1 reply to this topic

[Art Montemayor]

pea:

Don't allow yourself to get confused. This is quite a simple item to comprehend when designing and fabricating a U-tube heat exchanger.

I've often found (to my chagrin) that recent engineering authors - specifically Coulson & Richardson - like the authors that I studied 45 years ago - also fail to apply common engineering logic and explanations to equipment design. They also fail to explain accurately and clearly just what they propose and why. I attribute this to a lack of practical, hands-on experience and a willingness to simply repeat information from others or rely on hear-say that they read somewhere. This is where a Forum for Engineering Students - such as this one - is invaluable in getting to the actual, real truth, and in a rapid and authoritative manner.

What Coulson & Richardson are trying to say - but in a weak and mis-interpreted manner - is the following: when applying a tube bundle to a fixed diamter and fixed length heat exchanger shell, a U-tube design will inherently lend LESS tube heat transfer area than that of a straight-tube, single tube pass design due to the fact that the U-tube necessitates a minimum bend radius. This "bend radius" is the mechanical limit that you can bend a tube without deforming it (or "kinking" it) and plugging flow.

You don't use less tube heat transfer area in a U-tube design (you probably have to use MORE tube area due to the "f" factor); what you do to overcome the tube bend restriction is that you make the shell diameter bigger - or you make the bundle longer! You are absolutely correct in believing that you must make up the "additional" area - perhaps even more! This is the effect of an engineering trade-off.

As a student, bear in mind that the real way to analyze, design, and fabricate a heat exchanger is to carefully study and apply the many different manners that the heat transfer can take place within a shell & tube heat exchanger. Always take into consideration that the theoretical area given by A = Q/U deltaT is subject to a lot of mechanical constraints and phenomena: the number & spacing of baffles, the type of baffle, the internal leakage inherent to each type of baffle, the shell & tube side turbulence, the fluid velocity and its effect on fouling, etc., etc. All of this ultimately affects the required size of the exchanger and it is vital for engineers to understand these effects.

One way to start mastering the subject of heat transfer in a shell & tube aparatus is to read the TEMA standards - with special emphasis on understanding the various TEMA configurations of shell designs. This type of analysis will quickly orient you to the practical and real world of what you have to confront when applying a shell & tube heat exchanger and the many trade-offs that exist in this area. I realize that merely studying the theory behind the heat transfer is enough to give you multiple headaches - without getting into the actual, real fabrication world. But sooner or later you are going to have to confront reality, and the sooner you learn to visualize and understand the hydraulics and heat transfer that is occurring inside the shell & tube, the better you will dominate the subject. Besides, I firmly believe you will find it fun and very interesting to investigate the real fabrication challenges. This is the fun & enjoyable part of engineering.

I hope I helped to settle down your confuzzed state.

Art Montemayor