Article number 8:
ovens: Are they more uniform?
often convection is additionally introduced in a radiation oven/furnace. Does this
help the uniformity?
following experiment describes results of just such an investigation.
The pictures below show a 550C multi-rack hybrid oven fitted with a VTA-750-2GC (on the rear of the furnace) to provide convection. The independent control
panel for the oven and VTA assembly is also shown. The conclusions discuss the uniformity increase in the ove. The conditions selected are when a radiative heater could see distrupted line of sight heating.
above show the front of a typical oven with the door opened and the rear where the
VTA convector has been fitted. The picture on the extreme right is the
MHI control panel for the furnace.
of test hybrid oven
to wall chamber size: H 600mm X W 700mm X D600mm
2” vacuum formed alumina silica boards on all sides
of oven: 550
° C maximum. (other ovens tested at
350C and 900C please contact MHI for test reports). Review design criteria.
MHI heating coils partially embedded in fiberboards on two sides of the oven,
delivering 14kW in 3-phase configuration.
MHI Model VTA750-2GC hot air blower capable of producing amaximum of 900°
C air temperature.
2” dia vent adequately located. Location of the vent is very important.
As shown in picture above (3-phase system). VTA control set in control panel. VTA speed set to give temperature
control features of the oven
- The 14kW radiative heaters are connected
in 6-wire delta / star configuration. The temperature controller is provided
with a high alarm. This alarm is set 50 °
C less than the desired set point. At start-up, the heaters come ON
in delta configuration. This provides full power for rapid ramp up to temperature.
Once the temperature reaches the alarm set value, the alarm output relay switches
the heating elements into star configuration where the heating elements run
at half the rated capacity in order to save energy.
- With the 50% power control provided
by the 6-wire delta / star configuration, a + / - 1 °
C control accuracy can be achieved by simple ON/OFF control.
- In the delta configuration on start
up, the oven reached 320°C in eight minutes. The alarm (for change over from delta to star was set30 °
C below the set value of 350 ° C).
At 320°C, the alarm kicked in and
changed the heating element load to a star configuration thus reducing the
demand power by half. The large oven reached the set value of 350°C in the 12th minute. From there on, the control system t/c
controlled the oven with a + / - 1 °C control accuracy.
- The temperature range chosen for the
tests (i.e. between a 100°C and 900°C) is not the best for only radiative heating
insofar as uniformity is concerned. Note complexity of trays and shelves
inside the oven. Thus it was meaningful to analyze the effect of VTA
convection on the uniformity.
- Thermocouples were located at six
positions namely corners, face-walls, center and of-center locations in he
Conclusions of uniformity improvement with a Airtorch type convection generator. Consider also steam as a process gas
- During the first run, the
VTA was not turned ON. During this run, a variation of + / - 25 to 30 °
C was observed on the 6 point temperature measurement.
- When the VTA was turned during
a subsequent run the furnace variation reduced dramatically to + / - 6 °
C. Note that this is a very acceptable value for such
a large oven in the mid temperature range.
- The conclusion is that the
automatic switch over to a star configuration reduced the energy demand and
the VTA assisted dramatically in improving the uniformity without adding in
any significant manner to the power demand.
Althought this is a report submitted to MHI, there is no independent authetication of the data or experiment by MHI. The amount of uniformity enhancement, if any, will vary from installation to installation.