King Abdulaziz University Mechanical Engineering Department MEP 460 Heat Exchanger Design Design and rating of Shell and tube heat Exchangers Bell-Delaware method March 2019 1 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 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 2- Main flow streams for shell and tube heat exchanger 4 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 Main flow streams for shell and tube heat exchanger Stream E Stream A By pass between the baffle and the shell leakage between tubes and baffle 6 Using sealing strip to reduce bundle-shell by pass 7 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 3-Ideal shell side heat transfer coefficient and correction factors Pitch angle ππ = 30° 9 3-Ideal shell side heat transfer coefficient and correction factors Pitch angle ππ = 45° 10 3-Ideal shell side heat transfer coefficient and correction factors Pitch angle ππ = 90 ° 11 3-Ideal shell side heat transfer coefficient and correction factors Heat transfer coefficient ππ = π1 1.33 ππ Τππ π π ππ π3 π= 1 + 0.14 π ππ Friction coefficient ππ = π1 1.33 ππ Τππ π4 π π ππ π3 π= 1 + 0.14 π ππ βππ = ππ πΆππ παΆ π΄π ππ π2 −2Τ3 π2 π4 ππ ππ ,π€ 0.14 12 Ideal heat transfer and pressure drop factors 13 Shell side heat transfer coefficient βπ = βππ π½π π½π π½π π½π π½π Jc, Jl, Jb, Js, Jr Correction factors The ideal heat transfer coefficient for pure cross flow is given by βππ = ππ πΆππ παΆ π π΄π ππ −2Τ3 παΆ π παΆ π πΊπ = = π΄π ππ ππ ππ ,π€ 0.14 14 Heat transfer correction factors βπ = βππ π½π π½π π½π π½π π½π factor Due to Typical values Jc baffle cut and spacing (heat transfer in the window) 0.53-1.15 Jl leakage effects (streams A &E) 0.7-0.8 Jb bundle bypass flow ( streams C & F streams) 0.7-0.9 Js variable baffle spacing in the inlet and outlet sections( 0.85-1.0 Jr adverse temperature gradient build-up (Res<100) 1.0 for Res>100 Product of all factors (for well design HX 0.6 π½π π½π π½π π½π π½π 15 Shell side Reynold’s number π ππ = ππ παΆ π π π π΄π As is the minimum flow area at the shell centerline π΄π = π·π − πππΆ ππ π΅ πππΆ = π·π ππ NTC number of tubes at the centerline of the shell ππ παΆ π π ππ = π π π΄π Notice that Reynolds number is based on the outside diameter of the tubes 16 Shell side pressure drop Δππ Internal Crossed Windows Entrance Δππ€ Δππ Δππ = Δππ + Δππ€ + Δππ 17 Pressure drop in cross flow between baffle edges Δππ = Δπππ ππ − 1 π π π π Δπππ πΊπ 2 = 4 ππ 2ππ ππ ,π€ ππ 0.14 π΅π Rl leakage correction factor due streams A and C (typical value between 0.4 and 0.5) Rb by pass correction factor due to stream C and F (typical value between 0.5 and 0.8) Nb is the number of baffles Ncis the number of tubes crossed between baffle tips 18 Pressure drop in the window Pressure drop in windows Δππ€π Δππ€π Δππ€ = Δππ€π ππ π π παΆ s2 (2 + 0.6 πππ€ ) = 2ππ π΄π π΄π€ ππ παΆ s π ππ ≥ 100 πππ€ π΅ παΆ s = 26 + 2 + π΄π π΄π€ ππ π΄π π΄π€ π π·π» − ππ π·π€ π ππ < 100 No of tube rows crossed from tip to tip of baffle π«π 1 − 2 ππ = πΏπ π·π ππ PT No. of tube rows in window πππ€ = 0.8πΏπ ππ 19 Window gross area Awg or Swg Areas in the window of a baffle Tube area in the window Awt or Swt Flow area in window=Aw=Awg-Awt Parallel and normal tube pitch definition 21 Pressure drop at the entrance and exit Pressure drop in entrance and exit ππ + πππ€ Δππ = 2Δπππ π π π π ππ Nc is the number of tube rows crossed in the heat exchanger (baffle tip-to-tip) Ncw is the number of tube rows crosses in the window Rs 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 Δππ = Δππ + Δππ€ + Δππ or Δππ = ππ − 1 Δπππ π π + ππ Δππ€π π π + 2Δπππ 1 + πππ€ π π π π ππ 22 Number of baffles Number of baffles can be calculated using ππ = πΏ − π΅π − π΅π +1 π΅ Bi and Bo are the baffle spacing at inlet and exit If Bi=Bo=B then πΏ ππ = − 1 π΅ 23 24 25 Example 9.4 continue π΄π = π·π − πππΆ ππ π΅ πππΆ π·π = ππ 26 Example 9.4 continue 27 Example 9.4 continue 28 Example 9.4 continue 29 Example 9.4 continue 30 Example 9.4 continue 31 Example 9.4 continue Shell side pressure drop using Kern procedure 32 Example 9.4 continued 33 34 Example 9.5 continue 35 Example 9.5 continue 36 Example 9.5 continue 37 Example 9.5 continue 38 Example 9.5 continue 39 Example 9.5 continued 40 41
