High Discharge Temperature on Screw Compressors: Causes and Fixes

The discharge temperature sensor sits near the airend outlet. On Atlas Copco GA machines the alarm is at 105°C, shutdown at 115°C. The reading is the temperature of the oil-air mixture leaving the compression chamber. The alarm is set to protect the oil, not the metal. Mineral oil past 110°C oxidizes fast enough that stated service life becomes irrelevant.

A competent technician can clear the thermostatic valve, the cooler, the fan, and the separator in about half an hour.

Thermostatic valve: infrared thermometer on the cooler inlet, cooler outlet, and bypass line. Fifteen degrees across the cooler, oil is flowing, valve is fine. Same temperature at inlet and outlet with a hot bypass, valve is stuck open. Wax-element types drift off their opening temperature. Some Ingersoll Rand models use a spring-diaphragm design that cracks instead of drifting. Either way, two-minute check, binary answer.

Oil cooler: blow out the fins, check the temperature drop across it, move on. Internal varnish on the tube walls is a thing and it does need a solvent flush when it gets bad enough, When it does degrade heat transfer the effect is maybe 5°C.

Fan: plastic film against the cooler intake face. Flat, good. Fluttering, weak. Lost phase on the motor is the usual cause, clamp meter confirms it.

Separator: the Elektronikon displays the differential. Past 1 bar, change the element. Done.

All of that gets covered, the work order gets closed, and for maybe a third of high discharge temperature complaints that is all it takes. The thermostatic valve was stuck, or the fins were caked with cotton fiber, or the separator had 1.3 bar of differential.

For the rest of them, the technician has now exhausted every item on the checklist and the temperature is still high. The machine gets restarted, runs at 99°C, the technician writes something about continued monitoring in the service report, and the case enters a loop. Next quarter, same visit, same checks, same result. Eventually the machine gets classified as a hot runner.

The answer in most cases is Vi mismatch, and in some cases it is oil thermal degradation. Neither one exists in any service procedure, on any control panel display, or in any technician training curriculum.

Vi is a property of the airend rotor set. It determines what discharge pressure the compressor is designed to produce. The pressure shown on the control panel can be set to whatever the operator wants, but the physical geometry of the rotors inside the airend was built for one specific pressure, and when the operating pressure does not match the built-in pressure, the difference becomes heat. Not a little bit of heat. On a bad mismatch, 15°C to 20°C of additional discharge temperature, permanently, on every cycle. No maintenance action affects it. No part replacement affects it. The heat is generated inside the compression chamber by the interaction between rotor geometry and a backpressure the rotors were not designed to see.

Atlas Copco builds the GA range in 7.5, 8.5, 10, and 13 bar pressure classes. The rotor profiles are different castings between classes. The 7.5 bar version and the 13 bar version of the same frame size share a motor and a control panel. This is not a software difference or a configuration setting. The metal is different.

A textile plant in Jiangsu, GA75, 7.5 bar variant, installed 2021. At commissioning the machine ran at 82°C discharge with about 33°C ambient, which is fine. Original air distribution was maybe 120 meters of 80mm pipe feeding a weaving hall with 48 air-jet looms.

In late 2022, second weaving hall, extension on the east side. The piping contractor ran 65mm pipe because 80mm was not on site and it was a Saturday. About 85 additional meters, nine elbows, and a ball valve someone put in as an isolation point. With both halls running at capacity, the looms at the far end of the new extension were getting 5.8 bar at the station regulators. Not enough. The air-jet insertion was intermittent, fabric faults on the loom controllers.

Maintenance changed the compressor unload setpoint from 7.5 to 9.5 bar. Far-end pressure went up to about 7.2 bar. Looms ran clean.

The discharge temperature went to 96°C, approximately. The Elektronikon logged it. The alarm is 105°C. There was no reason for the maintenance supervisor to be monitoring the Elektronikon discharge temperature log while he had 80 looms, a dye house, a boiler, and a wastewater permit renewal to deal with.

Spring 2023, new process required compressed air drying above 10 bar. Setpoint went to 10.5 bar. Discharge temperature 101°C or 102°C. Still no alarm.

August 2023, 41°C outside, compressor room on the south side under a corrugated metal roof, one louvered opening on the east wall half-blocked by a roll of roofing insulation since May. Room temperature close to 50°C. Machine tripped at 107°C.

Dealer tech came out. Oil change, fin cleaning, new thermostatic valve from the van. Machine restarted, ran to about 2 PM, tripped again. Back the next day, infrared-checked the new valve, confirmed working, checked the fan, checked oil level. Told the plant to move the insulation roll. They did. Machine survived the rest of the summer. The following winter, ambient 10°C, the Elektronikon showed 98°C at full load.

Quarterly PM report: "discharge temperature slightly elevated, recommend continued monitoring."

