Why compression creates a water problem
Atmospheric air always carries water vapour. When a compressor squeezes that air to around 8 bar absolute, it concentrates the vapour into about one-eighth of the volume. Hot air can hold the vapour, but as soon as it cools in the cooler, receiver or pipework, the excess condenses as liquid water.
The quantity surprises many plant engineers. In humid Indian conditions, a medium-sized compressor can produce several hundred litres of condensate a day. That water carries rust and pipe scale, mixes with oil from oil-injected machines, washes lubricant out of pneumatic tools, corrodes receivers and pipework from the inside and overloads dryers and filters. Removing it early, as liquid, is far cheaper than removing it later as vapour.
How much water are we talking about?
An illustrative calculation helps set expectations. Take a compressor delivering 10 m³/min FAD on a monsoon day, drawing air at 35 °C and 70% relative humidity, discharging at 8 bar(g).
| Point in the system | Air condition (illustrative) | Water vapour carried | Liquid formed at this stage |
|---|---|---|---|
| Compressor intake | 35 °C, 70% RH, atmospheric | About 16–17 kg/h | None |
| After the aftercooler | About 45 °C, saturated, 8 bar(g) | About 4–5 kg/h | About 12 kg/h, removed by the separator and drain |
| After a refrigerated dryer | Pressure dew point about +3 °C | Well under 1 kg/h | About 4 kg/h, removed by the dryer drain |
On these assumptions, the aftercooler and separator remove roughly 280–300 litres a day on a 24-hour duty, and the dryer about another hundred. The figures change with inlet humidity, temperature, pressure and cooler performance, but the pattern holds: most of the water should leave at the aftercooler separator, and the dryer should only deal with what remains.
How aftercoolers work, and what limits them
An aftercooler is a heat exchanger. Air-cooled aftercoolers, built into almost every packaged screw compressor, use a fan to blow ambient air across finned tubes. Water-cooled aftercoolers, common on larger machines and process plants, use cooling-tower or chilled water in a shell-and-tube or plate exchanger.
The important number is the approach temperature: how close the air outlet gets to the cooling medium. A clean air-cooled aftercooler typically discharges air within about 10–15 °C of the air it is cooled by, which is the room air, not the outdoor shade temperature. If the compressor room is at 48 °C on a Telangana summer afternoon because hot exhaust is recirculating, the aftercooler outlet rises with it, more vapour passes through, and the dryer receives a heavier load than it was sized for.
- Fouled fins from cement, cotton, flour or coal dust reduce heat transfer.
- Oil mist on fins traps dust and makes fouling worse.
- Scale, silt or biological growth in water-cooled exchangers raises outlet temperature gradually.
- Poor ducting that returns hot air to the cooler inlet defeats even a clean cooler.
Moisture separators: geometry, velocity and pressure drop
Condensate leaves the aftercooler as droplets and mist carried in the air. A separator changes the air’s direction and speed so droplets hit a surface, coalesce and fall to a sump. Common designs include centrifugal or cyclonic separators, baffle types and wire-mesh demisters.
Every separator has an operating envelope. At very low flow, a centrifugal separator spins the air too slowly to throw droplets out. At very high flow, droplets can be re-entrained from the sump. Choose a separator for the actual flow range and pressure, not by pipe size, and check its pressure drop at full flow. Each extra bar of discharge pressure needed to overcome restrictions costs about 7% more compressor power, so separation should never hide a restriction that the compressor then has to beat.
Drains complete the job
A separator that cannot empty its sump is not a separator. Liquid builds up until it is carried straight on into the receiver and dryer. Drains are the most neglected component in compressed-air treatment, and drain failure is one of the most common reasons for “wet air” complaints.
| Drain type | How it works | Strengths | Watch for |
|---|---|---|---|
| Manual valve | Operator opens it on a routine | Simple, no power | Forgotten, left cracked open (wasting air) or closed |
| Timer solenoid | Opens for a set time on a set interval | Low cost, easy to fit | Wastes air when set too long; floods when set too short or load rises in monsoon |
| Float | Float lifts a valve when liquid rises | No power, little air loss | Sticks with oily, dirty condensate; orientation sensitive |
| Electronic level-sensing (zero-loss) | Sensor opens the valve only when liquid is present | Minimal air loss, alarm outputs | Needs power, periodic cleaning, correct capacity |
Ball valves before drains help maintenance, but a closed isolation valve makes a healthy drain useless. A blocked strainer does the same. Include every drain in the preventive maintenance list and test it regularly.
