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Technical Notes




Traps & Filters

Trapping and Filtering

The distinction in operating pressure ranges between traps or filters is apparent from any consideration of their operating characteristics.

Traps have no direct line-of-sight between inlet and outlet. In molecular flow, gas phase collisions are non-existent, meaning every molecule/atom in the gas flowing through the trap moves in a straight line (without any diverting gas phase collisions) and must hit at least one surface to traverse the trap. If a molecule is absorbed by, or freezes on, the surface, then it is successfully trapped. That is, traps work in molecular flow conditions.

Filters also have no (or limited) line-of-sight between inlet and outlet. Here, however, molecular flow is not the issue. Dust particles do not "flow" unless transported by a gas. Even the smallest particle is much heavier than a gas molecule and responds to gravity. To give dust sufficient momentum to "flow", it must have many gas phase collisions, and the gas pressure must be high enough to supply those collisions. That is, filters work in transitional or continuum flow conditions.

The filter's no line-of-sight is formed by either a fibrous/open cell foam element or a cyclone separator. The tortuous paths through an element have little influence on a gas's progress—some molecules hit surfaces and deflect, bouncing against others and causing them to deflect without ever hitting the element. By contrast, a particle's high mass and momentum cause it to continue on its "present path" and strike the element's surface (or continue down the cyclone separator's wall).

Backstreaming

While foreline trapping of oil vapor backstreaming from rough pumps is well established, there are critical aspects that are often misunderstood. Oil vapor in the foreline is a mixture of regular and "cracked" (lower molecular weight) molecules. These molecules, particularly the lower mass ones, migrate toward the chamber by a combination of surface "creep" and vapor transport following evaporation. A common—mistaken—belief is: a trap is not needed because the diffusion or turbo pump is an effective barrier.

The fluid in a diffusion pump may have a vapor pressure 2 or 3 decades lower than a rough pump's fluid (at the same temperature). When the rough pump's oil reaches the diffusion pump's boiler, it preferentially boils. Cold wall condensation is less effective and the rough pump's oil rapidly backstreams into the chamber.

The turbo pump's rotor, while spinning, effectively blocks vapor migration, however, when stationary, oil vapor migrates to the low level rotors. If the pump is frequently vented, the rotor blades' temperature can be well above ambient, promoting oil evaporation from the lower blades to higher levels until it arrives in the chamber.

To prevent oil vapor from reaching the chamber, stop it before it reaches the high vacuum pumps. Correctly install a good foreline trap and rigorously maintain it. [Correct installation includes mounting a high-quality, bakeable, shut-off valve on either side of the trap. Rigorous maintenance includes: frequent bakeout at 150°C to 300°C with upstream valve closed and heated; low pressure N2 gas flow through the trap during bakeout; before switching off the pumps, closing both valves (to prevent water vapor from entering the trap) and; venting the rough pump inlet after switching off (since anti-suckback valves frequently fail).]

What Are Traps and Filters?

In general, a trap is a device that captures gases and vapors, while a filter captures particles, chemical "smokes", and aerosols. These distinctions imply the devices work under different pressure ranges, loosely defined as: traps operate best at 10-3 torr or lower and filters operate best at 10-2 torr or higher (see sidebar, Trapping and Filtering).

Traps are placed in two positions:
  1. Between the high-vacuum pump and the mechanical pump—foreline trap
  2. In or near the chamber—system trap
Filters are positioned:
  1. Ahead of the rough pump—foreline filter
  2. At the rough pump's exhaust port—oil mist eliminator

When the application demands, multiple stages of filtering and/or trapping may be housed inside the same container.

Traps

Foreline Traps

Foreline traps are placed between a high-vacuum pump and backing pump or between a chamber and roughing pump. The primary application is preventing oil vapor migration from the backing/rough pump backwards along the foreline (backstreaming). A secondary application is trapping condensible vapors from the chamber process entering the pump (see "Vapors from Process").


Vapors from Pump

To stop oil vapors backstreaming, the foreline trap is filled with (a) a highly absorbent material, (b) a fibrous "wool", or (c) a re-entry vessel filled with a cryogen.

