Fisher ALPHA 1-2 LDplus Operating Manual page 22

Operating Manual Freeze Dryer ALPHA 1-2 LDplus
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).
General Information on Freeze Drying • 15