Specifications

Operating Manual Freeze Dryer ALPHA 1-4 LD-2 / 2-4 LD-2, Version 0203, Page 31
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.
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 even when the gas ballast valve is closed (for several hours).
In general, operation with closed gas ballast valve is not necessary. The vacuum pump
used reaches with open gas ballast valve a final pressure which corresponds to the
water vapour partial pressure that can be reached.
The residual moisture of the dried product depends mainly on:
− the temperature of the dried product during the final drying process
− the final vacuum reached during the final drying process
The end of the main drying phase is reached, when the product temperature is nearly
the same as the shelf temperature (temperature difference between shelf and product
approx. 3°C to 5°C ). If the crystalline bound water is to be removed from the product,
the final drying phase can be started.
The following picture shows the drying process for a product containing approx. 10%
solid matter. During the first quarter of the main drying phase 50 % of the water
content is condensed. During the next quarter of the main drying phase 50 % of the
remaining water content is condensed. This continues until the drying curve
approaches the time axis asymptotically.