How does improper water treatment and scaling primarily affect a boiler?

Improper water treatment and scaling can lead to significant problems during both the shutdown and startup phases of boiler operation. When scale forms inside the boiler, it acts as an insulating layer on heat transfer surfaces. This buildup can create hot spots, leading to overheating and even potential failure of boiler tubes. During startup, the presence of scale can hinder the boiler's ability to heat water efficiently, causing delayed and problematic ramp-up times.

Additionally, during shutdown, if scales form on these critical surfaces, it can complicate the processes needed to safely cool the system down, as the boiler may not distribute heat evenly or effectively. This combined impact on both operational efficiency and safety during these transitions is substantial, emphasizing the importance of regular water treatment and maintenance to prevent scaling issues.

The other options do not accurately reflect the consequences of improper water treatment; for example, scaling does not improve thermal efficiency or enhance safety, nor does it promote long-term reliability of equipment. Instead, it poses significant risks and operational challenges that can undermine the overall function and safety of the boiler system.