The evaporator is the core heat exchanger in a refrigerated air dryer, where heat is exchanged between the refrigerant and the compressed air. Its airtightness is critical for maintaining complete separation between the refrigerant circuit and the compressed air circuit.
When an evaporator leak occurs, accurately determining whether it is an external leak or an internal leak is essential. Misdiagnosis can lead to unnecessary disassembly, increased maintenance time, and higher repair costs.
This article provides a practical troubleshooting guide to help maintenance technicians quickly identify the source of evaporator leaks and take the appropriate corrective action.
How to Check for an External Leak
External leaks are typically indicated by low or zero refrigerant pressure on the pressure gauge, often accompanied by visible oil stains inside the machine.
The following inspection procedures are recommended.
1. Visual Inspection
Open the dryer enclosure and carefully inspect the interior, paying particular attention to:
- Small copper capillary tubes
- Process tubes
- Copper flare fittings
- Compression nuts and threaded joints
Look for oil stains or oil streaks around these components.
When refrigerant leaks from the system, it usually carries compressor lubricating oil with it. Therefore, oil contamination is often a reliable indicator of the leak location.
Also inspect small-diameter copper tubing (typically less than 6 mm) for cracks, fractures, or mechanical damage.
2. Pressure Holding and Leak Detection
Pressure Holding Test
- After completely relieving the system pressure, slowly charge the refrigeration circuit with an inert gas.
- Nitrogen is strongly recommended. If nitrogen is unavailable, gaseous refrigerant may be used. Never use oxygen, as it creates a serious safety hazard.
- Pressurize the system to approximately 0.2 MPa (2.0 bar) and listen carefully for any audible gas leaks.
- If no leakage is detected, gradually increase the pressure to 0.4–0.5 MPa for further testing.
Soap Bubble Leak Test
While the system remains pressurized:
- Apply a soap solution with a sponge or brush to all suspected leak points, including fittings, flare connections, and threaded joints.
- The appearance of bubbles indicates the exact location of the leak.
How to Check for an Internal Leak
Internal leaks typically present one of the following symptoms:
- The dryer experiences abnormally high refrigerant discharge pressure during operation, triggering the high-pressure protection shutdown.
- After shutdown, the refrigerant pressure gradually rises until it approaches the compressed air pressure.
The following methods can be used to confirm an internal evaporator leak.
1. Independent Pressure Holding Test
- Isolate the evaporator from the refrigeration system.
- Seal the evaporator independently and perform a pressure holding test.
- Allow sufficient stabilization time while monitoring the pressure.
If the pressure drops noticeably during the test, the evaporator has an internal leak.
2. Compressed Air Verification Method
- Shut down the refrigerated air dryer.
- Completely recover or release the refrigerant until the pressure gauge reads zero.
- Close the service valve.
- Open the compressed air inlet valve so that compressed air flows through the evaporator.
- Observe the refrigerant pressure gauge.
If the refrigerant pressure gradually increases and eventually reaches the same pressure as the compressed air system, an internal leak has been confirmed.
This occurs because the evaporator is the only component where the refrigerant circuit and compressed air circuit are separated by a heat-transfer wall. Any pressure equalization between the two systems indicates that the evaporator has been internally breached.
Preventive Maintenance Recommendations
1. Perform Regular Cleaning and Inspection
- Clean dust, oil deposits, and other contaminants from the evaporator surface on a regular basis to maintain efficient heat transfer and minimize corrosion.
- Inspect copper tubing, flare connections, fittings, and compression nuts for looseness, cracks, or corrosion.
- Tighten loose connections promptly and replace damaged components when necessary.
2. Reduce Vibration and Mechanical Shock
- Ensure the refrigerated air dryer is securely installed on a stable foundation to minimize vibration that could lead to fatigue cracks or loose connections.
- Operate the air compressor and downstream equipment properly to avoid frequent start-stop cycles, sudden valve operations, or rapid fluctuations in air demand, all of which can create pressure shocks that shorten evaporator life.
3. Optimize the Operating Environment
- Install the refrigerated air dryer in a well-ventilated, dry location away from heat sources.
- Maintain sufficient clearance around the unit to ensure proper heat dissipation and prevent overheating, which may reduce evaporator performance.
- In areas with unstable power supplies, install a voltage stabilizer to protect the compressor and other electrical components from voltage fluctuations that may indirectly affect evaporator operation.
4. Control Operating Parameters
- Maintain an appropriate evaporating temperature to prevent ice formation inside the evaporator. Excessive icing can expand and rupture the copper tubes, resulting in internal leaks.
- Prevent excessively hot, humid, or oil-laden compressed air from entering the dryer, as these conditions accelerate corrosion and increase thermal and mechanical stress on the evaporator.
Conclusion
Whether the problem is identified as an external leak or an internal leak, the repair strategy should be based on accurate diagnosis. Avoid unnecessary disassembly or repeatedly recharging the refrigeration system without first locating the root cause.
If you lack experience with refrigeration system maintenance, it is strongly recommended to contact a qualified refrigeration service technician. Professional diagnosis and repair can prevent further equipment damage, reduce downtime, and ensure safe system operation.
