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Heat Pinching

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#1 goggles30


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Posted 18 April 2017 - 12:19 PM

I'm performing a heat pinching analysis for the heat exchangers in my process flow diagram. Most of the streams are multicomponent. Hence I plan to use Hysys to obtain the Cp values by taking the average value between the inlet and outlet for each stream. However, I'm concerned about streams for which the Cp is sensitive to temperature change, or the temperature difference between inlet and outlet is huge such that the Cp value varies greatly. Would taking the average Cp value lead to a huge error for this case? Would it be better to divide the stream into segments of different Cp values, considering each one as a new stream?



So for example if "hot 1" has a variable Cp value for the link above, should I split it into "hot1a" and "hot1b" with their respective Cp values for the temperature ranges which "hot 1" was split into?

#2 pavanayi


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Posted 19 April 2017 - 03:36 AM

It is really up to your judgement as to how you should proceed. If you plot the Cp vs T value for the streams of concern and see that the curve is not really straight, then you can make a judgement if you want to split (into 2, 3 etc) or not. If you are only interested in finding the pinch temperatures and targets itself, I don't think assuming an average value for Cp will affect your results significantly.


If you end up assigning heat exchanger matches, then you have to be careful. At best, the real duty/terminal temperatures will be slightly different from the calculated values due to the difference between the local specific heat and the average Cp for the temperature range as a whole. At worst, the duties will be very different.


if you are using Hysys as your simulator, then you can use the Aspen Energy Analyser program to extract the stream information automatically (and also auto segment it). 


Are you doing a manual/hand calculation?

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