heat transfer

1. Department of Pharmaceutics
Anita D. Shinde

2. Heat Transfer
⚫Definition: Heat transfer is the process of transfer of heat
from high temperaturesystem toa low temperaturesystem.
⚫In the thermodynamic system, heat transfer is the
movement of heat across the boundary of the system due to
temperature difference between the system and
surrounding.
⚫There are three modesof heat transfer :
⚫Conduction, convection and radiation.
⚫The process in which there is no transferof heat between
the system and its surrounding is called as adiabatic
process.

3. Application :
 Evaporation : Heat is supplied in order to convert a liquid into
vapour. The liquid present in material is evaporated with the help of
heating to get a concentrated product. E.g. preparation of vegetable
extracts.
 Distillation : Heat is supplied to liquid mixture for converting the
liquid into vapour so that individual vapour components are
condensed atanotherplace.
Drying : In the production of tablets, heat is passed through a
carrier gas over a bed of wet solid mass for achieving drying.
 Crystallisation : Saturated solution is heated to bring about
supersaturation, which promotes the crystallisation of drugs.
Sterilisation : For the sterilisation of pharmaceuticals , autoclaves
are used with steam as a heating medium. Dry heat is used for the
sterilisationof glass apparatus and othercontainers.

4. Objectives
⚫To reduce the heatorenergy lossand making energy
utilisation moreeffective.
⚫Insulation , it is to reduce the heat transferas much as
possibleacrossa system and its surrounding.
⚫Enhancement , it is the promotionof heat transferacross
a finite temperaturedifference.
⚫Temperaturecontrol, the temperatureof a region is
required to be maintained close tospecified value.
Thermoregulation of human body through more or less
blood flow to localised regions coupled with mechanisms
like sweating is an exampleof temperaturecontrol.

5. Mechanism of Heat Transfer
⚫Heat f lows from a region of high temperature to a region
of low temperature. Heat may flow by one or more of the
three basic mechanisms.
⚫Conduction
⚫Convection
⚫Radiation

6. Conduction:
⚫Conduction , is a process in which heat flows in a body is
achieved by the transfer of the momentum of individual
atomsor moleculeswithout mixing .
⚫Or Transfer of heat from one atom to another within an
object by in directcontactwith each other.
⚫Forexample, flowof heat through metal shell of a boiler
takes place by conduction as far as solid wall or shell is
considered.
⚫The flow of heat is depends on the transfer of vibrational
energy from one molecule to another , and in case of
metal the movementof freeelectrons.

7. Convection ;
⚫Convection , is process in which heat flow is achieved by
actual mixing of warmer portions with cooler portions of
same material.
⚫ It is the heat transferdue to bulk movementwithin
f luid such as gases and liquids.
⚫Forexample , heating of water by hotsurface is mainly by
convection.
⚫Natural convection(or freeconvection) refers toa case
where the fluid movement is created by the warm fluid
itself. The density of fluid decreases as it is heated.
Thus , hot fluidsare lighterthan cool fluid.

8. Cont ..
⚫Forced convection usesexternal meansof producing
fluid movement.
⚫Forced convection is what in winterdays , heat loss
from your body is increases due to the constant
replenishmentof cold air by thewind.
⚫Natural wind and fans are two mostcommon sources
of forced convection.

9. Radiation
⚫Radiation is a energy transfer process in which heat flows
through space by meansof electromagneticwaves.
⚫Radiative heat transferoccurswhen theemitted radiation
strikes another body and is absorbed .Weall experience
radiative heat transfer everyday ; solar radiation , absorbed
byourskin, iswhywe feel warmer in the sun in the shadow.
⚫Solar water heaters, solar cookers, microwave ovens,
microwavecookers, sonicator bathsetc., area few example
in which radiation is utilized forproducing heat.

