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Heat Exchanger Design

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Heat Exchanger Design Anand V P Gurumoorthy Associate Professor Chemical Engineering Division School of Mechanical & Building Sciences VIT University – PowerPoint PPT presentation

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Title: Heat Exchanger Design


1
Heat Exchanger Design
  • Anand V P Gurumoorthy
  • Associate Professor
  • Chemical Engineering Division
  • School of Mechanical Building Sciences
  • VIT University
  • Vellore, India

2
Heat Exchanger Classification
  • Recuperative
  • Cold and hot fluid flow through the unit without
    mixing with each other. The transfer of heat
    occurs through the metal wall.
  • Regenerative
  • Same heating surface is alternately exposed to
    hot and cold fluid. Heat from hot fluid is stored
    by packings or solids this heat is passed over
    to the cold fluid.
  • Direct contact
  • Hot and cold fluids are in direct contact and
    mixing occurs among them mass transfer and heat
    transfer occur simultaneously.

3
Heat Exchanger Standards and Codes
  • British Standard BS-3274
  • TEMA standards are universally used.
  • TEMA standards cover following classes of
    exchangers
  • Class R designates severe requirements of
    petroleum and other related processing
    applications
  • Class C moderate requirements of commercial and
    general process applications
  • Class B specifies design and fabrication for
    chemical process service.

4
Shell and Tube Heat Exchanger
  • 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 a good shape for pressure
    operation.
  • Uses well-established fabrication techniques.
  • Can be constructed from a wide range of
    materials.
  • Easily cleaned.
  • Well established design procedures.

5
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6
Types of Shell and Tube Heat Exchangers
  • Fixed tube design
  • Simplest and cheapest type.
  • Tube bundle cannot be removed for cleaning.
  • No provision for differential expansion of shell
    and tubes.
  • Use of this type limited to temperature
    difference upto 800C.
  • Floating head design
  • More versatile than fixed head exchangers.
  • Suitable for higher temperature differentials.
  • Bundles can be removed and cleaned (fouling
    liquids)

7
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).
  • Bell-Delaware method
  • Most widely used.
  • Takes into account
  • 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.

8
Construction Details Tube Dimensions
  • 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.

9
Construction Details Tube Arrangements
  • Tubes usually arranged in equilateral triangular,
    square or rotated square patterns.
  • Tube pitch, Pt, is 1.25 times OD.

10
Construction Details - Shells
  • Shell should be a close fit to the tube bundle to
    reduce bypassing.
  • Shell-bundle clearance will depend on type of
    heat exchanger.

11
Construction Details - Shell-Bundle Clearance
12
Construction Details Tube Count
  • Bundle diameter depends not only on number of
    tubes but also number of tube passes.
  • Nt is the number of tubes
  • Db is the bundle diameter (mm)
  • D0 is tube outside diameter (mm)
  • n1 and K1 are constants

13
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
  • Most commonly used baffle is the single segmental
    baffle.
  • Optimal baffle cut 20-25

14
Basic Design Procedure
  • General equation for heat transfer is
  • where Q is the rate of heat transfer (duty),
  • U is the overall heat transfer coefficient,
  • A is the area for heat transfer
  • ?Tm is the mean temperature difference
  • We are not doing a mechanical design, only a
    thermal design.

15
Overall Heat Transfer Coefficient
  • Overall coefficient given by
  • 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

16
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.

