High Discharge Temperature on Screw Compressors: Causes and Fixes

The discharge temperature sensor on an oil-injected screw compressor sits near the airend outlet and measures the temperature of the oil-air mixture. Atlas Copco GA series alarms at 105°C, some Ingersoll Rand R series models alarm at 110°C, Sullair LS/TS series mostly fall between 105 and 110°C, with safety shutdown about ten degrees above that. Discharge temperature reflects the difference between the heat generated during compression and the heat removed by the cooling system.

The alarm threshold is set well below what the metal parts and seals can handle. That margin is there to protect the oil. Mineral oil above 110°C oxidizes at an accelerating rate. Synthetic oil tolerates more heat, but the same pattern holds. A machine running at 95 to 100°C or above for extended periods without ever triggering an alarm looks fine to the operator. The oil is already aging faster than it should be.

Oil-Injected Screw Compressor

Airend Discharge Zone

The most frequently misreplaced part on any screw compressor. Atlas Copco OEM valves cost a few hundred to maybe a thousand in local currency, aftermarket ones even less. Easy to pull out, easy to put back in. Sometimes the discharge temperature drops after replacing it, maybe because the machine cooled down during the shutdown, maybe because someone topped off the oil at the same time. Either way the valve gets blamed.

To check whether the valve is actually faulty: infrared thermometer on the oil cooler inlet, oil cooler outlet, and bypass line. If the temperature drop across the cooler is fifteen degrees or more, oil is flowing through the cooler and the valve is fine. If the inlet and outlet are nearly the same temperature and the bypass line is hot, the valve is stuck in bypass position. Atlas Copco and Sullair mostly use wax-element thermostatic valves. As the wax element ages, the opening temperature drifts, not necessarily stuck fully closed, sometimes just a few degrees off calibration. Ingersoll Rand uses spring-diaphragm type on some models, different failure mode.

Air-side fin cleaning is straightforward. Blow them out regularly, nothing more to say about it.

The oil-side internal wall is where the overlooked problem lives. Oil flowing over aluminum or copper tube walls at high temperature for thousands of hours deposits a thin, varnish-like carbon film. This film has very poor thermal conductivity. You cannot see it from outside. It needs to be removed with a circulating oil-system flush using a dedicated cleaning solvent. Changing to fresh oil does not remove carbon film already bonded to the tube walls. Water-cooled units with plate heat exchangers have the same oil-side issue, plus water-side scaling. In areas with hard cooling water, acid washing every six months is not excessive.

MaintenanceOil-Side Carbon Film

ComponentThermostatic Valve

Vi is a parameter that almost never comes up in maintenance-level discussions. Most field technicians have never heard of it. It does not appear on the control panel. It is not in any routine maintenance checklist. It only shows up in the OEM sizing manual and on the airend nameplate.

Vi is the volume ratio: the ratio of the rotor pocket volume at suction close to the rotor pocket volume at the discharge port opening. It is fixed the moment the rotors are machined. Not adjustable. Each Vi value corresponds to a theoretical optimum discharge pressure. When the gas inside the rotor chamber is compressed to exactly the system backpressure by the time the discharge port opens, the gas flows out smoothly and nearly all compression work converts to useful pressure energy.

When system pressure is lower than what the Vi calls for, the gas has been compressed beyond what the system needs. It expands after leaving the discharge port, and all the extra compression work turns into waste heat. This is over-compression.

Atlas Copco's GA30 through GA90 range ships with airend Vi values roughly between 2.8 and 3.5, depending on the speed and displacement combination. This Vi range is designed to match the 7.5 to 10 bar working pressure segment. The GA series offers models at 7.5 bar, 8.5 bar, 10 bar, and 13 bar, with different airend rotor sets for the low-pressure and high-pressure variants. Take a 7.5 bar version of a GA75 and run it at 13 bar, and under-compression becomes severe. Ingersoll Rand's R series and SSR series are similarly split by pressure class with different airends. Sullair's TS/LS series, same story.

