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    MEC551THERMAL ENGINEERING

    4 0 Heat Exchangers

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    4.0 HEAT EXCHANGER

    4.1 Types of Heat Exchanger: Shell and

    Tube, Plate - Parallel Flow, Counter

    Flow, Cross Flow.

    4.2 Overall Heat Transfer Coefficient.4.3 Log-mean-temperature difference (LMTD)

    method & correction

    4.4 Effectiveness of NTU (e-NTU method4.5 Heat Exchanger Design and

    Consideration

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    4

    Heat Exchangers

    Heat exchangers are typically classified according to

    flow arrangement and type of construction.

    The simplest heat exchanger is one in which the hotand cold fluids move in the same direction (parallel

    flow) or opposite directions (counter flow) in a

    concentric tube (or double pipe construction).

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    Heat Exchangers(Parallel-flow)

    In the parallel-flow

    arrangement, the hot

    and cold fluids enter

    the same end, flow in

    the same direction, andleave at the same end.

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    Heat Exchangers(Counter-flow)

    In the counter-flow

    arrangement, the fluids

    enter at opposite ends,

    in opposite directions,

    and leave at oppositeends.

    out

    in

    cold fluid in

    warmed

    fluid out

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    Heat Exchangers(Cross-Flow)

    Alternatively, the fluids

    may move in cross-flow

    (perpendicular) to one

    another.

    Fluid motion over them

    may be mixed or

    unmixed. The fluid is

    unmixed, because fins

    inhibit motion in a

    direction that is

    transverse to the main

    flow direction (x)

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    Heat Exchangers(Shell and Tube Design)

    These designs contain a large number of tubes

    (packed in a shell) and heat transfer takes place as

    one fluid flows inside the tubes while the other fluid

    flows outside the tubes through the shell.

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    Heat Exchangers(Shell and Tube Design)

    Baffles are commonly placed in the shell to force the

    shell-side fluid to flow across the shell to enhance

    heat transfer and to maintain a uniform spacing

    between the tubes.

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    Heat Exchangers(Shell and TubeMulti-pass designs)

    Shell and tube heat

    exchangers are further

    classified by how many shell

    and tube passes are involved.

    One-shell pass and two-tube

    passes devices

    Heat exchangers in which all the

    tubes make 1 U-turn in the shell

    are called

    Two-shell passes and four-

    tube passes devices

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    Heat Exchangers(Plate and Frame)

    This is an

    innovative design

    which consists of

    a series of plates

    with corrugatedflat flow passages.

    Hot and cold fluids

    flow in alternative

    passages, thus the

    cold stream is

    surrounded by two

    hot streams.

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    14

    Heat Exchangers(Terminology)

    Heat exchangers are often given specific names to

    reflect the specific application for which they are

    used:

    Condenser

    A heat exchanger in which one of the fluids is cooled

    and condenses as it flows through the heat exchanger.

    Boiler

    A heat exchanger in which one of the fluids absorbs heat

    and vaporizes.

    Space radiator

    A heat exchanger that transfers heat from the hot fluid to

    the surrounding space by radiation.

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    4.2 Overall Heat TransferCoefficient U,W/m2.oC

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    Double-pipe Heat Exchanger

    A heat exchanger typically

    involves two flowing fluids

    separated by a solid wall.

    Heat is transferred1. from the hot fluid to the wall

    by convection,

    2. through the wall by

    conduction,3. from the wall to the cold fluid

    again by convection.

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    Inner tube (dimensions)

    DiDo

    L

    t

    Ao= DoL

    Ai= D

    iL

    t = (Do- Di)/2 = Ro- Ri

    Outer surface area

    inner surface area

    Wall thickness

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    Thermal resistances of

    the tube wall

    Rwall= ln (Do/Di)2 kL

    inner surface

    Ri= 1/hiAi ,

    outer surface

    Ro= 1/hoAo

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    Double-pipe Heat Exchanger

    The thermal resistance of the

    network is thus:

    oo

    DD

    ii

    totAhkLAh

    R io

    1

    2ln1

    So the thermal resistance in the path of heat flow from hot tocold fluid in a heat exchanger includes:

    i. Skin resistance associated with the flow

    ii. Scale resistance from wall fouling (to be discussed later)

    iii. Thermal resistance of wall material

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    Overall Heat Transfer Coefficient

