Troubleshooting Guide: High Discharge Temperature in Screw Compressors

1. Problem Description and Scope

This guide addresses the critical symptom of high discharge temperature in oil-lubricated screw compressors. An excessive discharge temperature is an indicator of anomalies in the compression system and, if not diagnosed and corrected promptly, can lead to accelerated degradation of the lubricating oil, premature wear of internal components (bearings, rotors, seals) and, in serious cases, to forced shutdown and catastrophic failure of the compressor element.

The scope of this document focuses on screw compressors typical of the Brazilian manufacturing industry, equipped with air or water cooling systems. The diagnostic methodology prioritizes the systematic identification of the most common root causes, including:

• Inadequate oil level in the crankcase.
• Obstruction or scaling in oil coolers (air/water).
• Thermostatic oil valve failure.
• Unfavorable environmental conditions in the compressor room.

Severity Rating:

• Critical: Discharge temperature above the maximum operating limit specified by the manufacturer (generally > 105°C or according to shutdown alarm). Requires immediate intervention to avoid permanent damage.
• Major: Discharge temperature consistently 5-10°C above normal or setpoint but below shutdown threshold. Indicates a developing anomaly that requires urgent investigation and correction.
• Minor: Sporadic or slight variations (< 5°C above normal) in the discharge temperature. Requires monitoring and may indicate the beginning of a problem.

2. Safety Precautions

CRITICAL SAFETY ALERT: Before beginning any inspection or maintenance procedure, the safety of personnel and the integrity of equipment are essential.

• Lockout and Tagout (LOTO - Lockout/Tagout): According to Regulatory Standards NR-10 and NR-12, ensure that the compressor is completely de-energized, isolated from all energy sources (electrical, pneumatic, hydraulic) and properly locked and tagged.
• Total Depressurization: Ensure that the entire compressed air system and oil reservoir are completely depressurized before opening any component. Stored pneumatic energy is dangerous.
• Hot Surfaces: Compressor components, especially the compressor element, coolers and piping, can reach high temperatures. Allow adequate cooling before handling or use Personal Protective Equipment (PPE) suitable for high temperatures.
• Mandatory PPE: Always wear safety gloves resistant to cuts and high temperatures, safety glasses, helmet and safety shoes with steel toes.
• Hot and Pressurized Oil: Lubricating oil may be hot and under pressure. Take extreme care when checking levels or draining oil as it can cause serious burns.
• DO NOT attempt to diagnose or intervene on an operating compressor if environmental or safety conditions are considered to be a risk.

3. Required Diagnostic Tools

For effective and safe analysis, the following tools are essential:

4. Initial Assessment Checklist

Before proceeding with in-depth diagnosis, carry out these preliminary checks to collect essential data and define the problem.

5. Systematic Diagnosis Flowchart

This flowchart guides the technician through a logical process to identify the root cause of elevated discharge temperature.

