3 replies to this topic

[gvdlans]

The temperature approach used in a shell and tube heat exchanger is simply the difference between the outlet temperature of one stream and the inlet temperature of the other stream.

You can see that there are two temperature approaches. For example if you use cooling water to cool a process stream from 95 to 40°C, where the cooling water warms up from 25 to 35°C. Here the greater temperature difference (Hot In Cold Out) is 95-35=60 °C, where the least temperature difference (Cold In Hot Out) is 40-25=15 °C.

[David Southall]

A temperature approach is just what its says - when the temperature of one stream approaches that of another. This occurs at the inlet and outlet of a shell and tube, and you can get temperature approaches part way along the heat curve if latent heat transfer is taking place.

The temperature approach is significant in that the closer the approach is, the more surface you require to progress up the heating curve/down the cooling curve. It is possible to find that the overall LMTD looks reasonable, but a very close approach (or pinch) part way up the heat curves causes problems, in that a lot of surface is required to progress past it.

Depending on what type of unit you are looking to design, you may end up looking at dividing the heat curve into small steps; this is when these pinches can be more significant. If you divide the curve into small enough steps that the heat transfer coefficient can be treated as approximately constant, you can establish the UA required for that region, and from this the area required. If you're at a close pinch, large requirements for surface area will show up.

Essentially, your LMTD gives you an idea of the driving force available to you; details of approach temperatures and pinches allow you to set an appropriate LMTD.