    The heat transfer is therefore:

    Where U = heat transfer coefficient

    Note:

    TAUTAUTUA

    R

    TQ ooii

    oioi

    ooii

    AAunlessUU

    AUAU

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    Overall Heat Transfer Coefficient

    In this equation, U is called the overall heat transfer

    coefficient.

    tot

    ooiis

    RAUAUAU

    111

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    Since:

    where Amis called the logarithmic mean area

    Overall Heat Transfer Coefficient

    ) ) ) ) )

    ) )

    ) mAA

    LR

    LR

    RRL

    io

    LR

    LR

    tthickness

    ioR

    R

    D

    D

    wall

    Ak

    t

    k

    t

    k

    t

    kLRR

    RR

    kLkLR

    iAoA

    io

    i

    o

    io

    i

    o

    i

    o

    i

    o

    -

    -

    -

    -

    ln

    2

    2ln

    2

    2

    2

    ,

    11

    2

    ln

    2

    ln

    2

    ln

    )i

    o

    A

    A

    io

    m

    AAA

    ln

    -

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    Overall Heat Transfer Coefficient

    For a thin tube:

    1

    11

    0ln1;0

    -

    oi

    oi

    i

    o

    i

    o

    hhUU

    D

    Dso

    D

    Dt

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    Overall Heat Transfer Coefficient

    When the tubes are finned on one side to enhance the

    heat transfer, the total heat transfer surface area on

    the finned side becomes:

    unfinnedfintotals AAAA

    SurfaceArea of fins

    SurfaceArea of Unfinned portion

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    Overall Heat Transfer Coefficient

    For short fins of high thermal conductivity, we can use

    this total area in the convection resistance relation:

    Since in this case the fins will be isothermal.

    Otherwise we should determine the effective surfacearea from:

    )unfinnedfinsconv

    AAhAhR

    11

    finfinunfinneds AAA

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    Effect of Fouling

    The performance of heat exchangers

    usually deteriorates with time as a result

    of the accumulation of deposits on heat

    transfer surfaces.

    The layer of deposits represents

    additional resistance to heat transfer

    and causes Q to decrease.

    The net effect is represented by thefouling factor (Rf) which is a measure of

    the thermal resistance introduced by

    fouling.

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    Effect of Fouling

    If we define the fouling factorson the inside and the

    outside surface to be Rfiand Rfo, respectively. Then the

    total thermal resistance (R) with fouling and the overall

    heat transfer coefficient becomes:

    Generally, Uois low for a fluid with low k-values (for example

    gases and oils).

    1

    0

    1ln2

    1

    11

    -

    o

    fo

    i

    oo

    i

    ofi

    i

    o

    i

    o

    oo

    fo

    mi

    fi

    ii

    hRD

    D

    k

    D

    D

    DRD

    D

    hU

    hAA

    R

    Ak

    t

    A

    R

    hAR

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    Overall Heat Transfer Coefficient(Example 11.1)

    Properties of water at 45C (Table A-9):

    Properties of oil at 80C (Table A-16):

    sm

    waterCmW

    water

    waterm

    kg

    water

    k 2

    3

    610602.0637.0

    91.3Pr990

    -

    sm

    oilCmW

    oil

    oilm

    kg

    oil

    k 2

    3

    6105.37138.0

    490Pr852

    -

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    Overall Heat Transfer Coefficient(Example 11.1)

    Since the thickness is small:

    The hydraulic diameter for a circular tube is the diameter of the

    tube itself, Dh, water = D = 0.02 m

    Therefore the mean velocity (Vm) for water is:

    oi hhU

    111

    )

    ) )

    sm

    m

    kg

    s

    kg

    waterhwater

    water

    cwater

    water

    waterm

    m

    Dm

    AmV

    61.102.0990

    5.02

    4

    1

    2

    ,41

    3

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    Overall Heat Transfer Coefficient(Example 11.1)

    The Reynolds number for the water is:

    Since Re > 4,000, which is the Recritfor a pipe, the flow of water is

    turbulent. The Nusselt number is thus:

    ) )490,53

    10602.0

    02.061.1Re

    26

    ,

    -

    s

    m

    sm

    water

    waterhwaterm

    water

    mDV

    ) ) 6.24091.3490,53023.0

    PrRe023.0

    4.08.0

    4.08.0

    waterwaterwater

    hwater

    k

    DhNu

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    Overall Heat Transfer Coefficient(Example 11.1)

    Then:

    )Cm

    WCmW

    water

    waterh

    waterwater

    m

    NuD

    kh

    2663,76.24002.0

    637.0

    ,

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    Overall Heat Transfer Coefficient(Example 11.1)

    Now we repeat the analysis for oil.