1. Initial Symptom: High Discharge Temperature (Panel Alarm or Abnormal Reading)
   1. Check Environmental Conditions and Oil Level:
      1. Measure Ambient Temperature at Compressor Air Inlet.
         • IF Ambient Temperature > 40°C OR Obstructed hot air outlet: Probable Cause: Unfavorable Environmental Conditions. (Proceed to 7.4)
         • ELSE: Continue to the next step.
      2. Check Oil Level on Display or Sensor.
         • IF Oil Level Below Minimum: Probable Cause: Low Oil Level. (Proceed to 7.1)
         • ELSE: Continue to the next step.
   2. Inspect Coolers and Oil Lines:
      1. Visually Inspect Oil and Air Cooler Fins.
         • SE Visibly Dirty, Obstructed or Damaged Fins: Probable Cause: Cooler Externally Obstructed. (Proceed to 7.2)
         • ELSE: Continue to the next step.
      2. WITH COMPRESSOR IN OPERATION AND WITH SAFETY (EPI): Measure Oil Temperature Differential Before and After the Oil Cooler with Infrared Thermometer/Thermocouple.
         • IF Temperature Differential < 10°C (or value specified by the manufacturer) AND Oil Cooler Outlet Temperature Still High: Probable Cause: Internally Obstructed Cooler. (Proceed to 7.2)
         • ELSE: Continue to the next step.
   3. Test Oil Thermostat:
      1. WITH COMPRESSOR IN OPERATION AND WITH SAFETY (EPI): Monitor the Compressor Discharge Temperature and the Cooler Oil Outlet Temperature.
         • IF Compressor Discharge Temperature High AND Cooler Oil Outlet Temperature Close to Compressor Discharge Temperature (indicating little cooling, or that oil is not passing through the cooler or the thermostat is stuck closed): Probable Cause: Thermostatic Oil Valve Failure (Stuck Closed). (Proceed to 7.3)
         • IF Compressor Discharge Temperature High AND Cooler Oil Outlet Temperature Too Low (unlikely, but would indicate thermostat stuck open too much): Probable Cause: Oil Thermostatic Valve Failure (Locked Open/Early Opening). (Proceed to 7.3)
         • ELSE: Continue to the next step (consider sensor failure or other less common causes).
   4. Check Sensors and Instrumentation:
      1. WITH COMPRESSOR LOCKED/TAGED: Test the Discharge Temperature Sensor (PT100, Thermistor) with a Multimeter.
         • IF Sensor Out of Manufacturer Specification: Probable Cause: Temperature Sensor Failure. (Replace and check.)
         • ELSE: Continue to investigate less common causes (worn compressor elements, internally leaking intake valves, etc., which are outside the scope of this initial guide).

6. Matrix of Failures and Probable Causes

This table details the symptoms, most likely causes (with probability rating), specific diagnostic tests, and expected results to confirm each cause.

7. Root Cause Analysis for Each Failure

7.1. Low Oil Level

Why it happens: The lubricating oil in a screw compressor plays a key role not only in lubricating the rotors and bearings, but also in absorbing and dissipating the heat generated during compression. A low oil level severely compromises the compressor's ability to remove heat from the compressed air and the compression process, resulting in overheating. Causes for low level may include external leaks (seals, gaskets, connections), excessive oil consumption due to a damaged or overloaded oil separator, or failure to replace during preventative maintenance.

How to confirm: In addition to visual inspection of the sight glass, a leak test with UV dye can identify small leaks. Oil analysis may indicate a high oil content in the compressed air, confirming a separator problem. An oil consumption higher than that specified by the manufacturer for a given period of operation is also a strong indication.

Potential damage if left unaddressed: Lack of adequate lubrication will lead to rapid wear of bearings, which can cause rotors to seize. Excessive overheating of compressed air can carbonize the oil inside the compressor element and in the circuits, forming deposits that reduce efficiency and can block critical passages, leading to complete failure of the compressor element. There is also a risk of fire due to the oil flash point being exceeded.

7.2. Oil Cooler Obstruction

Why it happens: Oil coolers (air-to-oil or water-to-oil) are heat exchangers that remove heat from the lubricating oil before it returns to the compressor element. Obstruction can occur in two main ways:

• External Obstruction (Air Side): Accumulation of dust, dirt, fibers and other debris on the air cooler fins, preventing the flow of cooling air. Common in industrial environments with high concentrations of particulates.
• Internal Obstruction (Oil/Water Side): Formation of scale, carbon deposits (due to degraded oil), sludge or contamination on the internal surface of the cooler tubes, reducing the efficiency of thermal exchange. In water chillers, calcium scale or microbiological growth is common.

How to confirm: Visual inspection of the air cooler fins is the first step. For internal obstructions, measuring the oil (or water) temperature differential between the cooler inlet and outlet will indicate low heat exchange efficiency. A thermal imaging camera can reveal irregular temperature patterns on cooler surfaces. In water coolers, a significant pressure drop across the cooler also suggests blockage.

