Specifications

The drying time depends heavily on the drying vacuum. The nearer
the vacuum is to the solidification point in accordance with the vapour
pressure curve above ice, the shorter the drying time is.
Interesting correlations :
1.0 gram of ice at
1.0 mbar assumes a volume of 1 m³ vapour
0.1 mbar assumes a volume of 10 m³ vapour
0.01 mbar assumes a volume of 100 m³ vapour
Heat Supply during Drying
The required heat supply to the product to be dried takes place
through direct heat contact in the drying chamber, heat conduction
through gas or through radiation. Heat transfer by direct contact and
heat conduction through gas are the most usual sources of heat in
today’s freeze dryers. The constraints caused by the former can be
seen in the following diagram.
Heat transfer takes place via the heated shelves by direct contact with
the bottom of the vessel and/or by convection via the shelf and vessel
or product.
At the beginning of sublimation the transfer of heat is very effective
from the wall of the vessel to the frozen product. However, soon an
area develops which is ice free, porous and dried and has a
corresponding temperature gradient between the wall of the vessel
and the product. The poor heat conductivity of the already dried
product can lead to an increase in temperature of the ice core. If the
core temperature rises above the solidification temperature, the
product begins to thaw. This applies especially to inhomogeneous
products and to great layer thicknesses. During this drying phase it is
important to regulate the heat supply and control temperature and
pressure precisely.
16 • General Information on Freeze Drying Operating Manual Freeze Dryer ALPHA 1-2 LDplus