Maintenance Guides 1 min read

Screw Compressor Troubleshooting: High Discharge Temperature

Technical analysis: Troubleshooting screw compressor high discharge temperature: oil level, cooler fouling, thermostat f

1. Description of the Problem and Scope of Application

This manual describes in detail the diagnostic and troubleshooting procedures associated with abnormally high discharge temperatures in screw compressors. A high discharge temperature is a critical indicator that can indicate a number of potentially serious problems that, if ignored, will lead to premature component wear, compressor failure, and significant repair costs.

Application: The manual is relevant for all types of oil-filled screw compressors used in industrial production, including air-cooled and water-cooled compressors.

Symptoms:

  • High discharge temperature indication on the compressor control panel.
  • Overheating alarm activation.
  • Automatic shutdown of the compressor due to high temperature.
  • The smell of overheated grease.
  • Reduction of compressor efficiency.

Severity classification:

  • Critical: The discharge temperature exceeds the cut-out threshold (typically 110-120 °C), causing the compressor to stop immediately. Requires immediate diagnosis and elimination.
  • Significant: The discharge temperature is consistently above the operating range (eg > 100 °C with a normal range of 70-95 °C), but does not reach the trip point. This leads to accelerated degradation of the lubricant and wear of components, requiring operative intervention.
  • Minor: The discharge temperature is only intermittent or slightly higher than normal. Requires monitoring and preventive measures.

2. Precautions

BEFORE BEGINNING ANY DIAGNOSTIC OR REPAIR WORK ON THE COMPRESSOR, THE FOLLOWING SAFETY PRECAUTIONS MUST BE PERFORMED TO PREVENT INJURY AND EQUIPMENT DAMAGE:
  • LABEL LOCKOUT / HANGING OUT (LOTO): Before opening any panels or accessing internal components, MUST disconnect the compressor from the mains and apply the LOTO procedure in accordance with DSTU EN 1037 and internal company standards. Make sure there is no voltage.
  • STORED ENERGY: Screw compressors contain a significant amount of stored energy in the form of compressed air and hot oil. Make sure the system is completely depressurized and the pressure is reduced to atmospheric. Use pressure relief valves.
  • HOT SURFACES AND OILS: Compressor and oil operating temperatures can reach over 90°C. Allow the equipment to cool before operating. Use appropriate personal protective equipment (PPE).
  • PERSONAL PROTECTIVE EQUIPMENT (PPE): Always use protective glasses, protective gloves (heat resistant, oil resistant), protective clothing and protective shoes.
  • CHEMICALS: Compressor oil is a chemical. Avoid contact with skin and eyes. In case of contact, immediately flush the affected area with water.
  • MECHANICAL MOVING PARTS: Always be careful of moving parts of the compressor (fan, rotors), even if the power is disconnected, there is a risk of accidental starting or residual rotation.

3. Necessary Diagnostic Tools

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

Name of the Tool Specification/Model Measurement range Purpose
Digital multimeter Class True RMS, CAT III 1000V (e.g. Fluke 179) Voltage: 0-1000V AC/DC; Resistance: 0-50 MΩ Measurement of supply voltage, sensor/thermostat resistance, electric motor current.
Thermal imager Resolution ≥160x120 pixels, sensitivity ≤0.1°C (for example, Flir E6) -20°C to +350°C Detection of local overheating, evaluation of cooler efficiency, monitoring of temperature distribution.
Contact thermometer (pyrometer) Digital with K-type thermocouple (e.g. Testo 905-T2) -50°C to +300°C Accurate measurement of the temperature of the pipelines, the oil separator tank, and the housing.
Manometer (for air/oil) Accuracy class 1.0 or higher, diameter 63 mm 0-16 bar (air); 0-25 bar (lubricant) Pressure measurement in the lubrication system, filters, cooler and injection line.
Tachometer (non-contact) With laser pointer (e.g. PCE-DT 62) 10-99999 rpm Checking the rotation speed of the cooling fan.
Air flow meter/anemometer Impeller or thermal (e.g. Testo 417) 0.3-20 m/s Evaluation of air flow through the cooler.

4. Initial Evaluation List

Before starting a detailed diagnosis, perform the following check:

Check/Action Expected Result/Value Record
Visual inspection of the compressor Absence of visible oil leaks, damage to pipelines, contamination on radiators. Mark all deviations.
Checking the oil level (pointer/dipstick) The oil level is between the minimum and maximum marks during operation and after stopping. Record the current level.
Record indicators from the control panel Injection pressure (bar), injection temperature (°C), working hours (motor hours), motor current (A). Record all current values.
Checking the history of accidents/alarms To determine the frequency and nature of previous overheating trips. Write down the error codes and the time of their occurrence.
Environmental conditions Air temperature in the room (°C), availability of sufficient ventilation, absence of heat sources nearby. Record the ambient temperature, assess ventilation.
Date of last service When was the last time you changed the oil, oil filter, air filter, or cleaned the cooler. Write down dates and work done.

