Why cooling water matters on water-cooled compressors
Water-cooled screw, reciprocating and centrifugal compressors reject almost all of their input power as heat, most of it through the oil cooler, intercoolers and aftercooler. When those exchangers foul or corrode, the heat stays in the machine. Oil-injected screw discharge temperature climbs above its normal band of about 75–95 °C, reciprocating interstage temperatures rise, centrifugal stages lose efficiency and surge margin, and the aftercooler sends warmer, wetter air to the dryer.
The consequences develop slowly and then suddenly: high-temperature trips on hot afternoons, shortened oil life, varnish on airend and bearings, overloaded refrigerated dryers, water in the distribution system and, eventually, tube leaks that let water into the oil or the air. Water chemistry is therefore a reliability topic, not just a utility-department topic.
Know the cooling system before reading a water report
The same analysis means different things in different systems. Identify which one you have:
| System type | How it behaves | Main risks |
|---|---|---|
| Open recirculating (cooling tower) | Evaporation concentrates dissolved salts; air contact adds oxygen, dust and organisms | Scale, corrosion, biofouling, silt from dusty air |
| Closed loop with dry cooler or plate exchanger | Little make-up water; chemistry stays stable if the loop is tight | Corrosion from oxygen ingress or depleted inhibitor, glycol degradation |
| Once-through (borewell, canal or municipal) | Fresh water each pass, no concentration | Hardness and iron from borewell water, silt, high water use |
In Telangana and Andhra Pradesh many plants use borewell make-up water, which can be hard and high in dissolved solids. Cooling towers in dusty industrial estates also pick up a heavy load of airborne silt in the dry months. Both factors push towards scale and deposits unless treatment and blowdown are managed.
Exchanger construction and materials
Water quality is only acceptable or unacceptable for a given exchanger. Record what the water actually touches:
- tube or plate material: copper, cupronickel, admiralty brass, carbon steel, stainless steel;
- shell, channel heads and tube sheets, and any coatings;
- gaskets, O-rings and brazing materials on plate or brazed exchangers;
- whether water runs in the tubes or the shell, and whether tubes can be mechanically cleaned;
- sacrificial anodes, if fitted, and their condition.
Copper alloys are sensitive to ammonia, sulphides and high velocity; stainless steels are sensitive to chlorides in stagnant or hot zones; carbon steel corrodes readily in oxygenated water without inhibitor. Brazed plate exchangers have narrow channels that block quickly with silt and cannot be opened for cleaning.
Which water tests matter and what they tell you
A single pH or hardness value cannot show whether water is suitable. A useful minimum panel, taken at a defined point and time, includes:
| Parameter | What it indicates | Typical concern |
|---|---|---|
| pH | Acidity or alkalinity | Low pH attacks metals; high pH with hardness favours scale |
| Conductivity / TDS | Total dissolved salts and cycles of concentration | Rising values show under-blowdown in a tower |
| Calcium and total hardness | Scale-forming potential | Carbonate scale on hot tube surfaces |
| Alkalinity (M and P) | Buffering and carbonate balance | Combined with hardness and temperature for scaling indices |
| Chloride | Aggressiveness to stainless steel and copper alloys | Pitting and crevice corrosion |
| Sulphate | Corrosivity and sulphate-reducing bacteria feedstock | Under-deposit and microbial corrosion |
| Iron and copper | Corrosion products in circulation | Active corrosion somewhere in the loop |
| Suspended solids / turbidity | Silt and debris | Deposits, erosion, blocked plate channels |
| Silica | Hard, glassy scale potential | Very difficult deposits at high concentration |
| Microbiological count | Bacterial and algal activity | Biofilm, slime, microbially influenced corrosion |
| Inhibitor residual / glycol strength | Whether treatment is present | Depleted protection in closed loops |
Scaling indices such as the Langelier or Ryznar index combine pH, hardness, alkalinity, TDS and temperature to show whether water tends to deposit or dissolve calcium carbonate. They are a guide, not a guarantee, because the hottest tube wall is much hotter than the bulk water sample.
