6. HEAT TRANSFER

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6. HEAT TRANSFER
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Heat Exchange

• There are two methods of heat exchange
• Indirect transfer
• The transfer of heat through a tube wall is
  called indirect transfer.
• Examples heat exchangers, furnaces, boilers
• Direct transfer
• Another method by which heat exchange can be
  accomplished is without the benefit of transfer
  surface. The type of heating/cooling is called
  direct transfer and brought about by
  intermingling the hot and cold fluids.
• Examples fractionators, cooling towers,
  strippers

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Heat Exchange

• Flow of heat
• Steady state
• Rate of heat transfer remain constant and is
  unaffected by time
• Unsteady state
• Rate of heat transfer at any point varies with
  time
• Examples batch process, cooling and heating of
  materials, certain types of regeneration, curing
  or activation process.

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Heat Transfer

• Methods of Heat Transmission
• Convection transports heat by carrying hot
  portions of a fluid from one place to another
• Example home water heater
• Q U A (t2 t1)

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Convection

• All system of heating and ventilating depend upon
  what are called convection currents, which, in
  turn, depend upon the expansion of liquids and
  gases.
• Any gas or liquid expands when heated, volume
  increases, density decreases.
• In a convection current, the lighter fluid is
  pushed upward by the heavier surrounding fluid.
• Since the rising part of a convection current is
  warmer than the returning part there is a
  transfer of heat from the heat source to the
  cooler parts of the fluid at the top.

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Heat Transfer

• Methods of Heat Transmission
• Conduction the passage of heat from one
• part of a substance to another part of the
• same substance or from one substance to
  another in physical contact with it, without
• the movement of particles of either
  substance.
• Example metal spoon in hot coffee
• Q K/L A (t2-t1)

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Conduction

• Heat that passes from one part of a substance
  to another part of the same substance or from one
  substance to another in physical contact with it,
  without the movement of particles of either
  substance, is said to flow by conduction.
• Day to day example of heat transfer by
  conduction
• - Metal handle of a sauce pan becomes
  extremely hot even though only the bottom of the
  pan is exposed to a flame
• - We often found the handle of spoon in hot
  coffee or soup too hot to be lifted from the cup
  barehanded.
• These experiences are example of heat being
  transmitted by conduction.

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Conduction

• There are some substances, such as wood, wool,
  cork which are poor conductor of heat. They are
  called heat insulators
• All metals such as silver, copper, brass, iron
  are good conductor as compared to non-metals.
• Metals vary in their respective abilities to
  conduct heat. This ability or inability is
  referred to as conductivity. This physical
  property to conduct heat is called coefficient of
  thermal conductivity of the materials.

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Heat Transfer

• Method of Heat Transmission
• Radiation the transfer of heat through space by
  heat waves that
• travel in straight lines.
• Example heat from an electrical light
  bulb
• Q o eA (T4r1 T4 r2)

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Radiation

• If an iron ball is heated and hung up in
  the room the heat can be felt
• when the hand is held under the ball.
• this cannot be due to convection because hot air
  currents rise
• It cannot be due to conduction because gases are
  poor conductors
• A lighted electric bulb feels hot if the
  hand is held near it, but when the light is
  turned off, the sensation stops very quickly
• - the glass in the bulb is a poor
  conductor and there is very little air in
• the bulb
• - Therefore, the sensation, of heat cannot
  be due neither to convection
• or conduction
• - if a book or screen is placed between
  heat source and the hand the
• sensation immediately ceases.
• These effects are caused by the travel of
  heat waves in straight lines through the
  atmosphere similar to light. Radiation is the
  transfer of heat through space by heat waves that
  travel in straight lines.

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Factors Affecting Heat Transfer

• Factors that effect the transfer of heat energy
  from one substance to another
• 1. Temperature
• Transfer of heat is proportional to its driving
  force temperature
• difference.
• Heat is always transmitted from a warmer body to
  a colder one
• The greater the difference in temperature of the
  2 bodies, the greater the amount of heat
  exchanged
• 2. Thermal Resistance
• Transfer of heat is opposed by a factor called
  thermal resistance
• Opposition is caused by the material of the
  container (heat exchanger) stagnant films, scale
  or dirt on the tube wall.

