LNG Carrier Auxiliary Engine Seawater Cooling Optimization

Scenario

A newly delivered 174,000 m³ LNG carrier operating in the warm, high-salinity waters of the Arabian Gulf. During sea trials, auxiliary diesel generators (800 kW each) experienced jacket water temperatures exceeding 85°C at 90% load — above design limit — due to undersized original seawater cooling capacity. Ambient seawater temperature reached 36°C, limiting ΔT availability. Constraints included: no pump replacement (existing 300 m³/h centrifugal pump fixed), no additional hull penetrations, and mandatory compliance with IACS UR Z17 corrosion guidelines.

Given Data

• Mass flow rate of jacket water: 4.2 kg/s
• Specific heat capacity of jacket water: 4178 J/(kg·K)
• Temperature difference of jacket water: 11.5 K (target inlet 92°C → outlet 80.5°C)
• Density of seawater: 1032 kg/m³
• Specific heat capacity of seawater: 3890 J/(kg·K)
• Temperature difference of seawater: 3.8 K (inlet 35.2°C → outlet 39.0°C)

Calculation

1. Heat duty to be rejected: $$ Q = 4.2 \times 4178 \times 11.5 = 202,700 , \text{W} $$

2. Required seawater mass flow rate: $$ \dot{m}_{sw} = \frac{202,700}{3890 \times 3.8} = 13.74 , \text{kg/s} $$

3. Required volumetric flow rate: $$ \dot{V}_{sw} = \frac{13.74}{1032} = 0.01331 , \text{m}^3/\text{s} = 47.9 , \text{m}^3/\text{h} $$

Result and Decision

The calculator yielded 0.01331 m³/s, confirming the existing 300 m³/h pump (0.0833 m³/s) was grossly oversized — but system pressure drop across the aged heat exchanger (measured at 1.8 bar) indicated severe internal restriction. Engineers concluded the bottleneck was not flow capacity, but flow distribution and fouling. They replaced only the heat exchanger core with a high-efficiency, corrosion-resistant CuNi 90/10 shell-and-tube unit featuring enhanced turbulence promoters, retaining the original pump and piping. Post-modification, jacket water ΔT stabilized at 11.4 K with seawater ΔT rising to 4.1 K — validating improved thermal effectiveness.

Lesson

A high-calculated seawater flow rate doesn’t always mean you need more flow — it may reveal hidden inefficiencies (e.g., fouling, poor distribution, or mismatched exchanger geometry); always validate with field-measured pressure drop and infrared thermography before upgrading pumping infrastructure.