Highly Porous Materials are "honeycombed" with molecule-sized pores, cavities, and channels, and have huge effective surface areas ranging typically from 40 to 400 m3/g. Examples of better-known materials are MICROMAZE® plates, molecular sieve (zeolite), activated charcoal, and activated alumina.

The MICROMAZE trap material (proprietary KJLC™) consists of highly porous plates that are assembled in a labyrinth design. This causes gas molecules in molecular flow to make numerous surface bounces between the trap's inlet/outlet. Molecular sieves are silicates or alumina-silicates developed, mainly, as catalysts for the petroleum industry. Molecular sieve pellets are poured loose into the trap or in a metal-mesh basket. Activated charcoal, in granular form, has been used for many decades as a highly adsorbent material (particularly for ‘‘gas masks"). Activated alumina is a more recent development and may have the smallest total surface area; however, it has one distinct advantage (see "Vapors from Process").

These materials must be regularly regenerated by baking to modestly high temperatures and pumping. Removal of desorbing vapor is assisted by a low pressure N2 flow through the trap during baking.

Metal Wools

Copper and stainless wools—dense balls of thin, machined turnings—are placed in traps to obscure the direct line-of-sight. These conversion traps have much smaller surface areas than the porous materials; however, the quantity of oil vapor retained may be comparable because the wool's surface coverage is not limited to mono-layers (as are highly absorbent materials). The disadvantage of the conversion trap is the oil is only loosely bound to the surface. Re-evaporation can cause oil vapor to appear upstream of the trap earlier than with porous material traps.

Conversion traps are particularly useful for mobile pumping carts. When not in operation, such carts are vented and left open to the atmosphere. Traps containing porous materials load with water vapor and must be regenerated before use. Conversion traps do not load with water vapor.

Permanently sealed conversion traps are discarded and replaced at the end of their useful life. Replaceable element conversion traps must have the element changed before vapor breakthrough. The time between replacement depends on the ambient temperature, distance between trap and pump, type of pump, type of oil, service-induced oil cracking, etc. It is rarely possible to give a useful answer except "prevention is better than cure"— that is, replace frequently.

Re-Entry Vessel

The trap's body surrounds an inner volume that may be a coiled tube mounted close to the trap's walls or another container with a large opening in the top. The tube type is cooled by a water flow or by blowing LN2 through the tube. The container type accepts alcohol/solid CO2 slush to cool the inner container's surface. The vacuum between the trap body and the inner volume reduces thermal conduction and convection heat transfer, lessening the cryogen requirements.

Vapors from Process

Condensible Vapors

All oil-sealed rough pumps and most dry rough pumps should not pump large quantities of condensible vapors. Ignoring any reduction in the oil's lubricity that might arise from vapor condensing in the oil, the immediate problem is the pump's compression-rarefaction mechanism, which causes the vapor to cycle between liquid and vapor states, creating unacceptably high foreline pressures.

At first sight, at least two types of foreline traps noted in "Vapors from Pump" seem to fit this application: the porous trap and an LN2 cooled re-entry trap. While often pressed into such service, in reality, they are not ideal. Trapping process vapors is difficult. First, in contrast to low mass transfer during pump vapor backstreaming, process vapors often present gas loads large enough to overwhelm the relatively small capacity of the foreline traps. Second, large mass flows from the process imply the foreline and trap are in transitional or viscous flow regimes. The predominance of gas phase collisions in these regimes causes some fraction of vapor molecules to pass through the trap and into the pump without hitting a trapping surface.


Corrosive Vapors

As a separate concern, allowing corrosive vapor into a pump—even an anti-corrosion version—should be avoided wherever possible. A corrosion-resistant pump filled with an inert fluid provides a longer life than a standard pump for any given corrosive condition. But it will not provide complete protection against every corrosive vapor that may enter it. Again, prevention is better than cure.

Traps designed specifically for removing process or corrosive vapors use, typically, large-area labyrinth structures with plates cooled by chilled heat-transfer liquids or gas cryogens like the blow-off gas from LN2 tanks. Occasionally, different trapping techniques are combined in series/parallel within one trap body - for example, a chiller stage to remove bulk vapor followed by a large capacity absorbent stage.