Where condensate collects across the system
Water does not stop forming at the aftercooler. It keeps condensing wherever air cools further:
- the wet receiver, which also acts as a secondary cooler and collector;
- dryer separators, especially on refrigerated dryers;
- coalescing filters, which collect both oil and water;
- low points, dead legs and long outdoor pipe runs that cool overnight;
- drops to users if they are taken from the bottom of the header instead of the top.
A wet receiver placed before the dryer adds cooling and dwell time and catches surges of liquid. A dry receiver after the dryer stores conditioned air for demand peaks. They do different jobs, and both need drains: even a dry receiver can collect water if the dryer is bypassed or fails.
Main headers should slope in the direction of flow, with drain legs at low points, and branches should leave from the top of the header. Cold starts after a weekend shutdown produce transient condensation in piping that was dry while warm, so drains must be in service before load returns.
Condensate is not just water
From an oil-injected screw compressor, condensate contains lubricant, sometimes as a stable emulsion, along with dust and rust. It must not go into a storm drain. An oil-water separator, usually a multi-stage unit with oleophilic media and activated carbon, separates the oil so the water fraction can be discharged according to site and local requirements. Synthetic lubricants and some food-grade oils form emulsions that standard gravity separators do not break, so the separator must suit the oil in use.
Condensate from oil-free compressors is free of compressor oil but can still be acidic and carry intake contaminants, so treat it according to site rules too.
Diagnosing wet air: check upstream first
When water appears at tools or in the process, the dryer often gets the blame. Work back through the chain first:
- Check compressor discharge temperature and aftercooler outlet temperature against ambient.
- Inspect cooler fins or water-side condition and room ventilation.
- Test every drain manually: separator, receiver, dryer and filters. Confirm isolation valves are open.
- Confirm the dryer bypass is closed and the dryer is running, with inlet temperature inside its rating.
- Look for cold points, low points and drops taken from the bottom of the header.
- Measure pressure drop across separators and filters at known flow.
- Only then measure the pressure dew point after the dryer and judge the dryer.
For critical air users, such as pharmaceutical processes or instrument air, drain alarms and duplicate drains on key points are worth considering, because one stuck drain can flood the whole downstream system.
Approach temperature: the number that sets the dryer’s load
An aftercooler’s approach temperature is its air outlet temperature minus the temperature of the cooling medium entering it: room air for an air-cooled unit, cooling water for a water-cooled one. Air leaving the separator is saturated (unless the intake air was very dry), so the outlet temperature alone fixes how much vapour goes on to the dryer. The intake humidity only decides how much liquid the aftercooler removes.
The table is calculated per 100 cfm free air delivery at 7 bar(g), using the Magnus vapour-pressure formula and an outlet pressure of 8.01 bar(a); the vapour column uses the monsoon-day air mass flow (about 2% lower on the dry day). Two Indian intake conditions are shown: a monsoon day (30 °C, 85% RH) and a dry summer afternoon (42 °C, 20% RH).
| Aftercooler outlet temperature | Vapour passed to the dryer, kg/h per 100 cfm | Liquid removed, monsoon intake (4.37 kg/h in) | Liquid removed, dry summer intake (1.92 kg/h in) |
|---|---|---|---|
| 40 °C | 1.10 | 3.27 kg/h | 0.83 kg/h |
| 45 °C | 1.44 | 2.94 kg/h | 0.51 kg/h |
| 50 °C | 1.86 | 2.51 kg/h | 0.09 kg/h |
| 55 °C | 2.39 (1.92 on the dry day: the air is not saturated) | 1.99 kg/h | Nil |
| 60 °C | 3.04 (1.92 on the dry day) | 1.33 kg/h | Nil |
Three practical conclusions follow:
- Every 5 °C rise in outlet temperature adds roughly 30% to the vapour the dryer must handle (1.10 to 1.44 kg/h is +31%; 1.44 to 1.86 kg/h is +29%).
