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OPERATION
THEORY OF OPERATION
Heating
When powered, the oven chamber heats to and then maintains the currently programmed temperature set
point. The set point may be adjusted by the operator using the Set Temperature controls.
Heating is controlled by a microprocessor controller board that stores the temperature set point. The
microprocessor senses the chamber air temperature via a solid-state probe located in the airstream on the
back wall of the chamber. When the processor detects that the chamber temperature has dropped below
the temperature set point, it pulses power to a heating element in a recirculation air duct space located
above the oven chamber.
The processor employs proportional-integral-derivative analytical feedback-loop functions when measuring
and controlling the chamber air temperature levels. PID-controlled heating pulse intensities and lengths are
proportional to the difference between the measured chamber temperature and the current set point. The
frequency of pulses is derived from the rate of change in the difference. The integral function slows the
rate of pulses when the temperature nears the set point to avoid overshooting.
These ovens rely on natural heat radiation for cooling.
When the oven is powered the chamber air temperature cannot go below the ambient room temperature
plus the internal waste heat of the oven. Waste heat is generated primarily by the operation of the internal
blower fan motor and the resulting air compression in the duct spaces. In practice, the temperature floor is
ambient +26°F.
The oven depends on the operation of the blower fan to maintain temperature uniformity and stability in
the chamber.
Air Circulation
The oven continually circulates air internally while powered in order to maintain temperature uniformity and
stability in the oven chamber and to speed drying rates. Air is forced through vent holes on the right side of
the chamber, blows across the shelf space, and is then pulled into a duct that makes up the left chamber
wall. From there, the air is drawn upward into a heating duct by the action of the blower fan. The oven is
intended to be run as a closed air-cycle system.
Vents – Intake and Exhaust
The oven is provided with an intake vent and an exhaust vent that may be opened or closed using
dampener slides located on the vents. The dampeners are intended to be opened after the heat treatment
or bake out phases of an application are complete. Opening the dampener vents during the treatment or
bake out may speed the rate of material drying, depending on the nature of the sample material,
outgassed byproducts, and ambient conditions. However, running the oven with the dampeners open
introduces a significant flow of cool air into the chamber while allowing heated air to exit. This will impact
the temperature uniformity and stability of the chamber and lower the operational temperature ceiling.
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