Figure 9-17.—Air filter using rotating blades as element.air lines can create problems which are potentiallyhazardous, such as the freezing of valves andcontrols. This can occur, for example, if very highpressure air is throttled to a very low pressure ata high flow rate. The venturi effect of the throttledair produces very low temperatures which willcause any moisture in the air to freeze into ice.This makes the valve (especially an automaticvalve) either very difficult or impossible tooperate. Also, droplets of water can cause seriouswater hammer in an air system which has highpressure and a high flow rate and can causecorrosion, rust, and dilution of lubricants withinthe system.For these reasons, air driers(dehydrator, air purifier, and desiccator are allterms used by different manufacturers to identifythese components) are used to dry the compressedair. Some water removal devices are similar indesign and operation to the filters shown in figures9-16 and 9-17. Two basic types of air dehydratorsare the refrigerated-type and the desiccant-type.REFRIGERATED-TYPE DEHYDRATORS.–In refrigerated-type dehydrators, compressed airis passed over a set of refrigerated cooling coils. Oiland moisture vapors condense from the air and canbe collected and removed via a low point drain.DESICCANT-TYPE DEHYDRATORS.– Adesiccant is a chemical substance with a highcapacity to absorb water or moisture. It also hasthe capacity to give off that moisture so that thedesiccant can be reused.Some compressed air system dehydrators usea pair of desiccant towers (flasks full of desiccant).One is kept in service dehydrating the compressedair, while the other one is being reactivated. Adesiccant tower is normally reactivated by passingdry, heated air through it in the direction oppositethe normal dehydration airflow.Another type of chemical drier is shown infigure 9-18. This unit consists of the housing, acartridge containing a chemical agent, a filter(sintered bronze), and a spring. Various types ofabsorbent chemicals are used by the differentmanufacturers in the construction of thecartridges. To ensure proper filtering, the air mustpass through the drier in the proper direction. Thecorrect direction of flow is indicated by an arrowand the word FLOW printed on the side of thecartridge.Figure 9-18.–Chemical drier.9-13
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