5. Systematic Diagnostic Algorithm

This algorithm will help to consistently determine the root cause of high injection temperature:

  1. SYMPTOM: High Discharge Temperature (> 95°C / Trip Trip)
    1. Oil Level Check:
      1. ACTION: Visually check the oil level on the dipstick or dipstick on the oil separator tank while the compressor is running and 2 minutes after stopping (while the system is not depressurized).
      2. IF THE RESULT: The oil level is below the minimum mark.
        • PROBABLE CAUSE: Low oil level.
        • GO TO: Section 7.1 Root Cause Analysis: Low Lubricant.
      3. IF THE RESULT: The oil level is normal.
        • GO TO: Item 1.2 "Oil and Air Cooler Inspection".
    2. Oil and Air Cooler Inspection:
      1. ACTION: Visually inspect the outer surfaces of the oil and air cooler for contamination (dust, dirt, lint). Use a thermal imager to measure the temperature difference at the inlet and outlet of the cooler.
      2. IF THE RESULT: The external surfaces are dirty, the thermal imager shows an uneven temperature distribution or insufficient temperature drop at the cooler outlet (for example, < 10°C). An increased pressure drop across the cooler (over 0.5 bar) is also possible.
        • PROBABLE CAUSE: Cooler contamination (external or internal).
        • GO TO: Section 7.2 Root Cause Analysis: Oil Cooler/Air Contamination.
      3. IF THE RESULT: The cooler is clean, the temperature difference is normal.
        • GO TO: Section 1.3 "Checking the Cooling Fan".
    3. Checking the Cooling Fan:
      1. ACTION: Visually check the operation of the cooling fan (rotation, absence of extraneous noises). Using a tachometer, measure the speed of rotation of the blades. Check the electrical connections and supply voltage to the fan motor with a multimeter.
      2. IF THE RESULT: The fan does not spin, spins slowly, has damaged blades, or no voltage to the motor.
        • PROBABLE CAUSE: Failure of the cooling fan or its drive.
        • GO TO: Section 7.5 Root Cause Analysis: Defective Cooling Fan.
      3. IF THE RESULT: The fan is working correctly, the rotation speed and voltage are normal.
        • GO TO: Item 1.4 "Checking the Oil Thermostat".
    4. Checking the Oil Thermostat:
      1. ACTION: Measure the oil temperature before and after the thermostat with a contact thermometer. (CAUTION: Hot grease!). Some thermostats can be tested for resistance (with the power off) or disassembled for testing in hot water (according to the manufacturer's instructions).
      2. IF THE RESULT: The temperature of the oil after the thermostat is significantly higher or almost the same as the temperature before it, indicating that the thermostat is not opening fully or is stuck in the closed position. Or if it is stuck in the open position, the compressor takes a long time to reach operating temperature.
        • PROBABLE CAUSE: Malfunction of the oil thermostat.
        • GO TO: Section 7.3 Root Cause Analysis: Oil Thermostat Malfunction.
      3. IF THE RESULT: The thermostat is working correctly, directing the oil to cool down when the operating temperature is reached.
        • GO TO: Clause 1.5 "Analysis of Environmental Conditions".
    5. Analysis of Ambient Conditions:
      1. ACTION: Measure the temperature of the ambient air at the point of intake by the compressor. Check the presence of obstacles to air circulation around the compressor, the efficiency of the exhaust ventilation of the room.
      2. IF RESULT: Ambient air temperature exceeds permissible values ​​(usually > 35°C) or room ventilation is insufficient for heat removal.
        • PROBABLE CAUSE: High ambient temperature or poor ventilation.
        • GO TO: Section 7.4 Root Cause Analysis: High Environmental Conditions.
      3. IF RESULT: Environmental conditions are normal.
        • GO TO: Item 1.6 "Oil Filter and Lubricant Type Check".
    6. Checking the Oil Filter and Lubricant Type:
      1. ACTION: Check the pressure drop reading on the oil filter (if equipped). Estimate the date of the last oil and filter change. Be sure to use the correct type and viscosity of oil recommended by the compressor manufacturer. Take a sample of the lubricant for analysis if degradation is suspected.
      2. IF THE RESULT: Increased pressure drop on the oil filter (> 0.5 bar), the lubricant has not been changed for a long time, or the wrong type of lubricant is used.
        • PROBABLE CAUSE: Clogged oil filter or degraded/incorrect oil.
        • GO TO: Section 7.6 Root Cause Analysis: Clogged Oil Filter / Degraded Oil.
      3. IF RESULT: All previous checks found no cause.
        • PROBABLE CAUSE: Other, less common causes are possible (non-return valve failure, wear of screw unit elements, compressor overload). In-depth diagnosis or referral to specialists is required.

6. Matrix "Symptom - Probable Causes"

Symptom Probable Causes (ranked by likelihood) Diagnostic Test Expected Result if Cause Confirmed
High injection temperature
  1. Low oil level (Very high)
  2. Contamination of the external surfaces of the cooler (High)
  3. Cooling Fan Malfunction (High)
  4. High ambient temperature / Poor ventilation (Medium)
  5. Malfunction of the oil thermostat (Medium)
  6. Clogged oil filter (Medium)
  7. Internal contamination of the cooler (Low)
  8. Degraded / Improper Lubrication (Low)
  9. Temperature sensor failure (Low)
  • Visual inspection, checking the oil level.
  • Visual inspection, thermal imager, pressure drop.
  • Visual inspection, tachometer, multimeter.
  • Thermometer, ventilation check.
  • Measurement of temperatures before/after, resistance.
  • Pressure drop across the filter.
  • Thermal imager, chemical washing.
  • Lubricant analysis, documentation.
  • Comparison of indicators with the control sensor, resistance check.
  • The oil level is below normal.
  • Pollution is visible, Toutput < Tin, low drop ΔT.
  • Fan not working/slow, damaged blades.
  • Texternal > 35°C, no air flow.
  • The thermostat does not open/close, the temperature before/after is uneven.
  • ΔP on the filter > 0.5 bar.
  • Low efficiency of heat transfer with a clean outer side.
  • Change in the color/odor of the lubricant, non-compliance with the characteristics.
  • Inconsistency of sensor readings with real temperature.

