In a fire tube boiler, tubes smaller than 4 inches are known as what?

In a fire tube boiler, the term used for tubes that are smaller than 4 inches in diameter is indeed flues. Flues are the passages through which the combustion gases from the fire travel. In fire tube boilers, these tubes allow the hot gases to transfer heat to the water surrounding them, which is essential for heating the water and generating steam.

Understanding the function of flues is important because they play a critical role in the boiler's heat transfer efficiency and overall performance. The design and size of these tubes are integral to the boiler's ability to produce steam efficiently and safely. Flues help facilitate the movement of gases while maximizing surface area for heat exchange, leading to effective thermal management within the boiler system.