Vapour Pressure of Ice

The driving force behind freeze drying is the pressure differential between the product chamber and the condenser: the greater the difference, the faster the flow of vapour away from the drying product. The condenser must be significantly colder than the product for effective freeze drying. The table below shows the pressures/temperatures at which ice will sublime directly into a gas.

Note that the lower the temperature, the smaller the decrease in relative pressure. For example, the difference in pressure between 0°C and -10°C is (6.108-2.597=) 3.511 mbar, whereas the difference between -50°C and -60°C is only (0.0393-0.0108=) 0.0285 mbar. Freeze drying at very low temperatures becomes increasingly slow and impractical.

More information about freeze drying: get a copy of our Introduction to Freeze Drying booklet here, or read through some common Misconceptions in Freeze Drying.