Heat transfer; Objectives; Applications; Heat transfer mechanism; Fourier's Law; Heat transfer by conduction, convection and radiation; Heat interchangers and exchangers

1. z Heat Transfer
Presented by- Sandhya Punetha

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Topics-
 Objectives
 Applications
 Heat transfer mechanism
 Fourier’s law
 Heat transfer by conduction , convection and radiation
 Heat interchangers and Heat exchangers

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Introduction
 Heat is a form of energy.
 According to the principles of thermodynamics, whenever
a physical or chemical transformation occurs, heat flows
into or leaves the system.
 Heat transfer-It is a physical process involving the transfer
of heat from hot area to cold area.
 Heat transfer occur due to temperature difference .

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Objectives
 Understand how thermodynamics and heat transfer are
related to each other.
 Distinguish thermal energy from other forms of energy and
heat transfer from other forms of energy transfer.
 Perform general energy balances as well as surface energy
balance.
 Understand the basic mechanism of heat transfer.
 Develop an awareness of the cost associated with heat
losses.

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Applications of heat transfer
 Evaporation- Heat is supplied in order to convert a liquid into vapour.
 Distillation- Heat is supplied to the liquid mixture for separation of
individual vapour component.
 Drying- Heat is transfer for drying the wet granules.
 Crystallization- Saturated solution is heated to bring out super
saturation, which promotes crystallization of drugs.
 Sterilization- Autoclaves are used with stream as a heating medium.
 Heat transfer is required for refrigeration.

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Heat Transfer Mechanism
 There are 3 basic mechanism of heat transfer-
1. Conduction
2. Convection
3. Radiation

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Conduction
 When heat flow in a body is achieved by the transfer of the
momentum of individual atoms or molecules without mixing, such a
process is known as conduction.
 It take place in solids and fluids whose movement is restricted.
 Eg- Flow of heat through a metal shell of a boiler takes place by
conduction as far as solid wall or shell is considered. No mixing is
involved and conduction is limited to solids and fluids whose
movement is restricted.

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 There is no actual movement of molecules. On receiving energy from
the surface of heat, molecules vibrates and pass on the energy to the
another molecule. When both the molecules have same temperature
heat transfer get stopped.
 Driving force- Difference of temperature between two end.

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Convection
 When heat flow is achieved by actual mixing of warmer and cooler
portion of the same material , the process is called as convection.
 There is actual physical movement of the molecules.
 It takes place in some fluids.
 Eg- When hot and cold water are mixed together than their will be
physical movement of molecule which may lead to heat transfer by
convection.

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Radiation
 When heat flows through space by means of electromagnetic waves ,
such energy transfer is called as radiation.
 Eg- Black surface absorbs most of the radiation received by it and
simultaneously the absorbed energy is quantitatively transfered into
heat.
 Eg- Solar cooker, Microwave oven etc.

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Questions-
1. Define heat transfer.
2. Write the objectives of heat transfer.
3. What are the application of heat transfer.
4. What are the mechanism of heat transfe

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Fourier’s Law
 Fourier law states that the rate of heat flow through a
uniform material is proportional to the area and the
temperature drop and inversely proportional to the length of
the path of flow.

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Heat Transfer by Conduction,
Convection and Radiation
 Conduction-
 Heat can flow only when there is a temperature gradient, i.e. Heat
flow from a hot surface to a cold surface.
 The basic law of heat transfer by conduction can be written in the
form of a rete equation as follows-

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 The driving force is the temperature drop across the solid
surface.
 The greater the temperature drop, the greater will be the
rate of heat flow.
 The flow of heat will also depend on the conductivity of the
material through which it is flowing.

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Convection-
 When heat flow is achieved by actual mixing of warmer
portions with cooler portion of the same material the
process is known as convection.
 Types of convection-
1. Forced convection
2. Natural convection

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Forced convection
 Mixing of fluid may be obtained by the use of a stirrer or agitator or
pumping the fluid for recirculation. This type of process is known as
forced convection.
 Example- In some type of tube evaporators, the evaporating liquid is
forced through the tubes under pressure. Therefore, forced
convection is observed.

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Natural Convection
 Mixing of fluid may be accomplished by the current set up,
when body of fluid is heated. Such process is known as
natural convection.
 Example- In pan evaporator, convection currents are set up
in the evaporating liquid.

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Radiation
 Radiation is a process in which heat flows.
 Heat is transfered through space by means of
electromagnetic waves .
 Thermal Radiation- Heat transfer by radiation is known as
thermal radiation.
 Radiation is effective across perfect vacuum and also
through layers of air.

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Questions
1. Define grey body
2. Define black body
3. Define resistance
4. Define driving force
5. Give general equation of resistance in series and parallel during heat
transfer
6. Give different types of resistance.
7. Define absorbivity and emmisivity
8. What do you mean by thermal conductivity
9. Give derivation for calculating rate of heat flow by conduction and
convection.

