» Table 1. Comparison of smooth tube and low-fin tube designs.
Heat Transfer Enhancement
Get more out of your heat exchangers with heat transfer enhancement: Part 3 – Use of externally finned tubes
In this series of articles we will look at the idea of heat transfer enhancement. The benefits of enhancement are that your heat exchangers will provide the same performance at a lower cost or provide better performance at the same or smaller overall size and footprint.
By Himanshu Joshi, Heat Exchanger Specialist, Lou Curcio, Heat Transfer Advisor, and Craig Thomas, Director of Technical Sales, NEOTISS Inc.
In the first two articles we looked at the different enhancement techniques which are commercially available. In this part we will take a detailed look at fins on the OD( outside) of heat exchanger tubes. Such tubes are also called Low-Fin tubes or Integral Fin Tubes( IFT).
When to use low-fin tubes There are several factors to consider as shown in Ref. [ 1 ] New vs Retrofit: The value proposition for low-fin tubes will vary depending on whether the project is for new equipment or a retrofit or debottlenecking of existing units. When designing a new heat exchanger, first consider if the shell side resistance is controlling( see Part 2 for an explanation of“ controlling resistance”), meaning the resistance to heat transfer is much higher on the shell-side compared to the tube-side. When retrofitting an existing heat exchanger with the goal of increasing the heat duty, low-fin tubes will increase the shell side surface area by 2.5 to 3 times without having to change the shell size or piping layout. Even if the shell side resistance is not controlling, low-fin tubes may still aid in the debottlenecking goal of a retrofit. Cost vs Benefit: For new equipment, if the shell side resistance is higher by a ratio of at least 3:1, this is typically the threshold where the added cost of the fin tube is more than offset by the reduced size and cost of the overall heat exchanger. There are exceptions to this rule of thumb when considering multiple shell designs and expensive materials of construction. Two examples of cost savings are shown below, using Ref. [ 1 ]: A compressor intercooler with a single-phase gas being cooled on the shell side with seawater on the tube side would easily meet the 3:1 ratio of shell side controlling resistance and designing with finned tubes would likely show a significant size, weight, and cost saving compared to a smooth tube regardless of the materials of construction. See Table 1, comparing two designs for cooling high pressure air with seawater, using 25.4 mm( 1 in.) Titanium tubes. There is an 18 % reduction in the shell diameter, 42 % reduction in the number of tubes, 33 % reduction in the quantity of seawater required to maintain a reasonable velocity, and a 3.6 ton lower weight. The weight savings could be important in certain cases, such as an offshore platform. Note that the surface area is based on the outside surface, with a fin density of 1181 fins / m( 30 fins / in). In the smooth tube case the heat transfer resistance ratio is about 10:1. We have assumed some fouling resistance in the evaluation. The second example is of a condenser with the controlling resistance on the shell side but by a smaller ratio of 2:1, which might normally not be a consideration for finned tubes. However, if a very expensive material of construction is required such as titanium, super duplex, or alloy 625, it may still be worth considering low-fin tube because as the material cost increases the ratio of finning cost proportionally decreases. For example, consider a 19 mm( 0.75 in.) tube:
• Smooth Alloy 625 tube: USD 20 / linear foot cost divided by the surface area 0.20( sqft / ft length) = USD 100 / sqft
• Finned: USD 25 / linear foot cost divided by the surface area 0.50( sqft / ft length) = USD 50 / sqft
• The finned tube, while 20 % more expensive per linear foot in this alloy type, is half the cost per square foot of external surface area.
Multiple Shells & Total Installed Cost: When a heat exchanger requires multiple shells, it is often a good case for finned tubes as a way to reduce the number of shells. When comparing a finned vs smooth tube design, it is important to evaluate the total installed cost, not just the fabricated heat exchanger cost. Let’ s say that the smooth tube design requires six shells in parallel, but the fin tube design allows four shells in parallel. Even if the fin tube design does not reduce the direct equipment cost compared to the smooth
Shell OD No. of Tubes Surface Area Heat Exchanger Weight( wet) Water Requirement
mm m 2 T T / hr
Smooth Tube 850 394 181 11.3 432
Finned Tube 700 232 275 7.7 288
42 Heat Exchanger World April 2025 www. heat-exchanger-world. com