Shell-and-Tube Heat Exchanger Laboratory

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Shell-and-Tube Heat Exchanger Laboratory

• Results and Discussion section
• ST_HE_INTRO.ppt

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Contents

• Background what is a shell-and-tube heat
  exchanger?
• Basic aim.
• Basic theory overall heattransfer coefficients
  from experiments correlations.
• Assistance with key concepts (analysis).
• Approach to writing task information given to
  you our suggestions.

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Background

• This is a heat-transfer experiment on a piece of
  equipment called a shell-and-tube heat exchanger.

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What is a Shell-and-Tube Heat Exchanger?

• This is a development of the double-pipe heat
  exchanger (2nd year lab) that gives lower heat
  losses more heat-transfer area in the same
  space.

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Double-Pipe Exchangers
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Development of Shell Tube Heat Exchangers

• Start with double-pipe exchanger (2nd year)
• Know that countercurrent arrangement gives best
  duty trombone or hairpin arrangement
  possible for large sizes.
• BUT
• costly
• poor volume utilisation (large)
• large external surface (heat losses).

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Development of Shell Tube Heat ExchangersShell
Around Tubes
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Development of Shell Tube Heat Exchangers

• Put shell around tubes (shell tube)
• Simple, more or less countercurrent
• BUT
• poor flow distribution (short circuiting)
• poor heat transfer (along tube) on shell side
• hard to get high enough shell-side flows

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Baffles in Shells to give high enough shell-side
velocities
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Development of Shell Tube Heat Exchangers

• Shells with Baffles
• Now, combination of
• crossflow, and
• co- or counterflow
• Not so easy to analyse.
• BUT
• have turbulent heat transfer to tubes even at low
  Reynolds number

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So What is a Shell-and-Tube Heat Exchanger?

• Like the last slide, it is a shell around tubes
  with baffles.

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Basic Aim of Experiment

• First, must establish reliability of the results
  by doing energy balances.
• Then, compare
• heat-transfer coefficients obtained from
  experiments with
• heat-transfer coefficients predicted from
  correlations.

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Overall Heat-Transfer Coefficients from
Experiments

• Measure temperatures, flowrates
• Can work out Q (W) - heat duty
• Can work out mean temperature difference - this
  is not simply the log mean of the terminal
  temperatures because the exchanger has some
  cocurrent flow some countercurrent flow.
• Know the area.

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Heat-Transfer Coefficients from Experiments
Design Equation

• Do not worry about how the mean-temperature
  difference was calculated (will be explained in
  the heat-transfer course) the design equation
  has already been used correctly to calculate
  these mean-temperature differences the overall
  heat-transfer coefficients (U) for all the
  experiments.

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Difference Compared with Second-Year Laboratory

• In second-year laboratory, we had pure
  counter-flow, so the mean temperature driving
  force was the logarithmic mean.
• In this experiment, closer to reality, we have a
  mixture of co- counter-flow, so the driving
  force is a modified version of the logarithmic
  mean.
• This modification has already been accounted for
  in the calculations.

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Overall Heat-Transfer Coefficients from
Experiments (U)

• These are essentially key outcomes from the
  experiments.
• They are uncertain because the measurements
  (temperatures flowrates) used to calculate them
  are uncertain.
• We will see later how uncertainties errors in
  measurements (like temperatures flowrates)
  propagate through calculations to give
  uncertainties errors in final results.

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Overall Heat-Transfer Coefficients from
Correlations

• We can calculate film heat-transfer coefficients
  inside and outside the tubes (?o outside, ?i
  inside)
• from Nusselt, Prandtl Reynolds numbers, which
  are functions of
• fluid flowrates (inside outside tubes), tube
  diameters, fluid properties, geometry, etc
• the appropriate correlations have been used for
  the data given to you.

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Estimating Overall Heat-Transfer Coefficients
from Correlations

• Heat transfer occurs through the tubes, so the
  overall heat-transfer coefficient is a
  combination of these film coefficients, fouling
  resistances (Rfi inside Rfo outside) thermal
  resistance of tube walls (L tube wall
  thickness, ? thermal conductivity of wall).

