What is the feature used to facilitate cleaning the inside of a water boiler?

The handhole is specifically designed to provide access to the interior of a water boiler, which is essential for cleaning and maintenance. Its design typically allows for a sufficient opening that can be easily covered and sealed when not in use, enabling operators to enter the boiler for inspection, cleaning, or repairs safely and effectively. This functionality is crucial in maintaining operational efficiency and ensuring the longevity of the boiler.

While an inspection plate can also provide access, it is typically smaller and may not allow for comprehensive cleaning tasks like the handhole. A flange is mainly used for joining sections of pipes or equipment together and does not facilitate direct access for maintenance. A nozzle, on the other hand, is generally related to the flow of fluids or gases and is not associated with access for cleaning purposes. Thus, the handhole stands out as the most appropriate feature for facilitating the cleaning of the inside of a water boiler.