Diagnostic Guide: High Discharge Temperature in Screw Compressors

1. Description and Scope of the Problem

High discharge temperature on an oil-injected lubricated screw compressor is a critical symptom that, if not addressed immediately, can lead to severe damage to internal components, reduced equipment life, and unplanned production shutdowns. This diagnostic guide is designed for maintenance technicians and reliability engineers who face this anomaly in screw compressors of various capacities, commonly found in manufacturing sectors in Spain and Latin America. We will address the most frequent causes: low oil level, fouling of the cooling system, failures in the thermostatic valve and adverse environmental conditions. The classification of this fault is CRITICAL, since prolonged operation under these conditions can irreversibly damage the rotor, screw unit and motor.

2. Safety Precautions

CAUTION: Working on air compressors presents significant risks. Always observe your plant's safety procedures and local regulations (UNE-EN ISO 12100).

• Lockout and Tagout (LOTO): Before any intervention, ensure that the compressor is de-energized, locked out and tagged out. Check the absence of voltage with a calibrated multimeter.
• Stored Energy: Compressed air systems store energy at high pressure. Completely depressurize the system before opening any ductwork or components.
• Hot Components: The oil and compressor surfaces can reach high temperatures (>90°C). Always use appropriate Personal Protective Equipment (PPE), including heat-resistant gloves (UNE-EN 407) and eye protection (UNE-EN 166).
• Chemical Substances: Cleaning coolers may involve the use of chemicals. Consult the material safety sheets (MSDS) and use the recommended PPE (chemical gloves UNE-EN 374, respiratory protection UNE-EN 149, eye protection UNE-EN 166).
• Electrical Risk: Electrical connections must be handled by qualified personnel. Avoid contact with live electrical components.

3. Required Diagnostic Tools

For an accurate diagnosis, it is recommended to have the following tools:

4. Initial Evaluation Checklist

Before starting any diagnosis, collect the following information. This methodical observation can significantly guide the resolution of the problem.

5. Systematic Diagnostic Flowchart

1. Symptom: High Discharge Temperature (>95°C or alarm)
   1. Check Oil Level:
      1. If the oil level is low:
         • Probable Cause: Oil leak or excessive consumption.
         • Go to section 7.1: Root Cause Analysis - Low Oil Level.
      2. If the oil level is correct:
         • Go to the next step.
   2. Inspect the Oil and Air Cooler:
      1. If the cooler beams are visibly dirty or blocked:
         • Probable Cause: Cooler fouling.
         • Go to section 7.2: Root Cause Analysis - Cooler Fouling.
      2. If the cooler appears clean externally:
         • Go to the next step.
   3. Test Thermostatic (Oil) Valve:
      1. If valve does not open or close correctly at its setting temperature:
         • Probable Cause: Thermostatic valve failure.
         • Go to section 7.3: Root Cause Analysis - Thermostatic Valve Failure.
      2. If the valve works apparently fine:
         • Go to the next step.
   4. Analyze Environmental Conditions and Ventilation:
      1. If ambient temperature is high (>35°C) and/or there is restriction in ventilation air flow (anemometer <1.5 m/s at chiller outlet):
         • Probable Cause: Environmental conditions adverse conditions or poor ventilation.
         • Go to section 7.4: Root Cause Analysis - Environmental Conditions and Ventilation.
      2. If ambient conditions and ventilation are optimal:
         • Go to the next step.
   5. Other Possible Causes (less likely, but to be considered):
      1. If differential pressure across the oil filter is high (>1 bar):
         • Probable Cause: Clogged oil filter.
         • Go to section 7.5: Root Cause Analysis - Clogged Oil Filter.
      2. If there is abnormal noise or low oil pressure:
         • Probable Cause: Oil pump failure.
         • Go to section 7.6: Root Cause Analysis - Oil Pump Failure.
      3. If the oil appears degraded or incorrect (viscosity, color):
         • Probable Cause: Incorrect or degraded oil.
         • Go to section 7.7: Root Cause Analysis - Incorrect or Degraded Oil.

6. Failure-Cause Matrix

7. Root Cause Analysis for Each Failure

7.1. Low Oil Level

• Why does it occur? An insufficient amount of oil reduces the lubrication and cooling capacity of the screw unit. The main causes include external leaks (gaskets, pipes, seals), internal leaks (oil passing into the compressed air system due to a damaged oil separator or improper drainage), or excessive consumption due to prolonged operation at high load or poor quality oil.
• How to Confirm? Check the oil level in the sight glass on the tank or on the electronic gauge. A level below the minimum, both in operation and at rest (after depressurization of the system), confirms the problem. Visually examine the compressor and surrounding pipes for evidence of oil leaks. Excessive oil carryover in the discharge compressed air, detectable by an air quality analyzer, can also indicate internal oil loss.
• Damage if not resolved: Lack of lubrication causes accelerated wear of the bearings and screw rotors, which results in increased friction, increased temperature, seizure of the compressor unit and, in serious cases, the total destruction of the element.

