Diagnostic and Resolution Guide: High Discharge Temperature in Screw Compressors

1. Description of the Problem and Scope

This guide addresses the symptoms and underlying causes of high discharge temperature in screw compressors, a critical operating condition that can lead to significantly reduced component life, premature failure, and unscheduled shutdowns. Oil injected screw compressors are the main equipment affected. A sustained discharge temperature above the manufacturer's specified limits (generally >100-110°C) is considered a high temperature alarm. We classify this issue as critical due to its potential to damage vital internal components, such as compression element bearings and oil seals, directly impacting reliability and operational continuity.

2. Safety Precautions

CRITICAL SAFETY WARNING: Before beginning any diagnostic or maintenance procedures, it is imperative to secure the equipment. Perform a Lockout/Tagout (LOTO) procedure in accordance with the EN 1037 standard for safe energy isolation. Verify the absence of electrical voltage using a calibrated multimeter. Consider the energy stored in the system, including residual pressure in the hot air and oil circuit, as well as the thermal energy of the compressor components. Allow the equipment to cool before handling any hot surfaces. Always wear appropriate Personal Protective Equipment (PPE), including heat-resistant gloves, safety glasses conforming to EN 166, and hearing protection (ear muffs or plugs) conforming to EN 352, especially during operation or testing with energized equipment.

3. Required Diagnostic Tools

The following table details the essential tools for accurate diagnosis of high discharge temperature:

4. Initial Evaluation Checklist

Before proceeding with a detailed diagnosis, complete the following checklist to gather crucial information about the compressor's operating conditions. This information is essential to guide the troubleshooting process.

5. Systematic Diagnostic Flowchart

This flow chart will guide the technician through a structured process to identify the root cause of the high discharge temperature.

1. Initial Observation: Confirmed High Discharge Temperature (>100-110°C)
   • Has an alarm been activated on the compressor controller?
   • Does the discharge temperature consistently exceed the normal operating threshold?
2. Step 1: Verification of Environmental Conditions and Ventilation
   1. Measure the ambient temperature of the room and at the compressor inlet.
   2. Check air flow through the air and oil cooler.
   3. Inspect room ventilation (exhaust fans, ducts) for obstructions or hot air recirculation.
   4. If the ambient temperature is excessively high (>40°C) or ventilation is poor: Proceed to section Resolution: Environmental Conditions.
   5. If environmental conditions and ventilation are adequate: Continue with Step 2.
3. Step 2: Checking the Oil Level and Condition
   1. With the compressor off and depressurized, check the oil level on the sight glass or dipstick.
   2. Visually examine the oil for discoloration, foam, or particles.
   3. If oil level is low: Proceed to section Resolution: Low Oil Level.
   4. If the oil is discolored, foamy, or has a burning smell: Proceed to section Resolution: Oil Degradation.
   5. If the oil level and condition appear correct: Continue with Step 3.
4. Step 3: Inspect the Oil and Air Cooler
   1. Visually inspect the external fins of the oil and air cooler for obstructions (dust, dirt, debris).
   2. Measure oil/air temperatures before and after the cooler with a pyrometer or contact sensor. Calculate the temperature drop.
   3. If the cooler fins are visibly dirty or clogged: Proceed to section Resolution: Cooler Fouling.
   4. If the temperature drop across the cooler is less than specified (e.g. <10-15°C): This indicates poor heat transfer. Suspect internal cooler fouling or low flow. Continue with Step 4 to discard the thermostat.
   5. If the cooler appears clean and the temperature drop is adequate: Continue to Step 4.
5. Step 4: Checking the Thermostat (Thermostatic Valve)
   1. With the compressor running hot, use a pyrometer to measure the temperature of the oil line before and after the thermostat.
   2. The temperatures before and after the thermostat should be significantly different if the thermostat is closing the bypass and directing the oil to the cooler.
   3. If temperatures indicate that oil is not being directed to the cooler effectively (e.g., little difference or the bypass is hot while the cooler is not hot enough): Proceed to section Resolution: Thermostat Failure.
   4. If the thermostat is working properly: Continue with Step 5.
6. Step 5: Less Common / Internal Causes
   1. Check for possible restrictions in the discharge line (line filters, clogged condensate traps).
   2. Consider the possibility that the airend is failing (worn bearings, bad seals), generating excessive heat internally. This is usually accompanied by unusual vibrations or mechanical noises. Use a vibration analyzer (ISO 10816).
   3. If an internal airend problem is suspected: Contact a UNITEC service specialist.

