Calculation of td_log in heat exchangers with constant temperature difference

[dk-teknologisk-enp]

Currently it seems that if ttd_u == ttd_l for a heat exchanger, then td_log ends up as nan, and thus also the kA.

However, this is a perfectly valid situation, for example in a heat exchanger with purely boiling/condensing on both sides.

I suggest making the following changes to base.py, such that td_log is set to the constant temperature difference in case ttd_u == ttd_l:

(base.py:838-840)

[fwitte]

Dear @dk-teknologisk-enp,

that sounds like a very good idea. It seems I have never come across this bug in 6 years of development and using the software. Thanks a lot. Do you like to make the changes in a pull request yourself? I‘ll be in holidays one more week and can only implement the changes once I get back to work.

best

Francesco

[dk-teknologisk-enp]

Hi Francesco,

I have made this PR: #396

I have not added any tests, but that might be a good idea.