Vacuum Freeze-Drying Technology-Vacuum Pumps
Vacuum freeze-drying technology, a high-tech processing technology, is considered as the best method to produce high-quality dehydrated foods. The principle is that the water in ice state in previously frozen object is distilled directly in vacuum without getting melted according to sublimation so that the material is dried rapidly at a low temperature. This method is known as vacuum freeze-drying and food produced with this method is called freeze-dried food.
The most important equipment in freeze-dried food production is a vacuum freeze dryer unit, of which the performance, the energy consumption and the automation take a decisive part in the production technology of freeze-dried food manufacturers. Food freeze dryer contains as far two kinds, the intermittent and the continuous, which is rare in the production now. A intermittent freeze dryer is made up of five parts, drying oven, heating system, vacuum system, refrigeration system, control system.
Drying ovens are formed usually in two shapes, round and square. Each one has its own advantage and disadvantage. The round is easier to be manufactured and has less space to be used while the square offers more space for use on the contrary and is more difficult to fabricate.
Most of heating systems transfer heat by radiation. The radiation plate is made of anodized aluminum. Thermal conductive medium includes thermal oil, water saturation, secondary steam, secondary steam, organic solvent such as propylene glycol, glycerol, etc.. And all heat sources come from high-pressure steams.
The vacuum system is set up on mechanical vacuum pumps. Two settings of vacuum systems are widely used in process, Roots vacuum pump+oil sealing vacuum pump and Roots vacuum pump+Roots vacuum pump +steam ejector vacuum pump+liquid ring vacuum pump. The main difference between these two systems concern if it is capable to pump steam. The former can not pump water vapor (it requires a higher cold trap efficiency)and it consume loss energy while the latter could pump a small amount of water vapor at higher energy consumption.
The cooling system consists of chillers and cooling exhaust tubes. As a steam trap (also known as the cold trap) according to their position relative to the radiation plate assembly, there are several forms of cooling systems: ① bottom-mounted: placed in the bottom of the radiation plate assembly; ② side-mounted: placed on both sides of the radiation plate assembly; ③ rear-mounted: placed behind the radiation plate assembly; ④separately mounted: placed in another container which connects with a short and thick pipe.
The control system includes manual control and automatic control. Automatic control contains instrumentation automatic control and PLC programmable control. PLC programmable control is the most advanced technology. A customized freeze-drying process control instrument is designed at the base of PLC as freeze-drying process requests. It is capable to do the complicated control operations in freeze-drying process on its own, and to store the data as well.