Engineering task and calculation objective
The CIRC module calculates heat transfer and pressure drop in the shell side of shell-and-tube heat exchangers with disk-and-doughnut baffles. In this design, central disks alternate with annular (doughnut) baffles, so the shell-side fluid is guided alternately radially outwards and back inwards through the central ring opening — an axisymmetric alternative to segmental baffles.
The calculation is based on the methods from "Verfahrenstechnische Berechnungsmethoden, Teil 1" (VCH Weinheim), a standard German reference for process engineering calculation methods. From the mass flow, fluid properties such as density, thermal conductivity, viscosity and Prandtl number, and the geometry of shell, bundle, disk and doughnut, the module determines the shell-side heat transfer coefficient and the pressure drop — both quantities are prerequisites of any thermal-hydraulic design. The tube layout data can be transferred from the SPIE module; for the overall design of a heat exchanger, CIRC can be integrated into the WTS program package.
In practice, disk-and-doughnut baffling is used where a uniform shell-side flow pattern with few dead zones is desired, for example with fouling-sensitive media or vibration-prone bundles.
Standard and calculation basis: Verfahrenstechnische Berechnungsmethoden, Teil 1, VCH Weinheim
Calculation workflow
- Specify operating data and fluid properties: Mass flow or volume flow, inlet pressure, and the inlet and outlet temperatures of the shell-side fluid are entered. The fluid properties — density, specific heat capacity, thermal conductivity, viscosity and Prandtl number — are evaluated at the mean temperature; for the wall correction, the dynamic viscosity at wall temperature is additionally required.
- Describe the bundle and baffle geometry: The inside shell diameter, bundle diameter, outside tube diameter, transverse tube pitch and bundle length define the tube field. The baffling is defined by the hole diameter of the doughnut, the disk diameter, and the pitch and number of baffles.
- Determine flow zones and velocities: The module divides the shell side into longitudinal flow zones (around the disk and through the doughnut opening) and cross flow zones. From the associated areas, the velocity around the disk, the velocity in the doughnut opening and the governing cross flow velocity are obtained.
- Calculate the heat transfer coefficient: Using the Reynolds and Prandtl numbers, Nusselt correlations for the cross flow and longitudinal flow zones are evaluated and combined into an effective shell-side heat transfer coefficient. The wall correction (η/η_W)^m accounts for the viscosity ratio between the bulk flow and the wall.
- Determine the pressure drop: From friction factors for the cross flow and turnaround zones, the number of baffles and the inlet and outlet nozzle diameters, the total shell-side pressure drop is summed up.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Total pressure drop: Δp = Δpc + Δpl + Δpn | Δp | Pa |
| Friction factor of the cross flow | ξq | - |
| Number of tube rows in cross flow zone (average) | nq | - |
| Density | ρ | kg/m³ |
| Tube pitch (crosswise) | t | m |
| Outside tube diameter | da | m |
| Reynolds number | Req = vq∙da/ν | - |
| Cross flow velocity | vq = V/Aq | m/s |
| Kinematic viscosity | ν | m²/s |
| Friction factor of the longitudinal flow | ξl | - |
| Dynamic viscosity | η | mPa·s |
| Dynamic viscosity at wall temperature | ηW | mPa·s |
| Baffle pitch | S | m |
| Number of baffles | N | - |
| Average velocity in baffle opening | vl = V/Al | m/s |
| Pressure drop of the cross flow | Δpq | Pa |
| Pressure drop of the longitudinal flow | Δpl | Pa |
| Cross flow area | Aq = LE∙S | m² |
| Exponent of the wall correction (η/ηW)m | m | - |
| Hole diameter of doughnut | dR | m |
| Disk diameter | dS | m |
| Number of tubes in longitud. flow zone doughnut | nR | - |
| Number of tubes in longitudinal flow zone disk | nd | - |
| Area of longitudinal flow zone through doughnut | AR | m² |
Calculation options
Fluid liquid /gaseous?
liquid · 1
Frequently asked questions
When should disk-and-doughnut baffles be preferred over segmental baffles?
The disk-and-doughnut arrangement produces an axisymmetric flow without the pronounced bypass and leakage streams along the segment edge. It is suitable where uniform heat transfer around the circumference, fewer dead zones or reduced vibration excitation of the bundle are desired. Its drawbacks are more elaborate fabrication and poorer drainability and venting of the shell side in horizontal installations.
Why is the viscosity at wall temperature required?
With heated or cooled walls, the viscosity in the near-wall boundary layer differs from that of the bulk flow, which measurably changes the heat transfer. The correlations correct for this with the factor (η/η_W)^m. If the wall viscosity is not set, the correction is omitted — for viscous liquids with large temperature differences, this can significantly distort the calculated heat transfer coefficient.
Which geometry inputs are, in practice, the most common source of error?
The assignment of tube counts to the flow zones: the number of tubes in the longitudinal flow zone of the disk and in the longitudinal flow zone of the doughnut, as well as the average number of tube rows in the cross flow zone, must match the actual tube layout. Inconsistent inputs lead to wrong flow areas and hence wrong velocities. Transferring the data from the tube layout module SPIE avoids these errors.
Does CIRC also calculate the tube side or the complete exchanger?
No, CIRC deals exclusively with the shell side (the outer space). For the tube-side heat transfer and for combining both sides into the overall heat transfer coefficient, surface area and duty balance, the module is integrated into the WTS program package, which handles the overall design of the heat exchanger.