11. Conduction
⚫Thermal conduction is the transferof heat (internal energy)
by microscopic collision of particles and movement of
electronswithin a body.
⚫The basic lawof heat transfer byconductioncan bewritten
in the form of rateequation as follows:
⚫ Rate = driving force
resistance
Driving force is the temperature drop across solid surfaces,
the greater the temperature drop, the greater will be the rate
of heat flow.
The flow of heat will also depend on conductivity of
material through which it is flowing.
(1)

12. Cont ..
⚫Forexample , conductionof heat is fasterthrough an
iron rod than though wooden log. This factor is
represented by the term resistance, which can be
quantitativelyexpressed by Fourier's law.
⚫ Resistance = Thickness of the surface (m)
Proportionalityconstant Χ Areaof surface
= L
Km. A
This equation forresistance which obtained from
Fourier’s law.
(2)

13. Fourier’s law – Conduction of heat through Metal wall
⚫Fourier’s law states that rate of heat flow through a
uniform material is proportional toarea and temperature
dropand inversely proportional to the length of path of
flow.
⚫ Rateof heat f lowα Area (m2) Χ Tempdifference (Δt)
Thickness (m)
q α A . Δt
L
q = Km. A. Δt
L
Where, Km =mean proportionalityconstant,W/m.K
(3)

14. Derivation :
⚫Fourier lawcan beapplied toa metal wall through
which theconduction of heat taking place.
⚫Areaof wall = A,m2
⚫Thickness of wall= L, m
⚫Face of wall (HH) is maintained at uniform, definite &
higher temperature = t1, K
⚫Face of wall (CC) is maintained at a lower , but uniform
temperature = t2 , K
⚫The heat flow will be at right angle to the plane A & is
assumed to be in steady state.

15. Cont..
⚫Consider thin section of thicknessdL atan intermediate
point in thewall.
⚫ for this section , Fourier’s law may beapplied as given :
dQ = -k.A .dt
dθ dL
Where, Q= Heat transfer
θ = Time,s
K = Proportionalityconstant , W/m.K
t = Temperature ,K
The ‘minus’ sign indicate thedecrease in temperature in
directionof flow.
in equation (dt/dL) represents temperaturegradient.
(4)

16. Cont..
⚫ For the steadystate heat transfer, thisequationchanges to :
dQ/dθ = constant = q = -K.A.dt/ dL---------(5)
Where , q= rateof heat transfer, J/s (or W)
Rearranging equation (5) gives,
q = -Km.A. Δt --------------------(6)
L
Where , Km= mean proportionalityconstant , W/m.K
in steadystate heat transfer, ‘q’ remainsconstant.
Rearranging equ(3) bycomparing itwith rateequ.
q = Δt
L ...........................(7)
Km.A

17. Cont..

18. Cont..
By Comparing aboveequation with rateexpression ,
term Δt indicate thedriving force.
Resistance = L
Km.A ----------- (8)
Fourier’s law is thus used todefine the resistance in
quantitativeterm.
The thermal conductivity (K) is thequantityof heat
transmitted due to unit temperaturegradient, in unit
time understeadyconditions in direction normal toa
surfaceof the unitarea.
SI unitof thermal conductivity iswattsper meter-
kelvin (W/m.K)

19. CONVECTION
⚫Convection is a process in which heat flow is achieved by
actual mixing of warmer portion with coolerportionsof the
same material.
⚫ Forced convection : It is defined as heat transferconvection
process in which mixing of fluid may be obtained by the use
of stirreroragitatoror pumping the fluid forrecirculation.
⚫For example , in forced circulation evaporators, the
evaporating liquid is forced through tube underpressure.
⚫Therefore forced convection is observed.
⚫Natural convection : is defined as heat transferconvection
process in which mixing of fluid may be accomplished by
currentsset up, when bodyof fluid is heated .

20. Cont..
⚫Forexample , in pan evaporator, convectioncurrentsare set
up in theevaporating liquid.
⚫In general , fluid flow may be described as either laminar
or turbulent.Thesecreates problems in estimation.
⚫When heat is passed through the tube, stagnant films
determine the rateof heat transfer.
When fluid flow is viscous, velocity is zero at actual surface
of thewall. It means that layerof f luid adjacent towall actsas
stagnant film.
Comparativelystagnant film can beobserved even in
turbulent flow.

