Operating Manual Freeze Dryer
ALPHA 1-4 LD-2 / 2-4 LD-2
, Version 0203, Page 30
4.2
Main Drying
The vacuum pump is switched on.
Please note:
Defrosting during the drying process is possible (visible foaming) when drying products
containing e. g. solvents or high salt concentrations. In this case it is necessary to
freeze the product as low as possible, e.g. in liquid nitrogen.
Warning:
Acidic products or products with a high solvent concentration cannot be dried without
special protective measures and devices e.g. a cooling trap for protection of the
vacuum pump (if necessary contact our company).
Special precautions are necessary when using azides because a dangerous
explosive develops in combination with copper or non-ferrous metals! It is
absolutely essential to consult our company!
As soon as sublimation of the water vapour from the frozen product begins, heat is
extracted and consequently the product continues to cool down.
The maximum rate of sublimation is reached at the start of the drying process.
Depending on the rate of sublimation the ice condenser temperature and thus the
pressure in the drying chamber respectively ice condenser chamber rises.
The duration of the main drying phase depends mainly on:
−
the layer thickness of the product
−
the solid content of the product
−
the heat supplied to the product during the drying process
−
the pressure inside the drying chamber during the drying process
With increasing pressure (not vacuum!) the rate of sublimation rises and the drying
period is shortened.
The water vapour generated during the main drying phase is not pumped off by the
vacuum pump but collected by the ice condenser.
The purpose of the vacuum pump is to lower the partial pressure of the non-
condensable gases so that the water vapour can be transported from the product to
the ice condenser.
However, small quantities of water vapour are also pumped off by the vacuum pump.
Therefore, the vacuum pump is equipped with a gas ballast device.
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