What condition may cause the stack temperature of a boiler to rise?

The stack temperature of a boiler rising can be attributed to the condition where the fireside of the boiler becomes dirty. When the fireside surfaces, including the internal components like heat exchangers and tubes, accumulate soot, scale, or other contaminants, their ability to transfer heat effectively is reduced. This buildup acts as an insulating layer, preventing efficient heat transfer from the flue gases to the water/steam in the boiler. As a result, the flue gases remain hotter for a longer time since they cannot lose heat quickly enough to the water, leading to an increase in stack temperature.

In contrast, excessive fuel input can lead to incomplete combustion and possibly lower stack temperatures due to unburned fuel lowering the overall thermal efficiency of the combustion process. Improper ventilation typically causes a reduction in combustion efficiency and can produce lower temperatures as well. Low steam pressure usually indicates that the system is not functioning at its optimal design, which may relate to inadequate heat transfer, but it is not a direct cause of a rise in stack temperature. Thus, the build-up on the fireside distinctly points to a situation where the stack temperature would significantly increase.