Firetube boilers have their tubes fixed into flat steel plates known as?

In firetube boilers, the tubes are connected to flat steel plates that serve as structural support and water-tight barriers, which are known as tube sheets. These tube sheets are critical to the operation of the boiler because they hold the tubes in place, allowing the hot gases produced during combustion to pass through the tubes while the water surrounds them. This setup maximizes heat transfer and ensures efficiency in heating the water. The design and integrity of the tube sheets are vital for maintaining boiler safety and performance, as any leaks or structural failures in this area could lead to operational issues.

The other choices do not accurately reflect the terminology used in boiler construction. Tubular sheets, tubing membranes, and plate panels are not recognized terms in the context of firetube boilers, which is why they do not represent the correct answer.