DRYERS AND DRYING 1.Tobacco in a warehouse, held at 30 C and 40% relative humidity, is placed in a room at 32 C and 70% relative humidity preparatory to being worked on. For each 50 kg of tobacco moved from the warehouse, what is the bone-dry weight? What is the actual weight of this quantity of tobacco after staying in the working room? Solution: kg m 50 1 = For tobacco at 40% RH % 30 . 13 1 = gain Re at 70% RH % 00 . 25 2 = gain Re (a) 1 1 gain Re Bdw m Bdw − = Bdw Bdw − = 50 1330 . 0 kg Bdw 13 . 44 = (b) 2 m = actual weight ( ) ( ) ( ) ( ) kg Bdw Bdw gain Re m 16 . 55 13 . 44 13 . 44 25 . 0 2 2 = + = + = 2.Air enters an adiabatic drier at 6 m/s through a 2-m diameter duct at 29 C dry bulb and 22 C wet bulb temperatures. It is heated to 80 C before reaching the material to be dried and leaves the drier at 44 C and 80% RH. The material enters the drier with a moisture content of 24%, and leaves with a moisture content of 8%. Determine (a) the mass of water removed per kg of dry air, (b) the volume flow ofair entering the reheater, (c) the kg of water evaporated per second, (d) the mass flow rate of material leaving the drier, and (e) the heat requirement of drier per kg of water evaporated. Solution:
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1. Tobacco in a warehouse, held at 30 C and 40% relative humidity, is placed in aroom at 32 C and 70% relative humidity preparatory to being worked on. For each
50 kg of tobacco moved from the warehouse, what is the bone-dry weight? Whatis the actual weight of this quantity of tobacco after staying in the working room?
Solution:
kgm 501 =
For tobacco at 40% RH
%30.131 =gain Re
at 70% RH
%00.252 =gain Re
(a)1
1
gain Re
Bdwm
Bdw
−
=
Bdw Bdw −= 501330.0
kg Bdw 13.44=
(b) 2m = actual weight
( )( ) ( )( ) kg Bdw Bdwgain Rem 16.5513.4413.4425.022 =+=+=
2. Air enters an adiabatic drier at 6 m/s through a 2-m diameter duct at 29 C dry bulb
and 22 C wet bulb temperatures. It is heated to 80 C before reaching the material
to be dried and leaves the drier at 44 C and 80% RH. The material enters the drierwith a moisture content of 24%, and leaves with a moisture content of 8%.Determine (a) the mass of water removed per kg of dry air, (b) the volume flow of
air entering the reheater, (c) the kg of water evaporated per second, (d) the massflow rate of material leaving the drier, and (e) the heat requirement of drier per kg
3. A drier is to be designed to reduce the water content of a certain material from55% to 10%. Air at 29 C dry bulb temperature and with a humidity ratio of 0.005
kg/kg is heated to 50 C in a reheater before entering the drier. The air leaves thedrier at 38 C with 70% relative humidity. On the basis of 1000 kg of product per
hour, calculate (a) the volume flow rate of air entering the reheater, and (b) the
(a) Volume flow rate of air entering the reheater = ( )( ) smvmV a
3
11 65.9862.02.11 === &&
(b) Heat supplied in the reheater = ( ) ( ) kW hhma8.240425.632.11
12 =−=−= &
4. A dryer is to deliver 1000 kg/hr of palay with final moisture content in the feed is
15% at atmospheric condition with 32 C dry bulb and 21 C wet bulb. The dryer ismaintained at 45 C while the relative humidity of the hot humid air from the dryer
is 80%. If the steam pressure supplied to the heater is 2 MPa, determine thefollowing:
(a) Palay supplied to the dryer in kg/h.(b) Temperature of the hot humid air from the dryer in C.
(c) Air supplied to dryer in cu m/h.(d) Heat supplied by the heater in kW.