System Traps

System traps are adopted here as the collective name for cold traps placed in or near the chamber. They are arbitrarily divided into (a) traps associated with high-vacuum pumps, (b) baffles used with, or substituting for, the traps in category (a), (c) cooled surfaces or shrouds mounted inside the chamber, often custom-built for a specific applications, and (d) Meissner traps mounted inside the chamber.

The lower the trap's surface temperatures, the higher the chances of residual gas molecule sticking, first bounce, on that surface. Expressed another way— at low temperature, the molecule's internal energy is low and is less likely to have sufficient energy to break the surface-molecule bond.

Water vapor is the major residual gas component in a normal vacuum system. If the trap's temperature is low enough to cause water vapor molecules to stick semi-permanently, the trap acts as a pump and the chamber pressure may be significantly reduced.

Traps for Pumps

These (commercially available) traps are most frequently used in conjunction with diffusion pumps. The trap is positioned between chamber and diffusion pump to prevent oil vapor backstreaming into the chamber. (In this respect, it is not unlike a foreline trap.) An additional benefit is water vapor condensed on an LN2 surface has a vapor pressure of 10-11 Torr. Providing the LN2 level is maintained, this trap pumps water vapor and reduces the chamber's total pressure.

As noted in "Trapping with LN2", this trap is not recommended for applications that generate or employ large quantities of condensible vapor. In addition, most turbo pump manufacturers do not recommend placing LN2 traps above their pumps. The argument is, when the system is vented, water frozen on the trap's surface thaws and drips into the pump. While it's reasonable to protect the pump, if there is sufficient water to drip, perhaps an in chamber LN2 trap should be used.

Baffles

A baffle is a trap shaped like a short tube with vanes formed into a chevron or double Venetian blind pattern. The vanes enable no line-of-sight but still allow a high gas conductance. The vanes are cooled, often by water circulating through the vanes or through a serpentine pipe connected to the vanes. Some baffles are designed for refrigerant gas cooling using a closed-circuit compressor system.

Cooled baffles are sometimes installed as a lower cost (but less effective) substitute for an LN2 system trap. However, water-cooled baffles are mostly used as thermal barriers, say between LN2 system traps and diffusion pumps.


Trapping with LN2

All traps that have a contained volume of LN2 require rigorous, yet simple maintenance—the LN2 level must be kept constant. Allowing long periods between fills causes large level changes and runs the risk of releasing vapors previously frozen on the trap's surfaces. With the re-evaporation of vapor comes two risks: (a) vapor entering the pump or (b) pressure buildup (if the system is shut off but not vented).

Obviously, (b) may cause an explosion at a structural weak point such as a glass envelope ion gauge or viewport. Less obvious but perhaps more dangerous is the potential chemical, toxic, or fire hazard created as the operator opens the chamber connected to a trap that is now at ambient temperature. As a safety precaution, any application in which questionable vapors are frozen should be thoroughly examined by the facility's safety group.

LN2 Shrouds

LN2 cooled shrouds have many applications, some of the more common ones being (a) space simulation chambers formed by a surrounding LN2 shroud just inside the main chamber wall, (b) titanium sublimation pumps with LN2 cooled shrouds have higher pumping speeds than room temperature shrouds, (c) beam trapping in molecular beam experiments, and (d) LHe cryostats are thermally shielded by an LN2 shroud.


Meissner Traps

When pumping a chamber from atmosphere, between ~10-3 to ~10-8 Torr, the major residual gas component is water vapor. A Meissner trap is, essentially, a coil of tubing through which a liquid/gas cryogen flows to maintain a low surface temperature. Given a low enough surface temperature, it is an efficient pump for water vapor.

Custom-built Meissner traps are often multiple loops of copper tube through which an LN2 is blown. Commercial Meissner coils, however, have a distinct advantage for certain applications. The cryogen flow from a (cooler/heater) compressor cools the coil from ambient to operating temperature in just a few minutes. This short delay to reach operating temperature and high water vapor pumping speed (examples are quoted from ~10,000 L/sec to >100,000 L/sec) makes these Meissner traps particularly useful for rapid pumpdown in chambers. Heating the trap from operating temperature to ambient takes a similarly short time, making these traps ideal for pumping water vapor in chambers that are frequently vented to atmosphere.


