Compressed Air Best Practices - March 2008 - (Page 42)

| 03/08 Focus Industry FOOD PROCESSING | REAL WORLD BEST PRACTICES MEASURING PRESSURE DEWPOINT IN A COMPRESSED AIR SYSTEM The initial cost of this type of dewpoint monitors is usually higher than other types. The calibration costs are similar. Chilled mirror units are very well adapted for laboratories use and/or where pressure dewpoint control is very, very critical, such as when compressed air used with chlorine, or other volatile chemicals. Aluminum Oxide/Ceramic In this type, an aluminum oxide probe measures the water vapor pressure, which is directly proportional to the dewpoint. The sensor is made of an aluminum strip, which is anodized to create the oxide layer. Then a very thin film of gold is evaporated onto the oxide layer. This creates an aluminum oxide capacitor. Water vapor is transported through the gold layer and equilibrates onto the pore walls of the oxide layer. The amount of water molecules absorbed onto the oxide layer determines the conductivity of the pore walls. Each value of wall resistance equals a specific value of electrical impedance, which is related to water vapor pressure. The ceramic probe works the same way, but instead of an aluminum core, a chemically inert ceramic substrate is used. These probes are relatively sturdy and they are not as sensitive to dirt and debris as some other types of probes. As with all PDP sensors, a coalescing/particulate filter is recommended before the sample air reaches the probe. These filters should be inspected on a regular basis and replaced as necessary. The recommended calibration interval is one year. The initial cost for types of probes is lower than the chilled mirror type. The calibration and maintenance cost may be somewhat lower. This type of probe has a very accurate and fast response time for dewpoint readings. They are capable of having saturated air reach the probe and not cause damage. They will dry out in a relatively short period of time when the saturated air is replaced with dry air. The drying time is relative to dryness of the air passing over the probe. Practically speaking — if the dryer upsets the system and the sensor gets saturated, you merely have to correct the dryer problem and once the system air is dry, the probe will dry itself in a reasonable amount of time. Continued saturation of these sensors will ultimately lead to deterioration and a replacement will be required. Aluminum oxide and ceramic sensors are often recommended for use in the production facilities, due to the low initial cost, durability, accuracy and repeatability. They can be used in either desiccants or refrigerated dryer systems. Depending on the calibration range. Thin Film Polymer The thin film polymer sensor consists of two parts. One is a capacitive polymer bonded with a temperature sensor. The polymer measures the amount of moisture in relative humidity. The temperature sensor measures the temperature of the polymer sensor. From these two measurements, the onboard microprocessor calculates the dewpoint. Polymer sensors have a high tolerance to corrosive chemicals and are immune to condensation. They also have a long-term stability and accuracy. Some manufacturers offer an auto calibration feature on their monitors to compensate for wide changing dewpoint temperatures and at very low dewpoints. As the Relative Humidity reaches zero, very small changes in RH% result in large 42 www.airbestpractices.com View inside the aluminum oxide probe Schematic of the aluminum oxide sensor 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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