Flow Meter Tutorial - Ultrapure Apps - Flow Control Septembe

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Flowmetering in Ultra-Pure Applications Run New Search by: Walt Boyes Pages: 41; September, 2000

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The enormous expansion of the semiconductor industry has created a market for flow control devices all its own. There are significant differences between the semiconductor "ultra-pure" industry, and other high-purity flow control industries, such as bottled water, pharmaceutical or food. There are similarities, but enough differences exist that make it necessary to treat applications in these markets differently. In the food and pharmaceutical industries, there are basic standards of cleanliness, along with the ability to be Cleaned-in-Place, that a manufacturer of flowmeters or controls must meet. Usually, stainless steel is the material of choice for both piping and sensors. In addition, in the food industry, the object is to keep a reasonably clean product, with a relatively short shelf life, from becoming contaminated during processing. In the pharmaceutical industry, there are additional criteria, including maintaining chemical purity, documentation and validation and accuracy in batch processing applications. Semicon Strict

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It is in the semiconductor industry, however, that we find the most rigorous requirements for material compatibility and purity, product performance and price sensitivity. In the manufacture of semiconductor wafers, the presence of contaminating material in the submicron range can ruin an entire run of chips. This has led to an increasing search for piping, fittings and valves and meters that are compatible with the widest possible range of fluids, from ultra-pure deionized water, to high-purity chemicals, to silicon slurry. The industry has migrated from commonly using PVC to PVDF (often sold under its original trade name, Kynar), to PTFE (originally, Teflon), to PFA. Now, the semiconductor industry uses these three basic materials: PVDF, PTFE and PFA.

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PFA is preferred wherever possible, and PVDF is only used where either PTFE or PFA are unsuitable for mechanical reasons or expense. For example, PTFE is very hard to injection mold, while PVDF and PFA are relatively simple molding materials. PTFE machines easily, but does not take pressure well, and

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is porous to some commonly used high-purity chemicals, such as hydrofluoric acid. PFA also machines well, and is much less porous, but is often 10 times the cost of an equivalent amount of raw PTFE stock. The semiconductor industry, surprisingly, is extremely cost conscious. Traditionally, most liquid flow applications were done with paddlewheel flowmeters, usually made of PVDF. Many applications still are. The problem with these flowmeters is their basic inaccuracy, and the tendency to shed debris, both from the bearings and from the PVDF itself. Even shafts made of rare alloys and ceramics shed large enough particles to be dangerous to the product. The semiconductor industry has long looked for a flowmeter that would meet some very basic requirements: • No moving parts; • Capable of being made entirely of PFA; • Reasonably high accuracy (one percent of span or better); • Able to handle clean liquids and slurries and • Very low cost. Three of a Kind Three types of flowmeters meet most of these requirements. None, however, meets them all. These meters are the Transit-Time ultrasonic; the Vortexshedding and the Coanda effect. The Coanda effect meter has not yet been made in high-purity materials. This leaves Transit-Time ultrasonic and Vortexshedding flowmeters as the preferred methodologies. Two basic methods are being used to make Transit-Time work in ultra-pure flows: embed the sensor in an axial or coaxial spool section; or make a sensor assembly capable of being non-invasively attached directly to a piece of existing PFA pipe or tubing. These meters only handle clean liquids, and are not low cost. (The cost of using Transit-Time flow in ultra-pure applications has reduced significantly over the last four years.) Vortex-shedding flowmeters have the advantage of being able to work in both clean liquids and some slurries, although when they are exposed to abrasive slurries, they quickly become worn and inaccurate. Vortex-shedding flowmeters also have the advantage of requiring less costly electronics than Transit-Time. Transit-Time meters, on the other hand, use multiplexed controllers. Most Vortex-shedding flowmeters are manufactured in PVDF. If they were made in PFA, their cost advantage over Transit-Time flowmeters would almost be entirely eliminated. PFA Vortex-shedding meters have reached the market just recently. Enter the Paddlewheel The issue of reducing the cost of high-purity measurement of flow is one that still needs to be addressed. A PVDF paddlewheel is still commonly used in applications, even where the product is inappropriately applied. There are plenty of stories about paddlewheels being used in scrubber slurry flow applications. They are used because they are inexpensive.

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Single-jet Pelton-wheel style flowmeters, from a variety of manufacturers in PVDF, PTFE and PFA, are common in the semiconductor industry. These units share the extremely inexpensive nature of paddlewheel flow sensors. They even, in some cases, share the paddlewheel. These meters can be used, but they are invasive. For high accuracy and wide range, they need jeweled bearings and abrasion-resistant shafts, such as zirconium or silicon carbide. However, these parts can produce pockets that are difficult to keep clean, as well as submicron debris. Meeting the performance criteria of the semiconductor industry's ultra-pure water requirements in an ongoing challenge for flowmeters manufacturers. Subscribe | Email | Site Map | Search | FAQ | Advertise

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