LMTD, Effectiveness, Fouling, NTU, Overall Heat Transfer Coefficient, CFD
Due to the modernization and globalization, the energy resources are depleting very rapidly and very
vastly of this world. Thus the price of these precious resources going very high on the international
market. So the need to use energy more efficiently has become a necessity. The recovery of waste
heat from exhaust gases has become essential due to declining energy resources and production cost.
A major result of the energy conversion drive is the development of process recovery aimed at
reducing the amount of waste heat discharged to the environment thus increases the overall efficiency
of various processes and systems. Heat recovery conserves energy, reduces the overall operating costs
and thereby reduces peak loads.
1.1. Background of Study
Heat exchangers are devices in which heat is transferred from one fluid to another fluid. Heat
exchangers are widely used equipment in various industries such as power generation, refrigeration
industry, transportation and process. Heat exchangers are classified on the basis of nature of heat
exchange process, relative direction of fluid motion, design and constructional features and physical
state of fluids.
a) Nature of Heat Exchange Process:
i. Direct Contact Heat Exchanger: - In this type of heat exchanger, heat transfer takes place by
direct mixing of hot and cold fluids. In this process mass exchange also takes place
ii. Indirect Contact Type Heat Exchanger: - In this type of heat exchanger heat is transferred
through transmission by wall which separated two fluids.
b) Relative Direction of Fluid Flow:
i. Parallel Flow Heat Exchanger: - In the parallel flow heat exchanger two fluids travels in same
direction. Both fluids enter at one end and leaves at the other end.
ii. Counter Flow Heat Exchanger: - In the counter flow heat exchanger hot and cold fluids enter
at opposite ends and flow in opposite directions.
iii. Cross Flow Heat Exchanger: - In cross flow two fluids (hot and cold) cross each other in
space usually at right angles.
c) Design and Constructional Features
i. Concentric Tube: - Two concentric pipes are used each carrying one of the fluids. The
direction of flow may be parallel or counter flow. The effectiveness of heat exchangers is
increased by using swirling flow.
ii. Shell and Tube: - In such type of heat exchanger one of the fluids flow through a bundle of
tubes enclosed by a shell. The other fluid is forced through the shell and it flows over the
outer surface of the tubes.
In addition for transferring the heat for obtaining the basic needs there are certain additional
requirements which need to be further specific for the industry in which they are employed, e.g. the
exchanger used in automotive and aviation industry need to be light weighted. These exchangers and
the exchangers which are used in commercial and domestic refrigeration tends to have the same type
of fluid in many applications. The exchangers which are used in chemical process industry are also
have a very wide variety of fluid types with different degree of cleanliness. But with the contrast, the
exchangers used in cryogenic applications handles relatively clean fluids. These and other similar
industry specific requirements have resulted in development of different types of exchanger ranging
from the conventional shell and tube heat exchanger to other tubular and non tubular exchangers of
varying degree of compactness.
Shell and Tube Heat Exchangers: - The basic principle of operation of these exchangers are very
simple and easy, as the two fluids with different temperatures brought into close contact but separated
from mixing by some physical barrier. Then the temperature between the two fluids tends to equalize
by transfer of heat through the tube wall. This principle is similar to the zeroth law of
thermodynamics. The fluids can be either liquids or gases on either the shell or the tube side. In order
to transfer heat efficiently, a large heat transfer area should be used, leading to the use of many tubes.
In this way, waste heat can be put to use. This is an efficient way to conserve energy.
Shell and tube heat exchangers (STHXs) are widely used in many industrial areas, such as power
plants, chemical engineering, petroleum refining, food processing, and etc. A large percentage of
world market for heat exchangers is served by the industry workhouse, the shell and tube heat
exchanger. According to Master B.I. et al. 2006 more than 35-40% of heat exchangers are of the shell
and tube type due to their robust geometry construction, easy maintenance and upgradation. Rugged
and safe construction, availability in a wide range of materials, mechanical reliability in service,
availability of standards for specifications and designs, and long collective operating experience and
familiarity with the designs are some of the reasons for its wide usage in industry. Recent
developments in other exchanger geometries have penetrated in various industry applications. Thus
the shell and tube heat exchanger still remains the industry choice because of their reliability and
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