Desiccant

   Externally Heated and Blower Purge Desiccant Air Dryers

Airtek heat reactivated desiccant air dryers use the adsorption method to remove moisture from compressed air. Pressure dew points ranging from minus 40 O F (-40 O C) to minus 100 O F (-73 O C) are achieved by directing the flow of saturated compressed air over a bed of desiccant. The most commonly used desiccant is activated alumina, a spherically shaped, hygroscopic material, selected for its consistent size, shape and extreme surface to mass ratio. This physically tough and chemically inert material is contained in two pressure vessels commonly referred to as "dual" or "twin" towers. As the saturated compressed air flows through the bed of the "on-line" tower, its moisture content adheres to the surface of the desiccant. The dry compressed air is then discharged from the vessel into the distribution system.
 

A master controller cycles the flow of compressed air between the towers. While one tower is "on-line" drying, the other tower is "off-line" regenerating. Regeneration, sometimes referred to as "purging", is the process of stripping the accumulated moisture from the "off-line" bed. Both types of Airtek heat reactivated dryers combine heat with either a small portion of the dried compressed air or with forced ambient air to affect regeneration. As heated, low pressure, purge air flows gently through the regenerating bed, it desorbs the moisture that had accumulated on the surface of the desiccant during the drying cycle and exhausts it to the atmosphere.

   Flow Schematic

Airtek's patented Multi-Port Regeneration System (TWP Series) affects superior desiccant bed regeneration and, as a result, provides better and more consistent performance. The Multi-Port Regeneration System injects heated purge air at precise points up and down the towers length providing a more balanced distribution of heat. This system prevents the desiccant on top from prematurely deteriorating while providing the bottom of the chamber with enough heated purge air to allow complete regeneration on every cycle.

The energy saving temperature monitor-ing system senses the exiting purge air temperature. When the purge air temperature increases to a pre-set point at which the desiccant bed is fully heated and regenerated, the blower and heater are turned off. Airtek's Secondary Blowdown System is standard on all TWP Series and TWB Series heat reactivated air dryers 1000 SCFM and larger. It improves perfor-mance and efficiency while increasing desiccant life. The depressurization stage strips moisture from the bottom of the tank through a purge muffler. Once depressurization is complete, the system switches to the main exhaust where final regeneration is accomplished with low pressure purge air. Bypassing the exhaust mufflers eliminates back pressure and allows for more thorough regeneration.

   Complete Air Treatment System

Without proper filtration, desiccant air dryers will not work. Desiccant dryers are designed to adsorb vapor from compressed air; they are not designed for liquid. When liquid, especially oil, is allowed to enter the desiccant chamber, it coats the desiccant material preventing any further adsorption. Oil coated desiccant can not be regenerated, and must be replaced. To protect the desiccant from contact with liquids, a coalescing pre-filter is required. The pre-filter must be properly sized and properly installed with a dependable automatic drain and a visual indicator to determine element condition. To protect downstream equipment from potential damage caused by the abrasive effects of desiccant dust, a particulate after-filter is also required.

Most field problems experienced with desiccant air dryers are the result of improper filter selection, installation, maintenance, and/or draining of condensate.

Considering the importance of filtration to dryer performance, Airtek recommends that all desiccant dryers be ordered as a complete, factory assembled Air Treatment System. The Optional Airtek Package "E" includes: properly sized, factory installed coalescing prefilter and particulate after filter with electronic drain system (No Loss Demand Drains standard on 400 SCFM and larger), and integrated digital element condition indicators.

Factory packaging, with matched components and single point connections reduces installation costs, ensures performance and allows Airtek to assume total responsibility for system integrity.