7. Root Cause Analysis for Each Malfunction

7.1. Low Lubricant Level

Explanation: The oil in a screw compressor performs several critical functions: lubrication, cooling, and sealing. An insufficient level of lubricant leads to a decrease in its circulation and cooling efficiency. This causes an increase in temperature in the screw unit due to increased friction and insufficient heat dissipation.

How to confirm: Visual inspection of the oil level indicator (as a rule, during the operation of the compressor, the level should be within the MIN and MAX marks, or by the dipstick after the pressure has stabilized). If the level is below the minimum mark by 1/3 of the volume or more, this is a confirmation.

Consequences, if not eliminated: Insufficient lubrication will lead to accelerated wear of bearings, seals and rotor profiles, increased clearances, increased energy consumption, and ultimately - to jamming of the screw block, which is the most expensive element of the compressor.

7.2. Oil Cooler and Air Contamination

Explanation: Cooler radiators (oil and air) are designed to remove heat from the oil and compressed air to the environment. External contamination (dust, dirt, lint) reduces the heat exchange surface area, preventing effective heat transfer. Internal contamination (oil sludge, scale from hard water in water-cooled compressors) creates an insulating layer that also reduces cooling efficiency.

How to confirm:

  • External: A layer of dust or dirt is visually visible on the radiator fins. The thermal imager will show areas with a higher temperature on the surface of the cooler, where contamination is the densest, as well as insufficient temperature difference of the air passing through the radiator.
  • Internal: After external cleaning, the problem persists. Measuring the oil pressure drop across the cooler (ΔP > 0.5 bar) may indicate internal deposits.

Consequences, if not eliminated: Constant overheating of the lubricant leads to its rapid degradation (oxidation, loss of viscosity), the formation of varnishes and deposits that can clog the oil lines and the oil separator. The overall efficiency of the compressor decreases, electricity consumption increases, and the service life of components shortens.

7.3. Malfunction of the Oil Thermostat

Explanation: The oil thermostat regulates the flow of oil through the cooler, maintaining its temperature in the optimal range. If the thermostat is stuck in the closed position, the oil does not enter the cooler or does not get into it in sufficient quantity, which leads to its overheating. If the thermostat is stuck in the fully open position, the compressor may be operating at too low a temperature, which is a less common cause of overheating, but can cause excessive water condensation in the oil and oil degradation.

How to confirm:

  • Stick in closed position: Measure the oil temperature before and after the thermostat with a contact thermometer. If the temperature to the cooler is much higher than the operating temperature, and the temperature after the thermostat does not decrease properly, this indicates its malfunction.
  • Checking resistance: If it's an electric thermostat, check its resistance with a multimeter.

Consequences, if not eliminated: Permanent overheating of the lubricant and its rapid degradation, as described above. Increase in the amount of condensate in the system when jammed in the open position.

7.4. High Environmental Conditions / Poor Ventilation

Explanation: Screw compressors are designed to operate in a certain range of ambient temperatures (usually from +5°C to +40°C according to ISO 1217). If the temperature of the air entering the compressor for cooling is too high (eg > 35°C), or if the room is not well ventilated, the efficiency of the chillers is significantly reduced. This leads to an increase in the operating temperature of the lubricant and injection air.

How to confirm: Measure the temperature of the air at the intake point of the compressor and around it with a thermometer. Check the operation of the supply and exhaust ventilation, the absence of obstacles to the air flow, the correct placement of the compressor and other heat sources. Estimate the volume of air passing through the vents using an anemometer.

Consequences, if not eliminated: Overheating of the compressor, frequent shutdowns due to temperature, shortening the service life of components, increased energy consumption due to inefficient operation.

7.5. Defective cooling fan

Explanation: The cooling fan is responsible for ensuring the necessary air flow through the radiators of the coolers. Failure of the fan motor, broken drive belt, damage to the blades or clogging of the protective grid lead to a decrease in the volume of air passing through the coolers and, as a result, to an increase in temperature.

How to confirm:

  • Visual inspection: Check the integrity of the fan blades, the presence of a belt (for belt drives), the absence of foreign objects that prevent rotation.
  • Rotation: Make sure the fan rotates when the compressor is running.
  • Rotation speed: Measure the rotation speed with a tachometer. Compare with passport data.
  • Electrical check: Using a multimeter, check the supply voltage and current consumption of the fan motor. A faulty motor may have abnormal current or no voltage.

Consequences if not eliminated: Insufficient cooling, similar to the contamination of coolers, with all the resulting consequences for the lubrication and life of the compressor.

7.6. Clogged Oil Filter / Degraded Oil

Explanation: The oil filter removes contaminants from the oil, protecting the compressor components. A clogged filter creates excessive resistance to oil flow, reducing oil circulation and cooling efficiency. This can cause the filter bypass valve to open, allowing raw oil to circulate through the system. Degraded lubricant (oxidized, with lost viscosity) loses its lubrication and heat dissipation properties, contributing to overheating.

How to confirm:

  • Pressure drop: High pressure drop across the oil filter (> 0.5 bar) if the compressor is equipped with appropriate sensors.
  • Filter/lubricant age: Check date of last change.
  • Visual assessment of lubricant: Color change (darker than normal), appearance of sediment, specific smell.
  • Lubricant analysis: Laboratory analysis will confirm degradation, the presence of water, metal particles and changes in viscosity.

Consequences if not eliminated: Accelerated wear of moving parts, damage to bearings and screw unit due to dirty or ineffective lubrication. Reducing the service life of the oil separator.

8. Step-by-step Removal Procedures

Make sure all precautions in Section 2 are followed before performing any procedures.

