The Aluminum Bronze That Has Served Seawater for Decades
Aluminum bronze is a copper-based alloy containing 5–11% aluminum, with additions of iron, nickel, and manganese. The alloy forms a protective aluminum oxide film that resists seawater corrosion, and the film is self-healing if it is damaged. The corrosion rate in clean seawater is typically less than 0.025 mm per year, which provides a service life of 20–30 years for a typical tube wall thickness.
Aluminum bronze is used in seawater heat exchangers because it combines adequate corrosion resistance with high thermal conductivity and moderate cost. The thermal conductivity is approximately 60 W/m·K, which is roughly 240 times higher than PTFE. The high conductivity means that aluminum bronze heat exchangers are compact and relatively inexpensive.
The limitation of aluminum bronze is its susceptibility to erosion-corrosion at high flow velocities and to sulfide attack in polluted seawater. The erosion-corrosion limit is typically 1.5–2.5 m/s for seawater service, above which the protective film is mechanically removed and the corrosion rate increases sharply. The sulfide attack occurs when the seawater contains hydrogen sulfide, which is common in polluted harbors and estuaries. The sulfide reacts with the copper in the alloy and forms copper sulfide, which is not protective.
The PTFE That Eliminates the Corrosion Mechanisms
PTFE is resistant to seawater at all concentrations and temperatures within its service range. The material does not corrode, and the service life is determined by mechanical factors rather than chemical attack. PTFE is not susceptible to erosion-corrosion at high flow velocities, and it is not attacked by sulfides or chlorides.
The elimination of the corrosion mechanisms is the primary reason PTFE is specified for seawater service where the aluminum bronze would fail. The PTFE unit does not develop the erosion-corrosion or sulfide attack that limits the aluminum bronze service life, and the heat transfer performance remains stable over the service life.
The Thermal Conductivity Gap
The thermal conductivity of aluminum bronze is approximately 60 W/m·K, compared to 0.25 W/m·K for PTFE. The ratio is approximately 240:1.
For a heat exchanger rejecting 100 kW in seawater at 30°C, an aluminum bronze unit requires approximately 5 m² of surface area, based on an overall heat transfer coefficient of 1,000 W/m²·K and a log mean temperature difference of 20°C. A PTFE unit for the same duty requires approximately 25 m² of surface area, based on an overall coefficient of 200 W/m·K and the same temperature difference.
The PTFE unit is 5 times larger than the aluminum bronze unit. The larger size increases the capital cost, the floor space, and the fluid volume. The lower heat flux reduces the tube wall temperature, which is beneficial for the process, but the larger surface area also provides more surface for deposits and requires more time to clean.
The Erosion-Corrosion Limit of Aluminum Bronze
The erosion-corrosion limit is the flow velocity above which the protective film on the aluminum bronze is mechanically removed and the corrosion rate increases sharply. The limit depends on the alloy composition, the seawater chemistry, and the tube geometry.
For standard aluminum bronze in clean seawater, the erosion-corrosion limit is typically 1.5–2.5 m/s. Above this velocity, the corrosion rate can increase by a factor of five to ten, and the service life is reduced from 20–30 years to 3–5 years.
The erosion-corrosion limit is a critical design parameter for aluminum bronze heat exchangers. The tube-side velocity must be maintained below the limit, which may require a larger tube diameter or a lower flow rate. The larger tube diameter increases the heat exchanger size and the capital cost.
PTFE does not have an erosion-corrosion limit in the same sense. The material is not susceptible to mechanical removal of a protective film because there is no protective film. The PTFE heat exchanger can operate at higher flow velocities without the risk of erosion-corrosion, which provides more flexibility in the design.
The Sulfide Attack on Aluminum Bronze
Sulfide attack occurs when the seawater contains hydrogen sulfide, which is common in polluted harbors, estuaries, and industrial discharge areas. The sulfide reacts with the copper in the aluminum bronze and forms copper sulfide, which is not protective. The corrosion rate increases sharply, and the service life is reduced from 20–30 years to 1–3 years.
The sulfide attack is a localized phenomenon that initiates at the tube ends and at the tube-to-tube sheet joints, where the flow velocity is low and the sulfide concentration is high. The attack manifests as pitting, which penetrates the tube wall and causes a leak.
PTFE is not attacked by sulfide. The material is resistant to hydrogen sulfide at all concentrations and temperatures within its service range. This is a primary reason PTFE is specified for seawater service in polluted harbors and estuaries.
Comparing the Two Materials in Seawater Heat Exchange
The following table summarizes the performance and cost characteristics of PTFE and aluminum bronze heat exchangers in seawater heat exchange service at 30°C with clean seawater and a flow velocity of 1.5 m/s.
