Heat Exchange of Food Machinary

2. Heat Exchanger
Definition
Heat exchangers are devices built for efficient heat
transfer from one fluid to another. Widely used in
engineering processes.
A heat exchanger is a device in which heat is
transferred from one fluid stream to another either
of the fluids may be a liquid or a gas. In food
processing the purpose is to heat or cool a liquid
food in bulk.
Examples : intercoolers, preheaters, boilers ,
evaporators and condensers in the power plants.

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

4. 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.

5. 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.

6. Direct contact type Heat Exchanger:
- is one in which two 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.

7. 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

8. 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

9. Fig. Plate Heat Exchanger

11. 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 (counterflow).
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.

12. Tubular Heat Exchanger

13.  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

14. Fig. (a) one shell two tube (b) Two shell four Tube –
shell and tube heat exchanger

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

17. 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)

18. 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.

19. 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.

20. 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 heating 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.

22. 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

23. Design of a Heat Exchanger
One of the key objectives in calculations involving a heat
exchanger is to determine the required heat transfer area
for a given application.
Considering the following assumptions:
1. Heat transfer is under steady-state conditions.
2. The overall heat-transfer coefficient is constant
throughout the length of pipe.
3. There is no axial conduction of heat in the metal pipe.
4. The heat exchanger is well insulated. The heat
exchange is between the two liquid streams flowing in
the heat exchanger. There is negligible heat loss to the