# [Simscape] Why does the thermal liquid pipe calculate incorrect temperatures?

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Andreas on 11 Nov 2013
Answered: Andreas on 18 May 2015
Hello everyone,
I am currently trying to build a heat exchanger with the standard elements from the thermal and thermal liquid foundation libraries of simscape and am facing big problems with temperature calculations.
Have a look at the attached model. There I included a tube. Inlet temperature: constant 6°C. Wall temperature: temperature profile from signal builder block.
How can the outlet temperature of the tube be less than input and wall temperature (@ ca. 150 s)???
Any idea? I kinda misused the tube for a quite flat plate heat exchanger geometry, so don't wonder about the strange mask parameters.
Bonus: Can anyone explain me the parameter "shape factor"? 64 sounds ... erm... ok for round tubes, but what about more square shapes (think of 0.2*0.005 m² for heat exchangers)? How do you calculate this parameter?

Andreas on 23 Dec 2013
Edited: Andreas on 23 Dec 2013
I'll put the support answer here:
"The temperature of the port A of the block “Pipe (TL)2” is indeed rigidly specified by the block Controlled Temperature Reservoir (TL)1 to vary extremely rapidly at some points.
What happens is similar to what we described in the previous email (inverting the outlet and the inlet): “The temperature increases more rapidly at the outlet (port A) than the center (port W), and the only way for this to hold true (given that the mass flow is also fixed) is if the inlet (port B) has a decrease in temperature.”
It is true that the temperature of the thermal port W is not rigidly specified but still it cannot increase as rapidly as the temperature of port A."

Andreas on 18 May 2015
Btw: the shape factor seems to have nearly no effect on heat transfer, which is quite strange.
Imagine two pipe segments with the same surface area and perimeter. One of them is perfectly round, one is only a small gap. The latter one has much more turbulent flow and boundary layer area than the round pipe and thus much more heat transfer through convection.
I cannot figure out, how to model that properly...