Marine Engine Cooling System HT LT Circuit

Modern 2‑stroke /4‑stroke marine main engines use two separate closed fresh‑water glycol circuits: HT (High‑Temperature) circuit and LT (Low‑Temperature) circuit. Neither circuit lets seawater touch engine components directly; seawater only cools the LT loop via the central cooler.

 

HT Circuit (Jacket Water Circuit)

Operating temperature: 75‑88 °C (typical set‑point ~82‑85 °C) Main cooled components:

Cylinder liners, cylinder heads, exhaust valves, engine jacket

Some two‑stage CAC designs: high‑temperature first‑stage charge‑air cooling

Function:

Keep hot combustion‑zone parts at stable working temperature.

Avoid cold corrosion: if jacket water runs too cold, sulphuric acid condenses from heavy‑fuel exhaust and attacks liners.

Prevent local boiling / cavitation if temperature gets too high.

Flow path: HT pump → engine cylinder jackets & heads → thermostatic 3‑way valve. Part of water recirculates back to pump for fast warm‑up; excess heat transfers to LT circuit via heat exchanger / mixing valve.

LT Circuit (Low‑Temperature Fresh Water)

Operating temperature: 32‑42 °C inlet to consumers Main cooled components:

Charge air cooler (second‑stage / full‑LT CAC)

Lube oil cooler

Turbocharger bearings, generator sets, compressors, auxiliaries.

Function: Supply cold treated fresh water for components that need much lower temperature than jacket water, especially the charge‑air cooler to get compressed intake air down to 45‑55 °C before cylinders.

Flow path: LT pump → charge air cooler + lube oil cooler + auxiliaries → central cooler (seawater‑fed plate/shell‑tube heat exchanger). Seawater removes heat from LT fresh water; cooled fresh‑water returns to LT pump. 3‑way thermostat mixes bypass flow to stabilise LT temperature regardless of seawater temperature.

Item	HT Circuit	LT Circuit
Medium	Closed treated fresh‑water / glycol	Closed treated fresh‑water / glycol
Typical working temp	75‑88 °C	32‑42 °C
Primary loads	Cylinder jacket, heads, exhaust valves	Charge air cooler, lube oil cooler, auxiliaries
Heat rejection	Rejects heat into LT circuit	Rejects total system heat to seawater via central cooler
Risk focus	Cold corrosion at low load	Fouling on CAC / lube oil cooler

 

How they work together

HT loop absorbs combustion heat from engine block and heads.

HT excess heat is dumped into the LT circuit (heat‑exchanger or controlled mixing).

LT circuit carries all combined heat (from HT + CAC + lube oil) to the central cooler.

Seawater passes through central cooler, takes heat away and discharges overboard.

Cooled LT fresh‑water loops back to CAC and other consumers.

 

Common practical issues

LT water too warm: charge‑air outlet temperature rises → higher NOₓ, engine derate. Often caused by dirty central cooler, sea‑water filter blockage.

HT temperature too low at low load: risk of sulphuric acid cold corrosion on cylinder liners.

Charge‑air cooler fouling (air‑side soot/oil) increases heat load on LT circuit.