1. Problem and scope of intervention
A high discharge temperature in a lubricated screw compressor is a critical indicator of impending failure or operational inefficiency. This guide covers oil-injected screw compressors used in the aerospace and energy industries. A temperature above the nominal thresholds accelerates the degradation of the lubricant, causes the formation of varnish, increases mechanical clearances and can lead to an emergency shutdown of the system. This guide is classified as a major corrective maintenance procedure.
DANGER: RISK OF BURNS AND OVERPRESSURE. Any intervention on the delivery circuit or the oil circuit must be preceded by a lockout procedure (LOTO) and a total depressurization of the system. Wearing full PPE (heat-resistant gloves, protective glasses) is mandatory. Wait for the compressor block to cool completely (temperature < 40°C) before opening.
3. Required diagnostic tools
| Tool | Recommended specification | Measuring range | Usage |
|---|---|---|---|
| Thermal camera | Min resolution 160x120, sensitivity < 0.1°C | -20°C to +400°C | Identify hot spots on the cooler and block |
| Digital multimeter | True RMS accuracy, category CAT III 600V | Resistance: 0.1 Ω to 40 MΩ | Check the temperature probe (PT100/PT1000) |
| Pressure gauge | Class 1.0, calibrated | 0 to 25 bar | Measure the pressure drop across the oil filter/cooler terminals |
| Anemometer | Industrial measuring range | 0 to 20 m/s | Check cooling air flow |
4. Initial checklist (before diagnosis)
| Checkpoint | Action | Registration |
|---|---|---|
| Ambient temperature | Measure the temperature in the technical room | ___°C |
| Oil level | Check the level cold and hot | ___ mm |
| Alarm history | View the controller's logbook | Code/Date |
| Filter status | Check clogging indicators (oil/air) | ΔP (bar) |
5. Systematic diagnostic flow
- Symptom: High temperature alarm
- Check the actual temperature vs probe display.
- If significant difference (> 5°C), replace the probe (Aeronautical sector: compliance ISO 9001).
- Check the cleanliness of the cooler (external/internal).
- If dirty, carry out high pressure cleaning (air/water according to manufacturer recommendations).
- Check the operation of the oil thermostat.
- If the thermostat does not open, the by-pass is active. Test by measuring the temperature at the cooler inlet/outlet.
- Analyze the ambient conditions.
- If the ambient temperature > 40°C, check the ventilation of the technical room.
- Check the actual temperature vs probe display.
6. Fault-Cause Matrix
| Symptom | Probable causes (Order) | Testing | Expected result if cause confirmed |
|---|---|---|---|
| Temperature > 105°C | Clogged cooler (1) | Thermal camera on the cooler | High temperature difference between inlet and outlet (> 15°C) |
| Temperature > 105°C | Thermostat failure (2) | T° measurement at the cooler inlet | Cooler inlet T° < Opening setpoint T° |
| Temperature > 105°C | Low oil level (3) | Visual check / level sensor | Level outside operational zone |
| Temperature > 105°C | High ambient temperature (4) | Local T° measurement | Local temperature > 40°C |
7. Root cause analysis
Oil thermostat failure
The thermostat regulates the oil temperature by diverting part of the flow to the cooler. If it remains stuck in the closed position, oil does not pass through the cooler, causing a rapid rise in temperature. A failure can be confirmed by a high discharge temperature while the chiller remains cool to the touch.
Clogged cooler
In industrial environments (even aerospace), dust or oil vapor can clog cooler fins. This reduces the efficiency of heat exchange. Confirmation is done by thermal imaging showing cold (obstructed) areas alternating with hot areas.
8. Resolution procedures
- Replacing the thermostat
- Isolate and depressurize the compressor.
- Partially drain the oil circuit.
- Disassemble the thermostat body.
- Inspect the seat and spring. Replace with an original UNITEC-D part (ref. TH-AERO-2026).
- Reassemble, purge the circuit and carry out an operating test (discharge T° measurement).
- Cleaning the cooler
- Isolate the system.
- Apply a degreaser compatible with the material (Aluminium/Copper).
- Clean against the current (compressed air 2 bar max or low pressure water).
- Dry the exterior and check for deformation of the fins.
9. Preventive measures
| Cause | Prevention strategy | Tracking method | Interval |
|---|---|---|---|
| Clogging | Regular external cleaning | Visual inspection and ΔP | Quarterly |
| Thermostat failure | Preventive replacement | Oil Analysis (Spectrometry) | Annual |
10. Spare parts and components
All components must meet CE standards and UNITEC-D technical specifications.
| Description | Specification | Replacement frequency | UNITEC category |
|---|---|---|---|
| Oil thermostat | Calibration 75°C | 10,000 hours | Thermal components |
| Oil filter | 10 microns absolute | 2,000 hours | Filtration |
| Lubricant | High temperature synthetic | 4,000 hours | Lubricants |
To order these parts, visit: https://www.unitecd.com/e-catalog/
11. References
- NF EN 1012-1: Compressors and vacuum pumps - Safety requirements.
- ISO 1217: Positive displacement compressors - Acceptance tests.
- ATEX directives (if applicable).
- OEM compressor technical manual (maintenance chapter).
