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HEAT TRANSFER
(MEng 3121)
Debre Markos University
Mechanical Engineering
Department
Prepared and Presented by:
Tariku Negash Demissie
Sustainable Energy Engineering (MSc)
E-mail: thismuch2015@gmail.com/ tariku_negash@dmu.edu.et
Lecturer at School Mechanical and Industrial Engineering, Institute of Technology,
Debre Markos University, Debre Markos, Ethiopia
Jan, 2020
HEAT EXCHANGER
Chapter 7
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 Recognize numerous types of heat exchangers, and classify them.
 Develop an awareness of fouling on surfaces, and determine the
overall heat transfer coefficient for a heat exchanger.
 Perform a general energy analysis on heat exchangers.
 Obtain a relation for the logarithmic mean temperature difference for
use in the LMTD method, and modify it for different types of heat
exchangers using the correction factor.
 Develop relations for effectiveness, and analyze heat exchangers
when outlet temperatures are not known using the effectiveness-NTU
method.
 Know the primary considerations in the selection of heat exchangers.
Objectives
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7.1 Introduction of heat exchanger
 The process of heat exchange between two fluids that are at different
temperatures and separated by a solid wall occurs in many engineering
applications.
 The device used to implement this exchange is termed a heat exchanger, and
specific applications may be found in space heating and air- conditioning,
power production, waste heat recovery, and chemical processing.
 For example, in a car radiator, heat is transferred from the hot water flowing
through the radiator tubes to the air flowing through the closely spaced thin
plates outside attached to the tubes.
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7.2 Types of heat exchanger
 Different heat transfer applications require different types of hardware
and different configurations of heat transfer equipment.
 The attempt to match the heat transfer hardware to the heat transfer
requirements within the specified constraints has resulted in numerous
types of innovative heat exchanger designs.
 Therefore, heat exchangers are typically classified according to flow
arrangement and type of construction.
A. Double-pipe (Concentric tube) heat exchanger.
The simplest heat exchanger is one for which the hot and cold fluids move
in the same (parallel flow) or opposite (counter flow) directions in a
concentric tube (or double-pipe) construction as shown on a fig a and b
respectively.
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B. Compact heat exchanger.
 Which is specifically designed to realize a large heat transfer surface
area per unit volume, is called area density 𝜷.
 A heat exchanger with 𝜷 > 𝟕𝟎𝟎 𝒎 𝟐/𝒎 𝟑 is classified as being
compact heat exchanger.
 Examples of compact heat exchangers are
Car radiators (𝜷 ≈ 𝟏𝟎𝟎𝟎 𝒎 𝟐
/𝒎 𝟑
),
Glass ceramic gas turbine heat exchangers (𝜷 ≈ 𝟔𝟎𝟎𝟎 𝒎 𝟐
/𝒎 𝟑
),
Regenerator of a Stirling engine (𝜷 ≈ 𝟏𝟓, 𝟎𝟎𝟎 𝒎 𝟐
/𝒎 𝟑
), and
Human lung (𝜷 ≈ 𝟐𝟎, 𝟎𝟎𝟎 𝒎 𝟐
/𝒎 𝟑
).
 Compact heat exchangers enable us to achieve high heat transfer rates
between two fluids in a small volume, and they are commonly used
in applications with strict limitations on the weight and volume of
heat exchangers
7
For example a gas-to-liquid compact heat
exchanger for a residential air-conditioning
system.
Cross-flow: In compact heat exchangers, the
two fluids usually move perpendicular to each
other. The cross-flow is further classified as
unmixed and mixed flow.
Different flow configurations in
cross-flow heat exchangers.
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 Shell-and-tube heat exchanger: The most common type of heat
exchanger in industrial applications.
C. Shell-and-tube heat exchanger
 Heat transfer takes place as one fluid flows inside the tubes while the
other fluid flows outside the tubes through the shell.
 Baffles are commonly placed in the shell to force the shell-side fluid to
flow across the shell to enhance heat transfer and to maintain uniform
spacing between the tubes.
 They contain a large number of tubes (sometimes several hundred)
packed in a shell with their axes parallel to that of the shell.
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 Heat exchangers in which all the tubes
make one U-turn in the shell, for
example, are called one-shell-pass and
two tube-passes heat exchangers.
 Likewise, a heat exchanger that involves
two passes in the shell and four passes in
the tubes is called a two-shell-passes and
four-tube-passes exchanger
Multi-pass flow arrangements in shell- and-tube heat
exchangers.
 Shell-and-tube heat exchangers are further classified according to the
number of shell and tube passes involved.
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 A plate heat exchanger, PHE, is a compact heat exchanger where thin
corrugated plates (some 0.5 mm thick, bended 1 or 2 mm) are stacked in
contact with each other, and the two fluids made to flow separately along
adjacent channels in the corrugation (Fig.).
D. Plate heat exchanger
 The closure of the staked plates may be by clamped gaskets, brazing
(usually copper-brazed stainless steel), or welding (stainless steel, copper,
titanium), the most common type being the first, for ease of inspection and
cleaning.
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Additionally, a frame (end-plates and fixing rods) secures together the plate stack
and connectors (sometimes PFHE, standing for plate-and-frame heat exchanger,
is used instead of PHE).
 The PHE was developed in the 1920s in the food industry (for the
pasteurization of milk), but they are taking over all markets now because of
its compactness and efficiency (3 to 10 times more than STHE).
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Flow patterns
A Parallel Flow B. Counter Flow
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Cold and hot flow fluid through Plate heat exchanger
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Plateheatexchanger
17
 Regenerative heat exchanger: or more commonly a regenerator, is a type of
heat exchanger where heat from the hot fluid is intermittently stored in a thermal
storage medium before it is transferred to the cold fluid. To accomplish this the
hot fluid is brought into contact with the heat storage medium, then the fluid is
displaced with the cold fluid, which absorbs the heat. It involves the alternate
passage of the hot and cold fluid streams through the same flow area.