- A fouled air-cooled aftercooler with a 20 °C approach in a 35 °C compressor room delivers 55 °C air. That is about two-thirds more vapour than a clean unit with a 10 °C approach (2.39 against 1.44 kg/h), at the same time as the dryer’s own condenser is struggling with the heat.
- Many refrigerated dryers are rated at 35 °C inlet and are commonly derated to roughly 0.8 at 40 °C and 0.65–0.7 at 45 °C; check the maker’s table with the dryer correction factor worksheet. A few degrees saved at the aftercooler is cheaper than a larger dryer.
Targets that are realistic in Indian plants: a clean air-cooled aftercooler gives an approach of about 10–15 °C; a water-cooled shell-and-tube unit with clean tubes gives about 5–10 °C above the cooling water inlet.
Air-cooled or water-cooled aftercooler
| Factor | Air-cooled | Water-cooled |
|---|---|---|
| Typical approach | About 10–15 °C above cooling air | About 5–10 °C above cooling water |
| Outlet temperature in a 45 °C summer room | About 53–60 °C, unless fresh air is ducted in | Set by cooling tower water, typically 32–35 °C inlet, so outlet about 37–45 °C |
| Utilities | Fan power only | Cooling water, pump, cooling tower and water treatment |
| Main fouling risk | Dust, lint, oil mist on fins | Scale and biological fouling in tubes; hard borewell water scales fast |
| Failure that wets the air | Fan stopped or hot-air recirculation | Tube leak; water enters the air side when the compressor stops |
| Best suited to | Most packaged machines up to medium size; sites without treated water | Large stations, hot or enclosed rooms, existing cooling water circuits |
On an air-cooled unit the cooling air inlet temperature is what matters, not the outdoor shade temperature. If the compressor draws air that has already passed through its own cooler, or a neighbour’s, the approach is measured from a much hotter starting point. Ducting the hot cooler discharge out of the room often recovers 5–10 °C at the outlet.
Sizing an aftercooler: worked example (illustrative)
Duty: 500 cfm (500 ÷ 35.315 = 14.16 m³/min) free air delivery, 7 bar(g), intake 35 °C and 70% RH, air entering the aftercooler at 90 °C from an oil-injected screw, target outlet 45 °C (10 °C approach to 35 °C cooling air).
- Dry air mass flow: about 935 kg/h, or 0.260 kg/s.
- Sensible heat from the air: 0.260 × 1.005 kJ/kg·K × (90 − 45) = 11.75 kW.
- Water vapour carried in: 23.5 kg/h. Vapour that can remain at 45 °C and 8.01 bar(a): 7.0 kg/h. Condensed: 16.4 kg/h.
- Latent heat of that condensation: 16.4 ÷ 3,600 × 2,400 kJ/kg = 10.96 kW. Sensible heat of the vapour itself adds about 0.55 kW.
- Total aftercooler duty: 11.75 + 0.55 + 10.96 = about 23.3 kW.
Two points stand out. First, in humid weather the condensing water nearly doubles the duty compared with dry air alone, so an aftercooler checked only on sensible heat will run hot in the monsoon. Second, the cooling medium needed: a water-cooled unit with a 5 °C water temperature rise needs 23.3 ÷ (4.18 × 5) = 1.11 kg/s, about 67 L/min; an air-cooled unit with a 10 °C cooling-air rise needs 23.3 ÷ (1.005 × 10) = 2.3 kg/s, about 7,400 m³/h of cooling air.
The same 500 cfm from a two-stage piston compressor discharging at about 150 °C raises the sensible load to 0.260 × 1.005 × 105 = 27.4 kW, and the total to about 40 kW. Discharge temperature must therefore be part of any aftercooler enquiry.
Aftercooler and oil cooler in screw compressors
Oil-injected screw packages usually carry the oil cooler and the aftercooler as two sections of one combined cooler, sharing the fan. The oil cooler removes most of the compression heat, typically 70–80% of package input power; the aftercooler’s sensible load is much smaller, about 12% of input for the machine above (assuming about 95 kW package input). Because both sections share one face and one fan, dust or lint on the core raises the oil temperature and the air outlet temperature together. Clean both sides of the core, and check the air outlet temperature as well as the oil temperature, after any cooler service.