Potential damage if left unresolved: A clogged cooler will result in constant overheating of the oil, accelerating its degradation (oxidation, formation of acids and sludge). Degraded oil loses its lubricating and cooling properties, leading to the same damage as low oil, but with the addition of widespread system contamination. High temperature can also compromise the integrity of seals and gaskets.

7.3. Oil Thermostatic Valve Failure

Why it happens: The thermostatic oil valve (also known as a minimum temperature valve or by-pass valve) directs the flow of oil. When the oil is cold, it bypasses the cooler, allowing rapid heating to the ideal operating temperature. When the oil reaches operating temperature, it gradually opens, allowing the oil to pass through the cooler. If the thermostatic valve fails, getting stuck in the "closed" position (full cooler bypass), hot oil will not be directed to the cooler, resulting in overheating. The failure may be mechanical (broken spring, damaged thermostatic element) or due to contamination by particles that prevent it from operating freely.

How to confirm: With the compressor in operation and high discharge temperature, check the temperatures of the oil piping before and after the thermostatic valve and cooler. If the valve is stuck in the by-pass position, the oil temperature after the valve and before the compressor element will be very high, and the cooler inlet piping may be cold or significantly colder than expected for operating conditions. A visual inspection of the valve (if accessible and after LOTO) may reveal wear or obstruction.

Potential damage if not resolved: Failure of the thermostatic valve results in continuous overheating of the oil, with all negative consequences for the useful life of the lubricant and internal components of the compressor, such as accelerated wear of the compressor element, bearing failure and oil carbonization. Failure to properly cool the oil can quickly lead to dangerous temperatures.

7.4. Analysis of Environmental Conditions

Why it happens: The compressor discharge temperature is directly influenced by the inlet air temperature and the efficiency of heat dissipation into the environment. Unfavorable environmental conditions include:

• Excessively High Ambient Temperature: Compressor rooms without adequate ventilation, exposed to direct sunlight, or located close to heat sources (furnaces, boilers). Compressors are designed to operate in specific ambient temperature ranges (generally up to 40-45°C).
• Inadequate Ventilation: Failure of the room's exhaust fans, obstructed air inlets/outlets, recirculation of hot compressor discharge air, or incorrect sizing of the ventilation system for the room's thermal load.
• Incorrect Installation: Compressor too close to walls, other heat-generating equipment, or in confined spaces without air renewal.

How to confirm: Measure the air temperature at the compressor air filter inlet. Compare this reading with the ambient temperature of the room and the outside temperature. Check airflow through the compressor room by inspecting fans, ducts and openings. Use a thermal camera to identify heat accumulation points in the room or hot air recirculation areas.

Potential damage if not resolved: Chronic overheating due to environmental conditions drastically reduces the useful life of the compressor, its internal components and the lubricating oil. Increased energy consumption due to lower compression efficiency and greater load on cooling fans. Recurrent failures and high maintenance costs will be a direct consequence.

8. Step-by-Step Resolution Procedures

Perform these procedures only after confirming the root cause and with the compressor properly de-energized and blocked (LOTO).

8.1. Resolution for Low Oil Level

1. SAFETY ALERT: Ensure LOT and total depressurization of the compressor.
2. Locate and Fix Leaks: Carefully inspect all connections, seals, piping and the compressor block for leaks. Retighten connections or replace damaged seals/gaskets.
3. Top up Oil Level: Add lubricating oil of the exact type and specification recommended by the manufacturer (ex: ISO VG 46 or 68, according to ABNT NBR 10100) up to the maximum level mark on the display.
4. Check Oil Separator: If excessive consumption is suspected, inspect and replace the oil separator element if it is clogged or damaged, according to the manufacturer's recommendations.
5. Monitoring: After correction, monitor the oil level and discharge temperature for several hours of operation to ensure stability.

