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HEAT EXCHANGERS

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Title: HEAT EXCHANGERS


1
HEAT EXCHANGERS
  • ? ?
  • March 21, 2005

2
Content
  • Instruction
  • Heat Transfer and Fluid Flow
  • Heat Exchangers
  • Heat Exchangers and Systems
  • Summary

3
Instruction
  • Heat transfer is an important function of many
    industrial processes
  • Transfer heat from one process to another
  • Types of heat exchangers
  • pipe coil exchangers, double-pipe heat
    exchanger, shell and tube heat exchanger,
    reboiler, plate and frame heat exchanger, air
    cooled heat changer, spiral heat exchangers

4
Heat Transfer and Fluid Flow
  • Conduction
  • Convection
  • Radiant
  • Laminar
  • Turbulent

5
Conduction
  • Heat energy is transferred through solid objects
  • Molecules absorb heat energy from a hotter source
    and then vibrate, so transfer the energy from the
    hot side to the cooler side
  • What is more, there are free electrons moving in
    the metal, so metal have better capability of
    conduction

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7
Convection
  • Convection occurs in fluids when warmer molecules
    move toward cooler molecules
  • The movement of the molecules sets up currents in
    the fluid that redistribute heat energy
  • The result is that the energy is equally
    distributed

8
Radiant Heat Transfer
  • Heat is conveyed by electromagnetic waves
  • Radiant heat transfer is a line-of-sight process
  • Radiant heat transfer is not used in a heat
    exchanger

9
Laminar Flow
  • Liquid flowing are in parallel fashion
  • Laminar flow usually exists at low flow rate
  • Laminar flow promotes the development of static
    film, which acts as an insulator

10
Turbulent Flow
  • Turbulent flow is random movement or mixing of
    fluids
  • Turbulent flow allows molecules of fluid to mix
    and absorb heat more readily than does laminar
    flow
  • Turbulent flow decreases the thickness of static
    film, increasing the rate of heat transfer

11
Heat Exchangers
  • Double-pipe heat exchanger
  • Shell and tube heat exchanger
  • Plate and frame heat exchanger
  • Spiral heat exchanger
  • Pipe coil exchanger
  • Air-cooled heat exchangers

12
Double-Pipe Heat Exchanger
  • A double-pipe exchanger has a pipe inside a pipe
  • The outside pipe provides the shell. And the
    inner pipe provides the tube
  • The warm and cool fluids can run parallel flow or
    countercurrent,because it is more efficient
  • Flow rates are very low in a double-pipe heat
    exchanger

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14
Double-Pipe Heat Exchanger
  • The chemical processing industry commonly uses
    hairpin heat exchangers
  • Hairpins are typically rated at 500psig shellside
    and 500 psig tubeside
  • Fins can be added to the internal tubes external
    wall to increase heat transfer

15
Double-Pipe Heat Exchanger
  • Advantages
  • Its excellent capacity for thermal expansion
  • It is easy to install and clean
  • Its modular design makes it easy to add new
    sections
  • Replacement parts are inexpensive and always in
    supply
  • Disadvantages
  • It is not as cost effective as most shell and
    tube exchangers
  • it requires special gaskets

16
Shell and Tube Heat Exchangers
  • The shell and tube heat exchanger is the most
    common style found in industry
  • As the tubeside flow enters the exchanger, flow
    is directed into tubes that run parallel to each
    other. these tubes run through a shell that has a
    fluid passing through it
  • Heat energy is transferred through the tube wall
    into the cooler fluid
  • Heat transfer occurs primarily through conduction
    and convection

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19
Shell and Tube Heat Exchanger---Head
  • The head can be classified as front-end or
    rear-end types, the front-end head has five
    primary designs, the rear-end has eight possible
    designs

20
Shell and Tube Heat Exchanger---Shell
  • In most cases, the shell is designed to withstand
    the greatest temperature and pressure condition
  • The shell is the largest single part of the heat
    exchanger
  • The shell can be classified as single-pass,
    double-pass, split flow, double split flow,
    divided flow, cross-flow

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22
Shell and Tube Heat Exchanger---Tube
  • Tube can be plain or dinned
  • Plain tubes are commonly used in fabrication
  • Finned tube are starting to make an impact, fins
    can be located externally or internally
  • Tube materials include brass, carbon,

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24
Shell and Tube Heat Exchanger---Tube Sheet
  • Tube sheets are often described as fixed or
    floating, single or double
  • Tube sheets have carefully drilled hole, the ends
    of the tubes in a heat exchanger are fixed by
    rolling, welding, or both
  • Double tube sheets are used to prevent tubeside
    leakage of highly corrosive fluids

25
Shell and Tube Heat Exchanger---Tube Sheet
  • Engineering specifications take into account
    thermal tube expansion
  • If the tube sheet is welded or bolted to the
    shell, it is called fixed
  • If the tube sheet is independently secured to the
    tub head and is allowed to move freely inside the
    shell, it is called floating

26
Shell and Tube Heat Exchanger---Baffle
  • Baffles provide the framework to support and
    secure the tubes and prevent vibration
  • The baffle layout increases or decreases fluid
    and directs flow at specific points
  • Tubeside baffles are built into the heads to
    direct tubeside flow
  • In multipass exchangers, cost goes up with each
    pass, provide adequate fluid velocities to
    prevent fouling and to control heat transfer