Separator efficiency and what reaches the dryer
A moisture separator does not remove vapour; it removes liquid droplets formed in the aftercooler. Its efficiency is quoted as the fraction of that liquid captured at rated flow. In the example above, with 16.4 kg/h of liquid formed:
- At 99% efficiency, 0.16 kg/h of liquid passes on, about 4 L/day over 24 hours.
- At 95% efficiency, 0.82 kg/h passes on, about 20 L/day, as liquid slugs the refrigerated dryer is not designed to receive.
Centrifugal separators need velocity to work: below about a third to a half of rated flow, droplets are not spun out efficiently; above rated flow, liquid is re-entrained from the bowl. Size to the actual flow range, not the pipe size, and fit a drain that matches the condensate rate. A float or timer drain that is adequate in May may be overwhelmed by several times the liquid in August (compare the two intake columns in the approach-temperature table).
Aftercooler and separator faults: symptom, cause and check
| Symptom | Likely cause | Check |
|---|---|---|
| Liquid water downstream of the separator | Drain blocked, strainer choked or drain undersized; separator flooded | Open the manual drain bypass; time how long liquid runs; test the drain on a timed cycle |
| Outlet more than 15 °C above cooling air | Fins choked with dust or lint, fan fault, hot-air recirculation | Measure cooling air inlet and air outlet with one thermometer; clean the core; check fan direction |
| Water-cooled outlet hot | Scale in tubes, low water flow, hot cooling tower water | Water inlet and outlet temperatures and flow; tube inspection |
| Water in the air line after a stoppage (water-cooled) | Tube leak letting water into the air side when air pressure falls | Bubbles in cooling water return when running; pressure test the bundle |
| Separator drains nothing in summer | Often normal: dry intake air is not saturated at the outlet (see table). Otherwise a blocked drain | Calculate from intake RH; open manual bypass to confirm |
| Wet air on monsoon mornings only | Pipework downstream colder than the aftercooler outlet; no drain legs | Pipe surface temperature; slope and drain points on the header |
| Milky or oily condensate | Oil carry-over from a worn separator element or high oil level | Separator differential, oil level, oil consumption |
How Reddy Compressors helps
We service the complete moisture-management chain on all major brands, including ELGi, Atlas Copco, Ingersoll Rand, KAESER and Chicago Pneumatic, as an independent multi-brand specialist. Our engineers clean and inspect air- and water-cooled aftercoolers, check separator performance, replace or upgrade drains, and supply oil-water separators and spare parts. Where a dryer is struggling, we check what is reaching it before recommending repairs or replacement.
For portable diesel compressors used for sandblasting or cable blowing, we supply hire machines with aftercoolers and moisture separators fitted from our own rental fleet. Our office is at Tarnaka, Secunderabad; service teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment and spares across India.
What to send us
- Compressor make, model and serial number, with a nameplate photograph.
- Discharge pressure and typical running hours per day.
- Where water appears, and photographs of drains, separators and the compressor room.
- Dryer type and model, and any dew-point readings.
- Summer compressor-room temperature, if known.
- Lubricant type, for oil-water separator selection.
Frequently asked questions
Do I still need a dryer if my compressor has an aftercooler?
For most plants, yes. An aftercooler removes the bulk liquid, but the air leaving it is still saturated at its outlet temperature. As it cools in the receiver and pipework, more water condenses. If tools, instruments or the process need dry air, a refrigerated dryer, giving a pressure dew point of about +3 °C, or a desiccant dryer for lower dew points, is still required.
Why is there water in my compressed air lines even with a dryer?
The most common causes are failed or isolated drains, a bypass valve left open, a dryer overloaded by hot inlet air from a poorly ventilated room or fouled aftercooler, and drops taken from the bottom of the header. Check upstream equipment and every drain before blaming the dryer. Measure the pressure dew point after the dryer to confirm its actual performance.
How much condensate does an air compressor produce?
