Troubleshooting Screw Compressors: High Discharge Temperature

1. Description of the Problem and Scope of Application

This manual is intended for diagnosing and troubleshooting problems associated with high discharge temperatures in screw compressors. A high discharge temperature is a critical indicator that can lead to accelerated component wear, lubricant degradation, increased energy consumption and, in the worst case, to an emergency stop of the compressor. This manual covers single and twin screw compressors used in industrial compressed air systems.

Symptoms:

• Activation of the emergency signal "High discharge temperature".
• Constantly high temperature readings on the display of the compressor controller (above the normalized range, for example, >95°C).
• Reduction of compressor efficiency.
• A change in the color or smell of the compressor oil.

Severity Classification:

• Critical: The discharge temperature exceeds the set emergency stop value (for example, 100-110°C), which leads to an immediate shutdown of the compressor. Requires immediate intervention.
• Significant: The discharge temperature is consistently above the operating range (eg 90-95°C) but below the emergency threshold. Needs urgent identification and elimination of the cause to prevent damage.
• Minor: The discharge temperature periodically or slightly exceeds the norm (for example, 85-90°C). Indicates the initial stages of a malfunction, requires monitoring and proactive diagnostics.

2. Precautions

BEFORE BEGINNING ANY DIAGNOSTIC OR REPAIR WORK ON THE COMPRESSOR EQUIPMENT, ALL PERSONNEL MUST COMPLY WITH THE SAFETY RULES LISTED BELOW.

LOCKOUT/TAG-OUT (LOTO): Shut down the compressor and ensure it is locked out using the LOTO system in accordance with DSTU EN 1010-1:2004 and internal plant procedures. Make sure all power sources (electrical, pneumatic, hydraulic) are isolated and locked out.

STORED ENERGY: Compressor systems can contain significant amounts of stored energy (compressed air, hot oil, hot surfaces). BEFORE DISASSEMBLING OR OPENING ANY ASSEMBLIES, make sure the system is depressurized to atmospheric and the temperature has cooled to a safe level. Use safety valves and appropriate pressure indicators.

PERSONAL PROTECTIVE EQUIPMENT (PPE): Always use appropriate PPE when working:

• Safety glasses or a protective mask (EN 166).
• Protective gloves (heat-resistant and oil-resistant, EN 388, EN 407).
• Protective clothing (special clothing).
• Safety shoes with a metal toe (EN ISO 20345).

HOT SURFACES AND LIQUIDS: Compressors operate at high temperatures. Compressor oil and cooling system elements can be very hot. Allow equipment to cool before servicing. Avoid contact with skin.

CHEMICALS: Some compressor lubricants and cleaners can be toxic or irritating. Use PPE and follow the material manufacturer's instructions and Material Safety Data Sheets (SDS).

3. Necessary Diagnostic Tools

4. Initial Evaluation Checklist

Before starting a detailed diagnosis, perform the following actions:

