Abstract:
In hot air dryers, only a small percentage of the provided thermal energy is use
d for the
drying process, while a large fraction is lost via the exhaust air. To recycle waste heat
from the exhaust air, the present study aimed to develop a solar dryer equipped with a
novel heat recovery system. The designed dryer comprised of a solar a
ir collector, a
drying chamber, an internal closed
-
loop air circuit and an open
-
duct heat recovery
system. The evaluation tests were conducted at different allowable
r
elative
h
umidities
(RH) and mass flow rates of the recirculating air. The results indicat
ed that the best solar
fraction was at the highest RH and air flow rate. Increasing the
RH
from 7 to 17% caused
a reduction of 51% in electricity consumption. Furthermore, electrical energy needed for
drying increased by 24% with raising the air flow rate
from 0.008 to 0.016 kg s
-
1
. A
minimum specific energy consumption of 7.54 MJ kg
-
1
was observed at the highest
RH
and the lowest air flow rate. At a constant RH, reduction of the air flow rate led to an
increasing trend in lightness and decreasing trends in
browning index of the products.
Moreover, increasing the
RH
from 7 to 17% increased lightness and decreased browning
index. In general, it can be stated that the best colour quality was achieved when the
minimum air flow rate and the maximum
RH
were used
for the solar drying.
Keywords
: Browning index, Energy recycling, Specific energy consumption, Surface colour
