UNIT-II of Pharmaceutical Engineering for second year B Pharm as per PCI subjec

1. HEAT TRANSFER
Ms. Mandakini Sampat Holkar
(M. Pharm.)
For Second Year B. Pharm. Program as
per PCI syllabus, New Delhi
Unit-II

2. Syllabus
Objectives, applications & Heat
transfer mechanisms. Fourier’s law,
Heat transfer by conduction,
convection & radiation. Heat
interchangers & heat exchangers.

3. MECHANISMS OF HEAT TRANSFER
Heat flows from a location at higher temperature to a region at low
temperature. The flow can occur either by a single or a combination of
any of the three basic mechanisms.
(a) Conduction:
When the flow of heat takes place in a solid body by the
transference of the momentum of individual particles,
atoms or molecules without actual mixing, the process is
called as conduction. It mainly occurs in solids and those
fluids where the movement is restricted due to high
viscosity. For example, flow of heat through a metal sheet,
wall or shell of boiler, evaporator and heat exchanger.
Conduction also occurs through a very thin layer of liquid
film which is formed just adjacent to the metal wall and is
not moving i.e., static liquid films.

4. (b) Convection:
In this form heat flows in fluid by actual mixing of the higher
temperature portions from one location with the cooler portions
at next location.The moving fluid contains high energy in it,
which is transferred to the particles at lower energy. For example,
heating of water in a vessel and heating of air in a room, fumace
or air oven. The main driving force of circulation of fluid is
considered to be the difference in the density between the two
locations.

5. (c) Radiation:
This involves the transfer of heat by means of
electromagnetic waves through space. Radiate means to
spread from a central location or origin. Thus, radiation
is spread of energy from an origin to the space
surrounding it. This mechanism does not involve any
movement or the interaction of the matter and energy is
purely carried by the electromagnetic waves.
The hotter the body the more it radiates.
The best example of radiation can be the heat
transferred from the sun to earth through space. Some of
the other examples are hot air ovens, infrared heaters,
microwave ovens and sonicator baths etc. Which utilize
radiation for producing heat.

6. Heat Exchangers and heat Interchangers
A wide variety of heat transfer equipments are used in
pharmaceutical industries. They
he used to heat fluids i.e., liquids and gases and sometime even
solids. Heating media
d is generally the hot fluid or steam or direct heating using electric
heated coils. Some of processes which involve heat transfer in
pharmaceutical industry are:
❖Evaporators
❖Distillation
❖Drying
❖Crystallization
❖Preparation of semisolid dosage forms- creams, ointments and
pastes etc.
The equipments used for the transfer of heat are known either as
heat exchangers or
heat interchangers.

7. (a) Heat exchangers: These are the devices used to transfer heat
from one fluid to the other liquid through a metal wall. So,
generally liquids are heated using steam as heating media in
such equipments.
(b) Heat interchangers: These are the devices which transfer the
heat from one liquid to another liquid or from one gas to the
another gas through a metal wall.
The two terms are also used interchangeably as the classification
is not exactly marked. Therefore, it is suggested to better call
them as heat transfer equipments.

8. Heat Exchangers or Heaters
The term heater or heat exchanger covers many
devices that exchange the heat between
two fluids of different temperatures that are separated
by a solid wall.
Some heat transfer equipments under this category
are:
❖Single pass tubular heater (shell and tube heater)
❖Two pass floating head heater
❖Multi-pass heat transfer heater

9. ❖Single pass tubular heater (shell and tube heater)

10. The heater consist of an outer cylindrical shell or casing C,
with an inlet and outlet for the fluid to be heated.
Bundle of relatively thin walled parallel tubes A is enclosed in
this casing.
The ends of these tubes are expanded into tube sheets B1 and
B2.
Two distribution chambers, D1 and D2 are provided at each
end of casing.
Fluid to be heated is allowed to enter in D2 chamber through
an inlet, H, and heated fluid is taken out from D1 chamber
through an outlet.
Steam or any other heated fluid is introduced into the tubes
through an inlet, F.
Non-condensable vapours and the condensate from steam are
allowed to escape/drain from K and G respectively.
Construction

11. Working: Steam is passed through F into the tubular
structure and is allowed to flow down the tubes so that the tubes
get heated up. Vent, K, is opened to escape the non-condensed
gases and condensate from steam is drained out from G.
Fluid to be heated is introduced through H to enter distribution
chamber D2. It flows through the heated tubes and reaches the
other distribution chamber D1. The steam and the fluid are
physically separated by the metal wall of the tubes but they are
in thermal contact with each other. Fluid gets heated up due to
conduction of heat across the metal wall of tube and finally
convection occurs to spread the heat to the entire fluid. The total
heat transfer occurs only by single passage of fluid through the
tubes, hence the name, single pass heater.
From Di, the heated fluid leaves the casing through outlet, I

