Vacuum
dryer

Principle

In vacuum dryer, material is dried by the application of
vacuum.

When vacuum is created, the pressure is lowered so that
water boils at a lower temperature. Hence, water evaporates faster. The heat
transfer becomes efficient i.e. rate of drying enhances substantially.

Construction

• It is made up of a cast iron heavy jacketed vessel. It is
so strong that it can withstand high vacuum within the oven & steam
pressure in the jacket.

• The enclosed space is divided into a number of portions by
means of 20 hollow shelves, which are part of jacket. These shelves provide
large area 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 air tight
seal. Oven is connected to a vacuum pump by placing condenser in between

Working

• Material to be dried is spread on trays. Trays are placed
on the shelves

• Pressure is decreased up to 30 to 60 kilopascals by means
of vacuum pump. Door is closed firmly

• Steam or hot air is supplied into the hollow space of
jacket & shelves. Heat transfer by conduction takes place

• At this vacuum, evaporation of water from the material
takes place at 25-30°C, on account of lowering of boiling point.

• Water vapour passes into the condenser where condensation
takes place

Advantages

1. Large surface area for heat transfer

2. Handling of material, trays & equipment is easy

3. Easy to switching over to next material

4. Hot water of desired temperature can be supplied

5. Electrically heated hollow shelves can be used

Disadvantages

1. Heat transfer coefficient are low

2. Limited capacity & used for batch process

3. More expensive than tray dryer, Labour & running cost
is also high

4. There is danger of overheating as the material is in
contact with steam heated surface for longer period

Uses

1. Heat sensitive materials, which undergo decomposition

2. Dusty & hygroscopic material

3. Drugs containing toxic solvents can be separated into
closed containers.

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

5. Drugs which required as porous end products