# Logarithmic Mean Temperature Difference

1. Dec 14, 2013

### Woopydalan

1. The problem statement, all variables and given/known data

2. Relevant equations

3. 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??

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2. Dec 14, 2013

### SteamKing

Staff Emeritus
3. Dec 14, 2013

### Staff: Mentor

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

Chet

4. Dec 14, 2013

### Woopydalan

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??

Last edited: Dec 14, 2013
5. Dec 14, 2013

### SteamKing

Staff Emeritus
Read the attachment to Post #2.

6. Dec 14, 2013

### Staff: Mentor

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.

7. Dec 14, 2013

### Woopydalan

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