8.1. Eliminating Low Lubricant Level

  1. SAFETY CHECK: Apply LOTO, depressurize the system.

  2. Determine the cause of the low oil level (leakage, entrainment). Repair the leak, if any.
  3. Fill in new compressor oil recommended by the manufacturer (UNITEC offers a wide range of compatible oils). The volume of lubricant should be up to the MAX mark on the level indicator.
  4. Start the compressor, let it work for 5-10 minutes, then stop.
  5. Check the oil level 2 minutes after stopping (before depressurization). If necessary, top up to the optimal level.
  6. VERIFICATION: Monitoring of discharge temperature after start-up. It should stabilize in the standard range (70-95°C).

8.2. Oil and Air Cooler Cleaning/Flushing

  1. SAFETY CHECK: Apply LOTO, depressurize system, allow cooler to cool.

  2. External cleaning: Using compressed air (maximum 2-3 bar), blow the cooler fins from the opposite direction of the air flow. Use a soft brush or vacuum cleaner to remove stubborn dirt.

    CAUTION: Do not damage the lamellae!

  3. Internal cleaning (for heavily contaminated): If external cleaning did not help, the cooler must be dismantled. Flush it with a special oil system cleaning solution or chemical descaling agent (for water coolers) according to the manufacturer's instructions. Rinse with clean water, then blow dry.
  4. Install the cooler in place, check all connections for tightness.
  5. VERIFICATION: After starting the compressor, monitor the discharge temperature and the temperature drop on the cooler using a thermal imager or a contact thermometer. The air temperature drop through the radiator should be at least 10°C, and the discharge temperature should be within the normal range.

8.3. Replacing a Defective Oil Thermostat

  1. SAFETY CHECK: Apply LOTO, depressurize system, allow to cool.

  2. Drain the oil from the compressor (or part if the thermostat is high).
  3. Disconnect the pipes from the thermostat.
  4. Disassemble the faulty thermostat.
  5. Install a new thermostat (UNITEC offers original spare parts and quality analogues), make sure the orientation is correct (flow arrows). Tighten the connection according to the tightening torque specified in the manual (usually 30-50 Nm).
  6. Fill in the oil, check the level.
  7. VERIFICATION: Start the compressor. Monitor the oil temperature. It should gradually increase to the working range, and then stabilize, which indicates the correct operation of the thermostat.

8.4. Optimization of Environmental Conditions

  1. SAFETY CHECK: General safety rules must be observed when working near the compressor.

  2. Measure the temperature of the surrounding air. If it exceeds 35°C, measures must be taken to reduce the temperature in the room or to ensure that cold air is supplied to the compressor.
  3. Check the efficiency of supply and exhaust ventilation. Make sure the exhaust fans are running and the intake vents are not blocked. The volume of exhaust air should be 10-15% greater than the volume of supply air to create a small rarefaction.
  4. Remove any obstructions to the free flow of air around the compressor (minimum recommended clearance from walls: 1 meter).
  5. Make sure that the hot air drawn from the compressor does not recirculate back to the compressor intake.
  6. VERIFICATION: Repeated measurement of the ambient air temperature, which should be within the permissible values. Compressor discharge temperature monitoring.

8.5. Repair/Replacement of Cooling Fan

  1. SECURITY CHECK: Apply LOTO, let cool. Beware of moving parts.

  2. Visual inspection: Check the fan blades for damage.
  3. Checking the drive: For belt drives - check the tension and condition of the belt. For direct drives — check for foreign objects.
  4. Electrical check: Using a multimeter, check the supply voltage to the fan motor. If there is voltage and the motor does not turn, check the windings for an open or short circuit.
  5. Replace the faulty motor, belt, or fan blades. Install components that meet OEM specifications.
  6. VERIFICATION: Start the compressor, make sure the fan is rotating at the correct speed (use a tachometer). Check the discharge temperature.

8.6. Replacing the Oil Filter and/or Lubricant

  1. SAFETY CHECK: Apply LOTO, depressurize system, allow to cool.

  2. Drain all old oil from the compressor.
  3. Remove the old oil filter.
  4. Install a new original or certified analogue UNITEC oil filter. Apply a thin layer of new oil to the filter O-ring, hand tighten, then tighten 3/4 or 1 turn according to the manufacturer's instructions.
  5. Fill in new compressor oil recommended by the manufacturer of the appropriate viscosity grade (eg ISO VG 46 or VG 68). The volume of lubricant should be up to the MAX mark.
  6. Run the compressor for a short time (1-2 minutes), then stop.
  7. Check the oil level and the absence of leaks. Top up if necessary.
  8. VERIFICATION: Injection temperature monitoring. It should return to normal.

9. Preventive Measures

Regular maintenance is key to preventing high discharge temperatures and ensuring compressor longevity.

The root cause Prevention Strategy Monitoring method Recommended Interval
Low oil level Regular checking of the oil level and topping up. Elimination of leaks. Visual inspection of the level indicator/dipstick. Daily/Weekly.
Contamination of the cooler Scheduled external cleaning of coolers. Visual inspection, thermal imaging control, pressure drop control. Monthly/Quarterly (depends on conditions).
Thermostat malfunction Scheduled check of the functionality of the thermostat (if provided by the manufacturer). Temperature measurement before/after the thermostat. Once a year or at each scheduled maintenance.
High environmental conditions / Poor ventilation Maintenance of proper ventilation of the room, temperature monitoring. Measurement of room temperature, inspection of ventilation systems. Daily/Monthly.
Defective cooling fan Regular inspection of the fan, checking of the electrical parameters of the motor. Visual inspection, rotation speed control, motor current measurement. Monthly/Quarterly.
Clogged oil filter / Degraded lubricant Timely replacement of the oil filter and lubricant in accordance with the manufacturer's regulations. Using the recommended lubricant. Control of run-in, visual evaluation of lubricant, control of pressure drop on the filter. According to the manufacturer's regulations (usually 2000-4000 hours or 1 year).

