Error diagnosis when the compressor end temperature of screw compressors is too high

Description of the problem & scope of application

This guide covers the systematic diagnosis and elimination of the critical condition of excessive compressor end temperature in oil-lubricated screw compressors. Exceeding the specified final temperature - typically >100°C to 110°C, depending on the compressor oil and manufacturer - affects operational safety, increases wear on components and can lead to unplanned downtime. All industrial screw compressor systems that are used to generate compressed air are affected. Failure to comply with temperature limits poses risks of oil carbonization, seal failure, rotor blockages and, in extreme cases, fire hazards. The fault is classified as critical because it significantly shortens the service life of the compressor and endangers the process reliability of the connected consumers.

Safety precautions

WARNING: Before starting any work on the compressor, the system must be completely shut down in accordance with DIN EN 1037 (Safety of Machines - Avoidance of Unexpected Start-Up) and SECURE AGAINST RE-USE (LOTO - Lockout/Tagout).

• Disconnect: Interrupt the electrical power supply and secure it against being switched on again.
• Secure: Depressurize compressed air systems completely. Open condensate drains manually to relieve residual pressure.
• Protect against being switched on again: Lock the main switch, attach a label.
• Personal protective equipment (PPE): Suitable PPE must always be worn during diagnosis and maintenance, in particular safety glasses (DIN EN 166), protective gloves (DIN EN 388), hearing protection (DIN EN 352) and safety shoes (DIN EN ISO 20345).
• Hot surfaces: Compressor and components can reach operating temperatures of over 100°C. Avoid contact, observe cooling times.
• Hot oil: Compressor oil can cause burns at operating temperature. Be careful when handling oil drains and filter changes.

Required diagnostic tools

Checklist for initial assessment

Systematic diagnostic flow plan

1. Symptom: Compressor end temperature too high (>100°C)
   1. Check oil level: Switch off compressor, wait approx. 5 minutes, check oil level on sight glass/display.
      • Result: Oil level below minimum?
        • YES: Probable cause: Low oil level. Go to Root Cause Analysis & Remediation Procedure: Low Oil Level.
        • NO: Go to step 2.
   2. Check oil cooler surface: Visual inspection of the cooling fins. Non-contact temperature measurement of the cooler inlet and outlet sides (oil and air sides) with infrared thermometer.
      • Result: Oil cooler fins heavily soiled (dust, fibers, etc.) OR slight drop in temperature at the oil cooler (<10°C zwischen Ein- und Austritt) auf Ölseite?
        • YES: Probable cause: Contamination of the oil cooler Continue to cause analysis & remediation procedure: Contamination of the oil cooler.
        • NO: Go to step 3.
   3. Check cooling air flow: Measure the air flow through the oil cooler with an anemometer. Check the condition of the radiator fan (direction of rotation, blockages, wear).
      • Result: Cooling air flow significantly below nominal value OR fan defective/blocked?
        • YES: Probable cause: Insufficient cooling air flow/fan failure (sub-category contamination of the oil cooler, but also an independent defect). Continue to cause analysis & remedy procedure: Contamination of the oil cooler (focus on the fan).
        • NO: Go to step 4.
   4. Check thermostat valve (oil temperature control valve):
      1. Temperature measurement: Start engine, run compressor under load. Use an infrared thermometer to measure the temperature before and after the thermostatic valve.
      2. Electrical test (for electrically controlled valves): Measure the resistance of the temperature sensor, check the control of the valve (multimeter).
      3. Mechanical test (for thermally controlled valves): When the compressor is at a standstill and cooled down, visually check whether the valve is completely closed. When the oil heats up it should open.
      • Result: Temperature in front of the valve significantly higher than after the valve (with warm oil) OR valve remains closed/blocked OR electrical control faulty?
        • YES: Probable cause: Failure of the thermostatic valve. Go to Cause Analysis & Remedy Procedure: Thermostat Valve Failure.
        • NO: Go to step 5.
   5. Analyze ambient conditions: Measure the ambient temperature in the installation room (thermally encapsulated sensor, e.g. Testo 605i). Check supply and exhaust air paths for blockages and inadequate ventilation.
   • Result: Ambient temperature well above manufacturer's specification (>35°C) OR inadequate supply/exhaust air flow?
     • YES: Probable cause: Unfavorable ambient conditions. Continue to Root Cause Analysis & Remediation Procedure: Unfavorable Environmental Conditions.
     • NO: Systematic error isolated. Further detailed check (e.g. internal oil circuit blockage, compressor stage defective). Contact UNITEC Service for advanced diagnostics.

