Compressor oil coolers and aftercoolers

What do oil coolers and aftercoolers do?

In an oil-injected screw compressor most of the electrical energy put into the motor ends up as heat. The bulk of that heat is carried away by the injected oil, and the rest leaves with the compressed air. A typical package therefore has two heat exchangers doing two different jobs.

• Oil cooler: cools the oil returning from the separator tank before it is injected back into the airend. It keeps the oil within the range the manufacturer specifies, hot enough to avoid condensing water in the oil, cool enough to protect the oil and the bearings.
• Aftercooler: cools the hot discharge air leaving the minimum-pressure valve. As the air cools, water vapour condenses and is removed by a moisture separator and drain. Discharge air typically leaves a well-performing air-cooled aftercooler about 10 to 15 °C above the cooling-air temperature.

On air-cooled packages both cores are often mounted side by side or stacked, with a single fan drawing air through them. On water-cooled machines they are usually shell-and-tube or plate exchangers connected to a cooling-water circuit. Oil-free compressors also use intercoolers between stages, which are covered by the same principles of fouling and heat duty.

Types of cooler and where they are used

Water-cooled designs depend heavily on water quality. Hard borewell water, common in parts of Telangana, deposits scale quickly in narrow passages unless the cooling-water circuit is treated and filtered.

Why does my compressor run hot?

A high-temperature trip is one of the most common breakdown calls, particularly in Hyderabad summers when compressor-room temperatures climb well above design ambient. The cooler is often blamed, but it is one of several possible causes.

Running hot shortens oil life and speeds up varnish formation, which then affects the separator element, inlet valve and bearings. Treating the temperature as a symptom of the whole package, not just the cooler, avoids repeat trips.

Why is my compressed air hot and wet after the aftercooler?

A fouled or undersized aftercooler leaves the air hotter than it should be. Hot air carries much more water vapour, so more moisture reaches the dryer and the distribution system. A refrigerated dryer designed for a given inlet temperature can then struggle to hold its typical pressure dew point of around +3 °C, and you see water at the point of use.

Check these before blaming the dryer:

• Air temperature leaving the aftercooler, compared with the ambient air temperature at the cooler face.
• Condition of the aftercooler fins or water-side passages.
• Moisture separator and drain after the aftercooler: a blocked drain lets condensed water carry straight on.
• Room ventilation: if hot discharge air recirculates back into the cooler, everything downstream suffers.
• Dryer inlet temperature against the dryer’s design conditions.

Cleaning versus replacing a cooler

Many coolers can be returned to good performance by cleaning, but there is a point where replacement is the safer and more economical choice.

High-pressure washers and stiff brushes can bend fins and close up the very passages you are trying to clear. Isolate and depressurise the machine, allow it to cool, and protect the fan motor and electrical parts before any wet cleaning.

How we inspect and test a cooler

1. Record the machine identity and operating conditions: make, model, serial number, running hours, set pressure, ambient and room temperature.
2. Measure the temperatures under steady load: airend discharge, oil into and out of the cooler, cooling air in and out, and discharge air after the aftercooler.
3. Check the thermostatic valve to confirm oil is actually being sent through the cooler once warm.
4. Check airflow: fan direction, condition and current, ducting and room ventilation. For water-cooled units, water flow and inlet and outlet temperatures.
5. Inspect the core: fin condition, external oil wetting, leaks, corrosion, mounting and vibration.
6. Check the oil: level, colour, smell and, where needed, an oil analysis for oxidation or contamination.
7. Recommend: clean, flush, repair the surrounding system, or replace the cooler, and explain why.

Comparing the temperature rise and approach against the machine’s normal values tells us whether the cooler is genuinely losing performance or whether the problem lies elsewhere.

Choosing a replacement oil cooler or aftercooler

Two coolers can share the same frame size and bolt pattern and still be quite different inside. A replacement must suit the duty, not just the space.

• Heat duty: the core must reject the heat the package produces at its rated power and the site’s maximum ambient.
• Pressure rating: the oil and air passages must be rated for the compressor’s maximum working pressure with a suitable margin.
• Pressure drop: too much restriction on the air side reduces delivery; on the oil side it affects oil flow to the airend.
• Connections and orientation: port sizes, thread types, flange patterns and the position of inlet and outlet must match.
• Materials: aluminium, copper and steel behave differently with certain oils, cooling waters and corrosive atmospheres.
• Combined or separate cores: some packages use one combined oil and air block, others separate units.