None of the work the dealer tech did had any possibility of fixing this. Zero. The thermostatic valve was fine when it was replaced. The cooler was fine. The oil was fine, or at least it was fine as far as the mismatch goes, the oil had its own problems from running at 100°C for a year but even brand new oil would not have brought the temperature down because the heat source is geometric. It is inside the rotors. It happens on every compression cycle and it will keep happening until the setpoint comes down to match the airend or the airend gets replaced with one that matches the setpoint.

The plant cannot lower the setpoint. The looms in the new hall need the pressure. The maintenance supervisor has thought about this and the answer is the same every time: if the compressor goes back to 7.5 bar, the far end of the extension drops below 6 bar at peak demand and the fabric quality numbers go red. The plant manager does not want to see red numbers.

The piping is a mess and everyone knows it. The 65mm Saturday-pipe should be 100mm. The threaded fittings at the loom station drops leak. You can hear them from across the aisle on second shift when the hall is a bit quieter. There is probably 1.5 bar of recoverable pressure drop in the distribution network between the compressor and the far end, which would bring the needed setpoint well within the 7.5 bar airend's range.

The maintenance supervisor has proposed the pipe work twice. Costs around 130,000 RMB, which is a rough estimate that might be low. The plant manager declined both times. No production gaps in the calendar. The cost goes through the group finance office in Shanghai and it is not a priority up there.

An airend rebuild with a higher-Vi rotor set is about 170,000 RMB at OEM pricing for a GA75 in eastern China. Aftermarket rotors exist but the quality can be bad enough that the bearings fail within a year and then the whole cost gets paid twice. This is also a capital expenditure item, same approval chain, same finance office.

So the machine stays at 10.5 bar. Oil changes at 2500 hours instead of the OEM's 4000. Separator elements swapped more often because the high temperature accelerates varnish on the media. Thermostatic valves replaced periodically because the aftermarket quality the dealer stocks is inconsistent. Annual excess maintenance cost on this one machine is running 50% to 70% above what it would be at design pressure. Three years of that excess spend is in the ballpark of the pipe rework that keeps getting turned down. But the pipe rework is a lump sum that requires a signature from Shanghai, and the maintenance spend is a series of unremarkable monthly purchase orders for a few thousand RMB each that go through on autopilot.

Over-compression goes the other way. System pressure below Vi design point, gas squeezed past what is needed, excess becomes heat. Shows up when factories cut production and drop their setpoint.

Standard oil analysis is useless for catching the kind of degradation that raises discharge temperature. Viscosity, TAN, wear metals: these are lubricant metrics. The oil in a screw compressor is a heat transfer fluid. The specific heat capacity determines how many joules per kilogram per degree it can carry out of the compression chamber. As the oil oxidizes, specific heat drops while viscosity and TAN stay within spec. The lab report says serviceable. The technician reads it and crosses oil off the list. Discharge temperature stays 8°C or 10°C above where it should be.

Load-unload cycling on fixed-speed machines destroys oil thermal performance through microbubble oxidation at the surfaces of gas that foams out on every unload event. A machine cycling twenty times an hour is aging its oil at a rate that makes the OEM interval fiction. VFD machines do not have this problem. Humidity compounds it, and so does topping off with an incompatible base stock.

The diagnostic is crude and it works. Drain everything. Not a partial drain. Everything. Flush if possible. Refill with the correct fresh product. If discharge temperature drops on startup, the oil was the issue. Going forward, either add specific heat capacity to the analysis panel if the lab offers it, or cut the interval to 60% of OEM hours in tough environments.

Clearances open over years. Gas leaks back from the discharge side, gets recompressed. A few degrees per year. Output volume drops in parallel and the load factor increases. GA airends in clean environments have gone 50,000 hours, in quarries much less. Weekly logging of full-load discharge temperature and ambient on paper taped to the machine catches the trend. Clean oil and good intake filtration slow it down. Once the gaps are open, the fix is a rebuild.

Adiabatic compression amplifies intake temperature through the compression ratio. Twenty degrees of intake rise translates to about 35°C at the discharge. The compressor room is the usual culprit. A GA75 dumps tens of kilowatts continuously. Poor ventilation recirculates that heat back into the intake. Cold air in through a low opening, hot air out through a high opening on the opposite wall, or duct the cooler exhaust outside. Cheap, effective, gets deferred because it is a building modification and does not fit on a compressor service work order. Rooms that started fine get choked off as partitions go up and material gets stacked against vents, and the discharge temperature tracks the degradation month by month while the compressor gets serviced on schedule without anyone checking whether the louvers are still clear.

The dual temperature sensor on variable-speed cooling fans deserves a mention because it catches people off guard. Atlas Copco mid-range and larger machines have a sensor feeding the fan VFD that is separate from the Elektronikon discharge temperature display. If that sensor drifts low or has a wiring fault, the fan runs at minimum speed. The panel discharge temperature reads correctly because it uses a different sensor. The technician checks the panel reading against an infrared gun, gets a match, and concludes the sensors are fine. The fan sensor is wired into the VFD terminals and does not appear on any controller screen.