Description; Dryer Function; Automatic Purge Saving System - Gardner Denver DGH 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 -150°F (101°C) [1 ppb @100 psig, 7.0 kgf/cm
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, 7 kgf/ cm
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.
] are
2
) of the dried air to atmospheric
2

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 patented 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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3. Description
3. Description