The drying chamber of a freeze drying machine has to be vacuum tight. To freeze dry an object or solution it first has to be frozen so the water separates off from the other material as ice crystals. A vacuum is then created in the drying chamber that radically lowers the boiling point of the frozen water. Heat going into the frozen product while under vacuum promotes a process known as sublimation in which the ice comes away as vapour rather than melting. This leaves all the other material such as minerals, nutrients, volatiles etc. intact and eventually dry.
A separate, colder area of the freeze drying machine attracts the vapour to it where it condenses back to ice and is collected.
The factors determining this rate of sublimation are complex and the process is slow. The process becomes even slower however as the generation of vapour results in the evaporative cooling of the object being dried. To counter this cooling, the product must be given more heat. A surprising amount of heating can take place with the product remaining frozen.
A considerable amount of heat energy has to be removed by the condenser to turn this water vapour back to ice. The faster the drying cycle of the machine, the more powerful the condenser refrigeration has to be. Machines that operate on a 24 hour drying cycles have proportionally very powerful refrigeration systems.