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

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

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

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

Input Data

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: (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

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 2:

Step 3

Step 4

Step 5

Step 6

Step 7

Step 8 Heat Transfer Calcs

Step 8 Continued

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

Sum of Products Summary

End of Presentation Good luck on your exchanger designs. If you have any questions call rhawrela@xcelco.on.ca