The use of aluminum boilers in closed loops has increased in recent years. There are many benefits to using aluminum as a construction material. As compared to carbon steel, aluminum has a much lower density and much higher ability to transfer heat (thermal conductivity). Aluminum transfers heat at roughly three times the rate of carbon steel. As a result, aluminum boilers require much less surface area for heat transfer and a smaller overall footprint to provide the same water heating capacity as a steel boiler. In commercial and industrial applications, floor space is at a premium and the footprint of unit operations is a significant design consideration. Even though carbon steel is less expensive than aluminum, the higher heat transfer efficiency and smaller design makes aluminum an attractive choice for heat exchange (table 1).
While the improvement in boiler efficiency is beneficial, the integration of aluminum into closed-loop systems has resulted in a significant challenge to the water treatment industry. With the addition of aluminum heat exchangers and boilers to closed systems, there is an introduction of a strongly anodic metal. In the presence of iron and copper, which are less anodic, aluminum provides the potential to form galvanic corrosion cells with both free copper and free iron. In these multi-metal systems, it becomes increasingly important to control corrosion of all metal surfaces.