Error-cause matrix

Root cause analysis for each error

Low oil level

Why it happens: A low oil level is usually the result of oil loss due to leaks (seals, pipes, oil separators) or insufficient oil refill after maintenance work. An inefficient oil separator can also lead to excessive oil consumption. The compressor oil is not only used for lubrication, but also primarily to remove heat from the compressor stage. If there is too little oil, the heat cannot be adequately dissipated.

How to confirm: After switching off the compressor and waiting for about 5 minutes (for the oil to flow back into the collection container), the sight glass shows a level below the minimum mark. A review of the maintenance log can show when oil was last added or an oil change occurred. Leaks can be identified by visible oil stains on the compressor or on the floor.

What damage it causes: A permanently too low oil level leads to insufficient lubrication of the compressor rotors and bearings, which results in increased wear and even bearing damage or blockage of the compressor stage. The increased temperature can also chemically break down the compressor oil and shorten the life of the seals and oil separator.

Contamination of the oil cooler

Why it happens: The oil cooler (air/oil or water/oil) is responsible for dissipating the heat generated during the compression process. In air-cooled systems, dust, fibers, sand or other particles from the ambient air can clog the fine fins of the cooler. This reduces the air flow and the heat transfer area. In water-cooled systems, deposits (limescale, sludge, biofilm) on the water side can drastically reduce the efficiency of the heat exchanger. Internal contamination on the oil side can be caused by contaminated or aged oil as well as by mechanical abrasion.

How to confirm: External contamination is visually visible. A differential pressure measurement across the cooler (permissible differential pressure <0.5 bar) or an endoscopy can provide information about internal contamination on the oil side. For air-cooled systems, the cooling air flow must be measured with an anemometer; a reduced volume flow indicates a blockage. A temperature measurement with the infrared thermometer shows a lower temperature drop across the cooler than specified (<10°C Delta T).

What damage it causes: A dirty oil cooler directly leads to insufficient oil cooling, which increases the compressor final temperature. This accelerates oil aging and the formation of oil carbon, which in turn can clog filters and the oil separator. In the long term, the rotor bearings and seals can be damaged by the increased thermal load.

Thermostat valve failure

Why it happens: The thermostatic valve (also called the oil temperature control valve) regulates the flow of oil through the oil cooler. When the compressor is cold, it directs the oil past the cooler to quickly reach the optimal operating temperature. When the oil reaches a certain temperature (e.g. 75-85°C), the valve opens the way to the cooler. A failure can be mechanical (jamming due to dirt, spring breakage, wax element defective) or electrical (for electronically controlled valves: sensor error, actuator defective, control error). If it remains closed, the oil will not be cooled; If it remains open, it will take too long to reach operating temperature and the oil will be excessively cooled.

How to confirm: When the compressor is at operating temperature (>80°C oil temperature), the temperature downstream of the thermostat valve (towards the cooler) should be noticeably lower than the temperature of the oil coming directly from the compressor stage. If the temperature difference remains small or the valve gets stuck in the closed state, the cooler is bypassed and the oil overheats. Dismantling and functional testing (e.g. in a hot water bath for mechanical valves) can confirm function.

What damage it causes: A stuck thermostat valve prevents the oil from cooling, which leads to a continuously high compressor end temperature. This results in all of the consequential damage already mentioned due to overheating (oil aging, seal damage, bearing wear) and can lead to compressor failure.

Unfavorable environmental conditions

Why it happens: Screw compressors are designed to operate within a specific ambient temperature range (often 0°C to 35°C). Insufficient ventilation of the installation room or installation in locations with permanently high ambient temperatures (e.g. near heat sources, in poorly ventilated rooms) means that the air sucked in is already too warm and/or the oil cooler cannot effectively release its heat to the environment. A suction of warm exhaust air from the compressor itself (short circuit in the air duct) can also be the cause.

How to confirm: The ambient temperature in the compressor room should be measured with a calibrated thermometer. A temperature of over 35°C during operation or inadequate dimensioning of the room's supply and exhaust air openings confirms this cause. Check whether exhaust ducts are blocked or compressor exhaust is being drawn directly from other compressors or heat sources.

