Cryo:
You are on the right trail in questioning the configuration for a compressor intercooler. It’s a logical question and, as such, has a logical answer that is revealing and consistent with engineering logic.
The reasons for placing the gas on the shellside in a compressor intercooler are as follows. Some are economically driven; others are process-logic driven.
1) The shellside of a tubular heat exchanger inherently has the lower pressure drop – as compared with the tubeside. It’s logical to have the process gas on the shellside because you would want to keep your energy bill as low as possible;
2) The intercooler gas is usually separated of a condensable component – such as water – and the shell side, with its lower Reynolds Number and velocity, is the natural and simple way to handle the necessary phase separation and draining of the liquid portion;
3) The nature of a gaseous fluid gives it the notoriety for having a lousy conduction and film heat transfer coefficient; this forces the heat transfer designer to include tube fins in some cases – in order to compensate for the miserable gas film coefficient. The fins, of course, go on the gas side. If the gas is on the tubeside, this makes it troublesome and expensive. There are (and have been) some tubes fabricated with internal fins specifically for this type of application – however, this is limited to the larger sizes. And they are expensive to make – and more so to clean. The practical and logical solution is to place the gas on the shellside where the potential application of external tube fins is easy and conventional.
4) Although a liquid fluid has the potential for a desirable high heat transfer coefficient, it must undergo an increased pressure drop to accentuate the positive, resulting heat transfer. But that requires a high Reynolds Number and increased velocity – something that is difficult to achieve in the shellside without introducing horizontal and multiple baffles – something that every plant and maintenance engineer wants to avoid if possible. It's far more logical and reasonable to place an intercooler’s cooling water in the tube side where the ability to design an appropriate water flow is easier due to simply multiplying the number of tube passes – something that is more difficult and expensive when done on the shellside. It is also far easier to clean (or rod-out) exchanger tubes than to try to extract a tube bundle or acid-clean a fixed-tube sheet intercooler. A fixed-tube sheet cooler is far less expensive than one with a removable tube bundle and has a smaller diameter it therefore is a natural as an intercooler with the gas on the shellside. Let’s be practical: the fouling in a compressor intercooler better be restricted to the water side; if it occurs on the gas side, you’ve got more serious problems than just fouling.
5) For what it’s worth, the shell exterior acts as a heat loss for the hot gas and helps in cooling it.
The above are some of the more obvious reasons that I’ve come across in applying compressor intercoolers. I’ve designed and built about 15 to 20 of them in the field and I’ve always applied the same philosophy and general guidelines. The only times that I’ve placed gas in the tubeside of an intercooler is in the case of employing spiral-tube coolers such as the Graham Heliflow. This, I’ve done with relatively higher pressure gas – 150 to 5,000 psig pressures. However, when I’ve done this I have always had to compensate for the higher gas-side pressure drop. Usually this is in the form of the compression ratio and the energy bill.
I hope the above addresses and responds to your query.