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Heat Interchangers and Heat
Exchangers
 Heat Exchangers- These are the devices used for transfering heat from
one fluid to another fluid through a metal wall.
 Heat Interchangers- These are the devices used for transfering heat from
one liquid to another or from one gas to another gas through a metal wall.

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Heat Exchangers
1. Tubular heater ( Shell and Tube Heater)
2. Multipass heater
3. Two pass floating head heater

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Tubular Heater ( Shell and tube heater)
 It is the simplest form of a tubular heater. It is a single pass tubular heater.
 Construction-

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 It consists of bundle of a parallel tubes, which are relatively thin walled.
 The ends of these tubes are expanded into 2 tube sheets, B1 and B2.
 Bundles of tubes is enclosed in a cylinderical shell or casing C in which tube
sheets are fitted.
 Many heaters have a cast iron shell.
 2 distributing chambers ,D1 and D2 are also attached at each end of the casing.
 Fluid inlet is provided to the distributing chamber D2 while fluid outlet is
provided to the distribution chamber D1.
 Two covers , E1 and E2 are used to close the distribution chamber from the
sides.
 Steam or other vapours are introduced by a connection , F.
 Provisions are made for the escape of non condensable vapour K and
condensed vapour to drain to G.

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Working
 Steam are passed into the space surrounding the tubes by steam inlet.
 As the steam flows down the tubes ,the tubes get heated.
 The condensate vapours are drained from condensate outlet.
 Non condensate vapours are escaped from vent.
 The fluid to be heated is passed through inlet cold fluid into the
distributing chamber D2.
 The fluid and stream are physically separated but are in thermal contact
through the thin tube walls.
 The fluid in the tubes get heated by heat transfer by conduction through
metal wall.
 After reaching to the distribution chamber D1 ,the Hot fluid can be
obtained from outlet hot fluid.

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 In this type of heater , the cross section area of the tubes is
larger as a result of which velocity of fluid inside the tube is
low.
 Advantage- It has larger heating surface which can be
packed into a small volume.

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Disadvantages-
 Velocity of fluid flowing inside the tube is low.
 Expansion of the tubes and shell takes place due to
difference in temperatures. This may lead to the loosening
of the tube sheets or buckle the tubes.

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Questions-
1. Why velocity of fluid inside the tube become low.
2. Why their is an expansion of the tubes and shells takes place.
3. What is the other name of tubular heater.
4. Define heat exchangers with examples.
5. Define heat interchangers with examples.

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Multipass Heater
 In this type of heaters ,velocity of the fluid can be increased
as a result of which heat transfer coefficient also increases.
 In this the liquid to be heated is passed through several
times thats why these heaters are more superior from
single pass shell and tube heaters.

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Construction

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Construction
 It consists of bundles of parallel tubes.
 The end of these tubes is expanded into 2 tube sheets.
 The tube bundle is wrapped in a cylinderical casing.
 Two distributing chambers are provided at each end of casing.
 Since the heater is multipass, the same liquid has to flow through several
tubes back and forth.
 In order to facilitate this process, distributing chambers are partitioned
by means of baffles and their arrangements are different in thw two
chambers.

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Working
 Steam is introduced through the connection into the space
surrounding the tubes.
 As the steam flows down the tubes get heated.
 The condensed vapour is drained and non condensate vapours if any
are escape through the vent provided at the top of the casing.
 The fluid to be heated is pumped at high velocity into the right
distribution chamber through the compartment.
 During this process the fluid in the tubes get heated due to heat
transfer by conduction through metal wall followed by convection.

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Advantages
 Multipass construction decreases the cross section of the
fluid path, thereby increases the fluid velocity.

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Disadvantages
 Fabrication of multipass heater is more complicated.
 Pressure drop through the apparatus is increased because
of enhanced velocity of fluid flow.
 More number of exit and entrance points increase the
friction losses.This increases the cost of pumping of fluid.

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Floating head two pass heater
 In floating head two pass heater, the ends of the tubes are structurally
independent of the shell.
 Construction-

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 Two pass floating head heater consist of a bundle of parallel tubes.
 These are enclosed in a shell.
 Right side of the distribution chamber is partitioned and fluid inlet and
outlet are connected to the same chamber.
 Partition is such that both the chamber containes equal no of tubes.
 On left side distribution chamber is not connected to the casing.It is
structurally independent which is known as floating head.
 The other end of the tubes is embedded into the floating head.
 Steam or other vapours is introduced through inlet provided to the
shell.

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Working
 Steam is introduced through the inlet.
 As steam flows down the tubes get heated.
 Condensed vapours escape through the bottom of the shell while non condensate
gases, escape through the vent at the top of the shell.
 Fluid to be heated is introduced into the distributing chamber on right side of
heater.
 Fluid flows through few tubes present in the part of the partition.
 Fluid reaches the floating head and changes direction.
 Now it passes back to the next part of the partition chamber on the right side.
 During this process fluid get heated by conduction followed by convection.
 Then the fluid leaves the outlet provided in the shell.

