This document discusses different types of heat exchangers, including tube and shell and plate heat exchangers. It describes parallel flow, counter flow, and cross flow configurations. Counter flow heat exchangers are most efficient due to having the highest log mean temperature difference. The effectiveness-NTU method is introduced to calculate heat transfer rate when inlet/outlet temperatures are unknown. Heat exchangers are also classified as regenerative or non-regenerative based on whether the same fluid acts as both the heated and cooled fluid.

1. Performance & Analysis of
Counter flow & Parallel flow
Heat Exchangers , LMTD &
Effectiveness

2. Introduction
Need for heat transfer-

3. Heat Exchangers

4. Types of Heat Exchangers
Although heat exchangers come in every size and
shape imaginable, there are two basic types.
They are-
Tube & Shell Heat Exchanger
Plate Heat Exchanger

5. Tube & Shell Heat Exchanger
The most common type of heat exchanger is a shell
and tube heat exchanger. The system consists of a set
of tubes enclosed in a shell.

6. Cont..
• The fluid flowing inside the tubes is called the tube side fluid and
that flowing on the outside of the tubes is called the shell side fluid
• The tube side fluid is separated from the shell side fluid by a tube
sheet(s)
• The tubes are rolled and press fitted or welded into the tube sheet
to make it leak proof
• The higher pressure fluid is directed through the tubes and lower
pressure fluid is circulated through the shell side.This is based on
economy considerations as the tubes can be made to withstand
higher pressure than the shell of the heat exchanger at a much
lower cost

8. Plate Heat Exchanger
• Plate heat exchangers employs plates instead of
tubes to separate the hot and the cold fluid
• Because of large surface area, plates provide an
extremely large heat transfer area

9. Cont..
• Plates are smaller than tubular heat exchangers of
the same capacity because of the large heat transfer
area of the plates
• Plate heat exchangers are used for comparatively
low pressure processes
• The reliability of plates is less in context to leakage
as large gaskets used between plates

10. •
• fffffffffffffff plate heat exchanger

11. Classification of Heat Exchangers
The heat exchangers are categorized on the basis of-
1. Flow configuration
2. Regeneration

12. Flow Configurations
• Three basic flow arrangements are:-
1. Parallel Flow
2. Counter Flow
3. Cross Flow

13. Parallel Flow Heat Exchangers
• In parallel flow heat exchangers both hot and cold streams enter
the heat exchanger at the same end and travel to the opposite end
in parallel streams
• Energy is transferred along the length from the hot to the cold fluid
so the outlet temperature asymptotically approach each other
• Parallel flow results in rapid initial rates of heat exchange near the
entrance, but the transfer rates rapidly decrease as the temperature
of two streams approach one another
• Hottest cold fluid temperature is less than the coldest hot fluid
temperature

14. Fig- parallel flow heat exchanger

15. Counter Flow Heat Exchangers
• In a counter flow heat exchanger, two streams enter at
opposite ends of a heat exchanger and flow in parallel
but opposite directions
• Temperatures within the two streams tend to approach
in a nearly linearly fashion, resulting in a much more
uniform heating pattern
• In contrast to the co-current flow, the counter flow heat
exchangers can have the hottest cold fluid temperature
greater than the coldest hot fluid temperature

16. fig-counter flow heat exchanger

17. Cross flow heat exchangers
• Cross flow exists when one fluid flows
perpendicular to the second fluid
• In this arrangement one fluid flows through the
tubes and other fluid flows around the tube at 90
angle
• They find application when one of the fluid changes
state e.g. boilers, condensers
˚

18. Fig – cross flow heat exchanger

19. fig-Cross flow heat exchangers
Efficiency: 61%

20. Log Mean Temperature Difference
• Heat flows between the hot and the cold streams due
to temperature difference across the tube acting as a
driving force
• The temperature difference will vary along the
length of the heat exchanger

21. • Fig:
Fig-Temperature difference between hot and cold
process streams in parallel and counter flow

22. Cont..
• The integrate average temperature difference for
either parallel or counter flow can be written as:
• The effective temperature calculated from this
equation is known as log mean temperature
difference
θ1=ΔT2 θ2=ΔT1and Δθ=ΔTlmand

23. Comparison
• Each of the three types of heat exchangers have their
advantages and disadvantages
• Of the three the counter flow heat exchanger is the
most efficient when comparing heat transfer rate
per unit area
• The LMTD Δθ=ΔTlm is greater, than a similar
parallel or cross flow heat exchanger

