Talk:Heat transfer coefficient

Thermal resistance due to fouling deposits

Shift this section to Thermal Conductivity. The resistance is by conduction and not by convection and should not be in Heat transfer coefficient. 220.255.2.144 (talk) 05:50, 14 May 2011 (UTC)

Shift Thom correlation to Nucleate boiling

As much as convective heat transfer is significant in nucleate boiling, this article not about boiling. It would be better placed in the article of Nucleate boiling. Venny85 (talk) 07:44, 9 May 2011 (UTC)

Talk from before 2008

How can Q be a power input? Isn't power ${\dot {Q}}$, J/s?

answer: right. Q is the heat, P is the power
${\dot {Q}}={\frac {dQ}{dt}}=P$
[Q]=J and [P]=W=J/s

obviosly $Q=h\cdot A\cdot \Delta T$ is wrong
correct would be: $Q=h\cdot A\cdot \Delta T\cdot \Delta t$ to check this, you can simply put in the units: Q is in J; h is in W / (m² * K) where W=J/s; delta T is in K and delta t is in s. so:
$J={\frac {W}{m^{2}\cdot K}}\cdot m^{2}\cdot K$
$J=W=J/s$ is wrong. when you multiply it by s you get J = (J/s) * s = J which is right.

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Please!!! Someone help improve this. This topic deserves pages and pages, with links to all the different correlations and when they are applicable.

I don't think this is really what is needed. There are literally thousands of heat transfer correlations - perhaps this article could refer the reader to the "Handbook of Heat Transfer" by McGraw-Hill. What is needed is a much better explanation of the Heat Transfer Coefficient. Using internal flow as an example is a really bad idea, since it should include a correct treatment of log-mean temperature difference. External flow is much simpler, and would get the general concept across better -- which is a better objective, I think. If someone improves this, that's what I'd recommend. (BTW, the Dittus Boelter is kinda out of date anyway. There are now much better correlations for fully developed pipe flow. Dr. David Naylor

I can add more detailed explanations of the derivation of the heat transfer coefficient. But im not really good at typing maths on wikipedia! haha... Anyway, LMTD is used in heat exchanger calculations. Internal convective flow in pipes doesnt need LMTD, rather the film and bulk mean temperature is used. Venny85 (talk) 07:48, 9 May 2011 (UTC)

I came to this article for help and clarification, but it just confuses matters.

For a start, h can be a local HTC at distance L, or an average over 0 to L, but which one is never mentioned, so not only can the formulae not be made use of, but the reader is left ignorant of the possibility.

Secondly, in external flow, the free stream temperature doesn't change so it can be referred to by a single number, T_inf, but with internal flows, like in a pipe, once the boundary layers have joined up, there is no free stream temperature to plug into the formulae, and the bulk temperature is changing. Some sort of average or LMTD temperature might be the right thing to do, or it could also be entry temperature: the article doesn't say. If the internal flow isn't fully developed, then the mass flow through the duct, and the mass flow participating in the thermal boundary layer are probably different. Again, this is not discussed.

p.s. If Dittus Boelter is out date, as Dr Naylor states above, what and where is the up-to-date method?

JBel (talk) 23:25, 30 November 2018 (UTC)

LMTD

Please Help.

how can i measure the efficiency regarding heat input and output in shell and tube heat exchanger?

Properties table removed

I removed some text from the article. It was a table of Prandtl number and thermal conductivity of air. Not really relevant to the article. My reasoning is this, Prandtl number has its own article, and equating thermal conductivity and heat transfer coefficient is just plain wrong. Especially for air.Djd sd (talk) 05:12, 30 January 2008 (UTC)

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External Convection in Pipes

Is there any simple correlation for the heat transfer coefficient for the exterior of a pipe? (for instance when we have an insulated pipe exposed to the air)--190.160.181.235 (talk) 03:37, 11 September 2008 (UTC)

Numbers ?

This article is not very useful - there's no numbers. What's an example coefficient? 1? 10000? 10000000000000? and what are some common values and what they mean? —Preceding unsigned comment added by 203.206.137.129 (talk) 11:39, 7 November 2008 (UTC)

• "there's no numbers" - Here are some "numbers":

"Some typical heat transfer coefficients include:

• Air - h = 10 to 100 W/m²K
• Water - h = 500 to 10 000 W/m²K"

This values are indicative, and there are no other numbers, because the heat transfer coefficient is determined for each combination of materials, geometry, flow regime, and so on. You can found a number by yourself, applying the equation on the articles.

• "and what they mean?" - Read the article and say me the parts of it that you don't understand, I will try to write them more clearly. --Aushulz (talk) 13:33, 25 November 2008 (UTC)

No source was given for these numbers, and as the above note makes clear (but the article text does not!) the exact values depend on system geometry, so for microscopic distances you expect much larger values, and over hundreds of meters much smaller ones. The source for the above numbers appears to be this page where the context is residential or commercial walls and heat exchangers - i.e. a few cm to 1 m in size. This should be made clear in the article (or the numbers removed altogether).

