The internal heated design
is one of the most efficient reactivation systems using about 3% of the process air/gas for purge and the lowest heater kW.  This unit can also be adapted very easily for liquid applications, such as ethanol and solvents.

The exhaust heated purge design
uses about 6%+ to 8%+ of the process air/gas for purge. 

A blower-purge design does not use any process air or gas for purge, so there is not process air/gas loss.  This design can also be used for liquid applications depending on the process requirements.

A heatless dryer diverts about 15% of the dried air from the dryer outlet at a nominal 100 PSIG operating pressure.

Heatless dryer purge losses can be reduced to about 3% when coupled with a vacuum source for reactivation.  This can be added as a redesign option for existing dryers.


Open-system regenerative dryers are restored with either an internal or external heater, external heater-blower purge, or a heatless method.

An internally heat reactivated dryer uses a small portion of the dried air or gas for purge.  The heat source is composed of electric or steam units distributed in the desiccant beds that heat the adsorbent directly, thus vaporizing the adsorbed moisture.  When electric heating elements are used, they are located inside a well and are isolated from the desiccant.  When a steam line is used, the steam is not sent directly into the tower but is directed through a coil in the bed.  The dry purge gas is used to sweep the moisture from the bed and is independent of the inlet flow and pressure.

An exhaust heated-purge dryer heats a portion of the process air for purge before entering the regenerating tower.  This air can hold more moisture and thus picks up more water from the bed than the previous methods.  However, this method uses more purge air as a result because it has to heat the bed by convection, making it less efficient than the internal design; and it heats the bed to lower reactivation temperatures and therefore has a lower design bed capacity.  The purge is also based on the inlet flow rate so higher inlet flow rates equates to higher purge flows and lower net outlet flow rates.  Also, the purge rates have to be adjusted to higher rates for lower effluent dew point requirements.

A blower-purge dryer forces ambient air through an external heater with a blower, which is then heated and routed through the desiccant.  The hot air heats the desiccant and picks up water from the bed and then is discharged back to the atmosphere.

A heatless dryer diverts a slip stream of dry air for reactivation, and there is no reactivation heat source required.  Drying is done at pressure, and reactivation is done at atmospheric pressure.  The change in pressure is the primary driving force for reactivation.  Purge flows are a volumetric equivalent to the process flow and are calculated based on the system pressure.  (Higher system pressures result in lower purge flows; conversely, Lower system pressures result in higher purge flows.)  These dryers are the least efficient dryers compared to heat reactivated dryers; however, they are also the lowest in initial capital cost.  These dryers are best suited for intermittent operation, small compressor systems, and low flow rates.

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