In a previous article we focused on problems such as corrosion, erosion and thermal fatigue in the tubes of Shell & Tube heat exchangers.
In this month’s article, we aim to provide you with an overview of some of the different types of finned tubes (extended surface tubes) you may encounter and the applications or duties they suit.
Finned tube heat exchangers generally use air to cool or heat fluids such as air, water, oil or gas, or they can be used to capture or recover waste heat. These heat exchangers can used in a broad range of industries including oil & gas, power generation, marine and HVAC&R.
Finned tube heat exchangers have a wide range of applications, a few of which are:
- diesel charge air coolers;
- oil coolers;
- hydrogen coolers;
- waste heat recovery;
- air conditioning;
- air heaters;
- steam condensers;
- generator coolers
Finned tube heat exchangers are often used in circumstances where air is the preferred medium for the cooling or heating, particularly where there is limited or poor quality water.
In a finned tube heat exchanger, heat is exchanged between a thermally efficient fluid that transports heat efficiently, such as a liquid which has some viscosity, and a fluid that does not, such as air or gas with little density. On the ‘air side’, the tube surface is enhanced by the addition of fins or other elements such as looped wires, designed to increase the surface area of the tube and improve its thermal performance.
Fins can range in height (high-fin to low-fin) and the fins can be either pressure connected to the outer surface of the tube or formed into the tube surface.
Depending on the intended duty and the environment in which they are to operate, finned tubes can be manufactured in numerous designs and incorporate a combination of differing materials for both the tubes and the fins. The types and combinations of tubes and fins is significant, but in this article, we will explore only the more common types.
The profile of the fins has significant effect of the performance of a finned tube heat exchanger. It is important to ensure each fin has a tight connection on the tube surface to provide maximum thermal conductivity.
The larger the fins and the tighter the fin pitch, the more thermal conductivity is achieved. The trade-off may be an increase in pressure drop which may, in turn, adversely affect performance. A balance between the two opposing functions is vital for effective and optimal thermal performance and equipment function.
1. Elfin Technology
‘Elfin’ finned tubes are used extensively in hydro power generator coolers and have been chosen by hydro power stations such as Snowy Hydro, Hydro Tasmania and Origin Energy, to provide long lasting and reliable cooling of their important generators.
With the Elfin computer-generated technology, each fin strip is mechanically forced over the outside of tube, producing a tight and thermally efficient bond. This process ensures, not only excellent adhesion of the fin to the tube, but the vital inner tube wall is not compromised in any way.
Unlike Elfin tubes, bulleted tube fins are formed by passing a bullet inside the tube to enlarge and force its wall outward into the fins to form a bond with the fins.
As can be seen in Fig. 4 the fins have been precision punched to leave an ‘L’ shaped lip that ensures exact fin pitch spacing between fins. The computerized punch process can adjust the tube pitch minutely to 0.01mm tolerance to conform with the exacting computer calculations. Elfin finned tubes are produced in blocks which are custom sized to fit the exact duty requirements and dimensions of the heat exchanger.
Bulleting require tubes that are sufficiently thin and ductile to allow for expansion however Elfin technology allows tubes of any wall thickness and material to be used and ensures the inner surface of the tube is not compromised and the strength of both the tubes and the fins is enhanced.
2. Bulleted Fins
With an external appearance similar to Elfin Finned Tubes, bulleted finned tubes are a common and cost-effective way to attach strip fins to tubes. This is achieved by manually placing the fins over the tubes and pushing or pulling a ‘bullet’ through the tube (“bulleting”) to expand the tube wall out into the fins, locking them in place.
Bulleting is commonly used in Fin Coil units found in applications such as HVAC&R and is a cost effective process which requires tubes that are of a material and thickness and sufficiently ductile to enable the tube to be expanded into the fins.
Round or Helical Fins come in a number of geometries commonly identified by a letter corresponding to the profile of the base of the fin where it connects with the tube.
