Heat Transfer Equipment

Heat Transfer Equipment
© 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Heat Exchangers Heat Transfer Basics Tubular Exchangers
Heat Exchanger Design Compact Heat Exchangers © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Three Mechanisms of Heat Transfer
Conduction Affects wall resistances, which are usually negligible for heat transfer equipment Convection Usually the governing mechanism in most process applications Radiation Important in fired heaters Q = U A DT Q = heat duty A = area T = absolute temperature k = thermal conductivity U = heat transfer coefficient σ = Stefan Boltzman const. ε = transmission factor Q = A σ ε (ΔT4) © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Convective Heat Transfer in a Tube
Dittus-Boelter Equation: (for inside h.t.c. hi based on inside diameter di) Collect variables: So if we increase then hi will: Fluid thermal conductivity, k increase Fluid heat capacity, Cp increase Fluid density, ρ increase Velocity, v increase Fluid viscosity, μ decrease Pipe diameter, di decrease Convection heat transfer is the second type to be discussed. In convection the heat transfer is found empirically for each type of geometry. This equation covers single phase heat transfer in a circular tube. The heat transfer coefficient is a function of the physical properties of the fluid. The table on the left shows the effect of changing each of the variables in the equation. Note that the two factors that hurt heat transfer are increasing diameter and increasing viscosity. Heat transfer is worse in larger tubes and with more viscous fluids. The last equation corrects this coefficient, which is calculated based on inside diameter to the outside diameter of the tubes. All of the heat transfer equations refer to the area across which the heat is transferred. To make that a consistent area is used, all calculations are based on the area of the outside diameter of the tubes. Therefore, this inside coefficient must be corrected. © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Combined Conduction and Convection
h1 hot h2 cold L For a flat plate, overall resistance is the sum of the individual resistances Hence overall heat transfer coefficient, U is given by k T Th Combining the conduction and convection problems. Consider a flat surface with fluids on each side. The heat is transferred from the hot fluid to the surface by convection. Conduction occurs in the surface. A second convection rate is on the other side of the surface. This looks similar to the series conduction problem. Tc x © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Cylindrical Geometry (Tubes)
By convention, U is based on outside diameter Add terms for fouling: ro ri Outside h.t.c. ho depends strongly on equipment type: see Chapter 12 for correlations © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Counter Current Heat Transfer
Tc, in Th, in Tc, out Th, out Cold End ΔTc = Th,out – Tc,in Hot End ΔTh = Th,in – Tc,out Q = U A ΔTm For perfect counter current flow, ΔTm is the log mean temperature difference: Easiest to remember as: © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Counter Current Heat Transfer
Tc, in Th, in Tc, out Th, out Hot End Approach ΔTh = Th,in – Tc,out Cold End Approach ΔTc = Th,out – Tc,in © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Counter-current Flow in Real Exchangers
Most real heat exchangers do not have pure counter- current flow We apply a correction factor for this in design Q = U A F ΔTlm F is usually > 0.8 unless the design is poor (see later) F is sometimes called Ft However, few if any exchangers are really using counter current flow. This diagram shows a more typical exchanger. The tube side fluid travels the length of the exchanger twice. One trip in the bottom half and the return in the top half. At the same time, the shell side fluid travels once along the length of the exchanger, but many times back and forth across it. Again, which temperature difference is used here? cold in out hot © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Shell and Tube Heat Exchangers
How do we turn this - Into this - The design process for a heat exchanger starts with the process design. The two streams that need to be exchanged are determined. The next biggest step is turning that concept in a physical reality with real equipment. The physical reality of exchangers provides many of the limitations on what is a possible/practical process design. © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Shell and Tube Heat Exchangers
The exchange has many parts. It is important to learn the terminology. The AES type shown is the most common exchanger in the refinery The next biggest step is turning that concept in a physical reality with real equipment. The physical reality of exchangers provides many of the limitations on what is a possible/practical process design. Source: Perry’s Chemical Engineers Handbook, McGraw-Hill © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Shell and Tube Exchangers
Source: Riggins Company: © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

S&T Exchanger Construction
Baffle assembly Inserting tubes Welding the shell Final product Tubesheet Source: Bos-Hatten Inc.: © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Tube Bundles U-tubes Tubesheet Baffles Source: UOP

