Heat Transfer Equipment

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1 Università di Pisa Facoltà di Ingegneria Heat Transfer Equipment Unit Operation I Prof. Cristiano Nicolella

2 Typical overall heat transfer coefficients

3 Fouling factors

4 Frank nomograph

5 Fouling factors

6 Exchanger types: fixed-tube tube-sheet Simple and cheap Limited to clean fluids (difficult cleaning) No provision for differential expansion (Dtmax=80 C)

7 Exchanger types: U tube Cheaper than floating head Reduced number of joints (reduced capital and maintenance costs in high-pressure contructions) Limited to clean fluids (difficult cleaning)

8 Exchanger types: Internal floating head Removable tube bundle Provision for differential expansion Low tube-shell clearance (split ring assembly) Possible leakages

9 Heat-exchanger exchanger standards The mechanical design features, fabrication, materials of construction and testing of tube and shell heat exchangers are covered by international standrds: Tubular heat exchanger manufacturer association (TEMA) British standards

10 TEMA designations for shell-and tube exchangers

11 Tubes Tube diameters: 6-50 mm Most often: mm Small diameters (up to 25 mm) give more compact (cheaper) exchangers Larger diameters used for fouling fluids (easier to clean) Tube thickness: Internal and external pressure Corrosion allowance Preferred lengths : 1, 2, 3, 6 m Size often determined by plant maintenance department standards 19 mm good first trial diameter for design calculations

12 Tube arrangements Triangular and rotated square patterns Higher heat transfer rate Higher pressure drop Square and rotated square patterns: P t = 1.25 d e Fouling fluids (mimimun clearance between tubes: 6 mm)

13 Shells Diameters BS: up to 1070 mm TEMA: up to 1520 mm Minimum clearance with tube bundle (reduced bypassing)

14 Tube count d b = d e N K t 1 1 n 1 d b : bundle diameter d e : tube outside diameter N t : number of tubes In U-tube exchangers, the spacing between the two central rows is determined by the minumum allowable radius for the the U-bend. The number of tubes in the central row is d b /p t.

15 Shell types One shell pass most commonly used. Two shell passes used for temperature crosses (more commonly two one shell pass exchangers in series) Divided flow and split flow arrangements used to reduce shell pressure drop (when pressure drop rather than heat exchange is the controlling factor)

16 Baffles Baffles are used to improve the rate of heat transfer by: directing the fluid stream across the tubes; Increasing the fluid velocity. Segmental baffles (a) most commonly used. Baffle cut: percentage of height of the segment removed to form the baffle to diameter of the baffle disc Baffle cuts: % Optimum cut: % (good heat transfer rate without excessive pressure drop) Support plates: similar to baffles, closer tolerances (0.4 vs 0.8 mm). With gases: trimmed baffles (bottom) for condensate flow With liquids: trimmed baffles (top) for noncondensable flow Baffle spacing: d s /5 (minimum 50 mm) d s Optimum spacing: 0.3d s -0.5 d s Support plate spacing: 1 m (d e =19 mm); 2m (d e =25mm)

17 Temperature correction factor

18 Tube-side heat transfer coefficient For water: t: temperature, C u t : velocity, m/s d i : tube inside diameter, mm

19 Tube-side pressure drop P t : tube-side pressure drop, Pa N P : number of tube side passes

20 Shell-side heat transfer coefficient A u d d s s eq eq Re = = s = ( p d ) W ρa 2 π 3pt d 2 = π 2 de 2 ρusdeq = µ t s s p e t e d s l d 2 π 4 pt d 4 2 πd 2 e 2 e square pitch triangular pitch A s : cross flow area, m 2 u s : velocity, m s-1 d eq : equivalent diameter, m Re s : Reynolds number p t : tube pitch, m d e : tube outside diameter, m d s : shell diameter, m l d : baffle spacing, m W: mass flow rate, kg s -1 ρ: density, kg m -3

21 Shell-side heat transfer coefficient

22 Shell-side pressure drop

23 Equipment sketches

24 Data sheets

25 TEMA designations

26 Finned tubes Longitudinal fins Transverse fins

27 Transverse fin efficiency

28 Transverse fin heat transfer and pressure drop

29 Transverse fin heat transfer and pressure drop D D e ' ev = = 2 ( A + A ) A f f πp 4V + A o o A f : fin surface area (both sides) A o : bare outside tube surface p: projected perimeter V: net free volume (volume between the center lines of two vertical banks of tubes less the volume of the alf tubes and fins within the central lines)

30 Crossflow arrangements

31 Crossflow temperature difference correction

32 Crossflow temperature difference correction

33 Jacketed vessels Spirally baffled Dimple Half pipe

34 Coiled vessels Single helix Vertical pipes Helix + pancake

35 Heat transfer coefficients from jackets and coils

36 Heat transfer coefficients from jackets and coils h j D k j = 0.36 L 2 2 Nρ µ 3 cµ k 1 3 µ µ w 0.14 jackets h c D k j = L Nρ µ 2 3 cµ k 1 3 µ µ w 0.14 coils D j : vessel inside diameter L: agitator paddle length N: agitator angular velocity