 Dynamic-type regenerator: involves a rotating drum and continuous flow of the
hot and cold fluid through different portions of the drum so that any portion of
the drum passes periodically through the hot stream, storing heat, and then
through the cold stream, rejecting this stored heat.
 Condenser: One of the fluids is cooled and condenses as it flows through the
heat exchanger.
 Boiler: One of the fluids absorbs heat and vaporizes.
 A space radiator is a heat exchanger that transfers heat from the hot fluid to the
surrounding space by radiation.
D. Others Types of Heat Exchanger
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Classification of heat exchangers depending on their applications.
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7.3.1 The Overall Heat Transfer Coefficient
7.3 Design consideration in Heat Exchanger
• A heat exchanger typically involves two
flowing fluids separated by a solid wall.
• Heat is first transferred from the hot fluid to
the wall by convection, through the wall by
conduction, and from the wall to the cold
fluid again by convection.
• Any radiation effects are usually included in
the convection heat transfer coefficients.
Double-pipe heat exchanger.
(1)
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U is the overall heat transfer coefficient, W/m2C.
When
 The overall heat transfer coefficient U is dominated by the smaller convection
coefficient. When one of the convection coefficients is much smaller than the
other (say, hi << ho), we have 1/hi >> 1/ho, and thus U  hi and vise verse
 This situation arises frequently when one of the fluids is a gas and the other is a
liquid. In such cases, fins are commonly used on the gas side to enhance the
product UA and thus the heat transfer on that side.
(2)
(3)
21
The overall heat transfer coefficient ranges
from about 10 W/m2C for gas-to-gas heat
exchangers to about 10,000 W/m2C for heat
exchangers that involve phase changes.
For short fins of high
thermal conductivity, we
can use this total area in the
convection resistance
relation Rconv = 1/hAs,
since the fins in this case
will be very nearly
isothermal. Otherwise, we
should determine the
effective surface area As
from
When the tube is finned on one
side to enhance heat transfer, the
total heat transfer surface area on
the finned side is
(4)
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7.3.2 Fouling Factor
 The performance of heat exchangers usually deteriorates with time as a result of
accumulation of deposits on heat transfer surfaces.
 The layer of deposits represents additional resistance to heat transfer. This is
represented by a fouling factor Rf.
 The fouling factor increases with the operating
temperature and the length of service and
decreases with the velocity of the fluids.
(5)
𝑇𝑎𝑏𝑙𝑒 ≈ 10−4
𝑚2
℃/𝑊
which is equivalent to the
thermal resistance of a 0.2-
mm-thick limestone layer (k
= 2.9 W/m ·°C) per unit
surface area.
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•Chemical fouling: when chemical changes within the fluid cause a fouling
layer to be deposited onto the tube surface. A common example of this
phenomenon is scaling in a kettle or boiler caused by “hardness” salts depositing
onto the heating elements as the solubility of the salts reduce with increasing
temperature. This is outside the control of the heat exchanger designer but can be
minimized by careful control of the tube wall temperature in contact with the
fluid. When this type of fouling occurs it must be removed by either chemical
treatment or mechanical descaling processes (wire brushes or even drills to
remove the scale or sometimes high-pressure water jets).
•Biological fouling: this is caused by the growth of organisms within the fluid
which deposit out onto the surfaces of the heat exchanger. Again this is outside
the direct control of the heat exchanger designer, but it can be influenced by the
choice of materials as some, notably the non-ferrous brasses, are poisonous to
some organisms. When this type of fouling occurs it is normally removed by
either chemical treatment or mechanical brushing processes.
Common Types of Fouling
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•Deposition fouling: this is when particles contained within the fluid settle out
onto the surface when the fluid velocity falls below a critical level. To a large
extent, this is within the control of the heat exchanger designer, as the critical
velocity for any fluid/particle combination can be calculated to allow a design to
be developed with minimum velocity levels higher than the critical level.
Mounting the heat exchanger vertically can also minimize the effect as gravity
would tend to pull the particles out of the heat exchanger away from the heat
transfer surface even at low velocity levels. When this type of fouling occurs it is
normally removed by mechanical brushing processes.
•Corrosion fouling: this is when a layer of corrosion products build up on the
surfaces of the tube forming an extra layer of, usually, high thermal resistance
material. By careful choice of materials of construction the effects can be
minimized as a wide range of corrosion resistant materials based on stainless steel
and other nickel-based alloys are now available to the heat exchanger
manufacturer.
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Example 7.1
Water at an average temperature of 107°C and an average velocity of 3.5
m/s flows through a 5-m-long stainless steel tube (k =14.2 W/m ·°C) in a
boiler. The inner and outer diameters of the tube are Di = 1.0 cm and Do =
1.4 cm, respectively. If the convection heat transfer coefficient at the outer
surface of the tube where boiling is taking place is h = 8400 W/𝑚2
°C,
determine the overall heat transfer coefficient 𝑈𝑖 of this boiler based on the
inner surface area of the tube when,
a) Negligible fouling factor and
b) Assume fouling factor 𝑅𝑓,𝑖 = 0.0005 m𝑚2
℃/𝑊 on the inner surface
of the tube.
Assumptions
1 Water flow is fully developed.
2 Properties of the water are constant.
3 The heat transfer coefficient and the fouling factor are constant and
uniform.
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7.4 Analysis of Heat Exchangers
An engineer often finds himself or herself in a position
1. to select a heat exchanger that will achieve a specified temperature change in
a fluid stream of known mass flow rate – the log mean temperature difference
(or LMTD) method.
