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3.0 Description
3.1 Function
Heat of Compression 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 des-
iccant. 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
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 based on a fixed time,
the time required to heat the off-line tower (standard) or
dew point basis (optional).
1. Fixed Cycle
When a tower is configured for regeneration, the
desiccant is regenerated by driving off (desorbing)
the water collected on its surface. Regeneration is
accomplished with the hot air from the final stage
of compression. The elevated air temperature and
resultant low relative humidity creates the necessary
dry purge air for regeneration. The hot purge air
flows down through the desiccant bed, desorbs the
moisture from the desiccant, and carries the desorbed
water out of the tower. Heating continues until the
minimum drying time has expired at which time the
towers switch.
2. Standard Cycle
When a tower is configured for regeneration, the
desiccant is regenerated by driving off (desorbing)
the water collected on its surface. Regeneration is
accomplished with the hot air from the final stage
of compression. The elevated air temperature and
resultant low relative humidity creates the necessary
dry purge air for regeneration. The hot purge air
flows down through the desiccant bed, desorbs the
moisture from the desiccant, and carries the desorbed
water out of the tower. Heating is “complete” when
the temperature of the air exiting the bottom of the
bed sensor rises to a preset level. The heating flow
rate and thus heat time will vary depending upon the
downstream air demand. High air demand will shorten
the time and low air demand will lengthen the heat
time. If heating completes quickly, purge air continues
to flow through the off-stream tower until the Minimum
Drying Time set point been met.
3. Dew Point Control (Option)
The regenerating tower is heated through in exactly
the same way as described previously; however, the
Dew Point Control makes use of an outlet dew point
transmitter sampling the dried air to monitor the outlet
air dew point and extend the drying period until the
bed capacity is exhausted before tower switchover is
initiated. Longer drying times extend bed life by mini-
mizing thermal cycling of the adsorbent. In addition,
extended drying times also minimize the frequency
of switchover temperature & dew point “spikes” and
increase overall reliability by minimizing valve cycles.
4.0 Installation
4.1 System Arrangement
Install the dryer as close as possible downstream of the
compressor to minimize temperature drop between the
compressor outlet and the dryer inlet. Air to the dryer must
be no more than 450°F and contain no oil liquids or vapors.
The standard dryer package includes a water-cooled heat
exchanger followed by a moisture separator to cool the
air and remove condensed water prior to passing into the
drying tower. The system is designed for 85°F maximum
cooling water temperature to achieve a maximum 95°F air
temperature to the drying bed. Temperatures above 95°F
(35°C) reduce drying capacity. Contact your local distribu-
tor for information on proper dryer sizing at elevated water
and/or inlet air temperatures.
To ensure downstream air purity (prevent desiccant dust
from traveling downstream) adequate filtration down-
stream 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.
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 separator bowl, auto drains, and lower piping
with inlet switching valves is necessary to prevent conden-
sate from freezing. If installing heat tracing, observe elec-
trical class code requirements for type of duty specified.
NOTE:
Low ambient temperature increases heat loss
from the vessel shell, resulting in lower adsorbent regen-
eration temperature and higher outlet dew points. The
tower insulation option is recommended for installations
exposed to temperatures below 50°F.