24. Mathematical explanation
• The following elaboration shows that LMTD counter
flow heat exchanger is greater than LMTD parallel
flow or cross flow heat exchanger

28. • The results demonstrate that given the same
operating conditions, operating the same heat
exchanger in counter flow manner will result in a
greater heat transfer rate than operating in parallel
flow

29. Actually what is done…
• Most large heat exchangers are not purely parallel or
counter or cross flow, rather a mixture of two or three
heat exchangers is used
• The reason for this mixture is to maximize the efficiency
of heat exchangers within the restrictions placed on
design such as size, cost, operating pressure, required
efficiency, type of fluid processed and temperature
• Two fluids are made to pass each other several times
within a single heat exchanger to increase the
performance

30. Cont…
• When the fluids pass each other more than one time it is
called multi-pass heat exchanger
• If the fluids pass each other single time it is called
single-pass heat exchanger
• Commonly the multi pass heat exchangers reverses the
flow several times by the use of “U” tubes
• A second method to achieve multi passes is by the use of
baffles on the shell side of the heat exchangers

32. Regeneration
Heat exchangers are also classified by their function
in a particular system as
1. Regenerative heat exchanger
2. Non-Regenerative heat exchanger

33. Regenerative Heat Exchanger
• A regenerative heat exchanger is the one in which
the same fluid is both the cooling fluid and the
cooled fluid
• i.e. hot fluid gives heat to incoming cool fluid
heating it and itself being cooled
• Usually found in high temperature systems

34. Fig – Regenerative heating

35. Non-Regeneration
• In a non regenerative system, the hot fluid is cooled
by fluid from a separate system
• The energy removed is not returned to the system

36. Effectiveness-NTU Method
• NTU or number of transfer units is used to calculate
the rate of heat transfer in heat exchangers
• NTU method used when there is insufficient
information to calculate the LMTD
• So when the inlet and outlet temperatures are not
specified properly, the NTU method is use

37. The Method
• To define the effectiveness of a heat exchanger, we need
to find the maximum possible heat transfer that can be
hypothetically achieved in a counter flow heat
exchanger of infinite length
• Therefore one fluid will experience the maximum
possible temperature difference , which is
(temperature difference between the inlet temperature
of hot stream and cold stream)
• The method proceeds by calculating the heat capacity
rates( mass flow rate multiplied by specific heat) andСһ Сс

38. Cont..
for the hot and cold fluid, and denoting the smaller
one as
A quantity
=
Is found out where Ɋmax is the maximum heat that
could be transferred between the fluids in unit time
must be used as it is the fluid with the lowest
heat capacity that undergoes max possible
temperature change
Сmin
Ɋmax

39. Cont…
• The other fluid changes temperature more slowly
along the heat exchanger length
• We are concerned only with the fluid undergoing
the maximum temperature change
• The effectiveness(E), is the ratio of heat transfer rate
and maximum possible heat transfer rate
E = Ɋ/ Ɋmax
Where Ɋ= =

40. Cont…
• Effectiveness is a dimensionless quantity between 0
and 1
• If we know E for a particular heat exchanger and
we know the inlet conditions of the two flow
streams, we can calculate the amount of heat being
transferred between the fluids by
Ɋ=E
For any heat exchanger it can be shown that
E=

41. Cont…
• For a given geometry, E can be calculated using
correlations in terms of “ heat capacity ratio”
Heat capacity ratio=
and the NTU
Where U = overall heat transfer rate
A = overall heat transfer area

42. Cont…
• The effectiveness of a parallel flow heat exchanger is
calculated with
E
Or the effectiveness of a counter flow heat exchanger
is calculated with
E

43. Cont…
For heat capacity ratio =1
E=
Similarly effectiveness relationships can be derived for
other heat exchangers also and are differentiated
from one another depending on the flow regime,
number of passes and whether a flow stream is
mixed or unmixed.
1+

44. Cont…
The heat capacity ratio is =0, is a special case of
evaporation or condensation where phase change
takes place in a heat exchanger. Hence in this
special case the heat exchanger behaviour is
independent of flow arrangements. Therefore the
effectiveness is given by
E=

45. Conclusion
• Heat exchangers serve as the backbone of the
industry in which it is used
• Their effectiveness directly relates to the economy of
the organization and the tasks performed with them
• So the arrangements of heat exchangers are very
important
• The proper functioning of the units are necessary as
they save energy, cost & product damage

46. Thank you