According to the freezing place of the material, it can be divided into freeze-drying integrated type and freeze-dried separation type. Freeze-drying combined type means that the freezing and drying of materials are carried out in the freeze-drying box. The shelf in the freeze-drying box can be cooled and heated. The freeze-dried material is placed on the shelf, which mainly relies on conduction for pre-treatment Freezing and heating, radiation heat transfer is secondary. Most of the freeze dryers used in medicine are freeze-drying combined type, the purpose is to keep the materials clean and hygienic, and prevent pollution.
The freeze-drying separation type means that the pre-freezing is carried out in a cold storage, quick-freezer or liquid container, and then the pre-frozen material is moved to the drying box (bin) during drying. The purpose is to save energy, save drying time, and improve production efficiency. To reduce product cost, this kind of freeze dryer mainly uses radiation heating, which is mostly suitable for food freeze dryers.
According to the freezing method, it can be divided into static freezing, dynamic freezing, centrifugal freezing, shell freezing, spin freezing, spr.ay freezing, airflow Freezing (airblast freezing), etc.
The specific choice of freezing method should be determined according to the nature of the material and the requirements of the freeze-drying process.
According to the position where the water catcher (water vapor condenser) is placed, it can be divided into two types: built-in type and external type. The built-in water catcher is placed in the freeze-drying box, and can be placed on both sides, the bottom or the rear of the shelf of the freeze-drying box.
The built-in freeze dryer saves floor space, the resistance of the pumped gas to the water catcher is small, and the connecting pipes and valves between the drying box and the water catcher are omitted, which is economical. However, the distance between the built-in water catcher and the shelf is relatively short, which affects the heating uniformity of the shelf, is prone to thermal bridges, and affects the water-catching characteristics of the water catcher. Some built-in water catchers need to stop working when defrosting. Affected the production cycle and production efficiency. The external water catcher is placed outside the freeze-drying box and requires an independent shell. The freeze-drying box and the water catcher are connected with pipes and valves. Its advantages and disadvantages are just opposite to those of the built-in type.
According to the vacuum system used, there are two types: water vapor jet pump type and water trap plus mechanical pump type. The water vapor jet pump vacuum system does not need a water trap, and can directly extract a large amount of water vapor. The air extraction volume is large, the efficiency is high, and the air extraction volume can be controlled. However, water vapor is needed as power, and the pressure of water vapor generated by the boiler is easy to fluctuate, which affects the performance of the pump. , Burning boilers is likely to cause environmental pollution, so only a small amount of food uses this system. In the water trap plus mechanical pump system, a refrigeration system needs to be configured for the water trap.
Generally, in the refrigeration system of single-stage compressor, the minimum surface temperature of the evaporator is -35°C; in the refrigeration system of two-stage compressor, it can reach -50°C; in the refrigeration system of cascade compressor, it can reach -80°C. According to the need of vacuum degree, Roots pump plus water ring pump unit, Roots slide valve pump unit, Roots rotary vane pump unit can be configured in front of the water trap, or only various mechanical vacuum pumps that can directly discharge the atmosphere can be configured.
According to the heating method on the shelf in the freeze-drying box, it can be divided into direct heating and indirect heating.
Direct heating is to directly heat the shelf with electricity or various heat-carrying media, which has a fast heating speed and saves energy, but it is easy to cause uneven temperature on the shelf and is difficult to control. Indirect heating requires a heat exchanger. After the heat medium is heated by the heat exchanger, it is pumped into the shelf for circulating heating. Its advantages and disadvantages are just opposite to those of direct heating.
According to the working medium used for heating, it can be divided into oil, water, steam heating and electric heating.
The price of oil is high, the heat capacity is small, and the circulation volume is large; water is easy to scale in the shelf flow channel, which reduces the cross-sectional area of the channel and affects the heat exchange effect; the temperature and pressure of water vapor are not easy to control; the temperature uniformity of the electric heating shelf not good. Therefore, in recent years, various factories have prepared various cold and hot medium working fluids suitable for refrigeration and heating, such as ethylene glycol, Rll, etc. ⑨ According to the production method, it can be divided into two types: continuous type and intermittent (periodic) type.
The continuous freeze dryer has high cost and large production capacity, and is suitable for large-scale production of a single material; the advantages and disadvantages of the batch freeze dryer are just opposite to those of the continuous freeze dryer.