It depends on inlet humidity, temperature, flow and pressure. As an illustration, a 10 m³/min compressor running on a humid 35 °C day can condense roughly 12 kg of water an hour at the aftercooler separator, close to 300 litres over a 24-hour shift pattern, with more removed in the dryer. Monsoon months produce far more than dry winter months.
Which type of automatic drain is best for a compressor?
Electronic level-sensing drains generally waste the least air because they open only when liquid is present, and many give an alarm output. Timer drains are cheap but waste air if set too long and flood if set too short. Float drains need no power but can stick on oily, dirty condensate. Choose by condensate volume, contamination, pressure and criticality.
Can compressor condensate be discharged into the drain?
Not directly from an oil-injected compressor. Its condensate contains lubricant and must pass through a suitable oil-water separator first, with the water fraction then discharged according to site and local requirements. Emulsifying oils need a separator designed for them. Condensate from oil-free compressors has no compressor oil but should still be handled according to site rules.
Why does my aftercooler outlet temperature rise in summer?
An air-cooled aftercooler can only cool the compressed air to a margin above the air it receives. When ambient rises, or hot exhaust recirculates into the compressor room, the outlet rises with it. Dust and oil on the fins make it worse. Clean the cooler, duct hot air out of the room and confirm adequate fresh-air intake.
What is aftercooling in an air compressor?
Aftercooling is cooling the hot compressed air immediately after the compressor, so that most of its water vapour condenses while it can still be separated and drained. Air leaves an oil-injected screw at about 80–95 °C and a piston compressor at 150 °C or more. An aftercooler brings it to within about 10–15 °C of the cooling air, or 5–10 °C of the cooling water. In humid Indian weather this removes roughly two-thirds or more of the water before the dryer, and protects the dryer from overload.
What is the difference between an aftercooler and an oil cooler on a screw compressor?
The oil cooler cools the injected oil, which carries most of the compression heat, typically 70–80% of the input power, and returns it to the airend. The aftercooler cools the compressed air leaving the separator tank so that water condenses out. On most packaged screws both are sections of one combined cooler sharing a fan, so a dirty core raises both the oil temperature and the air outlet temperature. Clean the whole core and check both temperatures after service.
What approach temperature should an aftercooler achieve?
A clean air-cooled aftercooler usually discharges about 10–15 °C above the temperature of the air entering its fins. A clean water-cooled aftercooler usually discharges about 5–10 °C above the cooling water inlet. A larger gap points to fouled fins or tubes, low fan or water flow, or hot-air recirculation. It matters because every 5 °C higher outlet temperature sends roughly 30% more water vapour to the dryer, which may then fail to reach its rated dew point.
Why does my moisture separator not drain any water in summer?
On a hot, dry day it can be normal. At 42 °C and 20% RH intake, air compressed to 7 bar(g) and cooled to 50 °C is barely saturated, so very little liquid forms and the separator stays almost dry; the water passes on as vapour for the dryer to remove. If the intake air is humid, or the aftercooler outlet is below about 45 °C, there should be liquid. Then open the manual bypass: if water runs out, the automatic drain or its strainer is blocked.
Related pages
Field guide
The seven stages of compressed-air treatment
Air leaving a compressor is compressed, but rarely clean. It carries heat, water vapour and condensate, oil aerosol and vapour, dust, rust and, in sensitive work, microorganisms. Compression concentrates these contaminants, and as the air cools the water condenses. Treatment is a layered defence: each stage removes one kind of contamination and prepares the air for the next.
Not every system needs all seven. The order and the number of stages follow the air quality the process requires.