12. Floating head two-pass heater

13. In floating head two pass heater, the ends of the tubes are
structurally independent of the shell.The construction of
this heater is similar to that of tubular heater with slight
modifications.
Construction: It consists of a bundle of parallel tubes.
Tubes are enclosed in a shell. The right side of the
distribution chamber is partitioned and opening for inlet
and outlet of cold fluid are connected to the same chamber.
The partitioning is so done that both the sections have
same number of tubes. The left side of the casing has the
distribution chamber,
which is not connected to the shell. Since, it is independent
of the shell structurally, it is known as Floated head. The
left end side of the tubes is embedded into the floating
head.

14. The tubular bundle has openings for inlet of steam or hot
vapour and outlet for draining the
the condensate.
A vent is also provided at the top for the escape of non-
condenšable gases.

15. Woking- Steam or hot vapour is introduced through the
inlet port into the tubes and is allowed to flow down the
tubes so that the tubes get heated up. Vent is opened to
escape the non-condensed gases and condensate from
steam is drained out from the outlet.
The fluid to be heated (cold fluid) is introduced into the
distribution chamber on right side of the heater. The
portion directs the fluid to pass through the heated tubes
and fluid reaches the floating head. The fluid then
changes its direction and moves back to the second part
of the partition chamber on right side. Thus, the fluid
has to flow twice through tubes hence the name two pass.
Fluid gets heated up due to conduction of heat across the
metal wall of tube and finally convection occurs to spread
the heat to the entire fluid. Then the fluid leaves the
shell through the outlet provided in the shell.

16. Multipass heater

17. Heat interchangers
When heat is to be transferred from one liquid to
another or from one gas to another through metal wall
the device is known as heat Interchanger

18. Double-pipe heat interchangers

19. Double-pipe heat interchanger: In this type of heat interchanger, the fluid
to be heated passed single time through the heated tubes before it is
discharged
Construction-
This is a concentric tube construction where two pipes are used ,one is
inserted in to the other.
The inner pipe is carry the fluid to be heated and the outer pipe acts like a
jacket through which hot liquid is circulated.
The outer pipes are connected through the annular spaces.
Generally there are few pipe sections joined and the length of the pipe is
also less.
A proper number of such pipes are connected in parallel
and then stacked vertically. Flow in a double-pipe heat
interchanger can be co-current or counter-current.
Two flow configurations: co-current is when the flow of the
two streams is in the same direction,
counter current is when the flow of the streams is in
opposite directions.

20. Working:
The hot liquid is pumped through the jacketed section
and is allowed
circulate through the annular spaces between them and
passed from one section to the other. In doing so, the
inner pipes get heated up and the hot liquid looses its
heat so that its temperature falls.
The liquid to be heated i.e., the cold fluid is pumped
through the inner pipe in such a way that either its flow
is co-current or counter current to the flow of hot fluid. In
diagram counter-current flow configuration. As the liquid
passes in the pipes, it gets heated up and flows through
the bent tubes into the next section of the pine Incase the
flow is counter current, the fluid gets heated further
every time it reaches the next section. This continues
the fluid is exited from the outlet on right side of the
interchanger.

21. Heat transfer by conduction
Heat can only flow whenever there is a temperature gradient i.e. Hot
and cold regions under steady state conditions, the rate of heat
transfer by condition can be written in the form of basic rate equation
Rate = Driving force/resistance------------------------------1
The driving force is the temperature drop across the solid surface
which is in direct relation to the rate of transfer.
The more is the temperature drop, the more will be the rate of heat
flow. resistance is the impeding factor which obstructs the flow and is
related to the thickness , surface are and thermal conductivity of
material This factor can be quantitatively expresses by Fourier law
Resistance = Thickness of the surface (L)
Mean proportionality constant(km) * surface area (A)

22. R= L/ Km A..............................................1
Fourier's law states that the rate of flow of heat through a uniform
material is proportional to the area and temperature drop and is
inversely proportional to length of path of flow i.e. Thickness of the
surface
Rate of heat flow α (Area * Temperature drop )
Thickness
dQ/dθ =q α (A. ∆t) / L or
q= Km.A. ∆t / L ....................2
Here Q= quantity of heat
Θ=time
A =area
∆t = Temperature drop
L= thickness
q= quantity of heat transferred in unit time
Km=mean proportionally constant

