Chapter 5: Heat Exchangers

CHAPTER 5: HEAT EXCHANGERS
5.1 INTRODUCTION
A heat exchanger is a device that is used to transfer thermal energy (enthalpy) between two or
more fluids, between a solid surface and a fluid, or between solid particulates and a fluid, at different
temperatures and in thermal contact. In heat exchangers, there are usually no external heat and work
interactions. Typical applications involve heating or cooling of a fluid stream of concern and evaporation
or condensation of single- or multicomponent fluid streams. In other applications, the objective may be
to recover or reject heat, or sterilize, pasteurize, fractionate, distill, concentrate, crystallize, or control a
process fluid.
5.2 CLASSIFICATION OF HEAT EXCHANGERS
In a few heat exchangers, the fluids exchanging heat are in direct contact. In most heat
exchangers, heat transfer between fluids takes place through a separating wall or into and out of a wall
in a transient manner. In many heat exchangers, the fluids are separated by a heat transfer surface, and
ideally they do not mix or leak. Such exchangers are referred to as direct transfer type, or simply
recuperators. In contrast, exchangers in which there is intermittent heat exchange between the hot and
cold fluids—via thermal energy storage and release through the exchanger surface or matrix are
referred to as indirect transfer type, or simply regenerators. Such exchangers usually have fluid leakage
from one fluid stream to the other, due to pressure differences and matrix rotation/valve switching.
Common examples of heat exchangers are shell-and tube exchangers, automobile radiators,
condensers, evaporators, air preheaters, and cooling towers. If no phase change occurs in any of the
fluids in the exchanger, it is sometimes referred to as a sensible heat exchanger. There could be internal
thermal energy sources in the exchangers, such as in electric heaters and nuclear fuel elements.
Combustion and chemical reaction may take place within the exchanger, such as in boilers, fired
heaters, and fluidized-bed exchangers. Mechanical devices may be used in some exchangers such as in
scraped surface exchangers, agitated vessels, and stirred tank reactors. Heat transfer in the separating
wall of a recuperator generally takes place by conduction. However, in a heat pipe heat exchanger, the
heat pipe not only acts as a separating wall, but also facilitates the transfer of heat by condensation,
evaporation, and conduction of the working fluid inside the heat pipe. In general, if the fluids are
immiscible, the separating wall may be eliminated, and the interface between the fluids replaces a heat
transfer surface, as in a direct-contact heat exchanger.
A heat exchanger consists of heat transfer elements such as a core or matrix containing the heat
transfer surface, and fluid distribution elements such as headers, manifolds, tanks, inlet and outlet
nozzles or pipes, or seals. Usually, there are no moving parts in a heat exchanger; however, there are
exceptions, such as a rotary regenerative exchanger (in which the matrix is mechanically driven to rotate
at some design speed) or a scraped surface heat exchanger.
The heat transfer surface is a surface of the exchanger core that is in direct contact with fluids
and through which heat is transferred by conduction. That portion of the surface that is in direct contact
with both the hot and cold fluids and transfers heat between them is referred to as the primary or direct
surface. To increase the heat transfer area, appendages may be intimately connected to the primary
surface to provide an extended, secondary, or indirect surface. These extended surface elements are
referred to as fins. Thus, heat is conducted through the fin and convected (and/or radiated) from the fin
(through the surface area) to the surrounding fluid, or vice versa, depending on whether the fin is being
cooled or heated. As a result, the addition of fins to the primary surface reduces the thermal resistance
on that side and thereby increases the total heat transfer from the surface for the same temperature
difference. Fins may form flow passages for the individual fluids but do not separate the two (or more)
fluids of the exchanger. These secondary surfaces or fins may also be introduced primarily for structural
strength purposes or to provide thorough mixing of a highly viscous liquid.Not only are heat exchangers
often used in the process, power, petroleum, transportation, air-conditioning, refrigeration, cryogenic,
heat recovery, alternative fuel, and manufacturing industries, they also serve as key components of
many industrial products available in the marketplace. These exchangers can be classified in many
different ways. We will classify them according to transfer processes, number of fluids, and heat transfer
mechanisms. Conventional heat exchangers are further classified according to construction type and
flow arrangements. Another arbitrary classification can be made, based on the heat transfer surface
area/volume ratio, into compact and noncompact heat exchangers. This classification is made because
the type of equipment, fields of applications, and design techniques generally differ. Additional ways to
classify heat exchangers are by fluid type (gas–gas, gas–liquid, liquid–liquid, gas two-phase, liquid two-
phase, etc.), industry, and so on, but we do not cover such classifications in this chapter.
5.3 COUNTER-FLOW HEAT EXCHANGER
Counter-flow heat exchanger is used in proposed system as a solution-solution heat exchanger.

In a counterflow or countercurrent exchanger, the two fluids flow parallel to each other but in opposite
directions within the core.{ The temperature variation of the two fluids in such an exchanger may be
idealized as one-dimensional. As shown later, the counterflow arrangement is thermodynamically
superior to any other flow arrangement. It is the most efficient flow arrangement, producing the highest
temperature change in each fluid compared to any other two-fluid flow arrangements for a given overall
thermal conductance (UA), fluid flow rates (actually, fluid heat capacity rates), and fluid inlet
temperatures. Moreover, the maximum temperature difference across the exchanger wall thickness
(between the wall surfaces exposed on the hot and cold fluid sides) either at the hot- or cold-fluid end is
the lowest, and produce minimum thermal stresses in the wall for an equivalent performance compared
to any other flow arrangements. However, with plate-fin heat exchanger surfaces, there are
manufacturing difficulties associated with the true counterflow arrangement. This is because it is
necessary to separate the fluids at each end, and the problem of inlet and outlet header design is
complex. Also, the overriding importance of other design factors causes most commercial heat
exchangers to be designed for flow arrangements different from single-pass counterflow if extremely
high exchanger effectiveness is not required.
Here 𝐶ℎ = (𝑚̇𝑐𝑝 )ℎ is the heat capacity rate of the hot fluid, Cc is the heat capacity rate of the cold fluid,
and specific heats cp are treated as constant. The symbol T is used for temperature; the subscripts h and
c denote hot and cold fluids, and subscripts i and o represent the inlet and outlet of the exchanger.

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CHAPTER 5: HEAT EXCHANGERS

5.2 CLASSIFICATION OF HEAT EXCHANGERS

5.3 COUNTER-FLOW HEAT EXCHANGER

Counter-flow heat exchanger is used in proposed system as a solution-solution heat exchanger.

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