Gjahab

1. HEAT TRANSFER
Heat is the form of energy , and when it transform from
one place to another , it is called heat transfer. It is
though medium (solid ,liquid ). And it may be also
transfer though surrounding (air). The specific
mechanisms are usually referred to as convection,
thermal radiation, and conduction.

2. OBJECTIVE
1. Model basic heat transfer processes and identify
modes
2. Calculate thermal resistances
3. Perform an energy balance to determine temperature
and heat flux
4. Identify fins and calculate fin performance
5. Use shape factors for 2-D conduction
6. Solve lumped parameter transient heat transfer
problems
7. Solve distributed parameter transient heat transfer
problems

3. 8. Recognize basic convective heat transfer and apply
appropriate methods for quantifying convection
9. Calculate convective heat transfer coefficients for
internal flow
10. Calculate convective heat transfer coefficients for
external flow

4. Modes of Heat Transfer
- There are basically three modes:
1. Conduction 2. Convection and
3.Radiation.

5. - Conduction refer to the thermal energy transfer that
will occur across a medium, which may be a solid or a
fluid due to a temperature difference.
- Convection refer to the thermal energy transfer
between a surface and a moving fluid when they are at
different temperature levels.
-Thermal radiation refer to the net transfer of thermal
energy between two surfaces at different temperature
levels, in the absence of an intervening medium
between them. This occurs due to electromagnetic
waves emitted from a hot body.

6. Physical mechanisms for different modes of HT
CONDUCTION -Thermal conduction is the transfer of
thermal energy from the high energetic to the low
energetic particles of a stationary medium(solids, liquids
or gas) due to interactions between the particles.
- In solids ,conduction may be attributed to atomic
activity in the form of lattice vibrations and energy
transport by the free electrons.
-In fluids, conduction occurs due to the collisions and
diffusion of the molecules during their random motion.

7. The basic equation for thermal conduction is the
Fourier’s law. - It states that the heat flux(Heat Transfer
rate per unit area) is directly proportional to the
temperature gradient.
q ∞ dT/dx or q ═ - k dT/dx
Where, k - thermal conductivity (W/m.K)
dT/dx - temperature gradient
q - heat flux (W/sq. m)
𝐐 = −𝐤𝐀 *𝐝𝐓/ 𝐝 X

8. Assumptions of Fourier’s Law
1. Steady state conduction – temp. does not change with
time.
2. Uni-directional heat flow.
3. Constant temp. gradient and linear temp. profile.
4. No internal heat generation.
5. Bounding surfaces are isothermal in character.
6. Isotropic and homogeneous material

9. Thermal Conductivity(k)
-It is the measure of the ability of a material to conduct
heat.
-It is one of the transport properties of a material.
-Its unit is W/m0C or W/m. K
-For Solids and Liquids, k = f(T)
-For gases/vapor, k = f(p, T)

10. - Metals are best conductors.
-Alloying of metals will reduce ‘k’
- ‘k’ decreases with increase in temp in most of metals
except Aluminum and Uranium.
- In Aluminum. ‘k’ is constant between -1300C to 3700C -
In Uranium it is increases with temp.
-Gases with high molecular weight have small ‘k’.
- Materials having crystalline structure have high ‘k’ than
amorphous form.

11. CONVECTION- Convection refer to the thermal energy
transfer between a solid surface and a moving fluid
when they are at different temperature levels.
-It involves the combined effect of conduction and fluid
motion.
-Heat is transfer from the solid surface to the fluid layer
which is in contact with it by conduction.
-Then to the adjacent layers heat will transfer by the
random molecular motion

12. - Thermal energy transfer by convection is classified as:
1. Natural (Free) convection and
2. Forced convection.
- Forced convection is the transfer of thermal energy when
the flow is caused by external means, such as a fan, a
pump or atmospheric winds.
-Natural convection is induced by buoyancy forces due to
density variations as a result of temperature differences.
-There are thermal energy convection by latent heat
exchange. This latent heat is due to change of phase from
liquid to vapor or vice-versa.
- Boiling and condensation are examples for such
processes.

