3-1-1 Heat Transfer Theory

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    Heat Transfer TheoryIntroduction

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    The heat lost by the hot mediumis equal to the amountof heat gained by the cold medium, except for losses tothe surroundings.

    There must be a temperature differencebetween the twomedia for heat transfer to take place.

    Fundamentals of Heat Transfer Theory

    Three basic natural laws of physics:

    Heat will always be transferred from a hot mediumto acold medium, until equilibrium is reached.

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    Modes of Heat Transfer

    Three modes:

    Radiation = Electromagnetic waves

    Conduction = Molecular or atomic vibrations

    Convection = Transport of small mass elements

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    Convection

    Is the most efficient way of transporting heat

    This is what we want in the heat exchanger

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    Flow Principles: Laminar

    Parabolic velocity profile:

    friction close to wall -> lower velocity

    centre of tube -> higher velocity

    Low velocity and low Reynolds number -> low pressure drop

    Distinct parallel fluid layers -> no mixing between layers

    Only conduction -> poor heat transfer efficiency

    Flow profile Velocity profile

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    Nearly constant velocity profile

    High velocity and high Reynolds number -> high pressure drop

    Turbulent flow -> cleaning effect -> less fouling

    No orderly flow -> mixing of fluid mass elements

    Mainly convection -> high heat transfer efficiency

    Flow Principles: Turbulent

    Flow profile Velocity profile

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    Even in turbulent flow, there is always a layer closest to the wall with

    laminar film where heat is transferred only by conduction

    Flow Principles: Turbulent

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    Plate pattern

    Turbulent flow

    Herring bone pattern -> many

    contact points where the fluid

    is forced to turn direction ->

    induced turbulence ->convection -> high heat

    transfer efficiency

    Cleaning effect

    High pressure drop

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    Laminar Film

    Thickness of the laminar film depends on

    Fluid velocity

    Fluid viscosity

    Plate pattern

    The thinner the laminar film, the less resistance to heat transfer,

    the more efficient heat exchanger

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    Two heat exchanger types

    Direct

    Principle: Product and service medium are in direct contact

    Example: Water and air in a cooling tower

    Indirect

    Principle: Product and service medium are separated by a wall

    Example: Hot water and product in a plate heat exchanger

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    Counter-current v.s Co-current

    Counter current flow:

    medium driving force all over plate

    possible with crossing temperatures

    larger LMTD -> smaller area

    T2outT1out

    Co current flow:

    high driving force in beginning

    very low in the endbetter control of wall temperature

    lower pressure drop in

    condensing duties

    T2outT1out

    T2in

    T1in

    T2in

    T1in

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