We identify the cooler from the machine nameplate and the existing part, including any part number on the core, and confirm fit and rating before quoting. Where a genuine part is unavailable or uneconomic, we can propose a named aftermarket equivalent of matching specification, and the quote states clearly that it is not genuine.

Preventing cooler problems

• Keep the compressor room ventilated and duct hot cooling air outside, especially from March to June.
• Inspect and clean cooler fins at intervals that suit the site; dusty sites such as construction and stone-crushing yards need much more frequent attention.
• Keep intake and cabinet filters clean so dust does not reach the cores.
• Use the lubricant type the manufacturer specifies and change it on time.
• Fix oil leaks promptly: oil mist on fins traps dust and builds a hard layer.
• For water-cooled machines, treat and filter cooling water and record water temperatures.
• Log oil and discharge temperatures so a slow rise is noticed before it becomes a trip.

Where the heat goes: Cooler heat-load arithmetic

In an oil-injected screw compressor, almost all the shaft power becomes heat, and the coolers have to reject most of it. The split below is a typical range. The maker’s heat-balance data for your model is the figure to use when you have it.

Motor losses and the cooling-fan power come on top of the shaft power. They also heat the cooling air and the room.

Worked example (illustrative): a 55 kW shaft-power package.

• Oil cooler: 55 × 0.75 = 41.25 kW. Aftercooler: 55 × 0.13 = 7.15 kW. Total through the coolers: 48.4 kW.
• Air-cooled: cooling air flow = heat ÷ (density × specific heat × temperature rise). At 35 °C the density of air is 1.146 kg/m³ and its specific heat is 1.006 kJ/kg·K. For a 15 K rise across the cores: 48.4 ÷ (1.146 × 1.006 × 15) = 2.8 m³/s, about 10,100 m³/h. If the air may only rise 12 K, the flow needed is 3.5 m³/s.
• Water-cooled: water flow = 48.4 ÷ (4.18 × 10) = 1.16 kg/s, about 4.2 m³/h, for a 10 K rise in water temperature.

This is why recirculation matters so much. On a 45 °C May afternoon, cooling air enters 10 K hotter than at 35 °C, and every temperature in the package rises with it. The fan also moves about 3% less mass, because hot air is less dense. If the room’s exhaust duct cannot carry 10,000 m³/h or more out of the building, the package is breathing its own exhaust.

Approach temperature: What it tells you

Approach is the outlet temperature of the hot fluid minus the inlet temperature of the cooling medium. For an air-cooled aftercooler, it is the air outlet temperature minus the cooling-air inlet temperature. Approach largely strips out the effect of the weather, so a reading in March can be compared with one in May. Record it at full load once the machine is stable, using the same sensors or contact thermometer points each time. Write down the value when the cooler is new or freshly cleaned. That baseline is your reference. A rise of 5 °C or more above the baseline, at similar load, is a reliable signal to act.

As a guide, a well-performing air-cooled aftercooler usually runs at an approach of 10–15 °C. A water-cooled unit with clean tubes is often under 10 °C. Above about 20 °C on air-cooled units, a dryer sized for standard inlet conditions is likely to be overloaded.

Aftercooler outlet temperature and condensate load

Worked example (illustrative): a compressor delivers 10 m³/min FAD at 7 bar(g) (8.013 bar(a)). It draws monsoon air at 32 °C and 80% relative humidity.

1. Saturation vapour pressure at 32 °C = 4.76 kPa, so the partial pressure of water vapour is 0.80 × 4.76 = 3.81 kPa.
2. Moisture content = 0.622 × 3.81 ÷ (101.33 − 3.81) = 24.3 g per kg of dry air.
3. Dry air flow = 97.52 kPa × 10 m³/min ÷ (0.287 × 305.15 K) = 11.13 kg/min. Water drawn in = 11.13 × 0.0243 × 60 = 16.2 kg/h, about 130 litres in an 8-hour shift.
4. Saturated air at the aftercooler outlet can hold only 0.622 × p_sat ÷ (801.3 − p_sat), with pressures in kPa. The rest condenses and must be drained at the separator.

