Classification of Heat Exchangers

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 di fferent temperatures and in thermal contact. In heat exchangers, there are
usually no external heat and work interactions. In a few heat exchangers, fluids the
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, thefluids 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.
In general, if the fluids are immiscible, the separating wall may be eliminated, and the
interface between thefluids replaces a heat transfer surface, as in a direct -contact 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 coldfluids 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 fins. to asThus, heat is
conducted through the fin and convicted (and/or radiated) from thefin (through the surface
area) to the surrounding fluid, or vice versa, depe nding on whether the fin is being cooled or
heated. These exchangers can be classi fied in many di fferent ways. We will classify them
according to transfer processes, number fluids,
of 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. All these Classification are summarized in Fig. 1.1
1.2 CLASSIFICATION ACCORDING TO TRANSFER PROCESSES
Heat exchangers are classified according to transfer processes into indirect -and direct-
contact types.
1.2.1 Indirect-Contact Heat Exchangers: In an indirect-contact heat exchanger, thefluid
streams remain separate and the heat transfers continuously through an impervious
dividing wall or into and out of a wall in a transient manner. Thus, ideally, there is no
direct contact between thermally interactingfluids. This type of heat exchanger, also
fied into direct -transfer
referred to as a surface heat exchanger, can be further classi
type, storage type, and fluidized-bed exchangers.
 FIGURE 1.3 Fluidized-bed heat exchanger
1.2.2 Direct-Contact Heat Exchangers
In a direct-contact exchanger, twofluid streams come into direct contact, exchange
heat, and are then separated. Common applications of a direct-contact exchanger
involve mass transfer in addition to heat transfer, such as in evaporative cooling and
rectification; applications involving only sensible heat transfer are rare. The enthalpy
of phase change in such an exchanger generally represents a significant portion of the
total energy transfer. The phase change generally enhances the heat transfer rate.
Compared to indirect-contact recuperators and regenerators, in direct-contact heat
exchangers, (1) very high heat transfer rates are achievable, (2) the exchanger
construction is relatively inexpensive, and (3) the fouling problem is generally
nonexistent, due to the absence of a heat transfer surface (wall) between the two
fluids. However, the applications are limited to those cases where a direct contact of
two fluid streams is permissible. The design theory for these exchangers is beyond the
scope of this book and is not covered. These exchangers may be further classi fied as
follows
1.2.2.1 Immiscible Fluid Exchangers. In this type, two immiscible fluid streams are brought
into direct contact. Thesefluids may be single -phase fluids, or they may involve
condensation or vaporization. Condensation of organic vapors and oil vapors with
water or air are typical examples.

