Talk:Logarithmic mean temperature difference
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It's probably clearer to express the LMTD in terms of temperatures for 'left' and 'right' ends of the HE. This way the same formula applies to both co- and counter-current systems, and also the cross-flow case. Need to explain that the LMTD follows from a simple 1-D analysis (Incropera & DeWitt 4th Ed, sec 11.3.1).
- I agree with this, IMHO the whole first section should be scrapped and rewritten with a nice diagram and the use of 'left' and 'right' temperature differences. Also I think calling the log mean temperature difference a 'logarithmic average' is a bit of a misnomer. LMTD is just the mean temperature difference (ie, just an arithmetic mean), it just turns out the arithmetic mean using infinitesimal steps has a log in it (see the derivation section)! Calling it a logarithmic mean just confuses the issue and makes it appear more abstract than it actually is.
'F' is a 'correction factor'. But doesn't really come in to the definition of the LMTD.
Reasonable values of LMTD for typical types of heat exchangers would be useful.
Derivation of LMTD
edit2021 April, please check this alternative (causal) LMTD heat exchanger formula Introducing the analytical causal closed-form LMTD
Just added a quick derivation of LMTD as a distraction from study. Maybe someone can make a quick diagram up and fix any errors in the formulas (maybe fleshing bits out that I neglected). —Preceding unsigned comment added by 220.239.133.247 (talk) 12:43, 11 March 2008 (UTC)
good —Preceding unsigned comment added by 203.235.241.70 (talk) 03:08, 2 April 2008 (UTC)
In the case of counter-current flow HE with equal heat capacity fluids, one case to watch for is when ka and kb are equal and opposite sign. The result is capital K is zero and the final equation for LMTD becomes undefined (delta Ta and delta Tb are equal and the ln of the ratio is zero in the denominator. Of course simple inspection reveals this case and the mean temperature difference equals delta Ta equals delta Tb. 71.226.185.80 (talk) 00:44, 25 November 2010 (UTC)
Assumptions and Limitations
editThere are mistakes in "assumptions and limitations" in this article "A particular case where the LMTD is not applicable are condensers and reboilers" & "It has also been assumed that the heat transfer coefficient (U) is constant, and not a function of temperature"- are both incorrect. Somebody needs to read the original research paper by Colburn where LMTDs are introduced ( Colburn, I.P; Mean Temperature Difference and Heat Transfer Coefficient in Liquid Heat Exchangers. Ind. Eng. Chem., 1933, 25 (8), pp 873–877) and summarise amongst other things the assumptions, to quote Colburn: "Assumptions: (1) The over-all heat transfer coefficient, U, is a linear function of temperature t; i. e., U = a (1 + bt), where a and b are constants, and t is the temperature of one of the fluid streams. The specific heats are constant, and heat losses are negligible. (2) The derivation is limited to the following cases: (a) Two heat transfer mediums in countercurrent flow are changing in temperature in such a way that the decrease in sensible heat of one is equal to the increase in sensible heat of the other; or (b) One heat transfer medium is at a constant temperature, such as a condensing vapor or boiling liquid, and the other is changing in sensible heat as indicated by a temperature rise or fall" — Preceding unsigned comment added by 130.95.54.63 (talk) 08:36, 19 August 2011 (UTC)
See also section
editI miss the See also section. Specifically, I'd like to see the NTU method being mentioned at least somewhere in the article. — Preceding unsigned comment added by Hummeling (talk • contribs) 12:18, 18 June 2015 (UTC)