2. to predict the outlet temperatures of the hot and cold fluid streams in a
specified heat exchanger – the effectiveness-NTU method.
1st law of thermodynamics rate of heat transfer from the hot fluid be equal to the
rate of heat transfer to the cold one. (HE is insulated):
Two fluid streams that have the same capacity rates experience the same temperature
change in a well-insulated heat exchanger.
(6)
(7)
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Heat capacity rate 𝑚 is the rate of evaporation or
condensation of the fluid,
.
• Note that the only time the temperature rise of a
cold fluid is equal to the temperature drop of the
hot fluid is when the heat capacity rates of the
two fluids are Equal to each other
 Two special types of heat exchangers
commonly used in practice are condensers and
boilers.
 One of the fluids in a condenser or a boiler
undergoes a phase-change process, and the rate
of heat transfer is expressed as
𝑚 is the rate of evaporation or condensation of the fluid,
hfg is the enthalpy of vaporization of the fluid at the
specified temperature or pressure.
(8)
(9)
28
Tm is an appropriate mean (average) temperature difference between the
two fluids.
The heat capacity rate
of a fluid during a
phase-change process
must approach infinity
since the temperature
change is practically
zero.
An ordinary fluid absorbs or releases a large amount of heat essentially at
constant temperature during a phase-change process, as shown on the fig a & b
The rate of heat transfer in a heat exchanger
(10)
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7.5 The Log Mean Temperature Difference Method (LMTD)
Variation of the fluid
temperatures in a parallel-
flow double-pipe heat
exchanger.
Assuming the outer surface of the heat
exchanger to be well insulated so that any heat
transfer occurs between the two fluids, and
disregarding any KE and PE
An energy balance
(11)
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Integrating from the inlet of the heat exchanger to its outlet, we obtain
(12)
Finally, solving Eqs. 6 and 7 for and
(13)where
Log mean temperature difference which is the
suitable form of the average temperature
difference for use in the analysis of heat
exchangers.
and substituting into Eq. (12)
Arithmetic mean temperature difference:
(14)
(15)
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 The logarithmic mean temperature difference
Tlm is an exact representation of the average
temperature difference between the hot and
cold fluids.
 Note that Tlm is always less than Tam.
Therefore, using Tam in calculations instead
of Tlm will overestimate the rate of heat
transfer in a heat exchanger between the two
fluids.
 When T1 differs from T2 by no more than
40 percent, the error in using the arithmetic
mean temperature difference is less than 1
percent.
 But the error increases to undesirable levels
when T1 differs from T2 by greater
amounts.
Cont.
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 For specified inlet and outlet temperatures, the Tlm for a counter-flow
heat exchanger is always greater than that for a parallel-flow heat
exchanger.
 That is, Tlm, CF > Tlm, PF, and thus a smaller surface area (and thus a
smaller heat exchanger) is needed to achieve a specified heat transfer rate
in a counter-flow heat exchanger.
 A condenser or a boiler can be considered to be either a parallel- or
counter- flow heat exchanger since both approaches give the same
result
7.6 Counter-Flow Heat Exchangers
 In the limiting case, the cold fluid will be heated to the inlet temperature of the
hot fluid. However, the outlet temperature of the cold fluid can never exceed
the inlet temperature of the hot fluid.
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7.7 Multi-Pass and Cross-Flow Heat Exchangers:
Use of a Correction Factor
 The log mean temperature difference ∆𝑻𝒍𝒎
relation developed earlier is limited to
parallel-flow and counter-flow heat
exchangers only.
 Similar relations are also developed for cross-
flow and multi pass shell-and-tube heat
exchangers, but the resulting expressions are
too complicated because of the complex flow
conditions.
 The correction factor F depends on the geometry of the heat exchanger and the
inlet and outlet temperatures of the hot and cold fluid streams.
 ∆𝑻𝒍𝒎.𝑪𝑭 is the log mean temperature difference for the case of a counter-flow
heat exchanger with the same inlet and outlet temperatures and is determined
from Eq. (14).
(16)
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F for common shell-and-tube heat and cross-flow exchanger configurations is
given in Fig. versus two temperature ratios P and R defined as
1 and 2: inlet and outlet
T and t: shell- and tube-side
temperature . respectively
F = 1 for a condenser or boiler
HOW?
(17)
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Example 7.2
Ashell-and-tube heat exchanger with 2-shell passes and 8-tube passes is used to
heat ethyl alcohol (𝐶 𝑝 = 2670 J/kg · °C) in the tubes from 25°C to 70°C at a
rate of 2.1 kg/s. The heating is to be done by water (𝐶 𝑝= 4190 J/kg ·°C) that
enters the shell side at 95°C and leaves at 45°C. If the overall heat transfer
coefficient is 950 W/m · °C, determine the heat transfer surface area of the heat
exchanger.
Assumptions
1 Steady operating conditions exist.
2 The heat exchanger is well-insulated so
that heat loss to the surroundings is
negligible and thus heat transfer from the hot
fluid is equal to the heat transfer to the cold
fluid.
3 Changes in the kinetic and potential
energies of fluid streams are negligible.
4 There is no fouling.
5 Fluid properties are constant
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7.8 The Effectiveness–NTU Method
The LMTD method is very suitable for determining the size of a heat exchanger to
realize prescribed outlet temperatures when the mass flow rates and the inlet and outlet
temperatures of the hot and cold fluids are specified.
With the LMTD method, the task is to select a heat exchanger that will meet the
prescribed heat transfer requirements. The procedure to be followed by the selection
process is:
1. Select the type of heat exchanger suitable for the application.
2. Determine any unknown inlet or outlet temperature and the heat transfer rate using
an energy balance.
