The Role of Thermal Shrouds
A thermal shroud is a large panel made of a vacuum compatible material, most commonly either a stainless steel or aluminium alloy, and can be heated and cooled using resistance heaters and cryogens, respectively. The shroud provides radiative heating/cooling, interacting with items (e.g., satellite instruments) located within the chamber, typically this is referred to as the Item Under Test, or IUT.
In developing AEON’s in-house bakeout facility, engineers were tasked with the design, manufacture and installation of both the thermal shrouds and their control systems.
By performing radiative analyses, it was evident that curved thermal shrouds provided the best view factor to the IUT, this means that the heating/cooling is much more efficient by being purposefully directed to the centre of the vacuum chamber.
AEON’s Engineers separated the thermal shrouds into four independently controlled heating/cooling elements which allows for specific directed control of the flow of heat from/to each quadrant. In large space simulation chambers, it is not uncommon to have several temperature settings across several shrouds to help achieve the required temperature of the IUT. It’s important to remember that, under vacuum there is no heat flow due to convection (air flow) and only very limited conduction (contact), therefore the major mechanism for the transfer of heat is radiation. It’s best to think as radiative heating as ‘line of sight’ transfer of heat, and if you can’t ‘see’ the object being heating, then it simply isn’t being heated. This is what engineers call view factor. The other major contributors to how effective a shroud is, is it’s emissivity, or ability to radiate heat in the infrared. This is mostly linked to surface finish.
As an internal project for AEON’s CERES facility, the design and manufacture of the thermal shrouds included design of insulative attachment points, permitting the shrouds to be fitted, but minimising how much contact they have with the chamber wall.
Any kind of assembly point can be a challenge to design, but its also important to understand that attachments have to survive reasonably large temperature swings, from -196°C through to +200°C.
Furthermore, AEON’s shrouds were designed with longevity and operational redundancy in mind, this meant increasing the quantities of heating elements and designing harnesses to route electrical power across multiple control and power systems.
This type of failsafe means that, should a heater fail during a long bakeout, it does not cause a failed process, as simply a redundant heater can be powered on and take the place of the failed unit.
AEON have a dedicated Engineers Rule of Thumb case study which talks you through the do’s and don’ts of UHV Chambers and maintaining good practices.