10. Spare Parts and Components

For quick and efficient troubleshooting, it is recommended to have the following spare parts in stock. UNITEC-D GmbH offers a wide range of high-quality components for screw compressors, certified by CE and UkrSEPRO.

Description of the Part Specification When to Replace Category UNITEC
Compressor oil As per OEM recommendations (ISO VG 46, VG 68, Synthetic/Mineral) According to the regulations (2000-8000 hours) or during degradation Lubricants and Technical Fluids
Oil filter Original OEM number or certified equivalent According to the regulations (2000-4000 hours) or with increased ΔP Filters
Air filter Original OEM number or certified equivalent According to the regulations (1000-4000 hours) or with increased ΔP Filters
Oil thermostat Opening temperature, original OEM number In the event of a malfunction (spell, incorrect operation) Valves and Control Elements
Cooling fan (motor/impeller) Power, voltage, size, original OEM number In the event of a malfunction or significant wear and tear Electric motors and drives
Drive belt (if equipped) Type, length, profile (eg SPB, A) With wear, cracks, stretching Belt Transmissions
Oil separator Original OEM number or certified equivalent According to the regulations (4000-8000 hours) Filters
Temperature sensor Type (PT100, NTC), range, original OEM number In case of incorrect readings Sensors and Automation

For ordering and detailed spare parts catalog visit: www.unitecd.com/e-catalog/

11. Links

  • DSTU ISO 1217:2018 (ISO 1217:2009, IDT) Volumetric compressors. Admission test.
  • DSTU EN 1037:2018 (EN 1037:1995, IDT) Machine safety. Prevention of unexpected start.
  • Compressor manufacturer (OEM) operation and maintenance manuals.
  • Relevant standards of the ISO 4406 series (purity of hydraulic fluids), ISO 2909 (determination of the viscosity index of lubricants).
  • UNITEC-D GmbH: Manuals for maintenance of pneumatic systems.

Related Articles

Maintenance Guides 2 min read

Screw Compressor Troubleshooting: High Discharge Temperature

Technical analysis: Troubleshooting screw compressor high discharge temperature: oil level, cooler fouling, thermostat f

1. Description of the Problem and Scope of Application

This manual is intended for systematic diagnosis and troubleshooting of high discharge temperatures in screw compressors. High discharge temperatures exceeding the manufacturer's limits (typically >100°C) can lead to accelerated component wear, lubricant degradation, increased energy consumption, and potential compressor shutdowns that critically affect production processes. This manual applies to all types of oil-filled screw compressors used in industrial production.

  • Affected equipment: Screw compressors of any capacity used for the production of compressed air or process gas.
  • Symptoms: Indication of high temperature on the compressor control panel, frequent shutdowns due to overheating, specific smell of overheated oil, change in color of oil, increased oil consumption.
  • Severity classification:
    • Critical: The discharge temperature exceeds the emergency threshold, the compressor shuts down. Immediate intervention.
    • Major: The discharge temperature is constantly higher than the operating norm, but lower than the emergency threshold. Requires scheduled diagnosis and elimination.
    • Minor: Episodic or slight increase in temperature. May indicate the initial stage of the malfunction.

2. Safety Precautions

CAUTION: Before performing any diagnostic or repair work on the compressor equipment, be sure to observe the following safety precautions. Failure to follow these instructions can result in serious injury or death.

  • LOCKOUT/TAGOUT (LOTO): Before opening any guards or working inside the compressor, turn off the power supply and lock it in the off position according to LOTO (DSTU EN 1037) procedures. Make sure the power source is turned off and cannot be turned on accidentally.
  • STORED ENERGY: Compressors contain a significant amount of stored energy in the form of compressed air and hot oil. Completely depressurize the system (air and oil) before starting work. Check the absence of pressure using pressure gauges.
  • HOT SURFACES AND LIQUIDS: Compressor components and lubricant can be very hot (>100°C). Allow equipment to cool before handling. Use appropriate personal protective equipment (PPE), including heat-resistant gloves, safety glasses, and overalls (DSTU EN ISO 11612).
  • MOVING PARTS: Always watch out for moving parts of the compressor. Never remove protective covers while the equipment is in operation.
  • CHEMICALS: Compressor oil and other fluids can be harmful if in contact with skin or inhaled. Use PPE (protective gloves, goggles) and follow the safety instructions in the material safety data sheet (MSDS/SDS).

3. Necessary Diagnostic Tools

Tool Specification / Model Range of Measurements Purpose
Pyrometer / Infrared thermometer Range -50°C to +500°C, accuracy ±1.5°C -50°C ... +500°C Surface temperature measurement (oil separator, radiator, pipelines)
Thermal imager Resolution 320x240, thermal sensitivity <0.05°C at 30°C -20°C ... +650°C Visualization of temperature distribution, detection of local overheating in coolers
Control manometer Accuracy class 0.6, range 0-16 bar (0-230 psi) 0 ... 16 bar Checking the injection pressure, the pressure in the lubrication system, the pressure drop on the cooler
Multimeter True RMS, AC/DC voltage, AC/DC current, resistance measurements Voltage up to 1000V, current up to 10A, resistance up to 40 MΩ Checking electrical components (thermistors, temperature sensors, control relays)
Lubricant analyzer (portable) Measurement of viscosity, water content, acidity According to the parameters of the lubricant Express analysis of the condition of the compressor oil on site
Air flow speed meter (anemometer) Range 0.3-30 m/s, accuracy ±3% 0.3 ... 30 m/s Checking the air flow through the cooler

4. Initial Assessment Checklist

Before starting a detailed diagnosis, conduct a visual inspection and collect primary data.

