Compressed Air Best Practices - June 2008 - (Page 18)

| 06–07/08 | PHARMACEUTICALS TALKING DEW POINT How is dew point measured and monitored in most facilities? When discussing a typical facility’s compressed air system, it’s helpful to divide the entire network into two separate subsystems: the supply side and demand side. The supply side consists of the compressors and air treatment equipment up to the flow/pressure controller. The demand side consists of the distribution and storage systems or everything after the flow/pressure controller. On the supply side, dew point transmitters providing analog signals can be built into the dryer control system or can be installed in-line before or after the receiver tank. On the demand side, fixed mount instruments providing a local display, alarm relays and data logging capability are quite common throughout the distribution network and before critical end-use applications to give operators and plant personnel a quick assessment of dew point conditions at specific points in the system. This helps ensure that the dew point level of the air being produced at the dryers is maintained through the entire facility and to the end use points. Portable devices are an excellent tool for verifying dryer performance, conducting quality audits and checking the calibration of fixed mount instruments. dried exiting air. Some towers switch based on a timer, regardless of whether the desiccant has been fully saturated. By integrating a dew point sensor with the dryer control system, the towers will not switch until the dew point transmitter senses a degrading dew point temperature, thus ensuring full utilization of each desiccant tower and minimizing wasted purge air. How is dew point measured in point-of-use applications? For point-of-use dew point measurements, generally there are two options available — direct in-line insertion or sample extraction. Each method offers advantages and disadvantages that should be considered carefully. Direct insertion involves installing the probe through a threaded connection or “T” in the line. The benefits of this approach are ease of installation with no accessories required and no venting or loss of the compressed air. Line pressure fluctuations and sensor removal however can present drawbacks. The best installation for a dew point instrument isolates the sensor from the main line using a stainless steel sample line and sample cell. This setup allows for “valving off” from the main line and the ability to regulate the pressure, which has a considerable affect on the dew point reading. Easy installation and removal of the sensor can also be an important advantage. How is dew point measured by refrigerated air dryers? Refrigerated dryers operate by using a refrigerant to cool the supply air with heat exchangers (usually to between 35° F to 40° F) and condense out water vapor for removal by a moisture separator and drain. Due to their relatively low initial cost, long-term reliability and minimal maintenance requirements, refrigerated dryers often do not integrate a dew point transmitter into their design for monitoring or control purposes. What are the different prevalent technologies used to measure dew point? With the vast number of different hygrometer technologies currently available on the market for measuring a wide range of dew points suited to different applications and industries, it would be difficult to cover all of them here in any detail. I’ll limit the scope of the discussion to briefly address only the most common sensor types used in compressed air measurement. Condensation hygrometers, often called “chilled mirrors” operate by cooling a surface in a controlled manner until condensation begins to form; this temperature is recorded as the dew point temperature of the air. The most common detection method for determining when liquid water has begun to form is optical reflectance, which uses a light source to measure the amount of reflected light from the surface. These devices are well known for their high accuracy (usually +/-0.2° C dew point) but generally require more maintenance to keep their reflective surface clean. They become prohibitively expensive for measuring very low dew point temperatures. How is dew point measured by desiccant air dryers? Desiccant air dryers can benefit from a dew point sensor for monitoring dryer performance, controlling desiccant tower regeneration or both. Most regenerative desiccant type dryers (heated or heatless) produce a dew point of around -40°C /°F. Installing a dew point instrument with a display or with built-in alarm relays to measure the exit air from the dryer is a smart way to ensure good dryer performance. However, dryer efficiency can be significantly improved by using a dew point device to control the regeneration cycle — known as Dew Point Demand Switching (DDS). Desiccant dryers operate using two separate towers containing desiccant — one tower is always in operation while the other tower is being regenerated or purged using a portion of the 18 www .airbe stprac tices com ai rbestp ractic es.com http://www.airbestpractices.com

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Compressed Air Best Practices - June 2008 Contents From the Editor Utility-Air News A Pharmaceutical Compressed Air System Audit Talking Dew Point Puerto Rico, Pharmaceuticals and Airequipo Inc. Oil-Free Rotary and Oil-Free Centrifugal Compressor Comparison SMC: Managing Energy and Dew Point in Pneumatic Systems Utility Grade Compressed Air Systems Training Calendar & Product Picks Wall Street Watch Advertiser Index Job Market

Compressed Air Best Practices - June 2008

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