Sublimation is when a solid (ice) changes directly to a vapor without first going through a liquid (water) phase. Thoroughly understanding the concept of sublimation is a key building block to gaining knowledge of Standard-Type-Experimental-Vacuum-Freeze-Dryer.

Sublimation is a phase change and heat energy must be added to the frozen product for it to occur.

Sublimation in the freeze drying process can be described simply as:

FREEZE - The product is completely frozen, usually in a vial, flask or tray.
VACUUM - The product is then placed under a deep vacuum, well below the triple point of water.
DRY – Heat energy is then added to the product causing the ice to sublime.

FREEZE DRYING EQUIPMENT

The main components of freeze drying equipment are:

• Refrigeration System
• Vacuum System
• Control System
• Product Chamber or Manifold
• Condenser 

The refrigeration system cools the (ice) condenser located inside the freeze dryer. The refrigeration system can also be employed to cool shelves in the product chamber for the freezing of the product.

The vacuum system consists of a separate vacuum pump connected to an airtight condenser and attached product chamber.

Control systems vary in complexity and usually include temperature and pressure sensing ability. Advanced controllers will allow the programming of a complete “recipe” for freeze drying and will include options to monitor how the freeze drying process is progressing. Choosing a control system for the freeze dryer depends on the application and use (i.e. lab vs. production).

Product chambers are typically either a manifold with attached flasks, or, a larger chamber with a system of shelves on which to place the product.

The purpose of the condenser is to attract the vapors being sublimed off of the product. Because the condenser is maintained at a lower energy level relative to the product ice, the vapors condense and turn back into solid form (ice) in the condenser. The sublimated ice accumulates in the condenser and is manually removed at the end of the freeze drying cycle (defrost step). The condenser temperature required is dictated by the freezing point and collapse temperature of the product. The refrigeration system must be able to maintain the temperature of the condenser substantially below the temperature of the product.

In shelf freeze dryers, the condenser can be located inside the product chamber (internal condenser) or in a separate chamber (external condenser) connected to the product chamber by a vapor port.

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