Parameter / Sign action Expected Result / Comment
Terms of Use Record the ambient temperature, humidity. Temperature <40°C, relative humidity <80%.
Current Readings Record the readings of manometers and thermometers on the control panel: injection temperature, oil temperature, injection pressure, pressure in the oil system. Injection temperature >100°C; injection pressure is within normal limits.
Lub Level Visually check the oil level through the inspection window (with the compressor stopped and the pressure released). The level should be between the MIN and MAX marks. If below MIN - possible cause.
Coolers Visually inspect the outer surface of the air and/or oil cooler for contamination (dust, lint). Clean, without visible contamination.
Fans Cooling Check the operation of the cooling fans (rotation, noise level). They work smoothly, provide sufficient air flow.
Temperature Sensors Check the connection of the wires to the temperature sensors. Reliable connection, no corrosion.
Alarm / Accident History View the alarm log on the compressor control panel. Record the code and time of the "High discharge temperature" alarm.
Recent Changes / Maintenance Find out if the compressor system has been serviced, repaired or changed recently. For example, replacement of lubricant, filters, repair of the cooler.

5. Scheme of Systematic Diagnostics

Use this diagram to consistently locate the root cause of a high injection temperature.

  1. Lubricant Level Check

    1. IF the lubricant level is below the minimum mark:

      • PROBABLE CAUSE: Insufficient lubrication level.
      • GO TO: Section 7.1.

    2. ELSE IF the oil level is normal:

      • CONTINUE TO: Item 2.
  2. Checking the Cooling System

    1. Visual inspection of the cooler (oil and/or air).

      1. IF the outer surface of the cooler is dirty (dust, lint, debris):

        • PROBABLE CAUSE: Contamination of the outer surface of the cooler.
        • GO TO: Section 7.2.1.

      2. ELSE IF the outer surface is clean:

        • CONTINUE TO: Clause 2.2.

    2. Measurement of the inlet and outlet temperature of the cooler (using a pyrometer or thermal imager).

      1. IF the temperature drop across the oil cooler is too small (<10°C) AND/OR the pressure drop across it is abnormally high (>0.5 bar):

        • PROBABLE CAUSE: Internal contamination (clogging) of the cooler.
        • GO TO: Section 7.2.2.

      2. ELSE IF temperature and pressure drop is normal:

        • CONTINUE TO: Item 3.
  3. Thermostat/Thermostatic Valve Check

    1. Measure the oil temperature at the inlet and outlet of the thermostatic valve/thermostat and the oil temperature returning to the compressor unit.

      1. IF the oil leaving the thermostat/thermostat valve and entering the compressor unit has a temperature close to the discharge temperature AND/OR the temperature of the oil after the valve does not decrease when the discharge temperature increases:

        • PROBABLE CAUSE: Thermostatic valve sticking in the closed position or its failure.
        • GO TO: Section 7.3.

      2. ELSE IF the thermostat works correctly, sending oil to the cooler when the threshold temperature is reached:

        • CONTINUE TO: Clause 4.
  4. Analysis of Ambient Conditions

    1. Measure the temperature of the ambient air at the place where the compressor is installed.

      1. IF the ambient air temperature constantly exceeds the maximum allowable value for the compressor (>40°C, or according to the manufacturer's instructions) AND/OR there is no proper ventilation of the compressor room:

        • PROBABLE CAUSE: Insufficient ventilation or excessive ambient temperature.
        • GO TO: Section 7.4.

      2. ELSE IF environmental conditions are normal:

        • PROBABLE CAUSE: Other, less common causes (see Section 6 - Fault Matrix, "Other Causes").
        • GO TO: Chapter 6.

6. Matrix of Malfunctions and Causes

This table summarizes the main symptoms, possible causes, diagnostic methods, and expected results.

Symptom Probable Causes (ranked by probability) Diagnostic Test Expected Result if Cause Confirmed
High injection temperature (on the board >100°C) 1. Low level of lubrication
2. Contamination of the external cooler
3. Internal contamination of the cooler
4. Thermostatic valve/thermostat failure
5. High ambient temperature / Insufficient ventilation
6. Malfunction of the temperature sensor
7. Cooling fan failure		 1. Visual inspection of the lubrication inspection window
2. Visual inspection of the cooler
3. Measurement of temperature/pressure drop across the cooler
4. Measuring the oil temperature before/after the thermostat
5. Measurement of intake air temperature and ventilation efficiency
6. Checking the sensor with a multimeter, comparing it with the reference
7. Visual inspection, fan motor current measurement		 1. Lubrication level < MIN
2. Clogging of the edges of the cooler with dust
3. Temperature drop <10°C, pressure drop >0.5 bar
4. The temperature of the oil to the compressor block is almost equal to the discharge temperature
5. Room temperature >40°C, no air flow
6. The readings of the sensor do not correspond to the actual temperature
7. The fan does not rotate or rotates slowly

7. Root cause analysis

7.1. Low Lubricant Level

Explanation: The oil in a screw compressor performs several critical functions: lubricating the moving parts, sealing the gaps between the rotors, and most importantly, removing the heat generated when the air is compressed. An insufficient level of lubrication leads to the fact that a smaller volume of lubrication circulates, accordingly, its heat capacity decreases, and it cannot effectively remove heat from the compressor element. This causes a rapid rise in temperature.