Heat Exchanger Design Heat exchange design must:
Provide required area Contain process pressure Prevent leaks from shell to tubes or tubes to shell Allow for thermal expansion Allow for cleaning if fouling occurs Allow for phase change (some cases) Have reasonable pressure drop S&T heat exchangers are built to standards set by the Thermal Exchanger Manufacturers Association (TEMA) © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

TEMA Nomenclature: Front Heads
A Type Easy to open for tubeside access Extra tube side joint B Type Must break piping connections to open exchanger Single tube side joint C Type Channel to tubesheet joint eliminated Bundle integral with front head N Type Fixed tubesheet with removable cover plate D Type Special closures for high pressure applications TEMA produces a standard for the design of heat exchangers. This is a group of exchanger suppliers getting together to define terminology and practices that all suppliers can agree to. One of the most useful things in the TEMA standards is a code to describe the overall configuration of shell & tube heat exchangers. They can come in a vast variety of designs and using this code, the configuration is readily conveyed. The code consist of 3 letters covering the front head, shell and rear head of the exchanger. © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

TEMA Nomenclature: Shells
E Type Most common configuration without phase change F Type Counter current flow obtained. Baffle leakage problems. G Type Lower pressure drop H Type Horizontal thermosyphon reboilers J Type Older reboiler designs K Type Phase separation integral to exchanger X Type Lowest pressure drop, low F factor Shell Types © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