Sampling that gives a fair picture
- Name the sampling point: make-up, tower basin, exchanger inlet, exchanger outlet or closed-loop drain.
- Flush the sample line before collecting, and use clean bottles suitable for the tests requested.
- Record date, time, water temperature, compressor load and recent dosing or blowdown.
- Take a make-up sample at the same time as the recirculating sample so cycles of concentration can be worked out.
- Send samples promptly, and repeat across seasons: a monsoon result can look very different from a May result.
- Keep results in one register alongside compressor temperatures so trends can be compared.
Scale, corrosion, fouling or simply low flow?
A rising discharge temperature or a warmer aftercooler outlet does not prove that the exchanger is scaled. Several causes give the same symptom:
- Scale: hard mineral layer on hot surfaces; water-side pressure drop changes little at first; worse in summer and at high cycles.
- Fouling: silt, debris, corrosion products or biofilm; pressure drop rises and flow falls; often patchy.
- Insufficient water flow: partly closed valves, worn pumps, air locks, blocked strainers or competing users on the same header.
- Warm inlet water: an undersized or poorly maintained cooling tower on a hot, humid day.
- Air-side or oil-side causes: higher compressor load, wrong oil, failed thermostatic valve, or a sensor in the wrong place.
Separate these before choosing acid cleaning or replacement. Cleaning a clean exchanger does not cure a low-flow problem, and repeated acid cleaning shortens tube life.
Measuring exchanger performance: approach temperature and heat balance
The most practical indicator is approach temperature: the difference between the cooled fluid outlet (air or oil) and the cooling-water inlet. Record it with the compressor at a known load and compare it with the commissioning figure or the maker’s design data. A steady rise in approach at similar load and water temperature points to fouling or flow loss.
A heat balance adds confidence. Water-side heat duty is flow × specific heat × temperature rise. For water, roughly 4.19 kJ/kg·K, so 1 m³/h of water warming by 5 K carries about 5.8 kW. If the measured duty is far below what the compressor power suggests, either the flow measurement is wrong or heat is leaving by another route. Use calibrated thermometers at proper wells, and record water flow from a meter or a measured fill time, not from a valve position.
Water-side pressure drop
Log the pressure difference across each exchanger. A rising difference at constant flow means deposits or blockage; a falling difference with lower cooling may mean bypassing or a pump problem. Never exceed the exchanger’s rated water-side pressure when isolating or flushing.
Treatment, cleaning and repair decisions
| Finding | Typical action | Caution |
|---|---|---|
| Rising TDS and hardness in tower | Adjust blowdown and dosing with the water-treatment supplier | Over-concentration saves water but scales exchangers |
| Silt and debris | Side-stream filtration, strainers, basin cleaning, mechanical tube cleaning | Brazed plate units cannot be opened; protect them with strainers |
| Carbonate scale | Controlled chemical descaling with suitable inhibited acid | Check compatibility with copper, brass and gaskets; neutralise and flush |
| Biofilm or slime | Biocide programme, physical cleaning of tower and dead legs | Biofilm shields deposits from inhibitors |
| Pitting, thin walls, leaking tubes | Pressure test, tube plugging within limits, retubing or replacement | Tube leaks can put water into oil or air |
| Closed-loop inhibitor depleted | Restore treatment and glycol strength; find the leak or oxygen source | Repeated top-up hides a loop leak |
Repair or replace when cleaning no longer restores approach temperature, when pressure testing shows leaks, or when wall thinning makes further chemical cleaning unwise. For a water-cooled compressor in a hot, dusty location, converting to air cooling or a closed loop with a dry cooler is sometimes the better long-term answer; that is a system decision worth evaluating with heat-load figures.
A practical monitoring routine
- Daily: water inlet and outlet temperatures, compressor discharge and oil temperatures, any alarms.
- Weekly: approach temperatures at known load, strainer condition, tower basin and fill condition.