A fouled aftercooler that lets the outlet drift from 40 °C to 55 °C more than doubles the dryer’s water load (7.9 ÷ 3.5 = 2.3 times). The dryer’s refrigeration load rises at the same time. The figures assume the separator catches all the free water. Real separators let a little through, so the dryer’s load is higher still. To rate a refrigerated dryer at those inlet conditions, use the dryer correction factor worksheet. The theory is set out on the aftercooling and moisture separation page.

Descaling, water quality and leak testing

For air-cooled cores, add three checks to the cleaning method above. Use a pH-neutral degreaser on aluminium cores, because strong alkaline cleaners etch aluminium. Make sure the fan shroud seals against the core, because air that leaks around the core does no cooling. Measure the approach again under load to confirm the cleaning worked.

Descaling a water-cooled cooler

1. Record the approach and the water temperature rise before cleaning, so the result can be proved.
2. Isolate and drain the water side. Remove strainers and clean them.
3. Circulate an inhibited descaler that the cooler maker approves for its tube, plate and braze materials. Strong uninhibited acids attack copper alloys, aluminium and brazed joints. Keep the circulation slow and in the reverse direction to normal flow.
4. The treatment is finished when gas no longer forms and the acid strength stops falling. Then neutralise, flush with clean water until the pH is neutral, and refill.
5. Pressure-test, restart, and record the new approach.

Water quality. Scale forms faster as the water gets hotter, so keep the water outlet within the maker’s limit. Many makers set a limit of about 40–45 °C. Hard borewell water used in once-through cooling is the worst case. On a cooling-tower circuit, control blowdown, dose scale and corrosion inhibitors and a biocide, and monitor pH, hardness, conductivity, chlorides and suspended solids against the cooler maker’s water specification.

Leak testing a cooler

• Signs of a leak: an oil sheen on the cooling-tower water or in the drain (oil is usually at higher pressure than the water while running), milky oil or a rising oil level (water entering the oil when the machine stands), bubbles in an aftercooler’s water return, or a patch of caked dust on an air-cooled core.
• Test: blank one side and fill it with water. Pressure it to no more than the test pressure marked on the cooler or in its data, and hold it for 15–30 minutes while watching the gauge and the other side. Small cores can be bubble-tested underwater with low-pressure dry air or nitrogen. Use water for higher-pressure tests, because it stores far less energy.

Intercooler service on two-stage piston compressors

In a two-stage piston compressor, the intercooler cools the air between the low-pressure and high-pressure cylinders. When it works, the high-pressure stage takes in cool, dense air, shares the work evenly and runs at a lower discharge temperature. For equal pressure ratios, the ideal interstage pressure is √(P_suction × P_discharge) in absolute terms. For a final pressure of 12 bar(g): √(1.013 × 13.013) = 3.63 bar(a), or 2.6 bar(g). For 8 bar(g) the figure is about 2.0 bar(g). The cylinder sizes set the actual value, so record the gauge reading on a healthy machine and use it as your reference.

Service the intercooler at each valve service. Clean the fins from the clean side and drain the interstage condensate. Leak-test the tube joints with soapy water at working interstage pressure. Remove carbon from the tubes and bench-test the interstage safety valve, which must never be plugged. Replace any cooler with cracked tubes or headers. A brazed patch on a vibrating tube bundle rarely lasts. The valve diagnosis is covered under reciprocating compressor valve engineering.

How Reddy Compressors helps

Reddy Compressors, based at Tarnaka in Secunderabad and serving industry since 2002, is an independent multi-brand specialist. We work on compressors from Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic, Gardner Denver, Sullair, Doosan, Kirloskar, Bobcat and many others, without being an authorised distributor of these brands.

For cooling problems we diagnose high-temperature trips on site, clean and flush coolers, check thermostatic valves, fans and room ventilation, and supply replacement oil coolers, aftercoolers and combined cores once fit and rating are confirmed. We also look at the effect on dryers and condensate drains downstream. Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply compressors, rentals and spare parts across India. If a cooler has to be replaced and production cannot wait, we can discuss a rental compressor in the meantime.

What to send us

• Compressor make, model and serial number, with a photo of the nameplate.
• Photos of the cooler core, any label or part number, and the connections.
• Whether the machine is air-cooled or water-cooled.
• The symptom: high-temperature trip, oil leak, hot or wet discharge air.
• Oil and discharge temperatures from the controller, plus room temperature if known.
• Running hours, last service date and your site location.

