What are the dryers?
Compressed air dryers remove moisture that might otherwise condense in airlines, air tools, and pneumatic instruments. This condensate can cause damage to equipment from corrosion, freezing, and water hammer, and can cause malfunctioning of instruments and controls.
Three types of air dryers are available: adsorption, deliquescent, and refrigeration.
Types of Dryers:
Adsorption Type:
The adsorptive or desiccant dryer contains a bed of an inert desiccant material, either silica gel or activated alumina, which has a high adsorptive surface area for a given weight and volume. This area is in submicroscopic cavities that can hold water vapour removed from the air. When the adsorptive desiccant is completely saturated with water, the water can be driven off again by heating. An airstream passed through the desiccant will carry away the released water vapour restoring the desiccant to its initial adsorptive condition.
Adsorption type dryers generally consist of two drying towers, each containing an adsorbent, plumbed in parallel. The dryer towers are cycled either manually, semi-automatically, or automatically so that one drying tower is on stream while the other tower is being reactivated. Reactivation is accomplished by means of electric or steam heaters embedded in the adsorbent or by passing dried process air through the unit.
Deliquescent (Absorption) Type:
The deliquescent or absorption dryer is lowest in initial cost but requires continual replenishment of the drying medium.
Simple in design, this type of dryer is a pressure vessel in which a bed of crystalline solids is placed on top of a screen which is located close to the bottom of the vessel. Wet air from the aftercooler and separator enters the bottom of the vessel and flows upward through the bed.
As it passes through the bed, the liquid water and vapour present in the air, dissolve the drying medium in what is termed a deliquescent effect. The resulting solution trickles to the bottom of the dryer where it is removed by a trap. The frequency with which the crystalline absorbent material must be replaced is a function of the design thickness of the bed and the amount of water and vapour present in the air entering the dryer.
Refrigeration Type:
Type. Dryers that remove moisture from the air by condensation incorporate a mechanical refrigeration unit or cold water, if available.
Inlet air passes through the precooler/reheater to the air-to-refrigerant exchanger which contains the refrigeration coils. As the air passes over the coils, further cooling takes place and moisture condenses into droplets. The droplets of oil or water then pass through the moisture/oil separator and are collected and drained through a condensate
trap. The cool, dry air is then directed back through the precooler/reheater, warmed by the incoming air and discharged for reuse by the system.
