May. 13, 2024
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In part one of discussing which considerations should be taken into account when buying a vacuum furnace, we looked at a number of factors including pumping measurement systems, the potential for leaks and what to do about them as well as the importance of compliant welding.
In the second part of this article, we'll move on to discuss vacuum furnace components, seals and thermal transfer.
Before that, however, we'll focus on materials.
All vacuum furnaces that will meet the standards expected of modern industry must – like other pressurised equipment – use durable materials of greater thicknesses than other sorts of equipment.
Light alloys and certain grades of stainless steel do not necessarily cut it when it comes to vacuum furnace construction. Furthermore, as we discussed in the first part of this article, strong welding joints need to be in place to match the extra thick materials that will have been used for a high-quality furnace.
Initially, the design stage of any new vacuum furnace will involve deciding the exact dimension of the vessel in question.
In turn, this should determine the maximum working pressure of the furnace as well as the highest temperature of operation.
As the thermal chamber enters a more detailed design phase, so any possible openings of the furnace's insulation screen will be taken into account.
After all, it is from here that most radiated heat will be able to escape.
What it breaks down to is the resistor's three power supply passages because they are all opening sources. These should not be discarded as mere insulator irregularities but an integral part of the design itself.
In terms of materials, what buyers will be best advised to do is to choose high refractory metals with good levels of thermal conductivity. This will be certain types of steel, in the main.
Furthermore, opting for insulating screens rendered from rigid graphite fibres is also often the best approach to take, especially when these come in wafer form. This material choice should lead to a high level of insulation with a low coefficient of heat transfer.
In the past, molybdenum was often favoured for this purpose, but it is too fragile a material for many vacuum furnace applications nowadays. Even worse, it is prone to oxidising even at low temperatures which can result in undesirable high vapour pressure.
On the other hand, rigid carbon fibre constitutes a top-class material for wafer frames as well as stiffeners.
Much research into this material has been conducted in recent years.
This was primarily so that the thermal treatments of certain aeronautical components could be optimised.
As a result of such work, it has been proven that no chemical reaction between graphite and metal will occur even under furnace conditions.
As such, the utilisation of graphite as a material with exceptional levels of resistance for heating elements is now the norm among high-quality furnace suppliers.
It is crucial that buyers realise that any vacuum furnace they are considering purchasing has sealing elements that can withstand positive as well as negative pressures.
This is sometimes an area of weakness that is not immediately obvious, but it can soon become apparent after a period of continuous usage.
As such, O-ring seal cavities should be carefully considered in the design of the furnace.
What you want to see is a reliable avoidance of any unnecessary movement of the gasket.
This often happens with inferior furnaces when a vacuum state returns to one at normal pressure.
Furthermore, if the surface of the seal is so big that a strain is put on it, then adequate seals should be fitted throughout.
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For instance, look for double-lip seals which are often superior in handling strain.
Overall, however, the seals should be examined fully to ensure they are capable of being adapted to any vacuum seal request that might be made.
Find out what the specific loads of any seals are, usually measured in kilos per square centimetre.
Finally, where seals for rotating shafts are being looked into, it may be a good idea to have the ability to alter the oil pressure of the seal.
This will allow your operatives to compensate for pressure drops in the furnace if and when they occur. Additionally, it is beneficial for motor fluid to be made of a neutral gas where seals with axial movements are concerned since this will help to get around the possibility of minute amounts of air making their way through them.
Something that some buyers of furnaces underestimate the value of is the quality of components.
This is certainly the case with vacuum furnaces where small parts will come under greater strain than other types.
Things like flanges and clamps must be of a robust quality which will stand the test of time.
The same can be said of any valves that are fitted to the unit.
In addition, you should ensure that the measuring head of the furnace is correctly sized and able to withstand the level of pressure it will come under.
When these sorts of components are not specifically designed for use in vacuum furnaces, there can be a serious chance of danger occurring.
Anyone who is in the vicinity of a furnace when a total component failure occurs could easily suffer injury.
In the worst cases, a component failure might even lead to a business using vacuum furnace technology to face unrepairable damage.
Of course, such catastrophic failures are rare but this is because leading vacuum furnace makers only use components which are capable of withstanding the maximum temperature and lowest pressure of their ovens.
In other words, never compromise on component quality!
When it comes to heat, a vacuum furnace design should be focussed in two areas.
The latter should offer a construction which makes use of low conductivity materials, such as graphite wafers, as mentioned in section 4 of this article. It is here that the furnace's resistor is housed.
Whether or not the vacuum furnace can operate with partial gas or without it completely, thermal transferral will always be a consideration.
Simply put, this happens when molecules move from a heated section of the furnace to colder one.
What buyers should be looking for is a level of conduction, or thermal transferral, that is negligible.
Ideally, this will be the case irrespective of the vacuum level the furnace is operating at.
In other words, even at low pressures, the thermal transfer of the oven via conduction should be practically zero.
Bear in mind, however, that where a partial gas operation is happening, thermal losses due to irradiation may also occur. In such cases, the technicalities may be a little different but what you want to see is as much thermal uniformity as possible even where, for instance, water-cooled copper clamps are in use.
If what you have read about purchasing vacuum furnaces in the modern age has whetted your appetite, then you can find out a great deal more by downloading our buyer's guide to vacuum furnaces.
Click here to download the guide, alternatively, if you would like to progress to the third part of this article, where you will encounter three more important purchasing considerations connected to vacuum furnace technology, then read on.
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