Chemical Processing - February 2008 - (Page 22)

Pressure drop The effect of delta P across the piping system is best viewed by looking at the mass flow through a system. A simple material balance across any vacuum system tells us the mass flow out of the equipment must be matched by the mass-handling capacity of the vacuum source. Per the ideal gas law, the mass equals the product of the operational pressure and the pumped gas volume. So, for a piece of vacuum equipment operating at 10 Torr with 100 ACFM of vapor being removed by the vacuum system, mass flow is 1,000 Torr-CFM (10 Torr × 100 ACFM). The vacuum source would have to have an equivalent mass-handling capacity, 1,000 Torr-CFM. However, it would have to handle that mass of gas at the vacuum level of the user equipment plus the pressure drop across the piping connecting the equipment to the vacuum source. In our example, if there were a 1 Torr pressure difference due to the design and sizing of the pipe, the vacuum source would have to handle the flow at 9 Torr (the operational pressure of the equipment minus the pressure drop) because the pressure would have to be lower at the vacuum source for flow to be in the direction of the vacuum source. The same mass flow, 1,000 Torr-CFM, handled at 9 Torr produces a vacuum capacity requirement of 111 ACFM, not 100 ACFM (1,000/9 = 111). So, the 10% pressure drop increases the size of the vacuum source by 11%. The generally accepted criterion of a well-designed vacuum system is that the pressure drop across the entire system not exceed 10% of the operational pressure. Air in-leak The expansion of in-leaks makes the requirement of the tightness of a vacuum system much more critical than in a compressed air system. There’s debate in the vacuum industry over the acceptable amount of in-leak. Of course the deeper the >> Glaring gap Figure 4. Gap is a tip-off to use of raised face or pressure flanges, which will require “high durable” gasket for vacuum service. level of vacuum used the less in-leak that can be tolerated. Remember that, besides its impact on the vacuum system, an in-leak also may ruin the product. For example, in the semiconductor industry, oxygen from an air in-leak can chemically react with silane to produce sand, SiO2, not the desired semiconducting solid. Piping practices suitable for pressure operations often aren’t sufficiently tight for vacuum operation. One common example is the use of threaded connections (Figure 1). While they work well for applications at atmospheric pressure and above, they aren’t good for vacuum because it pulls apart the mating thread surfaces responsible for sealing. The sidebar provides some more tips about piping design, while Figures 2–4 illustrate some proper practices. High vacuum design As the operational pressure deepens all these effects continue to magnify. At really low levels of vacuum (below 0.1 Torr), what may seem like minor things begin to be very important in design: • Cleanliness. This is essential to reach and maintain a high vacuum. Any dirt, liquid or oil will slowly offgas unless it has a very low vapor pressure, degrading the vacuum. So, ordinary oils and lubricants won’t do. Before use in the high vacuum chamber they all must be “pre-outgassed” or held in a deep vacuum until all volatilization has ceased. Even small smudges or fingerprints on the vacuum side of a vessel will outgas. • Metal surface smoothness. Small nooks and crannies on the metal surface can adsorb air and other gases, which then offgas slowly, degrading vacuum level. To prevent adsorption, deep vacuum vessels usually must have the surfaces facing vacuum polished. • Elastomers. Flexible materials used in gaskets, o-rings and seals can outgas at deep vacuum. Elastomers also www.chemicalprocessing.com >> Stress removal Figure 3. Employ a metal bellows connector to take pipe stress out of a system with a piece of rotating equipment. 22 • February 2008 http://www.chemicalprocessing.com

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Chemical Processing - February 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Nanoparticle Safety Raises Questions Take the Pressure Off Vacuum Systems Achieve Optimum Centrifugal Pump Performance Rethink Batch-Manufacturing Alarm Systems Dr. Gooddata Orlando Plant Pioneers HMI Migration Strategy Process Puzzler Plant InSites Equipment & Services Ad Index Product Spotlight/Classifieds End Point

Chemical Processing - February 2008

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