Conduction Drying

CONDUCTION DRYING So far the drying considered has been by hot air. Other methods of drying which are qite common!y encontered are drying by contact with a hot srface" a continos #ersion of this is the drm or ro!!er dryer where the food is coated as a thin $aste o#er the srface of a s!ow!y re#o!#ing heated hori%onta! cy!inder. In sch a case& the food dries for as mch of one re#o!tion of the cy!inder as is mechanica!!y feasib!e& after which it is scra$ed off and re$!aced by fresh wet materia!. The amont of drying is sbstantia!!y contro!! ed by the rate of heat transfer and estimates of the heat transfer rate can be sed for ca!c!ations of the e'tent of drying. EXAMPLE 7.19. Moisture content of breakfast food after drum drying ( drm dryer i s being sed to dry a starch)based brea*fast food. The init ia! moistre cont ent of the food i s +,- on a wet basis& the drm srface tem$eratre is /01C and the food !ayer oter srface 221C. The estimated heat transfer coeffici ent from the drm srface to the drying food is 022 3 m )4 s ) 1C ) . (ssme that the thic*ness of the foo d on the drm i s 2./ mm and the t herma! condcti#i ty of the food i s 2.,, 3 m ) s ) 1C ) . If the drm& m diameter and m in !ength& is rotating at 4 re#5min and the food occ$ies three)qarters of the circmference& estimate the moistre content of the fi!m being scra$ed off. (ssme the cri tica! moistre con tent for the food materia! is 6- on a dry basis& and that condction heat transfer is throgh the who!e fi!m thic*ness to gi#e a conser#ati#e estimate. Initia! moistre content 7 +, - wet basis 7 2.+,58 )2.+,9 7 / *g *g ) dry basis. Tota! qantity of materia! on drm 7 8π ' D ' /569 ' ' 2.222/ m / 7 π ' ' /56 ' ' 2.222/ 7 +. ' 2 )6 m / . (ssming a densit y of the food $ast e of 222 *g m / & :eight on drm 7 +. ' 2 )6 ' 2 / 7 2.+ *g. O#era!! resistance to heat transfer& 5U 7 5022 ; 2.222/52.,, 7 .4, ' 2 )/ ; 2.,, ' 2 )/ 7 .0 ' 2 )/ Therefore U 7 ,,< 3m )4 s ) 1C ) q 7 UA ∆T 7 ,,< ' π ' D ' ' 2.+, ' 8/0 ) 229 7 6=.0 *3 s ) . >atent heat of e#a$oration of water 7 44,+ *3 *g )

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Conduction Drying