Frequently asked questions

Why does my screw compressor trip on high temperature?

The most common causes are dirty cooler fins, a hot compressor room where cooling air recirculates, a thermostatic valve stuck in bypass, low oil level or a fan fault. Internal fouling of the oil cooler and degraded oil also raise temperature. Start by checking the room temperature and fin condition, then measure temperatures across the cooler under load to narrow down the cause.

How do I clean a compressor oil cooler?

Stop, isolate and depressurise the machine and let it cool. Blow dry dust out with low-pressure air from the clean side, opposite to the normal airflow. For oily deposits, use a suitable degreaser and gentle water rinse, protecting the fan motor and electrics, then dry fully. Avoid high-pressure jets and wire brushes, which bend fins and reduce airflow.

What is the difference between an oil cooler and an aftercooler?

The oil cooler cools the lubricant that is injected into the airend, keeping the compression process and bearings at a safe temperature. The aftercooler cools the compressed air after it leaves the compressor, so water vapour condenses and can be drained before the dryer. They may share a fan and frame but handle different fluids at different pressures.

Why is there so much water in my compressed air?

Excess water often starts with air leaving the aftercooler too hot, either because the cooler is fouled, the room is too warm or the moisture separator drain is blocked. Hot air carries more vapour into the dryer, which may then fail to reach its typical dew point. Check aftercooler outlet temperature, drains and dryer inlet conditions before replacing the dryer.

Can a leaking compressor oil cooler be repaired?

Small leaks at external fittings or gaskets can usually be fixed by replacing seals or connections. Leaks from the core itself, or cracks at the headers, are rarely worth repairing in the field because the core must hold full working pressure safely. In those cases replacement with a correctly rated cooler is the reliable choice, after checking mounts and vibration.

How often should compressor coolers be cleaned?

It depends on the site. Clean indoor installations may need attention only at routine services, while dusty sites, textile plants and construction yards may need checks every few weeks, especially in summer. Watching the oil temperature trend is the best guide: a steady rise under similar load and ambient conditions usually means the cooler needs cleaning.

How much heat does a compressor oil cooler remove?

In an oil-injected screw compressor, the oil cooler usually removes about 70–80% of the shaft power as heat, and the aftercooler about 10–15%. On a 55 kW machine, that is about 41 kW through the oil cooler and 7 kW through the aftercooler. Rejecting about 48 kW with a 15 K rise in cooling-air temperature at 35 °C takes about 2.8 m³/s (10,100 m³/h) of cooling air. A water-cooled unit needs about 4.2 m³/h of water for a 10 K rise.

What is a good aftercooler approach temperature?

Approach is the air outlet temperature minus the cooling-medium inlet temperature. A healthy air-cooled aftercooler usually runs at 10–15 °C. A clean water-cooled unit is often under 10 °C. Rather than relying on a general figure, record your own approach at full load when the cooler is clean. A rise of 5 °C or more above that baseline at similar load means fouling, poor airflow or scale. Above about 20 °C on an air-cooled unit, a dryer sized for standard inlet conditions is likely to be overloaded with water.

How do you service an intercooler on a two-stage air compressor?

Clean the fins from the clean side and check the fan and belt. Drain the interstage condensate and leak-test the tube joints at interstage pressure. Remove any carbon from the tubes and bench-test the interstage safety valve. Compare the interstage gauge reading with the machine’s own healthy value. The ideal figure is about 2.0–2.6 bar(g) for 8–12 bar(g) machines, but cylinder sizing sets the real one. High interstage pressure points to high-pressure valve leakage, and low interstage pressure to low-pressure valve or ring wear. Replace a cooler with cracked tubes.

How do I know if my compressor oil cooler is leaking?

On water-cooled machines, look for an oil sheen in the cooling water or tower, and for milky oil or a rising oil level after the machine has stood. On air-cooled cores, look for an oily patch where dust has caked onto the fins. To confirm, isolate the cooler, blank one side and pressure-test it with water, no higher than its rated test pressure. Hold for 15–30 minutes while watching the gauge and the other side. A leaking core should normally be replaced, not patched.

Related pages

• Thermostatic valves
• Cooler and heat-exchanger service and fouling control
• Aftercooling, moisture separation and condensate removal
• Compressor lubricants
• Refrigerated compressed-air dryers
• Find my compressor part