Cooling Water System — Most Asked Questions
15 questions asked about cooling water system across the General and Fn 4b papers, heaviest first.
Learn the system first
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1 Check sea water in/out temperature difference. Les.s means poor efficiency of the cooler (must be high) Pe
Answer
Read cooler health in temperature difference: wide seawater rise with strong flow means full heat transfer; narrow means fouling.
- Compare inlet and outlet seawater at steady load with flow proved normal.
- High difference carries design heat away; low difference with hot outlets means scale or sludge throttling tubes.
- Confirm with charge-air, oil and jacket temperatures moving together before opening.
- Clean, vent and re-prove — the difference should climb back with flow unchanged.
Surveyor follow-up: difference low with outlets hot and flow normal — air lock or fouling, and which vent proves it?
Full theory: Marine Central Cooling Water System (HT / LT / SW) -
2 Feel over cooler shell, upper hot, middle warm, down cool is normal
Answer
A healthy cooler grades hot to cool down its shell: hot top, warm middle, cool bottom with even flow.
- Hot inlet top, tempered middle, cool outlet bottom marks full circulation without bypass.
- Cold spots mean starved passes; hot bottom means short-circuiting or reversed flow.
- Air locks flatten the gradient — vent until the profile returns with steady temperatures.
- Feel with flow proved and compare to baseline logs before condemning tubes.
Surveyor follow-up: whole shell uniformly hot with normal flow — bypass, air or overload, and what do you touch next?
Full theory: Marine Central Cooling Water System (HT / LT / SW) -
3 Cooler maintenance carried out for optimum efficiency
Answer
Coolers hold efficiency by clean tubes and live anodes: open, clean, prove zincs, box up tight.
- Sea side fouls with mud, shell and scale that insulate tubes — open water boxes and clean mechanically or by air.
- Oil side fouls with waxy and additive deposits that throttle heat transfer the same way.
- Renew wasted zinc anodes, prove gaskets and covers, and flush air before returning to service.
- Trend inlet-outlet temperature split at steady load — closing split condemns before alarms do.
Surveyor follow-up: temperature split closing a week after cleaning — re-fouling, airlock or overload, and what proves it?
Full theory: Plate Heat Exchangers: Working, Operation, Maintenance & Troubleshooting -
4 Essential cooler for optimum efficiency can be maintained by controlling of temperature of fluid or sea water
Also asked: “• Essential cooler for optimum efficiency can be maintained by controlling of temperature of fluid or sea water”
Answer
Hold coolers in their window by throttling sea or bypassing charge: steady temperatures, full flow, clean faces.
- Control sea-water flow or recirculate the cooled medium so outlet temperatures sit on maker figures.
- Never starve flow to chase temperature — low flow scales and overheats faster than high flow corrodes.
- Keep scavenge, jacket and oil temperatures married: one drifting pulls the other two off design.
- Log temperatures against load daily; a wandering controller fails the same way a fouled plate does.
Surveyor follow-up: outlet cool with inlet hot and flow throttled hard — controlled or starved, and what reading decides?
Full theory: Charge Air Cooler & Scavenge Air System — Density Thermodynamics, Fouling Diagnostics & Condensate Control -
5 To prevent thermal shocking of liner if temp is too low. Checking cooler efficiency
Also asked: “• To prevent thermal shocking of liner if temp is too low. Checking cooler efficiency”
Answer
Cold jackets crack liners: warm through gently, hold maker minimums, and prove cooler duty by temperature split.
- Low jacket temperature with hot firing shocks liner metal into cracking, especially thick sections.
- Warm up on bypass and low load until inlet, outlet and expansion-tank levels steady in range.
- Prove cooler efficiency by design temperature drop at rated flow — collapsed drop means fouling or bypassing.
- Never slam cold sea through a hot engine to cool it — the cure cracks what the heat only stressed.
Surveyor follow-up: liner cracked a week after a cold start with coolers efficient — overcooling, material or fuel, and what log proves it?