23. Derivation of Fourier’s Law
Consider a flat metal wall of surface area A and thickness L.
Let one face of the wall be maintained at uniform higher temperature
t1 and other face of the wall be at a uniform lower temperature t2
over the same area.
Since it is a flat wall a does not vary with L.
Heat flows at the right angles to the plan A and is assumed to be at
steady state.
If thin section of the thickness dL parallel to area A is considered at
some intermediate point in the wall and the temprature difference is
dt across this thin section, then Fourier’s law may be applied as
mentioned below:
q= - k. A.dt /dl..........3
K is constant known as thermal conductivity of the solid.
Negative sign indicates that the temperature is decreasing in the
direction of heat flow.

24. The term dt/dl is known as temperature gradient.
The exact value of dt is not known but the temperature of the
two faces of the wall are known.
rewrite the equation 3
q.dl/A =-k.dt.................................................4
Integration equation 4 between the limits
L=0 when t=t1
L=L when t=t2
we get
q 0∫L dL/A = - t1 ∫t2 kdt = t2∫ t1 kdt
qL/A= km (t1-t2) =km ∆t...............................5
Rearranging equation 5
q= km.A.∆t / L

25. q= km.A.∆t / L
q= ∆t / L/km.A
Comparing this equation with 1
shows that ∆t is driving force and
L/km.A is the resistance for rate
of heat flow.

26. Conduction through compound resistance in series
Consider a flat wall constructed of series of three layers of different
materials.
Let the thickness of each layer be L1 ,L2 and L3
And Thermal conductivity be k1 ,k2 and k3 respectively.
Area of the entire wall is A and
the temperature drop across each layer be ∆t1 , ∆t2 , ∆t3.
Resistance offered by each layer be R1,R2and R3.
if ∆t is the overall temperature drop over the entire wall then
∆t = ∆t1 + ∆t2 + ∆t3 .......................................................1
overall resistance R is equal to the sum of individual resistance of
each layer
R= R1+R2+R3..............................................................2

27. According to Fourier’s law
R= L / K.A
R= L1/ k1.A + L2/ k2.A + L3 / k3.A..................................3
Hence
∆t = q1.L1/ k1.A + q2. L2/ k2.A + q3.L3 / k3.A..............................4
Since entire heat must pass through the resistance in series,haet
transfer rate q can be written as
q=q1 + q2 + q3...............................................5
q= ∆t /R1+R2+R3 ....................................................6
Therefore for heat flow through a number of resistances in series
the contribution of temperature difference to the total
temperature and individual thermal resistances to the total
resistance can be expresses as
∆t; ∆t1; ∆t2: ∆t3::R:R1:R2:R3.....................................7

28. Gray Body
I t is defined as that body whose absorptivity remains
constant at all the wavelengths of radiation at a given
temperature.
consider a small cold body with a surface area A and
temperature T2 which is completely enclosed by a hot
body having temperature T1.
The amount of heat transferred for this system can be
obtained by Stefan-Boltzmann law which is
q=A.σ.(T14- T24)

29. Black Body
The amount of radiation energy emitted from a surface at a given
wavelength depends up on the material of the body and condition of its
surface and also surface temperature. Hence different bodies may emit
different amount of radiation per unit surface area through they are at
same temperature.
A blackbody may be defined as a perfect emitter and absorber of radiation.
It absorbs all the radiation falling on it regardless of its wavelength and
direction. also it emits the radiation uniformly in all the directions per unit
area emission. Therefore a blackbody is a diffusion emitter which is
independent of direction.

30. Stefan-Boltzman Law
Amount of themal radiation emitted from the body increases rapidly
with increase in temperature.Both stefan and Boltzman were
physicist.
They found that amount of radiant energy emitted is proportional to
the fourthpower of the absolute temperature of the heat source body.
Eb = σ.( T abs) 4
Eb emission power the gross energy emitted from an ideal surface per
unit area (A) time θ
Eb= Q/A. Θ =q/A
q= A. σ.( T abs) 4
According to this equation rate of heating depends on the temperature and
surface area of the emitter.

31. It also depends up on the absorption capacity of the material to be
heated
Actual bodies do not radiates as much as blackbody
Equation modified for actual bodies
q= E .A. σ.( T abs) 4
E= ENERGY EMITTED BY ACTUAL BODY / ENERGY EMITTED by blackbody