Diagnosis and elimination of dew point deviations in compressed air dryers

1. Problem description & scope of application

This guide addresses the critical issue of dew point exceedance in refrigerant compressed air dryers. A dew point deviation occurs when the measured pressure dew point of the processed compressed air exceeds the limit specified by the manufacturer (typically +3 °C at 7 bar overpressure according to DIN ISO 8573-1 Class 4). This leads to the undesirable formation of condensation in the downstream compressed air system.

All industrial applications that require dry and clean compressed air quality are affected, particularly in the food and beverage industry, pharmaceutical production, painting technology, instrument control and mechanical engineering. The consequences range from increased corrosion in pipes and devices to malfunctions in pneumatic components and loss of quality in end products.

Severity classification:

• Critical: Production stoppage, irreversible damage to sensitive machines, contamination of products.
• Major: Frequent failures of pneumatic components, increased maintenance requirements, reduced product quality, increased energy consumption.
• Minor: Occasional water leakage at tapping points, visible corrosion on non-critical parts.

2. Safety instructions

ATTENTION: Working on compressed air dryers, especially on refrigerant circuits and pressurized systems, involves considerable risks. Strict compliance with safety regulations is essential to avoid personal injury and property damage. Always consult the manufacturer's operating instructions.

• Switching off and locking (Lockout/Tagout – LOTO): Before any maintenance or repair work begins, the compressed air dryer must be disconnected from the power supply and secured against being switched on again. The compressed air system must be completely vented and depressurized. Check that there is no voltage using a suitable multimeter (VDE 0105-100).
• Personal Protective Equipment (PPE): Always wear safety glasses, chemical-resistant gloves (especially when handling refrigerants), hearing protection and safety shoes.
• Refrigerants: Refrigerants are under high pressure and can cause frostbite on contact. Work on refrigerant circuits may only be carried out by certified specialist personnel (DIN EN 378) using special tools. Refrigerants must be disposed of or recovered in accordance with environmental regulations (F-Gas Regulation).
• Residual energy: Pay attention to stored energy in compressed air tanks and in the refrigerant circuit. Compressed air systems must be completely depressurized before piping is opened.
• Electrical hazards: High-voltage areas and live components should be avoided. Use isolated tools only.
• Moving parts: Fans and compressors can continue to run even after switching off. Wait until all moving parts have come to a complete stop.

3. Required diagnostic tools

The following tools are essential for precise and systematic error diagnosis:

4. Checklist for initial assessment

Before starting a detailed diagnosis, a systematic initial assessment of the entire system must be carried out. This helps identify obvious problems and focus the diagnostic process.

5. Systematic diagnostic flow

This decision tree guides the technician through troubleshooting elevated dew point deviation. Always start with step 1.