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Thermal Resistance

• Stagnant Film
• In indirect heat transfer, flow of heat is
  initially being accompished by convection.
  However, as the tube wall is approached the
  effect of convection is steadily reduced until
  heat is transmitted through the stagnant fluid
  film by conduction almost exclusively. Since
  liquids or gases are relatively poor conductor,
  the flow of heat is impeded.
• Heat transfer by conduction is inversely
  proportional to the thickness through which it
  flows. Decreasing the stagnant film thickness is
  the only means by which greater heat flow can be
  effected.
• This can be done by increasing the speed or
  velocity at which the fluid is passing over the
  tube wall.

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Thermal Resistance

• 2. Scale
• After passing through the stagnant fluid film,
  heat encounters its second resistance scale.
• Here mechanism is heat flow by conduction.
• Since scale is relatively porous it is a good
  insulator, not a conductor.
• There are no means of reducing the scale
  thickness on the run. Steps must be taken to
  reduce the tendency of scale formation and
  periodically clean the tube surface.

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Thermal Resistance

• 3. Tube wall
• Thermal resistance offered by the tube wall is
  very often very negligible compared with
  resistance offered by the stagnant films and
  scales
• Heat flow through the tube wall is dependent upon
  conduction, this means heat transfer is improved
  by reduced thickness of impeding material. It is
  for this reason that thin-walled tubes are used
  in heat exchangers.
• Selection of better conductor for tube material
  can help reduce this resistance.

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Heat Transfer Coefficient

• In the study of heat exchange and the design of
  heat transfer equipment there must be a means of
  evaluating the performance of a given exchanger.
• In heat transfer work, this evaluating factor is
  called the overall heat transfer coefficient
• It indicates the amount of heat an exchanger can
  interchange during a given period of time for a
  given amount of surface and with a given
  temperature difference between the hot and cold
  fluids.
• In engineering practice this value is referred us
  U
• Unit of measure is BTU/Hr/Ft2/oF.

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Overall Heat transfer Coefficient
Clean coefficient U 90
(100 efficiency)
U
U 63 after 3 months ops
70 efficiency
Months in operations
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Fouling Factor

• After heat transfer equipment has been in service
  for some time, dirt or scale may form on the heat
  transfer surfaces, causing additional resistance
  to the flow of heat
• To compensate for this, a resistance called dirt,
  scale or fouling factor is included in
  determining an overall coefficient of heat
  transfer

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Heat Transfer Equipment

• The 3 general categories of heat transfer
• equipment are
• Exchangers transfer heat from a hot process
  stream to a cold process stream.
• Cooler transfer heat from a hot medium to
  cooling wate
• Condenser condense vapor

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Heat Transfer Equipment

• Process use
• Exchangers conserve heat that would otherwise
  be wasted i.e. taking heat from one liquid that
  needs cooling and giving it to another which
  needs heating
• Coolers to cool materials leaving a unit to a
  temperature which is safe for storage or loading.
• Condensers remove heat from vapors, condense
  these vapor and heat the cooling water/

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Heat Transfer Equipment

• As a background in the use of heat transfer
  equipment, an
• understanding of the various types of equipment
  on the market is
• helpful.
• Double pipe heat exchanger
• In this type, one fluid flows in the
  small, inner pipe and the other fluid in the
  space between the small, inner pipe and the
  larger, outer pipe.
• Finned tubes are often used to improve
  the heat transfer efficiency of a unit
• Air cooled Heat Exchanger
• This is an air cooled heat exchanger in
  which the hot fluid is cooled by air blown past
  the tubes by the fan below

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Heat Transfer Equipment

• Shell and Tube heat Exchanger
• The majority of heat transfer equipment
  is the shell-and-tube type of heat exchanger.
• A shell and tube exchanger consists of a
  number of parallel tubes enclosed in a
  cylindrical shell.
• One fluid flows inside the tubes and is
  called the tube side fluid
• The other fluid flows outside the tubes
  and is called the shell side fluid.
• All shell and tube heat transfer
  equipment is composed of the same basic parts,
  but some of these parts are arranged in such a
  way as to produce the desired results.

