|Vacuum Freeze-Drying, a Method Used to Salvage Water-Damaged Archival and Library Materials: A RAMP Study with Guidelines (UNESCO, 1987)|
|5. Alternate methods of drying|
The difference between vacuum freeze-drying and vacuum-drying is, in reality, a matter of pumps. In a freeze-dry system the pump has the capacity to pull a vacuum of 4mm Hg and less; at temperatures below 0°C (32 °F) water as a solid (ice) can, under certain conditions of pressure and temperature, pees to water as a vapor (sublimation). On the other hand, in vacuum-drying the pump has the capacity of pulling a vacuum of 5mm Hg and above; at temperatures greater than O °C water as a liquid passes to water as a vapor (evaporation). A graphic representation of this phenomenon can be seen if an imaginary line is drawn in Figure 1 to represent the levels of pressures and temperatures cited.
As we have seen, the boiling point of water as a liquid can be lowered if the air pressure is decreased. The reduced boiling point at low pressure finds considerable practical application in the field of vacuum evaporation (evaporation under low pressure). This process is of primary importance in the auger industry. Boiling off the water from the syrup at normal atmospheric pressure would char the sugar. However, the pressure is kept so low that the water may be removed at rather low temperatures.
In the aerospace industry, thermal-vacuum chambers are used to test the behavior of spacecraft such as weather satellites under simulated space conditions. Such chambers, rather large in capacity, are very effective for drying large quantities of wetted (not frozen) archival materials. In one such chamber there is a purged air system, a method where vacuum pressure at 45mm Hg is applied for about one and a half hours. Then the chamber is purged with dry air of less than 1 percent humidity; the air passes over heaters at a temperature of 60 °C (140°F). This operation continues for about two and a half days then the vacuum is applied again. The cycle lasts for about five days. Steam injection is used to eliminate evaporation from the wetted materials (15). Other methods of eliminating or trapping are available, for example, through the use of refrigerated condensers.