Why a hotter compressor is a question, not a diagnosis
A rising discharge temperature is one of the most common complaints on rotary screw compressors in India, especially in April and May when plant rooms in Telangana and Andhra Pradesh can exceed 45 °C. The reading alone does not say why it rose. The same symptom can come from a hotter intake, a blocked oil cooler, low oil level, a stuck thermostatic valve, a slipping fan belt, recirculating hot air, a higher pressure setting or simply a sensor that has drifted.
The workbook gathers the evidence needed to separate those causes. Its output is a supported investigation direction and a way to verify the repair, not a red warning light. It never replaces the machine’s own protective shutdowns or the manufacturer’s limits.
Map the heat path first
Nearly all the electrical energy that goes into an air compressor ends up as heat. In an oil-injected screw compressor most of it is carried away by the oil and removed in the oil cooler; the rest leaves through the aftercooler, the motor and the package surfaces. Every temperature reading needs a physical meaning on that path.
| Measurement point | What it tells you | Typical reference |
|---|---|---|
| Cooling air entering the package | The heat sink; often hotter than outdoor ambient | Compare with design ambient on the data sheet |
| Airend discharge (oil-injected screw) | Heat generation and oil cooling together | Normally about 75–95 °C |
| Oil after the cooler | Oil cooler performance and thermostatic valve action | Compare with cooling-air temperature (approach) |
| Compressed air after the aftercooler | Aftercooler performance; load on the dryer | Typically a modest approach above cooling air |
| Cooling water in and out (water-cooled) | Heat removed, if flow is also known | Read with flow, never alone |
| Hot air leaving the package | Whether it recirculates to the intake | Check ducting and room layout |
Inputs the workbook records
- Date and time of each reading, with a single agreed clock.
- Load state (loaded, unloaded, speed for variable-speed machines) and delivery pressure in bar(g) and psig.
- Cooling-air inlet temperature at the package, not the outdoor reading.
- Discharge, oil-after-cooler and air-after-aftercooler temperatures.
- For water-cooled machines, water inlet and outlet temperatures and water flow.
- Maintenance events: cooler cleaning, oil change, filter change, thermostatic valve replacement.
- Sensor details and any calibration or replacement.
The calculations
Temperature rise is outlet temperature minus inlet temperature for the same stream.
Cooler approach is the temperature of the fluid leaving the cooler minus the temperature of the cooling medium entering it. For an air-cooled oil cooler, approach = oil out − cooling air in. A rising approach at the same load points to the cooler or its airflow; an unchanged approach with a hotter outlet points to a hotter sink.
Margin is the applicable limit minus the measured value. It is the number that matters for whether the machine will trip.
Water-side heat duty for a water-cooled machine is:
Q (kW) = density × flow × specific heat × (water out − water in) ÷ 3,600
with flow in m³/h, density about 1,000 kg/m³ and specific heat about 4.18 kJ/(kg·K) for clean water.
Worked example A: hotter inlet or a fouled cooler?
These figures are illustrative. Three steady readings are taken at the same load, pressure and fan state on an aftercooler.
| Condition | Cooling air in | Air out | Approach | Change from baseline |
|---|---|---|---|---|
| Baseline | 30 °C | 40 °C | 10 K | Reference |
| Hotter day | 38 °C | 48 °C | 10 K | Outlet +8 K, approach unchanged |
| Hotter day, fouled cooler | 38 °C | 54 °C | 16 K | Outlet +14 K, approach +6 K |
In the second row the cooler is performing as before; the outlet rose because the cooling air did. In the third row 8 K of the rise is the hotter inlet and 6 K is extra approach, which directs attention to the cooler core, fan and airflow. If the applicable limit were 55 °C, the margins would be 15 K, 7 K and only 1 K. An unchanged approach does not restore lost margin, so the second case may still need better ventilation.
Worked example B: larger water rise, less heat removed
Illustrative figures for one cooling-water circuit:
| Run | Flow | Water in | Water out | Rise | Heat removed |
|---|---|---|---|---|---|
| Reference | 6.0 m³/h | 30 °C | 36 °C | 6 K | 41.8 kW |
| Later | 3.0 m³/h | 30 °C | 39 °C | 9 K | 31.4 kW |
The water rise is 50% larger, which might look like the cooler is “working harder”, but the heat removed is 25% lower because the flow has halved. Check strainers, valves, the cooling tower pump and scaling before blaming the compressor.
