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Quotes - Engineering - SalesF-Shell Two-Pass U-Tube Exchangers
Restoring Counterflow
The F shell exists to solve one problem. A U-tube bundle is inherently two-pass on the tube side, which means half the tube length runs against the shell flow and half runs with it. When the required duty needs a temperature cross, that arrangement cannot deliver it.
How the baffle helps. Dividing the shell lengthwise means the shell-side fluid also makes two passes. Two shell passes against two tube passes approximates true counterflow, and the LMTD correction factor recovers to something workable.
The leakage problem. The longitudinal baffle has to seal against the shell wall along its whole length. Any bypass short-circuits fluid from one pass to the other and erodes the benefit. Sealing strips help; thermal distortion and age work against them.
The alternative. Two E-shell exchangers in series achieve the same counterflow with no sealing problem, at the cost of an extra shell, extra nozzles and more plot space. On a severe cross that robustness is often worth buying.
Typical Construction
- Shell type: F - two pass with longitudinal baffle
- Purpose: Counterflow profile; permits a temperature cross
- Tube passes: Two, matched to the shell passes
- Correction factor: Substantially higher than an equivalent E shell
- Baffle sealing: Sealing strips or welded baffle; leakage degrades performance
- Pressure drop: Higher than E shell for the same flow
- Bundle removal: Longitudinal baffle arrangement must permit withdrawal
- Alternative: Two E shells in series
- Typical duty: Close approach and crossing temperature profiles
- Codes: ASME Section VIII, TEMA R, B or C
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When to Choose Series Shells Instead
If the cross is mild, an F shell is efficient and compact. If the cross is severe, or the fluids are such that a leaking baffle would be difficult to detect, two shells in series is the more dependable engineering.
Series shells also give you a maintenance advantage: each unit is smaller and lighter, and one can be isolated. Against that, you pay for two shells, two sets of nozzles and the interconnecting pipework.
