Heat Exchanger

The main objective of Unit operations in food process
engineering is
i. to study the principles and laws governing the physical,
chemical, or biochemical stages of different processes, and
the apparatus or equipment by which such stages are
industrially carried out.
Introduction to Unit Operations:
Fundamental Concepts

Unit Operations: Classifications
 Physical stages: grinding, sieving, mixture,
fluidization, sedimentation, flotation, filtration,
absorption, extraction, adsorption, heat exchange,
evaporation, drying, etc.
 Chemical stages: refining, chemical peeling.
 Biochemical stages: fermentation, sterilization,
pasteurization, enzymatic peeling.
Mass Transfer Unit Operations:
Heat Transfer Unit Operations:
Simultaneous Mass–Heat Transfer Unit Operations:

The definition of specific heat is the amount of heat required to raise the
temperature of 1g of the substance. The unit for specific heat is J/(g*K).
The definition of heat capacity is just the amount of heat required to raise the
temperature. The unit for heat capacity is J/K
In this sense, specific heat could be one kinds of many forms of heat capacity
such as molar heat capacity and volumetric heat capacity.

Heat Exchanger
Definition
Heat exchangers are devices built for efficient heat
transfer from one fluid to another.
May be defined as an equipment which transfers the
energy from a hot fluid to cold fluid, with maximum rate
and minimum investment and running cost.
Examples :
• intercoolers and preheaters,
• condensers and boilers in steam plant
•evaporators and condensers refrigeration units.

Broad classifications of heat Exchanger
Three broad classification
1.Direct transfer type
2.Storage type
3.Direct contact type

1. Direct transfer type
- is one in which cold and hot fluids flow
simultaneously through the device and heat is
transferred through a wall separating the fluids.

Storage Type Heat Exchanger:
-is one in which the heat transfer from hot fluids to
cold fluids occurs through a coupling medium in the
form of a porous solid matrix. The hot and cold fluid
flows alternatively through the matrix, the hot fluid
storing heat in it and cold fluid extracting heat from it.

Direct contact type Heat Exchanger:
- is one in which to fluids are not
separated. If heat is to be transferred between a
gas and a liquid, the gas is either bubbled
through the liquid or the liquid sprayed in the
form of droplets in to the gas.

Types of Heat Exchanger
Heat Exchanger
Recuperato
r
Regenerator
Indirect contact
type
Direct contact type
Ex- spray and tray
type condenser
Fixed matrix
regenerator
Rotary
generator
Drum
type
Disk
type
Tubular Extended
surface
Plate
Double
pipe
Shell and
tube
Spiral
tube
Gasketed
Plate
Spiral Plate Tube Fin
Plate Fin

1.Plate Heat Exchanger
 This heat exchanger consists of a series of parallel,
closely spaced stainless-steel plates pressed in a
frame.
 Gaskets, made of natural or synthetic rubber, seal
the plate edges and ports to prevent intermixing of
liquids.
These gaskets help to direct the heating or cooling
and the product streams into the respective alternate
gaps.
The direction of the product stream versus the

Tubular Heat Exchanger
 The simplest noncontact-type heat exchanger is a double-
pipe heat exchanger, consisting of a pipe located
concentrically inside another pipe.
 The two fluid streams flow in the annular space and in the
inner pipe, respectively.
 The streams may flow in the same direction (parallel flow)
or in the opposite direction (counter flow).
In this type of heat exchanger, product flows in the inner
annular space, whereas the heating/cooling medium flows in
the inner tube and outer annular space.

 Another common type of heat exchanger used in the food
industry is a shell-and-tube heat exchanger for such
applications as heating liquid foods in evaporation system.
one of the fluid streams flows inside the tube while the
other fluid stream is pumped over the tubes through the
shell.
By maintaining the fluid stream in the shell side to flow over
the tubes, rather than parallel to the tubes, we can achieve
higher rates of heat transfer.
 Baffles located in the shell side allow the cross-flow
pattern.
shell-and-tube heat exchanger

Based on the flow properties-
1. Parallel flow
2. Counter flow
3. Cross flow

Parameter considering for Heat exchanger design:
Overall Heat Transfer Co-efficient:
 The overall heat transfer coefficient is a measure of the overall
ability of a series of conductive and convective barriers to transfer
heat.
 It is commonly applied to the calculation of heat transfer in heat
exchangers, but can be applied equally well to other problems.
 For the case of a heat exchanger, can be used to determine the
total heat transfer between the two streams in the heat exchanger
by the following relationship:
Where q= heat transfer rate (W)
U = overall heat transfer coefficient (W/(m²·K))
A = heat transfer surface area (m2)
∆TLM = log mean temperature difference (K)

Log mean temperature difference:
 The log mean temperature difference (LMTD) is used to
determine the temperature driving force for heat transfer in
flow systems, most notably in heat exchangers.
 The LMTD is a logarithmic average of the temperature
difference between the hot and cold streams at each end of
the exchanger.
 The larger the LMTD, the more heat is transferred.
 The use of the LMTD arises straightforwardly from the
analysis of a heat exchanger with constant flow rate and fluid
thermal properties.

We assume that a generic heat exchanger has two ends (which we call
"A" and "B") at which the hot and cold streams enter or exit on either side;
then, the LMTD is defined by the logarithmic mean as follows:
 where ΔTA is the temperature difference between the two streams at
end A, and ΔTB is the temperature difference between the two streams at
end B.
With this definition, the LMTD can be used to find the exchanged heat in
a heat exchanger:
Where Q is the exchanged heat.
U is the heat transfer coefficient and
Ar is the exchange area. Note that estimating the heat transfer
coefficient may be quite complicated.
This holds both for parallel flow, where the streams enter from the same
end, and for counter-current flow, where they enter from different ends.

Fouling Factor:
In heating equipment, when a liquid food comes
into contact with a heated surface, some of its
components may deposit on the hot surface,
causing an increase in the resistance to heat
transfer. This phenomenon of product buildup on
the heat transfer surface is called fouling.
Fouling results from a complex series of reactions,
and in heating processes these reactions are
accelerated with temperature.
Consequently, the energy requirements to operate
heat exchange equipment increase significantly.

 Flow rates:
factors: excessive pressure drop, erosion, noise and vibration
must considered. Max velocity of liquid kept below 7.5m/s
Tube size and layout: tube dia 6 mm to 50mm . But usually
25mm tube size are prefered conserning on the mechanical
cleanness. If chemical cleanness is applied then smaller tube
can be used. Tube length 2 to 7 m is the normal range.
Effectiveness consideration:
Effectiveness : actual heat transfer / max possible heat
transfer.

Criteria should consider for selection of a Heat
Exchanger
 Materials of constructions
Operating pressure, temperature, temperature profile etc
Flow rate
Flow arrangements
Performance parameter –effectiveness, pressure drop
Fouling tendencies
 Maintenance, inspections, cleaning, extensions and repairing
possibilities
Overall economy
Fabrication technique
Intended applications

Heat Exchanger

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Heat Exchanger