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How Does a Cold Trap Work?

A cold trap is placed on a vacuum pump to protect the vacuum from damage and aid in performance and durability. While a vacuum pump sucks up dirt, debris and some solvents, the cold trap assists by trapping condensable gases, including water and solvents. Cold traps are able to trap these solvents by freezing them to a surface or in a chamber. The purpose of trapping solvents is to prevent them from contaminating the vacuum pump and the stream of air that runs through the pump, which can lead to mechanical issues.

Function

Cold traps can help to trap liquid and gas.

The general process of cold trapping is called sublimation.  Sublimation means that gases freeze onto a trap or a cold metal surface or chamber, thereby turning into frost. There are several types of cold traps, each with very slight differences.  Types of cold traps include dry-ice cold traps, liquid nitrogen cold traps and mechanical cold traps.

Dry-Ice Cold Traps

Dry-ice cold traps are the least expensive.  These cold traps combine dry ice and alcohol in a trap well that reaches -75 degrees C. This trap well captures gases and solvents, condensing and freezing them with dry ice.  The user can later lift the trapping well out, defrost the material and dispose of it.

Liquid Nitrogen Cold Traps

Liquid nitrogen cold traps work in conjunction with pumps to freeze gas molecules, transforming the molecules from a gas phase to a crystalline phase, and skipping the liquid phase that usually happens in between.  The pump works at high speeds to frost the molecules onto a cold surface, so the molecules do not interfere with the vacuum pump flow. The frost can then be disposed of by the user. 

Mechanical Cold Traps

Mechanical cold traps are the most expensive type.  These traps are essentially mini refrigerators that gather molecules and freeze them into a chamber-type trap. Users can then defrost and dispose of the material.  Like a refrigerator, these traps use a mechanism to create a cold temperature.

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