Operating Manual Freeze Dryer ALPHA 1-2 LDplus General Information on Freeze Drying • 15
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).
In general, operation with closed gas ballast valve is not
necessary. The recommended vacuum pump 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 K to
5 K). If the adsorptively 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. This typical
drying curve is due to the fact that the area of sublimation recedes
into the product and the water vapour still to be extracted has to
pass through the already dried layers. During the drying process
the inner resistance increases. Thus the drying curve is primarily
determined by the latent heat of sublimation and the water vapour
transport speed. In order to increase the specific heat conduction
properties of the product to be dried and to keep the water vapour
volume as low as possible it is necessary that drying takes place
as close as possible to the solidification point (eutectic point).
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