1. Symptom: Pressure dew point at the outlet of the dryer is increased (> +3 °Ctd).
   1. Measuring the compressed air inlet parameters:
      • Test: Measure the compressed air inlet temperature and the inlet pressure at the dryer.
      • Setpoint: Inlet temperature max. +35 °C, inlet pressure ± 0.5 bar of nominal pressure.
      • Result:
        • Within target range: Go to step 2.
        • Inlet temperature too high: Eliminate the cause of the overheating (e.g. defective precooler, ambient temperature too high, lack of ventilation in the compressor room). After resolving the problem, measure the dew point again.
        • Inlet pressure too low: Check the compressed air network for leaks or insufficient compressor performance.
   2. Test of the dew point measuring device:
      • Test: Calibration of the dew point measuring device last when? Comparison measurement with reference device, if available.
      • Result:
        • Device correct: Go to step 2.
        • Device faulty: Calibrate or replace the measuring device.
2. Check refrigerant circuit:
   1. Connect pressure gauge:
      • Check: Connect the digital pressure gauge set to the high and low pressure sides of the refrigerant circuit. Record the pressures and the refrigerant suction and liquid temperatures.
      • Setpoint (example for R134a): High pressure 12-15 bar (at approx. +25 °C ambient), low pressure 2-4 bar. Temperatures according to manufacturer data sheet.
      • Result:
        • Pressures and temperatures in target range: Continue with step 3.
        • Low pressure too low, high pressure also low to normal: This indicates lack of refrigerant. Proceed to Step 7.1 (Leak Detection).
        • Low pressure too high, high pressure too high: This indicates heat exchanger contamination (condenser) or overfilling. Proceed to step 2b.
        • Low pressure too high, high pressure normal/low: This may indicate a compressor failure.
   2. Condenser check (external heat exchanger):
      • Check: Visual inspection of the condenser fins for contamination (dust, oil film, lint). Measurement of air temperature before and after the condenser.
      • Setpoint: Slats free from contamination. Temperature drop across condenser 5-10 °C.
      • Result:
        • Clean: Continue to step 2c.
        • Dirty: Proceed to step 7.2.1 (Condenser Cleaning).
   3. Evaporator test (internal heat exchanger):
      • Test: Measurement of the temperature difference between compressed air inlet (after precooler) and refrigerant suction gas temperature (after evaporator). Use of the thermal imaging camera.
      • Setpoint: Small temperature difference (e.g. 2-3 °C). Even temperature distribution on the evaporator.
      • Result:
        • In target range: Continue with step 3.
        • Low temperature difference or uneven temperature distribution: This indicates internal heat exchanger contamination. Proceed to step 7.2.2.
3. Check condensate drain system:
   1. Functional test:
      • Test: Observe the function of the condensate drain. Does it drain regularly (for electronic drains based on cycle time, for float traps based on fill level) and completely? Is there a constant hissing sound? Manual operation of the test button.
      • Setpoint: Condensate is drained periodically and completely. No permanent leakage.
      • Result:
        • Works perfectly: Continue with step 4.
        • No drain, irregular drain or continuous leak: This indicates a condensate drain malfunction. Proceed to step 7.3.
4. Check load adjustment:
   1. Capacity adjustment:
      • Check: Compare the actual compressed air volume flow (measurement or via compressor control) and the compressed air inlet temperature with the nominal output of the compressed air dryer. Is the dryer's refrigerant compressor constantly running under high load?
      • Setpoint: Current volume flow and inlet temperature < rated power of the dryer. The dryer's compressor cycles or runs in the partial load range.
      • Result:
        • In Target Range: Continue to step 5 (more specific checks or more complex errors).
        • Volume flow or inlet temperature exceeds rated power, dryer compressor runs continuously: This indicates a load adjustment problem (overload). Proceed to step 7.4.
5. Other specific tests (if the cause has not yet been found):
   1. Electrical components:
      • Test: Measure the voltage supply to the compressor, fan motor and all control components. Check the winding resistance of the refrigerant compressor and fan motors with the multimeter.
      • Setpoint: Voltage according to nameplate (e.g. 400V AC ±10%). Resistance values ​​according to manufacturer's specifications.
      • Result:
        • Deviations: Error in the electrical supply or motor/compressor winding. Correct the electrical fault.

6. Error-cause matrix

This matrix presents the most common symptoms, the most likely causes, the necessary diagnostic test and the expected results to quickly isolate the source of the problem.

7. Root cause analysis for each error

7.1 Lack of refrigerant in the circuit

Why it happens: A lack of refrigerant, typically caused by small but persistent leaks in the refrigerant circuit. These can occur at soldering points, seals, O-rings (e.g. on Schrader valves, pressostats) or through mechanical damage to refrigerant lines. Insufficient initial filling during installation or after repair is also a possible cause. The refrigerant is the essential medium that absorbs the heat from the compressed air and dissipates it to the environment. A deficiency significantly reduces the cooling capacity of the system.

How to confirm:

• Manometer readings: The low pressure side shows abnormally low pressure (e.g. for R134a below 2 bar), and the high pressure is also below the set point or normal.
• Temperature difference: Small temperature difference between compressed air inlet and refrigerant suction gas.
• Icing: If there is a severe deficiency, icing can occur on the evaporator or on the suction gas pipe of the compressor, as the evaporation pressure and thus the evaporation temperature drop sharply.
• Leak detection: Definitive proof is provided by locating the leak using an electronic leak detector (according to DIN EN 14624) or leak detection spray at all potential locations.

What damage it causes if left uncorrected: Continued exceeding of the dew point leads to condensation. The refrigerant compressor has to work longer and harder to achieve the desired cooling capacity, resulting in increased energy consumption and overheating of the compressor. This significantly shortens the life of the compressor and can lead to its premature failure, as the refrigerant also serves to cool the compressor.

7.2 Heat exchanger contamination

7.2.1 External heat exchanger (condenser)

Why it happens: The condenser is the element that releases the heat absorbed by the refrigerant into the surrounding air. Dust, fibers, oil mist and other particles from the ambient air get stuck in the fine fins of the condenser. This layer acts as thermal insulation and hinders heat transfer. This means that the refrigerant cannot cool and condense efficiently enough.

How to confirm:

• Visual inspection: Clearly visible contamination of the slat surfaces.
• Refrigerant pressures: The high pressure in the refrigerant circuit is increased (e.g. for R134a over 18 bar) because the refrigerant cannot release its heat and thus the condensation temperature increases.
• Temperature: The air temperature after the condenser is only slightly higher than before it, the temperature difference across the condenser is too small.

