Basics of Heat Exchangers

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Basics of Heat Exchangers

• P M V Subbarao
• Professor
• Mechanical Engineering Department
• I I T Delhi

The story of a First Thermodynamic Device?!?!?!
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HEAT EXCHANGER
Human Need of Power is Responsible for Its
Innovation! Heat Exchanger Made Power Generation
Viable!!! A first Step towards Scientific
living Style A True Mediator !!!
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Invention of FIRE, FLAME and TORCH

• Fire is a discovery rather than an invention.
• Homo erectus probably discovered fire by
  accident.
• Fire was most likely given to man as a 'gift from
  the heavens' when a bolt of lightning struck a
  tree or a bush, suddenly starting it on fire.
• The flaming touch and the campfire probably
  constituted early man's first use of 'artificial'
  lighting.
• As early as 400,000 BC, fire was kindled in the
  caves of Peking man.
• Prehistoric man, used primitive lamps to
  illuminate his cave.
• Various Oils were used as fuels.

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Sharing of Skills
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The First Civilized Food Processing !!!!!
Fire Can only Heat Solids !!!!!
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A Search for Ubuntu Device .

• A generous, hospitable, friendly, caring and
  compassionate.
• They share what they have/get/earn..
• A person with ubuntu is open and available to
  others.
• Various religions identified them as Mediators.
• Business Mediators.
• Energy Mediators A first step in Civilization
  and Development

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EARLIEST TYPES OF HX COOKING

• Primitive humans may first have savoured roast
  meat by chance, when the flesh of a beast killed
  in a forest fire was found to be more palatable
  and easier to chew and digest than the customary
  raw meat.
• They probably did not deliberately cook food,
  though, until long after they had learned to use
  fire for light and warmth.
• It has been speculated that Peking man roasted
  meats, but no clear evidence supports the theory.
  
• During Palaeolithic Period, Aurignacian people of
  southern France apparantly began to steam their
  food over hot embers by wrapping it in wet
  leaves.
• Crude procedures
• as toasting wild grains on flat rocks and using
  shells, skulls,
• or hollowed stones to heat liquids.
• Introduction of pottery during the Neolithic
  Period.
• A paste, toasted to crustiness when dropped on a
  hot stone, made the first bread.

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• Heat Exchangers Enhance the Utility of Fire .
• Can they do so beyond stomach ????

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The Aelopile

• In 130BC. Hero, a Greek mathematician and
  scientist is credited with inventing the first
  practical application of steam power, the
  aelopile.

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Branca's Steam Turbine

• In 1629, Giovanni Branca, of the Italian town of
  Loretto, described, in a work' published at Rome,
  a number of ingenious contrivances.

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The Savery Engine
Thomas Savery, July 2, 1698, patented the design
of the first engine which had the most important
advance in actual construction. A working model
was submitted to the Royal Society of London.
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Newcomen Engine
The original Thomas Newcomen engine was invented
in 1712.
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James Watts Engine
James Watt radically improved Newcomen's engine
(1769) by condensing the steam outside the
cylinder .
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No Recognition to The Heat Exchanger !!!?!?!?
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Onset of Heat Exchangers
The Plain Cylinder Boiler
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The Scientific Development of HXs
The Cornish Boiler
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The Scotch Boiler
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The Scientific Engineering !!!!!!
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Progress in Rankine Cycle
Year 1907 1919 1938 1950 1958 1959 1966 1973 1975
MW 5 20 30 60 120 200 500 660 1300
p,MPa 1.3 1.4 4.1 6.2 10.3 16.2 15.9 15.9 24.1
Th oC 260 316 454 482 538 566 566 565 538
Tr oC -- -- -- -- 538 538 566 565 538
FHW -- 2 3 4 6 6 7 8 8
Pc,kPa 13.5 5.1 4.5 3.4 3.7 3.7 4.4 5.4 5.1
h, -- 17 27.6 30.5 35.6 37.5 39.8 39.5 40
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A Train of External HXs in A Power Plant
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Consequence of An Internally Efficient Power Plant
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Impact of Cycle Improvement on Capability of Fire
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Stockholm 1920The Ljungström Air Preheater
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Economic Impact of the Landmark