17
Typical Overall Coefficients
18
Typical Overall Coefficients
19
Fouling Factors (Dirt Coeffs)
  • Difficult to predict and usually based on past
    experience

20
Mean Temperature Difference (Temperature Driving
Force)
  • To determine A, ?Tm must be estimated
  • True counter-current flow logarithmic
    temperature difference (LMTD)

21
LMTD
  • LMTD is given by
  • where T1 is the hot fluid temperature, inlet
  • T2 is the hot fluid temperature, outlet
  • t1 is the cold fluid temperature, inlet
  • t2 is the cold fluid temperature, outlet

22
Counter-current Flow Temperature Proflies
23
12 Heat Exchanger Temperature Profiles
24
True Temperature Difference
  • Obtained from LMTD using a correction factor
  • ?Tm is the true temperature difference
  • Ft is the correction factor
  • Ft is related to two dimensionless ratios

25
Temp Correction Factor Ft
  • Temperature correction factor, one shell pass,
    two or more even tube passes

26
Fluid Allocation Shell or Tubes?
  • Corrosion
  • Fouling
  • Fluid temperatures
  • Operating pressures
  • Pressure drop
  • Viscosity
  • Stream flow rates

27
Shell and Tube Fluid Velocities
  • 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

28
Pressure Drop
  • As the process fluids move through the heat
    exchanger there is associated pressure drop.
  • For liquids viscosity lt 1mNs/m2 35kN/m2
  • Viscosity 1 10 mNs/m2 50-70kN/m2

29
Tube-side Heat Transfer Coefficient
  • For turbulent flow inside conduits of uniform
    cross-section, Sieder-Tate equation is
    applicable
  • C0.021 for gases
  • 0.023 for low viscosity liquids
  • 0.027 for viscous liquids
  • µ fluid viscosity at bulk fluid temperature
  • µwfluid viscosity at the wall

30
Tube-side Heat Transfer Coefficient
  • Butterworth equation
  • For laminar flow (Relt2000)
  • If Nu given by above equation is less than 3.5,
    it should be taken as 3.5

31
Heat Transfer Factor, jh
  • j factor similar to friction factor used for
    pressure drop
  • This equation is valid for both laminar and
    turbulent flows.

32
Tube Side Heat Transfer Factor
33
Heat Transfer Coefficients for Water
  • Many equations for hi have developed specifically
    for water. One such equation is
  • where hi is the inside coefficient (W/m2 0C)
  • t is the water temperature (0C)
  • ut is water velocity (m/s)
  • dt is tube inside diameter (mm)

34
Tube-side Pressure Drop
  • where ?P is tube-side pressure drop (N/m2)
  • Np is number of tube-side passes
  • ut is tube-side velocity (m/s)
  • L is the length of one tube
  • m is 0.25 for laminar and 0.14 for turbulent
  • jf is dimensionless friction factor for heat
    exchanger tubes

35
Tube Side Friction Factor
36
Shell-side Heat Transfer and Pressure Drop
  • Kerns method
  • Bells method

37
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38
Procedure for Kerns Method
  • Calculate area for cross-flow As for the
    hypothetical row of tubes in the shell equator.
  • pt is the tube pitch
  • d0 is the tube outside diameter
  • Ds is the shell inside diameter
  • lB is the baffle spacing, m.
  • Calculate shell-side mass velocity Gs and linear
    velocity, us.
  • where Ws is the fluid mass flow rate in the
    shell in kg/s

39
Procedure for Kerns Method
  • Calculate the shell side equivalent diameter
    (hydraulic diameter).
  • For a square pitch arrangement
  • For a triangular pitch arrangement

40
Shell-side Reynolds Number
  • The shell-side Reynolds number is given by
  • The coefficient hs is given by
  • where jh is given by the following chart

41
Shell Side Heat Transfer Factor
42
Shell-side Pressure Drop
  • The shell-side pressure drop is given by
  • where jf is the friction factor given by
    following chart.

43
Shell Side Friction Factor
44
(Figure 8 in notes)
45
(Figure 4 in notes)
(Figure 2)
46
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47
(Figure 9 in notes)
48
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49
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50
(Table 3 in notes)
(Figure 10 in notes)
51
(Figure 12 in notes)
52
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53
Bells Method
  • In Bells method, the heat transfer coefficient
    and pressure drop are estimated from correlations
    for flow over ideal tube banks.
  • The effects of leakage, by-passing, and flow in
    the window zone are allowed for by applying
    correction factors.

