Which method would be least effective in controlling final steam temperature?

Single element feedwater control is the least effective method for controlling final steam temperature because it relies on a single input – typically the steam flow or pressure – to regulate the amount of feedwater being supplied to the boiler. This method does not take into account variabilities in steam demand or the actual temperature of the steam produced. Consequently, it can lead to either overheating or inadequate heating of steam.

In contrast, methods like gas recirculation, gas bypass, and attemperation provide more nuanced control over steam temperature. Gas recirculation can help adjust the combustion process and mitigate temperature spikes. Gas bypass enables control over gas flow and can help maintain appropriate combustion temperatures. Attemperation allows for direct control of steam temperature by injecting water into the steam line to cool it down to a desired temperature. These options are more responsive to changes in steam demand, making them more effective in maintaining accurate and stable final steam temperatures.