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For a marine or industrial application requiring a thermal management solution, one of the most important decisions an engineer must make is between gasketed and brazed plate heat exchangers. Both technologies function using the same heat transfer principles: a series of corrugated metal plates in between which two fluids circulate in counter flow allowing heat to transfer between them. But their construction, design philosophy and operational limits are different. At SME, with over 500 jobs annually and supporting 1,000 plus vessels with spare parts and service, we have dealt with the results of incorrectly specifying the type of PHE. This guide will help simplify the options so that the technology matches the reality of your vessel.
Construction and Physical Integrity
Gasketed Plate Heat Exchangers: These consist of a frame (fixed and movable end plates) where a stack of corrugated plates is compressed by the compression bolts. Each plate bears an elastomeric gasket that separates the flow channels, directing fluids through alternating passages. Plates can easily be added or removed. The plates and the seals are secured and stressed with high torques.
Brazed Plate Heat Exchangers: These come without frame, no bolts, no extractable gaskets. Rather, a series of plates are assembled and vacuum-brazed by use of filler metal with either copper or nickel as the filler that solidifies across the contact between the plates. The final being a solid block without any elastomeric seals, also tightly and sealing closed from the outside atmosphere and hence having less space and weight for that given rating compared to a gasket type design.
Operating Limits: Pressure and Temperature
Brazed Units can usually accept much greater pressures ranging from 30 to 45 bar and temperatures of 200°C and higher with nickel brazed models as no elastomeric sealing may extrude or break for a system such as refrigerant circuits, high pressure hydraulic oil and engine heat recovery on new vessels.
Gasketed Units constraints are based on the gasket material: Our team of SME engineers often provide NBR, EPDM and FKM gaskets which will have a temperature operating range of between -15°C to in excess of 150°C and with top pressure ratings for gasketed marine equipment around the bar of 25 or above. This rating is fine for any standard seawater cooling duty and in most instances would suffice for standard fresh water generators, and lube oil systems on most ships but the bar pressure cannot be exceeded with a gasketed marine product.
Maintenance, Cleaning, and Serviceability
This is where the two approaches diverge so sharply from one another, quite often to the shipowner the overriding point.
Gasketed Heat Exchangers are Serviceable by Design. Whenever fouling has occurred (and in the marine arena, fouling happens when sea water, scale or heavy oil residues accumulate on the surfaces, it is not if, but when) the unit is unbolted, the plates can be splayed, and the gasket taken out. The individual plates can be cleaned either mechanically or chemically and the gasket replaced. This is the reason SME have a workshop and an on-site team, so that gasket replacements, plate cleaning and full retro-fit to restore a gasketed unit to 95%+ of its original design rating can be performed. In addition, you can change the type of gasket if your operating conditions or media changes.
Brazed heat exchangers are disposable. They cannot be disassembled to clean the passages between plates. Brazed PHE units cannot be disassembled and internal clean-outs using cleaning chemicals circulating through the unit may be tried, but may not adequately remove scale or carbon build-up. The primary disadvantage to brazed PHE units, however, is that if the unit becomes blocked with scale or debris, or a channel gets damaged due to thermal expansion cycles or corrosion over time, the only fix is to buy new. This can be quite expensive with a large brazed unit or due to lead times on new equipment if a vessel cannot be serviced quickly.
Leakage and Cross-Contamination Risks
For a gasketed PHE, any early trace leakage will typically be a small bleed to the outside - you see liquid dripping from the frame, before two fluid streams mix disastrously. Double-gasket design is another option for critical applications where the two media must remain truly separated (oil, seawater). One has to admit that, to actually have that 'second' gasket function, one should probably not wait till gasket completely blew up. If a gasket fails, you can replace it in situ.
In a brazed PHE an internal leak from inside one channel into another (a pinhole corrosion or crack) will contaminate one process stream into the other without the operator noticing a unit leak on the external faceplate. In an oil or water application one or both streams can be contaminated almost immediately upon the unit going into operation; in refrigeration it might not be known until performance degrades dramatically. The unit must be taken offline and replaced.
Application Suitability and Lifecycle Cost
Select Brazed PHEs for closed loops of clean fluids, like refrigerants, hydraulic fluid and deionized water. They are also suitable for space-constrained applications, such as engine rooms on smaller vessels where installation and service access may be limited. Understand they won't last as long overall but a lower upfront cost.
Why you should select Gasketed PHEs? For open-loop seawater, dirty applications, heavy fuel oils, or other similar process mediums, where longevity and maintenance is absolutely key, use Gasketed PHEs. The upfront cost is greater, however the cost of replacing gaskets and plates on these will prove far cheaper over 15-20 years service life.
In our many thousands of vessel calls at SME, it became our practice that our customer’s choice of a conventional gasketed heat exchanger would always win due to its transparency and ease of maintenance in day to day service in the standard mainstream market. Our 12-month service warranty on these projects, together with our ISO 9001, 14001 & 45001 certified procedures, places SME’s team of 100+ technical staff right where it assists shipyards globally making those vital decisions regarding any coolant aspect. Whether you need a gasket change-out in China to a complete technical appraisal, we can, with the help of the wider SME team, help take heat exchangers beyond just solving today's worries to ensuring the one we put in place today continues to do the same tomorrow.