This is especially prevalent in Chinese factories. Equipment managers tend to think of insufficient end-of-line pressure as a compressor setting problem rather than a piping problem. Calculating pipe pressure drop, finding leaks, upsizing bottleneck pipe sections, none of that is on the table. Adjusting the compressor pressure costs nothing. Fixing the piping costs money and downtime. So the compressor runs in under-compression indefinitely. When discharge temperature goes high, someone comes to work on the compressor. Nobody looks at the piping.

There is another way Vi mismatch gets introduced, at the sales stage. Dealers trying to win an order quote the low-pressure version of a machine because it is cheaper and the price looks better. The customer signs based on price. The machine goes in and runs at a pressure higher than its airend was designed for. Discharge temperature is uncomfortable from day one. The dealer's service team comes out a few times, cleans the cooler, changes the oil, cannot fix it. After a while the customer gets used to it and assumes this particular machine just runs a bit hot, an individual quirk.

Can you solve this by lowering system pressure? Of course. Drop the unload setpoint back to the airend's design range and discharge temperature comes right down. The tradeoff is that end-of-line equipment may be starved again. So the real solution is two steps: first, address the pressure drop in the piping network (fix leaks, upsize bottleneck sections, shorten runs, position receiver tanks properly), and once end-of-line pressure is adequate, lower the compressor setpoint back down. If the piping has been optimized and end-of-line pressure is still insufficient, the air demand has outgrown this machine and it is time to add capacity or go bigger.

If nobody is willing to touch the piping, the other path is to replace the airend rotor set with a higher Vi to match the higher discharge pressure. This is an airend-level rebuild. Not cheap. For Atlas Copco GA series, a complete airend assembly (rotors, bearings, seals) at the GA75 level costs roughly a third to half the price of a new machine, varying by size.

Vi mismatch also has a less commonly mentioned reverse situation: over-compression. A factory scaled down production, removed equipment, air demand dropped significantly, piping pressure loss shrank, end-of-line pressure was now higher than needed. The operator lowered the compressor setpoint. Now the system pressure is well below the airend's Vi-matched discharge pressure, and every cycle is doing over-compression. Discharge temperature elevated, power consumption also elevated. This scenario shows up in manufacturing plants during economic downturns. Production halved, same compressor, nobody thinks about whether the Vi still matches.

System Design

Piping Network & Pressure Drop

Factory-stated oil change intervals are laboratory numbers. Atlas Copco Roto-Inject Fluid, Ingersoll Rand Ultra Coolant, Shell Corena S4 R, Mobil Rarus SH, all of these will reach their rated life under stable conditions. The gap between field conditions and laboratory conditions determines how much of a discount the oil life takes, and how much depends on a few factors.

Frequent loading and unloading. Every time the compressor unloads, system pressure drops sharply. Gas dissolved in the oil comes out of solution as a swarm of microbubbles. Localized overheating and oxidation occur at the bubble surfaces. A fixed-speed machine with poorly configured controls cycling twenty-plus times an hour ages its oil far faster than a machine running at steady full load. VFD machines do not have this problem because load changes are continuous, no pressure shock from loading and unloading cycles.

High humidity intake air. Water in the oil accelerates oxidation. Machines in coastal and tropical regions take a hit on this one.

Mixed oil. Different brands, different base oil types poured into the same system, additive cross-reactions causing accelerated oil degradation. PAO and ester base oils mixed together are particularly sneaky, no visible symptoms for hundreds of hours, then discharge temperature starts a slow climb. A technician who does not have the OEM oil on hand and pours in "equivalent grade" from another brand, this happens every day.

Specific heat capacity and thermal conductivity degradation does not show up in viscosity or TAN numbers. A batch of oil can have viscosity and TAN well within acceptable limits, with the routine oil analysis report concluding "suitable for continued use." Meanwhile the specific heat capacity of that oil has dropped, it carries less heat per cycle out of the compression chamber, and discharge temperature sits several degrees or even ten-plus degrees above where it would be with fresh oil. The technician reads the oil analysis report, rules out oil as a cause, goes back to checking the cooler and the thermostatic valve, finds nothing.

