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U:\QA\Manualer\Arkiv\Manuals- Labogene-Origio-GKO\Manualer-WORD\ScanVac\CoolSafe
XS
XL Usermanual-Rev A5.docx
To understand the freeze-drying process, this section describes the theory for the
process in short.
For more detailed information about freeze-drying please consult available literature
e.g. “An Introduction to Freeze Drying” by James M. Flink & Henning Knudsen, ISBN
87-872-0084-8.
All material in free form will
exist in one of three states:
Vapour, liquid or solid.
As shown in the diagram for
water, the physical state is
dependent on the
surrounding pressure and the
temperature. The point
where all three phases of
water co-exist is called the
triple point.
Freeze-drying is basically a
change in state and
properties of the
material,(water), that is to be
removed. In principle the
process is very simple, but in reality it also can be very complex as the product can
consist of many components, which interact with each other.
Moving from one state to another requires the supply or removal of energy to or
from the product. This energy flux will have an impact on the structure of the
material being dried.
Freeze-drying is achieved using equipment that controls temperature and pressure
in the environment of the product and is a 3-step process.
In the first step, the product is frozen solid so that the water present in the
material is converted to ice (pre-freeze).
In the second step, the ice formed in pre-freezing is removed from the
product by direct conversion from solid to vapour. This process is called
sublimation. In order to start removing moisture in the sublimation process,
the pressure surrounding the material has to be brought down to a value
below that of the triple point, whilst keeping the temperature of the product
below its freezing point. The condenser surface temperature must also be
down to a temperature lower than the temperature of the material handled.
When the product is heated it will theoretically place itself somewhere on
the transition line between the solid and vapour states. The heating will
initiate sublimation of the moisture from the product to the surrounding
environment. The vapour will move to the condenser surface because of the
lower vapour pressure in the condenser.