The first three combinations of RF
and conventional drying use RF in part of the overall process
cycle with some significant reductions in drying times. It
makes sense that another option is to use RF and conventional
methods simultaneously during the whole process. This offers
the largest potential reduction in drying time of all the
methods, although it does complicate equipment design. Greenbank
Engineering and the Electricity Council of the United Kingdom
did a notable study in this area. The results of their work
on Air, Radio Frequency Assisted (ARFA) drying is shown below. It shows the significant reduction in drying time
with the addition of RF to the standard process using 180°C
air. Drying times with other hybrid systems have been reduced
from hours to minutes in many cases.
DRYING CURVE, FULL RF
+ CONVENTIONAL DRYING
The simultaneous use of RF and
convection has another interesting application for temperature
sensitive products. In this case, the RF is used to heat and
evaporate the water in the product and ambient air (rather
than heated air) is used to remove the moisture from the surface
and keep the product temperature lower. This combination offers
a “gentle” drying where fast drying is needed
but the product cannot be exposed to high temperatures.
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