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Roughing Components
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Q: What effect does a molecular sieve trap have on the pumpdown time of a vacuum system?
A: A general rule-of-thumb is that a molecular sieve trap will add approximately 25% of the time it takes to pump down an untrapped system. For example: if an untrapped system requires four minutes of rough pumping to reach crossover pressure, adding a molecular sieve trap would add approximately one minute to the roughing time.
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Q: Can a single piece coaxial trap be cleaned and reused?
A: Yes, although it may take awhile for the trap to be completely free of the cleaning solvent. To minimize downtime, MDC recommends having a second trap on the shelf that can be swapped with an oil-laden trap. The removed trap can be cleaned when convenient and placed on a shelf ready for the next swap.
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Q: What is the advantage of a coaxial trap with replaceable filter over a single piece coaxial trap?
A: With a two piece coaxial trap, the internal filter can be quickly removed and replaced to minimize downtime when a trap becomes oil-laden. The trap body does not have to be removed from the roughing line - a definite advantage especially when the connecting ports are metal seal flanges.
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Q: What is a foreline?
A: The "foreline" name originated from early vacuum pumping methods. The most common of the early high vacuum pumps was the diffusion pump. It worked by spraying heated oil downward in a pump enclosure, and the oil captured air from the vacuum chamber. A mechanical roughing pump was used to remove the captured air from the diffusion pump enclosure. The vacuum line connecting the mechanical roughing pump to the diffusion pump was called the foreline. With advances in high vacuum pump technology, the role of a foreline has changed. The foreline is now the vacuum line from a roughing pump to any part of a vacuum system, and the name roughing line is frequently substituted for foreline.
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Q: What is the advantage of a cryogenic sorption pump over a mechanical roughing pump?
A: Most mechanical roughing pumps require a lubricant for moving parts. The oil could possibly migrate into the vacuum system, which could be catastrophic to some applications. For a completely oil-free system, a cryogenic sorption pump offers an ideal solution. Advances in roughing pump technology have led to oil-free mechanical pumps. However, dry mechanical pumps may be cost prohibitive, in which case a cryogenic sorption pump may be suitable.
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Q: When would a PVC roughing line be appropriate?
A: PVC roughing lines would be most appropriate when the desired level of vacuum does not exceed the performance of the roughing line. If an application requires only low vacuum - no better than 5 x 10-3 Torr - there would be no real need for installing stainless steel roughing lines. PVC roughing lines are an economical solution to installations requiring long lengths of travel between the chamber and roughing pump. For example, a large facility that provides a "house vacuum" to a number of locations would find PVC rigid tubing a reasonable substitute for stainless steel tubing.
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Q: Could a section of rubber tubing be used for a roughing line?
A: Probably not, unless it is reinforced to prevent collapse when the line is under vacuum. PVC flexible hose has is wire reinforced to withstand the substantial force of air pressure when the interior of the hose is at less than atmospheric pressure.
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Q: Why have a special up-to-air valve?
A: An up-to-air valve provides a way of equalizing air pressure in a vacuum system so that the vacuum system may be opened. External air pressure presses on any surface with a 14.7 pounds per square inch force. That force is considerable on even a small 2" valve, for example, which would make that size valve impractical as a way to allow air to enter a vacuum system: Force = Area x psi = pi x r 2 x 14.7 = 3.14 x 1 x 14.7 = 46 lbs. An up-to-air valve allows air to enter a system gradually without the turbulence of a large influx of air that would accompany a bigger valve.
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