Freeze-dryer Gamma 1-16 LSCplus
Freeze-dryer Gamma 2-16 LSCplus
2 Layout and mode of operation
Version 04/2013, Rev. 2.10 of 09/03/2020
• sb-dc
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Translation of the original operating manual
2.2.2.3
Main drying
When the product is frozen, the main drying phase commences. The
vacuum pump is switched on. The pressure inside the drying chamber will
be lowered to the value that corresponds to the freezing temperature in
accordance with the vapour pressure curve for ice and water (sublimation
pressure curve). At the same time, thermal energy will be added to the
product. In the case of products in round-bottom flasks, wide-neck bottles,
etc., this is realised through the environment that is considerably warmer
(direct contact heat), in the case of unheated shelves by way of thermal
radiation from the environment, and in the case of temperature-controlled
shelves directly via the shelves. As a result, the sublimation process starts.
At the beginning of the drying process, the maximum drying rate will be
reached. The more the sublimation area recedes into the product, the
further the produced water vapour must pass through the layers that have
already been dried.
Under certain conditions, it is possible that the vacuum inside the ice
condenser chamber increases during the main drying phase (e.g. from 0.63
mbar to 0.47 mbar) although the valve towards the vacuum pump is closed.
From a physical point of view, this is due to the pumping effect of the ice
condenser ("cryo-pumping effect").
The required drying time depends strongly on the drying vacuum. At 1.0
mbar, one gram of ice takes up a volume of 1 m
3
of vapour, at 0.1 mbar a
volume of 10 m
3
of vapour, and at 0.001 mbar a volume of 100 m
3
. The
closer the vacuum is to the solidification point, the smaller is the resulting
vapour volume. The drying rate increases and the drying time decreases.
→
2.2.2.4
Final drying
Final drying is an option whenever one requires a product with minimal
residual moisture. In the physical sense, this process is a desorption
process, i.e. the removal of adsorptively bound water. Final drying is
performed under the lowest possible final pressure that depends on the ice
condenser temperature in accordance with the vapour pressure curve for
ice and water as well as on the final vacuum of the vacuum pump that is
used. The process is supported by a higher shelf temperature.
→
2.2.2.5
End of drying and aeration
The end of the drying process is reached when both the product and shelf
temperature are clearly in the positive range (+15 to +20°C) and if their
difference is not greater than 5 K.
Another indication of the end of the drying process is the behaviour of the
vacuum and of the ice condenser temperature. The ice condenser is no
longer subject to load and reaches the final temperature of approximately -
55°C or -85°C. The pressure in the drying chamber decreases in
accordance with the ice condenser temperature.
The vacuum pump will be switched off and the drying chamber will be
aerated via a rubber valve or via the aeration valve. The aeration valve can
also be used to flood the unit with nitrogen or another inert gas instead of
ambient air.
Then, the product can be removed from the unit.
