A direct cause of overheating of the boiler tubes is what?

The direct cause of overheating of boiler tubes is best attributed to scale formation on the tubes. When scale builds up on the internal surfaces of the boiler tubes, it acts as an insulating layer that reduces the efficient transfer of heat from the combustion gases to the water or steam inside the tubes. This insulating effect means that more heat is retained in the metal of the tubes rather than being used to convert water to steam. As a result, the metal can reach temperatures beyond its design limits, leading to overheating.

Effective maintenance, including regular cleaning and monitoring for scale accumulation, is crucial in preventing this situation. Removing scale not only helps maintain efficiency in heat transfer but also prolongs the life of the boiler components by preventing thermal stress and damage due to high temperatures.