Optimum Air or Nitrogen Flow Rates
for Thick Film
Firing Furnaces
LORAC believes that the following guidelines should apply to correctly set
atmosphere controls on continuous belt furnaces.
Entrance curtain and exit curtain: The curtain flows pressurize
the
inside the furnace, P1 being > P2 (outside the furnace),
the curtain
flow will be outward, preventing (along with the physical barrier of
the stainless baffle curtains) ambient from entering.
This can ideally
be verified while observing the effect of curtain flow on a piece of
dry ice, cigarette or other method
of producing a visible gas.
Burnout/Firing Sections: These sections of the furnace have a defined
length, width and height. This can be measured and expressed in terms
of volume. Furnace operators are encouraged
to determine the recommended
atmospheric changes per hour
(or minute) for their particular application.
Firing thick film materials,
the recommended flow should consider the
amount of material being fired, both in terms of thickness and printed
paste area. Once the volume of the furnace and the frequency of completely
replacing
the atmosphere within the furnace are known, it's fairly straightforward
to determine a flowmeter setting which will achieve the desired result.
For example, a furnace with a volume of approx. 1 cubic foot, could
be expected to receive a complete change of atmosphere once per minute
(60 times per hour) if the total burnout and firing section flows were
60 SCFH. Two changes per minute would be achieved with a total flow
of 120 SCFH. It's then useful to know the approximate TGA properties
of the materials being fired to know how to set the ratio of flow between
the burnout and firing sections.
Modern mythology, where the firing section flows are set higher
than
the burnout section, "to push the flow counter to belt travel"
is incorrect.
The majority of the flow should be in the burnout section where it is
required based on how the materials are reacting to an increase in temperature.
The direction of the flow within the furnace
is determined by the venturi
(exhaust) settings, not by other means.
The exhaust flows are ideally set by monitoring the temperature of the
exhaust gas as it exits the furnace. With a thermometer in the exhaust
and a piece of dry ice at the entrance curtain, gradually increase the
exhaust flow, until the temperature becomes constant or until the curtain
flow reverses and becomes drawn into the furnace.
With some experimentation, a combination of flow rates can generally
be achieved that will optimize both the exhaust and curtain flow performance!
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