Common thermal patterns and likely causes
| Pattern | Likely direction | First checks |
|---|---|---|
| All temperatures rise together with cooling-air inlet | Hot room or recirculation | Ventilation, ducting, hot-air discharge path |
| Oil cooler approach rises steadily over weeks | Cooler fouling on air or oil side | Clean core, check fan and belt, oil condition |
| Discharge high soon after start, oil cooler cold | Thermostatic valve stuck in bypass | Valve element, oil flow |
| Sudden step change with no change in duty | Sensor or wiring fault | Compare with a calibrated reference |
| Rise after pressure setting increased | Higher duty, more heat | Return to the pressure actually needed |
| High water outlet, low water flow | Water-side restriction | Strainers, valves, scaling, pump |
| Slow rise with darkening oil | Oil degradation or wrong oil | Oil analysis, change interval |
Step-by-step use
- Define the complaint: trip, rising trend, hot dryer inlet or a hot spot found by thermal imaging.
- Record the machine configuration and any recent maintenance.
- Take readings at matched load and pressure, with cooling-air or water inlet temperature each time.
- Screen out readings with missing data, unit errors or obvious sensor faults; do not fill gaps with straight lines.
- Calculate rise, approach and margin for each stream.
- Compare with a baseline taken at similar duty, ideally soon after a cooler clean.
- List competing causes, check the cheapest and safest first, and record the result.
- After repair, repeat readings at the same duty to confirm the change is larger than measurement uncertainty.
Limits
The workbook cannot predict exactly when a high-temperature trip will happen, and a single lower reading after service does not prove the repair worked. Infrared images show surface temperatures, which depend on emissivity and viewing angle, so they support but do not replace installed sensors. Always follow the manufacturer’s limits and site procedures for an active alarm.
Air-cooled and water-cooled packages
On air-cooled machines the most frequent root cause in Indian plant rooms is ventilation. Hot air leaving the cooler must be ducted out, and fresh air must enter at low level without being drawn from beside a boiler or furnace. Measure the air temperature at the package intake, not outside the building; the difference is often several degrees and explains many summer trips.
On water-cooled machines, record water flow as well as temperatures. Cooling-tower water in hard-water areas scales heat exchangers over time, and open circuits collect dust and algae. A steadily rising approach with normal water flow suggests fouling on the water side, while a sudden change usually points to a valve, strainer or pump. Keep the cooler service record in the workbook so each clean can be judged against the trend it was meant to correct.
How Reddy Compressors helps
Since 2002 Reddy Compressors, from our office at Tarnaka in Secunderabad, has serviced and overhauled compressors of all major makes, including ELGi, Atlas Copco, Ingersoll Rand, KAESER and Chicago Pneumatic, as an independent multi-brand specialist rather than an authorised dealer. With over 20 service engineers and technicians, our teams cover the whole of Telangana and Andhra Pradesh.
For overheating complaints we take readings at matched duty, check the cooling-air path and ventilation, inspect coolers, fans, thermostatic valves and oil, and verify sensors before recommending parts. We supply coolers, valves and other spares identified from the nameplate, and if a critical machine must come out of service, our rental fleet can provide temporary air while the work is done.
What to send us
- Make, model, serial number and a photo of the nameplate.
- Recent temperature readings or controller logs, with load and pressure.
- Plant-room temperature and photos of the room, ducting and intake.
- Recent maintenance history: cooler cleaning, oil and filter changes.
- For water-cooled machines, water temperatures and flow if known.
Frequently asked questions
Why is my screw compressor running hot in summer?
The most common reason is that the cooling air reaching the package is much hotter than the design ambient, often because hot exhaust air recirculates inside a closed plant room. Dirty oil cooler fins, a slack fan belt, low oil level and a stuck thermostatic valve add to it. Measure the cooling-air temperature at the package and the cooler approach to see which applies.
What is a normal discharge temperature for an oil-injected screw compressor?
