Twin-tower dryer for low pressure dewpoints (–40 to –100°F). Uses purge air to regenerate the offline tower. Required for outdoor lines, instrument air, paint, lab.
How it works
One tower dries while the other regenerates via pressure-swing purge. Towers swap every 5–10 minutes via a 4-way valve. Heatless models use ~15% of rated flow as purge air.
Key specs
- PDP–40°F standard, –100°F option
- Purge loss~15% of rated flow
- Cycle time5–10 min standard NEMA cycle
- Inlet temp max120°F
Sizing
- Size to actual CFM at inlet pressure; derate for higher temp and lower pressure.
- Account for purge loss: heatless ≈ 15%, heated ≈ 7%, blower-purge ≈ 2%.
- Target dewpoint determines bead bed depth: –40°F standard, –100°F = larger bed.
Installation
- MANDATORY upstream coalescing pre-filter (0.01 µm / 0.008 ppm oil) — oil destroys beads.
- Particulate after-filter downstream to catch bead fines.
- Bypass loop required — dryers fail closed.
- Vent purge mufflers outdoors or to a silencer; can be 95+ dB.
Preventive maintenance
- Daily: log dewpoint (≤ –40°F), verify tower switching, listen for purge.
- Monthly: drain pre-filter bowl, check pressure drop.
- Annually: replace pre-filter and after-filter elements; inspect check valves and purge mufflers.
- Every 3–5 yr: replace desiccant beads (activated alumina or molecular sieve).
Troubleshooting
Dewpoint rising / wet air downstream
Likely causes
- Oil contamination of beads (from upstream filter failure)
- Tower not switching (failed switching valve or controller)
- Insufficient purge (purge orifice plugged or purge valve leaking)
- Inlet temperature above 120°F
- Bead bed channeling or saturated
Fix
Verify pre-filter, confirm cycle timer/valve operation, measure purge flow, replace desiccant if oil-fouled (smells, brown).
Excessive purge / high energy use
Likely causes
- Purge valve stuck open
- Cycle timer set wrong
- Demand-cycle PDP sensor failed (energy-saver units)
Fix
Replace purge solenoid, reset cycle to factory NEMA cycle, calibrate or replace PDP sensor.