6. Matrix of Failures and Causes

The following table correlates high temperature symptoms with their probable causes, diagnostic tests, and expected results.

7. Root Cause Analysis for Each Failure

7.1 Low Oil Level / Oil Degradation

Explanation: A low oil level means that there is less volume of lubricant to absorb and dissipate the heat generated during compression. This reduces the cooling capacity of the system, causing an increase in the discharge temperature. Oil also degrades over time due to oxidation (varnish and sludge formation) and contamination. Degraded oil loses its lubricating and heat transfer properties, leading to overheating and increased internal friction.

How to Confirm: Check the oil level with the compressor off and depressurized; It should be between the minimum and maximum marks on the scope or rod. Perform oil analysis at a certified laboratory (ISO 4406) to evaluate viscosity, total acid number (TAN), water content, and the presence of wear metals. A high TAN, low viscosity or high water content indicate degradation.

Damage if not Resolved: Operation with insufficient or degraded oil accelerates the wear of the airend bearings, shaft seals and rotors, potentially causing catastrophic failure of the compressor element and the generation of carbon deposits that clog the internal passages of the oil system.

7.2 Oil and Air Cooler Fouling

Explanation: The cooler is the main component responsible for dissipating heat from the lubricating oil and, in some designs, from the compressed air. Fouling can be external (dust, dirt, fibers, oil film on the fins) or internal (carbon deposits, oil varnishes or sludge on the tubes). Both types of fouling dramatically reduce the heat transfer efficiency of the cooler, preventing the system from maintaining oil temperatures in a safe operating range.

How to Confirm:

• External: Visual inspection of cooler fins.
• Internal: Measure the temperature drop of the oil (and/or air) across the cooler. For an oil cooler, a temperature difference between inlet and outlet of less than 10-15°C under normal load, with adequate airflow, is a clear indication of internal fouling. A thermal imaging camera can reveal an uneven temperature distribution.

Damage if not Resolved: Lack of heat dissipation leads to a feedback loop where the oil degrades more quickly due to high temperatures, which in turn aggravates internal fouling. This results in reduced airend life, compressed air quality problems and increased energy consumption.

7.3 Thermostat Failure (Thermostatic Oil Valve)

Explanation: The thermostatic oil valve (thermostat) regulates the oil flow to maintain an optimal air injection temperature. When the oil is cold, it diverts the flow away from the cooler (bypass) so that the compressor quickly reaches its operating temperature. Once hot, the thermostat opens to direct the oil through the cooler. If this valve fails and becomes stuck in the bypass position (closed to the cooler or partially open), hot oil will constantly recirculate bypassing the cooler, causing an uncontrolled rise in discharge temperature.

How to Confirm: With the compressor running and already warm, compare the temperature of the oil line just before the cooler with the temperature of the thermostat bypass line. If the bypass line remains very hot while the cooler does not reduce the oil temperature, it is a strong indication that the thermostat is stuck or not working properly. It can be confirmed by removing the valve and testing it in a water bath with a thermometer.

Damage if Not Resolved: A faulty thermostat is a direct cause of overheating. This not only degrades the oil and damages the airend, but can also cause failure of temperature-sensitive components such as seals and gaskets, leading to leaks and eventual compressor shutdown.

7.4 Adverse Environmental Conditions

Explanation: Elevated ambient temperature in the compressor room reduces the ability of the cooling air to absorb heat from the chiller. If the compressor is located in a poorly ventilated area, where hot discharge air is recirculated to the compressor air inlet, or if the outside temperature is exceptionally high, the cooling system simply cannot dissipate enough heat.

How to Confirm: Use a thermometer or the compressor's ambient temperature sensor to record the air temperature at the air filter inlet and at different points in the room. Compare with the maximum operating temperature specified by the manufacturer (typically 40-45°C according to EN ISO 2151). An anemometer can check the speed of air flow through the room and chiller.

