Consider the last question, for example, that of cooling the flue gases too much. What
happens? Well, there is always a certain amount of water vapor in the flue gases, from the
air and from the hydrogen in the fuel. As the temperature of the gases is lowered, there
comes a time when the saturation point is reached and the moisture condenses. If, at the
same time there happens to be any sulfur in the gas, (and there usually is) sulfuric acid will
be formed resulting in the spread of a thin but extremely corrosive layer of liquid on the
inside surfaces of the flues.