54
Bells Method Shell-side Heat Transfer
Coefficient
  • where hoc is heat transfer coeff for cross flow
    over ideal tube banks
  • Fn is correction factor to allow for no. of
    vertical tube rows
  • Fw is window effect correction factor
  • Fb is bypass stream correction factor
  • FL is leakage correction factor

55
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56
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57
Bells Method Ideal Cross Flow Coefficient
  • The Re for cross-flow through the tube bank is
    given by
  • Gs is the mass flow rate per unit area
  • d0 is tube OD
  • Heat transfer coefficient is given by

58
Bells Method Tube Row Correction Factor
  • For Regt2100, Fn is obtained as a function of Ncv
    (no. of tubes between baffle tips) from the chart
    below
  • For Re 100ltRelt2100, Fn1.0
  • For Relt100,

59
Bells Method Window Correction Factor
  • Fw, the window correction factor is obtained from
    the following chart
  • where Rw is the ratio of bundle cross-sectional
    area in the window zone to the tube bundle
    cross-sectional area (obtained from simple
    formulae).

60
Bells Method Bypass Correction Factor
  • Clearance area between the bundle and the shell
  • For the case of no sealing strips, Fb as a
    function of Ab/As can be obtained from the
    following chart

61
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62
Bells Method Bypass Correction Factor
  • For sealing strips, for NsltNcv/2 (Ns is the
    number of baffle strips)
  • where a1.5 for Relt100 and a1.35 for Regt100.

63
Bells Method Leakage Correction Factor
  • Tube-baffle clearance area Atb is given by
  • Shell-baffle clearance area Asb is given by
  • where Cs is baffle to shell clearance and ?b is
    the angle subtended by baffle chord
  • ALAtbAsb
  • where ßL is a factor obtained from following
    chart

64
Coefficient for FL, Heat Transfer
65
Shell-side Pressure Drop
  • Involves three components
  • Pressure drop in cross-flow zone
  • Pressure drop in window zone
  • Pressure drop in end zone

66
Pressure Drop in Cross Flow Zone
  • where ?Pi pressure drop calculated for an
    equivalent ideal tube bank
  • Fb is bypass correction factor
  • FL is leakage correction factor
  • where jf is given by the following chart
  • Ncv is number of tube rows crossed
  • us is shell-side velocity

67
Friction Factor for Cross Flow Banks
68
Bells Method Bypass Correction Factor for
Pressure Drop
  • a is 5.0 for laminar flow, Relt100
  • 4.0 for transitional and turbulent flow, Regt100
  • Ab is the clearance area between the bundle and
    shell
  • Ns is the number of sealing strips encountered
    by bypass stream
  • Ncv is the number of tube rows encountered in
    the cross- flow section

69
Bells Method Leakage Factor for Pressure Drop
  • where Atb is the tube to baffle clearance area
  • Asb is the shell to baffle clearance area
  • AL is total leakage area AtbAsb
  • ßL is factor obtained from following
    chart

70
Coefficient for FL
71
Pressure Drop in Window Zones
  • where us is the geometric mean velocity
  • uw is the velocity in the window zone
  • Ws is the shell-side fluid mass flow
  • Nwv is number of restrictions for cross-flow in
    window zone, approximately equal to the number of
    tube rows.

72
Pressure Drop in End Zones
  • Ncv is the number of tube rows encountered in the
    cross-flow section
  • Nwv is number of restrictions for cross-flow in
    window zone, approximately equal to the number of
    tube rows.

73
Bells Method Total Shell-side Pressure Drop
74
Effect of Fouling
  • Above calculation assumes clean tubes
  • Effect of fouling on pressure drop is given by
    table above

75
Condensers
  • Construction of a condenser is similar to other
    shell and tube heat exchangers, but with a wider
    baffle spacing
  • Four condenser configurations
  • Horizontal, with condensation in the shell
  • Horizontal, with condensation in the tubes
  • Vertical, with condensation in the shell
  • Vertical, with condensation in the tubes
  • Horizontal shell-side and vertical tube-side are
    the most commonly used types of condenser.

