Vacuum Dryer.



  • Vacuum drying is generally used for the drying of substances which are hygroscopic and heat sensitive, and is based on the principle of creating a vacuum to decrease the chamber pressure below the vapor pressure of the water, causing it to boil. 

  • Hence, water evaporates faster, and the rate of drying increases.


  • The oven is divided into hollow trays which increases the surface area for heat conduction.

  • The oven door is locked airtight and is connected to a vacuum pump to reduce the pressure.

  • The materials to be dried are kept on the trays inside the vacuum dryer and pressure is reduced by means of vacuum pump.

  • The enclosed space ( approximately 1.5 meter cube ) is divided in to a number of portions by means of 20 hollow shelves, which are part of the jacket. 

  • These shelves provide a larger surface area ( about 45 to 50 meter square ) for conduction of heat. 

  • Over the shelves, metal trays are placed for keeping the material. 

  • The oven door can be locked tightly to give an airtight seal. 

  • The oven is connected to a vacuum pump by placing a condenser in between.


  • The trays that are present in the dryer are used to dry the material that is placed in the shelves and the pressure is reduced to 30 to 60 Kps by 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 evaporated under vacuum, the water is evaporated from the material at 25 - 30℃.

  • The vapor goes to the condenser.

  • After drying, the vacuum line is disconnected.

  • Then the materials are collected from the tray.


  1. Vacuum dryer can be used for drying of following:

    1. Heat sensitive materials, which undergo decomposition.

    2. Dusty and hygroscopic material.

    3. Drugs containing toxic solvents. These can be separated in to closed containers.

    4. Feed containing valuable solvents. These are recovered by condensation.

    5. Drugs which are required as porous end products.

    6. Friable dry extracts.


  1. Material handling is easy.

  2. Hollow shelves which are electrically heated can be used.

  3. It provides a large surface area. So the heat can be easily transferred through the body of the dryer and the last drying action takes place.

  4. Hot water can be supplied through the dryer, which help in drying process at the desired temperature.


  1. Batch type process.

  2. Low efficiency.

  3. More expensive.

  4. Labour cost is too high.

  5. High maintenance.

  6. Danger of overheating due to the vacuum.

Commonly Asked Questions.

  1. With a well labeled diagram describe construction and working of Vacuum Dryer.

  2. Write a short note on Vacuum Dryer.

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