5. Systematic Scheme of Diagnostics

1. Lubricant Level Check
   1. IF the lubricant level is below MIN:
      • Diagnosis: Insufficient volume of lubricant in the system.
      • Probable cause: Lubricant leakage, insufficient dose at the last filling, inappropriate replacement interval.
   2. IF the oil level is normal (between MIN and MAX):
      • Go to point 2.
2. Oil/Air Cooler Inspection
   1. IF Visually dirty radiator:
      • Diagnosis: Contamination of cooler fins on the outside.
      • Probable cause: Dust, dirt, fluff from the environment.
   2. IF external inspection revealed no contamination:
      1. Measure the oil temperature difference at the inlet and outlet of the cooler (using a pyrometer).
      2. IF oil temperature drop <10°C (at nominal load):
         • Diagnosis: Internal contamination of the oil cooler.
         • Probable cause: Deposition of lubricant degradation products, soot.
      3. IF oil temperature drop ≥10°C:
         • Go to point 3.
3. Checking the Thermostatic Valve (Thermostat)
   1. Measure the oil temperature before and after the thermostatic valve (using a pyrometer) while the compressor is running.
   2. IF the oil temperature after the thermostat does not increase or increases slightly (e.g. <5°C) when the operating temperature is reached (e.g. >75°C at the inlet to the thermostat):
      • Diagnosis: Malfunction of the thermostatic valve (remains in the closed position or does not open fully).
      • Probable cause: Wear, contamination, jamming, failure of the heat-sensitive element.
   3. IF the thermostat works correctly (lubricant enters the cooler when the opening temperature is reached):
      • Go to point 4.
4. Analysis of Ambient Conditions
   1. IF ambient temperature (at the compressor inlet) >40°C:
      • Diagnosis: Overheating due to high indoor temperature.
      • Probable cause: Insufficient ventilation of the compressor room, hot streams from other equipment.
   2. IF the ambient temperature is normal (<40°C):
      • Go to point 5.
5. Additional Causes (If the problem persists after checking the previous points)
   1. Checking the Cooling Fan
      1. IF the fan does not rotate or rotates slowly:
         • Diagnosis: Cooling fan or its drive failure.
         • Probable cause: Open motor winding, capacitor failure (for single phase), jammed bearings, relay/contactor failure.
   2. Floating Oil Level Check
      1. IF oil foamed in the sight glass (after compressor stop and pressure relief):
         • Diagnosis: Lubricant degradation or water/other liquid contamination.
         • Probable cause: Excessive watering, oxidation, use of incompatible lubricant.
   3. Checking the Oil Separator
      1. IF the pressure difference on the oil separator exceeds the norm (for example, >0.5 bar):
         • Diagnosis: Clogging of the oil separator.
         • Probable cause: Exceeded service life, high amount of solid particles in the system.

6. Matrix of Malfunctions and Causes

7. Root Cause Analysis for Each Malfunction

7.1. Low Lubricant Level

• Explanation: Insufficient volume of lubricant reduces its ability to absorb and dissipate heat released during air compression. Also, the volume of lubricant circulating through the cooler is reduced, which reduces cooling efficiency.
• Confirmation: The oil level on the sight glass or dipstick is well below the minimum mark. After topping up the lubricant, the injection temperature normalizes.
• Possible Damage: Accelerated wear of bearings, screw block due to insufficient lubrication, carbon deposits, overheating and degradation of lubricant.

7.2. Contamination of the Cooler

• Explanation:
  • External contamination: Accumulation of dust, dirt, oil mist, fluff on the outer fins of the heat exchanger creates an insulating layer that prevents the efficient transfer of heat from the oil/air to the environment.
  • Internal contamination: Deposits of oil oxidation products, soot, resinous substances on the inner walls of the cooler channels reduce the cross-section for oil/air and reduce the efficiency of heat exchange.
• Confirmation: Visually visible contamination of the radiator fins. The thermal imager shows significant temperature differences across the surface of the cooler, indicating areas with poor heat exchange. The oil temperature difference at the inlet and outlet of the cooler is significantly reduced (for example, <10°C).
• Possible Damages: Increased load on the cooling system, degradation of lubricant, accelerated wear of seals and bearings, reduction of compressor efficiency.

7.3. Malfunction of the Thermostatic Valve

• Explanation: The thermostatic valve is responsible for directing the oil through the cooler when it reaches a certain temperature. If the valve jams in the closed position or does not open fully, the oil will not be cooled properly, circulating mainly in the bypass circuit, which will lead to an increase in the discharge temperature.
• Confirmation: When operating the compressor in the operating temperature range (for example, oil temperature >75°C), the pyrometer shows that the oil temperature after the thermostatic valve is almost equal to the temperature before it, or the difference is minimal (for example, 0-2°C), and the cooler itself remains cold.
• Possible Damage: Overheating of the lubricant, loss of its lubricating properties, damage to the screw block, bearings and seals.

7.4. High Ambient Temperature / Ventilation Problems

• Explanation: Compressors are designed to work in a certain range of ambient temperatures (usually up to 40°C). If the temperature of the air entering the compressor exceeds this limit, the cooling system cannot dissipate the heat effectively, resulting in an increase in the discharge temperature. Insufficient ventilation of the compressor room creates stagnant zones of hot air that recirculates through the compressor.
• Confirmation: Air temperature measurement at the inlet to the compressor shows a value above 40°C. A thermal imager can detect areas of hot air around the compressor or indoors.
• Possible Damages: Increased wear of components due to constant overheating, reduction of lubricant service life, increased energy consumption.

