1. Purpose of Driers in Refrigeration Systems
Driers are essential components in any refrigeration or air conditioning circuit, including VRF systems like the Fujitsu AIRSTAGE VR-II, as they:
Remove moisture from the refrigerant and oil.
Filter solid contaminants, such as metal particles, sludge, or carbon from wear or breakdown.
Protect expansion valves, EEVs, and compressors from corrosion and blockages.
Moisture is highly detrimental to system reliability. When moisture mixes with refrigerant oil (particularly POE oil used in modern R410A/R32 systems), it forms acids and sludge, which can attack motor windings, bearings, and metallic surfaces.
2. When and Why to Replace or Install Driers
A. During Compressor Replacement
When replacing a compressor — whether due to mechanical failure, electrical burnout, or contamination — it is essential to install replaceable core liquid-line driers to:
Absorb residual moisture introduced during component replacement.
Capture contaminants that may have circulated in the refrigerant after the failure.
Protect the new compressor and other critical components from subsequent damage.
A replaceable core drier shell is recommended instead of a sealed drier because:
It allows inspection and core replacement after system cleaning or retesting.
The cores can be changed multiple times until moisture and acid levels are stable.
It is cost-effective and service-friendly for large VRF systems.
3. Special Considerations — Compressor Burnout
A. Nature of a Burnout
When a compressor experiences electrical burnout, the motor windings inside the compressor break down due to overheating. This causes:
The formation of acid, sludge, and carbonized oil.
Contamination of the refrigerant circuit, which can spread corrosive residues throughout the system.
B. Required Service Actions
In these cases, replaceable core driers are not optional—they are critical.
Technicians must:
Recover and discard all contaminated refrigerant.
Flush or replace affected components (especially suction lines and oil return paths).
Install suction and liquid line replaceable core driers to capture residual acid and moisture.
Perform an acid test to verify system cleanliness before recharging.
4. Acid Testing
A. Purpose
Acid test kits are used to detect the presence of acid in refrigerant oil after a compressor failure. This determines whether the system requires:
Further cleaning or flushing.
Replacement of additional components.
Core changes in the drier assembly.
B. Procedure
After initial clean-up and vacuuming, collect an oil sample from the compressor or line.
Use an acid test kit (color-change reagent type).
If acid is detected, continue the cleanup process — replace cores, re-vacuum, and retest.
Only when the acid test is neutral (clean result) should a new compressor be installed and the system recharged.
5. Recommended Procedure (Step-by-Step)
Recover all refrigerant and isolate the faulty compressor.
Inspect oil — dark or burnt smell indicates acid presence.
Flush — Flush indoors, RB units and piping if required.
Install replaceable core suction and liquid driers in accessible locations.
Evacuate system to deep vacuum (≤ 500 microns).
Charge with dry nitrogen and purge to assist cleaning.
Run the system for several hours with the new compressor and initial cores.
Test oil acidity again after 24–48 hours.
Replace drier cores as necessary until acid-free readings are achieved.
Seal and document the service completion.
6. Consequences of Omitting This Step
Failure to install or replace driers and test for acid following a compressor burnout can lead to:
Premature failure of the new compressor.
Clogged expansion valves or EEVs.
Corrosion of copper piping and internal surfaces.
Recurring acid contamination that perpetuates the damage cycle.
In a VRF system, where multiple indoor and outdoor circuits are interconnected, such contamination can cause extensive and costly damage.
7. Best Practice Summary
| Situation | Required Action |
|---|---|
| Normal compressor replacement (no burnout) | Install replaceable core liquid-line drier to remove moisture. Replace core after 24–48 hours of operation. |
| Compressor burnout | Install both suction and liquid-line replaceable driers. Perform acid test. Repeat until results are clean. |
| New system installation | Use standard filter driers for commissioning; remove or replace after vacuum and stabilization. |
8. Key Takeaway
Always treat a compressor replacement — especially after a burnout — as a contamination cleanup operation, not just a component swap.
Replaceable core driers and acid testing are your best tools to protect the new compressor, ensure system longevity, and maintain manufacturer warranty compliance.