    The mean velocity of the oil is:

    m

    DDD iooilh

    01.002.003.0

    ,

    -

    -

    ) )

    ) ) sm

    m

    kg

    s

    kg

    iooil

    oil

    coil

    oil

    oilm

    m

    DD

    m

    A

    m

    V

    39.202.003.0852

    8.0222

    41

    2241,

    3

    -

    -

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    Overall Heat Transfer Coefficient(Example 11.1)

    So the Reynolds number for oil is:

    Recritfor oil in a pipe is higher, so the flow of oil is laminar.

    The Nusselt number can be found from Table 13-3 (in text) for

    Dt/D

    s= 0.667 as:

    ) )637

    105.37

    01.039.2Re

    26

    ,,

    -

    s

    m

    sm

    oil

    oilhoilm

    oil

    mDV

    45.5oilNu

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    Overall Heat Transfer Coefficient(Example 11.1)

    Therefore:

    since

    Then the overall heat transfer coefficient (U) for this heat

    exchanger becomes:

    )Cm

    WCmW

    oil

    oilh

    oiloil

    mNu

    D

    kh

    22.7545.5

    01.0

    138.0

    ,

    Cm

    W

    CmW

    CmW

    oi hh

    U

    2

    22

    5.74

    2.75

    1

    663,7

    1

    1

    11

    1

    oi hhU

    111

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    Effect of Fouling(Example 11.2)

    Example 11.2 A double-pipe

    (shell and tube) heat exchanger

    is constructed of a stainless

    steel inner tube with inner

    diameter (Di) of 1.5 cm and

    outer diameter (Do) of 1.9 cm.and an outer shell of inner

    diameter 3.2 cm.

    For the convection values and

    fouling factors given, determine

    the thermal resistance of the

    heat exchanger and heat

    transfer coefficients Ui and Uo.

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    Effect of Fouling(Example 11.2)

    The areas are:

    The thermal resistance is:

    )ooo

    ofD

    D

    i

    if

    ii

    ooiis

    AhA

    R

    kLA

    R

    Ah

    AUAUAUR

    i

    o

    1

    2

    ln1

    111

    ,,

    ) )

    ) )

    2

    2

    0597.01019.0

    0471.01015.0

    mmmLDA

    mmmLDA

    oo

    ii

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    Effect of Fouling(Example 11.2)

    Therefore:

    )

    ) )

    ) ) ) )

    )

    W

    C

    WC

    Cm

    W

    WCm

    CmW

    WCm

    Cm

    W

    ooo

    ofD

    D

    i

    if

    ii

    mmm

    m

    m

    mm

    AhA

    R

    kLA

    R

    AhR i

    o

    0532.0

    001396.000168.00025.000849.002654.0

    0597.01200

    1

    0597.0

    0001.0

    11.152

    015.0

    019.0ln

    0471.00004.0

    0471.08001

    1

    2

    ln1

    22

    22

    ,,

    2

    2

    2

    2

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    Effect of Fouling(Example 11.2)

    Note that 19% of the total resistance in this case is due to fouling

    and about 5% of it is due to the steel tube separating the two

    fluids. The rest of the 76% is due to convective resistances on

    the two sides of the inner tube.

    Now know the thermal resistances, the overall heat transfercoefficients can be found:

    ) )

    ) ) CmW

    WC

    o

    o

    Cm

    W

    WC

    i

    i

    mARU

    mARU

    2

    2

    3150597.00532.0

    11

    3990471.00532.0

    11

    2

    2

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    4.3 Log Mean TemperatureDifference Method

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    Analysis of Heat Exchangers

    The first law of

    thermodynamics requires

    that the heat transfer to

    the cold fluid must be

    equal to the heat transferfrom the hot fluid.

    Where the subscripts cand h

    stand for cold and hot.