What damage it causes if left uncorrected: A dirty condenser leads to permanently increased high pressure. This requires more compression work from the refrigerant compressor, which increases energy consumption and increases the thermal load on the compressor. The result is premature compressor wear, lower cooling capacity and thus a dew point being exceeded.

7.2.2 Internal heat exchanger (evaporator)

Why it happens: Particles, oil residue from the compressor or rust deposits from the compressed air network can get stuck on the compressed air side of the evaporator. These deposits form an insulating layer on the heat exchanger surfaces. This makes the heat transfer from the warm, moist compressed air to the cold refrigerant inefficient and the compressed air cannot be cooled sufficiently.

How to confirm:

• Temperature difference: Too small a temperature difference between the incoming compressed air (after precooler) and the refrigerant suction gas.
• Thermal Imaging Camera: Thermal imaging analysis of the evaporator may reveal uneven temperature fields or cold spots that indicate internal blockage.
• Dew point: The dew point is elevated even though the refrigerant circuit is functioning nominally (pressures and temperatures are correct).

What damage it causes if left uncorrected: The compressed air is not cooled down sufficiently, which directly leads to the dew point being exceeded. Continuous stress on the system results in increased energy consumption as the dryer's compressor attempts to compensate for inefficient cooling. In the long term, the deposits can hinder the flow of compressed air and lead to increased differential pressure.

7.3 Condensate drain malfunction

Why it happens: Condensate drain valves, be they mechanical (float-controlled) or electronic (time-controlled), can become blocked by dirt particles, rust or oil residue. A blocked valve that remains closed will cause condensate to back up in the evaporator. This reduces the effective heat transfer area and can lead to condensate entering the downstream compressed air network. If a valve remains open permanently, this leads to unnecessary loss of compressed air.

How to confirm:

• Visual and audio test: Observe the draining process. A lack of draining noise, emptying only in droplets or a permanent hissing sound (in the case of permanent leakage) are indicators.
• Manual operation: Press the test button on the valve. The valve should open and drain condensate.
• Moisture in the system: Despite the nominal function of the refrigerant circuit, the compressed air is moist or condensation forms in the deepest places in the compressed air network.
• Electrical testing: For electronic drains, check the power supply and the control signal (multimeter).

What damage it causes if left uncorrected:

• With the valve closed/blocked: Condensate back-up in the evaporator hinders cooling and can lead to the compressed air line being flooded with liquid water. This causes corrosion on machines and tools, causes pneumatic components to malfunction and reduces the lifespan of the entire compressed air system.
• If the valve is permanently open: This represents a significant leak, which leads to massive loss of compressed air and thus to a dramatically increased energy consumption of the compressor.

7.4 Load adjustment problems (incorrect dimensioning)

Why it happens: Compressed air dryers are designed for a specific nominal volume flow, inlet pressure and maximum inlet temperature. A load matching problem occurs when actual operating conditions are consistently outside this design range.

• Overload: The compressed air volume flow or the inlet temperature exceed the rated output of the dryer. Common causes include retrofitting a larger compressor, expanding production with increased air consumption without adjusting the dryer, or a permanently high ambient temperature that increases the inlet temperature of the compressed air. The dryer can no longer provide the necessary cooling performance.
• Underload (rare with dew point problems): A dryer that is too large can lead to overcooling of the compressed air at very low load, which could theoretically lead to icing in the evaporator. However, this is less often the cause of elevated dew points.

How to confirm:

• Parameter comparison: The measurement of the compressed air volume flow and the inlet temperature shows that the dryer manufacturer's specifications are consistently exceeded.
• Compressor running time: The dryer's refrigerant compressor runs constantly under high load and still does not reach the target dew point.
• System analysis: Review of the entire compressed air production and distribution (compressor performance, consumer profiles).

What damage it causes if left uncorrected: Sustained overload directly leads to a permanent exceeding of the dew point and thus to condensation problems in the entire compressed air network. The dryer's refrigerant compressor continuously works at the performance limit, which dramatically increases energy consumption and greatly reduces the compressor's lifespan. This can ultimately lead to total failure of the dryer.

8. Step-by-step troubleshooting

8.1 Eliminate a lack of refrigerant

1. Safety measures: Switch off the system, disconnect it from the power supply and secure it against being switched on again (LOTO). Depressurize the compressed air system. Wear personal protective equipment (PPE).
2. Suck off refrigerant: The remaining refrigerant must be extracted into a separate container using a suitable recovery device (according to DIN EN 378).
3. Leak detection and repair: Determine the exact position of the leak using an electronic leak detector (according to DIN EN 14624) or leak detection spray. Correct the leak professionally (e.g. replace the soldering point, replace the seal or O-ring, replace the defective line).
4. System Evacuation: Thoroughly evacuate the system after repair to remove moisture and non-condensable gases. Achieve a low vacuum of less than 0.5 mbar and hold this for at least 30 minutes to confirm the tightness.
5. Refrigerant filling: Fill the system exactly according to the manufacturer's instructions with the specified refrigerant type and the correct filling quantity (precision scale ± 5g). Example: For a medium-sized dryer 0.8 – 1.2 kg R134a.
6. Commissioning and checking: Put the system back into operation. Continuously monitor and record pressures (high/low pressure), temperatures and the pressure dew point.

