Frequently Asked Questions
- What is the thermal influence of coating systems?
- What is corrosion and pollution's influence on heat exchange capacity?
- How is the microbiological growth reduced in air-conditioning systems?
- Corrosion resistance of heat exchangers, a comparison
WHAT IS THE THERMAL INFLUENCE OF COATING SYSTEMS?Heat exchangers are designed to exchange heat between media without direct contact between those media. Aluminum and copper are good materials for this purpose as they have high heat conductivity ratings. Standard liquid-to-air heat exchangers are made with copper tubes and aluminum fins. A weakness in this design is the joint between copper and aluminum. As long as the fins are tightly fixed to the copper tube without interference of organic layers or corrosion products the heat transfer will be optimal.
Coating protection to heat exchangers is realized by applying organic coatings to the metal surfaces. The different systems can be divided into two types: pre-coat and post-coat.
Pre-coated aluminum consist of a thin organic (usually vinyl or epoxy) layer that is applied to the aluminum sheets before fin fabrication. After cutting, bending and assembly there will be an organic layer between the copper tube and the aluminum fin. It is accepted that this pre-coat layer results into a capacity loss ranging from 10 -15 % of heat exchange capacity.
Post coat systems consist of an organic layer that is applied after the heat exchanger is produced. An organic layer seals the copper-aluminum joint and the rest of the metals.
The effect of the Blygold post coat systems on the initial heat exchange capacity is summarized in this document.
WHAT IS CORROSION AND POLLUTION'S INFLUENCE ON HEAT EXCHANGE CAPACITY?Heat exchangers are designed to exchange heat between media without direct contact between those media. Aluminum and copper are good materials for this purpose as they have high heat conductivity ratings. Standard liquid-to-air heat exchangers are made with copper tubes and aluminum fins. A weakness in this design is potentially the joint between the copper and aluminum. As long as the fins are tightly joined to the copper tube, without gaps or interference of organic layers or corrosive products, the heat transfer will be optimal. Pollution on the fin material will also influence the heat transfer of a heat exchanger.
The joint between the copper tubes and aluminum fins is one of the more corrosion sensitive parts of an air-conditioning unit. Aluminum and copper are incompatible metals. What we mean by this is that the metals have a different potential. When these metals touch each other and there is an electrolyte in the form of a conducting fluid present, a current (flow of electrons) will flow from the less noble metal (aluminum) to the more noble metal (copper). Once aluminum starts loosing electrons it begins to dissolve easily and reacts into an aluminum corrosion product. The joint that existed between copper and aluminum is now replaced by a copper aluminum oxide joint. Heat conducting capacity of aluminum oxide is much lower than that of un-corroded aluminum. Therefore, the heat transfer between copper tubes to aluminum fins is significantly decreased.
If pollution on the fins is limiting the airflow through the heat exchanger, the temperature of the air that is passing over the aluminum fins will increase (the same kW in less kg of air). This will cause the temperature difference between the liquid/gas in the copper tube and the air passing over the fins to decrease. A smaller temperature difference will result in reduced heat transfer.
HOW IS THE MICROBIOLOGICAL GROWTH REDUCED IN AIR-CONDITIONING SYSTEMS?Growth of legionella in cooling towers and humidifying sections has been a well known problem for many years. But there are more organisms present in several parts of the airconditioning system. These organisms might not be as lethal as legionella but are well capable of creating nasty smell, food contamination, allergies and other unhealthy effects. In this study the microbiological growth and the prevention of growth on the air-cooling heat exchanger are discussed.
Cooling of air is realized by transporting airflow through a cooled heat exchanger. Due to humidity in the air and the temperature differences, water will condense on the heat exchanging surface. At the wet surface, pollution and micro-organisms will adhere easily. Depending on the temperature present, several organisms will be able to reproduce rapidly under these circumstances.
Air-conditioning systems normally re-circulate air from the air-conditioned room to prevent extreme energy consumption. As a result of this, air passing contaminated heat exchangers will be re-contaminated. This re-contaminated air is brought back into the air-conditioned room. This may result into several problems, for example: cross contamination in food processing. Overall it can be said that the air-conditioning system is reducing the indoor air quality instead of increasing it.
