When diagnosing and repairing refrigeration system issues in refrigerated trucks, understanding the symptoms of insufficient or excessive refrigerant is crucial. Here’s a detailed analysis:
1. Symptoms of Insufficient Refrigerant:
- Evaporator not fully frosted: Only the rear part of the evaporator will frost, indicating insufficient refrigerant.
- Low suction pressure: The suction pressure on the compressor’s intake side is too low.
- Expansion valve adjustment ineffective: Even after increasing the flow through the expansion valve, cooling performance does not improve.
2. Symptoms of Excessive Refrigerant:
- Evaporator frost issues: The evaporator may only form light frost (called “false frost”) or no frost at all.
- High suction pressure: Suction pressure is higher than normal.
- Rapid increase in discharge pressure: If you decrease the expansion valve flow, the discharge pressure from the compressor increases rapidly, potentially triggering the pressure protection system to shut down the system.
Indicators of Refrigerant Problems (Either Insufficient or Excessive):
- Compressor Suction Side:
- Pressure: Decreased with low refrigerant, increased with excess refrigerant.
- Compressor Discharge Side:
- Pressure: Decreased with low refrigerant, increased significantly with excess refrigerant.
- Compressor Suction Temperature:
- Temperature: Increased with low refrigerant, decreased with excess refrigerant.
- Compressor Discharge Temperature:
- Temperature: Decreased with low refrigerant, increased with excess refrigerant.
- Cylinder Head Suction Side Temperature:
- Temperature: Increased with low refrigerant, decreased with excess refrigerant.
- Compressor Noise:
- Noise: Normal operation with proper refrigerant levels, but abnormal, dull sounds when there’s an issue.
- Evaporator Frosting:
- Frost Pattern: Only the initial part of the evaporator will frost with insufficient refrigerant, while the rest may remain unfrosted or barely frosted with excessive refrigerant.
- Expansion Valve Sound:
- Sound: With refrigerant issues, you may hear alternating gas-liquid flow sounds or a continuous liquid flow near the expansion valve.
Inspection Method:
- Symptom-based Diagnosis: Check the system using the symptoms mentioned above to determine whether there’s an issue with the refrigerant level.
- Touch Test on Condenser:
- By touching the surface of the condenser, you can roughly estimate the refrigerant level. When the compressor is running, the gaseous refrigerant entering the condenser is hot, while the liquid refrigerant is much cooler. The point where the temperature changes noticeably on the condenser surface is an indication of refrigerant levels.
- For shell-and-tube type condensers that also function as receivers, the refrigerant level should not exceed 15% of the condenser’s volume. This method provides a rough estimate of whether the refrigerant charge is correct.
This approach will help you assess the refrigerant status and make necessary adjustments to ensure the refrigerated system operates efficiently.