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In modern commercial vessels, plate heat exchangers (PHE) are a core component of the cooling circuit and play a vital role in the main system central cooling, main engine jacket cooling, fuel oil preheating and in fresh water production plants. Marine conditions nevertheless expose the equipment to the extreme: continuous vibration, interaction with the aggressive sea water environment, high thermal cycles and chronic biofouling. Using the wrong PHE can lead to time off-hire, cost and loss of thermal efficiency and early leakage. Since 2000, SME Group has helped the maritime industry with more than 500 projects a year, including retrofit, maintenance, and repair projects. Our 5,000 m² Nantong factory and our team of more than 100 electrical and mechanical engineers offers ISO 9001, 14001, 45001 – certified solutions (ANAB/IAF/UKAS) to shipowners and operators all over the world. To help you avoid these types of frustrations, in this guide, we'll discuss some of the most important considerations you need to make to ensure your plate heat exchanger lasts long and performs well in extreme marine environments.
Plate Material Selection and Corrosion Resistance
The service life a PHE will serve in a marine environment can be almost entirely dictated by the selection of the plate material. However, seawater-cooled applications have been the best applications for titanium (Grade 1 or 2) because of its outstanding resistance to chloride induced pitting and crevice corrosion even at high temperature. The initial cost of the titanium can be higher, but the material has an essentially infinite lifespan if the application is in seawater service and the sea-state in which the vessel operates is tropical or brackish, where corrosion rates are increased. In a freshwater or closed loop (jacket water/lubricating oil) system, standard types such as 316L or the higher corrosion resistant 904L stainless steel are often selected. It is important however, to check the material for the resistance to stress corrosion cracking particularly in high chloride settings and intermittent exposure to seawater contamination. Our technical support engineers fully understand the requirement to undertake a fluid analysis and corrosion risk assessment before selecting a plate material to ensure that plate material is appropriate for the given operating conditions of each vessel at SME.
Gasket Type, Elastomer Compatibility, and Leakage Prevention
Gaskets are considered the weakest link of a plate heat exchanger because pressure compaction can have an effect on its properties and it can be attacked by heat and chemicals during thermal operation. For marine applications the gasket material will be required to resist temperature and pressure of fluid, as well as the exposure to cleaning chemicals, fuel oils and lubricants. The resistance of nitrile (NBR) gaskets and Ethylene Propylene Diene Monomer (EPDM) gaskets allow them to be used in oil-related tasks up to moderate temperature and for seawater and water-glycol mixtures, respectively. If the temperature goes over 150°C, Viton® (FKM) or special fluorocarbon elastomers may be necessary. In addition to materials, gasket design plays an important role: Glued gaskets are very tight and difficult to replace in the field; clip-on gaskets or snap-in gaskets are easy to replace in the field, but can be susceptible to displacement caused by thermal cycling. It is also advised to use double-gasket sealing on PHEs at the inlet and outlet ports to offer another barrier between the two fluid circuits to prevent cross-contamination. SMEs’ service technicians are taught how to torque a gasket and how to prevent a gasket from leaking. This ensures reliable performance, extends the life of re-gaskets and delays the need to remove the cover.
Flow Configuration, Pressure Drop, and Thermal Performance
Thermal and hydraulic characteristics of a PHE depend on the design of its plates, the chevron angle used, and the flow set-up. For instance, in marine systems, where pumping energy is a major expense in operation, the proper selection of a PHE should consider the relationship between the pump pressure drop and heat transfer efficiency. High-chevron plates (usually 60° angle) create substantial turbulence creating a better heat transfer coefficient, but with the associated higher pressure drop. Conversely, low-chevron plates (approximately 30°) generate less turbulence, decreasing both heat transfer and pressure drop. In many uses of seawater as a heat exchanger the actual combination of high and low chevron plates on the same frame, called a "mixed" or "L/H" arrangement is an optimum compromise. In addition, the number of passes (single pass versus multi pass) should be determined for the available space and pressure loss desired. Multi-pass arrangements will provide more surface for the heat transfer, but make it more complex and difficult to clean. At SME, our engineering team uses thermal modelling software to simulate performance with various seawater temperatures and fouling factors to ensure the pump is not overloaded and suitable purposes the heat duty are achieved with the selected PHE.
Maintainability, Cleaning Access, and Spare Parts Availability
Among the many factors to consider when selecting a PHE for marine service, perhaps the most practical is maintainability. Ships are at a distance and it is critical to be able to clean or replace gaskets in-situ and rapidly. The PHE frame should be robust enough to compel the compression of other plates in future expansion; further key plates should be easily compressible with added length of tie-bolts and end plates of the frame should be strong enough to resist warping during repeated opening and closing operations. As sea water is a regular cleaning necessity because of scale and biofouling the PHE should also be provided with connection points for chemical cleaning circuits and the plate pack should be easily accessible for mechanical cleaning, which will be achievable by high pressure water jets. Most importantly, the supplier should have spare plates and gaskets in stock that can be accessed quickly, and establishes lead times and batch accuracy. We are in possession of a full line of gasket and plate for the most frequently used frame size, and our Nantong stock can also provide you with fast delivery of gasket and plate to ships in dry-dock or at anchor. The 12-month warranty for project services ensures fleet operators peace of mind that service on the project will be treated to speedy completion and perfection.
Choosing the right plate heat exchanger for use in severe marine environments is a task that involves material science, thermal dynamics and operational logistics. A key strategy for shipowners to ensure they get the right solution is to review the plate alloys, gasket compatibility, hydraulic design, and maintainability. SME Group is easily your trusted partner in this selection process by virtue of its many years of experience in marine engineering and its attendant broad service network around the world. Our goal is not merely to fix problems, but to deliver heat exchange solutions that perform reliably, voyage after voyage.