3. ‘L’ Finned Tubes
One common type of finned tube is the ‘L’ fin. Receiving its name from the letter it creates from the cross-sectional view, the ‘L’ fin relies on maximum surface contact between fin and tube which is ensured by tension-forming a fin strip helically around the base tube.
This type of connection maximizes the heat transfer capacity and enhances the corrosion protection of the tube. The ‘L’ fin accommodates temperatures between 150 to 170 °C and comes in mainly ductile metals such as aluminum or copper which are capable of withstanding the compression around the base of the fin and allow stretching on the outside during installation.
4. ‘LL’ Finned Tube
Manufactured in the same way as the ‘L’ finned tube, the ‘LL’ fin has overlapping feet to completely enclose the base tube, resulting in excellent corrosion resistance. The maximum operating temperature is approximately the same as the ‘L’ fin. This type of fin commonly available in aluminum and copper. The Overlapped “L” fin design has interlocking fins that are wound together to prevent movement and separation. The fins protect the entire tube and the designation works well for the applications where corrosion is an issue.
5. ‘KL’ Finned Tube
‘KL’ Fin Tubes are also called knurled finned tubes. The fin is wrapped around the tube and the foot is rolled into the outer surface of the pre knurled tube and secured at each end. The fins are manufactured from a strip of metal which is machined into an accurately controlled L shape foot, similar to the L type fin, then it is rolled into a taper causing it to curl. The tube surface is knurled by a rotating tool, then the foot of the fin is knurled into the base tube providing a tight bond that optimizes thermal transfer.
6. ‘G’ Embedded Finned Tube
The main design feature of Embedded Fin tubes involves the fin being inserted and welded into a helical groove cut into the tubes. G fins can be used in higher temperatures and are very durable. Embedded fins are best suited for use in high thermal cycling or high temperatures and where the fin side will be subjected to regular cleaning. This type of fin comes with a major limitation being the need for a minimum wall thickness of 1.65mm to accommodate the grooves. However, the ‘G’ type fin can withstand temperatures of up to 400°C and can incorporate carbon steel fins for better conductivity.
7. Extruded Finned Tube
This fin type is formed from a bi-metallic tube consisting of an aluminum outer tube and an inner tube of almost any material. The fin is formed by rolling material from the outside of the exterior tube to produce an integral fin with excellent heat transfer properties and longevity. Extruded fin offers excellent corrosion protection of the base tube and excludes virtually all exposure to any outside fluid.
Extruded finned tubes are used in high temperature conditions and corrosive atmospheric conditions such as:
- operating temperatures less than 300°C;
- offshore or other remote applications;
- heat pipes;
- dry air coolers for air, gas or oil;
- air to air heat exchangers for HVAC applications;
- air dehumidification in air treatment plants and
- energy recovery in air exhaust system.
8. Wire Finned Tube
Wire loop tube is a high efficiency tube consisting of a series of elongated wire loops enhancing the surface of the tube, spirally wound on to the tube wall and held in position with a binding wire at the base of the loops. The loops and binding wire are then soft-soldered to the tube wall to give a metallic bond between the wire loops and the tube.
The wire loop secondary surface gives these enhanced tubes excellent heat transfer characteristics due to its ability to promote turbulence in the fluid passing over it. Temperatures of up to 250 °C can be applied to this type of finned tube.
There are many variables to be considered to successfully select and design a finned tube heat exchanger including:
- the duty to be performed;
- type, style and number of tubes required;
- metals best suited for the tubes and the fins;
- type of tube enhancement – fins or wire;
- thickness of the tube walls;
- I/D and O/D of the tubes;
- pitch of the fins;
- type and number of fans to provide air flow;
- the environment in which the heat exchanger is to be used and
- the duty it is required to perform
To ensure you get the best finned tube heat exchanger for your needs requires high-end software calculations, experience and technical know-how to bring it all together into a reliable unit that will provide years of efficient and reliable service.
Fluid Dynamics and its highly skilled international partners have many decades of expertise in the design, manufacture and maintenance of finned tube heat exchangers. Get in touch to find out more.