Compressed air is used in almost every industrial plant. It is used for cooling components, actuating valves and other mechanical components, powering air-driven tools, purging enclosures, cleaning and blow-off, and pressure testing. Each use determines the quality of filtration and dryness required of the air supply. Generally, compressed air can be classified in terms of dryness and cleanliness: as Shop Air, Plant Air, and Instrument Air. (not standard industry terms.)
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| Refrigeration dryers |
Shop Air is the dirtiest and contains the most moisture. It has had only rough moisture separation, usually a condensate drain on the air tank. Plant Air has often been filtered, and typically passes through a refrigeration dryer. Instrument Air has been carefully filtered and normally dried to -40°F dew point or below using desiccant dryers. Compressed air is dried in one of three waysin refrigeration, regenerative, or deliquescent dryers.
In the refrigeration dryers, the air is chilled to condense the water and drain it away. The incoming air is cooled by the lower temperature air leaving the dryer in the air-to-air heat exchanger. The pre-cooled air also warms the outgoing air so the piping doesn't "sweat." The incoming air then passes over the cold section of the refrigeration coils, where the water condenses and is drained away. Since this type of unit would freeze up if the temperature dropped below 0°C, the controls are set to keep the coils above 3°C. The MMR 31 DewPro is ideal for monitoring this type of dryer.
Regenerative dryers use desiccant towers, which are packed with a chemical desiccant that absorbs the moisture from the air.
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| "Heatless" dryer |
One of the two types of desiccant dryers, "heatless" or "pressure swing" dryers has two towers or beds of desiccant alternating their duty cycles between drying the air and regenerating. The flow of the air at pressure is dried in one of the towers and sent out to the plant. In addition, a small purge flow of the dry air is sent through the regenerating tower at reduced pressure to dry the desiccant there. Drying during regeneration occurs due to the reduced pressure dropping the dew point below that of the dryer, thus removing the water from the off-line drying tower. This cycle is often controlled by a timer, which can switch at a rate ranging from minutes to several days. More sophisticated control systems use a moisture sensor to determine the switch point on dryer demand rather than on switching time. Several OEM manufacturers incorporate this technique.
The heat-regenerated dryer operates in the same fashion as the heatless model, except the towers or beds are heated by steam, gas burners or electricity during regeneration, accelerating the regeneration process. This dryer must have a cool-down cycle built into the control sequence to prevent the air from being overheated for plant use. Installation of our moisture probes in air systems using this type of dryer should have a cooling coil ahead of the flow cell. Both types of regeneration dryers are best monitored by the MMY 30 DewPro transmitter.
Deliquescent dryers are single-tower units charged with an absorbent salt that attracts moisture and gradually dissolves. The water is drained periodically and the chemical is recharged every couple of months. The MMR 31 DewPro transmitter is an excellent instrument for monitoring this type of dryer.
As businesses grow, their need for dry air increases. Eventually, this need may exceed the capacity of the dryer. At that point, the system begins to pump water as well as air. If the temperature drops in the piping, it is possible the lines will freeze. This possibility is increased if the piping is exposed to weather. The most likely time for freeze ups is when usage is low, e.g. during holidays and weekends.
Preventing just one freeze-up would more than pay for an instrument. Additional benefits of purchasing a General Eastern moisture instruments include being able to schedule maintenance on a demand basis, predicting the proper time to invest in additional dryers, and monitoring the start-up of new dryer installations.
Typical installations usually consist of either a DewPro transmitter or a single-channel analyzer and probe with a flow cell and diffusion tube. Sometimes a cooling coil is added. If multiple dryers are to be spot-checked, then a transmitter, or a probe and flow cell should be installed at each dryer. Probes used in this application need only to be recalibrated annually unless there are special circumstances.
Remember, for refrigeration and deliquescent dryers only, use the MMR 31. For desiccant dryers use the MMY 30 DewPro.