- Monthly: water analysis panel and inhibitor residual; compare with make-up water.
- At each service: water-side pressure drop, visual check of accessible tube ends, leak check of oil and air sides.
- Before summer: clean towers and strainers, review blowdown, and confirm cooler capacity for 40–45 °C ambient days, and above 45 °C on open sites.
How Reddy Compressors helps
From our office at Tarnaka, Secunderabad, our service teams support water-cooled screw, reciprocating and centrifugal compressors of all major brands across Telangana and Andhra Pradesh. We are an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands. For cooling problems we:
- log compressor and cooling-water temperatures, flows and pressure drops to separate exchanger fouling from flow, load or sensor problems;
- inspect, clean and pressure-test oil coolers, intercoolers and aftercoolers, and arrange specialist retubing where needed;
- supply replacement coolers, thermostatic valves, gaskets and related spares, identified from the machine and confirmed for fit before quoting;
- review whether air-cooled or closed-loop alternatives make sense for the site;
- provide rental compressors from our own fleet while exchangers are out for repair.
We work alongside your water-treatment supplier; we do not sell treatment chemicals. Call +91 99590 60718 or +91 92468 88812.
What to send us
- Compressor make, model, serial number and nameplate photograph.
- Type of cooling system and source of make-up water.
- Recent water analysis reports, with sampling point and date.
- Logged discharge, oil and water temperatures at known load.
- Water flow, if known, and exchanger pressure readings.
- Photographs of exchanger tube ends, deposits or leaks.
Frequently asked questions
How does cooling water quality affect a water-cooled compressor?
Poor water quality builds scale, silt, corrosion products or biofilm on the water side of oil coolers, intercoolers and aftercoolers. These deposits insulate the tubes, so less heat leaves the compressor. Discharge and oil temperatures rise, oil life shortens, the dryer receives warmer and wetter air, and high-temperature trips become common in summer. Corrosion can also cause tube leaks that mix water into the oil or the air.
What water tests should I do for compressor cooling water?
Test pH, conductivity or TDS, calcium and total hardness, alkalinity, chloride, sulphate, iron, copper, suspended solids, silica and microbiological activity, plus inhibitor residual or glycol strength for closed loops. Sample the make-up and the recirculating water at the same time, record the sampling point and temperature, and repeat across seasons. Compare the results with the exchanger materials, not with a generic limit.
How do I know if my compressor aftercooler is scaled?
Measure approach temperature: air outlet temperature minus cooling-water inlet temperature, at a known load. A steady rise compared with commissioning values suggests fouling. Then check water flow, water-side pressure drop and inlet water temperature, because low flow or a warm cooling tower gives the same symptom. Inspect accessible tube ends. Confirm the cause before choosing chemical descaling or replacement.
Can borewell water be used for compressor cooling?
It can, but many borewell supplies are hard, high in dissolved solids and sometimes high in iron, so they scale exchangers quickly, especially in a cooling tower where evaporation concentrates salts. Have the water analysed, treat or soften it if needed, control blowdown, fit strainers, and check exchanger approach temperatures regularly. A closed loop with a dry cooler avoids most of these problems.
Is acid descaling safe for compressor oil coolers?
It can be, when the acid and inhibitor are chosen for the tube, plate and gasket materials, the concentration and contact time are controlled, and the exchanger is neutralised and flushed afterwards. Repeated or uncontrolled acid cleaning thins tubes, attacks copper alloys and damages gaskets. Pressure-test after cleaning, and only descale when measurements show scale is actually the cause.
When should a compressor heat exchanger be replaced rather than cleaned?
Replace or retube when cleaning no longer restores approach temperature, when pressure testing shows leaking tubes beyond the number that can be plugged within the maker’s limits, or when wall thinning and pitting make further cleaning risky. Water entering the oil or air side is a clear sign. Compare the cost and lead time of replacement with the risk of repeated unplanned trips.
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.
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.
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.
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.