8. Step-by-Step Troubleshooting Procedures

8.1. Oil Level Recovery

1. SECURITY: Follow the LOTO procedure. Wait for complete cooling and pressure relief in the system.

2. Determine the type and specification of the compressor oil (refer to the operation manual). Use only the lubricant recommended by the manufacturer (ISO VG of the appropriate viscosity).
3. Open the filler neck of the oil tank.
4. Carefully add oil, checking the level on the sight glass or dipstick. Fill up to the MAX mark. Do not overfill.
5. Close the filler neck. Remove the LOTO.
6. Start the compressor, let it run for 5-10 minutes, stop it, release the pressure again and check the oil level. If necessary, repeat the topping up.
7. Resume normal operation and observe the discharge temperature.

8.2. Cleaning the Cooler

1. SECURITY: Follow the LOTO procedure. Wait for complete cooling and pressure relief in the system.

2. External cleaning:
   1. Using a brush with soft bristles or compressed air (maximum 2 bar), remove dust and dirt from the fins of the cooler. The direction of cleaning is opposite to the direction of the air flow during operation.
   2. For persistent contamination (oil, grease), use special detergents for radiators recommended by the manufacturer. Follow the cleaner manufacturer's instructions and rinse off the residue thoroughly with water under low pressure.
   3. Make sure the cooler is completely dry before starting.
3. Internal cleaning (if internal contamination is confirmed):
   1. Contact UNITEC-D service center or specialists. Internal cleaning often requires dismantling the cooler and using special chemical solutions or ultrasonic cleaning.
   2. Unauthorized cleaning can damage the heat exchanger.
4. Resume normal operation and observe the discharge temperature.

8.3. Replacement/Repair of Thermostatic Valve

1. SECURITY: Follow the LOTO procedure. Wait for complete cooling and pressure relief in the system.

2. Drain part of the lubricant from the oil tank to the level below the thermostatic valve (if possible, otherwise you will have to drain the entire volume).
3. Disconnect the pipelines from the thermostatic valve. Prepare a container for collecting the remaining grease.
4. Dismantle the faulty thermostatic valve.
5. Install the new thermostatic valve, making sure the connections are correctly oriented and tightened to the torque specified in the manual (eg 25 Nm).
6. Connect the pipelines.
7. Top up the lubricant to the required level (item 8.1). Remove the LOTO.
8. Start the compressor and check the operation of the thermostat by monitoring the temperatures before and after it.

8.4. Optimization of Ventilation and Ambient Temperature

1. Check the operation of the exhaust and supply fans of the compressor room. Ensure their continuous operation.
2. Eliminate sources of heat radiation near the air intake of the compressor.
3. Check and clean the compressor room air intake filters.
4. Provide sufficient supply air volume to the compressor room. The recommended volume of inflow should not be less than the total volume of air removed from compressors and ventilation systems, taking into account the reserve.
5. Install baffles or ducts to direct the hot air from the compressor directly to the outside, preventing it from recirculating.
6. Ensure that the air temperature at the compressor inlet does not exceed 40°C. If this is not possible, consider installing an additional supply air cooling system.
7. Resume normal operation and observe the discharge temperature.

9. Preventive Measures

10. Spare Parts and Components

Looking for quality spare parts for your compressor equipment? Visit the electronic catalog of UNITEC-D.

11. Links

• Standards:
  • DSTU ISO 8573-1:2018 "Compressed air. Part 1. Pollutants and purity classes".
  • DSTU EN 1010-1:2004 "Machine safety. Safety requirements for the design and construction of printing machines and machines for paper processing. Part 1. General requirements".
  • ISO 1217:2009 "Performance measurement of rotary positive displacement compressors".
  • EN 60204-1:2018 "Machine safety. Electrical equipment of machines. Part 1. General requirements".
• OEM Manuals: Refer to the operation and maintenance manual provided by the manufacturer of your particular compressor (eg Atlas Copco, Kaeser, Ingersoll Rand).
• UNITEC Companion Manuals:
  • Compressor Maintenance Basics
  • Analysis of Lubricant in Compressors
  • Ventilation of Compressor Rooms