Seawater, with its abundant availability and low cost, has become widely used in cooling and heating systems across multiple industries. However, its highly corrosive nature presents significant challenges for equipment materials, particularly in heat exchangers. Titanium has emerged as the material of choice for critical components like plate heat exchangers (PHEs) due to its exceptional corrosion resistance, finding extensive applications in chemical processing, power generation, and large-scale marine vessels.
Plate heat exchangers represent high-efficiency thermal transfer devices characterized by their parallel arrangement of metal plates. These plates form narrow channels where hot and cold fluids flow in opposite directions, facilitating heat exchange through the plate surfaces. Their compact structure, superior heat transfer efficiency, and ease of maintenance have made them indispensable in industrial applications.
Structure and Operation
The core design features multiple corrugated metal plates stacked together to create fluid channels. The corrugated pattern serves dual purposes: increasing surface area for enhanced heat transfer while reinforcing structural rigidity.
Industrial Applications
- Chemical Processing: Heating, cooling, condensation, and evaporation in various reaction processes
- Power Generation: Cooling water systems and waste heat recovery in power plants
- Marine Engineering: Engine cooling and desalination systems aboard ships
- Food Processing: Thermal treatment including pasteurization and temperature regulation
- HVAC Systems: Building climate control applications
Multiple variables influence heat exchanger efficiency, including fluid properties, flow velocity, plate material characteristics, and structural design. Optimizing these parameters for specific applications is essential for maximizing thermal performance.
Heat Transfer Mechanisms
- Single-phase forced convection: Cooling hot water with cold seawater
- Evaporation heat transfer: Converting working fluids to vapor using warm seawater, as in ocean thermal energy conversion (OTEC)
- Condensation heat transfer: Phase change from vapor to liquid
Performance Enhancement Benefits
- Energy conservation and emission reduction
- Operational cost reduction
- Equipment miniaturization and weight reduction
While researchers have developed various techniques to improve heat transfer efficiency, traditional approaches face challenges in durability, manufacturing complexity, and production scalability.
Surface Modification Techniques
- Spray coating: Application of stainless steel particles to create porous surfaces
- Electrodeposition: Copper coating deposition to generate micro-scale surface roughness
Technical Challenges
- Surface roughness degradation over time
- High processing costs
- Limited production throughput
Addressing these limitations, Kobe Steel developed an innovative manufacturing solution using pattern transfer rolling to create micro-scale surface roughness on titanium plates.
Technology Overview
The process employs specially engraved rollers to imprint microscopic patterns onto titanium sheets during rolling, creating permanent surface textures that enhance heat transfer while maintaining material integrity.
Key Advantages
- Integrated surface-texture durability
- Cost-effective manufacturing
- High-volume production capability
Leveraging this technology, Kobe Steel commercialized HEET®, a high-performance titanium plate featuring precisely arranged cylindrical micro-protrusions measuring approximately 25μm in height. Comparative testing demonstrates 10-40% improvement in evaporative heat transfer performance compared to conventional smooth titanium plates, particularly when groove patterns align perpendicular to fluid flow.
Field Validation
The technology has been successfully implemented at Okinawa's Kume Island OTEC demonstration plant, where it continues to operate effectively.
HEET® production involves a multi-stage process:
- Material preparation using CP titanium (ASTM G1)
- Thermomechanical processing through hot and cold rolling
- Pattern transfer rolling with engraved cylindrical micro-pattern rollers
- Post-processing including heat treatment and flattening
- Precision cutting to required dimensions
Rigorous testing across multiple heat transfer modes confirms HEET®'s superior performance:
Evaporation Heat Transfer
Using R134a refrigerant in dedicated test apparatus, HEET® demonstrated 24% higher performance versus standard titanium plates.
Condensation Heat Transfer
Similar testing showed 6% improvement in condensation efficiency.
Single-phase Convection
Hot/cold water testing revealed 11% enhanced performance.
Visualization studies using specially prepared HEET® samples with R134a revealed significantly increased nucleation site density, explaining the enhanced evaporative performance through promoted nucleate boiling.
Full-scale testing at OTEC facilities using 700mm × 2400mm heat exchangers confirmed approximately 20% performance improvement under actual operating conditions, matching laboratory results.
Ongoing research focuses on optimizing micro-surface geometries and expanding applications across various heat exchanger types to further improve energy efficiency in thermal systems.
