This is lecture of 1st year chemical engineering department

1. Heat Transfer Operations

2. Practically all of the operations carried out by the chemical engineer
involve the production and absorption of energy in the form of heat
Therefore, the laws that govern the transfer of heat are important
The apparatus used to control the flow of heat are important
Nature of Heat Flow:
When two objects are brought into thermal contact with each other,
heat would flow from the object having high temperature to the
object having low temperature
Heat always flow from a hot body to a cold
body

3. CONDUCTION
As the temperature of a solid increases, the molecules that
make up the solid experience an increase in vibrational kinetic
energy. Since every molecule is bonded in some manner to
neighboring molecules, this energy can be transmitted
through the solid. Thus, heating a wire at one end eventually
results in raising the temperature at the other end.
This type of heat transfer is defined as conduction and is the
principle mechanism by which solids transfer heat.

4. CONDUCTION
The rate of heat flow by conduction is given by Fourier's law:
where q = heat flow rate, Btu/h
x = direction of heat flow, ft
k = thermal conductivity, Btu/(h · ft
·°F)
Thermal conductivity k, is the ability of a material to pass heat from it

5. CONDUCTION
When temperature difference between a hot object and cold object is
zero.
Or the temperature of hot object is equal to the cold object after heat
transfer,
The system/objects are said to be in THERMAL EQUILIBRIUM

6. CONVECTION
Heat transfer through fluids is called convection heat
transfer
• When a pot of water is heated on a stove, the layer of
water the bottom of the pot is the first to gain a
temperature rise. Gradually, the water at the top will gain
an increase in temperature and become hotter. This heat
transfer due to the second mechanism of heat transfer,
namely, convection. As the water at the bottom is heated,
its density decreases. This results in convection currents as
gravity causes the low-density water to move upward and
be replaced by the higher-density cooler water from above.

7. CONVECTION
Natural convection: When convection heat transfer is
spontaneous and occurs naturally from a fluid to other fluid
i.e NO external force is applied
Forced Convection:
When external force is applied to have convection of heat.
i.e; A hot air fan, a hot air blower

8. RADIATION
In the heat transfer mechanisms of conduction and
convection, the movement of heat energy takes place
through a material medium, usually a solid in the case of
conduction and a fluid in the case of convection. A transfer
medium is not required for the third mechanism, where the
energy is carried by electromagnetic radiation. If a piece of
steel plate is heated in a furnace until it is glowing red and
then placed several inches away from a cold piece of steel
plate, the temperature of the cold steel will rise, even if the
process takes place in an evacuated container.

9. Heat Transfer operations
• In conduction, the molecular energy is directly exchanged, from the
hotter to the cooler regions, the molecules with greater energy
communicating some of this energy to neighbouring molecules with
less energy.
• Convection is the transfer of heat by the movement of groups of
molecules in a fluid. The groups of molecules may be moved by either
density changes or by forced motion of the fluid.
• Radiation is the transfer of heat energy by electromagnetic waves,
which transfer heat from one body to another, in the same way as
electromagnetic light waves transfer light energy.
• In general, heat is transferred in solids by conduction, in fluids by
conduction and convection. Heat transfer by radiation occurs through
open space.