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Marine engine cooling systems are the heart and soul of propulsion and power generation systems on a vessel. In an era with ever-increasing power densities and a more stringent emissions agenda, the significance and challenges associated with engine thermal management have expanded. The proliferation of Plate Heat Exchangers (PHE) as the optimal selection for engine cooling-owing to their heat exchange efficiency, and compact size, along with the need to precisely couple all the hydraulic, materials and operational properties of the system-for enhanced longevity have become standard practice.
Why Plate Heat Exchangers for Marine Engines?
Compared to a traditional shell and tube exchanger, a plate heat exchanger transfers up to five times the heat coefficient due to induced turbulence from corrugated plates. The efficiency of the PHE has the direct result of much smaller size and weight, a critical consideration for space-restricted marine engines. With their modular design, additional plates can easily be installed if greater heat transfer is needed due to the changing loads on the engine or the demand on the cooling system. The close temperature approach in PHEs facilitates the better cooling of the jacket water and the lubrication oil to optimize the performance and durability of the engine combustion system.
Key Integration Points in the Cooling Loop
Marine engines have a two-part cooling system, consisting of a fresh water high-temperature circuit that cools the engine block, and a low-temperature seawater circuit to remove the heat transferred from the fresh water in a central cooler. Plate heat exchangers may serve two purposes in this system, either as the main engine jacket water cooler or the lubricating oil cooler. As the main engine jacket water cooler, the PHE transfers heat from the fresh water circuit to the seawater, and maintains the jacket water within the operating range of 75 °C to 85 °C.
The PHE as the lubrication oil cooler controls the lubrication oil temperature at or below a specified temperature to maintain the correct viscosity.
In addition to cooling the jacket water and the lubrication oil, PHEs are also used to cool the turbocharger charge air in a similar loop, to increase the density of the air entering the engine and thereby improving combustion efficiency. Each of these applications has varying pressures, temperatures and fouling characteristics which dictate system sizing.
Material and Design Considerations for Marine Service
Because of the corrosive environment of seawater in a marine application, careful selection of the correct plate materials is critical. Titanium can offer the best protection against the chloride-induced pitting, crevice and galvanic corrosion typically found with a warm, chlorinated seawater stream. 316L stainless steel can often be specified on the fresh water side of the PHE at a much lower cost while providing good corrosion resistance. For more corrosive fresh water or lubricating oil streams, Alloy 254 or duplex steel can be specified for higher strength and corrosion resistance. The gasket material selection also needs to address combined factors such as cycling temperatures, fluid chemistry and the effect of compression set.
Commonly selected materials for PHEs are nitrile, EPDM and FKM (Viton) each having different operating temperature ranges and resistances to specific fluid chemistries. It should be noted that the pressure drop across a PHE should be limited to around 1-2 bar for both circuits to prevent pump overload or low flow conditions.
System Hydraulics and Installation Best Practices
The correct use of the plate pack of a PHE goes deeper. The system hydraulics and design have significant implications for the proper operation of the heat exchanger. It should be designed to reduce the amount of piping and to provide easy access to the plate bundle for maintenance. Optimal PHE orientation and positioning should minimize the run length of piping and allow easy access for cleaning of the plate bundle, as a PHE needs to be drained to allow for bundle withdrawal.
Standard practice dictates vertically positioning the PHE to allow proper drainage of either circuit and venting of trapped air. Strainers should be included in both fluid streams prior to the PHE inlet to protect the plates and gasket system from contaminants and damage. Temperatures and pressures should be monitored on both circuits via appropriately placed gauges and the thermal expansion of both fluid circuits accounted for with either expansion loops or flexible piping connections to prevent undue stress on the PHE tie bolts. For vessels with multiple engines and their associated PHEs, valving should be in place to allow isolation and service of one PHE without taking the vessel out of operation.
Performance Monitoring and Maintenance Integration
System integration of a PHE is not a "fit and forget" operation but must involve monitoring the PHE's operating parameters to ensure that it performs at its design efficiency. Indicators of concern are significant deviations from the desired setpoints on the outlet fluid temperature, increasing pressure drop across either of the circuits and monitoring the rate of seawater bleed into the fresh water circuit. If deterioration in performance is noted, a chemical clean of the PHE will return it to its original heat transfer efficiency for organic fouling and high pressure, high-temperature washing can be used to clean off scale. By including the scheduled of routine maintenance, to be executed at the same time as engine maintenance periods, you can reduce the amount of unscheduled downtime and increase the effective working life of plate and gasket material.
Conclusion
Choosing to integrate plate heat exchangers in their marine engines provides optimal conditions, ensuring the highest degree of heat transfer within the smallest spaces, all the while allowing for incredible levels of flexibility in terms of capacity. The SME Group has been actively engaged in marine engineering for over twenty years and has the assurance of an ISO certified quality system for entire cooling systems, which guarantees meticulous management, even when performing Retrofits. Moreover, a 5,000 m² facility with a hundred engineers and technicians enables SME to complete all of your marine projects under the "Beyond Concern” motto, helping you and servicing approximately one thousand vessels every year with systems and genuine spare parts. The SME Group is the ideal collaborator to optimize both performance and service life of your marine engines cooling systems; do not hesitate to contact us to discuss with us the details of your next projects, enabling us to provide you with a Marine Cooling System quotation.