Specifications

Operating Manual Freeze Dryer ALPHA 1-4 LD-2 / 2-4 LD-2, Version 0203, Page 33
Effects of freeze drying of a product in a dish
Heat transfer takes place via the heated shelves
1
, via direct contact with the bottom of
the vessel and/or via the gas between the shelves and vessel by means of convection.
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 melting 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 the temperature and pressure
precisely.
1
In so far as there are heated shelves, otherwise via radiation on the walls of the vessel or the
exposed surface of the product