7.2. Oil and Air Cooler Fouling

• Why does it happen? The cooler is responsible for dissipating heat from the lubricating oil and hot compressed air. The accumulation of dust, dirt, fibers, or carbonized oil deposits on the fins or inside the tubes dramatically reduces their heat transfer efficiency. This is common in dusty or poorly maintained environments.
• How to Confirm?
  1. Visual Inspection: Observe the external fins of the cooler. If they are clogged by dirt, heat dissipation will be poor.
  2. Differential Temperature Measurement: With the compressor operating at full load, use an IR thermometer to measure the temperature of the oil at the inlet and outlet of the cooler. A temperature differential (ΔT) of less than 10°C for the oil, or an unacceptably high discharge air temperature after the aftercooler, suggests a problem. A clean and efficient cooler should have an oil ΔT of at least 15-20°C.
  3. Thermographic Camera: Allows you to view restricted air or oil flow patterns, identifying areas of the cooler with abnormally high or low temperatures.
  4. Differential Pressure (oil side): Some compressors have intakes to measure the pressure before and after the oil cooler. A differential pressure >0.5 bar in the oil circuit may indicate an internal obstruction.
• Damage if not Resolved: An ineffective cooler allows the oil to operate at elevated temperatures, which accelerates its degradation, reduces lubrication and can lead to carbonization of the oil, forming varnishes and sludge that further clog the system, including filters and lubrication holes. Eventually, this leads to failure due to overheating of the screw unit.

7.3. Thermostatic Valve Failure (Oil)

• Why does it happen? The thermostatic valve (also known as a mixing or three-way valve) directs the flow of oil. At low temperatures, oil bypasses the cooler to quickly heat the system. As the temperature increases, the valve progressively opens to send more oil to the cooler, maintaining the optimal operating temperature (typically between 80-90°C). A failure may be due to: internal wear, accumulation of deposits that prevent movement, or loss of properties of the thermosensitive element (wax). If the valve becomes stuck in a position that restricts flow to the cooler, the oil will overheat.
• How to Confirm?
  1. Temperature Measurement: With the compressor on load, use an IR thermometer to measure the oil temperature at the inlet to the cooler and at the outlet of the bypass valve. If the valve is failing, the inlet temperature to the cooler will be high, but the bypass outlet temperature (which mixes with the refrigerated oil) will not drop sufficiently, or the oil temperature in the main circuit will be persistently high.
  2. Bench Test (if possible): Disassemble the valve and immerse it in a hot water bath with a thermometer. At the OEM specified opening temperature (e.g. 71°C or 80°C), the valve stem should begin to move. At full opening temperature, it should be fully open. If it does not react or reacts out of range, it is defective.
  3. Visual Inspection: Look for deposits or excessive wear on the plunger or valve seat.
• Damage if Not Resolved: A faulty thermostatic valve prevents effective oil cooling, leading to the same consequences as a clogged cooler: accelerated oil degradation, varnish formation, premature component wear, and catastrophic failure of the compressor unit.

7.4. Environmental Conditions and Poor Ventilation

• Why does it happen? Compressors are designed to operate within a specific range of ambient temperatures (typically 5°C to 40°C). If the compressor room temperature exceeds this limit, the refrigeration system may not be able to dissipate enough heat. Additionally, inadequate ventilation (insufficient air intake, blocked hot air outlet, compressor discharge air recirculation, or a faulty chiller fan) impedes the flow of fresh air through the chiller, decreasing its effectiveness.
• How to Confirm?
  1. Ambient Temperature Measurement: Record the temperature at different points in the compressor room, especially near the air inlet of the compressor. If the upper operating limit of the equipment is exceeded (>40°C for most).
  2. Visual Ventilation Inspection: Check if the air inlet and outlet grilles are clogged, if there are objects blocking the air flow, or if hot discharge air is being recirculated to the inlet. Make sure the compressor fan is working properly (inspect blades, listen for noises, measure motor current with multimeter).
  3. Air Flow Measurement: Use an anemometer at the hot air outlet of the compressor. A low airflow (<1.5 m/s) or a velocity significantly lower than the manufacturer's specification indicates a ventilation problem.
• Damage if Not Resolved: Continued operation in an overheated or poorly ventilated environment subjects all compressor components (motor, screw drive, oil, electronics) to excessive thermal stress. This reduces service life, increases power consumption, and can cause electrical or mechanical failure.

