https://www.youtube.com/channel/UC6pXCSnR2N9A5JZdHzOXVzQ/videos

1. BY Brhane Amha

2. To be covered
• Chemical Unit operations
• Heat transfer
• Different mode of heat transfer
– Conduction
– Convection
– Radiation
• Multimode heat transfer

3. What is a unit operation?
A unit operation is any part of potentially multiple-step process which can
be considered to have a single function.
It is a basic step in a process because large processes can be broken into
unit operations in order to make them easier to analyze.
It can involve a physical change or chemical transformation such as
separation, crystallization, evaporation, filtration, polymerization,
isomerization, and other reactions..

4. Cont’d..
• Chemical engineering unit operations can be grouped into five general
classes:
1. Fluid flow processes
2. Heat transfer processes
3. Mass transfer processes
4. Thermodynamic processes
5. Mechanical processes

5. 1. Fluid Flow Process
Fluid flow processes: deals about fluids transportation and its
dynamics.
It includes fluids transportation (pump, compressor, blowers, pipes
and fittings,), gas-liquid two-phase flow, filtration, solids fluidization,
mixing, etc.

6. 2. Heat Transfer Processes
Heat transfer is the exchange of thermal energy between physical
systems, depending on the temperature and pressure, by dissipating
heat.
It includes heat exchange, evaporation, and condensation.

7. 3. Mass Transfer Processes
• Mass transfer is the net movement of mass from one location to
another.
• It occurs in many processes, such as absorption, distillation,
extraction, adsorption, and drying.

8. 4. Thermodynamics Process
A thermodynamic process may be defined as the energetic
development of a thermodynamic system proceeding from an initial
state to a final state.
It includes Refrigeration and Air Conditioning (AC), gas liquefaction.

9. 5. Mechanical Unit Operation
• Mechanical unit operation includes:
Solids transportation: different types of conveyors
Crushing and pulverization: reducing sizes
Screening and sieving: separation of different particles based on their
size.

10. • There are also some chemical engineering unit operations which
involves more than one class such as distillation, and reaction
crystallization
• A "pure" unit operation is a physical transport process, while a mixed
chemical/physical process requires modeling.

11. Heat transfer
• As you know from thermodynamics:
“Heat is defined as the energy-In-transit due to temperature difference.
• Heat transfer take place whenever there is a temperature gradient
within a system.
• Heat transfer cannot be measured directly, but the effects produced by
it can be observed and measured. E.g. temperature change

12. Heat transfer…
• All heat transfer process must obey the first and second laws of
thermodynamics.
• In addition to the laws of thermodynamics it is essential to apply laws
of heat transfer to estimate heat transfer rate.
• Estimating rate of heat transfer is a key requirement in the design and
analysis heat exchanger, refrigeration and air conditioning.

13. Modes of Heat Transfer
• The basic modes are:
Conduction
Convection , and
Radiation
• In most of the engineering problems, heat transfer takes place by more
than one mode simultaneously. Such kinds of heat transfer problems are
called multi-mode heat transfer.

14. Conduction Heat Transfer
• Conduction heat transfer takes place whenever a temperature
gradient exists in a stationary medium.
• By direct contact of two systems
• On a microscopic level, conduction heat transfer is due to:
The elastic impact of molecules in fluids
Molecular vibration and rotation about their lattice positions in solids, and
Free electron migration in solids

20. Convective Heat Transfer
• Convection heat transfer takes place between a surface and a moving
fluid, when they are at different temperature.
• It is the transfer of energy between an object and its environment, due
to fluid motion.
• All convective processes also move heat partly by
diffusion/conduction, as well.
• Convective heat transfer consists of two mechanisms operating
simultaneously:

21. 1. Energy transfer due to conduction through a fluid
layer adjacent to the surface
• Hydrodynamic boundary layer: when fluid flow on the surface, fluid layer
adjacent to surface attain velocity of surface. If surface is stationary, then
fluid layer is stationay.i.e.no slip condition
• Similarly, when a fluid flow on surface whose temperature is different from
fluid surface, then the fluid layer adjacent to the surface attain temperature of
surface. This is also no slip condition. This is called thermal boundary layer.
• And the heat transfer is initially from surface to fluid stationary layer by
conduction. Then the heat transfer will be by motion of the bulk fluid.
• This show similarity between momentum transfer and convective heat
transfer

22. 2. Energy transfer by macroscopic motion of fluid particles by
using of an external force (due to a fan/pump or buoyancy).
If you are using external force e.g. due to a fan/pump/ stirrers or other
mechanical means, this is called forced convection.
If it is without using any external force, it is called free/natural
convection.
Free, or natural, convection occurs when bulk fluid motions (streams
and currents) are caused by buoyancy forces that result from density
variations due to variations of temperature in the fluid.