3. Calculate the log mean temperature difference Tlm and the correction factor F, if
necessary.
4.Obtain (select or calculate) the value of the overall heat transfer coefficient U.
5.Calculate the heat transfer surface area As.
The task is completed by selecting a heat exchanger that has a heat transfer surface
area equal to or larger than As.
38
A second kind of problem encountered in heat exchanger analysis is the determination of
the heat transfer rate and the outlet temperatures of the hot and cold fluids for prescribed
fluid mass flow rates and inlet temperatures when the type and size of the heat exchanger
are specified.
Heat transfer effectiveness 𝜺
The maximum possible heat transfer rate
Cmin is the smaller of Ch and Cc .
Kays and London came up with a method in 1955 called the effectiveness–NTU
method, which greatly simplified heat exchanger analysis.
(18)
(19)
(20)
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Actual heat transfer rate
 Therefore, the effectiveness of a heat exchanger enables us to determine the
heat transfer rate without knowing the outlet temperatures of the fluids
 The effectiveness of a heat exchanger depends on the geometry of the heat
exchanger as well as the flow arrangement.
 Therefore, different types of heat exchangers have different effectiveness
relations.
 Below we illustrate the development of the effectiveness 𝜺 relation for the
double-pipe parallel-flow heat exchanger.
(21)
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(12)
Equation (12) for a parallel-flow heat exchanger can be rearranged as
Also, solving Eq. (19) for 𝑻 𝒉,𝒐𝒖𝒕
(22)
Substituting this relation into Eq. (22) after adding and subtracting 𝑻 𝒄,𝒊𝒏 gives
which simplifies to
(23)
(24)
(25)
(29)
(30)
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We now manipulate the definition of effectiveness to obtain
Substituting this result into Eq. (25) and solving for gives the following relation
for 𝜺 the effectiveness of a parallel-flow heat exchanger:
Taking either 𝐶𝑐 𝑜𝑟 𝐶ℎ to be 𝐶 𝑚 (both
approaches give the same result), the
relation above can be expressed more
conveniently as for Cmin=Cc
(26)
(27)
(28)
42
Effectiveness relations of the heat exchangers typically involve the dimensionless
group UAs /Cmin.
This quantity is called the number of transfer units NTU.
For specified values of U and Cmin, the value of
NTU is a measure of the surface area As. Thus,
the larger the NTU, the larger the heat exchanger.
Capacity ratio
It can be shown that effectiveness of a heat exchanger is a function of the number
of transfer units NTU and the capacity ratio c.
7.8.1 Numbers of transfer unit: NTU
In heat exchanger analysis, it is also convenient to define another dimensionless
quantity called the capacity ratio c as
43
 Effectiveness relations have been developed for a large number of heat
exchangers, and the results are given in Table.
 The effectivenesses of some common types of heat exchangers are also
plotted in Figure.
44
 When all the inlet and outlet temperatures are specified, the size of the heat exchanger
can easily be determined using the LMTD method.
 Alternatively, it can be determined from the effectiveness–NTU method by first
evaluating the effectiveness ε from its definition and then the NTU from the
appropriate NTU relation.
45
(e.g., boiler, condenser)
46
 The value of the effectiveness ranges from 0 to 1. It increases rapidly with
NTU for small values (up to about NTU = 1.5) but rather slowly for larger
values. Therefore, the use of a heat exchanger with a large NTU (usually larger
than 3) and thus a large size cannot be justified economically, since a large
increase in NTU in this case corresponds to a small increase in effectiveness.
 For a given NTU and capacity ratio c = Cmin /Cmax, the counter-flow heat
exchanger has the highest effectiveness, followed closely by the cross-flow
heat exchangers with both fluids unmixed. The lowest effectiveness values are
encountered in parallel-flow heat exchangers.
 The effectiveness of a heat exchanger is independent of the capacity ratio c for
NTU values of less than about 0.3. (NTU<0.3)
 The value of the capacity ratio c ranges between 0 and 1. For a given NTU, the
effectiveness becomes a maximum for c = 0 (e.g., boiler, condenser) and a
minimum for c = 1 (when the heat capacity rates of the two fluids are equal).
Observations from the effectiveness relations and charts
47
7.9 Selection of Heat Exchangers
 The uncertainty in the predicted value of U can exceed 30 percent. Thus, it is
natural to tend to overdesign the heat exchangers.
 Heat transfer enhancement in heat exchangers is usually accompanied by
increased pressure drop, and thus higher pumping power.
 Therefore, any gain from the enhancement in heat transfer should be weighed
against the cost of the accompanying pressure drop.
 Usually, the more viscous fluid is more suitable for the shell side (larger passage
area and thus lower pressure drop) and the fluid with the higher pressure for the
tube side.
 The proper selection of a heat exchanger
depends on several factors:
•Heat transfer rate
•Cost
•Pumping power
•Size and weight
•Type and Materials
Annual cost of electricity associated with the
operation of the pumps and fans:
The rate of heat transfer in the
prospective heat exchanger is
48
49
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A shell-and-tube heat exchanger with 1-shell pass and 14-tube passes is used to
heat water in the tubes with geothermal steam condensing at 120ºC (ℎ 𝑓𝑔 =
2203 kJ/kg) on the shell side. The tubes are thin-walled and have a diameter of
2.4 cm and length of 3.2 m per pass. Water (𝐶 𝑝 = 4180 J/kg·ºC) enters the
tubes at 22ºC at a rate of 3.9 kg/s. If the temperature difference between the two
fluids at the exit is 46ºC, determine
(a) the rate of heat transfer,
(b) the rate of condensation of steam, and
(c) the overall heat transfer coefficient.
