Corrosion, scaling, and leaching in BPHEs - best practices for a happy ownership
By Marion Karlsson Faudot, Application Engineer - SWEP
The Paris agreement is an international treaty on climate change with the goal to limit global warming to well below 2 °C. To contribute to the Paris agreement different measures have to be undertaken. One way to help lower the carbon footprint is by increasing the efficiency of existing technology.
Brazed plate heat exchangers have one of the most efficient ways of transferring heat from one medium to another. Additionally, they consist of a minimum of material and require extremely small space compared to other heat exchanger technologies. In BPHE, around 95% of the material is used for heat transfer, leading to a light and compact product with a low environmental impact. Their plate geometry enables high turbulence implying not only an efficient heat transfer, but also a self-cleaning effect.
Securing an efficient product
Media and material interaction with the heat transfer surface and operating conditions can lead to undesirable outcomes such as corrosion, fouling and scaling, as well as leaching. To enable a BPHE to remain highly efficient, it is important to investigate issues that can arise and impact its performance. Once the risks are detected solutions to counter them can emerge.
Good media and material interaction with the heat transfer surface and operating condition is needed to avoid undesirable outcomes i.e corrosion, fouling, scaling and leaching.
Fouling is the formation of an unwanted layer on heat transfer surfaces. The general effect of fouling is a decreased heat transfer and an increased pressure drop, lowering the product’s efficiency. There are different types of fouling that are classified into four categories: biofouling, particulate, scaling, and corrosion.
Biofouling is the accumulation of microorganisms, plants, algae on the heat transfer area. It is common in cooling water systems. Particulate is when suspended solids settle, this is common in open systems with cooling towers and with surface water. Scaling is the deposit of inorganic salts in the water circuit when temperature of water increases and wall temperature exceeds 65 °C. This occurs commonly in boilers, hot water heat pump condensers, and desuperheaters. An added layer of corrosion product on the BPHE’s surface leads to what is called corrosion fouling. This it is common in all applications and is reduced with the help of a closed water system.
Heat transfer surface properties such as structure and geometry have a significant role in whether fouling is likely to happen. Smooth stainless steel minimizes the risk of surface adhesion and a BPHE with high chevron angle promotes turbulence and shear stress, reducing the likelihood of fouling. By keeping tabs on the operating conditions such as: flow velocity, temperature, turbulence level but also shear stress, the probability of fouling can be reduced. The last parameter influencing the prospect for unwanted fouling build up is media. The media’s properties (conductivity, hardness, alkalinity and so forth) interact with the heat transfer surface to promote or demote fouling formation.
To mitigate fouling water filtration and water treatment can be utilised. Biocides, corrosion inhibitors, magnet filter, scaling inhibitor and make up water to keep the conductivity and mineral at a good level are all different potential treatments. Widening the view to the system, a strainer in front of key equipment, water flow control and water quality measurement and monitoring can be of aid. By choosing the correct BPHE high turbulence is enabled leading to increase shear stress, hence mitigating fouling.
Cut-out showing deposit of inorganic salt; scaling, that occurs at high wall temperature when the media and operating conditions are not properly controlled.
Cleaning in place (CIP) can be used to remove fouling and scaling. With high flow cleaning, turbulence is created to increase cleaning efficiency. Another CIP method is reversed flow, in which the dirt is moved in the opposite direction of the normal flow. The use of detergents, for particulate fouling, and weak acids, for scaling, can be used to dissolve and loosen the blockage.
Indicators showing when CIP is required are when the temperature difference is smaller than specified or when higher pressure drop than expected are observed over the BPHE. A 30 % or higher increase than the nominal value is seen as a reference for conducting a CIP. Before deciding to clean the BPHE, it is crucial to make sure that filters are clean, and pumps are functional to rule out any other options for an increase pressure drop and decreased temperature difference.
CIP does not require disassembly of the heat exchanger and can be made at the installation site. It results in a uniform removal and lower overall operating cost. It has no risk of mechanical damage to the plates or gaskets and if fluids can pass through the BPHE, it is possible to effectively clean it.
