Quick answer
If compressed-air dew point rises while the dryer appears to be running, verify the measurement first and then separate dryer-capacity loss from bypass leakage, drain failure, high inlet temperature, excessive flow and regeneration/refrigeration faults. A RUN indication proves only that the machine is enabled; it does not prove that moisture is being removed effectively.
Problem / job
Plant air becomes wetter, drains carry more condensate, instruments stick or dew-point alarms rise although the dryer shows normal run status.
Evidence
Capture dew point, dryer inlet/outlet pressure and temperature, flow/load, bypass-valve position, drain behavior, regeneration/refrigeration data and ambient condition.
Decision
Separate a real dryer performance loss from bad dew-point measurement, bypass leakage, excess demand, hot inlet air and condensate-removal problems.
As-found evidence capture
Trend the dew point together with compressor load, dryer inlet temperature, plant-air demand and ambient conditions. Confirm where the dew-point sensor is installed and whether its sample flow is correct. Note recent maintenance on filters, drains, refrigeration circuits, desiccant towers or bypass valves.
Seven-step diagnostic workflow
1. Verify the dew-point indication
Check sensor condition, sample flow and calibration against an approved reference or analyzer procedure. Contaminated sensors can report falsely wet or respond slowly.
2. Confirm bypass valves are fully closed
A partly open manual or automatic bypass can mix untreated air with dry air while the dryer itself continues to run normally.
3. Check inlet temperature and flow
Dryer capacity falls when inlet air is hotter or flow exceeds design. Compare current conditions with the dryer rating and the historical healthy baseline.
4. Inspect condensate separation and drains
Failed separators or drains can send excess liquid water toward the dryer and overload it. Confirm upstream aftercooler performance and drain cycling without defeating automatic drains.
5. Check the dryer-specific process
For refrigerated dryers, inspect refrigeration temperatures/pressures, heat exchangers and condenser airflow. For desiccant dryers, inspect tower switching, purge flow, heater/regeneration operation, valves and desiccant condition according to the dryer design.
6. Check prefilters and afterfilters
High filter differential pressure can reduce flow or disturb dryer operation, while oil carryover can damage desiccant and sensors. Review filter DP and element condition.
7. Verify recovery under representative demand
After correction, trend dew point through normal compressor loading and plant demand. Confirm the outlet remains inside the approved plant/dryer target without relying on a no-load test.
Evidence-to-decision table
| Evidence | Likely direction | Next check |
|---|---|---|
| Dew point worsens only at peak air demand | Dryer overloaded or bypassing | Flow, capacity and bypass valves |
| Inlet temperature higher than historical | Reduced dryer capacity | Aftercooler and compressor room conditions |
| Liquid water reaches dryer inlet | Separator/drain problem | Aftercooler separator and drains |
| Sensor reading conflicts with dry-air behavior | Measurement issue | Dew-point sensor and sample path |
| One desiccant tower performs poorly | Valve/regeneration/desiccant issue | Tower switching and regeneration data |
Use pressure dew point together with actual dryer load
A dryer can appear healthy at low demand and still fail when plant flow rises. Compare dew point with actual compressed-air flow, inlet temperature and pressure during the same period. If dew point deteriorates only during peak demand, determine whether the dryer is being overloaded, whether a bypass is leaking, or whether hot/wet inlet air is exceeding the condition used for the original dryer sizing. Do not judge performance from the RUN lamp or one quiet-shift measurement.
For a refrigerated dryer, compare refrigeration behavior, condenser condition and outlet temperature with the healthy baseline. For a desiccant dryer, compare tower switching, purge or regeneration behavior, heater operation where fitted and whether one tower repeatedly underperforms. Keep the diagnosis specific to the installed dryer type rather than applying one generic fault list to both systems.
Field verification after the repair
- Confirm the dew-point sensor and sample path are clean, correctly located and responding normally.
- Verify dryer bypass valves are in the intended operating position.
- Record inlet pressure, inlet temperature, flow/load and dryer outlet dew point under representative demand.
- Check separator and automatic-drain operation through a normal compressor loading cycle.
- Confirm prefilter and afterfilter differential condition is acceptable.
- Trend dew point long enough to cover at least one normal high-demand period rather than stopping at a short no-load test.
If the dew point rises again after a temporary recovery, look for the repeat-failure mechanism: intermittent bypass leakage, unstable drains, seasonal inlet-temperature change, overloaded compressor sequencing, sensor contamination, refrigeration cycling problems or incomplete desiccant regeneration. A recurring dew-point alarm should produce a trend-based diagnosis, not repeated alarm resetting.
Return-to-service acceptance criteria
Accept the dryer when measured dew point is stable within the approved target at representative flow, bypass paths are closed, condensate drains work, inlet conditions are inside design limits and the dryer process shows normal temperatures/pressures or regeneration behavior.
Common mistakes
Do not accept a RUN lamp as proof of drying. Do not change the dew-point alarm before verifying the sensor. Do not ignore a cracked-open bypass valve. Do not replace desiccant before checking regeneration and oil contamination. Finally, interpret dew point at actual plant demand, not only during a quiet period.
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