Cold Traps

In vacuum applications, cold traps are added to vacuum pumping systems either to remove unwanted contaminants (e.g., water, solvents, acidic or alkaline compounds) from the gas stream or to prevent pump backstreaming. These conditions can cause loss of efficiency, poor product quality or damage to the vacuum pumping system. Let’s learn more.

In simplest terms, the process of cold trapping (trapping of condensable gases) is a form of sublimation. In essense, the vacuum furnace acts as a sublimation apparatus where contaminates in solid or liquid are exposed to heat at reduced pressure and subsequently volatize, forming a gas that is then carried out of the vacuum chamber by the pumping system only to condense out on the chilled cold-trap surface, often appearing as frost or solid residue on the trap.

Cold traps that are large enough and cold enough to collect the condensable vapors emanating from the chamber should be chosen. Cold traps and cold caps are also used to prevent oil vapors from backstreaming (i.e. oil migration from the pumps into the chamber). In such cases, 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 cryogenic liquid such as nitrogen or by use of an electrically driven Peltier element (a thermoelectric heat-pump device in which one side is cooled while the opposite side heats up when a voltage is placed across the device, transferring heat from one side of the device to the other), oil vapor molecules that strike the baffle vanes will condense and be removed from the pumped cavity.

Cold traps are also recommended in systems where a large amount of outgassing or contaminants may be present. Examples include systems where “dirty” parts are run, brazing applications where filler-metal vaporization is present and freeze drying where a large amount of liquid must be removed from the vacuum environment.

Types

Vacuum pumps will often perform better and last longer when used in conjunction with an appropriate inlet trap or filter. Cold traps work well for condensable gases such as water, solvents or oils but are often called upon to handle other forms of contamination such as solids (e.g., carbon in the form of soot).

Several types are in common use.

• Mechanical refrigeration traps: These traps are basically small refrigerators and vary in size depending on the amount of gas to be processed. They can achieve trap surface temperatures between -40 to -70˚C (-40 to -94˚F). Automatic defrost cycles and single- and two-stage cascade styles are available. Mechanical refrigeration is considered the most expensive type of cold trap but also the one needing the least attention. They are limited in size, generally to about 0.20 m³/min (7 ft³/min), due to cost considerations.

• Dry-ice (foreline) traps: Dry ice and alcohol are used to produce a slurry placed in a trap well, which allows the surface of the vessel to reach -75˚C (-103˚F). This is low enough to condense most volatile materials. The trapping surface of the center well is visible during operation through the top view ring. Defrost and cleanup is made easy by lifting out the trapping well after venting.

A typical vessel consists of an electropolished type-304 stainless steel with an outer wall in the vicinity of 1.65 mm (0.065 inch) thick with welded-in ports. Some designs use an acrylic plastic cover over the cold wall. These systems are typically lower in cost than many other cold-trap types.

• Liquid-nitrogen traps: Liquid-nitrogen cold traps prevent contaminants from the chamber to make their way into the pumps, where they will either contaminate the pump (Fig. 1) or cause breakdown of the pump fluid. In either case, severe loss of efficiency and poor vacuum levels result. In the example shown, the diffusion pump was only capable of reaching 4 x 10^-3 torr rather than the typical 1 x 10^-5 torr range achieved with a clean pump. In addition, these types of cold traps prevent backstreaming of the pump.

The nitrogen traps are typically small, efficient and maintenance-free but must be filled and defrosted either manually or automatically. Handling of liquid nitrogen is very easy but can be dangerous if safe handling procedures are not followed (see below).

Cold traps should be used in all high-vacuum systems to prevent backstreaming of the vapor from the diffusion pump into the system. Although most pumping systems have very low backstreaming tendencies, the ultimate vacuum, which can be achieved by a given pumping system, is generally in the 10^-6 torr range. When cold traps are provided just above the pump throat, oil vapors passing this point are condensed and returned to the pump. The pump will then be able to reach a lower pressure than would otherwise be possible.

When comparing clean, outgassed and tight vacuum systems with and without cold traps (Fig. 2), it can be noted that without a cold trap the ultimate pressure in the vessel being pumped is in the neighborhood of 10^-6 to 10^-7 torr. By placing a cold trap between the pump inlet and the vessel, the ultimate pressure will reach 10^-9 torr. It should be noted that the ultimate pressure attainable with a given pump depends to an extent on the type of pump fluid used. This gain in ultimate pressure, however, is accompanied by a lowering of the pumping speed – in this case by about 40%. One solution to this problem is to make the trap diameter larger than the pump throat.

When liquid nitrogen is used, it is necessary to keep the reservoirs in the cold trap at a reasonably constant level to ensure constant cooling of the baffles. This can be done manually, but an automatic filling device is preferred for large systems.

Safety Considerations

Cold traps should be checked frequently to make sure they do not become plugged with frozen material. After completion of a vacuum cycle in which a cold trap has been used, the system should be vented in a safe and environmentally acceptable way. Otherwise, pressure could build up, creating a possible explosion and sucking pump oil into the system. Cold traps under continuous use should be cooled electrically and monitored by low-temperature probes.
Appropriate training and personal protective equipment, including gloves and a face shield, should be used to avoid contact with the skin when using cold baths. Dry gloves should be used when handling dry ice. Lowering of the head into a dry-ice chest is to be avoided because carbon dioxide is heavier than air and asphyxiation can result. The preferred liquids for dry-ice cooling baths are isopropyl alcohol or glycols, and the dry ice should be added slowly to the liquid portion of the cooling bath to avoid foaming. The common practice of using a combination of acetone and dry ice as a coolant is not recommended. Dry ice and liquefied gases used in refrigerant baths should always be open to the atmosphere. They should never be used in closed systems, where they may develop uncontrolled and dangerously high pressures.

Extreme caution should be exercised in using liquid nitrogen as a coolant for a cold trap. If such a system is opened while the cooling bath is still in contact with the trap, oxygen may condense from the atmosphere. The oxygen could then combine with any organic material in the trap to create a highly explosive mixture. Thus, under no circumstances should a vacuum line with a cold trap be opened to air while the liquid-nitrogen source (e.g., Dewar) is in place until the trap has been removed because there is a potential for the formation of liquid oxygen in nitrogen-cooled vessels. Also, if the system is closed after even a brief exposure to the atmosphere, some oxygen (or argon) may have already condensed. Then, when the liquid-nitrogen bath is removed or when it evaporates, the condensed gases will vaporize, producing a pressure buildup and the potential for explosion.

In Conclusion

Cold traps are a welcome addition to the pumping-system arsenal, especially where low vacuum levels or large contaminant loads are present. They should be considered by heat treaters concerned with improving product quality and wanting to reduce the frequency of pump maintenance. IH

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