    )

    )outhinhphh

    incoutcpcc

    TTCmQ

    TTCmQ

    ,,

    ,,

    --

    -

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    Analysis of Heat Exchangers

    In heat exchanger analysis it is convenient to combine

    the product of the mass flow and the specific heat of a

    fluid into a single quantity called the heat capacity

    rate.

    )

    )outhinhhincoutcc

    TTCQ

    TTCQ

    ,,

    ,,

    --

    -

    phhh cmC pccc cmC

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    Analysis of Heat Exchangers

    Consider an incremental area of the heat exchanger surface (as

    shown for either a counter-flow or parallel flow heat exchanger).

    Parallel Flow Counter-flow

    To

    dA

    Hot

    Cold

    Th,in

    Tc,inTc,out

    TL

    To

    Th,in

    Tc,out

    Th,out

    TL

    Th,out

    Tc,in

    Hot

    Cold

    dA

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    Analysis of Heat Exchangers

    The heat transfer over the area (dA) can be expressed in three

    ways:

    1) The heat flow between hot fluids:

    2) The heat gain by the cold fluid:

    3) The heat given up by the hot fluid:

    TdAUQd

    cc dTCQd

    hh dTCQd -

    Log Mean Temperature

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    Log Mean Temperature

    Difference Method

    However, since the temperature difference (T)

    between the hot and cold fluids varies with distance

    along the heat exchanger. It is convenient to use a

    mean temperature difference (Tm) between the hot

    and cold fluids, so that:

    mTUAQ

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    LMTD Method

    Consider a parallel double-

    pipe heat exchanger. The

    heat transfer on each fluid in

    the differential section can

    be expressed as:

    cc

    hh

    dTCQ

    dTCQ

    -

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    LMTD Method

    The rate of heat transfer in the differential section can

    be expressed as:

    Substituting this into the previous equations gives:

    ) dATTUQ ch -

    )

    --

    --

    chch

    ch

    CCdAU

    TTTTd 11

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    LMTD Method

    Integrating from the inlet to the heat exchanger to its

    outlet, we get:

    )

    --

    - L

    ch

    L

    ch

    ch

    dACCUTT

    TTd

    00

    11

    + For parallel-flow

    - For counter-flow

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    LMTD Method

    Solving the integral for parallel flow

    (where T1=Th,in-Tc,inand T2=Th,out-Tc,out) we get:

    -

    -

    -

    -

    -

    hc

    h

    ch

    c

    ch

    ch

    ch

    T

    incinh

    T

    outcouth

    CC

    C

    CC

    CAU

    CC

    CCAU

    T

    T

    CCAU

    TT

    TT

    11ln

    11ln

    1

    2

    ,,

    ,,

    1

    2

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    LMTD Method

    )

    --

    -

    ---

    hinhouth

    incoutc

    cincoutc

    inhouth

    CTT

    TT

    CTT

    TTAUT

    T 11ln,,

    ,,

    ,,

    ,,

    1

    2

    Since:

    Then:

    )inhouth

    incoutc

    c

    h

    incoutc

    inhouth

    h

    c

    TT

    TT

    C

    Cand

    TT

    TT

    C

    C

    ,,

    ,,

    ,,

    ,,

    --

    -

    -

    --

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    LMTD Method

    Solving:

    ) )

    -

    ----

    -

    --

    QTTAU

    Q

    TTTTAU

    Q

    TT

    Q

    TTAU

    T

    T

    incinhoutcouth

    incoutcinhouth

    12

    ,,,,

    ,,,,

    1

    2ln

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    LMTD Method

    The log mean temperature difference method (LMTD) can be usedfor both parallel and counter-flow heat exchangers, if the

    following terms are used for T:

    Parallel Heat Exchangers Counter-flow Heat Exchangers

    Correction factors

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    Correction factorsfor LMTD Method

    The LMTD method is strictly limited to single pass parallel andcounter-flow heat exchangers only.

    However, similar relations are also developed for cross-flow

    and multi-pass shell and tube heat exchangers, but these are

    very complicated expressions.

    In these cases it is more convenient to relate the equivalent

    temperature difference relation for the counter-flow case as:

    flowcounterLMTDm TFT

    - ,

    Log Mean Temperature Difference

    for counter-flow heat exchanger

    Correction factor

    Correction factors

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    Correction factorsfor LMTD Method

    Correction factors (F) for several common

    configurations are available from standard charts.