7.5. Clogged Oil Filter

• Why does it happen? The oil filter retains particles and contaminants to protect lubricated components. Over time, it becomes clogged with the byproducts of oil degradation and component wear. A clogged filter restricts the flow of lubricating oil and coolant to the screw unit, limiting the system's ability to dissipate internal heat.
• How to Confirm? The most accurate way is to measure the differential pressure across the oil filter. Many modern compressors incorporate a differential pressure indicator or sensor that activates an alarm when the value exceeds the limit (typically between 0.8 and 1.2 bar, depending on the OEM). A visual inspection of the filter and its last change date are also relevant.
• Damage if Not Resolved: Restricted oil flow causes insufficient lubrication of the bearings and rotors, generating excessive friction and heat. Additionally, if the filter is completely clogged, oil can bypass through an internal filter bypass valve, allowing unfiltered and contaminated oil to circulate through the system, causing severe abrasive wear and premature failure of lubricated components.

7.6. Oil Pump Failure (if applicable)

• Why does it happen? Some larger or specifically designed screw compressors use an oil pump to ensure adequate flow under all conditions. A pump failure (gear wear, defective motor, blockage) would result in insufficient oil pressure, which compromises both the lubrication and cooling of the system.
• How to Confirm? Use a pressure gauge to check the oil pressure directly after the pump (if there is a test port). Compare the reading to the manufacturer's specifications. A significantly low or zero value indicates a pump failure. Also listen for abnormal noises (screeching, metallic noises) coming from the pump.
• Damage if not Resolved: It is a critical cause. A complete failure of the oil pump means the absence of lubrication and cooling, leading to instant or very rapid seizure of the screw unit and other moving components, resulting in catastrophic damage and costly repairs.

7.7. Incorrect or Degraded Oil

• Why does it happen? Compressor oil is an integral part of the refrigeration system. Using the wrong type of oil (e.g. with too low or high a viscosity), or an oil that has exceeded its useful life, affects its ability to transfer heat and lubricate efficiently. Degraded oils can form varnish and sludge, clogging the system and reducing heat transfer.
• How to Confirm?
  1. Maintenance History: Check the type of oil used in the last change and compare it with the manufacturer's recommendations.
  2. Visual Inspection: Degraded oil may be dark in color, have a burning smell, or have an unusual consistency.
  3. Oil Analysis: It is the most reliable confirmation. An oil sample sent to a laboratory will reveal viscosity, Total Acid Number (TAN, oxidation indicator), water content, and the presence of metallic particles. Values ​​outside acceptable limits for TAN (e.g. increase of >2.0 mg KOH/g over new) or viscosity confirm degradation.
• Damage if not Resolved: Incorrect or degraded oil compromises lubrication and cooling, leading to premature wear of the bearings and rotors, formation of deposits that clog the system (filters, cooler, thermostatic valve), and eventually, failure due to overheating and severe mechanical damage.

8. Step-by-Step Resolution Procedures

ATTENTION: Before beginning any repair procedure, ensure that the compressor is de-energized, locked out and tagged out (LOTO), and depressurized. Use appropriate PPE.

8.1. Resolution for Low Oil Level

1. Fill Oil: With the compressor stopped and depressurized, fill with the type of oil recommended by the manufacturer (see section 10) to the maximum level indicated in the sight glass or indicator.
2. Inspect for Leaks: Operate the compressor and carefully check all pipe connections, gaskets, shaft seals and drains for leaks. Use UV light if a leak detection additive is used.
3. Repair Leaks:
   • If external leaks, replace any damaged gaskets, seals or pipes. Ensure adequate torque on threaded connections (see OEM manual for exact values, typically between 20-50 Nm for 1/2" to 3/4" NPT connections).
   • If there is any indication of oil carryover, inspect the oil separator element and the separator drain line. Replace if necessary.
4. Check: Start the compressor and operate under load, monitoring the oil level and discharge temperature. Make sure the level remains stable and the temperature within the normal range (e.g., 85-90°C).

8.2. Resolution for Oil and Air Cooler Fouling

1. External Cleaning (Fins):
   • Use dry, clean compressed air (maximum pressure of 3 bar to avoid damaging the fins) or a low pressure pressure washer. Direct air or water flow in reverse of normal operating flow to dislodge dirt more effectively.
   • For stubborn dirt or oil deposits, use a metal-specific industrial degreaser (non-corrosive to aluminum and copper). Apply, let sit according to manufacturer's instructions, and rinse thoroughly.
2. Internal Cleaning (Oil/Air Circuits): If external cleaning is not sufficient and internal obstruction (high differential pressure) is suspected, it may be necessary to disassemble the cooler and send it to a specialized workshop for chemical or mechanical cleaning.
3. Check: Once cleaned and assembled, operate the compressor. Monitor the discharge temperature and oil temperature differential in the cooler. You should see an oil ΔT of at least 15-20°C and a significant reduction in discharge temperature.

