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June 12

Isothermal expansion or isobaric?

Can an isothermal expansion process occur at constant temperature? My question is in particular reference to Rankine cycle. Here when the heat is absorbed during isothermal process, is it at constant pressure? — Preceding unsigned comment added by Jazzy Prinker (talk • contribs) 12:59, 12 June 2014 (UTC) Jazzy Prinker (talk) 13:00, 12 June 2014 (UTC)[reply]

An engineer could give you a better answer, but I'd say: In the Rankine cycle, the isothermal process occurs at the "same" (iso) temperature. There are actually two points on the diagram in the article where there are horizontal lines - whenever a liquid boils or a vapor condenses, its temperature remains a constant while it does so, even as the pressure remains a constant. However, as they point out, the steam is generally superheated and so doesn't stick to an isothermal regime, and of course as some boiler explosions can attest, the pressure can also vary from the ideal. :) Wnt (talk) 16:05, 12 June 2014 (UTC)[reply]

Isothermal expansion is by definition at constant temperature. Spinning Spark 16:34, 12 June 2014 (UTC)[reply]

I am so sorry for the mistyping and that I failed to notice it. I meant to ask about "constant pressure". So here is my question all over again:

Can an isothermal expansion process occur at constant pressure? My question is in particular reference to Rankine cycle. Here when the heat is absorbed during isothermal process, is it at constant pressure? I hope someone will now give a simple answer.Jazzy Prinker (talk) 13:35, 13 June 2014 (UTC)[reply]

If both temperature and pressure are constant, and heat is nonetheless moving into or out of the working fluid, it means that the fluid is boiling or condensing. However, for it to do so it has to be free to change overall volume. For example, if you have a pot of boiling water on the stove, it is at 100C and 1 atmosphere (at sea level) but steam escapes. You could in theory cover it and run the steam through a machine to generate physical work (designing a slightly higher constant pressure for the operation of such a machine). Wnt (talk) 13:52, 13 June 2014 (UTC)[reply]