Design and rating of Shell and tube heat Exchangers Bell-Delaware method

Transcription

1 King Abdulaziz University Mechanical Engineering Department MEP 460 Heat Exchanger Design Design and rating of Shell and tube heat Exchangers Bell-Delaware method 1 April 2018

2 Bell Delaware method for heat exchangers 1-Introduction 2-Main flow streams 3-Ideal shell side heat transfer coefficient and pressure drop 4-Shell side heat transfer coefficient and correction factors 5-Shell side pressure drop and correction factors 6- Example 2

3 1-Intoduction Shell and tube heat exchangers are the most commonly used heat exchangers Use extensively in power plants, refineries and industrial and commercial sectors TEMA Standards of shell and tube layouts are available Very well known analysis methods: Simple Kern method Bell-Delaware method 3

4 2- Main flow streams for shell and tube heat exchanger 4

5 Main flow streams for shell and tube heat exchanger Stream A is the leakage between the baffle and tubes Stream B is the main effective cross flow stream over tube bundle Stream C is the bundle bypass between the tube bundle and the shell wall Stream E is the leakage between the baffle edge and the shell wall Stream F is the by pass stream in flow channel partition due to omissions of tubes in tube pass partition 5

6 Main flow streams for shell and tube heat exchanger Stream E By pass between the baffle and the shell Stream A leakage between tubes and baffle 6

7 Using sealing strip to reduce bundle-shell by pass 7

8 3-Ideal shell side heat transfer coefficient and correction factors The three common tube layout used in shell and tube heat exchangers are: 1-Triangular pitch 2-Square pitch 3-Rotated square pitch 8

9 3-Ideal shell side heat transfer coefficient and correction factors Pitch angle θ p = 30 9

10 3-Ideal shell side heat transfer coefficient and correction factors Pitch angle θ p = 45 10

11 3-Ideal shell side heat transfer coefficient and correction factors Pitch angle θ p = 90 11

12 3-Ideal shell side heat transfer coefficient and correction factors Heat transfer coefficient 1.33 j i = a 1 P T d o a Re s a 2 a = a Re s a 4 Friction coefficient 1.33 f i = b 1 P T d o b Re s b 2 b = b Re s b 4 h id = j i C ps m A s Pr 2 3 μ s μ s,w

13 Ideal heat transfer and pressure drop factors 13

14 Shell side heat transfer coefficient h o = h id J c J l J b J s J r J c, J l, J b, J s, J r Correction factors The ideal heat transfer coefficient for pure cross flow is given by h id = j i C ps m A s Pr 2 3 μ s μ s,w

15 Heat transfer correction factors h o = h id J c J l J b J s J r factor Due to Typical values J c baffle cut and spacing (heat transfer in the window) J l leakage effects (streams A &E) J b J s J r J c J l J b J d J s bundle bypass flow ( streams C & F streams) variable baffle spacing in the inlet and outlet sections( adverse temperature gradient build-up (Re s <100) Product of all factors (for well design HX for Re s >

16 Shell side Reynold s number Re s = d om s μ s A s A s is the minimum flow area at the shell centerline A s = D s N TC d o B N TC = D s P T N TC number of tubes at the centerline of the shell Re s = d om s μ s A s Notice that Reynolds number is based on the outside diameter of the tubes 16

17 Shell side pressure drop Internal Δp c Windows Δp w Entrance Δp e Δp s = Δp c + Δp w + Δp e 17

18 Pressure drop in cross flow between baffle edges Δp c = Δp bi N b 1 R l R b Δp bi = 4 f i G s 2 2ρ s μ s,w μ s 0.14 R l leakage correction factor due streams A and C (typical value between 0.4 and 0.5) R b by pass correction factor due to stream C and F (typical value between 0.5 and 0.8) N b is the number of baffles 18

19 Pressure drop in the window Pressure drop in windows Δp w = Δp wi N b R l Δp wi = m s 2 ( N cw ) 2ρ s A s A w Re s 100 Δp wi = 26 μ sm s A s A w ρ N cw P T d o + B D w 2 + m s A s A w ρ s Re s < 100 No of tube rows crossed from tip to tip of baffle N c = D s 1 2 L c D s P p P T No. of tube rows in window N cw = 0.8L c P p 19

20 Parallel and normal tube pitch definition 20

21 Pressure drop at the entrance and exit Pressure drop in entrance and exit Δp e = 2Δp bi N c + N cw N c R b R s N c is the number of tube rows crossed in the heat exchanger (baffle tip-to-tip) N cw is the number of tube rows crosses in the window R s is the correction factor entrance and exit section having different baffle spacing than internal section due existence of inlet and outlet nozzles Total Shell side pressure drop Δp s = Δp c + Δp w + Δp e or Δp s = N b 1 Δp bi R b + N b Δp wi R l + 2Δp bi 1 + N cw N c R b R s 21

22 Number of baffles Number of baffles can be calculated using N b = L B i B o B + 1 B i and B o are the baffle spacing at inlet and exit If B i =B o =B then N b = L B 1 22

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24 Example 9.4 continue A s = D s N TC d o B N TC = D s P T 24

25 Example 9.4 continue 25

26 Example 9.4 continue 26

27 Example 9.4 continue 27

28 Example 9.4 continue 28

29 Example 9.4 continue 29

30 Example 9.4 continue Shell side pressure drop using Kern procedure 30

31 Example 9.4 continued 31

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34 Example 9.5 continue 34

35 Example 9.5 continue 35

36 Example 9.5 continue 36

37 Example 9.5 continue 37

38 Example 9.5 continue 38

39 Example 9.5 continued 39

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