What is the maximum allowable gap between two sidewall tubes in a D. Type boiler?

The maximum allowable gap between two sidewall tubes in a D-type boiler is indeed 1/4 inch. This specification is important for maintaining the structural integrity and efficiency of the boiler. A gap that is too large can lead to inadequate heat transfer, which can diminish the boiler's performance and potentially lead to safety issues.

In this context, the design of D-type boilers takes into account various factors including thermal expansion, structural alignment, and the overall efficiency of heat exchange. A gap greater than 1/4 inch could result in issues such as leakage or structural weaknesses, which are not acceptable in the operational conditions of boilers. Hence, the specified maximum ensures that the design remains effective for safe and efficient operation.