Why is a row of tubes sometimes left out of an H.R.T. boiler?

Leaving a row of tubes out of a Heat Recovery Steam Generator (H.R.T.) boiler is primarily done to improve circulation. When a row of tubes is omitted, it allows for better fluid dynamics within the boiler, which enhances the movement of the steam and water. This improved circulation helps in maintaining effective heat transfer and assists in preventing hot spots or uneven heating, which can lead to reduced efficiency or even damage to the boiler. By facilitating better flow, the operational reliability and efficiency of the boiler can be significantly enhanced.

The other options don't adequately capture the primary reason for leaving out a row of tubes. Structural integrity and lowering production costs may be considerations, but they are not the main purpose. Similarly, saving materials can be secondary but does not address the immediate benefit of circulation improvements.