Topic
Drying with Air Re-circulation
A heater-dryer arrangement with air recirculation is shown in the above figure. The
flow of different streams is mentioned. In the following six questions, circle the
correct answer. Note that G represents the dry air flow in kg per hour.
Heat lost, Qd
Heat in, Qh
Recirculated air, G6
G2
G3
HEATER
DRYER
G4
G5
Fresh air, G1
Dried solid, X2
(a) X1 = X2
(a) H3 = H2
(a) H3 = H4
(a) H6 = H2
(a) H1 = H5
(a) G1 = G5
(b) X1 > X2
(b) H3 > H2
(b) H3 > H4
(b) H6 > H2
(b) H1 > H5
(b) G1 > G5
Wet solid, X1
(c) X1 < X2
(c) H3 < H2
(c) H3 < H4
(c) H6 < H2
(c) H1 < H5
(c) G1 < G5
(d)
(d)
(d)
(d)
(d)
(d)
can
can
can
can
can
can
not
not
not
not
not
not
say
say
say
say
say
say
The wet feed material to a continuous drier contains X1= 10 wt% water on a dry basis
(kg water per kg dry solid and dried to X2= 5 wt% by countercurrent air flow. The
feed rate is Ls = 500 kg dry solid/h. Fresh air to the system is at T G1= 25.6 C and has
a humidity of H1=0.007 kg water/kg dry air. The moist air leaves the dryer at TG4 =
37.8 C and H4 =0.022 and a part of it is re-circulated and mixed with the fresh air
before entering a heater. The heated mixed air enters the dryer at T G3 = 95 C and H3
=0.01. The solid enters at TS1 = 35 C and leaves at TS2 = 65 C. The heat capacity of
the dry solid is 1.5 kJ/(kg dry solid.K) and the heat capacity of the liquid moisture is
4.2 kJ/(kg H2O.K). Calculate
 the air flow-rate in the dryer
 the flow-rate of the fresh air
 the flow-rate of the recycled air
 the heat loss from the dryer in KW
 the flow-rate of wet solid feed entering the dryer in kg total solid per hour