Consider Finned Tubes to Increase Surface Area

This post will introduce the practice of adding fins to shell-and-tube heat exchanger tubes to increase the surface area, and discuss when to consider it and when to avoid it. This is not a post about air coolers.

Image of finned tubes: Heft Engineers Website

Review: When fluid flows through tubes in a heat exchanger, the total resistance to heat exchange is the sum of: tubeside fluid convection resistance, tube material conduction resistance, shellside fluid convection resistance. Often there is fouling, which will add tubeside and/or shellside fouling conduction resistance. The shell-side fluid resistance is governed by the equation:

q = h*A*(Tw-Tf)

Where q = duty, h = convection coefficient (depending on geometry, fluid conditions, etc.), A = surface area, and Tw-Tf = difference in temperature between tube wall and shell-side fluid. The tube wall temperature is obviously affected by the tubeside fluid temperature, usually calculated with a resistance-in-series model).

Now there are a variety of fins and spines that can be made, rectangular, parabolic, triangular, etc. and various equations to estimate their impact on efficiency, or vendors can help you investigate it. There are also different ways to mechanically bind the fins to the tubes. But what I want to keep in mind are some simple guidelines: when should I investigate adding finned tubes at all? What is a clue that I need to start considering this technology?

When you add fins, it is a gambit to increase A in the above equation. You are going to get a lot more surface area by adding these fins, without much increasing the size of your heat exchanger. This can improve your heat exchange for a given size of exchanger, or let you buy a smaller heat exchanger to do the same duty. However, improving A with fins on the outsides of the tubes is only going to help the shell-side fluid resistance. If your heat exchange problems are on the tubeside, then fins are not going to help much. Also, adding fins makes your exchanger cost more and may make it harder to clean. So you don’t want to add fins when you don’t need to.

Therefore the following general guidelines are recommended – this is a list of tips I’ve picked up from books and presentations.

Consider finned tubes when:

• The shell-side resistance to heat transfer is roughly two times the tube-side resistance. (Most commonly occurs in gas-to-liquid exchangers using natural convection on the shell-side, e.g. car radiators)
• The shell-side fluid is vapor
• The tube-side fluid is condensing water or steam
• The shell-side fouling is severe?? (Controversial – see below)

A successful use of fins can increase the surface area by 2.5 times or so, which can increase heat transfer rates by as much as 60-70%, if the applications really suits the use of fins.

However, if the resistance is 90% on the tubeside and 10% on the shellside, then increasing that shellside surface area does very little to help you. Probably fins will not be economical.

Also, installing fins in a shell-side fouling service can be detrimental. I have seen some literature suggest that installing fins in fouling service is a good idea. But others literature advises against it: the fins provide an ideal place for dirt and fouling build-up to gather and fins can actually make heat transfer worse by creating ideal conditions for a build-up of fouling. In some cases you can double the heat transfer by removing the fins in a fouling service. So proceed with caution.

Check out this for more background on fins:

http://www.cdeep.iitb.ac.in/nptel/Mechanical/Heat%20and%20Mass%20Transfer/Conduction/Module%203/main/Objectives_10.html

P.S. A parting thought: in fired heaters, fins may have to be made of stronger material than the tubes. This is because fins do not benefit as much from the cooling effects of tube fluid. (Recall that in fired heaters you are always worried with keeping the tubes wet to protect them from fire damage? Fins aren’t exactly wet…)