Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes

1. HEAT TRANSFER
Md. Saiful Islam
BPharm, MSc
North South University
Fb Group: Pharmacy Universe

2. Heat is a form of energy associated with
disordered and chaotic movement of
molecules and ions. The higher the
temperature of a material, the faster the
molecules and ions are moving, and
hence the greater the amount of energy
present as heat.
A substance will have no heat content only
if it is at a temperature of absolute zero (0
K).
Heat is a form of energy and hence can be
measured and expressed in Joules,
practically in Kilo joules (KJ; a 1000 times)
or Mega joules (MJ; 1000 000 times).
e.g. It takes about 65kJ of heat to raise
200mL of water from room temperature to
its boiling point.

3. Heat transfer is the exchange
or movement of heat energy
and will occur spontaneously
wherever there is a
temperature gradient. The
rate of heat transfer indicates
how quickly heat is
exchanged and is expressed
in J s-1 or watts (W).
Many Pharmaceutical processes
involve the transfer of heat:
• Melting materials.
• Creating an elevated
temperature during cream,
suppository or ointment
production.
• Controlled cooling of the
same products.
• Heating of solvents to hasten
dissolution processes.
• Sterilization of products e.g.
using steam in autoclaves.
• Heating or cooling of air in air-
conditioning plant.
• Drying granules for tablet
production.

4. Methods of heat transfer
Heat transfer can take place by three
methods:
1. Conduction
2. Convection
3. Radiation
1. Conduction – Heat transfer by
conduction in solids result from the
movement of heat energy to adjacent
molecules by vibrational energy transfer
and the motion of free electrons. No
mixing action is involved so that the
conduction is limited to solids and fluids
that are ‘bound’ in some way that
prevents free movement.
2.
The molecule/electron donating the heat
energy will subsequently vibrate to a lesser
extent and therefore cool down whereas
the molecule receiving the heat energy will
vibrate to a greater extent and therefore
increase in temperature.
In case of a solid no appreciable
movement of the molecules occurs.
Heat transfer due to electron movement
is generally a greater effect than that
due to vibration of atoms. Therefore,
metallic solids (more free electrons) are
better conductors of heat than non-
metallic solids.
In static fluid (and therefore through
boundary layers) the mechanisms are
virtually the same. Heat is transferred
between molecules as a result of
molecular collisions.
Gases become better conductors at
higher temperatures owing to the faster
movement of the molecules, whereas
most liquids (with the notable exception
of water) become poorer conductors at
higher temperatures.

5. Methods of heat transfer
2. Convection – Heat transfer by convection is due to the movement of
molecules and their associated heat energy on a macroscopic scale. It
involves the mixing of molecules and occurs within fluids, where the
molecules are free to move around. This process can occur naturally (e.g.
warm less dense air rises in the atmosphere to be replaced by colder more
dense air) and forcefully by moving the fluid mechanically using e.g. mixing
blade.
If forced convection also induces turbulent flow then the heat transfer process
is aided as there will be a reduction in the fluid boundary layer thickness. It is
a faster process compared to conduction.
3. Radiation – Heat transmission by radiation occurs by energy transfer
through space (even vacuum) by electromagnetic radiation. E.g. the energy
emitted by the sun. These waves can be reflected, transmitted or absorbed.
When they are absorbed by a material on which they fall, energy reappears
as heat and the material increases in temperature. Heat transfer by radiation
is only of pharmaceutical practical importance during microwave drying. Black
colored objects are the perfect emitter and absorber of radiation, although all
bodies radiate to some extent.

6. Heat energy from the gas burner is transferred by conduction through the
container wall to the water in the bath, which therefore increases in
temperature until its boiling point is reached. This heat gained is referred to
as sensible heat, as it produces an appreciable rise in temperature and
the change can be detected by the senses.
When the boiling point is reached, further heat generates steam without
further increase in temperature. This heat gain by the steam is termed
latent heat of evaporation or latent heat of vaporization.
Conduction

7. • The steam produced rises and contacts
the cool outer surface of the dish wall
and condenses giving up its latent heat
and forming a condensate layer on the
wall that runs down and drops back
into the water bath. Fresh condensate
is continually formed to take its place.
• The latent heat that is liberated by the
condensation passes by conduction
through the wall of the dish and into
the contents to be heated. Heat is then
transferred through the fluid by
natural convection and conduction.
• The advantage of using
steam as a heat transfer
agent in this setup:
– Causes indirect heating where
the temperature can never
exceed 100˚C (at atmospheric
pressure) and therefore there
is less chance of localized
overheating damaging the
product being heated.
– Since the steam circulates
over the whole dish surface,
heating is much more uniform
than It would have been if the
dish was heated directly over
the gas flame.

8. TS = steam temperature
TL = temperature of the
boiling liquid
TI and TO = temperature of the
inner and outer
surface of the dish
Above, heat transfer/flow undergoes three barriers which are the condensate layer, the dish
wall & the liquid side boundary.
The rate of heat transfer i.e. the quantity of heat transferred (Q, Joules) in
unit time (t, seconds) depends on the temperature difference between inner
and outer surface, the dish thickness LD and the area available for heat
transfer A.
Introducing a proportionality constant KD, the factors can be combined as:-
Q/t = KD A (TO – TI)/LD

9. • KD is also known as the co-efficient of thermal
conductivity and expressed in W/m K. It gives an
indication of the ability of the material to conducti heat;
higher the value, the more easily heat is conducted.
Question: Calculate the quantity of heat passing in
a period of 4 minutes through a pure Aluminium
dish (204 W/m K) whose effective heating surface
area is 30cm2 and whose thickness is 1.5mm, if the
temperature at the surfaces of the dish are 90˚C
and 75˚C respectively.