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Advantages
 Tubes and shells may get expanded due to difference in
temperature.
 Contraction are possible when heater is switched off.

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Questions
1. Write about multipass heater
2. Write about floating head two pass heater

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Heat Interchangers
 These are the devices used for transfering heat from one
liquid to another or from one gas to another gas through a
metal wall.
 In heat interchangers, the heating medium is a hot liquid.
 The liquid to be heated is the cold liquid.

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Heat interchangers
 Liquid to liquid interchangers
 Double pipe heat interchangers

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Baffles
 It consists of circular discs of sheet metal with one side cut away.
 These are perforated to receive tubes.
 To minimize leakage, the clearance between the baffles, shell and tubes
should be small.
 The baffles are supported by one or more guide rods, which are
fastened between the tube sheets by set screws.

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Working
 Baffles are placed outside the tubes.
 These increases the velocity of liquid outside the tubes.
 Baffles make the liquid flow more or less right angles to the
tubes, which create mor turbulence.
 This helps in reducing the resistance to heat transfer
outside the tube.
 Baffles constitute an important role in the heat transfer.

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Liquid- to- liquid interchangers
 The basic construction and working of any heat transfer
equipment more or less remains the same. Only a few
modifications are included.

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Construction
 Normally, tube sheets, spacer rods and baffles are assembled first
and then tubes are installed. The most important part in the
construction of the heat interchangers are the baffles.

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 Appropriate size of tube sheets is chosen for the fabrication.
 One or more guide rods are fixed to the tube sheets by means of set
screws.
 Baffles consists of circular discs of a metal sheet, with one side cut
away.
 Baffles are placed at appropriate places using guide rods.
 The baffles are arranged with appropriate spacing using short
sections of the same tubing.

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 Baffles have perforations through which tubes are inserted.
 The ends of tubes are expanded into the tube sheets.
 The shell has a provision for introducing the heating medium, hot
fluid.
 The outlet for the fluid is at right side top.
 On each side of the tubes, 2 distributing chambers are provided.
 Left side chamber contains an inlet for fluid to be heated and the
right side chamber contains outlet for hot fluid.

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Working
 The hot fluid is pumped from the left side top of the shell.
 The fluid flows outside the tubes and moves down directly to the
bottom.
 Then it changes the direction and rises again.
 This process is continued till it leaves the heater.
 Baffles increases the velocity of the liquid outside the tubes.
 Baffles also allow the fluid to flow more or less right angles to the tube,
which creates more turbulence.

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 These help in reducing the resistance to heat transfer outside the tubes.
 Baffles lengthen the path and decreases the cross section of the path of
the cold fluid.
 Baffles get heated and provide greater surface area for heat transfer.
 Simultaneously, during the flow the tubes also get heated.
 As a result, the film coefficient inside the tube also increases.
 The liquid to be heated is pumped through the inlet provided on left side
distribution chamber.
 The liquid passes through the tubes and gets heated and the heated
liquid is collected from the right hand side distribution chamber.

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Advantages
 Heat transfer is rapid.
 Fluid passes at high velocity outside the tubes.
 Fluid flows more or less at right angles to the tubes.

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Double Pipe Heat Interchangers
 In a liquid- to – liquid heat interchanger , the fluid to be heated is
passed only once through the tubes before it gets discharged.
 The heat transfer in this case is not efficient.
 When few tubes per pass is desirable, double pipe heat interchanger
is employed.

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Construction

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 In this, 2 pipes are used- one is inserted in the other.
 Inside pipe is used for the pumping of cold liquid to be heated.
 Outer pipe acts as a jacket for the circulation of the hot liquid.
 All jacketed sections are interconnected.
 Normally the number of pipe sections are few. The length of the pipe is
also less.
 Glass tube, standard iron pipe and graphite constructions are available.
 Standard metal pipes are assembled with standard return bends.
 A proper number of such pipes are connected in parallel and stacked
vertically.
 The pipes may have longitudinal fins on its outer surface.

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Working
 The hot liquid is pumped into the jacketed section.
 The hot fluid is circulated through the annular spaces between them
and carried from one section to the next section.
 Finally it leaves the jacket.In this process the pipes get heated while
the hot fluid looses its temperature.
 The liquid to be heated is pumped through the inlet provided at the
right side.
 The liquid gets heated up and flows through the bent tubes into the
next section of the pipe.
 The liquid further get heated and the same liquid continuous to flow
and finally leaves the interchanger through the exit point on the right
side.

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Uses
 Double pipe heat interchangers is useful when not more than 0.9
to 1.4 metre square of surface is required.

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“The education of a man is never be
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