21. Cont.
Sometimes, scalesaredeposited on the surfaceof the
metal wall and heat must be conducted through the
scales.
When steam gives up latent heat , waterwill condense
on the surface of the vessel (or tube ).Again the heat
must beconducted through thiswater film.
For heat transfer in tube , heat mustpass through
stagnant film byconduction.

22. Forced convection
⚫Forced convection is a heat transfer convection process
in which mixing of fluid may beobtained by the useof
stirreroragitatororpumping the f luid for
recirculation.
⚫Consider acaseof heat flowing from a hot fluid through
a metal wall intocold f luid .
⚫Variation of temperatureata specific point is observed.

24. Metal wall : Characteristics
⚫Dotted line HH and CC represent the boundariesof
the film in viscous flow on the hotand cold sides.
⚫The gradient through line tcand td caused by f lowof
heatwhoseconductivity is known.
⚫Metal wall thickness is L.
Hot fluid side :
⚫To the rightof HH, the f luid is in turbulent f lowon
the hotside.
⚫Ta is the maximum temperature in hot fluid.
⚫Tb is temperatureat boundaryon the hot
side(turbulent &viscous flow junction )

25. Cont..
⚫Tc is the temperatureat theactual interface.
⚫Curve ta,tb,tcrepresents temperaturegradient from the
bulk of hot fluid to the metal wall.
Cold fluid side :
To the leftof CC , the f luid is in turbulent f lowon thecold
side.
Tf is the minimum temperatureof cold fluid.
Te is the temp. At the boundaryon thecold side
Td is the temperatureat theactual interface. (between
fluid and solid)

26. Cont..
⚫Curve td, te, tf represents the temperaturegradient from the
metal wall to the bulk of thecold fluid.
⚫Surfaceor film coefficients:
Film coefficient is the quantity of heat flowing
through unitareaof the stagnant film perunitdrop in
temperature.
It is theconductivecapacityof the stagnant film for the
transferof heat.
Letq watt (joule persecond ) of heat is flowing from hot
f luid tocold one.
Same heat mustpass through stagnant fluid film on the hot
side, through the metal wall and through the stagnant film
on thecold side.

27. Cont..
⚫Let , area of the metal wall on the hotside=A1,m2
⚫Areaof the metal wall on thecold side = A2, m2
⚫Average area of the metal wall = Am,m2
Surfaceor film coefficienton the hotside: On the hot
side, the surfacecoefficient,h1,isdefined as:
Film coefficienton hot side Amountof heat f lowing (W)
(W/m2.K) = Area(m2)Χ diff. in temperature
H1 = q ----------- (9)
A1 (t1-tc)
From equation (9) and (6), it can be seen that surface
coefficient (h1)is analogous to the term , k/L for metal wall.

28. Cont..
⚫Since L/KA is the resistance term for metal wall.
⚫1/h1A1 is known as thermal resistance on hot side.
⚫The thermal resistance isdue to thecombine effectof
viscous film HH and turbulent core.
⚫Surfaceor film coefficienton thecold side :
= Amount of heat flowing
AreaΧ diff in temperature
h2 = q
and ,
A2 (td-t2)
1
h2 A2 is thermal resistance on cold side.

29. Overall coefficient :
⚫In theoverall heat transfer , three resistance termsare
involved in series,
1 is the resistanceon the hot fluid side.
h1 A1
L is the resistanceof the metal wall.
K.Am
1 is the resistanceon thecold fluid side.
h2 A2

30. Cont..
⚫Theoverall heat transferwritten as:
q = Δt ---------------(9)
1 L 1
h1A1 h2A2
If both numeratorand denominatorof the right side of
equation (9) are multiplied byA1, then
q = A1 Δt
1 + LA1 + A1
h1 k Am h2A2
Overall heat transfercoefficient U1 (W/m2.K) is defined as
onedivided by thedenominatorof aboveeq.
+ K.Am +

31. Cont...
⚫ q = U1 A1 Δt
Rateof heat transfer= Overall heat transfercoefficient
Χ Areaof heating surface Χ Temperaturedrop

