2
3.0 Description
3.1 Function
Externally Heated Purge Air dryers are an economical
and reliable way to dry compressed air to dew points
below the freezing point of water. 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. Adsorption continues until equilibrium is
reached between the partial pressure of the water vapor
in the air and that on the surface of the desiccant.
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. The switching from one tower
to the other is controlled by a solid-state controller on
either a fixed time basis (standard) or a demand basis
(optional).
1. Fixed Cycle Regeneration (Standard)
When a tower is placed off-line, it is slowly depressur-
ized and the desiccant is regenerated by driving off
(desorbing) the water collected on its surface. Regen-
eration is accomplished by expanding a portion of the
dried air to atmospheric pressure. This extremely dry
air (purge air) is then passed through a heater. This
heated air flows through the desiccant bed, desorbs
the moisture from the desiccant, and carries the de-
sorbed water out of the dryer. The heater is turned
off when the Heat Saver temperature sensor detects
bed heating is complete. The purge air continues to
flow through the off-stream tower, cooling the desic
-
cant bed and reducing the dew point spike after tower
change over. The tower is then repressurized to full
line pressure. This prevents desiccant bed movement
and downstream pressure loss when the tower goes
back on-line.
2. Optional Demand Cycle Regeneration
(With Venturi Blower)
When a tower is placed off-line, it is slowly depres-
surized and the desiccant is regenerated by driving
off (desorbing) the water collected on its surface. Re-
generation is accomplished by expanding a portion of
the dried air to atmospheric pressure through a venturi
blower. The venturi blower also draws ambient air into
the purge line. This combination of purge and ambient
air is then passed through a heater. This heated air
flows through the desiccant bed, desorbs the moisture
from the desiccant, and carries the desorbed water
out of the dryer. The heater and venturi suction valve
are turned off when the Heat Saver temperature sen-
sor detects bed heating is complete. Dry purge air
continues to flow through the off-stream tower, cooling
the desiccant bed and reducing the dew point spike
after tower change over. The tower is then repres-
surized to full line pressure. This prevents desiccant
bed movement and downstream pressure loss when
the tower goes back on-line.
4.0 Installation
4.1 System Arrangement
Install the dryer downstream of an aftercooler, separator,
receiver, and high-efficiency oil-removing filter(s) so that
the dryer inlet air is between 40°F (4.4°C) and 120°F
(49°C) and contains no liquid water or oil. Liquid water
and/or inlet air temperatures above 100°F (37.8°C) can
reduce drying capacity. Contact your local distributor for
information on proper dryer sizing at elevated inlet air
temperatures.
Adequate filtration is required upstream of the dryer in
order to protect the desiccant bed from liquid and solid
contamination. Use an Air Line Filter in systems supplied
by a non-lubricated (oil-free) air compressor. In systems
supplied by a lubricated air compressor, use a High Ef-
ficiency Oil Removal Filter. A coarser filter will be required
upstream of the Oil Removal Filter if heavy liquid or solid
loads are present.
To ensure downstream air purity (prevent desiccant dust
from traveling downstream) adequate filtration downstream
of the dryer is required. A High Temperature Afterfilter,
typically rated at 450°F (232°C) operating temperature
and capable of removing all desiccant fines 1 micron and
larger should be installed at the dryer outlet.
DANGER — This dryer must be fitted with a high
efficiency coalescing filter and liquid drainer that is
maintained properly. Failure to do so could result
in an in-line fire.
WARNING — The afterfilter, if installed, must be rated
for 450°F (232°C).
4.2 Ambient Air Temperature
Locate the dryer under cover in an area where the ambi-
ent air temperature will remain between 35°F (2°C) and
120°F (49°C).
NOTE:
If dryer is installed in ambients below 35°F (2°C),
low ambient protection requiring heat tracing and insula-
tion of the prefilter bowls, auto drains and/or sumps, and
lower piping with inlet switching and purge/repressurization
valves is necessary to prevent condensate from freezing.
If installing heat tracing, observe electrical class code
requirements for type of duty specified. Purge mufflers
and their relief mechanisms must be kept clear from snow
and ice buildup that could prevent proper discharge of
compressed air.
Summary of Contents for HPD Series
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