23. • Convective heat transfer, or convection, is the transfer of heat from
one place to another by the movement of fluids, a process that is
essentially the transfer of heat via mass transfer.
• Convection is usually the dominant form of heat transfer in liquids and
gases.
Convective Heat Transfer…

24. Convective Heat Transfer…
• Heat transfer rate by convection is written as:
=> This is called Newton’s law of cooling (basic equation for convective heat
transfer/ Convective cooling).
Where, ℎc= the convective heat transfer coefficient,
Tw=is surface temperature, and
T∞=is temperature of fluid in the free stream
• Newton’s law of cooling states “The rate of heat loss of a body is
proportional to the temperature difference between the body and its
surroundings.”

25. Convective Heat Transfer…
• However, by definition, the validity of Newton's law of cooling
requires that the rate of heat loss from convection be a linear function of
("proportional to") the temperature difference that drives heat transfer,
and in convective cooling this is sometimes not the case.
• In general, convection is not linearly dependent on temperature
gradients, and in some cases is strongly nonlinear.
In these cases, Newton's law does not apply.

29. Radiation Heat Transfer
• Radiation heat transfer does not require a medium for transmission.
And it is more effective in vacuum.
• It is the transfer of energy from the movement of charged particles
within atoms which is converted to electromagnetic radiation.
• Energy transfer occurs due to the propagation of electromagnetic
waves such as microwave and light wave. A body due to its
temperature emits electromagnetic radiation.
• It is propagated with the speed of light in a straight line in vacuum. Its
speed decreases in a medium but it travels in a straight line in
homogenous medium.

31. Remember these;
What are the different types of chemical unit operations?
What is heat transfer?
What are the different mode of heat transfer?
What are the basic governing equation for each types of heat
transfer modes?

32. To be covered
HE Design considerations
Key steps of the thermal design procedure for HE
Overall energy balance
Selection of flow arrangement
Preliminary geometrical design
Choice of fluid flow velocities
The log-mean temperature difference
Baffle design

33. Design Considerations
The problem of heat exchanger design is complex and
multidisciplinary.
The major design considerations for a new heat exchanger include:
process/design specifications, thermal and hydraulic design,
mechanical design, manufacturing and cost considerations, and trade-
offs.
Most of the other major design considerations involve qualitative and
experience-based judgments, tradeoffs, and compromises.
Therefore, there is no unique solution for designing a heat exchanger
for given process specifications.

34. Cont’d..
• Two most important heat exchanger design problems are the rating
and sizing problems.
• Determination of heat transfer and pressure drop performance of either
an existing exchanger or an already sized exchanger is referred to as
the rating problem.
• In contrast, the design of a new or existing-type exchanger is referred
to as the sizing problem.
• Performance problem Vs Design problem

35. Step-by-step procedure for the "sizing" problem
to determine Exchanger dimensions
The key steps of the thermal design procedure for a serpentine tube heat
exchanger are as follows:
1. From given parameters calculate unknown inlet or outlet temperatures
and flow rates of fluids and heat transfer rate of heat exchanger using
overall energy balance
2. Select a preliminary flow arrangement (i.e. based on the common industry
practice).
3. Design preliminary geometry parameters of heat exchanger. The work
includes selection of tube diameter, layout and pitch.
4. Choose fluid flow velocities.
5. Estimate the log-mean temperature difference.

36. Cont’d..
6. Estimate an overall heat transfer coefficient using appropriate
methods for heat transfer calculation for designed type of heating
surface.
7. Estimate required heat transfer area.
8. Calculate length of tubes, the number of serpentines or passes,
baffles etc.
9. Estimate outside dimensions of heat exchangers.
10. Calculate pressure drops for both fluids.
11. Repeat, if necessary, steps 3 to 9 with an estimated change in design
until a final design is reached that meets specified requirements.