For most oil-injected screw compressors the airend discharge temperature normally sits at about 75–95 °C in steady running, depending on design, load and cooling conditions. Check your manufacturer’s data for the exact range and the alarm and trip settings. A steady rise towards the alarm at the same load and ambient conditions is worth investigating before it causes a shutdown.
What is cooler approach temperature?
Cooler approach is the temperature of the fluid leaving a cooler minus the temperature of the cooling air or water entering it. For an air-cooled oil cooler it is oil-out minus cooling-air-in. Approach is useful because it removes most of the effect of ambient changes; if approach rises at the same load, the cooler or its airflow has probably deteriorated.
Does a higher cooling-water outlet temperature mean the cooler is working better?
Not necessarily. A higher water temperature rise often means water flow has fallen, and the heat actually removed may be lower. Heat removed depends on flow multiplied by the temperature rise. Measure or estimate flow before drawing conclusions, and check strainers, valves, pumps and scaling on the water side as well as the compressor itself.
Can thermal imaging replace installed temperature sensors?
No. A thermal camera is excellent for finding hot spots on coolers, motors, terminals and pipework, but it measures surface temperature, which depends on emissivity, angle and reflections. It cannot see oil or air temperature inside the machine. Use it alongside installed sensors and a calibrated reference probe, not as a substitute.
Does a lower temperature after service prove the repair worked?
Only if the readings before and after were taken at similar load, pressure and cooling-air temperature, and the change is larger than the measurement uncertainty. A cooler morning after service can make any repair look successful. Repeat readings over several days at matched duty, and compare cooler approach as well as absolute temperature.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Cooler approach and margin against a baseline
Compare a cooler reading now with a baseline taken at the same load, pressure and fan state. The tool splits the rise in leaving temperature into the part caused by a hotter cooling medium and the part caused by a worse cooler approach, and shows the margin to the limit.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Baseline: cooling medium entering (°C): 30
- Baseline: fluid leaving the cooler (°C): 40
- Now: cooling medium entering (°C): 38
- Now: fluid leaving the cooler (°C): 54
- Applicable limit for the leaving temperature (°C): 55
- Measurement uncertainty of the comparison (K): 1
Result
- Approach: baseline: 10 K
- Approach: now: 16 K
- Change in leaving temperature: +14 K
- of which hotter cooling medium: +8 K
- of which change in approach: +6 K
- Margin to limit: baseline: 15 K
- Margin to limit: now: 1 K
- Investigation direction: Approach has risen: look at the cooler core, fan, belt and airflow (or water-side fouling), and at the thermostatic valve and oil condition.
Method. Approach = fluid leaving the cooler − cooling medium entering it (for an air-cooled oil cooler: oil out − cooling air in). Rise in leaving temperature = change in cooling-medium inlet + change in approach. Margin = applicable limit − leaving temperature. A change in approach larger than the measurement uncertainty points to the cooler core, fan or airflow; an unchanged approach with a hotter outlet points to a hotter heat sink (ventilation, recirculation). Readings must be at matched duty.
What to take from it. An unchanged approach clears the cooler but does not restore lost margin. A hot room can still trip a healthy machine.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
Water-side heat duty of a water-cooled compressor
Compare two runs of the same cooling-water circuit. A larger water temperature rise does not mean more heat is being removed: heat removed depends on flow multiplied by rise.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Reference run: water flow (m³/h): 6
- Reference run: water in (°C): 30
- Reference run: water out (°C): 36
- Later run: water flow (m³/h): 3
- Later run: water in (°C): 30
- Later run: water out (°C): 39
Result
- Reference run: water rise: 6 K
- Reference run: heat removed: 41.8 kW
- Later run: water rise: 9 K
- Later run: heat removed: 31.4 kW
- Change in water flow: -50 %
- Change in water rise: +50 %
- Change in heat removed: -25 %
Method. Q (kW) = density × flow × specific heat × (water out − water in) ÷ 3,600, with flow in m³/h, density 1,000 kg/m³ and specific heat 4.18 kJ/(kg·K) for clean water.
What to take from it. If the rise goes up while the heat removed goes down, the flow has fallen. Check strainers, valves, pump and scaling before the compressor.