The frequency of ash removal in a coal-fired furnace with dumping grates should be determined by what factor?

The frequency of ash removal in a coal-fired furnace utilizing dumping grates is primarily influenced by the fluid bed thickness. In this context, the fluid bed refers to the layer of coal and ash that operates within the combustion chamber. The thickness of this fluid bed is crucial because it affects the combustion efficiency and the buildup of ash.

As the fluid bed reaches a certain thickness due to ash accumulation, it can impede the airflow necessary for proper combustion, leading to inefficient burning and potential operational issues. Consequently, a thicker fluid bed necessitates more frequent ash removal to maintain optimal operational conditions, ensuring that airflow is not restricted and that combustion remains efficient.

Choosing this option emphasizes the importance of monitoring the fluid dynamics within the furnace, which directly correlates with how often ash must be removed to ensure that the system operates effectively and safely. Other factors, such as grate width, fuel type, and air supply, contribute to the overall combustion process but are not as directly related to the critical need to manage ash accumulation in relation to fluid bed thickness.