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Our Assistance One Key Concept for Analysis

• What is the controlling heat-transfer coefficient
  or resistance (1/coefficient)? Inside or
  outside?
• Why is this a key concept?
• If a coefficient is controlling (e.g. inside),
  then errors or uncertainties in it will have a
  larger effect on the overall heat-transfer
  coefficient than other coefficients (e.g.
  outside).

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Another Key Concept

• Film coefficients vary with many aspects (like
  fluid flowrate, properties geometry).
• Here, only water is involved, so fluid properties
  are virtually constant (only slightly temperature
  dependent).
• Here, geometry is constant.

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• Only fluid flowrate varies here we normally
  express this as a dimensionless variable called
  the Reynolds number.
• Here there are two Reynolds numbers, one inside,
  the other outside, the tubes.
• If the film coefficients vary with the
  corresponding Reynolds numbers, then the overall
  heat-transfer coefficient should as well.
• Take care in plotting results for example,
  plotting the overall heat-transfer coefficient
  against the tubeside (inside) Reynolds number
  only makes sense if the shellside (outside)
  Reynolds number is virtually constant.

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How to Approach This Type of Task

• How will you give the reader confidence in your
  results? Energy balance.
• When theory (here U from correlations) disagrees
  with experiment, how can you explain it? Compare
  the two Us first.

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Information Given to You Suggestions

• Heat-transfer rates
• heat lost from hot fluid, heat gained by cold
  fluid
• suggested use energy balance, gives reader
  confidence in results
• what you have to do work out how to present the
  heat balance in the most effective way

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• Overall heat-transfer coefficients (from
  correlations experiments) corresponding
  inside outside Reynolds numbers
• suggested use compare correlations
  experiments, assess controlling coefficients,
  hence explain discrepancies between correlations
  experiments
• what you have to do work out how to compare,
  analyse in the most effective way

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So What? A Few Reminders About Writing Results
Discussion

• Your writing task is to create a results and
  discussion section from raw data given to you
  this explanation.

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Results Discussion A Brief Reminder

• Results explain what you are presenting why
  this order
• Discussion should discuss the significance of
  the results in the same order as the results are
  presented.

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What do the Columns in the Spreadsheet Mean?

• 1st column run number. H percentage of
  maximum hot-side flow rate C percentage of
  maximum cold-side flow rate these have no
  special significance except to show that the
  experimental conditions span a wide range of
  conditions.

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• 2nd 3rd columns hot (shell-side) cold
  (tube-side) stream mass flow rates. Again, these
  do not actually have great significance 6th
  7th columns hot (shell-side) cold (tube-side)
  stream Reynolds numbers are much more
  significant, since they are dimensionless
  flowrates, effectively normalised for fluid
  properties, geometry, etc.
• 4th 5th columns energy (heat) lost from hot
  stream energy (heat) gained by cold stream
  essential for energy balance.

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• 8th column U extracted from experimental
  heat-transfer data
• 9th - 12th columns U from adding up predicted
  (correlated) heat-transfer coefficients
• correlation is not theory exactly, but here it is
  the closest thing to theory
• Kern Bell are two methods for correlating
  shell-side heat-transfer coefficients the shell
  side is the most difficult to handle
  theoretically, because of the complex flow
  patterns here (between baffles, etc) tube side
  flow pattern is just flow through tubes

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• Minimum maximum fouling means using fouling
  resistances (Rfi inside Rfo outside) from
  textbook (e.g. Hewitt).
• Fouling resistances actually span a range Hewitt
  often quotes a range (e.g. 0.000175 m2KW-1 to
  0.00035 m2KW-1 for treated cooling water, as
  here).
• Minimum fouling using minimum fouling
  resistances on both tube shell side.
• Maximum fouling using maximum fouling
  resistances on both tube shell side.

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Conclusions

• What is a shell-and-tube heat exchanger?
• Basic aim theory overall heattransfer
  coefficients from experiments correlations.
• Key concepts (analysis).
• Approach to writing task information given to
  you our suggestions.