Diagnostic guide: Exceeding the dew point in compressed air dryers

1. Problem description & scope

This diagnostic guide addresses the critical issue of dew point exceedances in refrigeration dryers used in compressed air systems. An elevated dew point leads to condensation of water in the downstream compressed air network, which can result in corrosion, damage to pneumatic components, impairment of production processes and quality problems in end products. This guide is aimed at maintenance technicians and maintenance managers in manufacturing companies to enable systematic error detection and elimination.

Affected systems: Refrigerant compressed air dryers of all common types (e.g. direct evaporation, thermal storage).

Severity classification:

• Critical: Dew point permanently above +10 °C, immediate production impairment or high risk of system damage.
• Major: Dew point between +3 °C and +10 °C, increased wear, medium-term damage or reduction in quality.
• Minor: Dew point briefly or locally above the setpoint, but below +3 °C, indication of beginning problems.

2. Safety precautions

»ATTENTION SAFETY!«

• Electrical hazards: Before starting any work on the dryer, interrupt the power supply and secure it against being switched on again (lockout/tagout according to VDE 0105-100). Use an approved voltage tester (DIN EN 61243-3) to make sure that all phases are voltage-free.
• Compressed air: The compressed air system must be depressurized before working on the dryer. Residual pressure in lines or components can cause serious injury. Use a pressure gauge to check the pressure drop to 0 bar.
• Refrigerants: Refrigerants are under high pressure and can cause frostbite or chemical burns on contact. Work on the refrigerant circuit may only be carried out by certified specialist personnel (Category I according to EU Regulation 517/2014). Personal protective equipment (PPE): safety glasses (DIN EN 166), cold protection gloves (DIN EN 511), skin protection.
• Hot Surfaces: Compressors, condensers and refrigerant lines can reach high temperatures during operation. Danger of burns!
• Moving Parts: Fan motors and other moving parts may exhibit residual rotation even after switching off.

3. Required diagnostic tools

4. Checklist for initial assessment

Before the actual diagnosis begins, the following points must be checked and documented:

5. Systematic diagnostic flow chart

This flow chart enables structured troubleshooting for elevated dew points:

1. Start: Dew point at the dryer outlet too high (> +3 °C)
   1. Checking the basic functions:
      1. Is the dryer in operation and the power supply present?
         • Yes: Continue with 1.a.ii.
         • No: Check power supply (fuse, contactor), check control. Fix and test again.
      2. Is the dryer's compressor running?
         • Yes: Go to 1.a.iii.
         • No: Check whether the dryer requires a minimum inlet temperature or pressure. Check compressor control logic. Measure current consumption.
      3. Are the condenser fans working?
         • Yes: Continue with 1.b.
         • No: Check fan motor (voltage, current consumption, winding resistance). Check condenser pressure switch or regulator.
   2. Checking the refrigerant circuit:
      1. Refrigerant suction pressure too low (<2 bar for R134a, R407C)?
         • Yes: → Probable cause: lack of refrigerant/leak.
         • No: Continue with 1.b.ii.
      2. Refrigerant condensing pressure too high (>15 bar for R134a, R407C at 25 °C ambient)?
         • Yes: → Probable cause: Contaminated condenser.
         • No: Continue with 1.c.
   3. Checking the heat exchanger:
      1. Are the fins of the air condenser free of contamination (dust, oil)?
         • Yes: Continue with 1.c.ii.
         • No: → Probable cause: Contaminated condenser. Clean.
      2. Temperature difference between inlet compressed air and refrigerant suction side too low (< 5 K)?
         • Yes: → Probable cause: Dirty or iced-up refrigerant compressed air heat exchanger (evaporator).
         • No: Continue with 1.d.
   4. Checking the condensate management:
      1. Does the condensate drain valve (timer-controlled, level-controlled) function correctly?
         • Yes: Continue with 1.d.ii.
         • No (permanently open/closed, blocked)? → Probable cause: Condensate drain valve defective/blocked.
      2. Does compressed air with condensate escape continuously?
         • Yes: → Probable cause: Condensate drain valve defective/blocked.
         • No: Continue with 1.e.
   5. Load adjustment and sizing:
      1. Do the current compressed air inlet conditions (temperature, pressure, volume flow) correspond to the specifications of the dryer?
         • Yes: Continue with 1.e.ii.
         • No (e.g. increased inlet temperature or volume flow)? → Probable cause: overload / mismatch.
      2. Has the dryer recently been moved to a different environment or connected to a new compressor?
         • Yes: → Probable cause: sizing error or mismatch to new operating conditions.
         • No: Continue with 1.f.
   6. Further checks (if all of the above points were normal):
      1. Is the hot gas bypass valve (if present) functioning correctly?
         • Yes: Continue with 1.f.ii.
         • No: → Probable cause: Hot gas bypass valve faulty (stuck open).
      2. Is the dryer control (controller, sensors) functional?
         • Yes: Expert consultation necessary.
         • No: → Probable cause: Sensor or controller defective.