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28
Shell and Tube Heat Exchanger---Baffle
  • Each segmental baffle supports half of the tubes
  • Baffles are evenly spaced
  • Segmental baffles may be horizontal or vertical
    cut
  • Systems transferring large quantities of
    suspended solids may use vertical arrangement,
    which allows liquid and solids to flow around
    baffles
  • Horizontal baffles are used in clean service with
    notches at the bottom to allow liquid drainage on
    removal from serve

29
Shell and Tube Heat Exchanger---Baffle
  • Impingement baffles are used to protect tubing
    from direct fluid impact
  • Longitudinal baffles are used inside the shell to
    split or divide the flow, increase velocity, and
    provide superior heat transfer capabilities,
    longitudinal baffles do not extend the entire
    length of the exchanger since at some point the
    fluid must flow around it

30
Shell and Tube Heat Exchanger---Tie Rod
  • Tie rods and concentric tube spacers keep the
    baffles in place and evenly spaced
  • Each hole in the baffle plates is 1/64 larger
    than the tubes outside diameter

31
Shell and Tube Heat Exchanger---Nozzles and
Accessory Part
  • Nozzles and accessory part
  • Inlet and outlet nozzles are sized for pressure
    drop and velocity considerations
  • Thermowells, pressure indicator connections,
    safety and relief valves, product drains, vents,
    block valves and control valves

32
Shell and Tube Heat Exchanger---Fixed Head
  • In a fixed head, single pass shell and tube heat
    exchangers, the tubes are connected to two tube
    sheets which are firmly attached to the shell
  • in the multipass Shell and tube heat exchangers,
    the baffle added to the channel head and the lack
    of a tube side outlet on the discharge head
  • Temperature differential is less to 200?

33
Shell and Tube Heat Exchanger---Floating Head
  • One side of the tube bundle is fixed to the
    channel head, the other side is unsecured
  • Floating head exchangers, with their high
    cross-sectional areas(fins), are designed for
    high temperature differentials and high flow
    rate, produces the highest heat transfer
    efficiency

34
Shell and Tube Heat Exchanger---U-Tube
  • The tube sheet connects a series of tubes
    bent in a U shape, the ends of the tubes are
    secured to the tube sheet
  • The total number of tubes is limits
  • Large temperature differentials
  • Each complete U tube has a single fundamental
    frequency

35
Plate and Frame Heat Exchanger
  • It consist of a series of gasketed plates,
    sandwiched together by two end plates and
    compression bolts

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37
Plate and Frame Heat Exchanger
  • Easy to disassemble and clean and distribute heat
    evenly so there are no hot spots
  • Plates can easily be added or removed
  • Low fluid resistance time, low fouling, high heat
    transfer

38
Plate and Frame Heat Exchanger
  • If gaskets leak, they leak to the outside, and
    gasket easy to replace
  • Prevent cross-contamination of products
  • High turbulence and large pressure drop and small
  • This device is best suited for vicious or
    corrosive fluid slurries
  • High-pressure and high-temperature limitations
    (2.5MPa and 180?) for protect internal gasket
  • Gaskets are easily damaged and may not be
    compatible with process fluids

39
Spiral Heat Exchanger
  • Spiral heat exchangers are characterized by a
    compact concentric design that generates high
    fluid turbulence
  • Type 1 spiral flow on both sides
  • Type 2 spiral flow-cross-flow

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41
Pipe Coil Exchanger
  • Pipe coils are submerged in water or sprayed with
    water to transfer heat
  • This type of operation has a low heat transfer
    coefficient and requires a lot of space

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43
Air-Cooled Heat Exchanger
  • Air-cooled heat exchangers provide plain or
    dinned tubes connected to an inlet and return
    header
  • Air is used as the outside medium to transfer
    heat away from the tubes
  • Fans are used in a variety of arrangements to
    apply forced convection for heat transfer
    coefficients
  • Provides a 40? temperature differential between
    the ambient air and the exiting process fluid

44
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45
Air-Cooled Heat Exchanger
  • simple to construct and cheaper to maintain
  • Cannot fouling or corrosion
  • Low operating costs and superior high temperature
    removal
  • Limited in use
  • High outlet fluid temperature and high initial
    cost of equipment
  • In cases of loss of containment, they would be
    fire or explosion

46
Heat Exchangers and Systems ---Parallel and
Series Flow
  • in series flow, the tubeside flow in a multipass
    heat exchanger is discharged into the tubeside
    flow of the second exchanger
  • In parallel flow, the process flow goes through
    multiple exchangers at the same time

47
Heat Exchangers and Systems ---Cooling Towers
  • The system consists of a cooling tower, heat
    exchanger, pump
  • Cooling water is pumped into the shellside of a
    heat exchanger and returned (much hotter) to the
    top of the cooling tower

48
Heat Exchangers and Systems---Reboilers,
Distillation Column
  • Rebolier---add heat to a liquid that was once
    boiling until the liquid boils again
  • Associated with the operation of a distillation
    column, which is energy of heat balance,
    reboilers are used to restore this balance by
    adding additional heat for the separation
    processes

49
Heat Exchangers and Systems--- Reboilers,
Distillation Column
  • Take kettle reboiler for example, reboilers take
    suction off of the bottom products and pump them
    through their system, so column temperatures are
    controlled at established set-points
  • Other type of reboilers
  • vertical and horizontal thermosyphon reboilers
  • stab-in reboilers
  • hot oil jacket reboilers

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51
summary
  • The methods of heat transfer are conduction,
    convection, and radiation
  • The fluid flow are laminar and turbulent
  • The best conditions for heat exchangers are large
    temperature difference,high heating or coolant
    flow rates, large cross-sectional area of the
    exchanger, and others, so which heat exchanger is
    choose, it depends

52
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