    Figure 11-18 (in text) shown on the next slide, lists

    some for common shell and tube cross-flow heatexchangers. The correction factor used for this is:

    ) )

    sideshellp

    sidetubep

    cm

    cm

    tt

    TTR

    tT

    ttP

    -

    -

    -

    -

    12

    21

    11

    12

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    LMTD Method

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    LMTD Method(Example 6.3)

    Example 6.3 Alcohol is to be cooled at a rate of 0.2kg/s from 75C to 35C in a counter-flow heatexchanger. Cooling water enters the heat exchanger

    at 12C at a rate of 0.16 kg/s. The convectivecoefficient between alcohol and the tube wall andwater is 0.34 kW/(m2K) and between the tube walland water is 0.225 kW/(m2K). The tube may beassumed to be thin. The specific heat for the alcohol

    is 2.52 KJ/(kgK) and water is 4.187 KJ/(kgK). Calculate the capacity ratio (C), effectiveness (),

    and heat exchanger surface area (As).

    LMTD Method

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    LMTD Method(Example 6.3)

    Th,in= 75C

    Tc,out= ?

    Tc,in

    = 12C

    Th,out= 35C

    TTh,in

    Tc,out

    Th,out

    Tc,in

    T1

    T2

    Km

    kWo

    Km

    kWi

    h

    h

    2

    2

    225.0

    340.0

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    LMTD Method

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    LMTD Method(Example 6.3)

    Energy balance:

    ) ) )

    ) ) )

    CCT

    CC

    CT

    TcmTcm

    QQ

    outc

    Kkg

    kJ

    s

    kg

    outcKkgkJ

    s

    kg

    alcoholphhwaterpcc

    lossgain

    -

    -

    1.4267.0

    16.2012

    357552.22.0

    12187.416.0

    ,

    ,

    LMTD Method

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    LMTD Method(Example 6.3)

    The effectiveness is thus:

    Cminis used for Qmaxbecause the fluid with the smallerheat capacity rate will experience the largesttemperature change and thus be the first toexperience the maximum temperature, at which pointheat transfer stops.

    )

    ) ) )

    635.01275

    3575

    ,,m in

    ,,

    m ax

    -

    -

    -

    -

    CC

    CC

    TTC

    TTC

    Q

    Q

    incinh

    outhinhh

    e

    LMTD Method

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    LMTD Method(Example 6.3)

    The temperature differences are:

    Therefore the LMTD is:

    CCCTTT

    CCCTTT

    outcinh

    incouth

    --

    --

    9.321.4275

    231235

    ,,1

    ,,2

    ) ) CCCTTTC

    CT

    Tm -

    -

    7.27

    ln9.3223

    ln9.32

    2312

    1

    2

    LMTD Method

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    LMTD Method(Example 6.3)

    For a thin tube (as given) then rori, the overall heattransfer coefficient is:

    Finally, the surface area is:

    Km

    kW

    oi

    i

    hh

    U

    --

    21354.0

    225.0

    1

    34.0

    111 11

    ) )2

    495.02737.271354.0

    16.20

    2

    mK

    kW

    TU

    Q

    A

    Km

    kW

    ms

    LMTD Correction

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    LMTD Correction(Example 6.4)

    Example 6.4 Cooling water (Cpc=4.187 kJ/(kgK))flows through a two-shell pass, four-tube pass heat

    exchanger at the rate of 2 kg/s and temperatures at

    entry of 20and exit at 80C. Hot oil enters throughthe shell side of the heat exchanger at 140C andleaves at 90C. Calculate the heat exchanger surfacearea if the overall heat transfer coefficient (U) is 300

    W/(m2C).

    LMTD Correction

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    LMTD Correction(Example 6.4)

    Th,in= 140C

    Th,out= 90C

    Tc,out= 80C

    Tc,in= 20C

    LMTD Correction

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    LMTD Correction(Example 6.4)

    The LMTD is:

    ) )C

    CC

    CC

    CCCC

    T

    T

    TTTm

    -

    -

    ---

    -

    9.64

    80140

    2090ln

    801402090

    ln2

    1

    21

    LMTD Correction

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    LMTD Correction(Example 6.4)

    Parameters to use the LMTD correction chart:

