Perforated fins are provided to improve the capabilities of fin and tube type heat exchangers, and to adapt them for flow outside of the tube that is essentially parallel to the axis of the tube. The fins are made of a thermally conductive material, such as metal, with perforations in the fins. Fins can be of any shape. Typically, one or more tubes or binding posts pass through the fins. The fins are attached to the tube or post by press fitting, furnace or torch brazing, welding, or other method of mechanical bonding. The perforations allow heat exchange with the contents of a tube of a fluid flowing essentially parallel to the axis of the tube, in contrast to conventional fin-tube heat exchangers. The fins may also be bonded to a post or other securing means and inserted into the inside of a tube or other hollow body to improve efficiency of heat exchange. In addition, the fins may carry a catalyst, optionally carried on a washcoat or similar treatment to increase surface area.
Perforated L-foot Finned Tubes (sometimes referred to as wheel fin), which is identical in configuration to the regular L-foot fin, except that holes are perforated through the fins at regularly spaced radial intervals. Significant heat transfer advantages have been claimed for perforated L-foot finned tubes in air coolers, under the premise that interruptions in the fin surface reduce the thickness of the stagnant air film that builds up on the fin as air flows over it. This reduced film thickness results in an increased film coefficient, thereby increasing overall heat transfer rate. The improved heat transfer efficiency can reduce heat transfer surface area requirements and cooler size, or reduce airflow and fan horsepower requirements.
Perforated fins effect on the heat transfer rate from a circular tube by using wind tunnel
Aluminum L-Foot Finned Tubes consist of thin aluminum fin strip tightly wound helically around the tube circumference. An L-shaped foot, 1/16″ wide, is first formed on one side of the fin strip (hence the name L-Foot). The strip is then wound tightly around the tube, with the foot bearing on the tube outer surface. A typical fin spacing is 10 fins per inch of tube length — this can be varied. Tension in the fin strip as it is wrapped around the tube serves to seat the fin foot forcefully on the tube, and to hold the fin firmly in place.
Perforated fins effects on the heat transfer rate of a circular tube are examined experimentally. An experimental system is set up through the wind tunnel and equipped with necessary measurement tools. Hot water passes through the finned tube and heat transfers to the fin-side air created using the wind tunnel with different velocities. Two fin sets of identical weight are installed on a circular tube with different outer diameters of 22 and 26 mm. The experiments are conducted at two different mass flow rates of the hot water and six Reynolds number of external air flow. Considering the four finned tubes and one no finned tube, a total of 60 tests are conducted. Results showed that with increasing the internal or external flow rates, the effect of larger cross-sectional area is greater. By opening holes on the fins, in addition to weight loss, the maximum heat transfer rate for perforated fins increases by 8.78% and 9.23% respectively for mass flow rates of 0.05 and 0.1 kg/s at low external Reynolds number. While, at high external Reynolds number, the holes reduces heat transfer by 8.4% and 10.6% for mass flow rates of 0.05 and 0.1 kg/s, respectively.
This enhancement to the fin face creates slits much like the spokes of a wagon wheel or a small “tear drop.” The fin is preformed by a set of rollers and when it passes through the forming area of the spindle roll and primary forming roller the “tear drop” or spoke opens up. There is no loss of material.
The enhancement can be done in any type of Wrap-On (W/O), Overlapped Footed Fin (OLFF) or Imbedded aluminum or copper fin. Some tooling is different for W/O and for Imbedded perforated fins. Please refer to the appropriate section in this catalog.
The slit or “tear drop” increases heat transfer and can reduce the required amount of heat transfer surface. Each end user would need to determine the percentage of improvement offered by this enhancement over the plain fin. The industry has also accepted that this type of enhancement reduces the amount of air required and therefore decreases power costs.
Perforated Fin: Available for any Aluminum or Copper Fin
Fins per Inch: 4-12 FPI
Tube Material: Ferrous, Non-Ferrous & Some Alloys
Fin Contact: Primary Fin selection determines fin contact
Type of Service: Primary Fin selection determines service level
Maximum Tube Wall Operating Temperature: Primary Fin selection determines the temperature
Maximum Tube RPM on 1” Tube: 3000