 Primary drying

Primary drying is the freeze-drying stage in which ice is removed mainly by sublimation. Frozen water becomes water vapor without first becoming bulk liquid water. This distinguishes it from secondary drying, which addresses residual sorbed water after the main ice-removal stage.^[ref]

A simple physical balance explains why removing frozen water still requires energy. If a mass $m$ of ice sublimes and the relevant specific latent heat is $L_s$, the idealized energy requirement isFor an illustrative $m=0.20\mathrm{g}$ and assumed $L_s=2.8\mathrm{MJ}\mathrm{k}{\mathrm{g}}^{-1}$, this gives $Q=560\mathrm{J}$. The example neglects heating of the remaining material and heat losses; it is an energy inventory rather than a drying schedule.

Thus reducing pressure does not remove the need for heat transfer. The phase change consumes energy while vapor must leave the material. A decrease in sample mass can establish removal of matter under appropriate assumptions, but identifying primary drying also requires evidence that the removed water was initially ice and followed the sublimation route.