13. - The basic equation for convection heat transfer is
known as Newton’s law of cooling:
𝑄 = ℎ𝐴(𝑇𝑠 − 𝑇∞
Where, 𝑇𝑠 is the surface temperature,
𝑇∞ is the fluid temperature and
A is the surface area of the solid.
h is the convection heat transfer coefficient
h is also called film heat transfer coefficient or surface
conductance.

14. RADIATION
- Thermal energy transfer by radiation is caused by
electromagnetic waves(or photons).
- Thermal radiation is emitted by all surfaces which are
kept at a finite temperature level.
- This happens from solids, liquids and gases.
- Rate of emission increases with temp. level.
- Radiant energy does not require a material medium for
its transport.
-Moreover, radiation transfer will occur effectively in
vacuum.

15. - The mechanism of heat flow by radiation consists three
distinct phases:
1. Conversion of thermal energy of the hot source into
electromagnetic waves.
- Photons are propagated through the space as rays.
2. Passage of wave motion through intervening space
- Photons travel with unchanged frequency in straight
paths with speed equal to that of light.
3. Transformation of waves into heat.
- Reconversion of wave motion into energy occurs in the
receiving surface which may partly absorbed, reflected
or transmitted through

16. - The basic rate equation for radiation HT is the
StefanBoltzman law:
𝑬𝒃 = 𝝈𝒃𝑨𝑻 𝟒
Where, 𝐸𝑏 is the energy radiated per unit time.
T is the absolute temp of the surface
𝜎𝑏 is the Stefan-Boltzman constant
𝜎𝑏 = 5.67 𝑥 10−8 𝑊/𝑚2𝐾

17. In pharmaceutical industries many types of equipments
are used for transfer of heat, they can be classified as
follows,
A. Heat Exchangers.
B. Heat Interchangers.
Heat Exchangers:
These devices are used for transferring heat from a fluid
(Hot Gas or Steam) to another fluid (Liquid) through a
metal wall.
Heat Interchangers:
These devices are used for transferring heat from a One
liquid to another liquid or one gas to another gas
through a metal wall

18. HEAT EXCHANGERS;
The equipment used for heat transferring are known as
heat exchangers.
Some of the processes that involves heat transfer in
pharmaceutical industries are:
Preparation of starch paste (in steam jacketed kettle).
Crystallization.
Evaporation.
Distillation.

19. Tubular heater
Tubular heaters consist of circular tubes, one fluid flows
through the inner tube, while the other flows through
the outside space.
The heat transfer takes place across the wall of the
tube.
Shell and Tube heater is the simplest form of tube
heater.

20. 1. Single Pass Tubular heater:

21. Construction:
It consists of a bundle of parallel tubes, which are relatively
thin-walled. The ends of these tubes are fitted to two tube-
sheets B1 and B2.
The bundles of parallel tubes are enclosed in a cylindrical shell
or casing (C, made of cast iron) to which the tube-sheets (B1
and B2) are fitted.
Two distribution chambers D1 and D2 are provided at each end
of the casing (C).
Cold fluid inlet (H) is fitted with distribution chamber D2 and
hot fluid outlet (I) is fitted with the distribution chamber D1.
Steam inlet F, steam outlet K(called vent) and condensate
outlet G are fitted to the shell.

22. Working:
Steam is introduced through the steam inlet F into the space
surrounding the parallel tubes.
Heat is transferred to the cold liquid inside the tubes and steam
is condensed.
The condensate is removed through condensate outlet G
placed at the bottom of the casing. Non-condensable gases, if
any, escape through the vent K provided at the top of the shell.
The fluid to be heated is pumped through the cold fluid inlet
(H) into distribution chamber D1, flows through the tubes and
collects in the distribution chamber D2.
Heat is transferred from the steam to the cold fluid through the
metal wall. The hot fluid leaves the heater through outlet (I).

23. Advantages:
Large heating surface is packed into a small volume.
Disadvantages:
The velocities of the fluid flowing through these tubes
are low because of the large cross-sectional area or
surface area.
The expansion of the tubes and shell takes place due to
differences in temperatures. This may lead to loosening
of the tube sheets from the casing.
Initial cost and maintenance costs are very high.