    833.02080

    90140

    5.020140

    2080

    ,,

    ,,

    ,,

    ,,

    -

    -

    -

    -

    -

    -

    -

    -

    CC

    CC

    TT

    TTR

    CC

    CC

    TT

    TTP

    incoutc

    inhouth

    incinh

    incoutc

    LMTD Correction

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    LMTD Correction(Example 6.4)

    From chart (Figure 13-18 in text) the value F= 0.97

    0.97

    LMTD Correction

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    LMTD Correction(Example 6.4)

    Therefore the surface area (As) is:

    ) ) )

    ) ) )23

    ,,

    6.269.6497.010300

    2080187.42

    2

    mC

    CC

    TFU

    TTCm

    A

    Cm

    kW

    KkgkJ

    s

    kg

    m

    incoutcpcc

    s

    -

    -

    -

    Analysis of Heat Exchangers

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    Analysis of Heat Exchangers

    Two different design tasks:1) Specified:

    - the temperature change in a fluid stream, and

    - the mass flow rate.

    Required:

    - the designer needs to select a heat exchanger.

    2) Specified:

    - the heat exchanger type and size,

    - fluid mass flow rate,

    - inlet temperatures.

    Required:

    - the designer needs to predict the outlet temperatures and heat transferrate.

    Two methods used in the analysis of heat exchangers:

    the log mean temperature difference (or LMTD)

    best suited for the #1,

    the effectivenessNTUmethod

    best suited for task #2.

    LMTD

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    LMTD

    )1 2

    1 2ln

    s lm

    lm

    Q UA T

    T TT

    T T

    -

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    Correction factors

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    Correction factorsfor LMTD Method

    The LMTD method is strictly limited to single pass parallel andcounter-flow heat exchangers only.

    However, similar relations are also developed for cross-flow

    and multi-pass shell and tube heat exchangers, but these are

    very complicated expressions.

    In these cases it is more convenient to relate the equivalent

    temperature difference relation for the counter-flow case as:

    flowcounterLMTDm TFT

    - ,

    Log Mean Temperature Difference

    for counter-flow heat exchanger

    Correction factor

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    4.4 The EffectivenessNTU Method

    The NTU Method

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    The -NTU Method

    This method is useful when the LMTD method cannotbe determined. The required data may instead be

    determined from the -NTU method charts of several

    common configurations

    Definitions:

    The heat capacity of the cold stream is given by:

    The heat capacity of the hot stream is given by:

    pccc cmC

    phhh cmC

    The NTU Method

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    The -NTU Method

    The capacity ratio is defined as:

    ch

    c

    h

    ch

    h

    c

    CCifC

    CC

    CCif

    C

    CC

    ;

    ;

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    The NTU Method

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    The -NTU Method

    The maximum temperature difference possible in aheat exchanger is:

    The heat transfer in the heat exchanger will reach its

    maximum value when:

    1) The cold fluid is heated to the inlet temperature of the hot

    fluid.

    2) The hot fluid is cooled to the inlet temperature of the cold

    fluid.

    incinh TTT ,,m ax -

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    The NTU Method

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    The -NTU Method

    Therefore, the maximum possible heat transfer rate inheat exchanger is:

    )incinh TTCQ ,,minmax -

    hc CorCofsmallertheisCwhere m in:

    The NTU Method

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    The -NTU Method

    Therefore the heat exchanger effectiveness for a

    counter-flow heat exchanger is:

    )

    ) )incinh

    outhinhh

    incinh

    incoutcc

    TTC

    TTC

    TTC

    TTC

    ,,m in

    ,,

    ,,m in

    ,,

    -

    -

    -

    -

    e

    e

    ratetransferheatpossibleMaximum

    ratetransferheatActual

    Q

    Q

    max

    e

    The NTU Method

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    The -NTU Method

    NTUNumber of Heat Transfer Unit

    The NTU is a measure of physical size of the heat

    exchanger; the larger NTU, the larger heat exchanger

    size. It is defined as the ratio of the heat capacity ofthe heat exchanger to the minimum heat capacity of

    the flow.

    )minmin p

    ss

    CmAU

    CUANTU

    The -NTU Method

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    (Step-by-step Procedure)

    The -NTU Method Procedure

    1) Determine Cminand Cmaxfrom of the streams

    and calculate the capacity ratio C.

    max

    min

    C

    Cc

    pcm

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    The -NTU Method

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    89

    (Step-by-step Procedure)

    3) Charts

    Knowing , NTU, and C use

    the chart (Figure 11-26 in the

    text) to determine U and As

    in question.