Example 7.4
Assumptions
1 Steady operating conditions exist.
2 The heat exchanger is well-insulated so that heat
loss to the surroundings is negligible and thus heat
transfer from the hot fluid is equal to the heat transfer
to the cold fluid.
3 Changes in the kinetic and potential energies of fluid
streams are negligible. 4 Fluid properties
are constant.
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Example 7.5
Hot oil (𝐶 𝑝 = 2200 𝐽/𝐾𝑔℃) is to be cooled by water (Cp = 4180 J/kg · °C) in a
2-shell-pass and 12-tube-pass heat exchanger. The tubes are thin-walled and are
made of copper with a diameter of 1.8 cm. The length of each tube pass in the
heat exchanger is 3 m, and the overall heat transfer coefficient is 340 W/m · °C.
Water flows through the tubes at a total rate of 0.1 kg/s, and the oil through the
shell at a rate of 0.2 kg/s. The water and the oil enter at temperatures 18°C and
160°C, respectively. Determine,
a) the rate of heat transfer in the heat exchanger and,
b) the outlet temperatures of the water and the oil.
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Consider a water-to-water counter-flow heat exchanger with these specifications.
Hot water enters at 95ºC while cold water enters at 20ºC. The exit temperature of
hot water is 15ºC greater than that of cold water, and the mass flow rate of hot
water is 50 percent greater than that of cold water. The product of heat transfer
surface area and the overall heat transfer coefficient is 1400 W/𝑚2· ºC. Taking
the specific heat of both cold and hot water to be Cp= 4180 J/kg · ºC, determine
(a) the outlet temperature of the cold water,
(b) the effectiveness of the heat exchanger,
(c) the mass flow rate of the cold water, and
(d) the heat transfer rate.
Example 7.6
Assumptions
1 Steady operating conditions exist.
2 The heat exchanger is well-insulated so that heat
loss to the surroundings is negligible and thus heat
transfer from the hot fluid is equal to the heat transfer
to the cold fluid.
3 Changes in the kinetic and potential energies of fluid streams are negligible. 4 The
overall heat transfer coefficient is constant and uniform.
53
Summary
 Types of Heat Exchangers
 The Overall Heat Transfer Coefficient
 Fouling Factor
 Analysis of Heat Exchangers
 The Log Mean Temperature Difference
Method
 Counter-Flow Heat Exchangers
 Multipass and Cross-Flow Heat Exchangers: Use of
a Correction Factor
 The Effectiveness-NTU Method
 Selection of Heat Exchangers
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Reference
This heat transfer lecture power point adapted from
1. Yunus Cengel, Heat and Mass Transfer A Practical Approach, 3rd
edition
2. Frank P. Incropera, Theodore l. Bergman, Adrienne S. Lavine,
and David P Dewitt, fundamental of Heat and Mass Transfer, 7th
edition
3. Lecture power point of heat transfer by Mehmet Kanoglu
University of Gaziantep.
You can download the previous lecture slide by using below link:
https://www.slideshare.net/TarikuNegash/chapter-4-transient-heat-
condution?qid=06cf696e-622b-45ac-ae41-cc4aa8067154&v=&b=&from_search=1
https://www.slideshare.net/TarikuNegash/two-dimensional-steady-state-heat-conduction
https://www.slideshare.net/TarikuNegash/heat-transfer-chapter-one-and-two-81503043

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Heat Transfer and Heat Exchangers Explained

  • 1. HEAT TRANSFER (MEng 3121) Debre Markos University Mechanical Engineering Department Prepared and Presented by: Tariku Negash Demissie Sustainable Energy Engineering (MSc) E-mail: thismuch2015@gmail.com/ tariku_negash@dmu.edu.et Lecturer at School Mechanical and Industrial Engineering, Institute of Technology, Debre Markos University, Debre Markos, Ethiopia Jan, 2020 HEAT EXCHANGER Chapter 7
  • 2. 1/27/2020Heat Transfer 2  Recognize numerous types of heat exchangers, and classify them.  Develop an awareness of fouling on surfaces, and determine the overall heat transfer coefficient for a heat exchanger.  Perform a general energy analysis on heat exchangers.  Obtain a relation for the logarithmic mean temperature difference for use in the LMTD method, and modify it for different types of heat exchangers using the correction factor.  Develop relations for effectiveness, and analyze heat exchangers when outlet temperatures are not known using the effectiveness-NTU method.  Know the primary considerations in the selection of heat exchangers. Objectives
  • 3. 1/27/2020Heat Transfer 3 7.1 Introduction of heat exchanger  The process of heat exchange between two fluids that are at different temperatures and separated by a solid wall occurs in many engineering applications.  The device used to implement this exchange is termed a heat exchanger, and specific applications may be found in space heating and air- conditioning, power production, waste heat recovery, and chemical processing.  For example, in a car radiator, heat is transferred from the hot water flowing through the radiator tubes to the air flowing through the closely spaced thin plates outside attached to the tubes.
  • 4. 1/27/2020Heat Transfer 4 7.2 Types of heat exchanger  Different heat transfer applications require different types of hardware and different configurations of heat transfer equipment.  The attempt to match the heat transfer hardware to the heat transfer requirements within the specified constraints has resulted in numerous types of innovative heat exchanger designs.  Therefore, heat exchangers are typically classified according to flow arrangement and type of construction. A. Double-pipe (Concentric tube) heat exchanger. The simplest heat exchanger is one for which the hot and cold fluids move in the same (parallel flow) or opposite (counter flow) directions in a concentric tube (or double-pipe) construction as shown on a fig a and b respectively.