ArthurPSmith (talk) 20:18, 4 September 2009 (UTC)

Inverse

This article contained a number of wrong statements, which I corrected, including the following: "Heat transfer coefficient can be thought of as an inverse of thermal resistance." Despite being full of weasel terms (why not just "is""? Why "an" inverse - are there more than one!?), it was also factually wrong, or at least it disagreed with the article Thermal conductivity, to which thermal resistance redirects. I replaced the link with "thermal insulance", even though I believe that the reader learns more about insulance here than on the other page. Nevertheless, the other page lists the related quantities in context, so that's of some use. — Sebastian 03:02, 13 December 2008 (UTC)

Combining Heat Transfer Coefficients

Someone changed this section by switching the equations for adding heat coefficient in series vs. parallel. The section was correct before the change, so I changed it back. Check any college physics textbook. Heat transfer coefficients do not combine like electrical resistance. —Preceding unsigned comment added by 192.12.184.2 (talk) 16:17, 24 February 2010 (UTC)

Actually, they are additive! or rather the resistance. where q = deltaT/ Sum of resistances. In the term for resistances, you can add it all up. For 1D steady state convection through plane wall, its R convection = 1/hA. And it if its conduction, its R Conduction = L/KA . The resistances can be drawn like a thermal circuit whether in parallel or series much like an electrical circuit diagram. Venny85 (talk) 07:42, 9 May 2011 (UTC)

This article is very poorly written

For someone not already thoroughly familiar with the field this article is not very helpful. The introduction is OK but thereafter it goes downhill rapidly and the standard of technical writing is generally rather poor. Many terms are used in the equations without any explanation of what they are or the explanation occurs some distance after the terms are introduced. For example what is $\mathrm {k}$? What is ${T}_{s}$? What is ${T}_{\infty }$? What is a 'representative length'? What is $\mathrm {Ra}$? What is $\mathrm {Re}$? What is $\mathrm {Pr}$? What is $\mathrm {Gr}$? There is a figure with absolutely no explanation which is therefore useless.130.246.132.178 (talk) 10:34, 1 February 2012 (UTC)

Heat transfer coefficient of pipe wall

The section about heat transfer coefficient of pipe wall looks incomplete. Is there a missing equation before line "where k is the effective thermal conductivity of the wall material and x is the wall thickness"?. Gknor (talk) 10:30, 6 September 2013 (UTC)

Thermal convection correlation

Since natural and forced convection correlations are concerned with the thermal convection coefficient, which is much more specific than the overall heat transfer coefficient, I suggest to move it to a page linked with thermal conductivity: there are simmetries and complementary aspects in Newton cooling law and Fourier law, and so on. This is not teaching anything about the overall heat transfer coefficient, so is very confusing in the actual structure and it could bring to misunderstanding if one try to compute the overall h.t.c. with one of these correlations --2.156.167.55 (talk) 17:18, 22 December 2013 (UTC)

Merge with thermal transmittance article

I suggest merging all of the thermal transmittance article into the heat transfer coefficient article. As far as I can tell from reading the articles, the two terms are synonyms that both describe the same thing -- the relationship between the temperature difference across some object, and the rate of transfer of heat through that object -- using the same units -- W/(m²K).

If you think there is some difference between them, please mention that difference in the articles. Thank you. --DavidCary (talk) 16:25, 9 June 2014 (UTC)

I have read the intro to thermal transmittance and it seems to be what is typically called in Chemical Engineering "The Overall Heat Transfer Coefficient" of building materials. The heat transfer coefficient in this page is only on component of the overall heat transfer coefficient. Sturgman (talk) 00:50, 7 May 2015 (UTC)

That said, I think we should make the heat transfer coefficient page a redirect to Newton's law of cooling section in the convective heat transfer page. The material for heat transfer correlations in different situations can then be moved to a new page Correlations for the heat transfer coefficient. Sturgman (talk) 14:46, 7 May 2015 (UTC)

Agree on the merge, since this page is just the way that engineers talk about heat transfer coefficients. However, after some discussion, I have decided to (re)create the Newton's law of cooling page, since it really doesn't have a good home in the present Convective heat transfer article. Newton's law of cooling in today's language simply states that the heat transfer coefficient is constant. That's sometimes true, and sometimes not. The conditions where it is not, are better discussed separately. Certainly it isn't always strictly true in convective heat transfer, and even Newton recognized that. So, it's a special case and needs its own article (and history). Summaries of it can be put anywhere you like, with Newton's law of cooling as the main article for the section. SBHarris 02:02, 13 June 2015 (UTC)

Harris,that sounds like a reasonable approach. Are you suggesting removing Newton's law of cooling from the convection page? I think it should still be mentioned there and proper linking provided to the new page. — Preceding unsigned comment added by Sturgman (talk • contribs) 12:52, 14 June 2015 (UTC)

Yes, Sbharris did leave a short section on Newton's law of cooling on the convective heat transfer page. (I've also inserted a basic formula for heat transfer by convection.) Pololei (talk) 00:43, 18 September 2015 (UTC)

Don't merge—strongly oppose. Each of these physical properties is a subject in its own right. This seems clear from the articles themselves. Whilst the two properties share dimensions and units of measurement, they each have their own context of use. The overlap in the two subjects is comparatively small and arguably even an abstraction. The article on thermal transmittance includes a short statement distinguishing the property from the heat transfer coefficient. Pololei (talk) 00:14, 18 September 2015 (UTC)

Forgive me for bringing up a years-old discussion, but if it's different why is it stated in this article that: "The overall heat transfer rate for combined modes is usually expressed in terms of an overall conductance or heat transfer coefficient, U. In that case, the heat transfer rate is:" ?

Should this sentence be erased? I'm hesitant to do it because I admit I still don't fully understand the differences. Thariqziyad (talk) 08:36, 9 June 2022 (UTC)

As this proposal is quite old and there's no consensus—or, if anything, opinion is against merging—I'll remove the merge proposal tags and close this discussion. Pololei (talk) 19:23, 7 October 2015 (UTC)

Composition?

The section called Composition doesn't belong here. It relates to practical examples of convection in pipes and walls. It should be removed, or placed near the bottom of the article. It also needs a more specific title.Parveson (talk) 13:24, 7 June 2019 (UTC)

Inverse-square law

Is the Inverse-square law applicable to the HTC? Was this topic ever discussed in relation to HTC? I'm curious. All the best. AXONOV (talk) ⚑ 16:08, 27 May 2022 (UTC)