UNIT - 4

HEAT EXCHANGERS

The device used for exchange of heat between the two fluids that are at different temperatures, is called
the heat exchanger. The heat exchangers are commonly used in wide range of applications, for example, in a car
as radiator, where hot water from the engine is cooled by atmospheric air. In a refrigerator, the hot refrigerant
from the compressor is cooled by natural convection into atmosphere by passing it through finned tubes. In a
steam condenser, the latent heat of condensation is removed by circulating water through the tubes. The heat
exchangers are also used in space heating and air-conditioning, waste heat recovery and chemical processing.
Therefore, the different types of heat exchangers are needed for different applications.

The heat transfer in a heat exchanger usually involves convection on each side of fluids and conduction
through the wall separating the two fluids. Thus for analysis of a heat exchanger, it is very convenient to work
with an overall heat transfer coefficientU, that accounts for the contribution of all these effects on heat transfer.
The rate of heat transfer between two fluids at any location in a heat exchanger depends on the magnitude of
temperature difference at that location and this temperature difference varies along the length of heat exchanger.
Therefore, it is also convenient to work with logarithmic meantemperature difference LMTD,which is an
equivalent temperature difference betweentwo fluids for entire length of heat exchanger.

Classification of heat exchangers:

Heat exchangers are designed in so many sizes, types, configurations and flow arrangements and used for
so many purposes. These are classified according to heat transfer process, flow arrangement and type of
construction.

According to Heat Transfer Process:

(i) Direct contact type.In this type of heat exchanger, the two immiscible fluids atdifferent temperatures
are come in direct contact. For the heat exchange between two fluids, one fluid is sprayed through the
other. Cooling towers, jet condensers, desuperheaters, open feed water heaters and -scrubbers are the
best examples of such heat exchangers. It cannot be used for transferring heat
 between two gases or between two miscible liquids. A direct contact type heat exchanger (cooling
tower) is shown in Figure 6.1.

Figure 6.1: direct contact type heat exchanger (cooling tower)

(ii) Transfer type heat exchangers or recuperators:

In this type of heat exchanger, the cold and hot fluids flow simultaneously through the device and the heat is
transferred through the wall separating them. These types of heat exchangers are most commonly used in
almost all fields of engineering.
(iii) Regenerators or storage type heat exchangers.
In these types of heat exchangers,the hot and cold fluids flow alternatively on the same surface. When hot
fluid flows in an interval of time, it gives its heat to the surface, which stores it in the form of an increase in
its internal energy. This stored energy is transferred to cold fluid as it flows over the surface in next interval
of time. Thus the same surface is subjected to periodic heating and cooling. In many applications, a rotating
disc type matrix is used, the continuous flow of both the hot and cold fluids are maintained. These are
preheaters for steam power plants, blast furnaces, oxygen producers etc. A stationary and rotating matrix
shown in Figure 6.2 are examples of storage type of heat exchangers.
The storage type of heat exchangers is more compact than the transfer type of heat exchangers with more
surface area per unit volume. However, some mixing of hot and cold fluids is always there.
 Figure 6.2: Storage type heat exchangers
According to Constructional Features:
(i) Tubular heat exchanger.These are also called tube in tube or concentric tube or double pipe heat
exchanger as shown in Figure 6.3. These are widely used in many sizes anddifferent flow
arrangements and type.

Figure 6.3: Tubular heat exchanger

(ii) Shell and tube type heat exchanger.

These are also called surface condensers andare most commonly used for heating, cooling, condensation or
evaporation applications. It consists of a shell and a large number of parallel tubes housing in it. The heat transfer
takes place as one fluid flows through the tubes and other fluid flows outside the tubes through the shell. The
baffles are commonly used on the shell to create turbulence and to keep the uniform spacing between the tubes
and thus to enhance the heat transfer rate. They are having large surface area in small volume. A
typical shell and tube type heat exchanger is shown in Figure 6.4.The shell and tube type heat exchangers are
further classified according to number of shell and tube passes involved. A heat exchanger with all tubes make
one U turn in a shell is called one shell pass and two tube pass heat exchanger. Similarly, a heat exchanger that
involves two passes in the shell and four passes in the tubes is called a two shell pass andfour tube pass heat
exchanger as shown in Figure 6.5.

