1. Design of Heat Exchangers
Dick Hawrelak
4/26/2017
Design of Heat Exchangers
Dick Hawrelak
Presented to CBE 497 on 31 Oct 00 at UWO
Design of Heat Exchangers

2. Introduction Design using HTRI and based on TEMA Stds
TEMA Shell & Head Types, Perry VI, page 11-4
TEMA nomenclature, Perry VI, page 11-6
Liquid / liquid exchanger design example
RW Condenser example on CD-ROM

3. TEMA BEM Exchanger

4. Plant Design, (11) Exchangers
Heat Recovery Efficiency
Colburn heat transfer method for hi
CLMTD Correction Factor, Perry VI, p-10-27
Heat Exchanger Materials
Liquid – Liquid Exchanger design example
RW Condenser design example
Shell Size V1.1 for kettle shell diameter
Tube Count Exchanger Comparison

5. Approximate Design Method Tube Count Exchanger Comparison

6. Quick Approximate Method
Assume Design Ud values, Perry VI, p
BTU/hr & temperatures from process simulation
Assume heating or cooling temperatures
Calc LMTD, correct to CLMTD, if required
Calc Area = Q / Ud / CLMTD

7. Approx Method Continued
Assume tube od, BWG, tube length, to calc no. tubes (Table 11-2)
Assume no. tube passes. Determine shell diameter, Perry VI, Table 11-3 tube count
Assume materials & get cost estimate for exchanger

8. Pressure Drop Exchanger area vs pressure drop.
Economics often dictate pressure drop.
The designer sets the allowable pressure drops during simulation of process.
Confirm pressure drops during exchanger design.
Nozzle sizes, baffle spaces, tube dia., tube length, no. tubes per pass all affect pressure drop.

9. Fouling and Overdesign
Fouling factors are specified to give the exchanger a cleaning cycle (eg 1 year).
In clean hydrocarbon services, a dirt factor of is specified on both sides.
The combination of heat transfer coefficients, fouling and material resistance allow calculation of a clean heat transfer coefficient, Uc

10. Over-design Problems
Exchanger is designed with a Ud and a corresponding fouled CLMTD.
On start-up, the exchanger operates with a Uc and a clean CLMTD. This may result in flow problems for condensing systems.
Which steam pressure or refrigerant level should be used?

11. Temperature Profiles
Manual calculations use average in & out temperatures.
Subcooling affects LMTD.
Partial condenser temperature profiles with inert gases are difficult to model.
Good VLE data hard to obtain.

12. Mechanical Design
High RHO-V-SQUARE on inlet shell nozzle can rupture tubes.
Impingement plate design not well defined.
Tube vibrations with long tube spans.
How to join tubes to tubesheet?

13. Maldistribution
Shell side maldistribution with small window cuts. Use 20% baffle cuts.
Tube side maldistribution with low tube side pressure drops. Long tubes, small tube diameters.
Chinese hat diffusers on tube and shell sides.

14. Acoustics Shell side geometry can cause acoustic vibrations.
May require tuning baffles.

15. Entrainment
Minimize entrainment in Kettle refrigeration coolers. See Shell Size V1.2.
Entrainment levels often ignored on mass balances.
Kettle vapor outlets flow to KO pots in refrigeration compressor design.

16. Expansion Joints.
Expansion joints when shell and tubes are different materials.
Expansion joints are a hazard.
Expansion joints are fragile.
No. flexes per hour usually unknown.
Paper clip example.

17. Reboiler Recirculation Problems
Low Recirculation due to inert build-up in shell, high tube resistance, low liquid level in column.
Low recirculation promotes fouling and unwanted heavies production.
Seadrift EO tower explosion due to faulty reboiler design,

18. Thermosyphon Layout

19. Design of Heat Exchangers
Method by Lord, Minton and Slusser, of UCC
26 Jan 70, Chemical Engineering, p-96.
Methods suitable for all types of exchangers.
Method suitable for spreadsheet analysis.
See Liquid Liquid Exchanger and RW Condenser in Plant Design, Exchangers.
Alternatively, Process Heat Transfer by Kern

20. Input Data

21. Heat Balances Tubeside: (Wi)(ci)(tH – tL) = (hi)(A)(dTi)
Tube walls: ((Wi)(ci)(tH – tL) = (hw)(A)(dTw)
Fouling: (Wi)(co)(tH – tL) = (hs)(A)(dTs)
Shellside: (Wo)(co)(TH – TL) = (ho)(A)(dTo)
dTi + dTw + dTs + dTo = LMTD = dTM
dTi/dTM + dTw/dTM + dTs/dTM + dTo/dTM = 1

22. Heat Balances Continued
Tubeside: (Wi)(ci)(tH – tL) / [(hi)(A)(dTM)] +
Tube walls: ((Wi)(ci)(tH – tL) / [(hw)(A)(dTM)] +
Fouling: (Wi)(co)(tH – tL) / [(hs)(A)(dTM)] +
Shellside: (Wo)(co)(TH – TL) / [(ho)(A)(dTM)] +
= 1.0

23. Heat Transfer Coefficients
hi = 0.023ciGi/(ciui/ki)^0.67/(DiGi/ui)^0.2
hw = 24kw / (do – di)
ho = 0.33coGo(0.6)/(couo/ko)^0.67/(DoGo/ko)^0.2
hs = assumed value

24. Arrange Equations Into 4 Factors
For example for dTi/dTM for inside tubes, no phase change, liquid, Nre > 10,000
Numerical factor, f1 = 10.43
Physical Property Factor f2 = (Zi^0.467Mi^0.22)/si^0.89
Work factor f3 = Wi^0.2(tH – tL) / dTM
Mechanical Design Factor, f4 = di^0.8/n^0.2/L
dTi / dTM = (f1)(f2)(f3)(f4)
Similarly for hw, ho and hs

25. Pressure Drops
Tubeside pressure drop, psi, Eqn (21) DP = (Zi^0.2/si)(Wi/1000/n)^1.8((L/di)+25)/(5.4di)^3.8
Shellside pressure drop, psi, Eqn (25) DPs = (0.326)/So(Wo/1000)^2(L)/Ps^3/Ds

26. Step 1: Calculate Heat Duty

27. Step 2:

28. Step 3

29. Step 4

30. Step 5

31. Step 6

32. Step 7

33. Step 8 Heat Transfer Calcs

34. Step 8 Continued

35. Step 8 Continued dTi/dTM = (f1)(f2)(f3)(f4)
Similar Calculations for tube wall, fouling and shell side.

36. Sum of Products Summary

37. End of Presentation Good luck on your exchanger designs.
If you have any questions call