Chapter 4 Shell and Tube Heat Exchangers Dr. Rajan
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Shell and Tube Heat Exchanger •
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Most commonly used type of heat transfer equipment in the chemical and allied industries. Advantages: – The configuration gives a large surface area in a small – – – – –
volume. Good mechanical layout: layout: a good shape for pressure operation. Uses well-established well-established fabrication techniques. Can be constructed from a wide range of materials. Easily cleaned. Well established design procedures.
Shell and Tube Heat Exchanger •
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Most commonly used type of heat transfer equipment in the chemical and allied industries. Advantages: – The configuration gives a large surface area in a small – – – – –
volume. Good mechanical layout: layout: a good shape for pressure operation. Uses well-established well-established fabrication techniques. Can be constructed from a wide range of materials. Easily cleaned. Well established design procedures.
Types of Shell and Tube Heat Exchangers •
Fixed tube design – Simplest and cheapest type. – Tube bundle cannot be removed for cleaning. expansion of shell and tubes. – No provision for differential expansion difference upto – Use of this type limited to temperature difference 800C.
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Floating head design – More versatile than fixed head exchangers. – Suitable for higher temperature differentials. – Bundles can be removed and cleaned (fouling liquids)
Design of Shell and Tube Heat Exchangers •
Kern method: – Does not take into account bypass and leakage streams. – Simple to apply and accurate enough for preliminary design calculations. – Restricted to a fixed baffle cut (25%).
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Bell-Delaware method – Most widely used. – Takes into account: •
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Leakage through the gaps between tubes and baffles and the baffles and shell. Bypassing of flow around the gap between tube bundle and shell.
Stream Analysis method (by Tinker) – More rigorous and generic. – Best suited for computer calculations; basis for most commercial computer codes.
Construction Details – Tube Dimensions •
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Tube diameters in the range 5/8 inch (16 mm) to 2 inch (50 mm). Smaller diameters (5/8 to 1 inch) preferred since this gives compact and cheap heat exchangers. Larger tubes for heavily fouling fluids. Steel tubes – BS 3606; Other tubes – BS 3274. Preferred tube lengths are 6 ft, 8 ft, 12 ft, 16 ft, 20 ft and 24 ft; optimum tube length to shell diameter ratio ~ 5 – 10. ¾ in (19 mm) is a good starting trial tube diameter.
Construction Details – Tube Arrangements •
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Tubes usually arranged in equilateral triangular, square or rotated square patterns.
Tube pitch, Pt, is 1.25 times OD.
Construction Details - Shells •
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Shell should be a close fit to the tube bundle to reduce bypassing. Shell-bundle clearance will depend on type of heat exchanger.
Construction Details - Shell-Bundle Clearance
Construction Details – Tube Count •
Bundle diameter depends not only on number of tubes but also number of tube passes. 1 / n1
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N Db d 0 t K 1
Nt is the number of tubes Db is the bundle diameter (mm) D0 is tube outside diameter (mm)
n1 and K1 are constants
Construction Details - Baffles •
Baffles are used: – To direct the fluid stream across the tubes – To increase the fluid velocity – To improve the rate of transfer
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Most commonly used baffle is the single segmental baffle.
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Optimal baffle cut ~ 20-25%
Basic Design Procedure •
General equation for heat transfer is:
Q UAT m
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where Q is the rate of heat transfer (duty), U is the overall heat transfer coefficient, A is the area for heat transfer ΔTm is the mean temperature difference We are not doing a mechanical design, only a thermal design.
Tube Side Heat Transfer Coefficient
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Shell Side Heat Transfer Coefficient
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Figure
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Decision •
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The decision as to whether or not the exchanger is thermally suitable for a given service is based on a comparison of calculated versus required overall heat-transfer coefficients. The exchanger is suitable if the calculated value of the design coefficient, UD, is greater than or equal to the value, Ureq, that is needed to provide the required rate of heat transfer. If the converse is true, the exchanger is "not suitable". 16
Engineering Judgment? •
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The quotation marks are to indicate that the final decision to reject the exchanger should be tempered by engineering judgment. For example, it may be more economical to utilize an existing exchanger that is slightly undersized, and therefore may require more frequent cleaning, than to purchase a larger exchanger. In principle, the rating decision can be based on a comparison of heat transfer areas, heat-transfer rates, or mean temperature differences as well as heat-transfer coefficients. In fact, all of these parameters are used as decision criteria in various applications.
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Tube Side Pressure Drop
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Pressure Drop due to Minor loss and Nozzle
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Friction Factor
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U Shell and Tube HE •
U-tubes provide a less expensive alternative to a floating head. In common with type P and W floating heads (and all stationary heads), U-tube bundles have no internal gaskets where leakage and fluid mixing can occur.
The main disadvantages are: Cleaning interior tube surfaces is more difficult due to U-bends. Except for outermost tubes in bundle, individual tube replacement is not practical. Cannot be used if a single tube pass is required. 21
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Overall Heat Transfer Coefficient •
Overall coefficient given by:
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U 0
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h0
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hod
d 0 d 0 ln d i d 0 2 k w
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d i hid
d 0
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d i hi
h0 (hi) is outside (inside) film coefficient hod (hid) is outside (inside) dirt coefficient kw is the tube wall conductivity do (di) is outside (inside) tube diameters
Individual Film Coefficients •
Magnitude of individual coefficients will depend on: – Nature of transfer processes (conduction, convection, radiation, etc.)
– Physical properties of fluids – Fluid flow rates – Physical layout of heat transfer surface •
Physical layout cannot be determined until area is known; hence design is a trial-and-error procedure.
Typical Overall Coefficients
Typical Overall Coefficients
Fouling Factors (Dirt Coeffs) •
Difficult to predict and usually based on past experience
Fluid Allocation: Shell or Tubes? •
Corrosion
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Fouling
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Fluid temperatures
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Operating pressures
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Pressure drop
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Viscosity
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Stream flow rates
Shell and Tube Fluid Velocities •
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High velocities give high heat-transfer coefficients but also high pressure drop. Velocity must be high enough to prevent settling of solids, but not so high as to cause erosion. High velocities will reduce fouling For liquids, the velocities should be as follows: – Tube side: Process liquid 1-2m/s Maximum 4m/s if required to reduce fouling Water 1.5 – 2.5 m/s – Shell side: 0.3 – 1 m/s
Shell Side Friction Factor
Tube Side Friction Factor