How to confirm: Visual inspection of the inspection window of the oil level with the compressor stopped and de-energized (after the pressure has been released). The oil level should be between the MIN and MAX marks.

Potential damage: Long-term operation with a low level of lubricant may cause overheating of bearings, wear of rotors, damage to the compressor unit, formation of soot and varnish deposits in the system, which leads to expensive major repairs or replacements.

7.2. Contamination of the Cooler

7.2.1. External Contamination of Cooler (Air/Oil)

Explanation: The outer fins of air coolers or the outer surface of the water-to-oil heat exchanger can become contaminated with dust, lint, oil vapors and other particles from the environment over time. This layer of contamination acts as a thermal insulator, preventing the efficient transfer of heat from the hot oil (or compressed air) to the cooling medium (air or water).

How to confirm: Visual inspection of the outer surface of the cooler. Using a thermal imager will reveal areas with insufficient heat exchange (hotter areas on the radiator fins). With the help of an anemometer, you can measure the decrease in air flow through the cooler.

Potential damage: Decreased cooling efficiency, constant operation of the compressor at elevated temperatures, increased energy consumption, premature aging of lubricants and components.

7.2.2. Internal Contamination of the Cooler

Explanation: The internal channels of an oil or air cooler can become clogged with oil oxidation products (soot, sludge), small metal particles, or in the case of a water cooler, mineral deposits from water (scale). This reduces the cross-section of the channels, reducing the flow rate and heat exchange efficiency.

How to confirm: Measurement of oil temperature difference at the inlet and outlet of the cooler and pressure drop. The normal temperature difference of the lubricant is 15-20°C. The pressure drop across the contaminated cooler may exceed 0.5 bar. A thermal imager may show cold areas at the chiller outlet, indicating a lack of flow.

Potential Damage: Similar to external contamination, but with the added risk of blocked lines and much more difficult cleaning or the need to replace the cooler.

7.3. Failure of the Thermostatic Valve/Thermostat

Explanation: A thermostatic valve (or thermostat) controls the flow of oil through the cooler. At low temperatures, it directs the oil to bypass the cooler, allowing the compressor to reach optimum operating temperature more quickly. At high temperatures, it opens, directing all the oil through the cooler for maximum cooling. If the valve is stuck in the closed or half-closed position, the lubricant does not get to the cooler or passes through it in insufficient quantities, which leads to overheating.

How to confirm: Measure the temperature of the oil before the thermostatic valve, after the valve (into the cooler) and the oil returning to the compressor unit. If the temperature at the inlet to the compressor unit does not decrease when the discharge temperature increases, this indicates a valve malfunction. It is also possible to visually inspect the valve mechanism after its disassembly to check for carbon deposits or mechanical damage.

Potential damage: Constant overheating of the system, degradation of lubricant, formation of soot, damage to the compressor element.

7.4. Analysis of Environmental Conditions

Explanation: Screw compressors are designed to work in certain ranges of ambient temperatures (usually from +5°C to +40°C). If the compressor is operating in a poorly ventilated room or at air temperatures that exceed these limits, the compressor's cooling system will not be able to dissipate heat effectively. This leads to an increase in the temperature of the entire compressor unit and, as a result, the discharge temperature.

How to confirm: Measurement of air temperature at the entrance to the compressor and in the compressor room. Checking the supply and exhaust ventilation. Checking the presence of obstacles to the outflow of hot air.

Potential damage: Increased heat load on all compressor components, reduced service life, increased energy costs due to more frequent cooling cycles.

8. Step-by-Step Troubleshooting Procedures

8.1. Eliminating Low Lubricant Level

  1. CAUTION: Lockout/Tag (LOTO). Shut down the compressor, disconnect power, and apply LOTO procedures.

  2. CAUTION: Pressure relief. Wait for the system to completely release pressure. Check the pressure gauge readings (0 bar).

  3. Unscrew the filler neck of the oil tank.

  4. Add lubricant of the same type and viscosity grade recommended by the manufacturer (eg ISO VG 46 or ISO VG 68) up to the MAX mark on the sight glass. Use only original compressor oil recommended by UNITEC-D or the compressor manufacturer.

  5. Screw the filler neck.

  6. Verification: Remove LOTO, turn on the compressor. Control the injection temperature for 15-30 minutes. It should return to normal operating values ​​(eg 85-95°C). Check the oil level after reaching operating temperature - it may drop slightly.

8.2. Elimination of Contamination of the Cooler

8.2.1. Cleaning the Outer Surface of the Cooler

  1. CAUTION: Lockout/Tag (LOTO). Shut down the compressor, disconnect power, and apply LOTO procedures.

  2. CAUTION: Pressure relief. Wait for the system to completely release pressure.

  3. Remove the protective grilles or casings covering the cooler.

  4. Use compressed air (with a pressure of no more than 3 bar, from a distance of 15-20 cm) or a soft brush to remove dust and lint from the cooler fins. Blow the cooler in a direction opposite to normal air flow. Do not damage the thin fins of the cooler.

  5. To remove oil contamination, use special detergents for radiators recommended by the manufacturer. Follow the instructions for using the detergent.

  6. Replace the protective grilles/covers.

  7. Verification: Remove LOTO, turn on the compressor. Control the injection temperature. The air temperature difference before/after the cooler should increase, and the discharge temperature should decrease to normal.

