In low-temperature commercial and industrial refrigeration systems, scroll compressors often operate at high pressure ratios where elevated compression heat poses a risk to equipment longevity. Operating with discharge temperatures continuously above 130℃ leads to Polyolester (POE) lubricant thermal degradation, carbon deposit formation on scroll wraps, and eventual motor winding burnout. This diagnostic guide addresses root causes of overheating in Copeland scroll units and provides systematic troubleshooting procedures for Liquid Injection and Discharge Temperature Control (DTC) valve systems.
Primary Root Causes of Scroll Overheating
Excessive discharge temperatures are typically driven by abnormal suction vapor conditions or reduced mass flow rate:
- High Suction Superheat: Excessive suction superheat exceeding 15K, caused by improperly adjusted expansion valves or uninsulated suction lines, increases thermal loading before compression begins.
- High Compression Ratios: Operating with low evaporating pressures (below -30℃) alongside elevated condensing pressures forces compression ratios beyond 10:1, generating excess frictional and thermodynamic heat.
- Inadequate Subcooling: Liquid entering expansion devices with insufficient subcooling reduces effective mass flow, leading to higher discharge superheat.
DTC Injection Valve Diagnostic Protocol
Low-temperature scroll units rely on a DTC valve to inject liquid refrigerant directly into the intermediate compression stage when scroll temperatures reach 105℃.
- Sensor Bulb Positioning: Inspect the DTC thermal bulb mounted on the top cap well. Ensure the bulb uses approved thermal grease and is secured with heavy-duty mounting clips. Loose thermal contact causes delayed valve opening, driving discharge temperatures past safety limits.
- Liquid Line Restriction: Measure the pressure differential across the liquid line filter-drier supplying the DTC valve. A pressure drop exceeding 0.5 bar indicates clogging, which starves the injection circuit of liquid refrigerant.
- Valve Orifice Operation: Measure compressor body temperatures near the injection port. If the top cap exceeds 115℃ while the injection line remains cold, the internal capillary tube or thermal element has suffered mechanical blockage or lost its power element charge, requiring valve assembly replacement.
Preventative Maintenance and System Recovery
When resolving overheating incidents during routine compressor maintenance in cold storage applications:
- Inspect oil sight glasses for darkening or viscosity loss resulting from thermal oxidation.
- Adjust primary thermostatic expansion valves (TXV) to maintain a stable suction superheat between 5K and 8K at the compressor suction inlet.
- Verify that high-temperature limit cutouts trip reliably at 127℃ to prevent structural scroll wrap seizure.
Conclusion
Maintaining precise suction superheat and verifying DTC valve liquid injection functionality protects the refrigeration compressor from severe thermal degradation, ensuring continuous and energy-efficient performance in low-temperature cold storage environments.
