Vacuum Trap

In vacuum applications, a cold trap is a device that condenses all vapor except the permanent gases into a liquid or solid. The most common objective is to prevent vapors being evacuated from an experiment from entering a vacuum pump where they would condense and contaminate it. Particularly large cold traps are necessary when removing large amounts of liquid as in freeze drying.

Cold traps also refer to the application of cooled surfaces or baffles to prevent oil vapors from flowing from a pump and into a chamber. In such a case, a baffle or a section of pipe containing a number of cooled vanes, will be attached to the inlet of an existing pumping system. By cooling the baffle, either with a cryogen such as liquid nitrogen, or by use of an electrically driven oil vapor molecules that strike the baffle vanes will condense and thus be removed from the pumped cavity.

Applications

Pumps that use oil either as their working fluid (diffusion pump), or as their lubricant (mechanical rotary pumps), are often the sources of contamination in vacuum systems. Placing a cold trap at the mouth of such a pump greatly lowers the risk that oil vapours will backstream into the cavity.

Cold traps can also be used for experiments involving vacuum lines such as small-scale very low temperature condensations. This is accomplished through the use of a coolant such as liquid nitrogen or a freeze of dry ice in acetone or a similar solvent with a low melting point.

When performed on a larger scale, this technique is called freeze-drying, and the cold trap is referred to as the condenser.

Cold traps are also used in cryo pump systems to generate hard vacuum by condensing the major constituents of the atmosphere (nitrogen, oxygen, carbon dioxide and water) into their liquid or solid forms.