8.2.2. Cleaning the Inner Surface of the Cooler

  1. CAUTION: Lockout/Tag (LOTO). Follow LOTO procedures.

  2. CAUTION: Pressure relief and grease drain. Completely relieve pressure and drain the grease from the system.

  3. Dismantle the oil cooler from the compressor.

  4. Wash the internal passages of the cooler with a special solution for cleaning oil systems or hot water with an alkaline detergent (for water coolers). Follow the manufacturer's recommendations for detergents and washing procedures.

  5. Wash thoroughly with clean water (for water coolers) or degreaser (for oil coolers) and blow with compressed air until completely dry.

  6. Replace the cooler, replace all seals. Tighten the fastening bolts according to the manufacturer's instructions (for example, 25 Nm for M8 bolts).

  7. Fill in fresh compressor oil.

  8. Verification: Remove LOTO. Start the compressor, check for leaks. Monitor the discharge temperature and pressure drop across the cooler. The pressure should be within the normal range (0.2-0.4 bar).

8.3. Replacing the Thermostatic Valve/Thermostat

  1. CAUTION: Lockout/Tag (LOTO). Follow LOTO procedures.

  2. CAUTION: Pressure release and oil drain. Completely release the pressure and drain the oil (to a level below the thermostat).

  3. Dismantle the faulty thermostatic valve.

  4. Install a new thermostatic valve/thermostat (UNITEC Part No. XXXXXX, with an opening temperature of 71°C or as per compressor specification) using new O-rings.

  5. Tighten the fasteners according to the manufacturer's recommendations.

  6. Top up the oil to normal.

  7. Verification: Remove LOTO. Start the compressor. Control the injection temperature. It should stabilize in the working range. Check the temperature of the lubricant at the inlet to the compressor unit - it should correspond to the norm (for example, 80-90°C).

8.4. Optimization of Environmental Conditions

  1. CAUTION: Lockout/Tag (LOTO). When working with ventilation systems or moving the compressor.

  2. Ensure proper ventilation of the compressor room. Install additional supply and exhaust fans if necessary. The calculated ventilation performance should provide 3-5 times air exchange per hour. The air temperature at the inlet to the compressor should not exceed 40°C.

  3. Put the hot air from the compressor outside the room using air ducts if the compressor is placed in an enclosed space.

  4. Make sure that there are no obstacles to air circulation around the compressor (minimum distance to walls of 0.5 meters).

  5. Verification: Remove LOTO (if applicable). Start the compressor. Measure the temperature of the ambient air in the suction area of ​​the compressor and in the outlet air streams. The discharge temperature should decrease and stabilize.

9. Preventive Measures

The root cause Prevention strategy Monitoring method Recommended Interval
Low oil level Regular checking of the oil level and timely refueling. Elimination of leaks. Daily visual inspection of the viewing window. Daily / At every start
Contamination of the external cooler Regular cleaning of the outer surface of the cooler. Visual inspection, temperature drop measurement. Weekly / Monthly (depending on operating conditions)
Contamination of the internal cooler Regular replacement of lubricant and oil filter according to the manufacturer's recommendations. Use of high-quality lubricant. Flushing the system when changing the lubricant. Lubricant analysis (ISO 4406), pressure drop measurement on the cooler. Quarterly / Annually (lubricant analysis), Every 2000-4000 hours (lubricant change)
Failure of the thermostatic valve/thermostat Scheduled check of the functioning of the thermostatic valve. Replacement after a certain period of operation. Measurement of oil temperature before and after the valve. Annually / Every 8000 hours (scheduled replacement)
High ambient temperature / Insufficient ventilation Ensuring proper ventilation of the compressor room. Indoor temperature control. Monitoring of the air temperature in the room and near the suction of the compressor. Daily / Continuous automatic monitoring

10. Spare Parts and Components

To ensure trouble-free operation and prompt troubleshooting, it is recommended to have the following spare parts available. All UNITEC-D components comply with EN and ISO standards.

Description Details Specification When to Replace Category UNITEC
Compressor oil UNITEC SYNTHETIC ISO VG 46/68, 20 l canister, UkrSEPRO certificate According to the maintenance schedule (2000-4000 hours) or when the level drops. Lubricating materials
Oil filter UNITEC OEM Equivalent, 10 micron degree of filtration According to the maintenance schedule (1000-2000 hours) or when the contamination indicator is activated. Filter elements
Air filter UNITEC OEM Equivalent, Filtration Efficiency 99.9% According to the maintenance schedule (1000-2000 hours) or when the contamination indicator is activated. Filter elements
Grease separator UNITEC OEM Equivalent, Residual lubricant concentration <3 ppm According to the maintenance schedule (4000-8000 hours). Separators
Thermostatic valve (complete) UNITEC Thermostat Valve, Opening temperature 71°C or according to compressor specification When a malfunction is detected or scheduled (8000 hours). Valves and regulators
Thermostatic valve repair kit (thermoelement + seal) UNITEC Repair Kit, for valves DN20-DN50 In the event of a malfunction of the thermocouple or during scheduled maintenance. Repair kits
Temperature sensor UNITEC PT100/PT1000, range up to 150°C, accuracy class A In the event of a malfunction or suspicion of incorrect readings. Sensors

For ordering and detailed selection of spare parts, please visit our E-Catalog UNITEC-D.

11. Links

  • DSTU EN 1037: Machine safety. Prevention of unexpected start.
  • DSTU EN ISO 11612: Protective clothing. Clothing for protection against heat and flame.
  • ISO 4406: Hydraulic fluids. Coding of the level of pollution by solid particles.
  • Operation and maintenance manuals from compressor manufacturers (eg, Atlas Copco, Kaeser, Ingersoll Rand).
  • UNITEC-D internal standards for operation and maintenance of compressor equipment.

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