• The use of a Ljungström Air Preheater in a modern
  power plant saves a considerable quantity of
  fuel.
• So much that the cost of the preheater is
  generally recovered after only a few months.
• It has been estimated that the total world-wide
  fuel savings resulting from all Ljungström Air
  Preheaters which have been in service is
  equivalent to 4,500,000,000 tons of oil.
• An estimate shows that the Ljungström Air
  Preheaters in operation annually saves about 30
  Billion US.
• The distribution of thermal power capacity in
  which Ljungström Air Preheaters are installed
  over the world is shown in the table below.

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Heat Exchanger An Effective Landlord

• Creates a housing for both donor and Receiver.
• How to accommodate both in a single housing?
• Space Sharing Time sharing
• Space sharing Donor and Receiver are present
  always.
• Develop partition(s) in the house(HX).
• Time Sharing Donor And Mediator for sometime
  and Mediator and Receiver for sometime Repeat!
• Central Limit Theorem It is impossible to have
  time and space sharing in one system.
• Time Sharing Regenerators
• Space Sharing Recuperators
• Shell Tube HXs

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Design Considerations for Heat Exchangers

• When preparing to design a heat exchanger, do you
  ever wonder where to start? 
• You've done it before, but you hate that feeling
  of getting half way through the design and
  realizing that you forgot to consider one
  important element. 
• The thought process involved is just as important
  as the calculations involved. 
• Let's try to map out a heat exchanger design
  strategy. 
• We'll do so with a series of questions followed
  by information to help you answer the questions

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Is there a phase change involved in my system?

• A quick look at the boiling points compared with
  the entrance and exit temperatures will help you
  answer this question.

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How many "zones" are involved in my system?
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Various Simple Zones
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What are the physical properties of the streams
involved?

• Get the physical properties for each zone
  separately to ensure accuracy, but in some cases
  it is acceptable to use an average value.   
• Physical properties that you will want to
  collect for each phase of each stream will
  include  heat capacity, viscosity, thermal
  conductivity, density, and latent heat (for phase
  changes). 
• These are in addition to the boiling points of
  the streams at their respective pressures.

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What are the allowable pressure drops and
velocities in the exchanger?

• Pressure drops are very important in exchanger
  design (especially for gases). 
• The pressure drop and velocities must be
  limited. 
• The velocity is directly proportional to the heat
  transfer coefficient which is motivation to keep
  it high, while erosion and material limits are
  motivation to keep the velocity low. 
• Typical liquid velocities are 1-3 m/s. 
• Typical gas velocities are 15-30 m/s. 

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What is the estimated area of the exchanger?

• Unfortunately, this is where the real fun begins
  in heat exchanger design! 
• You'll need to find estimates for the heat
  transfer coefficients that you'll be dealing
  with. 
• Once you've estimated the overall heat transfer
  coefficient, use the equation Q UoADTlm to get
  your preliminary area estimate. 
• Remember to use the above equation to get an area
  for each zone, then add them together.

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What geometric configuration is right for my
exchanger?

• Now that you have an area estimate, it's time to
  find a geometry that meets your needs. 
• Once you've selected a shell diameter, tubesheet
  layout, baffle and tube spacing, etc., it's time
  to check your velocity and pressure drop
  requirements to see if they're being met. 
• Experienced designers will usually combine these
  steps and actually obtain a tube size that meets
  the velocity and pressure drop requirements and
  then proceed. 
• If your pressure drop requirements are low, avoid
  using four or more tube passes as this will
  drastically increase your pressure drop. 
• Now you have a geometry selected that meets all
  of your needs.
• Now that I have a geometry in mind, what is the
  actual overall heat transfer coefficient?
• What is the actual area of the exchanger using
  the 'actual' heat transfer coefficient?