C, refrigeration units for chamber and condenser. D, vacuum pump. It must be vacuum tight, refrigerated, and have an opening that provides easy accessibility. The important feature of a refrigerated condenser is that it should be located in the direct path of moving water vapor molecules where they can be trapped. When contact with the condenser surface is made, the water vapors give up their heat energy, turn to ice crystals, and are removed from the system and prevented from traveling to the vacuum pump.
Condensers are primarily of two types, internal or remote.
The choice depends on the application desired. In the remote type the condenser is housed in a vacuum chamber which is separate from the chamber that houses the frozen objects. This is shown in Figure 2. This type of condenser can be isolated by a valve that will permit defrosting. Smaller freeze-dryers can be defrosted with warm water or natural air.