Logarithmic Mean Temperature Difference

[member 392791]

Homework Statement

Homework Equations

The Attempt at a Solution

I am confused as to which streams I should use for the log mean temperature difference. I am pretty sure it should be the difference in temperature between the two streams on the same side, so I think it's

(700-450)-(1100-960) / ln [(700-450)/(1100-960)], but I am not sure.

The term wasn't well defined, and my search on wikipedia gave the definition as ''The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger.''

This is too vague, which end of the exchanger??

 

[SteamKing]

Perhaps this description can clear up your confusion:

http://yjresources.files.wordpress.com/2009/05/4-3-lmtd-with-tutorial.pdf

You have a counter flow heat exchanger; see p.2 of the link for the definitions; p.1 has the formula for LMTD.

 

[Chestermiller]

The way you calculated it is correct. You are not confused about it after all.

Chet

 

[member 392791]

I am confused, because the only reason I know it had to be that was because I would get a negative logarithm if I did it the other way. Suppose it was such that I didn't get a negative number in the logarithm either way that I subtracted it, then how would I do it??

 

[SteamKing]

Read the attachment to Post #2.

 

[Chestermiller]

I am confused, because the only reason I know it had to be that was because I would get a negative logarithm if I did it the other way. Suppose it was such that I didn't get a negative number in the logarithm either way that I subtracted it, then how would I do it??

You take hot temperature minus cold temperature at one end of the heat exchanger, and hot temperature minus cold temperature at the other end of the heat exchanger. How you do things after that doesn't matter.

 

[member 392791]

Okay great, I have convinced myself of that now. Thank you!