How To Choose The Right Size Of Marine Charge Air Cooler?
The sizing determines charge‑air outlet temperature, pressure drop, engine power output and NOₓ performance. Wrong size leads to over‑heating, power loss, high back‑pressure or condensation risk.
1. Collect core engine data (most critical)
Get these parameters from engine datasheet / original OEM drawing:
- Engine type: 2‑stroke main engine / 4‑stroke medium‑speed main / auxiliary generator engine
- Engine rated power & rated speed (kW / HP, RPM)
- Turbocharger model, mass flow rate of charge air (kg/s)
- Charge‑air inlet temperature (after turbocharger): typically 160 ~ 225 °C
- Required charge‑air outlet temperature: normally 35‑50 °C
- Allowable air‑side pressure drop:
- Large 2‑stroke main engine: ≤ 3‑5 mbar
- 4‑stroke medium‑speed engine: ≤ 5‑8 mbar
- Cooling medium conditions:
> Most marine CAC uses LT closed fresh water circuit: cooling water inlet temp, water flow rate, working pressure.
> For replacement case: Original CAC part number, core dimensions, fin density, tube count, flange size are the fastest reference. Never only copy outer housing size; core heat transfer area is decisive.
2. Calculate heat rejection duty
Heat load Q (kW) = air mass flow × specific heat of air × (T_air_in − T_air_out)
This Q value is the base for heat‑exchanger core sizing.
- Larger heat load → bigger heat transfer area (more tubes / longer tubes / larger fin area).
- Keep reasonable fin density: too dense fins = high air‑side pressure drop and easy fouling in marine dirty air environment.
3. Air‑side sizing rules (avoid excessive pressure drop)
1. Air velocity across fin tube bundle: keep 4‑7 m/s for marine charge air cooler.
- Too high: big pressure drop, turbo backpressure rises, engine power drops, fin erosion.
- Too low: need oversized unit, high cost, risk of water condensation inside CAC.
2. Fin pitch: marine service usually 2.0‑3.2 mm. Avoid ultra‑dense fins (<1.8 mm), they clog quickly with oil mist and soot.
3. Cross‑section air flow area: calculated from air mass flow and target velocity. This sets the core height × width.
4. Core tube length: determined by required heat transfer area and water‑side velocity limits.
4. Water‑side sizing rules
- Water velocity inside tubes: 1.2‑2.2 m/s for Cu‑Ni 90/10 tubes
- Too low: fouling and sediment build‑up.
- Too high: tube erosion, shorten service life.
- Check water‑side pressure drop to match ship LT cooling circuit pump capacity.
5. Material impacts on sizing
Same heat duty with different materials needs different core size:
- Cu‑Ni 90/10: high heat transfer coefficient → compact core, standard marine choice
- Titanium: lower thermal conductivity → need 10‑15% larger heat transfer area vs Cu‑Ni 90/10
- Stainless steel: also requires increased area compared to copper‑nickel.
6. Special marine derating factors (important for ship operating conditions
Add derating margin for real‑world marine conditions:
1. Fouling factor: marine CAC fouling factor 0.00017‑0.00035 m²·K/W, account for soot, oil mist, deposit. Do not use clean‑lab ideal calculation.
2. Warm sea water condition: in tropical zones, cooling fresh water inlet temp can go up to 38‑40 °C, reserve extra margin.
3. Partial load operation: engine runs below rated power most of ship service life; avoid excessive condensation at low load.
Recommended safety margin: 10 ~15 % extra heat transfer area over theoretical calculation.
7. Replacement vs new‑build selection difference
Replacement (drop‑in 1:1)
1. Priority: match original core heat transfer area, air‑side flow area, tube layout, fin pitch.
2. Housing dimensions & flange positions must fit existing ship piping.
3. If switching tube material (Cu‑Ni → Ti), increase core area within available housing space or modify housing.
4. Pressure test and thermal performance test must meet OEM specification.
New‑build custom CAC
1. Start from engine heat‑duty & flow data, do thermal simulation.
2. Balance heat transfer efficiency, pressure drop, footprint, weight, class requirement.
8. Common mistakes to avoid
❌ Only copy outer casing dimensions while changing tube / fin parameters → bad thermal performance
❌ Ignore fouling margin, works well on test bench but overheats after months on board
❌ Too dense fins for marine dirty charge air → rapid clogging
❌ Switch to titanium without upsizing core → high charge‑air outlet temperature
❌ Overlook allowable air‑side pressure drop, causes turbocharger operational issues
Information list you need to provide to heat‑exchanger manufacturer for correct sizing
1. Engine model, rated power, RPM
2. Charge‑air mass flow, air in / out temperature requirement
3. Cooling medium: medium type, inlet temp, available water flow
4. Original CAC part number / core drawing (if replacement)
5. Tube material selection (Cu‑Ni90/10, Ti, SS316)
