Engineering guide
The DRRB module calculates the pressure drop in cross-flow over finned tube bundles. Finned tubes multiply the heat-transfer surface on the gas side and are standard in air coolers, economizers, waste-heat boilers and condensers — the price of the finning is an increased flow resistance, which must be known for fan sizing and for assessing the pressure balance of the plant.
The calculation follows the methods in "Verfahrenstechnische Berechnungsmethoden, Teil 1" (VCH Weinheim), a German reference on process engineering calculation methods. The design type captures different fin geometries and tube arrangements (in-line or staggered); the medium may be gaseous or liquid. From the approach velocity, the fluid properties and the bundle geometry, the pressure drop of the finned tube bundle in cross-flow is obtained via the drag coefficient.
Anyone who wants to calculate the pressure drop across a finned tube bundle typically uses the module together with the thermal design of the finned-tube unit, in order to optimize heat duty against fan power.
Scope
- Pressure drop in cross flow over finned tube bundles
Typical workflow
Select design type and medium
First the design type of the finned tube bundle — fin geometry and tube arrangement — and the state of the medium (gaseous or liquid) are defined. Both choices determine which correlation set applies for the drag coefficient.
Enter geometry and operating data
Tube and fin dimensions, transverse and longitudinal pitch, and the number of tube rows in the flow direction describe the bundle. From the flow rate and the narrowest flow cross-section, the governing velocity in the minimum cross-section follows.
Determine Reynolds number and drag coefficient
The Reynolds number is formed with the fluid properties of the medium, and the drag coefficient of the finned bundle is calculated from the design-dependent correlation; the finning enters through geometry ratios such as fin height and fin pitch.
Sum up the pressure drop
The pressure drop results from the drag coefficient, the number of tube rows and the dynamic pressure of the velocity in the minimum cross-section. It serves as the basis for fan or pump sizing.
Frequently asked questions
Why is the velocity in the minimum cross-section governing rather than the approach velocity?
Between the finned tubes the flow cross-section narrows considerably — by the tubes themselves and additionally by the fins. The pressure-drop correlations are referenced to the dynamic pressure in the minimum cross-section because that is where the highest velocities, and hence the governing losses, occur. Using the free approach velocity instead drastically underestimates the pressure drop.
What influence does the tube arrangement (in-line vs. staggered) have?
Staggered arrangements force a stronger deflection of the flow and therefore deliver better heat-transfer coefficients, but also higher drag coefficients than in-line arrangements at the same pitch. The correlations consequently distinguish strictly between arrangements; a wrong assignment is one of the most common sources of error when re-checking a design.
Does the calculation also apply to fouled bundles?
The correlations apply to the clean bundle. Deposits between the fins narrow the flow passages further and can drive the pressure drop in operation well above the design value. In practice a margin or fouling reserve is therefore provided in the fan sizing.
Technical content source: LV-Solved-Problems-Content 628d313247e7