  • 6. 1/27/2020Heat Transfer 6 B. Compact heat exchanger.  Which is specifically designed to realize a large heat transfer surface area per unit volume, is called area density 𝜷.  A heat exchanger with 𝜷 > 𝟕𝟎𝟎 𝒎 𝟐/𝒎 𝟑 is classified as being compact heat exchanger.  Examples of compact heat exchangers are Car radiators (𝜷 ≈ 𝟏𝟎𝟎𝟎 𝒎 𝟐 /𝒎 𝟑 ), Glass ceramic gas turbine heat exchangers (𝜷 ≈ 𝟔𝟎𝟎𝟎 𝒎 𝟐 /𝒎 𝟑 ), Regenerator of a Stirling engine (𝜷 ≈ 𝟏𝟓, 𝟎𝟎𝟎 𝒎 𝟐 /𝒎 𝟑 ), and Human lung (𝜷 ≈ 𝟐𝟎, 𝟎𝟎𝟎 𝒎 𝟐 /𝒎 𝟑 ).  Compact heat exchangers enable us to achieve high heat transfer rates between two fluids in a small volume, and they are commonly used in applications with strict limitations on the weight and volume of heat exchangers
  • 7. 7 For example a gas-to-liquid compact heat exchanger for a residential air-conditioning system. Cross-flow: In compact heat exchangers, the two fluids usually move perpendicular to each other. The cross-flow is further classified as unmixed and mixed flow. Different flow configurations in cross-flow heat exchangers.
  • 8. 1/27/2020Heat Transfer 8  Shell-and-tube heat exchanger: The most common type of heat exchanger in industrial applications. C. Shell-and-tube heat exchanger  Heat transfer takes place as one fluid flows inside the tubes while the other fluid flows outside the tubes through the shell.  Baffles are commonly placed in the shell to force the shell-side fluid to flow across the shell to enhance heat transfer and to maintain uniform spacing between the tubes.  They contain a large number of tubes (sometimes several hundred) packed in a shell with their axes parallel to that of the shell.
  • 10. 1/27/2020Heat Transfer 10  Heat exchangers in which all the tubes make one U-turn in the shell, for example, are called one-shell-pass and two tube-passes heat exchangers.  Likewise, a heat exchanger that involves two passes in the shell and four passes in the tubes is called a two-shell-passes and four-tube-passes exchanger Multi-pass flow arrangements in shell- and-tube heat exchangers.  Shell-and-tube heat exchangers are further classified according to the number of shell and tube passes involved.
  • 11. 1/27/2020Heat Transfer 11  A plate heat exchanger, PHE, is a compact heat exchanger where thin corrugated plates (some 0.5 mm thick, bended 1 or 2 mm) are stacked in contact with each other, and the two fluids made to flow separately along adjacent channels in the corrugation (Fig.). D. Plate heat exchanger  The closure of the staked plates may be by clamped gaskets, brazing (usually copper-brazed stainless steel), or welding (stainless steel, copper, titanium), the most common type being the first, for ease of inspection and cleaning.
  • 12. 1/27/2020Heat Transfer 12 Additionally, a frame (end-plates and fixing rods) secures together the plate stack and connectors (sometimes PFHE, standing for plate-and-frame heat exchanger, is used instead of PHE).  The PHE was developed in the 1920s in the food industry (for the pasteurization of milk), but they are taking over all markets now because of its compactness and efficiency (3 to 10 times more than STHE).
  • 13. 1/27/2020Heat Transfer 13 Flow patterns A Parallel Flow B. Counter Flow
  • 14. 1/27/2020Heat Transfer 14 Cold and hot flow fluid through Plate heat exchanger
  • 17. 17  Regenerative heat exchanger: or more commonly a regenerator, is a type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid. To accomplish this the hot fluid is brought into contact with the heat storage medium, then the fluid is displaced with the cold fluid, which absorbs the heat. It involves the alternate passage of the hot and cold fluid streams through the same flow area.  Dynamic-type regenerator: involves a rotating drum and continuous flow of the hot and cold fluid through different portions of the drum so that any portion of the drum passes periodically through the hot stream, storing heat, and then through the cold stream, rejecting this stored heat.  Condenser: One of the fluids is cooled and condenses as it flows through the heat exchanger.  Boiler: One of the fluids absorbs heat and vaporizes.  A space radiator is a heat exchanger that transfers heat from the hot fluid to the surrounding space by radiation. D. Others Types of Heat Exchanger
  • 18. 1/27/2020Heat Transfer 18 Classification of heat exchangers depending on their applications.
  • 19. 1/27/2020Heat Transfer 19 7.3.1 The Overall Heat Transfer Coefficient 7.3 Design consideration in Heat Exchanger • A heat exchanger typically involves two flowing fluids separated by a solid wall. • Heat is first transferred from the hot fluid to the wall by convection, through the wall by conduction, and from the wall to the cold fluid again by convection. • Any radiation effects are usually included in the convection heat transfer coefficients. Double-pipe heat exchanger. (1)
  • 20. 1/27/2020Heat Transfer 20 U is the overall heat transfer coefficient, W/m2C. When  The overall heat transfer coefficient U is dominated by the smaller convection coefficient. When one of the convection coefficients is much smaller than the other (say, hi << ho), we have 1/hi >> 1/ho, and thus U  hi and vise verse  This situation arises frequently when one of the fluids is a gas and the other is a liquid. In such cases, fins are commonly used on the gas side to enhance the product UA and thus the heat transfer on that side. (2) (3)
  • 21. 21 The overall heat transfer coefficient ranges from about 10 W/m2C for gas-to-gas heat exchangers to about 10,000 W/m2C for heat exchangers that involve phase changes. For short fins of high thermal conductivity, we can use this total area in the convection resistance relation Rconv = 1/hAs, since the fins in this case will be very nearly isothermal. Otherwise, we should determine the effective surface area As from When the tube is finned on one side to enhance heat transfer, the total heat transfer surface area on the finned side is (4)
  • 22. 1/27/2020Heat Transfer 22 7.3.2 Fouling Factor  The performance of heat exchangers usually deteriorates with time as a result of accumulation of deposits on heat transfer surfaces.  The layer of deposits represents additional resistance to heat transfer. This is represented by a fouling factor Rf.  The fouling factor increases with the operating temperature and the length of service and decreases with the velocity of the fluids. (5) 𝑇𝑎𝑏𝑙𝑒 ≈ 10−4 𝑚2 ℃/𝑊 which is equivalent to the thermal resistance of a 0.2- mm-thick limestone layer (k = 2.9 W/m ·°C) per unit surface area.