    OR from standard derivation

    Tables 11-4 & 11-5

    The -NTU Method

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    (Step-by-step Procedure)

    4) Compute the heat transfer rate:

    )incinh TTCQ ,,min - e

    The -NTU Method

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    91

    (Step-by-step Procedure)

    5) Calculate the outlet temperature:

    c

    incoutc

    h

    inhouth

    C

    QTT

    C

    QTT

    -

    -

    ,,

    ,,

    The -NTU Method

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    (Example 6.5)

    Example 6.5 A single, pass counter-flow shell-and-tube heat exchanger is used to cool hot mercury

    (Cph=1.37 kJ/(kgK)) from 110C to 70C flowing at arate of 1 kg/s with water (Cpc=4.187 kJ/(kgK)) enteringat 30C and flowing at a rate of 0.2 kg/s. Calculatethe heat transfer surface area required and the exit

    temperature of the water if the overall heat transfer

    coefficient (U) is 250 kW/(m2K).

    The -NTU Method

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    30C

    70C110C

    (Example 6.5)

    The -NTU Method

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    (Example 6.5)

    The temperature difference (T2) is:

    The heat capacities are:

    CCC

    TTT incouth

    -

    -

    403070

    ,,2

    ) ) sK

    kJKkg

    kJs

    kg

    pccc

    sKkJ

    KkgkJ

    s

    kg

    phhh

    CmC

    CmC

    836.0187.42.0

    37.137.11

    The -NTU Method

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    95

    (Example 6.5)

    Since Ch> Cc, therefore:

    61.0

    37.1

    836.0

    836.0

    370.1

    max

    min

    min

    max

    C

    CC

    CC

    CC

    sK

    kJ

    c

    sKkJ

    h

    The -NTU Method

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    (Example 6.5)

    The total heat transfer (Q) is:

    The maximum heat transfer is:

    ) ) ) )

    kW

    K

    TTCQ

    KkgkJ

    outhinhh

    8.54

    2737027311037.1

    ,,

    -

    --

    --

    ) ) ) )

    kW

    K

    TTCQ

    sKkJ

    incinh

    9.66

    27330273110836.0

    ,,m inm ax

    -

    -

    The -NTU Method

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    97

    (Example 6.5)

    The heat exchanger effectiveness is:

    Now consult the -NTU chart (Figure 11-26 in text)

    82.0

    9.66

    8.54

    max

    kW

    kW

    Q

    Q

    e

    The -NTU Method

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    98

    (Example 6.5)

    0.82

    2.7

    From the chart:

    NTU = 2.7

    The -NTU Method

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    99

    (Example 6.5)

    Therefore the surface area (As) is:

    The exit temperature of the water is thus:

    ) 2min 03.9250

    836.07.2

    2

    mU

    CNTUA

    Km

    kW

    sKkJ

    s

    )

    ) CorKKkW

    TCQT

    TTCQ

    sK

    kJ

    inc

    c

    outc

    incoutcc

    -

    6.956.36827330836.0

    8.54

    ,,

    ,,

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    4.5 Heat Exchanger DesignConsiderations

    Design Considerations

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    Design Considerations

    Some design considerations are:

    Heat transfer rate

    This is the most important quantity. A heat exchanger

    must be capable of transferring heat at a specified rate in

    order to achieve the desired temperature change of thefluid at a specified mass flow rate.

    Cost

    Budgetary limitations always play an important role.

    Operating and maintenance costs are also a factor.

    Pumping Power

    The fluids are normally forced by pumps or fans which

    require a pump with associate electrical costs.

    Design Considerations

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    Design Considerations

    Size and weight Normally the smaller and lighter, the better. This is

    especially true for automotive and aerospace industries.

    Type

    The type of heat exchanger depends on therequirements, fluids involved, size and weight limitations

    etc.

    Materials

    The materials in the heat exchanger construction may bean important consideration, especially if thermal

    stresses are an issue.

    End Of Heat Exchanger SectionC C

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    End Of HeatExchanger SectionC C

    End Of LecturesC C

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    End Of LecturesC C

    Study hard for your finals

    I hope you all do well