Letters

Steam Concepts

I enjoy reading your magazine and find many of its articles informative and helpful. I noticed several errors in the [Facts at Your Fingertips] article, Steam Concepts, [February 2017, p. 32] and wanted to bring them to your attention:

In the section “Pressure-temperature relation” the author states: “enthalpy, a thermodynamic property of a fluid that is defined as the internal energy of the fluid added to the product of its pressure and temperature.” This is incorrect. Enthalpy is the internal energy added to the product of its pressure and specific volume.

In the second paragraph under “Industrial practice” the author states: “Lower heat transfer rates occur because condensed water can form a film on heat-transfer surfaces, which reduces heat transfer because thermal conductivity of water is much lower than that of steam.” The observation is correct but the reasoning is incorrect. The thermal conductivity of liquid water is in fact significantly higher than steam. This is true for most liquids when compared to gases. The reason for the lower heat transfer rates when a film of liquid water forms on the heat transfer surfaces is as the author stated earlier in the article: the heat transfer coefficient (capacity) for condensing steam is significantly higher than for liquid water (at the same temperature and pressure). And this higher heat-transfer capacity is mainly due to it changing phase from a gas to a liquid where large amounts of energy are released. So in fact liquid water must form when steam is condensing on heat transfer surfaces. The trick is to design the surfaces so that this condensate drains away quickly and that steam traps are suitably sized so that condensate doesn’t backup onto the heat transfer surfaces.

Auckland University of Technology

Editor’s note:Thank you for pointing out the errors in the column. Regarding the definition of enthalpy, you are correct­— the inclusion of temperature for volume was a typographical error. Regarding the discussion of water films on heat-transfer surfaces, the author was trying to condense a longer explanation, and it was indeed flawed. Thank for you clarifying this. The online version has been updated to reflect these corrections, and can be found at www.chemengonline.com/steam-concepts/.

Making ammonia at milder conditions

There were errors in two chemical formulas in the Chementator article, Making ammonia at milder conditions, ( Chem. Eng., November 2016, p. 11). The formulas shown as Ru/CO2 and Ry/MgO should be Ru/CeO2 and Ru/MgO. The corrected version of the story can be found online at www.chemengonline.com/making-ammonia-milder-conditions.

Thank you to our long-time reader, Sorab R. Vatcha, consultant from Mountain View, California, for finding the error and bringing it to our attention.