Description - SPX Hankinson HHE Series Instruction Manual

3.

DESCRIPTION

3.1 Dryer Function
• Dual tower regenerative desiccant dryers are an economical
and reliable way to dry compressed air to dew points below the
freezing point of water (dew points as low as -100°F (-73.3°C)
[0.2 ppmV @ 100 psig, 6.9 barg] are possible) or reduce the
moisture content of compressed air when used in critical process
applications.
• These dryers continuously dry compressed air by using two
identical towers, each containing a desiccant bed. While one
tower is on-stream drying, the other tower is off-stream being
regenerated (reactivated, i.e., dried out). The towers are
alternated on- and off-stream so that dry desiccant is always in
contact with the wet compressed air. In this way a continuous
supply of dry air downstream of the dryer is possible.
• Desiccant dryers lower the dew point of compressed air by
adsorbing the water vapor present in the compressed air onto
the surface of the desiccant. Desiccant is a highly porous solid
containing extensive surface area.
• Adsorption occurs until the partial pressure of the water vapor
in the air and that on the surface of the desiccant come into
equilibrium. As adsorption occurs, heat is released (referred to
as the heat of adsorption) and is stored in the bed for use during
regeneration.
• Desiccant is regenerated by driving off (desorbing) the water
collected on its surface. Pressure-swing (also called heatless or
heater-less because no outside heat is added) dryers regenerate
by expanding a portion (approximately 14 -15% at 100 psig,
6.9 barg) of the dried air to atmospheric pressure. This "swing in
pressure" causes the expanded air to become very dry (have a
very low vapor pressure). This very dry air (called purge air) plus
the stored heat of adsorption allows the moisture to desorb from
the desiccant. The purge air then carries the desorbed water out
of the dryer.
3.2 Automatic Purge Saving System
Featured with the Level 2 Controller, the Automatic Purge Saving
System is designed to save energy (purge air) when pressure-swing
dryers are operated at reduced loads.
The Purge Saving System operates by monitoring the changes
in temperature within the desiccant beds. These changes in
temperature are the result of heat (thermal energy) that is released
when a bed is on-line drying (heat of adsorption), and the heat
that is used when a bed is off-line being regenerated (heat of
desorption). The magnitude of these changes in temperature is an
indirect measure of the water vapor content in the air being dried.
This information is used to determine the time a tower stays on line
during the drying cycle.
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