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  • Subject area(s): Engineering
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  • Published on: 7th September 2019
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LMTD, Effectiveness, Fouling, NTU, Overall Heat Transfer Coefficient, CFD  

1. INTRODUCTION

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

simultaneously.

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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