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HEAT EXCHANGE EQUIPMENT

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Heat transfer coefficients in shell and tube heat exchangers: In a shell-and-tube exchanger, ... – PowerPoint PPT presentation

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Title: HEAT EXCHANGE EQUIPMENT


1
HEAT EXCHANGE EQUIPMENT
2
  • A 'heat exchanger' may be defined as an equipment
    which transfers the energy from a hot fluid to a
    cold fluid. Here, the process of heating or
    cooling occurs. In heat exchangers the
    temperature of each fluid changes as it passes
    through the exchangers.

3
  • General design of heat exchange equipment
  • The design of heat exchange equipment is based on
    general principles.
  • From mass and energy balance HT area is
    calculated.
  • Quantities to be evaluated are U,LMTD.
  • In simple devices these quantities can be
    calculated accurately but in complex processing
    units the evaluation may be difficult and the
    final design is always a compromise based on
    engineering judgment to give best overall
    performance.

4
  • TYPES OF HEAT EXCHANGERS
  • 1.Doble pipe heat exchangers

5
  • It consists of concentric pipes with standard
    return bends.
  • One fluid flows through inside metal pipe and
    the second fluid flows through the annulus
    between the outside pipe and inside pipe.
  • The flow directions may be either parallel or
    counter fashions.
  • These exchangers are used when heat transfer area
    required is not more than 150 sq.ft

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  • Advantages
  • 1. Simple in construction
  • 2. Cheap
  • 3. Very easy to clean
  • 4. Very attractive when required Heat transfer
    areas are small.
  •  
  • Disadvantages
  • 1. The simple double pipe heat exchanger is
    inadequate for large flow rates
  • 2. If several double pipes are used in parallel,
    the weight of metal required for the outer tubes
    becomes so large.
  • 3. Smaller heat transfer area in large floor
    space as compared to other types
  • 4. Leakage are more.
  •  

8
  • Shell and tube heat exchanger
  • The simple double pipe heat exchanger is
    inadequate for large flow rates. If several
    double pipes are used in parallel, the weight of
    metal required for the outer tubes becomes so
    large.
  • When large areas are required we go for shell and
    tube heat exchangers.
  • It is the most common type of heat exchanger in
    oil refineries and other large chemical processes.

9
  • Shell and tube heat exchanger consists of a shell
    with a bundle of tubes inside it.
  • One fluid flows through the tubes (the tube
    side) and the other fluid flows outside the tubes
    but inside the shell (the shell side).
  • Heat is transferred from one fluid to the other
    through the tube walls, either from tube side to
    shell side or vice versa.
  • The fluids can be either liquids or gases on
    either the shell or the tube side.

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12
  • Construction and parts
  • Shell
  • It is usually a cylindrical casing through
    which one of the fluid flows. Shell is commonly
    made of carbon steel. The minimum thickness of
    shell made of carbon steel varies from 5 mm to 11
    mm depending upon the diameter.
  •  

13
  • Tubes
  • Standard heat exchanger tubes which are used in
    many industrial processes may be of various sizes
    and lengths.
  • The wall thickness of tubes is usually expressed
    in terms of Birmingham Wire Gauge (BWG).
  • The thickness depends upon material of
    construction and diameter.
  • Standard lengths of tubes for heat exchanger
    construction are 8, 12, 16 and 20 ft.

14
  • Tube pitch
  • The shortest centre-to-centre distance between
    the adjacent tubes is called as tube pitch.
  • Tubes arranged in a triangular or square layout,
    known as triangular or square pitch.
  • Square pitch gives lower shell side pressure drop
    than triangular pitch.
  • Square pitch is good for easy cleaning whereas
    triangular pitch gives more number tubes for same
    space available
  • Unless shell side fluid fouls badly, triangular
    pitch is used.

15
TEMA standards specify a minimum center to center
distance 1.25 times outside diameter of the tubes
for triangular pitch and a minimum cleaning lane
of ¼ inch for square pitch.
16
  • Tube sheet
  • It is essentially a flat circular plate. A large
    number of holes are drilled in the tube sheet
    according to the pitch requirements.

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18
  • Baffles
  • The baffles are installed in the shell
  • To increase the rate of heat transfer by
    increasing the velocity and turbulence of the
    shell side fluid
  • It helps as structural supports for tubes and
    dampers against vibration.
  • The baffles cause the fluid to flow through shell
    at right angles to the axes of the tubes (Cross
    flow).
  • OR
  • They promote cross flow

19
  • To avoid the bypassing of the shell side fluid
    the clearance between the baffles and shell, and
    baffles and tubes must be minimum.
  • The centre-to-centre distance between adjacent
    baffles is known as baffle spacing or baffle
    pitch.
  • The baffle space should not be greater than the
    inside diameter of the shell and should not less
    than the one-fifth if the inside diameter of the
    shell.
  • The optimum baffle spacing is 0.3 to 0.50 times
    the shell diameter

20
25 cutoff baffles
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22
  • Single pass 1-1 exchanger

23
  • Limitations
  • It occupy more space
  • Cannot obtain high velocities hence low heat
    transfer coefficients.
  • No solution for expansion problems.

24
  • MULTI PASS HEAT EXCHANGERS
  • Multi pass construction decreases the cross
    section of the fluid path and increases the fluid
    velocity and corresponding HT Coefficient
  • Advantages
  • High velocities
  • Short tubes
  • Solution to expansion problems

25
  • Disadvantages
  • Exchanger is more complicated
  • Friction loss are increased because of high
    velocities, longer path ,multiplication of
    entrance and exit losses

26
  • 1-2 heat exchangers

27
  • 2-4 Heat exchangers
  • 1-2 heat exchanger has an important limitation.
  • Because of parallel flow pass ,the exchanger
    is unable to bring one of the fluid very near to
    the entrance temperature of the fluid.
  • OR
  • The heat recovery is poor.
  • So we go for 2-4 heat exchanger
  • It gives high velocity and large HTC than 1-2
    Exchanger with same flow rates.

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30
  • Heat transfer coefficients in shell and tube
    heat exchangers
  • In a shell-and-tube exchanger, the shell-side
    and tube side heat transfer coefficients are of
    comparable importance and both must be large if a
    satisfactory
  • overall coefficient is to be attained.

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33
Gb
Gc
Gb
34
Exchanger Fouling
Electron microscope image showing fibers, dust,
and other deposited material on a residential air
conditioner coil and a fouled water line in a
water heater.
35
Exchanger Fouling
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