76
Heat Transfer Mechanisms
  • Filmwise condensation
  • Normal mechanism for heat transfer in commercial
    condensers
  • Dropwise condensation
  • Will give higher heat transfer coefficients but
    is unpredictable
  • Not yet considered a practical proposition for
    the design of condensers
  • In the Nusselt model of condensation laminar flow
    is assumed in the film, and heat transfer is
    assumed to take place entirely by conduction
    through the film.
  • Nusselt model strictly applied only at low liquid
    and vapor rates when the film is undisturbed.
  • At higher rates, turbulence is induced in the
    liquid film increasing the rate of heat transfer
    over that predicted by Nusselt model.

77
Condensation Outside Horizontal Tubes
  • where (hc)1 is the mean condensation film
    coefficient, for a single tube
  • kL is the condensate thermal conductivity
  • ?L is the condensate density
  • ?v is the vapour density
  • µL is the condensate viscosity
  • g is the gravitational acceleration
  • G is the tube loading, the condensate flow per
    unit length of tube.
  • If there are Nr tubes in a vertical row and the
    condensate is assumed to flow smoothly from row
    to row, and if the flow is laminar, the top tube
    film coefficient is given by

78

79
Condensation Outside Horizontal Tubes
  • In practice, condensate will not flow smoothly
    from tube to tube.
  • Kerns estimate of mean coefficient for a tube
    bundle is given by
  • L is the tube length
  • Wc is the total condensate flow
  • Nt is the total number of tubes in the bundle
  • Nr is the average number of tubes in a vertical
    tube row
  • For low-viscosity condensates the correction for
    the number of tube rows is generally ignored.

80
Condensation Inside and Outside Vertical Tubes
  • For condensation inside and outside vertical
    tubes the Nusselt model gives
  • where (hc)v is the mean condensation coefficient
  • Gv is the vertical tube loading, condensate per
    unit tube perimeter
  • Above equation applicable for Relt30
  • For higher Re the above equation gives a
    conservative (safe) estimate.
  • For Regt2000, turbulent flow situation analyzed
    by Colburn and results in following chart.

81
Colburns Results
82
Boyko-Kruzhilin Correlation
  • A correlation for shear-controlled condensation
    in tubes simple to use.
  • The correlation gives mean coefficient between
    two points at which vapor quality, x, (mass
    fraction of vapour) is known.
  • 1,2 refer to inlet and outlet conditions
    respectively
  • In a condenser, the inlet stream will normally be
    saturated vapour and vapour will be totally
    condensed. For these conditions

83
Flooding in Vertical Tubes
  • When the vapor flows up the tube, tubes should
    not flood.
  • Flooding should not occur if the following
    condition is satisfied
  • where uv and uL are velocities of vapor and
    liquid and di is in metres.
  • The critical condition will occur at the bottom
    of the tube, so vapor and liquid velocities
    should be evaluated at this point.

84
Condensation Inside Horizontal Tubes
  • When condensation occurs, the heat transfer
    coefficient at any point along the tube will
    depend on the flow pattern at that point.
  • No general satisfactory method exists that will
    give accurate predictions over a wide flow range.

85
Two Flow Models
  • Two flow models
  • Stratified flow
  • Limiting condition at low condensate and vapor
    rates
  • Annular flow
  • Limiting condition at high vapor and low
    condensate rates
  • For stratified flow, the condensate film
    coefficient can be estimated as

86
  • Condensation of steam
  • For air-free steam a coefficient of 8000 W/m2-0C
    should be used.
  • Mean Temperature Difference
  • A pure, saturated, vapor will condense at a
    constant temperature, at constant pressure.
  • For an isothermal process such as this, the LMTD
    is given by
  • where Tsat is saturation temperature of vapor
  • t1 (t2) is the inlet (outlet) coolant
    temperature
  • No correction factor for multiple passes is
    needed.
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