Specifications

Operating Manual Freeze Dryer ALPHA 1-4 LD-2 / 2-4 LD-2, Version 0203, Page 32
This typical drying curve is due to the fact that the area of sublimation recedes into the
product and the water vapour still to be extracted has to pass through the already dried
layers. During the drying process the inner resistance increases. Thus the drying curve
is primarily determined by the latent heat of sublimation and the water vapour transport
speed. In order to increase the specific heat conduction properties of the product to be
dried and to keep the water vapour volume as low as possible it is necessary that
drying takes place as close as possible to the eutectic point.
The drying time depends heavily on the drying vacuum. The nearer the vacuum is to
the eutectic 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
Energy transport 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.