| Characteristic | Aluminum Bronze Heat Exchanger | PTFE Heat Exchanger | Advantage |
|---|---|---|---|
| Thermal conductivity | 60 W/m·K | 0.25 W/m·K | Aluminum bronze |
| Overall heat transfer coefficient | 1,000 W/m²·K | 200 W/m²·K | Aluminum bronze |
| Surface area required for 100 kW | 5 m² | 25 m² | Aluminum bronze |
| Chemical resistance to clean seawater | Excellent | Excellent | Equal |
| Chemical resistance to sulfide-contaminated seawater | Poor | Excellent | PTFE |
| Erosion-corrosion limit | 1.5–2.5 m/s | None | PTFE |
| Maximum operating temperature | 200°C | 260°C | PTFE |
| Mechanical strength | High | Moderate | Aluminum bronze |
| Thermal shock resistance | Moderate | High | PTFE |
| Repairability | Limited | Good | PTFE |
| Service life in clean seawater | 20–30 years | 15–20 years | Aluminum bronze |
| Service life in sulfide-contaminated seawater | 1–3 years | 15–20 years | PTFE |
| Capital cost for 100 kW duty | $45,000 | $52,000 | Aluminum bronze |
| Installation cost | $6,000 | $5,000 | PTFE |
| Annual maintenance cost | $2,000 | $3,000 | Aluminum bronze |
| Five-year total cost (clean seawater) | $91,000 | $92,000 | Equal |
| Five-year total cost (sulfide-contaminated) | $180,000 | $92,000 | PTFE |
The five-year total cost values assume a production value of $600 per hour and include the capital cost, the installation cost, and the maintenance cost over five years. The sulfide-contaminated case assumes one heat exchanger replacement over five years for the aluminum bronze unit.
When Aluminum Bronze Is the Better Choice
Aluminum bronze is the better choice in several specific applications.
Clean seawater service. When the seawater is clean and the flow velocity is below the erosion-corrosion limit, aluminum bronze provides excellent performance at a lower capital cost.
Low-velocity service. When the flow velocity is below 1.5 m/s, the erosion-corrosion risk is low, and aluminum bronze is the better choice.
Space-constrained installations. When the available space is severely limited, the smaller aluminum bronze unit may be the only feasible option. The PTFE unit would require 5 times the surface area, which may not fit in the available space.
High-pressure service. Aluminum bronze has higher mechanical strength than PTFE and can withstand higher pressures. For seawater service at pressures above 10 bar, aluminum bronze may be the only viable option.
When PTFE Is the Better Choice
PTFE is the better choice in several specific applications.
Sulfide-contaminated seawater. PTFE is resistant to sulfide, while aluminum bronze is attacked. For seawater service in polluted harbors and estuaries, PTFE is the better choice.
High-velocity service. PTFE does not have an erosion-corrosion limit, which makes it suitable for service at high flow velocities.
Thermal cycling service. PTFE has higher thermal shock resistance than aluminum bronze, which makes it suitable for service with frequent temperature changes.
Repairable service. PTFE heat exchangers can be repaired on site by a qualified technician, which reduces the downtime and the production loss.
Long service life requirement in contaminated seawater. When the seawater is contaminated and the heat exchanger is expected to have a service life of 10 years or more, PTFE is the better choice because it resists the chemical attack over a longer period.
Design Features That Extend Service Life
Several design features extend the service life of both PTFE and aluminum bronze heat exchangers in seawater service.
Flow velocity control. Maintaining the flow velocity below the erosion-corrosion limit reduces the corrosion rate of the aluminum bronze.
Sulfide monitoring. Monitoring the sulfide concentration in the seawater allows the operator to switch to a different cooling source if the sulfide concentration rises above the threshold.
Cathodic protection. Sacrificial anodes or an impressed current cathodic protection system can reduce the corrosion rate of the aluminum bronze.
In-place cleaning provisions. Connections for cleaning solution supply and return allow the heat exchanger to be cleaned without disassembly, which reduces the downtime and the production loss.
Temperature monitoring. Temperature sensors at the heat exchanger inlet and outlet allow the heat transfer performance to be monitored, which allows early detection of fouling or degradation.
Specification Parameters for Seawater Heat Exchange
Equipment engineers evaluating PTFE and aluminum bronze heat exchangers for seawater heat exchange should confirm the following parameters with the supplier.
Seawater chemistry. Specify the chloride concentration, the sulfide concentration, and any other contaminants.
Operating temperature. Specify the maximum operating temperature and the expected temperature profile.
Flow velocity. Specify the maximum flow velocity and the erosion-corrosion limit for the aluminum bronze.
Operating pressure. Specify the maximum operating pressure.
Heat load and surface area. Specify the heat load and the required surface area for each material option.
Space constraints. Specify the available space and the maximum allowable footprint.
Maintenance capability. Specify the maintenance capability of the facility, including the ability to repair the heat exchanger on site.
Service life requirement. Specify the expected service life and the replacement schedule.
Matching Material Selection to Seawater Service Conditions
The selection between PTFE and aluminum bronze heat exchangers in seawater heat exchange depends on the seawater chemistry, the flow velocity, the space constraints, and the service life requirement. Aluminum bronze has a higher thermal conductivity and requires less surface area, but it is susceptible to erosion-corrosion and sulfide attack. PTFE has a lower thermal conductivity and requires more surface area, but it is resistant to all seawater contaminants and flow velocities.
The selection decision should be based on a facility-specific analysis that incorporates the capital cost, the installation cost, the maintenance cost, and the production loss exposure. A simple spreadsheet model with these cost components provides the information needed to make the decision.
For seawater heat exchange facilities evaluating heat exchanger options, the material selection should be based on the total cost of ownership over the expected service life, not just the capital cost or the thermal performance. An aluminum bronze unit with a lower capital cost may have a higher lifetime cost if the seawater is contaminated and the unit requires frequent replacement.
Equipment engineers and procurement specialists evaluating PTFE heat exchanger specifications for seawater heat exchange or other corrosive service can request a design review covering seawater chemistry, operating temperature, flow velocity, operating pressure, and space constraints to confirm the material selection is matched to the actual service conditions and cost structure of the facility.
