In a vacuum dryer, the materials are dried by vacuum. Due to vacuum, there is a decrease in pressure. Therefore water boils at a lower temperature and then the evaporation of water takes place faster.
Construction of Vacuum Dryer
It consists of a heavy steam jacketed container made of cast iron. The closed chamber consists of shelves that are used to place metal trays consisting of material. The shelves provide a large area for heat conduction. The chamber is so strong to withstand under vacuum. The oven can be closed by a door. The oven is connected through a condenser and liquid receiver to a vacuum pump.
Working of Vacuum Dryer
The trays that are present in the dryer are used to dry the materials that are placed on the shelves and the pressure is reduced to 30 to 60 kps by the vacuum pump. The door closes firmly and steam passes through the jacket space and the shelves. So the heat transfer is carried out by the conduction mechanism. When evaporating under a vacuum, the water is evaporated from the material at 25-30°C. The vapor goes to the condenser. After drying the vacuum line is disconnected. Then the materials are collected from the trays.
Advantages of Vacuum Dryer
- Material handling is easy.
- Hollow shelves which are electrically heated can be used.
- It provides a large surface area. So the heat can be easily transferred throughout the body of the dryer and fast-drying action takes place.
- Hot water can be supplied throughout the dryer, which helps in the drying process at the desired temperature
Disadvantages of Vacuum Dryer
- A dryer is a batch-type process.
- It has low efficiency.
- It is more expensive.
- Labor cost is too high.
- Need high maintenance.
- There is a danger of overheating due to vacuum.
A vacuum dryer can be used for the drying of heat-sensitive materials, dusty materials, hygroscopic materials, toxic materials, feed materials containing the solvents, drugs which are required as porous end products, and for friable dry extracts.
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