6. Error-cause matrix

7. Root cause analysis for each error

7.1. Lack of refrigerant / leakage

Why it happens: Leaks in the refrigerant circuit often occur at solder joints, screw connections, compressor shaft seals or due to material fatigue/vibrations. A gradual loss of refrigerant reduces the cooling capacity of the system because less refrigerant is available for heat transfer. This leads to an increase in the suction pressure at the evaporator and consequently to insufficient cooling of the compressed air.

How to confirm:

• Suction and condensing pressure measurement: Both values ​​are typically lower than the setpoint, but the suction pressure drops more.
• Increased degree of overheating at the evaporator outlet.
• Use of an electronic refrigerant leak detector (according to DIN EN 14624) to precisely locate the leak point.
• Visually check for traces of oil on components, as refrigerant often leaks with oil.

What damage does it cause if unresolved: A permanent lack of refrigerant leads to inadequate cooling of the compressor (due to hot gas return), which significantly shortens its service life and, in the worst case, leads to compressor failure. The high dew point in the compressed air network causes corrosion and increases the maintenance intervals of the downstream systems.

7.2. Dirty heat exchanger (condenser or evaporator)

Why it happens:

• Condenser (air side): Dust, fibers and oil mist from the ambient air settle on the fins of the air condenser. This hinders the heat transfer from the hot refrigerant to the ambient air. As a result, the condensing pressure increases, the cooling capacity decreases and the dew point increases.
• Evaporator (refrigerant-compressed air heat exchanger, air side): Although rarer, particles from the compressed air (especially if pre-filters are missing or defective) or oil films can coat the inside of the evaporator and hinder the heat transfer from the compressed air to the refrigerant. Icing of the evaporator, often due to a stuck hot gas bypass valve or inadequate defrost cycles, also leads to a drastic reduction in cooling performance.

How to confirm:

• Capacitor: Visual inspection for contamination of the fins. Measurement of the temperature difference between the air inlet and outlet at the condenser - a small difference indicates poor heat transfer. High condensing pressure.
• Evaporator: Measurement of the temperature difference between the inlet compressed air and the refrigerant suction side - a small difference combined with a high dew point indicates poor heat transfer. Visual inspection for ice formation (through an endoscope or with the housing open).

What damage it causes if unresolved: A dirty condenser leads to increased power consumption of the compressor and a reduction in its lifespan due to overheating and high pressure. A dirty or icy evaporator results in inefficient drying operation and, in extreme cases, can lead to a drop in pressure in the system.

7.3. Condensate drain valve defective/blocked

Why it happens: Condensate drain valves (mechanical, electronic or level controlled) are critical components. They can be blocked by dirt particles, rust or oil emulsions from the compressed air. A permanently open valve leads to unnecessary compressed air losses. A blocked, closed valve leads to the accumulation of condensate in the evaporator, which is then absorbed by the compressed air flowing through it (carry-over effect), which drastically increases the dew point.

How to confirm:

• Manual test: For electronic valves, activate the manual drain function and observe the condensate drain. For level-controlled valves, test the function by simulating condensate accumulation (if possible).
• Visual control: Observe whether condensate is drained regularly and completely or whether compressed air is constantly escaping.
• Noise: A constant hissing indicates an open valve.
• Endoscopy: If valves are difficult to access, an endoscope can make it easier to check for blockages.

What damage it causes if left unsolved: A defective drain valve is a main cause of water entering the compressed air network. This leads to corrosion, damage to pneumatic cylinders, valves and tools, as well as an increased reject rate in sensitive processes (e.g. paint shops, food production). Permanent compressed air losses also increase operating costs significantly.

7.4. Overload/maladaptation

Why it happens: A refrigeration dryer is designed for specific operating conditions (inlet temperature, inlet pressure, compressed air volume flow and ambient temperature). If these conditions are permanently exceeded, the dryer can no longer provide the required cooling capacity to reach the dew point. Common causes are:

• Increase in the ambient temperature in the installation room.
• Connection of a larger compressed air generator (compressor).
• Increase in the inlet temperature of the compressed air (e.g. due to insufficient after-cooling of the compressor or lines that are too long and uninsulated).
• Increase in total compressed air requirements for production.

