What type of solution is effective for scale prevention in an open system such as cooling towers and evaporator condensers?

In an open cooling system, such as cooling towers and evaporator condensers, the buildup of scale due to mineral deposits is a significant concern. The use of Acid-Chromate solutions is effective in such applications because they not only help to control scale but also provide corrosion inhibition for the metallic parts of the system. Acid-Chromate operates by forming a protective layer on the surfaces where scaling tends to occur, thereby reducing the potential for mineral deposits to adhere.

This option stands out because it addresses both issues of scale and corrosion, which are critical for the efficient operation of these systems. The other choices either do not effectively address scale prevention in the context of open systems or may introduce additional problems. For instance, sulfuric acid is primarily used for pH control and might not effectively prevent scale accumulation. Similarly, calcium carbonate is a scaling compound itself, which could exacerbate the problem, while magnesium sulfate doesn't provide the necessary properties for effective scale prevention or corrosion control in such systems.