32. Fluid in Natural Convection/Free convection
⚫Natural convection is defined as heat transfer
convection process in which a fluid is heated , the
currentsset up maycause mixing of fluid .
⚫This is typeof heat transport , in which fluid motion is not
generated by external source (like pump, fan, suction
deviceetc)
⚫In natural convection, fluid surrounding heat source
receives heat and by thermal expansion becomes less
denseand rises.
⚫The surrounding ,cooler fluid is then moves to replace it .
⚫This cooler fluid is then heated and the process continues,
forming convection current:

33. Cont..
⚫This process transfers heatenergy from the bottomof the
convection cell to top.
⚫Thedriving force for natural convection is buoyancy, a
resultof difference in fluid density.
⚫This processcontinues therebyeffecting the mixing of hot
and cold fluids.
⚫Application :
⚫Natural convection is observed when extractsare
evaporated in open pans.

34. Modes of feed -heat transfer
⚫Heat transfer by convection is involved between two fluids.
⚫Parallel heat flow- Variation in temperature
⚫Countercurrent heat f low- Temperaturegradient.
⚫Parallel heat flow:
When the hot fluid and thecold fluid enter theapparatus
from the same end, the flow is parallel to each other. This
arrangement is known parallel flow.
The temperatureof the hot fluid inside a pipe decreases
from T1 toT2 by transferring heat tocold fluid outside the
pipe .

35. cont..

36. Parallel heat flow:

37. Cont..
⚫As a result, the cold fluid temperature is increased from t1
to t2.
⚫The temperature drop at left is much greater than at right
end. It means that heat transfer is faster at left –side than
thatof the right –side.
⚫Mathematically , heat transfer in parallel flow of liquid can
be written as:
dq= U.A.Δt ------------(10)
Eq.(10) is based on two assumption
a) Theoverall coefficient(U) is considered constant
throughout theequipment.
b) The specific heatof each fluid isconsidered constant.

38. Cont..
Integrating equation
q= UaL Δt1- Δt2
In Δt1 ---------------(11)
Δt2
Where L= length of pipe,m
a= areaof the pipe,m2
Comparing eq. With q= UA Δt
Δtm= Δt1- Δt2
In Δt1 ---------------(12)
Δt2

39. Cont..
⚫Logarithmic mean temperaturedifference (Δtm) is
used. the total heating surface (A)is equal to aL.
⚫Heat transferequation in parallel flow heatexchanger
is: q= UA Δtm --------------- (13)
⚫If the temperaturedrop is nearlyequal (Δt1≈Δt2), then
arithmeticaverage temperature (Δtm)

40. Counter –current heat flow –temperature gradient.
⚫When hot fluid is passed through one end of theapparatuswhile
cold fluid is passed through theotherend, fluid pass in opposite
direction.This arrangement is known as counter -current or
counter–flow.
⚫From figure itcan be concluded that the temperaturedrop along
the length of theapparatus is nearlyconstant.
⚫In otherwords, amountof heat transferperunitarea is
substantiallysameat both ends.
⚫The heating surface is nearlyconstant throughout theapparatus.

41. Counter –current heat flow.

42. Cont..
⚫In counter –current heat flow, the exit temperature of
the hot f luid is considerably less than exit temperatureof
thecold fluid .
⚫If the Δt1 ≈ Δt2, temperature (Δt) can be takenas
arithmeticaverage.
Δtave= Δt1 + Δt2 -------------(14)
2
The heat transfer equation for counter current heat flow can be:
q= UA. Δtave

43. Radiation :
⚫Radiation is a energy transfer process in which heat is
transferred through space by meansof electromagnetic
waves .
⚫Thermal radiation :
⚫Heat transfer by radiation is known as thermal radiation .
⚫Radiation is effectiveacross perfectvacuumand also through
layers of air.
⚫All solid bodies radiates energy when their temperatureare
above theabsolutezero.
⚫The amountand kind of thermal energy radiated increases
rapidlywith temperature.

44. Cont...
⚫Thermal radiation usuallyoccurs simultaneouslywith heat
transfer by convection and conduction .
⚫Advantages :
⚫The radiation sourcecorresponding towavelength from
0.8 to 400 um is used for the thermal radiation.
⚫Radiantsenergy penetrates a shortdistance (1 to 2 um )
into material.