| Stage | Job | Without it |
|---|---|---|
| 1. Aftercooler | Cuts air temperature to near the cooling medium and turns vapour into removable liquid. Air-cooled, water-cooled, built in or external. | More moisture downstream, heavier dryer load, condensation in receivers and pipes, corrosion |
| 2. Moisture separator | Removes bulk liquid by centrifugal action or impingement; condensate leaves through a drain (timed, float, level-sensing or zero-loss). | Water in pipelines, corroded valves and actuators, paint defects, freezing in cold sites |
| 3. Refrigerated dryer | Cools air in a refrigeration circuit, drains the condensate and reheats the air. Typical pressure dew point about +3 °C. | Wet air for general work; not enough for freezing pipes or very dry air |
| 4. Pre-filter | Coalescing filter takes liquid droplets, oil aerosol, rust and scale before the dryer. | Oil coats desiccant, dew point rises, desiccant and dryer fail early |
| 5. Adsorption dryer | Twin towers of alumina, silica gel or molecular sieve; one dries while the other regenerates. Typical dew points −20, −40 or −70 °C. | Air not dry enough for instruments, pharma, electronics, outdoor lines |
| 6. After-filter | Catches desiccant dust and fine particles leaving the dryer. | Dust reaching valves, cylinders, nozzles and product-contact points |
| 7. Point-of-use filter | Final protection at the process: particulate, coalescing, carbon, sterile, local separators, regulators. | Pipe rust, scale and condensate picked up in distribution reach the process |
Choosing an adsorption dryer
- Heatless uses a share of the dried air to regenerate. Simple and reliable, but it consumes purge air (see the purge worksheet).
- Heated purge adds a heater to cut the air needed for regeneration.
- Blower purge heats ambient air for regeneration and nearly removes compressed-air loss.
- Heat of compression uses the compressor’s own heat and can be very efficient where the installation suits it.
Compare them on flow, pressure, inlet temperature and moisture load, required dew point, energy, regeneration method, duty cycle and ambient conditions.
Receivers and condensate
A wet receiver ahead of the dryer steadies pressure, adds condensate separation and cuts compressor cycling; a dry receiver after the dryer buffers demand. Every receiver needs a relief valve, gauge, drain, isolation valve, inspection access and certification. Condensate holds water, oil, solids and corrosion products, so it needs reliable automatic drains, drain monitoring and, for oil-lubricated machines, oil-water separation before disposal. One failed drain can defeat a well-designed train.
Specify by measurable class
“Clean”, “dry” and “oil-free” are not specifications. ISO 8573-1 classifies air by solid particles, water and total oil. A workshop, a pharmaceutical filling line, a paint booth and instrument air need different classes. The sequence is: find the most sensitive process, set the ISO 8573-1 class, set the pressure dew point, choose stages, calculate pressure loss, size for actual conditions, then verify by measurement.
Pick a dew point below the lowest temperature anywhere in the downstream piping, or water will condense after the dryer. Monitor it where it matters: pharma, instrument air, outdoor lines, electronics, food and critical coating.
The energy cost of treatment
Every element adds pressure drop, and the compressor must run at a higher pressure to make up for it. Watch differential pressure, change filters on condition, size equipment for real flow, keep drains working and avoid over-drying. The aim is the required air quality at the lowest practical lifecycle energy cost.
Eight common mistakes
- Sizing equipment from pipe size instead of flow, pressure, inlet temperature, ambient and dew point.
- Ignoring pressure correction factors (see the correction worksheet).
- Fitting a dryer without pre-filtration.
- Neglecting condensate drains.
- Choosing a lower dew point than the process needs.
- Overlooking filter and dryer pressure drop.
- Giving every application the same air quality.
- Not monitoring dew point, differential pressure and drains.
For critical sites, plan redundancy: duty and standby dryers, duplex filtration, redundant drains, remote alarms and continuous dew-point monitoring.
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.
Moisture load at the dryer inlet
Estimate how much water a compressor puts into the air system each hour, and how much a dryer or aftercooler must remove.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Compressor flow (m³/min free air): 12
- Intake air temperature (°C): 35
- Intake relative humidity (%): 70
- Delivery pressure (bar g): 7
- Air temperature at dryer inlet (°C): 38
Result
- Water entering with intake air: 19.9 kg/h
- Water still held at dryer inlet: 4.1 kg/h
- Condensate removed before the dryer: 15.8 kg/h (about 379 L per day)
Method. Water in = air × relative humidity × saturation vapour density at intake (Magnus formula). Water the air can hold after compression = humidity ratio at saturation at the dryer inlet temperature and pressure × dry-air mass per m³ of intake air. Condensate = the difference. Flow is free air at intake conditions; indicative.
What to take from it. This water must be removed somewhere: aftercooler separator, drains, dryer. A failed drain sends it into the pipework.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.