Heat Exchanger Design Lab (LMTD & Effectiveness-NTU) Community
Questions, answers and tips from people who use the Heat Exchanger Design Lab (LMTD & Effectiveness-NTU). Switch between counter-flow and parallel-flow, size a real heat exchanger, and watch the hot and cold temperature profiles run the length of the unit while effectiveness-NTU and LMTD agree on the same heat duty..
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This page shows only real posts from real people, so it starts empty. The guide below has the questions people most often have about the Heat Exchanger Design Lab (LMTD & Effectiveness-NTU). Pick one to start a thread, or write your own.
From the tool guide: common questions about Heat Exchanger Design Lab (LMTD & Effectiveness-NTU)
The items below are not posts by community members. They come from the written guide for this tool and are here to get a conversation started. Use “Ask a follow-up” to open a thread about any of them.
Why does counter-flow always transfer more heat than parallel-flow for the same size and flow rates?
In parallel-flow both fluids move the same direction, so their temperature difference is largest at the inlet end and shrinks steadily, capping how close the outlet temperatures can get; in counter-flow the fluids move opposite directions, keeping a more even temperature difference along the whole length, which lets the cold outlet temperature actually exceed the hot outlet temperature in some cases - something parallel-flow can never achieve.
From the tool guide Ask a follow-upWhy does parallel-flow effectiveness plateau instead of climbing toward 1 as NTU increases?
As NTU grows, both fluids approach a common temperature at the outlet since they can never cross paths (same direction of travel), and that common-temperature limit fixes the maximum possible heat duty at a fraction 1/(1+Cr) of the theoretical maximum - adding more area beyond that point does not transfer any additional heat.
From the tool guide Ask a follow-upKey ideas
- The effectiveness-NTU method rates a heat exchanger by NTU = UA/Cmin and the capacity-rate ratio Cr = Cmin/Cmax, giving an effectiveness that directly yields the actual heat duty without needing to guess outlet temperatures first.
- For the same NTU and Cr, a counter-flow exchanger always reaches a higher effectiveness than a parallel-flow exchanger of identical size.
- Parallel-flow effectiveness is capped at 1/(1+Cr) no matter how large NTU gets, because the two fluids' temperatures can only approach each other, never cross, as they travel the same direction; counter-flow has no such hard ceiling.
- The log-mean temperature difference (LMTD) method gives the same heat duty as effectiveness-NTU when both are done correctly - q = UA times LMTD equals q from epsilon times Cmin times the maximum possible temperature difference.
Things to try
- Try the counter-flow effectiveness-above-0.80 challenge.
- Switch to parallel-flow, push U and A to maximum, and watch effectiveness plateau at the 1/(1+Cr) ceiling.
- Compare the same U, A and flow rates between counter-flow and parallel-flow and see the heat duty difference.
- Watch the hot and cold temperature curves on the chart - see them stay apart in parallel-flow but nearly cross in counter-flow.
Where this is used
Process, chemical and HVAC engineers use exactly this effectiveness-NTU and LMTD cross-check when sizing shell-and-tube or plate heat exchangers for power plants, chillers, radiators and industrial process cooling, since choosing counter-flow over parallel-flow can meaningfully shrink the exchanger needed for the same duty.
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