Sublimation Front Temperature too High

After evacuation and at the start of heating, the trend of Tice gets to close to the maximum tolerable Tice. A reduction in operation control pressure (pc) will immediately stop this trend (Fig. 2.37.1). (If the desired Tice is not reached pc could be raised.) As shown in Fig. 

2 the slope of the function TKt = f (pc) can become unfavorably steep. In this case, the shelf temperature should also be lowered or raised, but the effect of this change will take time before an equilibrium state is reached again (e. g. 0.5-1 h) depending on the heat transfer conditions and the heat capacity of the shelves and the heat transfer medium. 

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Note for the last two points A too high or too low ice temperature at the sublimation front can be changed by two operation data By changing the shelf temperature or by changing the heat transfer from the shelf to the bottom of the vials or trays. As shown in Table 1.10 Klol can vary from =117 kJ/m2 h °C at = 0.5 mbar to = 62 kJ/m2 h° C at 0.36 mbar (all other data constant). The change of Tice with the pressure is rapid as shown in Fig. 2.37.1. Figure 2.37.2 shows, that the range of control is approx. 10 °C (-28 °C to -18 °C) under the conditions of these runs, in which all other data have been constant. 

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