Fisher ALPHA 1-2 LDplus Operating Manual page 21

14 • General Information on Freeze Drying
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!
The gas ballast valve can only be closed for final drying since there is
not much water vapour left during this section of the drying process.
During the main drying phase the moisture is removed by sublimation,
during final drying the bound moisture is removed by desorption.
This small quantity of water vapour generated during the final drying
phase can be pumped off by the vacuum pump even when the gas
ballast valve is closed (for some hours).
Operating Manual Freeze Dryer ALPHA 1-2 LDplus