SHELLKERN Heat-Transfer Coefficients in Shell Tube Exchangers

1. SHELLKERN.ppt Heat-Transfer Coefficients in Shell & Tube Exchangers

2. Components of Course: What Stage are We Up To? Types of exchangers, revision of OHTCs, fouling factors. Heat exchanger selection. Thermal performance analysis (NTUs) for co- & counter-current exchangers. Multi-pass exchangers (S&T). Condensation & boiling. Radiation.

3. Content Tube-side coefficients “easy” to predict using familiar correlations. Shell-side coefficients more difficult Kern’s method adequate for rough estimates. Need to consider flow patterns in more detail (Bell’s method).

4. Outcomes Be able to calculate tube-side heat-transfer coefficients. Be able to estimate shell-side coefficients using Kern’s method. Appreciate need to consider flow in more detail & overall basis of Bell’s method.

5. Summary So Far Not quite true in shell & tube arrangement 1-2 exchanger:

7. U for Shell-&-Tube Exchangers Look at individual film heat-transfer coefficients

8. Tube Side Single-phase flow in tubes not difficult situation Sieder-Tate equation for fully-developed turbulent flow familiar many variations also Dittus-Boelter (v. similar)

9. Shell Side - More Difficult: Ideal Flow Situations Flow across single tubes More complex... Flow across single tube row More complex... Flow across tube bank

10. Ideal situations studied experimentally Give basis of correlations for tube bundles in shell-&-tube heat exchangers BUT Real life even more complex

11. Do not have ideal cross flow: number of tubes in row across shell diameter varies (cross-sectional area for flow varies with depth into tube bundle)

12. Kern’s Correlation Re based on de = hydraulic mean diameter for flow parallel to tubes. (This is not physically realistic)

13. Kern’s Correlation: Example; Triangular Pitch

14. Kern’s Correlation: Example; Triangular Pitch

15. Kern’s Correlation: Example; Square Pitch

16. Kern’s Correlation: Example; Square Pitch

17. Example Consider 3600 kg hr-1 of water passing through 300 mm i.d. shell with baffle spacing of 60 mm (8 mm tubes on 10 mm square pitch) What is shell-side heat-transfer coefficient at 20oC?

18. Kern

21. Kern Integral method (1940’s-1950’s) Based mainly on practical experience, not on detailed analysis of flow patterns Results: “slightly unsafe” (10% undersize) to “very safe” (60% oversize)

22. Semi-Analytical Methods: Bell’s Method University of Delaware, 1960’s onwards Significant improvement for heat-transfer prediction Slight increase in complexity Hewitt et al pp. 275-285 (Bell’s method for calculation of shell-side heat-transfer coefficients)

23. Bell’s Method: Steps 1. Very detailed experimental studies on ideal tube banks to get ideal j-factors. Recall from Chem. Eng. 2

24. 2. j-factor then modified to account for flow non-idealities using semi-empirical graphical correlations. Combined effect of all corrections for well-designed exchanger typically 40-50%

25. Bell’s Method Results Results: slightly optimistic (15% undersize) to somewhat conservative (25% oversize) Application of technique to previous example gives Nu = 36; very close to Kern

26. Conclusions Tube-side coefficients: Dittus-Boelter, Sieder-Tate, etc. Shell-side coefficients: Kern’s method: slightly unsafe (-10%) to very safe (60%). Bell’s method accounts for flow patterns in more detail (shell-side flow complex).