TEMA Nomenclature: Rear Heads
L Type Same as A type front head M Type Same as B type front head N Type Same as N type front head P & W Types Rarely used S Type Floating head with backing ring T Type Floating head pulls through shell U Type Removable bundle without floating head Rear Head Types © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Selection of Exchanger Type: Examples
1) Feed preheater Low pressure Tubeside - Steam Shellside – Naphtha 2) Crude preheat train Low pressure Tubeside – Vacuum Residue Shellside – Crude oil BEU AES or AET 3) Reboiler Medium pressure Tubeside - Steam Shellside - Kerosene 4) Sterilizer Preheat Low pressure Tubeside - Milk Shellside - Steam Selection Example #1 BKU or BHM AET © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Lmtd Correction Factors
E Shell - 1 Shell Pass: Similar correlations exist for other shell arrangements T1 T2 t1 t2 R = (T1 - T2) (t2 – t1) Source: Perry’s Chemical Engineers Handbook, McGraw-Hill Factor to correct Log Mean Temperature Difference (LMTD) due to shell side flow pattern and number of tube side passes. E Type Shell © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Temperature Cross When Th, out < Tc, out we have a “temperature cross” Temperature cross causes problems if exchanger is not counter-current and gives low F factors If Tc,out – Th,out > 5% of Lmtd then F < 0.8 and it is usually best to split the exchanger into multiple shells in series Number of shells can be estimated by stepping off on T-H diagram T Duty, or Enthalpy, H T H Note: Real exchangers can have non-constant Cp Size & duty of HX in series is not necessarily the same Large number of shells in series approximates pure counter-current exchange © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Tube Pitch Triangular or square pitch, each with two orientations
TEMA minimum pitch is 1.25 x tube outside diameter Sometimes use larger pitch for easier cleaning (but bigger shell, lower shellside h.t.c.) Pitch is related to the angle based on the direction of flow. Square is much easier to clean but triangular tube layouts are cheaper. © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Baffle Types & Shell Flow Patterns
empty space FULL TUBEFIELD PARTIAL max unsupported tube span The most commonly used shell side baffle is the single segmental baffle. The baffle consists of alternating flat plates with a portion cut off. The flow is block by the solid portion of the baffle but allows flow to pass it through the cut out window. Tubes occupy the entire shell circle. The tubes in the center of the baffle are supported by each baffle. However, the tubes in the window are supported by every other baffle. A variation is the double segmental baffle. In this case the baffle plate is cut so that the flow is over both sides of some baffles and through a central gap in the other baffles. The next variation is the use of single segmental baffles but tubes are eliminated in the windows. This is a No Tube In Windows design. It is used for exchangers with vibration problems as each baffle doubles as a tube support. © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Selection of Sides Process Fluid Side Selection Reason Fouling fluid
Tube Easier to clean Viscous fluid Shell Lower Δp Suspended solids No dead spots for settling Highest T Cheaper, mechanically stronger Highest pressure Cooling water Corrosive fluid Cheaper, easier to replace tubes Much larger flow Condensing fluid Drains better © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Heat Exchanger Design Determine duty, check for temp cross
Estimate U and hence calculate area Determine exchanger type and tube layout Pick d, L and calculate number of tubes, hence shell diameter Calculate hi, ho and confirm U. Return to 2 if needed. Calculate Δp. Return to 3 if needed. Q = (m Cp ΔT) + (δm ΔHvap) Q = U A F ΔTlm © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Approximate Heat Transfer Coefficients More examples (in metric units) in Chapter 12
Fluid h (Btu/(hr ft2 F)) Shell-side Tube-side Liquids Water solutions, 50% water or more Alcohols, organic solvents Light Hydrocarbons (naphtha, gasoline) Medium Hydrocarbons (kerosene, diesel) Heavy oils (gas oils, crude oil) Vapors Air, 10 psig Hydrogen, 50 psig Hydrogen, 300 psig Hydrogen, 500 psig Hydrocarbon vapor, 50 psig Noncondensible gas, 2 psig Typical heat transfer coefficients for rapid checking of field surveys. Approximate only. Note: Coefficients are based on 3/4 inch diameter tubes. For Tube side flows, correct by multiplying by 0.75/Actual OD. Estimated accuracy is 25%. For 50% hydrogen in vapor, reduce h to 2/3 of pure H2 value. © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Approximate Fouling Factors
Fluid f (Btu/(hr.ft2.F)) River water Sea water Cooling tower water Town water (soft) Town water (hard) 300 Flue gas Steam Steam condensate Light & medium hydrocarbons 400 Heavy oils Boiling organics Aqueous salt solutions 600 Fermentation broths 300 © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Hydraulics & Pressure Drop
Heat exchanger design is a trade off between better heat transfer (high velocity, low diameter) and pressure drop In early stages of design, we usually allow for a “typical” pressure drop: 5 psi shell-side 10 psi tube-side But we have to calculate Δp rigorously where it is critical to performance, e.g. thermosyphon reboilers In detailed design, use correlations or simulation programs to more rigorously optimize if pressure drop is important to process performance – see Chapter 12 for examples © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Example: S&T HX Design What size of exchanger is needed to heat 375,000 lb/h of naphtha from 150F to 300F using medium pressure steam at 360F? Q = m.Cp.ΔT = 375 x 103 x 0.5 x 150 = MMBtu/h Put steam shell-side, oil tube-side, TEMA BEU Estimate hi = 190 Btu/(h.ft2.F), ho = 1500 Btu/(h.ft2.F) Estimate fi = 400 Btu/(h.ft2.F), fo = 1000 Btu/(h.ft2.F) Estimate 1/U = 1/ / / /1500 U = 106 Btu/(h.ft2.F) R ≈ 0 (negligible ΔT on shell-side) so F = 1.0 Lmtd = (60 – 210)/ln(60/210) = 119.7°F Hence A = Q/U.F.Lmtd = 28.1 x 106 / (106 x 1.0 x 119.7) = 2215 ft2 For 20’ long 3/4” tubes area/tube = 3.9 ft2, hence need 564 tubes From table in Perry’s Hbk, 1” triangular pitch TEMA U, nearest shell size is 29” i.d., with 648 tube count. We can now use correlations to confirm design and check hydraulics © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Hairpin Exchangers When small duties are required, hairpin exchangers are specified: cheaper than very small shell and tube highly effective (single pass, true countercurrent) 75  ft2 surface area 4  16" shell diameter, 20 ft long Hair pin exchangers consist of two parallel tubes acting as two shells in series with U shaped tubes and connecting bonnet head. The flow pattern is nearly countercurrent so it is very thermally efficient. If the tube side fluid flows through only a single tube it is called a double pipe exchanger. If the the tube side flow uses many tubes, it is a multitube hairpin. Hairpins are less expensive than shell and tube exchangers in smaller sizes. This design is used for double-pipe and multi-tube exchangers. © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Plate & Frame Exchangers
Plates Gasket Source: Alfa-Laval, © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Plate & Frame Exchangers
Advantages Close to counter-current heat transfer, so high F factor allows temperature cross and close temperature approach Easy to add area Compact size Relatively inexpensive for high alloy Can be designed for quick cleaning in place Disadvantages Lots of gaskets Lower design pressure, temperature External leakage if gaskets fail Applications Food processing, brewing, biochemicals, etc. Design method: see Chapter 12 Source: Alfa-Laval, Source: Alfa-Laval © 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

Air Fin Heat Exchangers

Heat Transfer Equipment

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