172
FREEZE-DRYING
.
prdposed methods is not the limiting factor for the drying rate but the ow of water vapeur
frointhe surface of the particles to some removal system has to happen even with the most
sophistiCated heating methods. A look at measured or estimated drying rates of various
‘
systems shows thispicture :. average drying rates as indicated by many authors as per
example J'
.
B. Lambert 13“, E. Petersen 13”, E. Seltzer 130, G. W. Oetjen
S. C. Tease 136,
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FIGUREI4. Patent drawing
22
a freeze—dryier in which the product is transported by vibration.
vary between approximately. -8
and 3.3 kg/m2 h, dependingon the type of trays;
the product, the applied te‘m‘peratures, and the applied pressures. At the higher rates
"approximately 7,000 m3/hï of water vap0ur have‘to be removed from —1 m2 if the assumed
operation pressure is approXimately 500 microns. Even if. a cr0ss-section between particles
10 %— of the whole heat—ing area would be available f0r the transportation of water vap0ur
:the average vetpour speedmust”be appto‘ximatèly20 to30 m/sec. If the gures’published by
:Ro"c‘kWæäll
per time and heated area becomes approxima—
Îtht'i‘s known:thàttbeevaporation rates atthe…beginning of the process are
'
timesäigher thanfthe average rates. Theräore, v‘apour velocitiesare
réaeh‘ed whichçwillblow away any dust or small particles, especially at the beginning of the
"Îdrying period. The increased drying rates therefore force on the: other hand to reduce the
.
layer fof:products:to avoid too many restrictions of
particles.lt
:shoul‘d‘be emphasized that neither heat transfennorîwapou‘r transportation inside the already
taken into
to point? out the‘necessary compromise more