The freeze-drying of a freeze-drying machine is a technique that uses the principle of sublimation for drying. It is the process of quickly freezing the dried substance at low temperature, and then directly sublimating the frozen water molecules into water vapor under an appropriate vacuum environment. The product obtained from freeze-drying is called freeze-drying, and this process is called freeze-drying. The substance is always frozen at low temperature before drying. At the same time, ice crystals are evenly distributed in the substance. During the sublimation process, no concentration will occur due to dehydration, avoiding the side effects such as foam and oxidation caused by water vapor. The dry substance appears as a porous sponge, with a basically unchanged volume, and is highly soluble in water to restore its original state.

Intermittent freeze-drying machines are suitable for small batch production of multiple varieties, especially in the food industry for seasonal food production. Adopting single machine operation, if one device malfunctions, it will not affect the normal operation of other devices. Intermittent freeze-drying equipment is convenient for controlling the heating temperature and vacuum degree requirements at different stages of material drying, and the processing, manufacturing, and maintenance of the equipment are easy to implement.

In recent years, continuous freeze-drying machines have been explored and used. The characteristic of continuous equipment is that it is suitable for the production of products with sufficient variety and raw materials, and is particularly suitable for the production of pulp and granular products. Continuous equipment is easy to achieve automated control, simplifying manual operations and management, but its main drawback is high cost.

With the end of GMP certification, domestic freeze-drying equipment has fully entered the modernization stage, with complete functions, reliable operation, and stable performance. It can achieve online cleaning (CIP) or steam disinfection and sterilization (SIP), and all technical indicators can meet the needs of biological products and drug freeze-drying production. In contrast, there are more varieties and specifications of freeze-drying equipment in foreign countries than in China, with complete supporting equipment, more refined energy-saving structures, and a larger output of continuous freeze-drying equipment. To ensure the quality and energy efficiency of freeze-drying products, a combination of freeze-drying equipment and other drying equipment is often used.

How to improve the efficiency of freeze-drying, shorten drying time, and save energy while ensuring product quality in the future will be the common goal of workers in the freeze-drying industry.

During the sublimation process of freeze-drying, although there is a significant temperature difference between the shelf and the product, due to the fact that the plate temperature, condenser temperature, and vacuum temperature remain basically unchanged, the sublimation heat absorption is relatively stable, and the product temperature is relatively constant. As the product dries layer by layer from top to bottom, the resistance to ice sublimation gradually increases. The temperature of the product will also slightly increase accordingly. Until the existence of ice crystals is no longer visible to the naked eye. At this point, over 90% of the moisture has been removed. The process of large-scale sublimation has basically ended. In order to ensure that the entire box of products is fully sublimated, the plate temperature still needs to be maintained for one stage before proceeding to the second stage of heating up. Residual water and free state water have different physical and chemical properties, including chemically bound water and physically bound water. Due to the binding force of residual moisture, its saturated vapor pressure decreases to varying degrees, resulting in a significant decrease in drying rate. Although increasing the temperature of the product promotes the gasification of residual moisture, if the temperature exceeds a certain limit, the biological activity may also sharply decrease. Ensure that the drying temperature of the product is determined by experiments. Usually, in the second stage, the plate temperature is increased by around 30 ℃ and kept constant. In the early stage of this stage, due to the increase in plate temperature, there is less residual moisture and it is not easy to gasify, so the temperature of the product rises rapidly. But as the product temperature gradually approaches the plate temperature, the heat conduction becomes slower and requires patience to wait for a considerable period of time. The dry substance appears as a porous sponge, and the drying time of residual moisture is almost equal to the time of a large amount of sublimation, sometimes even exceeding.