  • 23. 1/27/2020Heat Transfer 23 •Chemical fouling: when chemical changes within the fluid cause a fouling layer to be deposited onto the tube surface. A common example of this phenomenon is scaling in a kettle or boiler caused by “hardness” salts depositing onto the heating elements as the solubility of the salts reduce with increasing temperature. This is outside the control of the heat exchanger designer but can be minimized by careful control of the tube wall temperature in contact with the fluid. When this type of fouling occurs it must be removed by either chemical treatment or mechanical descaling processes (wire brushes or even drills to remove the scale or sometimes high-pressure water jets). •Biological fouling: this is caused by the growth of organisms within the fluid which deposit out onto the surfaces of the heat exchanger. Again this is outside the direct control of the heat exchanger designer, but it can be influenced by the choice of materials as some, notably the non-ferrous brasses, are poisonous to some organisms. When this type of fouling occurs it is normally removed by either chemical treatment or mechanical brushing processes. Common Types of Fouling
  • 24. 1/27/2020Heat Transfer 24 •Deposition fouling: this is when particles contained within the fluid settle out onto the surface when the fluid velocity falls below a critical level. To a large extent, this is within the control of the heat exchanger designer, as the critical velocity for any fluid/particle combination can be calculated to allow a design to be developed with minimum velocity levels higher than the critical level. Mounting the heat exchanger vertically can also minimize the effect as gravity would tend to pull the particles out of the heat exchanger away from the heat transfer surface even at low velocity levels. When this type of fouling occurs it is normally removed by mechanical brushing processes. •Corrosion fouling: this is when a layer of corrosion products build up on the surfaces of the tube forming an extra layer of, usually, high thermal resistance material. By careful choice of materials of construction the effects can be minimized as a wide range of corrosion resistant materials based on stainless steel and other nickel-based alloys are now available to the heat exchanger manufacturer.
  • 25. 1/27/2020Heat Transfer 25 Example 7.1 Water at an average temperature of 107°C and an average velocity of 3.5 m/s flows through a 5-m-long stainless steel tube (k =14.2 W/m ·°C) in a boiler. The inner and outer diameters of the tube are Di = 1.0 cm and Do = 1.4 cm, respectively. If the convection heat transfer coefficient at the outer surface of the tube where boiling is taking place is h = 8400 W/𝑚2 °C, determine the overall heat transfer coefficient 𝑈𝑖 of this boiler based on the inner surface area of the tube when, a) Negligible fouling factor and b) Assume fouling factor 𝑅𝑓,𝑖 = 0.0005 m𝑚2 ℃/𝑊 on the inner surface of the tube. Assumptions 1 Water flow is fully developed. 2 Properties of the water are constant. 3 The heat transfer coefficient and the fouling factor are constant and uniform.
  • 26. 1/27/2020Heat Transfer 26 7.4 Analysis of Heat Exchangers An engineer often finds himself or herself in a position 1. to select a heat exchanger that will achieve a specified temperature change in a fluid stream of known mass flow rate – the log mean temperature difference (or LMTD) method. 2. to predict the outlet temperatures of the hot and cold fluid streams in a specified heat exchanger – the effectiveness-NTU method. 1st law of thermodynamics rate of heat transfer from the hot fluid be equal to the rate of heat transfer to the cold one. (HE is insulated): Two fluid streams that have the same capacity rates experience the same temperature change in a well-insulated heat exchanger. (6) (7)
  • 27. 1/27/2020Heat Transfer 27 Heat capacity rate 𝑚 is the rate of evaporation or condensation of the fluid, . • Note that the only time the temperature rise of a cold fluid is equal to the temperature drop of the hot fluid is when the heat capacity rates of the two fluids are Equal to each other  Two special types of heat exchangers commonly used in practice are condensers and boilers.  One of the fluids in a condenser or a boiler undergoes a phase-change process, and the rate of heat transfer is expressed as 𝑚 is the rate of evaporation or condensation of the fluid, hfg is the enthalpy of vaporization of the fluid at the specified temperature or pressure. (8) (9)
  • 28. 28 Tm is an appropriate mean (average) temperature difference between the two fluids. The heat capacity rate of a fluid during a phase-change process must approach infinity since the temperature change is practically zero. An ordinary fluid absorbs or releases a large amount of heat essentially at constant temperature during a phase-change process, as shown on the fig a & b The rate of heat transfer in a heat exchanger (10)
  • 29. 1/27/2020Heat Transfer 29 7.5 The Log Mean Temperature Difference Method (LMTD) Variation of the fluid temperatures in a parallel- flow double-pipe heat exchanger. Assuming the outer surface of the heat exchanger to be well insulated so that any heat transfer occurs between the two fluids, and disregarding any KE and PE An energy balance (11)
  • 30. 1/27/2020Heat Transfer 30 Integrating from the inlet of the heat exchanger to its outlet, we obtain (12) Finally, solving Eqs. 6 and 7 for and (13)where Log mean temperature difference which is the suitable form of the average temperature difference for use in the analysis of heat exchangers. and substituting into Eq. (12) Arithmetic mean temperature difference: (14) (15)
  • 31. 1/27/2020Heat Transfer 31  The logarithmic mean temperature difference Tlm is an exact representation of the average temperature difference between the hot and cold fluids.  Note that Tlm is always less than Tam. Therefore, using Tam in calculations instead of Tlm will overestimate the rate of heat transfer in a heat exchanger between the two fluids.  When T1 differs from T2 by no more than 40 percent, the error in using the arithmetic mean temperature difference is less than 1 percent.  But the error increases to undesirable levels when T1 differs from T2 by greater amounts. Cont.
