Engineering guide
This module calculates the shell-side pressure drop across tube bundles in shell and tube heat exchangers with and without baffles according to Section L1.5 of the VDI-Wärmeatlas (VDI Heat Atlas, 12th edition, 2019). Besides heat transfer, the shell-side pressure drop is the second central quantity of the thermal-hydraulic design: it determines the required pumping power on the shell side and, in many plants, limits the permissible baffle spacing and flow velocity.
The real shell-side flow is considerably more complex than ideal cross-flow: baffles force the fluid alternately crosswise through the bundle and lengthwise through the window zones; in addition there are leakage streams through the clearances between tubes and baffle holes and between baffle and shell, as well as bypass streams between bundle and shell wall. The VDI Heat Atlas calculation method therefore decomposes the shell side into cross-flow, window, inlet and outlet zones and corrects the ideal bundle pressure drop with factors for leakage and bypass.
Anyone who wants to calculate the shell-side pressure drop of a heat exchanger obtains with this module the contributions broken down by zone and can identify specifically whether baffle spacing, clearance dimensions or sealing strips dominate the hydraulics.
Scope
- Pressure drop tube overflowed bundles in shell and tube heat exchangers with and without baffles
Typical workflow
Enter the shell-side geometry
Shell diameter, tube diameter, pitch and arrangement, baffle spacing and baffle cut as well as the clearance dimensions (tube/baffle hole, baffle/shell, bundle/shell) are defined – they determine the split of the partial streams.
Calculate the ideal cross-flow pressure drop
For the cross-flow zone between two baffles, the pressure drop of the ideally cross-flowed bundle is determined using the plain tube bundle correlations (Section L1.4), with the velocity in the narrowest cross-section and the number of tube rows crossed.
Apply leakage and bypass corrections
Correction factors account for the fact that part of the mass flow passes through the clearances at tubes and baffles (leakage) and around the bundle (bypass) and is therefore not effective as cross-flow; sealing strips reduce the bypass share and enter the factors.
Calculate the window zones
In the baffle windows the fluid is turned and guided lengthwise through the bundle; the window pressure drop is determined from a combined cross-flow/longitudinal-flow relation with the mean velocity in the window.
Form inlet/outlet zones and total pressure drop
Separate approaches apply to the first and last zones because of the nozzle inflow and the differing number of tube rows. The total shell-side pressure drop is the sum of all cross-flow, window and end zones plus the nozzle losses.
Frequently asked questions
Why is the real shell-side pressure drop considerably lower than that of the ideal cross-flow bundle?
Because leakage and bypass streams divert a substantial part of the mass flow past the cross-flow – through the clearances between tubes and baffle holes, between baffle and shell, and in the peripheral gap between bundle and shell wall. Depending on the manufacturing clearances, these partial streams can together account for 30 to 50 %; they lower the pressure drop but at the same time worsen the heat transfer.
What influence does the baffle spacing have?
A smaller spacing increases the cross-flow velocity and improves heat transfer, but makes the pressure drop grow disproportionately, since more turns and window passages occur. Too large a spacing, on the other hand, promotes longitudinal flow and vibration-prone, poorly swept zones. Spacings between about 0.2 and 1.0 shell diameters are common.
What is the purpose of sealing strips in the bypass gap?
Pairs of sealing strips force the bypass stream between bundle and shell wall back into the bundle. They increase the effective cross-flow and thus the heat transfer, while the pressure drop rises at the same time. In the correction factors of the method they enter via the ratio of sealing strip pairs to the number of tube rows; from about one pair per five tube rows the bypass influence is largely suppressed.
Does the method also apply without baffles?
Yes, the module also covers the case of the longitudinally flowed bundle without baffles; the pressure drop is then calculated in the manner of a duct flow using the hydraulic diameter of the intertube space. It is considerably lower than for cross-flow – as is the heat transfer, which is why baffle-free designs are chosen primarily for pressure-drop-critical applications.
Technical content source: LV-Solved-Problems-Content 628d313247e7