Which statement is true regarding flue gas flow in a fire tube boiler?

In the context of a fire tube boiler, the statement that the cooler the flue gas, the greater the amount of heat transferred is true because of the principles of heat transfer and thermal dynamics.

As the flue gas passes through the tubes of the fire tube boiler, heat is conducted from the hot flue gas to the water or steam surrounding the tubes. For effective heat transfer, it is beneficial to maintain a temperature gradient; the greater the difference between the temperature of the flue gas and the surrounding medium (water or steam), the more efficient the heat transfer process will be.

When the flue gas cools down, it allows for more heat to be extracted from it before it exits the boiler system. This means that if the flue gas is cooler as it exits, this indicates a better heat exchange has occurred, maximizing the energy transferred to the water or steam.

In contrast, when flue gas is hotter, it could mean that not all usable heat energy has been transferred. Thus, understanding this relationship is crucial in optimizing boiler performance and efficiency.