45. Fundamental concepts
⚫Thermal radiationobeys same laws of light ,namely –
a) It travels in a straight line
b) It may be reflected from thesurface.
Suppose the cold substance is placed in the sightof hot body
insidean enclosed space .
Thecold body intercept the radiation emitted by the hot body .
The fraction of radiations falling on the body may be reflected ,
which is known as reflectivity, ρ.
The fraction that isabsorbed is known as absorptivity, α.
The fraction that is transmitted is known as transmissivity ,τ.

46. Cont..
⚫The sum of these fractions must be unity :
α + ρ+ τ = 1
Black body :
All solid bodies radiates energy at temperature above the
absolute zero.
For the purpose of heat transfer ,a theoretic substance is
proposed and designated as black body.
Black body is defined as body that radiates maximum
possible amount of energy at a given temperature.

47. Cont..
⚫Black surfacesare betteremitters of heat radiation than
polished surfaces.
⚫Furtherthe term ‘black’ is nothing todowith thecolourof
the body .
In theory , a black body is considered to be an enclosed
spacewith a small (negligible) opening.
The temperature in the enclosed space should be constant
and uniform, because the amount of energy escaping
through a small opening is negligible.
In practice , a convenient black body is made from the tube
of carbon.

48. Cont..
⚫Both theends are plugged , with a small holeat thecentreof
oneend.
⚫When viewed through this small hole , the insideenclosed
space (furnace) is considered as black body, provided the
temperature is uniform.
⚫Similarlyall objectswithin the furnacecan beconsidered as
black bodies.
⚫A good absorberof heat is agood emittertoo.
Converselya poorabsorber isa pooremitter.

49. Rate of radiation:
⚫Normally , hot bodiesemit radiation. Stefan –Boltzman law
gives the total amountof radiation emitted bya black body.
q= bAT4
Where, q= energy radiate persecond.W(or J/s)
A= Areaof radiating surface ,m2
T= Absolute temperatureof the radiating surface, K
b= Constant, W/m2.K4
Kirchoff’s law :
This law establishes relationship between emissive power of
surface to its absorptivity.

50. Cont..
⚫E, emissive powerof the body is the radiantenergy
emitted from unitarea in unit time.
⚫Itstates that , the ratioof emissive powerto the
absorptivity is same for all bodies in thermal
equilibrium .’
⚫ E1 = E2
α1 α2
E1 and E2 = Emissive powerof two bodies
α1 and α2 = Absorptivityof two bodies

51. Grey body
⚫A grey body is defined as that bodywhoseabsorptivity is
constant atall wavelengthsof radiation ata given
temperature.
⚫Consider , a small cold bodywith surfacearea of A and
temperatureof T2 is completelysurrounded bya hot black
bodyat temperatureT1.
⚫Theamountof heat transferred in such a process is expressed
by Stefan law.
⚫ q= bA (T14 – T24)

52. Heat exchanger
⚫Thesearedevices used to transferring heat energy from
one fluid(hot gas or steam) to another fluid (liquid )
through a metal wall.
⚫Heatexchangercan be classified on the basisof :
⚫1. Type of f luid f lowarrangement
⚫2. Method of heat transfer.
1. Typeof f luid f lowarrangement :
 Parallel flow heatexchangers
 Counterflow heatexchanger
 Cross flow heatexchangers.

53. Parallel flow heat exchanger
⚫It isalso referred as cocurrentorparallel stream exchanger.
⚫The hot and cold f luid streams enter togetherat one end, f low
parallel toeach other in the samedirection, and leave togetherat
otherend.
⚫With parallel flow the temperaturedifference between the two
fluid is large at theentrance end, but it becomes small at theexit
end .
⚫Theoverall measureof the heat transferdriving force , the log
mean tempdifference is less than forcounter f low, so the heat
exchanger surface area requirement will be larger than for
counter flow heatexchanger with same inletand outlet temp for
hot and cold fluid.