Figure 6.4: Shell and tube type heat exchanger: one shell and one tube pass

Figure 6.5: Multipass flow arrangement in shell and tube type heat exchanger
(iii) Finned tube type. When a high operating pressure or an enhanced heat transfer rateis required, the extended
surfaces are used on one side of the heat exchanger. These heat exchangers are used for liquid to gas heat exchange.
Fins are always added on gas side. The finned tubes are used in gas turbines, automobiles, aero planes,heat pumps,
refrigeration, electronics, cryogenics, air-conditioning systems etc. The radiator of an automobile is an example
of such heat exchanger.
(iv) Compact heat exchanger. These are special class of heat exchangers in which theheat transfer surface area
per unit volume is very large. The ratio of heat transfer surface area to the volume is called area density. A heat
exchanger with an area density greater than 700 m2/m3 is called compact heat exchanger. The compact heat
exchangers are usually cross flow, in which the two fluids usually flow perpendicular to each other. These heat
exchangers have dense arrays of finned tubes or plates, where at least one of the fluid used is gas. For example,
automobile radiators have an area density in order of 1100 m2/m3.
According to Flow Arrangement:
(i) Parallel flow: The hot and cold fluids enter at same end of the heat exchanger, flowthrough in same direction
and leave at other end. It is also called the concurrent heatexchanger Figure 6.6.
(ii) Counter flow: The hot and cold fluids enter at the opposite ends of heat exchangers, flow through in opposite
direction and leave at opposite ends Figure 6.6.

Figure 6.6: Concentric tube heat exchanger

(iii) Cross flow:The two fluids flow at right angle to each other. The cross flow heatexchanger is further classified
as unmixed flow and mixed flow depending on the flow configuration. If both the fluids flow through individual
channels and are not free to move in transverse direction, the arrangement is called unmixed as shown in Figure
6.7a. If any fluid flows on the surface and free to move in transverse direction, then this fluid stream is said to be
mixed as shown in Figure 6.7b.
 Figure 6.7: Different flow configurations in cross-flow heat exchangers.

Fouling factor:

Material deposits on the surfaces of the heat exchangertube may add further resistance to heat transfer in additionto
those listed below. Such deposits are termed foulingand may significantly affect heat exchanger performance.
We know, the surfaces of heat exchangers do not remain clean after it has been in use for some time. The
surfaces become fouled with scaling or deposits. The effect of these deposits affecting the value of overall heat
transfer co-efficient. This effect is taken care of by introducing an additional thermal resistance called the fouling
resistance.
 Scaling is the most common form of fouling and is associated with inverse solubility salts.
Examples of such salts are CaCO3, CaSO4, Ca3(PO4)2, CaSiO3, Ca(OH)2, Mg(OH)2,
MgSiO3, Na2SO4, LiSO4, andLi2CO3.
 Corrosion fouling is classified as a chemical reaction which involves the heat exchanger
tubes. Many metals, copper and aluminum being specific examples, form adherent oxide
coatings which serve to passivate the surface and prevent further corrosion.
 Chemical reaction fouling involves chemical reactions in the process stream which results in
deposition of material on the heat exchanger tubes. When food products are involved this may
be termed scorching but a wide range of organic materials are subject to similar problems.
  Freezing fouling is said to occur when a portion of the hot stream is cooled to near the freezing
point for one of its components. This is most notable in refineries where paraffin frequently
solidifies from petroleum products at various stages in the refining process, obstructing both
flow and heat transfer.
 Biological fouling is common where untreated water is used as a coolant stream. Problems
range from algae or other microbes to barnacles.
Heat Exchanger Analysis:

Log mean temperature difference (LMTD) method for parallel& counter flow heat exchangers
Rearranged,
Effectiveness-NTU method: