Compressed Air Best Practices - March 2008 - (Page 41)

Focus Industry FOOD PROCESSING | 03/08 | Anytime pressure dewpoint is critical to production, it should be measured and monitored at the process. Measurement and monitoring ahead of the process will often alert personnel to a potential system malfunction before process deterioration, this will also help pinpoint the source of the problem for timely correction. The new aluminum oxide probes or sensors used today have dramatically changed this. They are much less sensitive and more important and will “dry themselves” if just left in a dry line. Plants with pressure dewpoint monitors disabled and not in use should review this new opportunity. Chilled Mirror Several companies offer chilled mirror type of dewpoint monitors. These are very accurate and have high repeatability if they are well maintained. The operation of this type sensor uses a polished metal surface such as Rhodium or Stainless Steel attached for a Peltier-cooling module. These metals are used, because of their excellent thermal conductivity. Current is supplied to the Peltier, by a servo controller, which causes the mirror to cool. An LED is used to illuminate the mirror and the reflected light is picked up by a photo diode. When water vapor condenses on the mirror, some of the light is scattered and not detected by the photo diode. The servo controller reduces the current to the Peltier causing the metal to heat up. The control system will modulate the amount of current to the Peltier to maintain a temperature to where the mass of water on the mirror is constant. The temperature of the mirror, then, is dewpoint. These chilled mirror systems do require a higher level of maintenance than some of the other types of sensors. They are sensitive to dirt and other contamination in the air lines. As the mirror gets dirty, the readings will become inaccurate. Most of these include self-compensating electronics to correct for the dirt on the mirror. Eventually it sets off an alarm to indicate a dirty mirror that needs cleaning. Over time, the surface may become “etched” by some contaminates and the mirror will have to be replaced. Some models do have field replaceable mirrors. Most Common Dewpoint Sensors Used Through The Late 1980’S And Early 1990’S: Hydroscopic Film Type Probe These sensors use a polystyrene core coated with a hygroscopic film wrapped with a bifilar winding of palladium wire. This sensor’s operation is based on its ability to change electrical resistance with small changes in humidity. The sensor usually contains a thermistor, which changes resistance proportionately to temperature changes. The resistance is measured in terms of electrical current. As the humidity changes so does the electrical resistance. This creates a linear relationship between current and dewpoint. This system incorporates an internal vacuum pump to pull the air samples from the main system. These types of sensors are very low maintenance, and cost due to the ability to change the probe in the field with no recalibration required. However, this technology is being replaced with newer, more accurate and repeatable types of probes, such as aluminum oxide, ceramic oxide, thin film polymer, etc. These are generally not recommended for continuing use in an industrial setting because the life span is shorter compared to the others. If this type of sensor ever becomes saturated it must be replaced. Aluminum Oxide (early versions) These were the probes of choice for almost all leading desiccant dryer manufacturers to monitor their -40 ˚F to -100 ˚F class pressure dewpoint performances. Through the early 1990’s, most commercial desiccant dryers particularly were equipped with dewpoint demand controllers and dewpoint monitors utilizing these types of sensors, which were very sensitive to transient moisture loads usually disabling the equipment until the probe could be replaced or serviced. Subsequently, many users tired of what they perceived as nagging problems and just quit repairing and using the controllers and monitors. www.airbestpractices.com 41 http://www.airbestpractices.com

Table of Contents Feed for the Digital Edition of Compressed Air Best Practices - March 2008

Compressed Air Best Practices - March 2008 Contents From the Editor Utility-Air News The Compressed Air Audit of the Month: Saving Energy with Blowers in Canada Do You Provide World-Class Service? Managing Rotary Screw Air Compressors at Midwest Bakery The Difference Between Acfm and Scfm for Proper Vacuum Sizing Best Practices — Multi-Module Process Air Dryers The Vacuum Formation Process in Building Compressed Air Filter Elements Solberg Manufacturing Real World Best Practices: Measuring Pressure Dewpoint in a Compressed Air System Resources for Energy Engineers: Training Calendar & Product Picks Wall Street Watch Advertiser Index Job Market

Compressed Air Best Practices - March 2008

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