The Operational Zone

 

Notice that water only exists as ice below about -60C (-76F) regardless of pressure.

Notice that water does not exist as a liquid below about 611 Pascals (0.088 psi)

These two observations are important to freeze drying.  If we hold the system below 611 Pascals in pressure, we can be sure that water cannot be a liquid anywhere in the system, no matter what temperature the system is.  This allows us to use temperature alone to determine the state of water.

If you follow the heavy black line, you can see the process of freeze drying on the diagram.  

Starting at the top right point on the line, product is brought into the freeze dryer at room temperature and atmospheric pressure.  As the product freezes, it follows the line leftward until it is frozen, then we begin drawing air out of the freeze dryer.  This is indicated by the line going downward.  We keep drawing air out until we cross the sublimation line.  At that point, the water begins sublimating into water vapor.  All energy we put into the system (both temperature and vacuum) will be spent on latent heat.  When either the pressure drops well below the sublimation line, or the temperature rises, we can be sure the water has been removed from the product.

It is important to note, while water is changing states, the system will be near one of the state transition lines.  Only when the systems moves into  a region can we be sure the state transition has completed.  Don't forget, when the state of water is changing, ALL energy goes into latent heat.  That means the temperature does not change and neither does the pressure.  Those energies are being spent on transitioning the state of water.