If the situation keeps coming up where oil analysis is normal but discharge temperature is elevated and everything else has been checked, try a complete oil change. Not a top-off, drain all of it and refill with fresh oil. If discharge temperature drops immediately, the problem was oil thermal performance. The long-term countermeasure is to ask the oil analysis provider to add specific heat capacity or thermal conductivity to the test panel. The cost of this additional test is low. Or in harsh operating conditions with frequent load/unload cycling and high humidity, cut the oil change interval to sixty or seventy percent of the OEM-stated hours.

Rotor-to-housing clearances increase over time. Compressed gas leaks from the high-pressure side back to the low-pressure side through the enlarged gaps, gets compressed again, adds heat. This process is measured in years. Discharge temperature might rise two or three degrees per year, not enough to notice in any single snapshot. At the same time, output volume is slowly declining and the compressor spends a growing percentage of its running time at full load. Many operators attribute the increasing full-load ratio to growing air demand rather than declining compressor efficiency.

The gradual rise in discharge temperature and the gradual decline in output volume are synchronized because they share the same root cause. Record full-load discharge temperature and ambient temperature once a week. Over time, the trend line is a more accurate indicator of when a specific machine needs overhaul than the OEM's fixed-hour recommendation. Plenty of Atlas Copco GA airends have gone forty or fifty thousand hours in clean environments without needing a rebuild. Same model in a dusty quarry might be done at twenty thousand.

Wear PatternRotor-to-Housing Clearance

OverhaulAirend Inspection

A three-phase fan motor with one phase lost still spins. Torque drops significantly, airflow may be cut by more than half. In the noise of a compressor room, you cannot hear the difference. Hold a thin plastic film strip against the cooler intake face. Normal airflow pins it flat. Reduced airflow from a lost phase lets it flutter. Or use a clamp meter on all three phases and check for balance.

Variable-speed cooling fans have a different hidden failure. The cooling system has its own temperature sensor wired to the VFD, separate from the discharge temperature sensor displayed on the control panel. If the cooling system's sensor drifts or its signal wire has a poor connection, the VFD thinks the temperature is low and keeps the fan at minimum speed. The service technician checks the panel discharge temperature sensor with an infrared gun, readings match, sensor is fine, now what. Nobody checks the other sensor because nobody knows it exists. Atlas Copco and Ingersoll Rand both use this dual-sensor architecture on their mid-range and upper-range models.

Separator element clogged, tank pressure rises, airend backpressure rises, compression ratio forcibly increased, discharge temperature goes up. Same mechanism as Vi mismatch.

Most machines mount the discharge pressure gauge downstream of the separator. The panel reads 8 bar, separator differential pressure is 2 bar, the airend discharge port is actually seeing 10 bar. Atlas Copco's Elektronikon controller and Ingersoll Rand's Xe controller both display separator differential pressure. Older or lower-end models may not have this reading, in which case a manual pressure gauge on each side of the separator to get the differential is needed.

New element differential pressure is typically a few tenths of a bar. Above one bar, it should be replaced. The clogging process is slow, half a year to gain one bar, discharge temperature creeps up with it. One hot day the ambient temperature stacks on top and the machine shuts down. The technician who shows up calls it a high ambient temperature issue.

Intake temperature's impact on discharge temperature is underestimated. The compression process is nearly adiabatic. Each degree of intake temperature rise translates to more than one degree of discharge temperature rise because the compression ratio amplifies it. Intake going from 25°C to 45°C can push discharge temperature up by thirty to forty degrees.

Cold air in from a low opening on one side, hot air out from a high opening on the opposite side, or ductwork taking the cooler exhaust directly outside. A few hundred in materials for the ductwork. Permanent effect. The best input-to-output ratio of any measure against high discharge temperature.

This problem is common. The compressor room had adequate ventilation when it was built. Then someone put up a partition wall next to it, stacked materials against the exhaust vent, blocked half the airflow path. It gradually turned into an oven and nobody noticed. Discharge temperature kept climbing, and the compressor kept getting serviced.

Room Design

Ventilation & Heat Rejection

Infrared thermometer and clamp meter. Do not take anything apart yet.