Technical data

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 vapor 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
vapor 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 vapor 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 vapor
transport speed. In order to increase the specific heat conduction
properties of the product to be dried and to keep the water vapor
volume as low as possible it is necessary that drying takes place
as close as possible to the solidification point (eutectic point).
Operating Manual Freeze Dryer ALPHA 1-4 LSC / ALPHA 2-4 LSC General Information on Freeze Drying • 15