Where We Go Wrong In Line Sizing - University of Western Ontario

Where We Go Wrong In Line Sizing - University of Western Ontario

Design of Heat Exchangers Dick Hawrelak Presented to CBE 497 on 31 Oct 00 at UWO 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 TEMA BEM Exchanger

B E M 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 Approximate Design Method Tube Count Exchanger Comparison Exchanger Shell Side Tubeside No. Exchangers Q, mm BTU/HR T1, hot in T2, hot out t1, cold in

t2, cold out LMTD = Area, sf Tube L, ft. Tube OD, in. No. Tubes No. Tube Passes Tubesheet Tube Pitch Shell ID tubes Shell ID expanded Flux = Q/A = U Back Calc'd = Dow RD E-202 Steam CTW Surf Cond 1

286.48 130 130 82 106.9 34.05 23600 36 1.00 2,504 1 Fixed 1.25" 30 Tri 70 96 12,139 356.55 Dow FS E-652

Steam CTW Surf Cond 1 87.000 106.35 96.35 72 80.6 25.04 9,743 24 1.00 1,551 1 Fixed 1.25" 30 Tri 54.41 74.62 8,929

356.55 Quick Approximate Method Assume Design Ud values, Perry VI, p-10-44. BTU/hr & temperatures from process simulation Assume heating or cooling temperatures Calc LMTD, correct to CLMTD, if required Calc Area = Q / Ud / CLMTD 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 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. 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 0.001 is specified on both sides. The combination of heat transfer coefficients, fouling and material resistance allow calculation of a clean heat transfer coefficient, Uc 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?

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

Thermosyphon Layout 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

Article Example - Liquid / Liquid Exchanger Input Data Conditions Flowrate, lb/hr = Inlet Temperature, C = Outlet Temperature, C = Viscosity, Centipoise, Z = Specific Heat, btu/h/F = Molecular Wt. = SG Ref to Water, s = Allowable DP, psi = Maximum Tube Length, ft = Minimum Tube Dia. inches = Inside ID, inches = Material Construction = Thermal Conductivity, metal = No. Passes, n = Fouling, h =

Therm Cond, btu/hr/sf/(F/ft.) = Baffl e Spacing, inches = Tubeside Shellside 307,500 105 ? 1.7 0.72 118 0.85 10 12 0.625 0.495 cs

26 1 1000 0.102 32,800 45 90 0.3 0.9 62 0.95 10 cs 26 1 0.183 5.000

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 Heat Balances Continued

Tubeside: 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

(Wi)(ci)(tH tL) / [(hi)(A)(dTM)] + 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 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

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 Step 1: Calculate Heat Duty Step 1 Calc Heat Transfer, Q = (m)(cp)(DT) from shellside data. Note Temps in Deg C require 1.8 factor for deg F. Q = = (32,800)(0.9)((90 - 45)(1.8) Q = 2.39E+06 BTU/hr Step 2:

Calc Temp Decrease Hot Liq = Q / m / cp for the tubeside DT = DT = (2.39E+06) / (307,500) / ((0.72) C 6.00 Tubeside Inlet Temp, Ti = Tubeside Outlet Temp, To = C 105 99 C C Step 3 Calculate LMTD

FT fr CLMTD = 1.00 (May Have to Correct if 1-2 Exchanger) see CLMTD program 105 90 15 CLMTD = 99 45 54 30.45 C Corrected LMTD = CLMTD = LMTD*FT

Step 4 Assume Ud and make First Approximation Of Area Assume Ud = A = Q / Ud /LMTD = 250 btu/hr/sf/F 174.52 (See Perry 6, Page 10-44) sf Step 5 Calculate No. Tubes for L = Area / Tube = (Do)(L) = No. Tubes =

No tubes per pass = 88.88 12 ft 1.9635 sf No. tubes = A / Area per tube 89 Re = 6.31W/d/cP = 25,908 Step 6

Approximate Shell ID = 1.75(OD)(Nt)^0.47 Shell ID = 9.01 Inches or (See Perry 6, 11-13) Step 7 Calc Max No Tubes That still gives turbulent Re > 12,600 Nt max = Wi / (2*di*Zi) Nt Max = 183 To Keep Re above 12,600

Step 8 Heat Transfer Calcs Step 1 Tubeside - Use Eqn (1) DTi / DTm = Numerical Factor, f1 10.43 DTi / DTm = 10.43 Phys Prop Factor, f2 (Zi^0.467Mi^0.22/Si^0.89) 4.27

Step 8 Continued Work Factor, f3 (Wi)^0.2(th-tL)/LMTD 0.620 Mechanical Design Factor, f4 (di^0.8/n^0.2/L 0.0193 Step 8 Continued dTi/dTM = (f1)(f2)(f3)(f4)

dTi/dTM = (10.43)(4.27)(0.62)(0.0193) = 0.5339 Similar Calculations for tube wall, fouling and shell side. Sum of Products Summary Trial >>> Total No. Tubes = No. Tube Passes No. Tubes per pass = Shell Dia., inches = Baffl e Spacing, inches Exchanger Area, sq. ft. = Product Of Factors Eqn (1) Tubeside Eqn (11) Shellside Eqn (17) Tube Wall

Eqn (19) Fouling Sum of products, SOP = SOP Message Eqn (21) Tubeside DP = Eqn (25) Shellside DP = Case 1 89.00 1 89.00 9.02 5.0 174.8 Case 2 109 1

109 9.92 5.0 214.0 Case 3 109 1 109 9.92 3.5 214.0 Case 4 121 1 121 10.42 3.5 237.6

0.5339 0.4228 0.0520 0.2497 1.2584 Add Area 14.31 3.93 0.5127 0.3655 0.0424 0.2039 1.1246 Add Area 9.93 3.57 0.5127

0.2951 0.0424 0.2039 1.0541 Add Area 9.93 10.42 0.5021 0.2738 0.0382 0.1837 0.9978 Exchgr OK 8.23 9.92 End of Presentation

Good luck on your exchanger designs. If you have any questions call [email protected]

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