Full theory: Cylinder Liner Wear, Calibration & Cold Corrosion -
6 When burning high viscosity, carbon rich fuels, it is of paramount importance that the correct operating limits be maintained in the engine, Scavenge air temperature, cooling water temperature and cylinder oil feed rates
Also asked: “• When burning high viscosity, carbon rich fuels, it is of paramount importance that the correct operating limits be maintained in the engine, Scavenge air temperature, cooling water temperature and cylinder oil feed rates must all be carefully monitored and adjusted. In order to cope with the possible problems associated with deterioration in fuel quality, the normal well-proven standards applica”
Answer
Heavy fuel forgives nothing: hold scavenge, jacket and oil feed on figures or wear and fouling run away.
- Keep scavenge temperature on design so charge density and ignition stay predictable.
- Hold jacket temperatures above acid dew point with matched alkaline cylinder oil at load-rated feed.
- Feed cylinder oil by brake power — too little scuffs rings, too much lacquers grooves and fouls ports.
- Watch exhaust temperatures per unit: one climbing cylinder betrays its injector, rings or oil feed first.
Surveyor follow-up: one unit hot with scavenge and jackets on figures — fuel valve, rings or oil feed, and what do you cut first?
Full theory: Charge Air Cooler & Scavenge Air System — Density Thermodynamics, Fouling Diagnostics & Condensate Control -
7 Expansion tank reasons
Answer
The expansion tank holds a constant head on the closed cooling system and takes up the water as the engine warms — that is its whole job.
- Designed to maintain a constant head and provide an air cushion, and to allow an increase in water volume as the engine warms up.
- It is not there to reduce temperature — the small heat loss from its surface is incidental and the tank is not insulated for it.
- Nor is it there to reduce turbulence; a small tank can actually increase it, which is why size is chosen for the volume, not the flow.
- Its level is the system’s expansion indicator, so a rising or falling level is a fault in the system or a water loss.
Surveyor follow-up: expansion tank level falling steadily with the engine at temperature — is it a leak, a vacuum or the vent?
Full theory: Cooling, Lubrication and Fuel — Systems and the Tanks Checked Before Starting -
8 Operation Of Shell & Tube Type Cooler
Answer
Seawater runs inside the tubes and the oil or water to be cooled runs outside them; baffles direct the flow across the tube bundle.
- Seawater is in contact with the inside of the tubes, tube plates and water boxes; the fluid cooled is outside them in the shell.
- Baffles direct the shell-side liquid across the tubes as it flows through, so the whole surface is used rather than a fraction.
- A two-pass flow arrangement is shown in the diagram, though straight flow is common in small coolers.
- Flow is controlled by a bypass or a control valve, often a wax-operated type actuated by a temperature sensor.
Surveyor follow-up: cooler losing efficiency with a clean tube bundle — is the baffle, the valve or the flow rate?
Full theory: Shell & Tube Heat Exchangers: Working, Construction, Overhaul & Retubing -
9 Purpose of division plate in cooler
Answer
A division or baffle plate in a cooler turns the flow across the tubes and supports the stack — without it the fluid takes the easy path.
- Brass circular baffles give radial flow to the fluid and support the tube stack at the same time.
- More recent designs use guided flow: radial fins between the tubes, so flow is guided alternately out and in from section to section.
- That gives greater heat transfer surface, better transfer, and a lower metal surface temperature.
- For oil heaters it also reduces the risk of the oil cracking and the fouling that cracking causes.
Surveyor follow-up: cooler short-circuiting with a missing plate — is the transfer, the fouling or the metal temperature?
Full theory: Shell & Tube Heat Exchangers: Working, Construction, Overhaul & Retubing -
10 Purpose of intercooler
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11 Two copper-base alloys have been widely used for seawater handling, namely aluminum brass and 90- 10 Cu-Ni although in recent years, particularly for large diameter piping there has been a strong trend towards 90-10 Cu-N
Also asked: “Two copper-base alloys have been widely used for seawater handling, namely aluminum brass and 90- 10 Cu-Ni although in recent years, particularly for large diameter piping there has been a strong trend towards 90-10 Cu-Ni. When using non-ferrous piping the system must be designed on the basis of water velocity in order to avoid impingement attack, even when some turbulence (which will give rise to”
Answer
Aluminium brass for ordinary seawater service, 90-10 copper-nickel where cavitation and corrosion bite hardest — the trend has moved to Cu-Ni.
- Both alloys are used for seawater handling; aluminium brass suits less severe service and 90-10 Cu-Ni the harder service.