54. Counter flow heat exchanger
⚫It is also known as countercurrentexchanger.
⚫ in this type hot and cold fluid flow parallel but in opposite
directions.
⚫The hot fluid entering at one end of the heat exchanger
f low path and the cold f luid entering at other end of the
flow path.
⚫Advantage:
⚫It is thermodynamicallysuperior toanyother f low
arrangement.
⚫It produces highest temperaturechange in each f luid .

55. Cross flow heat exchanger

56. Cont..
⚫In cross flow type , one fluid flows perpendicular to the
second fluid , i.e one fluid flow through tube and second
fluid passes around the tubeat right angle.
⚫Cross flow arrangement , mixing of either fluid stream may
or may notoccur.
⚫These heatexchangerare typically used for heat transfer
between a gas and a liquid .

57. Classification of heat exchanger according to transfer process.
⚫According to transfer process heat exchangercan bedivided
into two majorcategories:
 Indirectcontact type
 Directcontact type
I. Indirectcontact type :
 Theseare also referred toas surface heatexchanger.
In this type ,fluid streams remain separate , and the heat
transfer takes place continuouslythrough separating wall.
There are nodirect mixing of the fluid becauseeach fluid
flows in separate fluid passages .

58. Classification of indirect contact type
⚫A. Direct transfer type
⚫B. Storage type.
⚫C. Fluidised bed exchanger
 A. Direct transfertype:
In this type, the hot and cold fluid flow simultaneously through
the device and heat is transferred through a wall separating the
fluids.
Examplesare – Tubular , plate –type, and extended surface
exchangers
 B. Storage type.
Thesearealso referred as regenerative heatexchanger.

59. Cont..
⚫This type of heat exchanger has a heat transfer surface (flow
passage) which is generally cellular in structure and is referred
toas Matrix or it is porous solid material.
⚫In this , both fluid flow alternativelythrough the same flow
passages.
⚫When hotgas flows over the matrix, the thermal energy from hot
gas is stored in the matrixwall, and thus hot gas is being cooled
during matrix heating period .
⚫When cold gas flows through same matrix, the matrixwall gives
up thermal energy , which isabsorbed by thecold fluid.
⚫Thus, in this type , thecorresponding thermal energy is
alternativelystored and released by the matrixwall.

60.  C. Fluidised bed exchanger
⚫In fluidised –bed heatexchanger , one sideof a two-fluid
exchanger is immersed in bed of sand orcoal particles.
⚫Atoptimum fluid velocity , the bed particles floats and the
condition is referred toas fluidized condition .
⚫Under thiscondition very high heat transfercoefficients
areachieved.
⚫Application: In drying, mixing, adsorption, reactor
engineering, coal combustion.

61. II. Direct contact type
⚫Twofluids are notseparated bya wall. The heat transfer
takes place between two immiscible fluids like gas and
liquid coming intodirectcontact.

62. Direct transfer type
⚫Direct transfer type heatexchangersare widelyused
in industries. Two important types
⚫Types
⚫A. Tubularheatexchanger
⚫B. Plate heatexchanger
⚫A. Tubularheatexchanger:
 Shell and tubeexchanger
 Double pipe heatexchanger.

63. A. Tubular heat exchanger
⚫Shell and tube heater is thesimplest formof a tubular
heater(heatexchanger).
 Shell and tubeexchanger:
 Construction :
 Tubular heaterconsists of a bundle of parallel tubes inside the
cylindrical shell.
 Two distributionchambers,D1 and D2 are provided ateach end
of thecasing C.
 Fluid inlet is provided atchamber D2 and outlet at D1.
 Steam orothervapour is introduced byconnection ,F.
 Provision forescape the non-condensable vapour K and
condensed vapourtodrain at G.

