Which of the following is an indicator of excessive scale buildup in a water tube boiler?

An indication of excessive scale buildup in a water tube boiler is often represented by an increased chimney temperature. This occurs because scale deposits on the heat transfer surfaces reduce the efficiency of heat exchange within the boiler. As the scale acts as an insulating layer, the boiler has to work harder to produce steam, leading to elevated flue gas temperatures as more fuel is consumed to generate the required operating steam output.

When scale is present, the heat that could be effectively transferred to the water is impeded, causing less heat to be absorbed. As a result, the combustion gases exit at higher temperatures rather than transferring that heat to the water effectively. Monitoring chimney temperature can thus serve as a useful diagnostic tool to determine boiler performance and efficiency, particularly in identifying underlying issues such as scale buildup.

This choice highlights the importance of maintaining the cleanliness of the boiler's interior surfaces to ensure optimal thermal conductivity and overall system efficiency.