How to confirm:

• Measurement of the actual inlet temperature and volume flow of compressed air.
• Compare these measured values ​​with the technical data of the dryer (name plate, operating instructions).
• Checking the dew point curve over a longer period of time (e.g. 24 hours) to detect peak loads.

What damage it causes if left unsolved: Permanent overload not only leads to a high dew point, but also to a permanently high load on the compressor in the dryer. This leads to increased wear, more frequent failures and inefficient operation. The negative consequences of water in the compressed air network remain.

8. Step-by-step troubleshooting

8.1. Eliminate refrigerant shortage/leakage

1. Safety: Switch off the dryer and empty the compressed air system. »Put on PPE (protective goggles, cold protection gloves)!«
2. Locate leakage: Systematically check all solder joints, screw connections, valves and the compressor shaft seal using an electronic leak detector or soap solution.
3. Seal leakage: Depending on the type of leakage (e.g. tighten screw connection, repair solder joint). In the event of complex leaks, a certified refrigeration technician must be called in.
4. Evacuation: After the repair, the affected circuit must be evacuated using a vacuum pump to an absolute pressure of less than 0.5 mbar (leak test by maintaining a vacuum).
5. Filling: Refill the system with the specified refrigerant and the exact filling quantity (according to the manufacturer's instructions). Measurement using calibrated refrigerant scales.
6. Functional test: Put the dryer into operation and check the dew point, suction and condensing pressure. Check refrigerant again for leaks.

8.2. Clean contaminated heat exchanger

1. Safety: Switch off the dryer and empty the compressed air system.
2. Provide access: Remove the dryer side panels to gain access to the condenser and evaporator.
3. Cleaning the capacitor:
   • First remove loose dirt with compressed air or a soft brush.
   • Then spray a special cleaner for refrigeration technology (non-corrosive) onto the slats and, after allowing it to take effect, rinse with water (garden hose, not high pressure!). Rinse from the inside out to push the dirt out.
   • Allow to dry thoroughly.
4. Check evaporator (and clean if necessary):
   • Visual check for contamination or ice formation.
   • If there is icing, check the cause (e.g. hot gas bypass valve).
   • If there is severe internal contamination, chemical cleaning or replacement of the evaporator is required.
5. Assembly & functional test: Install the panels, put the dryer into operation, check the dew point and pressures.

8.3. Repair/replace condensate drain valve

1. Safety: Switch off the dryer from the power supply. »Drain the compressed air system and depressurize it (especially the line to the drain valve)!«
2. Check valve: Functional check of the valve (control manually or electrically).
3. Cleaning: Try to clean the valve (e.g. with mechanical floats or solenoid valves). Dismantling and cleaning the internal components (floats, seals, magnet armatures) can often resolve the problem. Clean clogged drain lines.
4. Replacement: In the event of irreparable damage or defects, replace the condensate drain valve with a new, specification-compliant replacement part.
5. Functional test: Pressurize the dryer, check the function of the drain valve. Ensure that there is no permanent air leakage and that condensate is drained reliably.

8.4. Fix overload / adjust dryer

1. Analyze operating conditions: Measurement and documentation of the real inlet temperatures, pressures and volume flows of the compressed air as well as the ambient temperature.
2. Check options:
   • Reduction of the inlet temperature: Improvement of the aftercooling on the compressor (larger aftercooler, better ventilation) or insulation of long compressed air lines.
   • Reduce volume flow: Check the compressor control (optimize load/idle times).
   • Additional dryer: Installation of a dryer connected in parallel to increase capacity.
   • Larger Dryer: Replacing the existing dryer with a more powerful model designed for current operating conditions.
   • Optimize environmental conditions: Improve ventilation in the dryer installation room, air conditioning.
3. Implementation & Verification: Implement the selected measure and monitor the dew point over a longer period of time to confirm success.

9. Preventive measures

10. Spare Parts & Components

For a detailed selection of spare parts and components, visit our UNITEC-D e-catalogue.

11. References

• DIN ISO 8573-1: Compressed air – Part 1: Impurities and purity classes.
• VDE 0105-100: Operation of electrical systems.
• EU Regulation No. 517/2014: F-Gas Regulation on Fluorinated Greenhouse Gases.
• Manufacturer maintenance instructions for specific dryer models.
• VDI 2067: Economic efficiency of systems – compressed air systems.