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Shell and Tube Exchange
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Shell and Tube Exchanger
Parts Identification
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2, 2 heat exchangers
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Reboiler
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Baffles

• Baffles are used to increase the rate of heat
• transfer by increasing the velocity of the shell
  side
• fluid.
• Two (2) general types of baffles
• 1. Transverse
• 2. Longitudinal
• In order to be effective, baffles must be
  installed so that there is a minimum of
  by-passing around baffles. This is done by
  reducing to a minimum the clearance between the
  baffles and the shell side fluid.

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Baffle Arrangements in Exchangers
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Cooler
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Cause of Fouling

• Fouling of a heat exchanger may result from
  scale, or particles lodging in the exchanger.
• Water residue may foul the tubes of coolers and
  condensers.
• Usually, oil side fouling material cannot be
  readily removed short of mechanical means. When
  this is necessary, the tube bundle must be
  removed.
• However, water residue can be removed
  without pulling the tube bundle from the shell.
  Water residue may be dirt, mud, silt, and/or
  scale.

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Cause of Fouling

• IF THE FOULING MATERIAL IS DIRT
• The use of backwash connections can substantially
  improve operating efficiency
• By reversing the direction of flow momentarily,
  the force of the water will knock the adhering
  trash from the tube ends and remove some mud and
  silt from the tube walls.
• Such an arrangement makes it possible to regain a
  degree of design performance without a shutdown

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Causes of Fouling

• Water scale formation can be removed without
  dismantling the exchanger by utilizing chemical
  cleaning. Water residue is removed mechanically
  and is accomplished by water jets or by drilling
  through which water passes as a scavenging agent.
• Mechanical cleaning of oil side scale can be done
  in a number of ways. Drilling, steaming, and
  sandblasting are used to clean the tube side. The
  shell side scales are usually hand sawed and
  washed with high pressure water. It is also
  possible to chemically remove oil scales.

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Tips to Reduce Maintenance

• OPERATION TECHNICIANS ARE IN A POSITION TO
  MAKE A SUBSTANTIAL CONTRIBUTION TO REDUCE
  MAINTENANCE AND IMPROVED OPERATING EFFICIENCY. By
  RECOGNIZING PROPER OPERATING PROCEDURES AND
  REALIZING THE EFFECT OF INCORRECT PROCEDURES ON
  EQUIPMENT LIFE, HE/SHE CAN PREVENT MATERIAL
  FAILURE AND EQUIPMENT INOPERABILITY
• Some example of recommended procedures by
  which we can help reduce maintenance and extend
  the life and efficiency of exchangers
• Maintain proper cooling water outlet temperature
  (not in excess of 125oF)

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Tips to Reduce Maintenance

• Failure to observe this recommendation may
  result in
• 1. excessive scaling of tube wall.
• 2. accelerated corrosion of exchanger
  parts.
• 3. mechanical failure of exchanger in tube
  rolls.
• Avoid introducing a high temperature stream into
  an exchanger before circulation of the cooling
  medium has been established. This is to prevent
  undue stress on the metal.
• Maintain adequate flow rates through exchangers
  to wash fouling media from the bundle and
  maintain heating rates within the limits of
  thermal design to prevent overheating.
• Operate within mechanical design limits to
  prevent overstress of metals.

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Tips to Reduce Maintenance

• Utilize steam traps on steam heaters to maintain
  maximum steam pressure within exchanger. By
  passing traps results in excessive steam
  consumption and reduced overall transfer
  coefficient.
• Guard against the use of superheated steam in
  exchangers designed for saturated steam heat.
  Superheated steam in such tube bundles reduces
  the overall transfer rate 10 times and may
  results in over heated equipment.