Damage if not Resolved: Continuous operation in high ambient temperature conditions causes constant thermal stress on all compressor components, reducing its useful life. Increases the likelihood of oil oxidation and seal failure, requiring more frequent and costly maintenance.

8. Step-by-Step Resolution Procedures

8.1 Resolution: Low Oil Level

1. Isolation and Depressurization: Perform LOTO. Completely depressurize the system. Allow the compressor to cool. WARNING: Hot oil under pressure can cause serious burns.
2. Leak Identification: Visually inspect all oil lines, fittings, seals, cooler and oil separator for leaks. Use a UV leak detector if the system has fluorescent dye. Repair any leaks found.
3. Oil Refill: Add oil of the correct type and grade (refer to the manufacturer's manual and UNITEC specifications) until the level is between the minimum and maximum marks on the sight glass or dipstick.
4. Check: Start the compressor and operate under load. Monitor discharge temperature and oil level. Check for new leaks.

8.2 Resolution: Cooler Fouling

1. Isolation and Depressurization: Perform LOTO. Depressurize the system. Allow the compressor to cool.
2. External Cleaning: Use dry, low-pressure compressed air (max. 2 bar) to blow the cooler fins from the inside to the outside. If the contamination is greasy, use a specific degreaser for coolers, following the manufacturer's instructions, and then rinse with low pressure water. Make sure water does not get into electrical components.
3. Internal Cleaning (if necessary): For severe internal fouling, it may be necessary to disassemble the cooler and send it to a specialized workshop for chemical or mechanical cleaning. In some cases, cleaning in situ can be performed with specific chemicals, circulating the product through the cooler. WARNING: Chemicals can be corrosive and require specific PPE.
4. Check: Once clean, reassemble and start the compressor. Monitor discharge temperature and temperature difference across the chiller. The temperature drop must be at least 10-15 °C.

8.3 Resolution: Thermostat Failure

1. Isolation and Depressurization: Perform LOTO. Depressurize the system. Allow the compressor to cool. Drain a small amount of oil to avoid spills when removing the thermostat.
2. Removal and Testing: Locate the thermostatic valve. Disassemble the housing and remove the thermostatic element. Immerse the item in a hot water bath along with a thermometer. Observe the temperature at which the element begins to open and at which it is fully open. Compare to manufacturer's specifications (e.g. opening at 70°C, fully open at 80°C).
3. Replacement: If the thermostat does not operate within specifications, replace it with a new one (reference UNITEC-D). Install with new gaskets, applying recommended tightening torque (see manual).
4. Check: Refill oil if necessary. Start the compressor and monitor the discharge temperature. Verify that oil is properly circulating through the cooler once the compressor has reached operating temperature.

8.4 Resolution: Adverse Environmental Conditions

1. Ventilation Optimization: Ensure that the air inlet and outlet openings in the room are adequate and free of obstructions. Consider installing additional thermostatically controlled exhaust fans.
2. Hot Air Separation: Implement hot air discharge ducts to expel it directly to the outside, avoiding recirculation in the room.
3. Thermal Insulation: If the room is exposed to direct solar radiation or external heat sources, consider improving thermal insulation.
4. Relocation (if feasible): In extreme cases of persistently hostile environments, relocating the compressor to a cooler, more ventilated area may be a long-term solution.
5. Verification: Measure the ambient temperature at the compressor inlet after applying the improvements. It must be reduced and maintained within the operating limits for the compressor.

9. Preventive Measures

Implementing a preventive and predictive maintenance plan is crucial to prevent the recurrence of high temperature problems.

10. Spare parts and components

Having the appropriate spare parts is essential for rapid intervention. The following components are available in the UNITEC-D e-catalogue:

Explore our complete catalog of compressor parts and components in our e-catalog: https://www.unitecd.com/e-catalog/

11. References

• UNE-EN Standard ISO 2151: Acoustics. Noise test codes for compressors and vacuum pumps.
• UNE-EN Standard ISO 1037: Safety of machinery. Prevention of unexpected start-up.
• UNE-EN Standard ISO 10816: Mechanical vibration. Evaluation of machine vibration through measurements on non-rotating parts.
• Compressor Manufacturer (OEM) Operation and Maintenance Manuals.
• Maintenance Guides for Industrial Oils for Compressors.
• Other UNITEC-D Maintenance Guides