Corrosion is the term for a chemical or electrochemical reaction between a material, usually a metal, and its environment, which produces a deterioration of the material and its properties. Pitting and crevice corrosion are essentially the same phenomenon. Pitting appears on exposed surfaces, for example attacking stainless steel if the passive layer is damaged. The passive layer is a protective surface film that is formed spontaneously when stainless steel is exposed to air. The attack can be sudden and quickly cause leakage. Crevices occur in welds that fail to penetrate, in flange joints and under deposits on the steel surface.
BPHE exposed to media with major sulphate content and high conductivity in order to trigger copper corrosion.
General corrosion is a deterioration distributed uniformly over a surface. It is more predictable than pitting; if a device has corroded 0.1 mm in one year, it will most likely corrode 0.2 mm in two years. This unwanted occurrence that leads to decreased product efficiency is governed by the choice of material, media and operating conditions. The material of the plates play an important role in whether corrosion will occur. Stainless steel is for instance sensitive to high chloride content and copper is sensitive to both high sulphate content and high conductivity.
Leaching or metal migration
Leaching or metal migration is the release of metal ions from heat transfer surface into the media. The focus is on the effect that the released ions have on the quality of the media, as certain applications are affected by media degradation. Drinking water applications requires extreme cleanliness and has a lot of health-related guidelines. For applications where pure or deionized water is used, the purity of the water should not be affected and leaching needs to be avoided at all costs. Copper ions negatively affect the heat transfer and lubrication properties of oils. Media degradation caused by metal migration affects the efficiency of the BPHE.
A way to attenuate corrosion and leaching is to apply chemical or mechanical treatment to the heat transfer media. Corrosion inhibitors, oxygen binding chemicals and traditional filtration can be used. Avoiding stagnation for longer time periods and monitoring but also controlling the water quality is helpful. Ensuring that the selected product is made of correct material or is using surface treatment will further help reduce corrosion and leaching.
Coating the product with thin-film technology to seal the inner surfaces of the BPHE with a protective layer of SiO2 enhances the resistance to corrosion, scaling and leaching. The ceramic nature of SWEP’s SiO2 based Sealix® layer, for example, improves corrosion stability and there are indications that the organic functionalities improve the surface behavior in terms of scaling. The metal leaching is significantly reduced by sealing the surfaces in contact with water and oil. These benefits are achieved without affecting the thermal and hydraulic performance.
Using coatings opens doors to new markets where a BPHE can contribute to improving a system’s efficiency. With tap water applications a coating serves as protection against nickel leaching and reducing scaling. In district water it reduces the tendencies for scaling. Having a protective layer can allow the product to withstand aggressive media with chlorides up to 1000 ppm at 60°C, reduce scaling tendency, and offer enhanced corrosion protection, allowing the technology to be used in cooling tower applications.
All-Stainless is another SWEP product that can be used with corrosive heat transfer media such as ammonia, deionized water, or biogas. This is used to circumvent copper corrosion as it leads to system failure and shorter lifetime. All-Stainless is an enabler when using deionized water systems with conductivity below I µS / cm. It also helps against media with high sulphury such as biogas and it can be used in different oil applications as it prevents copper leaching into the oil. The pharmaceutical process industry may require extreme cleanliness and that contamination of metals is kept to minimum, making the All-Stainless a product of choice.
All-Stainless products combine high corrosion resistance, low contamination tendencies and high thermal strength without compromising on its thermal performance.
BPHEs bring benefits in terms of thermally efficient and compact products. Through their efficiency they help reduce the global carbon footprint and contribute to reaching the Paris agreement goal. Some obstacles with keeping the BPHE’s good performance throughout their lifetime are the interactions between media, heat transfer surface, and operating conditions. When these parameters interact in an undesirable way it can yield corrosion, fouling, scaling and leaching concerns. These in turn decrease the performance and efficiency of the exchanger. By looking at understanding these processes, solutions, and ways to mitigate their occurrence can be researched. SWEP’s Sealix® and All-Stainless product lines allow for better resistance to corrosion, scaling and leaching, and open markets that were previously closed. Being able to replace older technology with newer, more efficient versions is an important part of reaching a sustainable society.