I. Classification of laboratory-series freeze dryers:
Laboratory-series freeze dryers prioritize compactness, light weight, versatility, stable performance, and high measurement accuracy. Designed for multi-purpose use, they accommodate freeze-drying experiments for a wide range of materials. The primary classification methods for laboratory freeze dryers are:
1. Classification by structure
① Bell-jar type freeze dryer: Features a stacked structure with a separate freeze-drying chamber and cold trap; the freeze-drying chamber lacks a pre-freezing function. Manual intervention is required to transition from the pre-freezing stage to the drying stage. Most laboratory freeze dryers are of the bell-jar type due to their simple structure and low cost. They often utilize a transparent acrylic cover, allowing for easy observation of the material during the freeze-drying process.
② In-situ type freeze dryer: Features two independent chambers—the freeze-drying chamber and the cold trap—where the shelves within the freeze-drying chamber are equipped with cooling capabilities. Once the material is loaded, both the pre-freezing and drying processes proceed without manual intervention. Although manufacturing is complex and costly, the in-situ type represents the future direction of freeze-dryer technology. It is an ideal choice for developing freeze-drying processes and is particularly suitable for pharmaceuticals, biological products, and other specialized materials.
2. Classification by function
① Standard shelf type: Materials are loaded in bulk onto trays; suitable for freeze-drying foods, traditional Chinese medicines, and powdered materials.
② Models with a stoppering mechanism: Suitable for drying materials in vials. During preparation, materials are dispensed into vials and the stoppers are placed loosely on top before freeze-drying begins. Once drying is complete, the stoppering mechanism presses the stoppers firmly into place; this prevents secondary contamination and moisture re-adsorption, facilitating long-term storage.
③ Manifold models: Flasks are attached to the exterior of the drying chamber to dry materials that have been shell-frozen onto the inner walls of the flasks. In this setup, the flask serves as the container connected to a manifold outside the drying cabinet, and the material inside is heated by ambient room temperature. A manifold valve system allows flasks to be removed or attached as needed without shutting down the machine.
④ Models with a pre-freezing function: The cold trap serves as the pre-freezing chamber during the material pre-freezing stage and acts as a water vapor condenser during the drying stage, capturing moisture released from the material. In these models, both pre-freezing and drying take place within the same unit, resulting in high operational efficiency.
II. Key parameters to consider when purchasing a freeze dryer:
1. Cold trap temperature
The cold trap is the component that captures moisture during the freeze-drying process. Theoretically, a lower cold trap temperature results in a greater water-trapping capacity; however, lower temperatures impose stricter demands on the refrigeration system, leading to higher equipment and operating costs. Laboratory-scale freeze dryers typically offer cold trap temperatures in ranges such as approximately -50°C and -80°C. A cold trap temperature of -50°C is suitable for products that are relatively easy to freeze-dry, whereas -80°C is appropriate for specialized products. Experiments regarding the impact of temperature on water-trapping capacity show a significant increase in performance as the temperature drops from -35°C to -55°C, but diminishing returns in capacity improvement are observed below -55°C. Therefore, unless there are specific requirements necessitating lower temperatures, a cold trap temperature of approximately -50°C is the ideal choice.
2. Cooling Rate
The cooling rate reflects the refrigeration system's cooling capacity. Under no-load conditions, the cold trap temperature should reach the specified value within one hour. For example, for a freeze-dryer with a rated cold trap temperature of ≤-50°C, the time required to reach -50°C—measured from the moment refrigeration is activated—should not exceed one hour.
3. Ultimate Vacuum Level
The ultimate vacuum level reflects the freeze-dryer's leak integrity and the vacuum pump's pumping efficiency. The vacuum level within the freeze-drying chamber should fall within a reasonable range. Excessively high vacuum levels hinder heat transfer and actually reduce drying speed; however, the no-load ultimate vacuum level of the chamber must reach at least 15 Pa.
4. Evacuation Time
Regarding the evacuation speed of the empty freeze-drying chamber, the pressure should be reduced from atmospheric pressure to 15 Pa within 30 minutes.
5. Control System
Freeze-dryer control systems vary in type and functionality. Laboratory-scale freeze-dryers are primarily used for optimizing freeze-drying processes and conducting small-scale trial production. Therefore, the control system should be capable of: displaying and automatically recording freeze-drying process parameters in real-time; setting, modifying, and effectively executing freeze-drying process programs; and providing communication interfaces to facilitate data acquisition and storage.