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Heat exchangers

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Heat exchangers Heat exchangers Device that facilitate the exchange of heat between fluids that are at different temperatures while keeping them from mixing with each ... – PowerPoint PPT presentation

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Title: Heat exchangers


1
Heat exchangers
2
Heat exchangers
  • Device that facilitate the exchange of heat
    between fluids that are at different temperatures
    while keeping them from mixing with each other.
  • Heat exchanger involves convection in each fluid
    and conduction through wall that separating the
    two fluids
  • It is convenient to use an overall heat transfer
    coefficient (U)

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HEAT EXCHANGERS
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Heat Exchangers
  • Parallel flow
  • Counterflow
  • Crossflow

Ref Incropera Dewitt (2002)
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Cross Flow Heat Exchanger
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HEAT EXCHANGERS, Shell and Tube
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HEAT EXCHANGERS, U-Tube
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HEAT EXCHANGERS, Plate Design
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HEAT EXCHANGERS, Condenser
Watt Equipment, Inc., Equip. No. E-203-TR, 2690
Sq. Ft. Surface, 400 PSI Shell, 400 PSI Tubes,
3/4" Stainless Steel Tubes, Stainless Steel Shell.
17
HEAT EXCHANGERS
APV condenser/shell and tube heat exchanger. The
shell side is 316L and the tube side is Titanium.
It is 18 foot 10 in. and has a tube surface of
1,752 square feet. There are (2)16 in, shell side
inlets. There are (558) 3/4 in. od tubes, and the
shell is 31 in. inside diameter.
Heat exchanger stainless steel, serial no. 3410,
170 tubes 1 inch diameter x 10 foot long, overall
length of 124 inches.
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Temperature profile in double-pipe heat exchanger
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Heat Exchanger Analysis
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Heat Exchanger Analysis
Counterflow
Parallel
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Counterflow Heat Exchangers
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Counterflow Heat Exchanger
  • What about crossflow heat exchangers?
  • ?tm F ?tm,cf

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Overall heat transfer coefficient in heat
exchanger
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Example
  • Hot oil is to be cooled in a double-tube
    counter-flow heat exchanger. The copper inner
    tubes have a diameter of 2 cm and negligible
    thickness. The inner diameter of the outer tube
    (the shell) is 3 cm. Water flows through the tube
    at a rate of 0.5 kg/s, and the oil through the
    shell at a rate of 0.8 kg/s. Taking the average
    temperatures of the water and the oil to be 45C
    and 80C, respectively, determine the overall heat
    transfer coefficient of this heat exchanger.

36
Estimation of h
  • For turbulent flow
  • Nu 0.023Re0.8Pr0.4
  • For laminar flow

Di/Do Nui Nuo
0.00 0.05 1.00 0.25 0.50 1.00 - 17.46 11.56 7.37 5.74 4.86 3.66 4.06 4.11 4.23 4.43 4.86
37
Analysis of heat exchangers
  • Select a heat exchanger when specified
    temperature change in a fluid stream of known
    mass flow rate ?log mean temperature difference
    method (LMTD)
  • Predict the outlet temperature of hot and cold
    fluid streams in a specified heat exchanger
    ?effectiveness-NTU method

38
Log mean temperature difference
  • c cold fluid, h hot fluid

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Example-the condensation of steam in a condenser
  • Steam in condenser of a steam power plant is to
    be condensed at a temperature of 30?C with
    cooling water from a nearby lake, which enters
    the tube of the condenser at 14?C and leaves at
    22?C. The surface area of the tubes is 45 m2 and
    the overall heat transfer coefficient is 2100
    W/m2.?C. Determine mass flow rate of cooling
    water needed and the rate of the steam in
    condenser.

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Example-heating water in a counter-flow heat
exchanger
  • A counter-flow double-pipe heat exchanger is to
    heat water from 20?C to 80?C at a rate of 1.2
    kg/s. The heating is to be accomplished by
    geothermal available at 160?C at a mass flow rate
    of 2 kg/s. The inner tube is thin-walled and has
    a diameter of 1.5 cm. If the overall heat
    transfer coefficient of the heat exchanger is
    640 W/m2.?C, determine the length of the heat
    exchanger required to achieve the desired heating.

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The effectiveness-NTU method
  • Dimensionless parameter
  • Actual heat transfer rate
  • Maximum possible heat transfer rate

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Example-heating water in a counter-flow heat
exchanger
  • A counter-flow double-pipe heat exchanger is to
    heat water from 20?C to 80?C at a rate of 1.2
    kg/s. The heating is to be accomplished by
    geothermal available at 160?C at a mass flow rate
    of 2 kg/s. The inner tube is thin-walled and has
    a diameter of 1.5 cm. If the overall heat
    transfer coefficient of the heat exchanger is
    640 W/m2.?C, determine the length of the heat
    exchanger required to achieve the desired heating.

51
Example-cooling hot oil by water in a multipass
heat exchanger
  • Hot oil is to be cooled by water in a
    1-shell-pass and 8-tubes passes heat exchanger.
    The tube are thin walled and are made of copper
    with an internal diameter of 1.4 cm. The length
    of each tube pass in the heat exchanger is 5 m,
    and the overall heat transfer coefficient is 310
    W/m2?C. Water flows through the tube at a rate of
    0.2 kg/s, and the oil through the shell
    at a rate of 0.3 kg/s. The water and the
    oil enter at temperature of 20?C and 150?C,
    respectively. Determine the rate of heat transfer
    in the heat exchanger and the outlet temperature
    of the water and the oil.

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