I want to minimize irreversibility losses in the heat exhanger. So i want to know if this can imply to improve exergetic efficiency . if yes how. Thank you
One of the crucial ways to minimize the irreversibility within a heat exchangers is reducing the temperature difference (the temperature profiles should be matched in such a way that the thermal heat conversion become maximum)
Please see the pinch point technology in some well-known books.
Agree with the comments above. In Pinch Technology, being able to match streams appropriately to minimize these reversibilities is the objective. Streams with extremely large temperature differences may not be well matched, or not matched at all. Usually taking a close look at these matches and improving them using the Pinch Heuristics will result in less reversibility and in many cases reduced utility usage for overall cost savings. The details can get very complicated depending on the number of exchangers and other process specifics, but this can be done.
The irreversibilities that matter in heat exchangers are primarily due to heat transfer over large temperature differences, as mentioned above. These can become especially large in heat exchangers where the capacity rates (mdot * cp) are quite different on either side of the heat exchanger, or where there is a phase change taking place, or where a fluid has large changes in the specific heat capacity (eg around the critical point). Such things result in lines of significantly different gradients on T vs H plot (temperature vs enthalpy rate, mdot * h). The pinch point will limit how close together the hot-side and cold-side curves can approach each other.
Another source of irreversibility relates to flow friction in the heat exchanger channels. This manifests as higher required pumping power, but in an exergy analysis can be directly connected to destruction of exergy through friction.
Edit: to answer your specific question, namely "does reducing irreversibilities imply improvement to exergetic efficiency", the answer is yes. Exergy efficiency for a heat exchanger is the exergy out (cold side exergy gain) divided by the exergy in (hot side exergy supplied). Exergy that doesn't go the cold side is either lost eg by conduction to the surroundings, or destroyed within the system, via the heat-transfer-through-a-temperature-difference stuff, described above. Exergy destruction is equivalent to irreversibility, so if you can reduce exergy destruction (or also exergy loss) then you improve exergy efficiency.
Thank you.
I have already determined the overall exergy efficiency of a system, with contain several heat exchangers, based on internal system temperatures. I think that the differents possibilities where I have a maximum exergy efficiency that means I have reduced the total irreversibility of the system. what do you think? thank you
There are different forms of second law analysis. For systems and thermodynamic design of systems we have exergy analysis as the most common form. For fluid flow and heat transfer we have flow exergy analysis and entropy generation analysis. Since all forms are derived basically from the second law all are the same.
However the level of details in each approach is different. Let me explain the following example:
You are going to design or analysis of a cycle which includes some heat exchangers. for these you will conduct a thermodynamic design at first or the highest level using for example exergy analysis. At this point you will have some main design variables for each components. Then for each components you will go through a different design process to achieve the design variables from the first step. Now for heat exchagers you need to design it in more details which gives you the designed performance in the cycle. At this point you might use the entropy generation method or flow exergy analysis.
Your goal of the analysis or design and its level of details is not clear. But generally, when you reduce the irreversibilities of a system, the efficiency can be higher and your method is just OK in my idea.
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