Martin Christ Beta 2-8 LDplus - Manual (Page 23)

Operating Manual Freeze Dryer BETA 1-8 LDplus / BETA 2-8 LDplus General Information on Freeze-drying 23
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.