Filters

Foreline Filters

Foreline filters (often called inlet filters) are designed to stop/trap dust particles made in the vacuum process from reaching the pump. There are two basic forms. The first has a replaceable element (not unlike the air filter in a car), and the second uses a centrifugal principle to force the particles into a storage volume.

As noted earlier, it is not uncommon to combine different filtration elements (or methods) in one container. An example is filtering particles with a wide range of sizes. For this, a course filter element, to remove the larger particles, is placed ahead of a fine filter element, to capture the smaller particles. Such an arrangement has a longer service life before either element must be replaced.

But, it should be recognized that, in practice, most processes produce a wide range of particle diameters, and accepted that a fraction of the lightest, smallest particles will not be filtered from the gas stream.


Foreline Filters—Element

Elements are typically cylindrical shapes made from woven fabric, fibrous mattes, or open cellular materials. The materials used include cellulose (paper), polyester fiber, and glass fiber. Gas flow is often from outside the cylinder into the center through pores in the material that provide no line-of-sight. At the pressures needed to "suspend" particles and make them flow, the material does not seriously limit the gas conductance. However, the particles' momentum means they are less easily diverted and hit the element with which they become mechanically entangled.

The filter's gas conductance depends to some degree on the particle size it is designed to remove. A common polyester filter will stop particles 5 micron in diameter or larger and has a conductance from 35 cfm to 175 cfm (16 L/s to 64 L/s). Note that the element must be replaced at regular intervals because capturing dust reduces its gas conductance.


Foreline Filters—Cyclone

In the cyclone filter, dust-laden gas enters a filter's cylindrical body tangentially. The gas is pumped from a top, central exhaust while the particle's momentum causes it to continuously swirl around the chamber walls and eventually fall under gravity to the bottom of the filter body.

Cyclone filters are particularly suited to removing particles of higher mass or higher density which typically, but not necessarily, means larger diameter.


Mist Eliminators

The gas exiting a rough pump's exhaust valve bubbles up through the oil reservoir, often creating an aerosol of oil droplets. When displacing large gas volume, these droplets are visible as a mist at the exhaust. Mist eliminators are designed to stop these aerosols from entering the atmosphere.

The typical mist eliminator mounts above the pump's exhaust port and has a filter element that causes the mist to coalesce and drip back into the pump's oil reservoir. The element may be a ball of metal shavings, an inorganic wool, or an open cellular foam. Like trapping particles, noted above, the intent is to make the exhaust gas's path so torturous, the heavy aerosol droplets cannot follow the gas path and strike solid surfaces. Some designs use electrostatic fields to ionize and electrostatically deflect the droplets, but they are not yet widely used.


Pump Exhausts

Avoid breathing the aerosols that escape from a pump's exhaust port, particularly when the pump is under a heavy gas load. While no specific health hazard is known to this writer, breathing in the mist is unlikely to be beneficial to the lung's performance. An oil mist eliminator may trap a high percentage of the aerosol droplets, but any untrapped mist will enter the atmosphere surrounding the pump.

It cannot be overemphasized: mist eliminators do not stop the exhaust gases/vapors from any vacuum process—some of which are extremely hazardous. It is always prudent to assume the worst and duct gases exiting a mist eliminator from the room where the pump is installed to a proper exhaust abatement system or the atmosphere as required by the appropriate health and safety codes.

Oil Filters

Particles entering an oil-sealed rough pump may or may not chemically react but will certainly form sludge. An oil filter, primarily, removes particles to reduce wear on close-tolerance pump surfaces. Typically, filter elements trap particle sizes >0.2 microns. Activated alumina is often used to neutralize hydrous acid and Lewis acids that are part of the process exhaust gases.

A few large-capacity pumps have small oil filters integrated into the pump body. But for those that do not, external oil filtration units are recommended. One or more filter elements are housed in separate canisters and connected by pipes to the pump's oil reservoir. An electrically driven gear pump circulates the oil from the pump through the elements and back. As an incidental benefit, the filter provides cooling by circulating the oil for several seconds outside the body of the mechanical pump.

In multi-element designs, different filter elements may be chosen—one for particle removal and another for acid removal. Element materials have included activated alumina, cellulose (paper) fibers, Fuller's earth, and fiberglass.

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