INSTALLATION, START-UP & MAINTENANCE MANUAL
SD-220 TO SD-600
REV 04 – 10 Oct 2003
Page 12 of 32
3.2. DESCRIPTION OF OPERATION
The Sullair SD-220 to SD-600 adsorption dryer functions according to the pressure change
principle, using dry air regeneration. Two adsorption vessels filled with desiccant are
installed parallel to each other for this purpose. While the medium is being dried in one
vessel, regeneration takes place in the other vessel. The changeover from one vessel to
the other is performed in a fixed rhythm by means of a time-dependent control system. The
adsorption dryer operates continuously and fully automatically.
3.2.1. Description of the Operation
• The standard Sullair SD dryer operates on a 10 minute NEMA cycle. In this
configuration each desiccant chamber is drying for 5 minutes. This is known as a 5
minute half cycle. (EDP Dryers operate on a 4 minute NEMA cycle or 2 minute half
cycle).
• Wet air enters through the inlet switching valve and into the on line desiccant chamber
through the bottom. It is dried as it flows upward through the desiccant bed. Dry air
exits the dryer through the outlet check valve.
• At the outlet of each chamber, a portion of the dry air (purge air) is diverted to the
regenerating chamber. This dry air, expanded to near atmospheric pressure, will
remove previously adsorbed moisture as it flows downward through the desiccant bed
exiting through the purge exhaust mufflers.
• The purge air is controlled by a purge adjustment valve and purge orifices located in
the purge line. Purge pressure can be monitored on the purge pressure gauge. Refer
to section on purge adjustment for instructions on setting the purge rate.
• Near the end of each half cycle, the chamber being regenerated will be re-pressurized.
For this to occur, the exhaust valve of the chamber being regenerated closes while
purge air continues to flow. Re-pressurizing the regenerated chamber to operating
pressure before placing it into service minimizes fluidization of the desiccant and helps
prevent desiccant abrasion which causes desiccant dust to prematurely block pilot and
after-filter elements.
• Dryers operating at pressures greater than 120-125 PSIG or in an accelerated NEMA
cycle may be fitted with a re-pressurization valve. This valve assists the regenerated
chamber to reach dryer operating pressure before chamber switchover.
• Chamber switchover takes place when both towers are at full line pressure to minimize
desiccant movement.
• When switchover is complete, the regenerated tower will be placed into service and the
exhausted tower will depressurize and begin the regeneration cycle.
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