Considering the lyophilization of vials, three different modes of heat transfer occur during the Freeze-Drying process:
✓ Conduction (Solid to Solid, Solid to Liquid)
✓ Convection (Liquid to Liquid)
✓ Radiation (Thermal Energy Transmission by Electromagnetic Waves)
∆ Conduction
Vials are solid glass containers containing material inside them. Therefore, vials and their material come in direct thermal contact with the shelves on which they are placed. Heat transfer starts from shelves, then to vials, and then to subsequent material under drying.
∆ Convection
This occurs due to bulk fluid motion. Circulating pressure enhances the rate of convective heat transfer, which speeds up the drying process.
∆ Radiation
Microwaves or thermal radiation occur inside the lyophilization chamber because of the heat emitted by the open space of the shelves. The vacuum also plays an important role in carrying electromagnetic waves.
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