8.3. Resolution for Thermostatic Valve Failure

1. Valve Replacement: In most cases, the thermostatic valve is not repairable and must be replaced.
2. Oil Drain: Drain enough oil from the reservoir so that the level is below the valve.
3. Disassembly: Disconnect the oil lines and remove the faulty valve. Pay attention to the mounting orientation.
4. Assembly: Install the new thermostatic valve (original part or equivalent of certified quality, CE/AENOR). Make sure new gaskets or seals are installed correctly. Tighten the connections to the OEM specified torque (e.g. 30-60 Nm).
5. Fill and Bleed: Fill the missing oil. Start the compressor and allow the oil to circulate to bleed any air bubbles from the system.
6. Check: Operate the compressor at full load. Verify with the IR thermometer that the valve is correctly modulating the oil flow to the cooler, maintaining the discharge temperature within the optimal range (85-90°C).

8.4. Resolution for Environmental Conditions and Poor Ventilation

1. Ventilation Optimization:
   • Fresh Air Inlet: Ensure that the air inlet grilles are large enough and not obstructed. Consider installing ducts to bring in fresh air directly from the outside if the room is overheated.
   • Hot Air Outlet: Direct the hot air discharged from the compressor to the outside through ducts. Avoid recirculation of hot air towards the compressor inlet. Use deflectors or louvers to guide airflow efficiently.
   • Additional Fans: If natural or compressor ventilation is insufficient, consider installing additional exhaust fans in the room.
2. Temperature Monitoring: Install environmental thermometers and/or sensors that alert when the room temperature approaches the compressor's operating limits.
3. Verification: Measure the ambient room temperature and air flow at the chiller outlet with an anemometer. The compressor discharge temperature should stabilize in the normal range (85-90°C).

8.5. Resolution for Clogged Oil Filter

1. Filter Replacement: With the LOTO compressor depressurized, drain the oil from the crankcase if the filter is below the oil level.
2. Disassembly: Unscrew the old oil filter. Make sure the rubber gasket is not stuck to the head.
3. Installation: Lubricate the rubber gasket of the new filter with clean oil. Screw on the new filter by hand until the gasket contacts, then tighten an additional 3/4 to 1 turn (see OEM manual). Do not overtighten.
4. Filling and Checking: Refill the oil if it was drained. Start the compressor and check for leaks around the filter. Monitor differential pressure and discharge temperature. The differential pressure should be minimal and the discharge temperature normalized.

8.6. Resolution for Oil Pump Failure

1. Pump Replacement: Due to complexity and criticality, a faulty oil pump usually requires complete replacement.
2. Oil Drain: Drain all oil from the system.
3. Disassembly: Remove the connected pipes and the faulty pump, following the instructions in the manufacturer's manual.
4. Installation: Fit the new oil pump, ensuring correct alignment and the use of new gaskets. Tighten all bolts to the OEM specified torque.
5. Filling and Start-up: Fill the system with new oil of the correct type and quantity. Perform an air bleed if the system requires it.
6. Check: Start the compressor and immediately monitor the oil pressure and discharge temperature. Make sure the oil pressure is within the manufacturer's specifications and the discharge temperature is normal.

8.7. Resolution for Incorrect or Degraded Oil

1. Complete Oil Change: Drain all oil from the compressor, including the oil from the cooler and lines if possible.
2. System Flushing: If the oil was severely degraded or sludge/varnish formation is suspected, consider flushing the system with a suitable flushing fluid before refilling with new oil.
3. Refill with Correct Oil: Refill the system with new oil of the EXACT type and viscosity specified by the compressor manufacturer (see section 10).
4. Check: Monitor compressor discharge temperature. It should return to normal values. Consider an oil analysis at a reduced interval to confirm the effectiveness of the change.

9. Preventive Measures

10. Spare parts and components

Find all the original and certified equivalent spare parts for your compressor in the E-Catalog of UNITEC-D.

11. References

• UNE-EN ISO 12100:2012 - Machine safety. General principles for design. Risk assessment and risk reduction.
• UNE-EN 407:2020 - Protective gloves against thermal risks (heat and/or fire).
• UNE-EN 166:2002 - Individual eye protection. Requirements.
• UNE-EN 374:2017 - Protective gloves against chemicals and microorganisms.
• UNE-EN 149:2009 - Filtering half masks for protection against particles. Requirements, tests, marking.
• UNE-EN ISO 2941:2009 - Hydraulic fluids. Filter elements. Verification of resistance to collapse/breakage.
• Manufacturer's Operation and Maintenance Manual (OEM) for your specific compressor.
• Screw Compressor Maintenance Guides - Guidelines from compressed air associations (e.g., CAGI - Compressed Air and Gas Institute).
• Industrial Oil Analysis Guides - Recommendations from lubricant manufacturers and analysis laboratories.