10. Steam as a heating medium
• The most commonly used heating medium. The reasons for
continued widespread use of steam include:
1. The raw material (water) is cheap and plentiful
2. It is easy to generate, distribute and control
3. It is generally cheaper than viable alternative forms of
heating e.g. electricity.
4. It is clean, almost colorless and tasteless, and so
accidental contamination of the product is less likely to
be serious like diesel and patrol. It is therefore also
environment friendly.
5. It has a high heat content (in the form of latent heat)
and can heat materials very quickly.
6. The heat is given up at a constant temperature, which
is useful in controlling heating processes and in
sterilization.

11. From kinetic theory of heat, a vapor contains heat in two
forms:-
i) Sensible Heat:- can be detected by the sense, i.e. a
temperature change when sensible heat is taken up or
given out.
ii) Latent Heat:- invisible heat, not detected as a
temperature change. So it is taken up or given out at
constant temp. as a change of phase between solid or
liquid or vapor.
Both of these heat contents are very important properties
of steam.

12. Converting ice to steam
Initially the heat supplied will be used to convert ice to liquid
without any rise in temperature (latent heat of fusion).
Latent heat of fusion - The heat absorbed as a substance
changes phase from solid to liquid. Specific latent heat of water
(LF) is equal to 334 kJ kg-1.
So, for 1kg water, latent heat of fusion = 334 kJ
F
F

13. The specific heat capacity equation can be used to calculate
the amount of energy required to raise the temperature of liquid
water to its boiling point.

14. All the heat supplied will be sensible heat to rise the temperature of the
water (0˚C) to the boiling point 100˚C.
The heat required is given by –
mass of water x specific heat capacity x temp. rise
1kg 4.2 KJ 100˚C
So, 420 KJ requires to raise 1kg water to its boiling point. Once the water
has reached boiling point, further heat energy input will not raise the
temperature of the water but will convert the boiling water at 100˚C to
steam at 100˚C. Steam at a temperature corresponding to the water boiling
point at that pressure (as in this case) is referred to as saturated steam.

15. The steam at this instance contains 2260 kJ of latent heat energy.
Steam in this state is referred to as dry saturated steam, as all the
liquid water has been converted to steam. This form of steam should
ideally be used for heating and sterilization processes as it contains
maximum latent heat energy and no associated air or water.
So only if further heated, will some of water convert into steam by supplying
latent heat of vaporization at constant temp (100˚C).
Latent heat of vaporization - The heat absorbed as a substance changes
phase from liquid to vapor. Specific latent heat of water (LV) is equal to 2260
kJ kg-1. So, for 1kg liquid water at 100˚C to convert to steam at 100˚C it would
require 2260 kJ of energy.
V
V

17. Dryness fraction: It is the fraction of water converted to steam
at particular time. Latent heat of vaporization is 2260 kJ/kg, so
it requires 0.5x2260 = 1130 kJ heat to produce dryness
fraction of 0.5.
Once all the water has been converted to steam, any further
heat energy input increases the steam temperature i.e. the
steam gains sensible heat. Steam at a temperature above the
saturation temperature is called superheated steam.

18. Design of Heating Equipment
Considering the factors affecting heat transfer, a number of precautions
must be taken in designing equipment for heating operation, especially
when steam is the source of heat.
1. AREA - Heating should take place over as surface as possible.
Q/t=Kd A(To – Ti)/Ld
2. TEMPERATURE GRADIENT - a suitable temperature gradient should
be employed, theoretically, as great as possible. But this is not in
practice-
a) Many pharmaceutical substances are thermo-labile & would be
damaged with a contact of high temperature surface.
b) Liquids boiling on hot surface form irregular streams of vapor
bubbles. Each stream originates from a point on the surface, called
nucleate boiling (Boiling in which bubble formation is at the liquid-
solid interface rather than from external or mechanical devices).
Above a critical surface temperature however, evolution of vapor is so
rapid that it cannot escape and the surface acquires a blanket of vapor
which forms an additional resistance to heat transfer, known as film
boiling.

19. 3. MATERIALS OF CONSTRUCTION – The plant should be made from
materials of suitable thermal conductivity.
4. GENERAL DESIGN - The design of plant construction should be such
that resistance due to surface layers are minimized.
a. Air Removal – Elimination, as far as possible, of air in steam is of
extreme importance. Film of air if present reduces the overall co-
efficient of heat transfer (4% air reduces 3/4th of co-efficient value!).
Moreover, the total pressure will be a mixture of a partial pressure
of air and steam, i.e. the temperature of the mixture will be lower
than that of saturated steam at same pressure.
b. Cleanliness - The surfaces of the vessel should be kept clean and
free from deposits of solids or scale.
c. Condensate Removal – The system should be arranged to permit
correct drainage and removal of the condensate formed as the
steam gives up its heat. The system could include a steam trap, a
device to distinguish between water & steam, allowing the former to be
discharged & the latter retained.
d. Liquid Circulation – Turbulent flow should be maintained by
avoiding awkward shapes where stagnation might occur and using
forced circulation if natural circulation is inadequate due to density
or viscosity changes.