8.2 Clean/repair heat exchanger

8.2.1 Clean condenser (external).

1. Safety measures: Switch off the system, disconnect it from the power supply and secure it against being switched on again (LOTO). Depressurize the compressed air system. Wear PPE.
2. Cleaning: Carefully clean the condenser fins with compressed air (max. 6 bar, from the inside out to blow out the dirt) or with a special, biodegradable fin cleaner and water. Carefully straighten damaged slats with a slat comb.
3. Drying and checking: After cleaning, the condenser must dry completely. Visual check for residues.
4. Commissioning and monitoring: Put the system back into operation. Monitor high pressure and temperature before and after the condenser. The high pressure should now be within the target range and the dew point should be stable.

8.2.2 Clean the evaporator (internal).

1. Safety measures: Switch off the system, disconnect it from the power supply and secure it against being switched on again (LOTO). Depressurize the compressed air system. Wear PPE. If necessary, suck out refrigerant if the evaporator needs to be dismantled.
2. Cleaning: Internal cleaning is complex and should be carried out according to the manufacturer's instructions. Chemical flushing of the compressed air side of the evaporator with special cleaning agents is often required. If there is heavy oil incrustation, removal and professional cleaning by a specialist company may be necessary.
3. Rinsing and drying: After chemical cleaning, the evaporator must be thoroughly rinsed and completely dried.
4. Commissioning and monitoring: Put the system back into operation. Monitor pressure dew point and temperature differences.

8.3 Eliminate condensate drain malfunction

1. Safety measures: Switch off the system, disconnect it from the power supply and secure it against being switched on again (LOTO). Depressurize the compressed air system. Wear PPE.
2. Dismantling and inspection: Dismantle the condensate drain valve. Check for contamination, blockages, mechanical damage (e.g. defective float, broken spring) or worn seals.
3. Cleaning or replacement: Remove contamination. Replace defective seals or worn parts. In the case of electronic arresters, check the electronics for functionality. If the defect cannot be repaired, replace the entire valve. Make sure you use the correct spare part type.
4. Assembly and leak test: Install the cleaned or new valve. Check the tightness under system pressure (leak detection spray).
5. Commissioning and functional check: Put the system into operation. Observe the function of the drain over several cycles. There should be clear, periodic draining without permanent leakage. Check dew point again.

8.4 Correct load adjustment (incorrect dimensioning)

1. Data analysis: Confirm overloading by accurately analyzing the compressed air volume flow and inlet temperatures over a longer period of time. Compare this data with the technical specifications of the installed dryer.
2. Capacity adjustment options:
   • Additional dryer installation: Add another appropriately sized dryer in parallel to increase overall performance and create redundancy.
   • Existing Dryer Replacement: Replace the undersized dryer with a model with sufficient capacity to meet current and future needs. Take into account reserves for temperature fluctuations (DIN ISO 7183).
   • Upstream system optimization: Check whether the compressed air inlet temperature can be reduced through more effective pre-cooling or better ventilation of the compressor room. Optimizing compressor control can also help smooth out peak loads.
3. Validation: After adjustment, the pressure dew point must be continuously monitored to confirm the effectiveness of the measure.

9. Preventive measures

Implementing preventative maintenance strategies is critical to avoid dew point deviations and ensure operational safety and efficiency of the compressed air system.

10. Spare Parts & Components

Keeping critical spare parts on hand minimizes downtime and ensures rapid resolution of dew point deviations.

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11. References

• DIN ISO 8573-1: Compressed air – Part 1: Impurities and purity classes.
• DIN EN 378: Refrigeration systems and heat pumps – safety and environmental protection.
• DIN EN 14624: Refrigeration systems and heat pumps – leak detection and leak testing of refrigeration systems.
• VDE 0105-100: Operation of electrical systems - Part 100: General provisions.
• VDI 2067: Economic efficiency of technical building equipment systems.
• TÜV guidelines and country-specific pressure equipment guidelines.
• Manufacturer maintenance manuals for the relevant compressed air dryers (e.g. Kaeser Kompressoren, Atlas Copco, Parker Hannifin, Donaldson).
• F-Gases Regulation (EU) No. 517/2014 and national implementing laws.