8.2. Resolution for Oil Cooler Obstruction

1. SAFETY ALERT: Ensure LOT and total depressurization of the compressor. Be careful with hot surfaces.
2. External Cleaning (Fins):
   • Use dry, low-pressure compressed air (max. 2 bar) or pressurized water (max. 5 bar, with biodegradable detergent) to remove dirt and debris from the fins. Blow in the opposite direction to normal airflow.
   • Avoid damaging the fins. Use a soft brush if necessary.
3. Internal Cleaning (Tubes):
   • For air coolers: If internal obstruction is suspected, consider disassembling the cooler (if applicable) and cleaning it by high pressure blasting or with specific decarbonizing chemicals, strictly following the manufacturer's instructions.
   • For water coolers: Drain the water circuit. Circulate an acid or alkaline cleaning solution (depending on exchanger material and type of scale) to remove deposits, followed by thorough rinsing. CONTACT THE MANUFACTURER.
4. Assembly and Testing: Reassemble the chiller. Fill the system with oil (or water) and check for leaks. Operate the compressor and monitor the chiller temperature differential.

8.3. Resolution for Oil Thermostatic Valve Failure

1. SAFETY ALERT: Ensure LOT and total depressurization of the compressor. The oil may be hot.
2. Valve Replacement: In most cases, the thermostatic valve is a sealed unit and is not field repairable. It is essential to replace it with an original part (OEM) or certified equivalent, with the same opening temperature.
3. Inspection and Cleaning of Area: Before installation, clean the cavity where the valve sits to remove any debris that may have contributed to the failure.
4. Assembly and Check: Install the new valve, ensuring adequate seals. Fill the system with oil if necessary and check for leaks. Operate the compressor and monitor the discharge temperature and chiller temperature differential to confirm proper operation of the new valve.

8.4. Resolution for Unfavorable Environmental Conditions

1. Ventilation Improvement:
   • Increase the flow rate of the exhaust fans or install additional fans, according to the dimensioning for the room's thermal load (considering ISO 1217 and ABNT recommendations).
   • Direct the hot compressor discharge air out of the room through ducts, avoiding recirculation.
   • Ensure adequate fresh air intake openings, preferably located at the bottom of the room and with filters, to create a unidirectional airflow.
2. Ambient Temperature Control:
   • If the outside temperature is persistently high, consider installing climate control systems (industrial air conditioning) to maintain the room temperature within the operating range recommended by the manufacturer.
   • Isolate heat sources near the compressor.
3. Reevaluation of the Layout: If the previous solutions are insufficient, it may be necessary to reevaluate the positioning of the compressor in a location with better natural ventilation or less exposure to heat sources.
4. Monitoring: Continuously monitor room ambient temperature and compressor discharge temperature to validate the effectiveness of improvements.

9. Preventive Measures

Prevention is essential to ensure the reliability and longevity of the screw compressor.

10. Spare Parts and Components

Always maintain a minimum inventory of critical parts to minimize downtime.

To purchase original, high-quality spare parts for your compressor, visit the UNITEC-D e-Catalog: www.unitecd.com/e-catalog/

11. References

• ABNT NBR 10100: Industrial Lubricating Oils – Specification.
• ABNT NBR 14781: Lubricants and Industrial Fluids – Oil Analysis in Service – Guide for Test Selection.
• ABNT NBR ISO 1217: Positive Displacement Compressors – Acceptance Tests.
• ABNT NBR ISO 8573: Compressed Air – Part 1: Contaminants and Purity Classes.
• Regulatory Standard NR-10: Safety in Electricity Installations and Services (Ministry of Labor and Employment of Brazil).
• Regulatory Standard NR-12: Safety at Work in Machines and Equipment (Ministry of Labor and Employment of Brazil).
• Original Equipment Manufacturer (OEM) Operation and Maintenance Manuals.