and Tungsten), all-metal hot zones produce parts that come out bright and clean – a common requirement in the medical industry.
So, to decide the right insulation panels for the heating chamber we have to go through all-metal hot zones comparing them to graphite ones – and that is what we shall do in the second part, entirely dedicated to all-metal hot zones, their characteristics and benefits compared to graphite hot zones.
Do not miss the next article where we'll analyze the all-metal solution.
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