Martin Christ Alpha 2-4 LDplus - Manual (Page 18)

18 General Information on Freeze-drying Operating Manual Freeze Dryer ALPHA 1-4 LDplus / ALPHA 2-4 LDplus
2.2. Preparation
In case the unit is equipped with a pressure control valve the
vacuum pump should be warmed up. The operation temperature
should be reached before loading the vacuum pump with
condensable gases. In this way, the service life of the vacuum
pump can be considerably extended.
The vacuum pump can be operated already during the freezing
process when the pressure control valve is closed. The vacuum
pump should be warmed up for at least 15 minutes or it should be
switched on at least 15 minutes before starting the main drying
process.
It may occur that the vacuum in the ice condenser chamber or in
the drying chamber goes up during the main drying process (e.g.
from 0.63 mbar to 0.47 mbar) although the valve to the vacuum
pump is closed. This can be explained physically by the pumping
effect of the ice condenser (“Cryo Pumping Effect”).
2.3. Freezing
Small product quantities are frozen directly inside the ice condenser
chamber of the freeze-dryer. Larger quantities are pre-frozen in a
deep-freeze.
If liquids are to be dried in bottles with a layer thickness of more
than 1 cm we recommend that pre-freezing is carried out with a
shell or spin freezing device in a cooling bath (see picture). Due to
the centrifugal force the liquid to be frozen will rise on the wall of the
bottle and freeze. With this freezing process the layer thickness is
reduced and thus the total drying period is shortened considerably.
Cooling bath with spin freezing device
Shell freezing diagram
Spin freezing diagram
Freezing inside the unit is not necessary if the product is pre-frozen
or stored in e. g. a deep-freeze. In this case, especially when
freeze-drying small quantities, it is advisable to pre-cool the shelves
in order to avoid partial thawing during the evacuation.