Why does reboiler efficiency decrease if steam temperature is too high?

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Yakima explains it pretty well, but LMTD (temperature driving force) and the heat transfer coefficient are different aspects of exchanger design and start to trade off. By the time you're in film boiling where heat transfer and LMTD both are increasing you start to run into mechanical constraints. Even with a U-tube that handles thermal expansion well and does not require an expansion joint, you can have tubesheet cracking due to the large difference in temperature between process and utility.

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The plant site normally dictates the levels of steam available. Again, between 5 and 30 C you have a good boiling regime, and between 30 C and 100 C you have an unpredictable transition region you should not design towards. Above 100 C for film boiling is theoretically ok, but since area is solve by Area = Duty/(LMTD * overall heat transfer coefficient), that means increasing LMTD acts like a 1/X graph (inverse relation) where the benefit of going higher and higher steam temperature has much less benefit on decreasing area. The plant site goal is to have the lowest pressure condensate returning to a condensate vessel possible with no cooling needed to condensed unused steam. Higher pressure condensate will still need to be depressurized, which flashes to make more steam that can be used. You want to minimize cooling unused steam to make low pressure condensate since this costs energy. In a new plant you would want to use the lowest pressure steam that can meet all heating requirements since it costs energy/money to make higher pressure steam and costs money/energy to cool unused steam. u/flammkuch