- In recent years, particularly for large diameter piping, there has been a strong trend towards 90-10 Cu-Ni.
- Cu-Ni resists corrosion and cavitation erosion far better, and has been proposed in higher-nickel alloys for the toughest duties.
- For ship’s side sea water inlet valves, steel or other ductile material is required; ordinary grey cast iron is unreliable.
Surveyor follow-up: aluminium brass suction line eroding at the impeller eye — is the material, the velocity or the impeller design?
Full theory: Marine Materials, Metallurgy & Mechanical Testing -
12 Valve Bodies – Non-Ferrous Systems
Answer
Non-ferrous valve bodies are used where salt water or corrosion decides it: bronze, gunmetal, copper-nickel and aluminium bronze.
- Bronze, gunmetal and aluminium bronze suit seawater service; Cu-Ni is chosen where cavitation and erosion bite hardest.
- For salt water, casing gunmetal is the usual choice where cast iron would corrode — impellers in aluminium bronze.
- Shafts are stainless and bearing-ring seals leaded bronze, matching the casing material to the service.
- Compatible metals, coatings or cathodic protection are the alternatives when the body itself has to be steel.
Surveyor follow-up: cast iron valve body in a seawater cooling line corroding at the flange — is the material, the isolation or the coating?
Full theory: Marine Materials, Metallurgy & Mechanical Testing -
13 cooling water tests
Answer
Cooling water is tested like any other system: chemistry for corrosion and scale, and gauges for the pressures and temperatures.
- Check pH, chloride and hardness on a cooled sample — the same tests the boiler water takes, for the same reasons.
- Watch the sea water side for scaling and the fresh side for corrosion, with a sample point at the cooler inlet and outlet.
- Test the conditioning chemicals — inhibitors, biocides and antiscalant — against the analysis, not on a fixed calendar.
- Read pressures and temperatures against maker figures at each load; a rising fresh-side temperature at constant load is the finding.
Surveyor follow-up: fresh-side temperature climbing with the sea strainers clean and chemicals on schedule — is the core, the sea or the load?
Full theory: Cooling, Lubrication and Fuel — Systems and the Tanks Checked Before Starting -
14 jacket Cooling water system,asked to explain
Answer
Jacket cooling is a closed loop: expansion tank, engine-driven pump, jackets, cooler and back — the head is constant and the expansion is absorbed.
- Each auxiliary has its own engine-driven pump, with the engine outlet led through a valve to the jacket water cooler and back to the pump suction.
- The expansion tank maintains a constant head and takes up the water as the engine warms, with an atmospheric vent to the tank.
- A connection to the main engine jacket system may be used for warming or as a standby source on a medium-speed set.
- Never steam a hot jacket: a hot engine is warmed through, because cold water on hot cast iron cracks the head.
Surveyor follow-up: expansion tank level dropping with the cooler clean and the pump proved — is it a leak, a vacuum or the vent?
Full theory: Cooling, Lubrication and Fuel — Systems and the Tanks Checked Before Starting -
15 o Heat transfer efficiency is lower compared to plate type cooler o Cleaning and maintenance is sometimes difficult since a tube HE requires enough space at one end to remove the tube nest o Capacity of tube HE cannot be
Also asked: “o Heat transfer efficiency is lower compared to plate type cooler o Cleaning and maintenance is sometimes difficult since a tube HE requires enough space at one end to remove the tube nest o Capacity of tube HE cannot be increased, once made. o Requires more space in comparison to plate HE for the same capacity”
Answer
Three limits of the tube cooler: lower efficiency, harder cleaning, and a capacity you can only raise by adding tubes.
- Heat transfer efficiency is lower than a plate type cooler for the same duty, so it needs more surface.
- Cleaning is difficult because there must be enough space at one end to withdraw the whole tube nest.
- Capacity cannot easily be increased by adding plates as it can in a plate cooler — a tube nest is a welded fabrication.
- Vent cocks are needed to purge air and a plug or cock at the bottom to drain, and the clearance for tube withdrawal is a design point, not a detail.
Surveyor follow-up: cooler sized with no room to draw the tube nest — what problem has the ship signed for?
Full theory: Plate Heat Exchangers: Working, Operation, Maintenance & Troubleshooting
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