What damage it causes: The increased ambient temperature forces the compressor to work at higher internal temperatures. This reduces efficiency, increases energy consumption and accelerates thermal wear on all components and seals in contact with oil. It can also affect the quality of compressed air as more moisture remains in the compressor.

Step by step fix procedure

Low oil procedure

1. SAFETY FIRST! Activate the compressor and secure it against being switched on again (LOTO). Depressurize.
2. Visually check for oil leaks at all connections, seals, sight glasses, drain valves and the oil separator container. Identify leaks and eliminate their cause.
3. Carefully open the vent screw on the oil tank to relieve any remaining pressure.
4. Open oil filler neck. Slowly top up with high-quality compressor oil according to the manufacturer's specifications (e.g. ISO VG 46 or VG 68 for screw compressors, DIN 51506 VD-L) up to the upper mark on the sight glass.
5. Close the oil filler neck and vent screw securely.
6. Start up the compressor and check for new leaks and the correct oil level under operating pressure. Monitor temperature progression.

Procedure for dirty oil cooler

1. SAFETY FIRST! Activate the compressor and secure it against being switched on again (LOTO). Depressurize.
2. For air-cooled coolers: Remove external contamination (dust, fibers) with compressed air (max. 2 bar, blowing from the inside out) or a soft brush. Be careful not to damage the slats. For stubborn dirt, a special radiator cleaner (ph-neutral) can be used, then rinse thoroughly with water and dry.
3. For water-cooled coolers: Disconnect the cooling water circuit. If limescale deposits occur, carry out chemical cleaning with a mild acid agent (e.g. phosphoric acid, according to the manufacturer's instructions), then neutralize and rinse thoroughly.
4. Internal contamination (oil side): If the differential pressure measurement indicates internal resistance, consider flushing the oil with fresh oil or, in extreme cases, chemical cleaning of the cooler (requires disassembly).
5. Check fan for free rotation and damage. If necessary, check motor bearings or replace fan motor.
6. After cleaning: Put the compressor into operation and monitor the temperature curve and the differential pressure on the cooler.

Procedure for failed thermostat valve

1. SAFETY FIRST! Activate the compressor and secure it against being switched on again (LOTO). Depressurize.
2. Drain oil (if necessary to dismantle the valve) or close suitable shut-off valves.
3. Remove thermostat valve. Caution: Hot oil can escape.
4. Visually check the valve for mechanical damage or jamming. For thermally controlled valves, check the wax element for functionality (e.g. in hot water). For electrically controlled valves, check the actuator and the associated temperature sensor for resistance and continuity using a multimeter.
5. Replace the defective thermostat valve with an original spare part from UNITEC-D. Specification and opening temperature must match exactly.
6. Install the valve with new seals (tightening torques according to manufacturer's instructions, e.g. 25 Nm for M10 screws).
7. Refill oil and start compressor. Monitor temperature progression before and after the valve to confirm correct function.

Procedure in unfavorable environmental conditions

1. Checking and optimizing the ventilation routes: Make sure that the supply air openings are not blocked and that the exhaust air can be discharged unhindered. If necessary, extend or insulate exhaust ducts to prevent warm air from recirculating.
2. Installation of additional ventilation systems: If ambient temperatures are consistently high (>35°C), the installation of additional exhaust fans or room air conditioning may be necessary to reduce the room temperature to an acceptable level.
3. Relocating the Compressor: As a last resort, if no other solution is practical, consider moving the compressor to a better ventilated or air-conditioned space.
4. Monitoring: Install permanent ambient temperature monitoring with an alarm function.

Preventive measures

Spare parts & components

For detailed specifications and ordering original spare parts, please visit the UNITEC-D e-catalog: www.unitecd.com/e-catalog/

References

• DIN EN 1037: Safety of machines – avoiding unexpected start-ups.
• DIN ISO 8573-1: Compressed air – Part 1: Impurities and purity classes.
• DIN 51506: Lubricants – compressor oils – minimum requirements.
• DIN ISO 10816: Mechanical vibrations – measurement and assessment of machine vibrations.
• VDMA 4363: Compressed air technology – screw compressors – performance data.
• Manufacturer-specific maintenance and operating instructions for the compressor.
• UNITEC-D maintenance manuals for compressed air systems.