Engineering task and calculation objective
The GGRI module calculates the shell-side heat transfer coefficient on low-finned tubes in shell-and-tube heat exchangers with segmental baffles. Low-fin tubes carry rolled circular fins with about 630 to 1000 fins per meter (16 to 26 fins/in) and fin heights of 1 to 2 mm; they increase the outside surface area by a factor of 2.5 to 3 compared with the plain tube and pay off whenever the shell-side heat transfer limits the design.
The calculation method follows the Heat Exchanger Design Handbook (HEDH): the correlations obtained for circular-finned tubes in crossflow are transferred to the case of low fins in the baffled shell side, with bypass and leakage streams across the segmental baffles accounted for as in the classical shell-side calculation. If you want to calculate heat transfer on a finned tube bundle, this gives you a consistent link to the design of shell-and-tube exchangers by the stream analysis method.
Important application limits: low-finned tubes are not suitable for laminar flow (Re < 1000), because the fin benefit is largely lost there, and they are not recommended for condensing steam, since the condensate film floods the spaces between the fins.
Standard and calculation basis: Heat Exchanger Design Handbook / Mechanical design of heat exchangers", Hemisphere Publishing Corporation, New York 1998
Calculation workflow
- Define the bundle and fin geometry: The inputs are the root tube diameter, fin height, fin density (fins per meter), fin thickness, as well as the tube pitch and layout angle of the bundle. From these, the area enhancement factor, the minimum flow cross-section and the characteristic length governing the dimensionless numbers are derived.
- Describe the shell-side geometry: The baffle cut, baffle spacing, shell and bundle diameters, and the clearances between tube and baffle and between baffle and shell define how the shell stream splits into crossflow, bypass and leakage fractions.
- Enter fluid properties and operating data: For the shell-side fluid, the mass flow, density, viscosity, thermal conductivity and specific heat capacity at the mean temperature are required; from these, the Reynolds and Prandtl numbers in the minimum cross-section of the finned bundle follow.
- Calculate heat transfer for the ideal crossflow finned bundle: Using the HEDH correlations for circular-finned tubes in crossflow, the Nusselt number of the ideally cross-flowed bundle is determined; the fin efficiency accounts for the fact that the fin temperature drops toward the fin tip, so the enlarged area is not fully effective.
- Correct to the real shell side and obtain the result: Correction factors for leakage, bypass and window flow reduce the ideal value to the effective shell-side heat transfer coefficient. This is referred to the chosen reference area (e.g. the root tube outside surface) and can be used directly in the overall heat transfer coefficient calculation of the exchanger.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Inside diameter of the shell | Di | m |
| Diameter of the baffle | Dl | m |
| Bundle diameter at cross flow zone | DB | m |
| Height of baffle cut | H | m |
| Baffle spacing | S | m |
| Number of tubes including blanks and support tubes | n | - |
| Number of tubes in the upper and lower windows | nF | - |
| Outside diameter of the tubes | da | m |
| Diameter of the bore-holes for the tubes in the baffle | dB | m |
| Tube pitch crosswise to direction of flow | s1 | m |
| Tube pitch in direction of flow | s2 | m |
| Number of sealing strip pairs | nS | - |
| Number of main resistances in a cross-flow zone | nW | - |
| Number of the shortest connection lines | nV | - |
| Volume flow | V | m³/s |
| Prandtl number | Pr | - |
| Prandtl number at wall temperature | PrW | - |
| Kinematic viscosity | ν | m²/s |
| Thermal conductivity | λ | W/(m·K) |
| Heat transfer coefficient | α | W/(m²·K) |
| Reynolds number | Reψ,1 | - |
| Shortest connecting path between tube and tube | e | m |
| Distance between boundary tubes and shell | e1 | m |
| Sum of the shortest connecting paths in the center | LE | m |
Calculation options
Fluid liquid /gaseous?
liquid · 1
Frequently asked questions
When do low-finned tubes pay off compared with plain tubes?
Whenever the shell-side heat transfer coefficient is significantly smaller than the tube-side one — typically with gases, viscous liquids or organic media of low thermal conductivity on the outside and water or steam on the inside. The 2.5 to 3 times larger outside area partially compensates for the poor external heat transfer. If both coefficients are of similar magnitude, or if fouling dominates anyway, the finning brings little benefit and only makes cleaning more difficult.
Why are low-fin tubes unsuitable for laminar flow and for condensing steam?
At Re < 1000, a largely stagnant flow forms between the closely spaced fins; the boundary layers of neighboring fins merge and the enlarged area is hardly used thermally. With condensing steam, the high surface tension of the condensate keeps the narrow spaces between the fins flooded, so the area advantage is negated by the thick condensate film. For organic media with low surface tension, finned-tube condensation can, by contrast, be very effective — but that is outside the scope of this module.
What role does the fin efficiency play?
The fin conducts the heat from the tip to the tube; along the fin, the temperature difference to the fluid decreases. The fin efficiency indicates what fraction of the fin area is effective compared with an isothermal fin. It depends on fin height, fin thickness, the thermal conductivity of the tube material and the heat transfer coefficient. For low fins made of well-conducting material it is usually above 0.9; for high-alloy steels with low thermal conductivity it can drop noticeably and must be taken into account in the design.
How do bypass and leakage streams enter the calculation?
As with a plain tube bundle, only part of the shell stream actually flows across the bundle; the rest passes through the clearances between tubes and baffle, between baffle and shell, and around the bundle edge. These streams contribute little to heat transfer. The method corrects the ideal value of the cross-flowed bundle with factors calculated from the clearance areas and the baffle geometry — tight manufacturing tolerances and sealing strips improve the result directly.