In heavy-duty industrial refrigeration and low-temperature cold storage applications, keeping compressor discharge temperatures within the original equipment manufacturer (OEM) design limits is critical to ensuring mechanical reliability. When a Bitzer semi-hermetic screw compressor operates with continuous discharge temperatures exceeding +105°C, synthetic Polyolester (POE) oil loses its kinematic viscosity, resulting in rotor wear, shaft seal degradation, and unexpected system shutdowns. Conducting systematic troubleshooting during routine compressor maintenance protects internal components and preserves long-term volumetric efficiency.
Primary Causes of Elevated Discharge Temperatures
Excessive discharge heat typically stems from three main sources: operation outside the standard design range, inadequate suction gas conditioning, or failure of auxiliary cooling systems:
- High Pressure Ratios: Operation at low evaporating pressures (below -30°C) paired with high condensing pressures (above +45°C) substantially increases the heat generated during compression.
- Excessive Suction Superheat: Underfeeding thermostatic expansion valves or poorly insulated suction piping push suction superheat above 15K, raising the inlet gas temperature before compression even starts.
- Liquid Injection System Failure: Malfunctioning liquid injection valves (LIV) or clogged injection nozzles stop liquid refrigerant from atomizing into the compression chamber to absorb compression heat.
- Oil Cooler Degradation: Oil temperatures entering the compressor (exceeding +60°C reduce internal heat dissipation across rotor flanks and bearings.
Step-by-Step Diagnostic and Inspection Workflow
Field engineers should follow a structured process to isolate thermal abnormalities and restore design operating parameters:
- Verify Operating Envelope Limits: Connect calibrated pressure transmitters to the suction and discharge service ports. Compare the actual pressure ratio against the operating envelope chart in Bitzer’s software to confirm the unit is not operating beyond the single-stage limits.
- Evaluate Suction Gas Conditions: Measure the pipe surface temperature 150 mm upstream of the suction shut-off valve. Adjust the expansion valve to stabilize suction superheat between 5K and 10K.
- Inspect Liquid Injection Cooling (CIC System): Confirm the solenoid valve coil is energized and the temperature sensor is calibrated correctly. Ensure the liquid injection valve opens when the discharge temperature reaches +100°C. Clean the internal line strainer to remove particulate debris that blocks refrigerant flow.
- Check Oil Supply and Thermal Bypass: Inspect the temperature of oil entering the compressor housing. Clean water-cooled shell-and-tube oil coolers or air-cooled oil radiator fins to restore effective oil cooling performance.
Preventative Guidelines for Long-Term Operation
Addressing thermal issues promptly prevents oil carbonization and rotor seizure. Installing automated discharge temperature transmitters connected to safety interlocks triggers an automatic shutdown before internal temperatures exceed +120°C, protecting the equipment through repeated demanding cooling cycles.
Do you need technical support, replacement parts, or liquid injection kits for your Bitzer screw compressor? Browse our component catalog or contact our technical engineering team for professional guidance.