Types of organisms found in air-conditioning installations: legionella, aspergillus, cladosporium, penicillium, staphylococcus, pseudomonas and salmonella.
Microbiological growth on cooling heat exchangers needs to be prevented or at least controlled. Simply disinfecting the surface is often considered to be a good way to stop microbiological growth. Thorough disinfecting a heat exchanger is very complicated as surfaces are difficult to reach. Besides that, one must consider the fact that surfaces are only hygienically clean temporarily. The moment air is passing the surface again it will be contaminated with organisms and pollution.
Thorough cleaning on a regular base will have much more effect than incidental disinfecting. By cleaning, the pollution on the surface (which serves as nutrition for organisms) is removed.
One of the most important steps in reproduction of airborne micro-organisms is the adhesion/settlement to a surface. As soon as there is some adhesion of organisms to a surface they tend to develop extra cellular polymers. These polymers will create a better adhesion to the surface but even more important to other airborne organisms as well. This way a bio-film will develop rapidly on a surface.
The best way to control or prevent microbiological reproduction is the prevention of this first adhesion of organisms to the surface. Three levels of surface treatment can be used to realize this:
- Prevention of rough and porous surfaces by using corrosion protection and suitable insulation materials.
- Creating biostatic surfaces by applying coatings with a low surface energy and a micro-organism repellent surface.
- Creating bioactive surfaces by applying coating with biocide leaching from the coating to actively kill organisms present on the surface.
Heat exchangers require special coating systems, as they must be heat conductive, flexible and chemical resistant at thin layers. Corrosion protection on heat exchangers can not be realized by just any paint. Blygold developed Blygold PoluAl to create corrosion resistant and smooth heat exchanger fins. This product is developed for aluminum fins but can also be used on copper. For galvanized steel another primer is necessary.
To create a biostatic surface on the heat exchanger fins Blygold developed PoluAl MB. This coating is not only creating a smooth corrosion protection layer, the special ingredients make the surface very repellent to mould, yeast and most bacteria as well. In this report the effectiveness and safety of these coatings is discussed.
CORROSION RESISTANCE OF HEAT EXCHANGERS, A COMPARISONHeat exchangers are designed to exchange heat between media without direct contact between those media. Aluminum and copper are good materials for this purpose as they have a high heat conductivity. Standard liquid-to-air heat exchangers are made out of copper tubes with aluminum fins. Bottle neck in this system is the joint between copper and aluminum. As long as the fins are tightly fixed to the copper tube without interference of organic layers or corrosion products the heat transfer will be optimal.
Furthermore, a layer of pollution on the heat exchanging surface will result in an increased thermal resistance of the material and therefore a much lower heat transfer.
Unfortunately the joint between copper and aluminum is also one of the most corrosion sensitive parts of an air-conditioning as two metals (copper and aluminum) with a different potential are connected.
The metals have a different potential and a current will flow as soon as there is a conducting fluid (electrolyte) present. This current (flow of electrons) will flow from the less noble metal (aluminum) to the more noble metal (copper).
Once aluminum starts loosing electrons it will dissolve easily and react into an aluminum corrosion product. The joint between copper and aluminum is now replaced by a copper aluminum oxide joint.
Heat conductivity of aluminum oxide is much lower than that of aluminum itself. The heat transfer between copper tubes to aluminum fins is significantly decreased. Pollution
If pollution is limiting the airflow through the heat exchanger, the temperature of the air that is passing the aluminum fins will increase (the same kW in less kg of air). The temperature difference between the liquid/gas in the copper tube and the air passing the fins will decrease this way. A lower temperature difference will result in a lower heat transfer.
The design and the type of material will determine the final corrosion resistance of a heat exchanger.
Fin distance: will influence the amount of pollution accumulation.
Air velocity: will influence the amount of pollution accumulation.
Fin fixation: will influence initial heat transfer from copper to aluminum and potential galvanic action
Fin material: type of metal will determine sensibility to corrosion of fins
Tube material: type of metal will determine sensibility to corrosion (tube and fins)
In this study results from various reports and tests are compared and summarized.
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