Hi ,
When designing a new exchanger, we typically add 10% design margin on flow rate or duty. Here we are planning for future capacity and uncertainty in process conditions and this is typically caught in the process design basis.
On the other hand, overdesign in exchanger area is a performance indicator that comes out of the thermal design itself, telling you how oversized (or undersized) your current exchanger geometry is for the given duty.
In other words, duty overdesign is an input you choose; area overdesign is an output you calculate. Mixing the two can lead to wrong expectations about how your exchanger will behave when flow increases. This is actually a common point of confusion where I have seen many process engineers fall into.
I broke this down with a worked example in this short article – including what happens to area overdesign when you increase flow by 10% on both sides of the exchanger:
👉 Read the full explanation here:
Overdesign on Exchanger Duty vs Area – Which to Consider
Happy design!
Mohamad
P.S. If you’d like a full, step‑by‑step workflow for thermal design of shell-and-tube exchangers using HTRI, you can check out our Heat Exchanger Thermal Design course.
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