Shel lab SVAC9-2, Installation And Operation Manual, Page: 29 / 46

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OPERATION
Heating in a Vacuum
In conventional ovens, a powered element transfers heat into the chamber air. The heated air then
circulates by natural convection or blower fan action and surrounds the product on the shelves,
gradually bringing it to temperature. In a vacuum oven, there is no atmosphere to transport heat
evenly from the elements to the product. Instead, heat transport takes place primarily by conduction.
The oven heating elements are located inside ducts and heat the chamber walls, which in turn
transfer heat to the shelves. Each shelf then transports heat to the products or samples resting on it.
Direct radiant heating through infrared emission in a vacuum environment provides poor
temperature uniformity compared to conductive heating.
Heating Control
The oven temperature controller stores a user-selected constant temperature set point. When
powered, the oven heats the chamber shelves to the set point. The controller board is wired to a
solid-state temperature probe located in the chamber on the rear wall. When the controller detects
that the shelf temperature has dropped below the temperature set point, it pulses power to the
heating elements.
The controller employs proportional-integral-derivative analytical feedback-loop functions when
measuring and controlling the shelving temperature. PID-controlled heating pulse intensities and
lengths are proportional to the difference between the measured shelf temperature and the current
set point. The frequency of pulses is derived from the rate of change in that difference. The integral
function slows the rate of pulses when the temperature nears the set point to avoid overshooting.
SVAC ovens rely on natural heat radiation for cooling. The oven can achieve a low-end operating
temperature of the ambient room temperature plus the oven waste heat.
The Over Temperature Limit System
When set, the mechanical OTL heating cutoff system prevents runaway heating in the oven chamber.
The OTL cutoff operates independently of the main controller and is provided with a separate,
hydrostatic temperature sensor probe located in the oven chamber. In the event the chamber
shelving temperature exceeds the current OTL setting, the OTL routes power away from the heating
elements.
The high limit is set by the end-user to a minimum of 10˚C above the highest temperature of the application
process the oven is currently being used for. Failure to set the high limit control system voids the oven
manufacturing defect warranty in the event of an overtemperature event.
After the OTL is tripped the temperature must drop below the current OTL setting and the red Reset
button must be pushed in order to restore power to the temperature controller and heating
elements. If the temperature controller screen is dark but the vacuum display is still powered, the
OTL may have tripped.