Water in compressed air can damage production machinery, resulting in downtime and spoiled product.
When pneumatic components wear or become corroded as a result of moisture, they consume more compressed air — and lose energy efficiency. When this wear or corrosion becomes great enough, components must be repaired or replaced — increasing operating expense.
The cost of replacement parts, labor, standby inventory, and downtime can have a devastating effect on the plant's bottom line. Eliminating even one of them by drying a system's compressed air will offset the cost of installing and operating the equipment to do the job.
Types of dryers
Dryers remove water vapor from the air, which lowers its dew point — the temperature to which air can be cooled before water vapor begins to condense. In broadest terms, there are four basic types of industrial compressed air dryers: deliquescent, regenerative desiccant, refrigeration, and membrane.'
Deliquescent dryers contain a chemical desiccant which absorbs moisture contained in the air, whether the moisture has already condensed or is still a vapor, Figure 1. The desiccant is consumed in the water-removal process and must be replenished periodically. The solution that must be drained from these dryers contains both liquid water and the deliquescent chemical, so disposal may be a problem. Local environmental regulations should be checked before disposal of this solution.
Deliquescent dryers reduce the dew point of the air 15° to 25° F below the inlet air temperature. If the incoming air has a dew point of 90° F, it will leave a deliquescent dryer with a dew point of about 65° F. Depending on operating conditions, some deliquescent dryers can produce dew points as low as 40° F; new deliquescent chemicals may produce even lower dew points. Two important points: desiccant level should not be allowed to fall below that recommended by the dryer manufacturer, and inlet temperature should be limited to 100° F or less to prevent excessive desiccant consumption.
Regenerative desiccant dryers remove water from air by adsorbing it on the surface of a microscopically porous desiccant, usually silica gel, activated alumina, or molecular sieve. The desiccant does not react chemically with the water, so it need not be replenished. However, it must be dried, or regenerated, periodically.
Heatless regenerative dryers use two identical chambers filled with desiccant. As wet air moves up through one chamber, a portion of the dry discharged air is diverted through the second chamber at close to atmospheric pressure, reactivating its desiccant. The moisture-laden purge air is vented to atmosphere. Some time later, air flow through the chambers is reversed.
Standard industry dewpoint ratings for these dryers is 40° F at pressure. By adjusting the flow rates and volume of purge air, 100° F pressure dewpoints can be achieved. These dryers, as with all desiccant dryers, should be supplied with oil-free air. Oil will greatly reduce the life expectancy of the desiccant.
Heat regenerative dryers also use two identical chambers, Figure 2. In this type, however, air flows through one chamber until its desiccant has adsorbed all the moisture it can hold. Then air flow is diverted to the second chamber. Internal heating elements or an external source of heat (steam or electricity) then dries the saturated desiccant in the first chamber. Because desiccant's adsorption capacity decreases as temperature increases, the dried desiccant bed must be cooled from the temperature it reaches during regeneration before it can be used again. The regeneration cycle in these dryers usually lasts several hours - 75% heating and 25% cooling.
Regenerative desiccant dryers can produce pressure dew points as low as 100° F. The type of desiccant used has a definite effect on the final dew point.
Refrigeration dryers condense moisture from compressed air by cooling the air in heat exchangers chilled by refrigerants. These dryers produce dew points in a range from 35° to 50° F at system operating pressure.
Most 20-scfm and larger refrigeration dryers reheat the cooled air after it has been dried, usually by routing it through heat exchangers in contact with the hot incoming air. Reheating the cooled air prevents condensation from forming on the exterior of air lines downstream from the dryer and also precools incoming air.
Standard refrigeration dryers should not be used where ambient temperature can drop below 40° F because lower temperature can freeze condensate. This will block air passages and could damage the dryer's evaporator. Dryers may be equipped with heat tracing packages for operating in ambient temperatures as low as 50° F.
Refrigeration dryers should not be operated in conditions where the incoming air and ambient air heat load is 15 to 20% of the rating — a 100-scfm rated dryer (100° F inlet and ambient) can freeze up if operated at 20 scfm and 40° F.