37. Overall energy balance
• Assumption; Energy losses to surrounding is neglected, i.e. heat energy
given off by hot fluid =heat energy absorbed by cold fluid
• Two energy conservation differential equations for an overall adiabatic two-
fluid exchanger with any flow arrangement are
dq = -ChdTh = +CcdTc
• The heat capacity rate is
Ci = mici
• The overall rate equation on a local basis is
dq = U(Th - Tc)dA = U∆TdA
• where dq=rate of HT,
• Ch and Cc =heat capacities of hot and cold fluid,
• ci - specific heat and
• U =heat transfer coefficient

38. .…Cont’d....
Integration of the above equations across the exchanger surface area
results in overall energy conservation and rate equations as follows.
q = Ch(Th,i - Th,o) = Cc(Tc,o - Tc,i)
and q = UA∆Tm
Here ∆Tm is the true mean temperature difference dependent on the
exchanger flow arrangement.
Aim of overall energy balance is to determine all external parameters
describing heat exchanger operation:
mass flow rates mh, mc
inlet temperatures Th,i, Tc,i
outlet temperatures Th,o, Tc,o
heat transfer rate

39. Depending on given parameters solving of
following tasks is possible:

40. • Basic flow arrangements of two fluids in heat exchanger are
counterflow, parallelflow, single-pass crossflow, multipass crossflow
and various combinations.
Selection of flow arrangement

41. Preliminary geometrical design
The work includes selection of tube diameter, layout and pitch.
Choice depends on many conditions i.e. type of fluid, its velocity, pollution by
particles and chemical impurities, design limitations etc.
Two layouts of tubes in bundle are:

42. Tube Diameters:
• The most common sizes used are Ø3/4" and Ø1".
• Use the smallest diameter for greater heat transfer area with a
minimum of Ø3/4".
• For shorter tube lengths say < 4ft can be used Ø1/2" tubes.

43. Choice of fluid flow velocities
Choice of fluid flow velocity according to common practice allows
reasonable and compromise values of overall heat transfer coefficient
and pressure drop already in the first approach to heat exchanger
design.
Recommended velocities of fluid flows are:

44. The log-mean temperature difference
 The driving force for any heat transfer process is a temperature
difference. For heat exchangers temperature difference between two
fluids across the heating area is not constant. It depends on heat
exchanger arrangement and value of Ch and Cc.

46.  Evaluation of an average temperature difference is necessary for
calculation of heat transfer rate.
 For cases with pure counter and parallel flow the temperature
difference is best represented by the log mean temperature difference
(LMTD or ΔTm ), defined in equation below.
 where:
ΔT1 = the larger temperature difference between the two fluid streams
at either the entrance or the exit to the heat exchanger
ΔT2 =smaller temperature difference

48.  Use of log mean temperature difference for evaluation of heat transfer
rate is valid on following conditions:
1. The heat exchanger is at a steady state.
2. Each fluid has a constant specific heat.
3. The overall heat transfer coefficient is constant.
4. There are no heat losses from the exchanger.
5. There is no longitudinal heat transfer within a given stream.
6. The flow is either parallel or counter.

49.  And the main basic heat exchanger equation becomes;
 For non-flow or batch heating the heat transfer rate is given by;
Q=m×cp×dT/t, where t is the time over which heating
process occurs
 For flow or continuous heating process;
Q=cp×dT×m/t

50.  In case of cross flow heat exchanger, fluid temperature differences are
illustrated as three-dimensional
 In these heat exchangers, the temperature difference is not possible to
calculate by previous method, the correction factor is usually used
where the log mean temperature difference is expressed as
where Ψ is the correction factor and ΔTm,CF is the log mean temperature
difference for a cross flow heat exchanger. Value of correction factor is
usually determined from diagrams relating to crossflow heat exchanger
arrangement.
Cross flow heat exchangers

51.  Baffles: are used to support tubes and enable a desirable velocity for
the fluid to be maintained at the shell side, and prevent failure of tubes
due to flow-induced vibration.
 There are two types of baffles; plate and rod.
Plate baffles may be single-segmental, double-segmental, or triple-
segmental
BAFFLE DESIGN

52.  The minimum spacing (pitch) of baffles normally should not be closer
than 1/5 of shell diameter (ID) or 2 inches whichever is greater.
 The maximum spacing (pitch) does not normally exceed the shell
diameter. Tube support plate spacing determined by mechanical
considerations, e.g. strength and vibration.
 No of baffles= (tube length/baffle spacing)-1

53. Remember these;
Major design consideration for a new heat exchanger
Key steps of thermal design procedure for heat exchangers
equations for energy balances in heat exchangers
Types of flow arrangements
Types of layouts of tubes
LMTD calculation
LMTD validity conditions
Baffle design

54. Example 1

55. Example 2

56. First calculate the required heat transfer rate for the above indicated
light oil:
Next calculate the log mean temperature difference (ΔTm):

57. Then,

58. • .
.