By Dan Herring
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 or damage when introduced into or emanating from the vacuum pumping system.
In simplest terms cold traps work by sublimating a gas molecule, that is, by transforming the molecule directly from the gas phase to the solid (crystalline) phase thus bypassing the liquid phase. The gas crystallizes out on a cold metal surface often appearing as "frost" on the trap.
Cold traps should be chosen that are large enough and cold enough to collect the condensable vapors in a vacuum system. Cold traps and cold caps refer to the application of cooled surfaces or baffles 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 and which transfers heat from one side of the device to the other), oil vapor molecules that strike the baffle vanes will condense and thus 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.
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 and can achieve trap surface temperatures between -40ºC (-40ºF) and - 70ºC (-94ºF). Automatic defrost cycles and single and two-stage cascade styles can be purchased. 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 is used to produce a slurry, then placed in the trap well, which allows the surface of the vessel to reach -75ºC (-103ºF), 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 clean up 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”) thick with welded-in ports. Some designs use an acrylic plastic cover over the cold wall. These systems are typically lower in cost than alternative cold trap designs.
Liquid Nitrogen Traps
Liquid nitrogen cold traps prevent products arising during the equipment operation from passing from the chamber into the pump where they could 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.
Liquid nitrogen cold traps use cryogenic nitrogen as a chilling media to provide the necessary trapping surface temperatures. The trapping of water and oil vapor is complete and irreversible at liquid nitrogen temperatures, allowing a vacuum in the high 10-6 Torr range or better to be achieved.
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).
In some designs, the liquid nitrogen reservoir, typically electropolished 304 stainless steel can be removed for cleaning from the top via a quick clamp O-ring flange. Side ports can be used for diagnostics, gauges or venting. For ease of installation mounting tabs can be provided on the trap body.
Figure 1 Contaminated Diffusion Pump
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 so that 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.
One such configuration employs a water or Freon cooled baffle located directly above the pump throat for control of the oil that comes up from the pump (particularly under high vacuum conditions). An additional trap introduced closer to the vacuum vessel, cooled by liquid nitrogen, will further improve the efficiency of the system and allow it to reach low ultimate pressure. In many cases copper chevrons are brazed between the tanks containing liquid nitrogen as a coolant. The chevrons are arranged so as to provide a complete optical baffle (in other words, no material can move in a straight line and pass through the trap without impinging on one or more of the plates). Under these conditions, all materials that are condensable at the temperature of the plates will condense and remain in the trap rather than entering the system or the pumps.
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 obtainable will reach 10-9 Torr. The ultimate pressure attainable with a given pump would depend to an extent on the type of pump fluid used. This gain in ultimate pressure is, however, accompanied by a lowering of the pumping speed in this case by about 40%.
Figure 2  - Typical Pumping Characteristics With and Without a Cold Trap
Cold traps will reduce the capacity of the pumps, by as much as 50% at low pressures, due to the obstructions they present but there is no choice if lower system pressures are desired. 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 insure constant cooling of the baffles. This can be done manually but for large systems an automatic filling device is preferred.
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. Past practices 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, a system that is connected to a liquid nitrogen trap, under no circumstances, should the vacuum line with the cold trap be opened to air while the liquid nitrogen source (e.g. Dewar) is in place until the trap has been removed as 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. The same explosion hazard can be created if liquid nitrogen is used to cool a flammable mixture that is exposed to air.
Cold traps are a welcome addition to any pumping system where low vacuum levels or large contaminant loads are introduced into the vacuum environment. They should be considered by heat treaters concerned with improving product quality and wanting to reduce the frequency of pump maintenance.
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.
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