64. Two fluids can exchange heat ,one fluid flows over the
outside of tube while second fluid flows inside tubes.

65. Working :
⚫Steam oranyvapour is introduced through a steam inlet F into
space surrounding the tube.
⚫The steam flows down the tubes. In this process, the tubegets
heated rapidlydue to highvalueof steam film coefficient.
⚫Thecondensed vapourdrain through condensed outlet at G.
⚫Non-condensable gases ,if any, escape throughvent K .
⚫The fluid to be heated is pumped through thecold fluid inlet H
intodistributing chamber D2.
⚫The fluid flows through tubes.
⚫The fluid in tubeget heated due to heat transfer byconduction
through metal wall , followed by stagnant layerand finally by
convection. Thusenhanced rateof heat transfer.
⚫Heated fluid reachesdistributing chamber D1 and leaves the
hot fluid outlet ,I.

66. Application :
⚫Used in pharmaceutical petroleum –refining and chemical
industries as a steam generators, condenser, boiler feed
water heater.
⚫Used in airconditioning and refrigeration applications.
Advantages :
 Theyare mostversatileexchangers , made from varietyof
metal and non-metal material.
 The tubesare replaceableand can becleaned easily.

67.  Double pipe Heat Exchanger
⚫Theseare simplestof all typeof heatexchangers.
⚫They are made up of two pieces of pipe –one inside the
other.
⚫On fluids flows through the inner pipe while second
fluid flows through annulus between the pipes.
⚫Flow inside double pipe heat exchanger can be co-
currentorcountercurrent.
⚫Advantage:
⚫Theyare inexpensive
⚫Easy todesign for high pressureservice.

70. Disadvantage
⚫Theyare difficult toclean
⚫Theyare suitableonly forsmall sizes.

71. Multipass Heater
⚫In multipass heater, thevelocity of fluid can be increased, this
causes increase in heat transfercoefficient.
⚫Liquid to be heated is passed through the tubes several times
before leaving equipment.
⚫This facilitates the effective heat transfer. Therefore multipass
tubular heatersare superiorto single pass shell and tube heater.
⚫Construction –
⚫It consist of numberof parallel tubes.
⚫The bundleof tube iswrapped in cylindrical casing.
⚫Two distributionchambers are provided ateach end of casing.
Since the heater is multipass, thesame liquid has to flow through
several tubes back and forth.

73. Working -
⚫The feed is entered intocompartment A of oneof the
head.
⚫Then it is passed through tubes intocompartment B of
other head.
⚫Then fluid back through othersetof tubes to
compartment C .
⚫And finally leaves through compartment I.
⚫The fluid is diverted by using baffles. Since heater is
multipass, so same liquid has to flow through several
tubes back and forth.

74. Advantage:
⚫Multipass tubularheatersare superiorto thesingle
pass shell and tube heater.
⚫Disadvantages:
⚫The fabrication of multipass heater is more
complicated .
⚫The pressuredrop through apparatus is increased bcz
of enhanced velocityof fluid flow.
⚫More numberof exit and entrancepoints increase the
friction losses.

75. B. Plate –type heat exchanger
⚫A plate exchanger consists of a series of parallel plates that are
placed one above the other so as to allow the formation of a
series of channels for fluids to flow between them.
⚫The space between two adjacent plates forms the channel in
which the fluid flows.
⚫Inlet and outlet holes at the corners of the plates allow hot and
cold fluids through alternating channels in the exchanger so
that a plate is always in contact on one side with the hot fluid
and theotherwith thecold.
⚫Fluidsaredivided into several parallel streamsand can produce
a perfectcountercurrent.

76. Plate type heat exchanger

78. Application :
⚫Theyare mostcommon in dairy ,juice, beverage,general
food processing and pharmaceutical industries.
⚫Used in synthetic rubber industry, paper mill.
⚫Advantage:
⚫It has high valueof overall heat transfercoefficient.
⚫Easy maintenanceand cleaning.
⚫There are no significant hotorcold spots in exchanger.

79. Heat interchanger
⚫These are device used for transferring heat energy from
one liquid to another liquid or from one gas to another gas
through a metal wall.
⚫In heat interchangers, the heating medium is hot liquid.
The liquid to be heated is a cold liquid.
⚫The film coefficient can be enhanced by increased by
increasing velocityof flow.
⚫But this is difficult from point of construction of the
device.
⚫The Shell and tube exchanger and Double pipe heat
exchangercan be used as heat interchanger.