Troubleshooting Screw Compressor High Discharge Temperature

1. Problem Description & Scope

High discharge temperature (HDT) in rotary screw compressors is a critical indicator of system inefficiency, component degradation, and potential catastrophic failure. This guide addresses air-cooled and water-cooled screw compressors operating in industrial environments. HDT is defined as operating temperatures exceeding the manufacturer’s recommended discharge threshold, typically resulting in automatic machine shutdown to protect the airend. Failure to address this condition leads to rapid oil oxidation, bearing failure, and premature mechanical wear.

Severity Classification: Critical. Continued operation above 105°C (221°F) compromises oil integrity and shortens component service life by 50% for every 10°C increase above normal operating limits.

2. Safety Precautions

WARNING: COMPRESSOR SYSTEMS CONTAIN STORED ENERGY. BEFORE PERFORMING ANY DIAGNOSTIC OR MAINTENANCE TASK, PERFORM FULL LOCKOUT/TAGOUT (LOTO) PROCEDURES. ISOLATE THE COMPRESSOR FROM ELECTRICAL POWER, VENT ALL AIR PRESSURE TO ATMOSPHERE, AND ALLOW THE SYSTEM TO COOL TO BELOW 40°C (104°F) TO PREVENT THERMAL BURNS. USE APPROPRIATE PPE, INCLUDING THERMALLY INSULATED GLOVES, SAFETY GLASSES, AND STEEL-TOED BOOTS.

3. Diagnostic Tools Required

4. Initial Assessment Checklist

5. Systematic Diagnosis Flowchart

1. Symptom: High Discharge Temperature Alarm
   • Step 1: Verify Actual Temperature. Use a calibrated digital thermocouple at the discharge pipe. If the sensor reading is significantly different from the gauge reading, the temperature sensor is faulty.
   • Step 2: Check Ambient and Ventilation. Verify the room temperature. Is the intake air restricted? If ambient is > 40°C, the system will overheat.
   • Step 3: Analyze the Oil Cooler. Use the infrared camera to check for cold spots on the radiator fins.
     • If cold spots exist, the cooler is fouled (internally or externally).
     • If the entire cooler is hot, the oil is not bypassing or the thermostat is stuck.
   • Step 4: Verify Oil Thermostat Operation. Check the temperature of the oil line leading to the cooler. If the line remains cool despite high discharge temperature, the thermostat is failed in the bypass position.

6. Fault-Cause Matrix

7. Root Cause Analysis for Each Fault

7.1 Oil Level and Quality

Oil serves as both lubricant and coolant in screw compressors. Low oil levels reduce the volume of heat-transfer fluid, leading to thermal overload. Furthermore, oil breakdown (sludge/varnish) increases friction and restricts heat transfer.

7.2 Cooler Fouling

External fouling occurs when intake filters fail, allowing dust and debris to plug the radiator fins, reducing heat exchange efficiency. Internal fouling occurs when oil degradation products deposit inside the radiator tubes, creating a thermal barrier.

7.3 Thermostat Failure

The oil thermostat regulates the flow of oil to the cooler. If it fails in the closed (bypass) position, hot oil is returned directly to the airend without passing through the cooler. This results in an immediate spike in temperature under load.

7.4 Ambient Conditions

Screw compressors are designed to operate within specific ambient temperature limits. High ambient temperatures reduce the ΔT available for heat transfer, forcing the compressor to run hotter.

8. Step-by-Step Resolution Procedures

8.1 Resolving Low Oil Level

1. Perform LOTO.
2. Allow system to depressurize completely.
3. Inspect for leaks (fittings, hoses, oil seal).
4. Add manufacturer-approved lubricant to the appropriate level.
5. Verify oil filter condition (if restricted, oil flow is impeded).

8.2 Resolving Cooler Fouling

1. Perform LOTO.
2. External: Use compressed air (below 2 bar) or a mild detergent solution to clean the fin pack. Do not use pressure washers as they can collapse the fins.
3. Internal: If suspected, flush the cooling system using a recommended oil-flush solvent according to the manufacturer’s technical bulletin.

8.3 Resolving Thermostat Failure

1. Perform LOTO.
2. Locate the thermostat housing (usually near the airend).
3. Remove the housing cover and extract the thermostatic element.
4. Test the element in a hot water bath (it should open at the rated temperature, usually 70-80°C).
5. Replace with a new element and new gasket. Tighten to torque specifications.

9. Preventive Measures

10. Spare Parts & Components

For all replacement parts, consult the UNITEC-D e-catalog: https://www.unitecd.com/e-catalog/

11. References

• ASME PTC 9: Performance Test Code for Compressors
• ANSI/CAGI B19.1: Safety Standard for Compressed Air Systems
• OEM Technical Service Manuals for Lubrication and Thermal Control