1.2.2.2 Gas–Liquid Exchangers. In this type, onefluid is a gas (more commonly, air) and
the other a low-pressure liquid (more commonly, water) and are readily separable
after the energy exchange. In either cooling of liquid (water) or humidification of gas
(air) applications, liquid partially evaporates and the vapor is carried away with the
gas. In these exchangers, more than 90% of the energy transfer is by virtue of mass
transfer (due to the evaporation of the liquid), and convective heat transfer is a minor
mechanism. A ‘‘wet’’ (water) cooling tower with forced-or natural-draft airflow is the
most common application. Other applications are the air-conditioning spray chamber,
spray drier, spray tower, and spray pond.
1.2.2.3 Liquid–Vapor Exchangers. In this type, typically steam is partially or fully
condensed using cooling water, or water is heated with waste steam through direct
contact in the exchanger. Noncondensables and residual steam and hot water are the
outlet streams. Common examples are desuperheaters and open feedwater heaters
(also known as deaeraters) in power plants.
1.3 CLASSIFICATION ACCORDING TO NUMBER OF FLUIDS
Most processes of heating, cooling, heat recovery, and heat rejection involve transfer
of heat between twofluids. Hence, two -fluid heat exchangers are the most common.
Three-fluid heat exchangers are widely used in cryogenics and some chemical
processes (e.g., air separation systems, a helium–air separation unit, purification and
liquefaction of hydrogen, ammonia gas synthesis). Heat exchangers with as many as
12 fluid streams have been used in some chemical process applications. The design
theory of three-and multifluid heat exchangers is alg ebraically very complex and is
not covered in this book. Exclusively, only the design theory for two-fluid exchangers
and some associated problems are presented in this book.
1.4 CLASSIFICATION ACCORDING TO SURFACE COMPACTNESS
Compared to shell-and-tube exchangers, compact heat exchangers are characterized
by a large heat transfer surface area per unit volume of the exchanger, resulting in
reduced space, weight, support structure and footprint, energy requirements and cost,
as well as improved process design and plant layout and processing conditions,
together with low fluid inventory.
A gas-to-fluid exchanger is referred to as a compact heat exchanger if it incorporates a
2 3
heat transfer surface having a surface area density greater than about 700 m /m or a
2 3
hydraulic diameter Dh < 6mm-1in. for operating in a gas stream and 400 m /m or
higher for operating in a liquid or phase-change stream. A laminar flow heat
exchanger (also referred to as a meso heat exchanger) has a surface area
2 3
density greater than about 3000 m /m or 100 mm < Dh < 1 mm. The term micro
heat exchanger is used if the surface area density is greater than about 15,000
2 3
m /m or1 mm < Dh < 100 mm. A liquid/two-phase fluid heat exchanger is
referred to as a compact heat exchanger if the surface area density on any one
2 3
fluid side is greater than about 400 m /m . In contrast, a typical process industry
2 3
shell and-tube exchanger has a surface area density of less than 100 m /m on one
fluid side with plain tubes, and two to three times greater than that with high-fin-
density low-finned tubing. A typical plate heat exchanger has about twice the average
heat transfer coefficient h on one fluid side or the average overall heat transfer
coefficient U than that for a shelland-tube exchanger for water/water applications.
1.5 CLASSIFICATION ACCORDING TO CONSTRUCTION FEATURES
Heat exchangers are frequently characterized by construction features. Four major
construction types are tubular, plate-type, extended surface, and regenerative
exchangers. Heat exchangers with other constructions are also available, such as
scraped surface exchanger, tank heater, cooler cartridge exchanger, and others
(Walker, 1990). Some of these may be classified as tubular exchangers, but they have
some unique features compared to conventional tubular exchangers. Since the
applications of these exchangers
1.5.1 Tubular Heat Exchangers
These exchangers are generally built of circular tubes, although elliptical, rectangular,
or round/flat twisted tubes have also been used in some applications. There is
considerable flexibility in the design because the core geometry can be varied easily
by changing the tube diameter, length, and arrangement. Tubular exchangers can
be designed for high pressures relative to the environment and high-pressure
differences between thefluids. Tubular exchangers are used primarily for liquid -to-
liquid and liquid-to-phase change (condensing or evaporating) heat transfer
applications. They are used for gas-to-liquid and gas-to-gas heat transfer applications
primarily when the operating temperature and/ or pressure is very high or fouling is a
severe problem on at least one fluid side and no other types of exchangers would
work. These exchangers may be classi fie d as shell-andtube, double-pipe, and spiral
tube exchangers. They are all prime surface exchangers except for exchangers having
fins outside/inside tubes.
1.5.1.1 Shell-and-Tube Exchangers. This exchanger is generally built of a bundle of round
tubes mounted in a cylindrical shell with the tube axis parallel to that of the shell. One
fluid flows inside the tubes, the other flows across and along the tubes. The major
components of this exchanger are tubes (or tube bundle), shell, front-end head, rear-
end head, baffles, and tubesheets, and are described in many books of Heat Transfer
A variety of different internal constructions are used in shell-and-tube exchangers,
depending on the desired heat transfer and pressure drop performance and the
methods employed to reduce thermal stresses, to prevent leakages, to provide for ease
of cleaning, to contain operating pressures and temperatures, to control corrosion, to
accommodate highly asymmetricflows, and so on. Shell -and-tube exchangers are
classified and constructed in accordance with the widely used TEMA (Tubular
Exchanger Manufacturers Association) standards (TEMA, 77/1999), HE-I81, DIN
and other standards in Europe and elsewhere, and ASME (American Society of
Mechanical Engineers) boiler and pressure vessel codes. TEMA has developed a
notation system to designate major types of shell-and-tube exchangers. In this
system, each exchanger is designated by a three-letter combination, the first
letter indicating the front-end head type, the second the shell type, and the third the
rear-end head type. Some common shell-and-tube exchangers are AES, BEM,
AEP, CFU, AKT, and AJW. It should be emphasized that there are other special
types of shell-and-tube exchangers commercially available that have front-and
rear-end heads Those exchangers may not be identi fiable by the TEMA letter
designation.
The three most common types of shell-and-tube exchangers are
 fixed tubesheet design
 U-tube design, and
 floating-head type.
 In all three types, the front-end head is stationary while the rear-end head can be
either stationary orfloating depending on the thermal stresses in the shell, tube, or
tubesheet, due to temperature differences as a result of heat transfer.
The exchangers are built in accordance with three mechanical standards that specify
design, fabrication, and materials of unfired shell-and-tube heat exchangers. Class R is
for the generally severe requirements of petroleum and related processing
applications. Class C is for generally moderate requirements for commercial and
general process applications. Class B is for chemical process service. The exchangers
are built to comply with the applicable ASME Boiler and Pressure Vessel Code,
Section VIII, IS 2825 ,IS: 4506 and other pertinent codes and/or standards. The
TEMA standards supplement andfine de the ASME code for heat exchanger
applications. In addition, state and local codes applicable to the plant location
must also be met.
The TEMA standards specify the manufacturing tolerances for various mechanical
classes, the range of tube sizes and pitches, baffling and support plates, pressure
classification, tubesheet thickness formulas, and so on, and must be consulted for all
these details. In this book, we consider only the TEMA standards where appropriate,
but there are other standards, such as DIN 28 008.
Tubular exchangers are widely used in industry for the following reasons. They are
custom designed for virtually any capacity and operating conditions, such as from
high vacuum to ultrahigh pressure [over 100 MPa (15,000 psig)], from cryogenics to
high temperatures [about 11008C (20008F)] and any temperature and pressure
differences between the fluids, limited only by the materials of construction. They can
be designed for special operating conditions: vibration, heavy fouling, highly viscous
fluids, erosion, corrosion, toxicity, radioactivity, multi-component mixtures, and so
on. They are the most versatile exchangers, made from a variety of metal and non-
metal materials (such as graphite, glass, and Teflon ) and range in size from small 0.1
2
m to supergiant
2.1.1 Direct-Transfer Type Exchangers. In this type, heat transfers continuously from the
hot fluid to the cold fluid through a dividing wall. Some examples of direct-transfer
type heat exchangers are tubular, plate-type, and extended surface exchangers
Heat exchangers can also be classified according to the process function, as outlined in Fig.
1.2. Additional ways to classify heat exchangers are by fluid type (gas –gas, gas–liquid,
liquid–liquid, gas two-phase, liquid two-phase, etc.)

FIGURE 1.2 (a) Classification according to process function; (b) classification of condensers;
(c) classification of liquid-to-vapor phase-change exchangers
FIGURE 1.2 (d) classi fication of chemical evaporators according to (i) the type of
construction, and (ii) how energy is (e) classification of reboilers