14 • General Information on Freeze Drying Operating Manual Freeze Dryer ALPHA 1-2 LDplus
Warning:
Acidic products or products with a high solvent concentration
cannot be dried without special safety measures and devices e.g.
an additional LN
2
cooling trap for protection of the vacuum pump
(if necessary contact our company). Another possibility is to use
chemical resistant vacuum hybrid pumps (e.g. RC-5).
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.
If the gas ballast valve is open the extracted condensable vapours
will be emitted via the exhaust pipe together with air.
For this reason the gas ballast valve must be open during the
main drying phase!
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