Sensitivity check for heat exchanger

[CloudNine1]

Hi folks,

As part of a theoretical exercise, I'm designing a double pipe, concentric heat exchanger.
The inner pipe carries water and the outer pipe carries hot thermal oil that heats the water up to a certain temperature.
There are a lot of variables in the problem (length of the pipes, flow rate of the oil, thickness of insulation, certain temperatures along the heat exchanger that need to satisfy physical conditions (e.g thermally developed flow)) that require exhaustive iteration solution.
I have managed to balance all the equations, but now I'm facing another issue: the system seems to be quite sensitive to temperature changes. Meaning, a 0.5C above the temperature that balanced everything, obviously causes a change in the heat transfer parameters, but it feels to me that such change is too high, thus making the system impossible to build in real life.
How do I quantify the sensitivity level to see if my gut feeling is right or wrong? Is there some kind of "thumb-rule" numbers/ratios that can assist me determine whether the change that the system experiences is bearable or not?
Sorry for the unprofessional expressiveness, it's a very new world to me and I couldn't focus myself on that by looking on Google.

Thanks!

 

[gerhardl]

Try to check what happens if you compare with results from a simplified, static, section of your system, calculated (by hand) with standard simple formula??

 

[IRstuff]

You need to provide more information, inlet/outlet temperatures, delta temperatures, etc.

It's likely that the last item is the cause. You need to have a temperature difference to achieve heat transfer, BUT, for your specific example, it's likely you are running a very small temperature difference, say 5 degrees, so 0.5 C represents a 10% change in operating characteristics. If you were able to run, say, a 50C delta, then 0.5C would only be a 1% change, but that's unlikely.

TTFN (ta ta for now)
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[CloudNine1]

@IRstuff, thanks for the answer.
The water's temperature is rising from 25C to 340C.
The water is flowing extremely slow (like 0.001 m/s - it is an external demand given in the exercise) and the oil is flowing quite fast - I guess it is also what makes the system so sensitive.