Freeze dryers (freeze dryers) typically do not directly cool or heat samples, but rather achieve dehydration by freezing the samples and removing the moisture in the form of solid water (ice). The main working principle of a freeze dryer includes three steps: freezing, vacuum drying, and dehydration.

1. Freezing: The sample is frozen, usually at extremely low temperatures, to freeze the moisture in it. This step transforms water from liquid to solid, forming ice crystals.

2. Vacuum drying: After freezing, the sample is placed in a vacuum state. In this step, frozen water (ice) will sublime directly, transforming from a solid state to a gaseous state, bypassing the liquid phase. This process is called sublimation drying, which occurs at low temperatures (usually during freezing) rather than through heating. That's why freeze dryers usually don't require heating.

3. Dehydration: The sublimated water vapor is discharged from the vacuum system to ensure that all moisture in the sample is removed, leaving behind a dry sample.

Although freeze-drying machines usually do not perform active cooling or heating operations, the processes of freezing and sublimation drying involve temperature control. Therefore, freeze-drying machines need to have refrigeration and vacuum systems to maintain the required temperature and vacuum conditions. These systems are typically adjustable to ensure appropriate temperature is maintained throughout the entire freeze-drying process, in order to effectively remove moisture and maintain sample stability.

In summary, freeze-drying machines achieve dehydration by freezing the sample, drying it in vacuum, and sublimating the moisture, typically without the need for direct cooling or heating of the sample. However, it requires precise temperature control to manage the freezing and sublimation drying processes.