24. 2. Floating-head two-pass heater

25. Construction: It consists of a bundle of parallel tubes.
They are enclosed in a shell (casing). The right side of the
distribution chamber is partitioned and fluid inlet and
outlet are connected to the same chamber. The partition
is such that both have equal number of tubes. On left
side the distribution chamber is not connected to the
casing. It is structurally independent, hence known as
floating head. The ends of the tubes are fitted with the
floating head. Casing is provided with steam inlet, vent,
and condensate outlet.

26. Working: Steam is introduced through the steam inlet of
the casing (shell). Steam heats the tubes. The
condensate escapes through the condensate outlet
fitted at the bottom of the casing. Non-condensable gas,
if any, escapes through the vent of the casing provided
at the top . The cold fluid is introduced through the cold
fluid inlet into the right-hand distribution chamber. The
fluid flows through few tubes present in the lower part
of the distribution chamber. The fluid reaches the
floating head and changes direction and flows through
the upper tubes again to the right-hand distribution
chamber. The hot fluid is taken out through the outlet.
During this process the fluid in the tubes get heated due
to heat transfer through the metallic wall.

27. Advantages: Due to differences in temperature the
tubes and shells may expand and the joints may get
loose. Since, the floating head part is independent of
shell (or casing) hence the problem of loosening is
prevented in this type of heater.

28. Liquid to liquid heat interchanger

29. Construction: Construction is same as that of single-
pass tubular heater, only difference is that it contains
baffles to lengthen the path of flow of outer liquid. A set
of parallel tubes are fitted to two tube-sheets at two
ends. The tubes and the tube-sheets are placed inside
the shell. Cold liquid inlet is fitted to the left-hand side
distribution chamber and outlet is fitted with the right-
hand side of the distribution chamber. Hot liquid is
entering through the right-hand top side of the shell
and leaving the shell through the outlet placed on the
top, left-hand side of the shell. Baffles consists of
circular metal sheet, with one side cut away. Baffles are
placed inside the shell at appropriate places. Baffles
have perforation on it through which the tubes pass .

30. Working: The hot liquid is pumped from the left-top of the
shell. The fluid flows through the shell (i.e. outside of the
tubes) and moves down directly to the bottom (due to baffle),
again moves up – like this it flows from the left to right-hand
side of the shell. Baffles increases the velocity of the hot fluid
outside the tubes, which creates more turbulence. This reduces
the film thickness at the outside of the tube and thus increases
the film coefficient and thus heat transfer increases. The baffles
also get heated and add to the heat transfer to cold liquid. The
cold liquid is pumped through the inlet at the left-hand side
distribution chamber. The liquid passes through the tubes and
gets heated. The heated liquid is collected from the right hand
side distribution chamber. 4. Double-pipe heat interchanger
N.B. In a liquid-to-liquid heat interchanger, the fluid to be
heated is passed only once through the tubes before it is
discharged, i.e. single pass. The heat transfer in this case is not
efficient.

31. 4. Double-pipe heat interchanger
N.B. In a liquid-to-liquid heat interchanger, the fluid to be heated is passed
only once through the tubes before it is discharged, i.e. single pass. The
heat transfer in this case is not efficient.
In double-pipe heat interchanger number of pass can be increased as
desired.

32. Construction: In this case two pipes are used – one is
inserted into the other. Cold fluid is passed though
inner tube. The outer pipe acts as a jacket for the
circulation of hot fluid. All jacketed sections are
interconnected. Normally the number of pipe-sections
are few. The length of the pipe is also less. The inner
tubes may be made of glass and standard iron. The
pipes are connected with standard return bends and
the pipes are stacked vertically. The pipes may have
longitudinal fins on its outer surface for better heat
transfer.

33. Working: The hot liquid is pumped into the jacketed
section. It 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. The liquid to be heated is pumped
through the inlet of the inner tube. The liquid gets heat
up and flows through the bend tube into the next tube-
section and finally leaves the exchanger.