  • 32. 1/27/2020Heat Transfer 32  For specified inlet and outlet temperatures, the Tlm for a counter-flow heat exchanger is always greater than that for a parallel-flow heat exchanger.  That is, Tlm, CF > Tlm, PF, and thus a smaller surface area (and thus a smaller heat exchanger) is needed to achieve a specified heat transfer rate in a counter-flow heat exchanger.  A condenser or a boiler can be considered to be either a parallel- or counter- flow heat exchanger since both approaches give the same result 7.6 Counter-Flow Heat Exchangers  In the limiting case, the cold fluid will be heated to the inlet temperature of the hot fluid. However, the outlet temperature of the cold fluid can never exceed the inlet temperature of the hot fluid.
  • 33. 1/27/2020Heat Transfer 33 7.7 Multi-Pass and Cross-Flow Heat Exchangers: Use of a Correction Factor  The log mean temperature difference ∆𝑻𝒍𝒎 relation developed earlier is limited to parallel-flow and counter-flow heat exchangers only.  Similar relations are also developed for cross- flow and multi pass shell-and-tube heat exchangers, but the resulting expressions are too complicated because of the complex flow conditions.  The correction factor F depends on the geometry of the heat exchanger and the inlet and outlet temperatures of the hot and cold fluid streams.  ∆𝑻𝒍𝒎.𝑪𝑭 is the log mean temperature difference for the case of a counter-flow heat exchanger with the same inlet and outlet temperatures and is determined from Eq. (14). (16)
  • 34. 1/27/2020Heat Transfer 34 F for common shell-and-tube heat and cross-flow exchanger configurations is given in Fig. versus two temperature ratios P and R defined as 1 and 2: inlet and outlet T and t: shell- and tube-side temperature . respectively F = 1 for a condenser or boiler HOW? (17)
  • 36. 1/27/2020Heat Transfer 36 Example 7.2 Ashell-and-tube heat exchanger with 2-shell passes and 8-tube passes is used to heat ethyl alcohol (𝐶 𝑝 = 2670 J/kg · °C) in the tubes from 25°C to 70°C at a rate of 2.1 kg/s. The heating is to be done by water (𝐶 𝑝= 4190 J/kg ·°C) that enters the shell side at 95°C and leaves at 45°C. If the overall heat transfer coefficient is 950 W/m · °C, determine the heat transfer surface area of the heat exchanger. Assumptions 1 Steady operating conditions exist. 2 The heat exchanger is well-insulated so that heat loss to the surroundings is negligible and thus heat transfer from the hot fluid is equal to the heat transfer to the cold fluid. 3 Changes in the kinetic and potential energies of fluid streams are negligible. 4 There is no fouling. 5 Fluid properties are constant
  • 37. 1/27/2020Heat Transfer 37 7.8 The Effectiveness–NTU Method The LMTD method is very suitable for determining the size of a heat exchanger to realize prescribed outlet temperatures when the mass flow rates and the inlet and outlet temperatures of the hot and cold fluids are specified. With the LMTD method, the task is to select a heat exchanger that will meet the prescribed heat transfer requirements. The procedure to be followed by the selection process is: 1. Select the type of heat exchanger suitable for the application. 2. Determine any unknown inlet or outlet temperature and the heat transfer rate using an energy balance. 3. Calculate the log mean temperature difference Tlm and the correction factor F, if necessary. 4.Obtain (select or calculate) the value of the overall heat transfer coefficient U. 5.Calculate the heat transfer surface area As. The task is completed by selecting a heat exchanger that has a heat transfer surface area equal to or larger than As.
  • 38. 38 A second kind of problem encountered in heat exchanger analysis is the determination of the heat transfer rate and the outlet temperatures of the hot and cold fluids for prescribed fluid mass flow rates and inlet temperatures when the type and size of the heat exchanger are specified. Heat transfer effectiveness 𝜺 The maximum possible heat transfer rate Cmin is the smaller of Ch and Cc . Kays and London came up with a method in 1955 called the effectiveness–NTU method, which greatly simplified heat exchanger analysis. (18) (19) (20)
  • 39. 1/27/2020Heat Transfer 39 Actual heat transfer rate  Therefore, the effectiveness of a heat exchanger enables us to determine the heat transfer rate without knowing the outlet temperatures of the fluids  The effectiveness of a heat exchanger depends on the geometry of the heat exchanger as well as the flow arrangement.  Therefore, different types of heat exchangers have different effectiveness relations.  Below we illustrate the development of the effectiveness 𝜺 relation for the double-pipe parallel-flow heat exchanger. (21)
  • 40. 1/27/2020Heat Transfer 40 (12) Equation (12) for a parallel-flow heat exchanger can be rearranged as Also, solving Eq. (19) for 𝑻 𝒉,𝒐𝒖𝒕 (22) Substituting this relation into Eq. (22) after adding and subtracting 𝑻 𝒄,𝒊𝒏 gives which simplifies to (23) (24) (25)
  • 41. (29) (30) 1/27/2020Heat Transfer 41 We now manipulate the definition of effectiveness to obtain Substituting this result into Eq. (25) and solving for gives the following relation for 𝜺 the effectiveness of a parallel-flow heat exchanger: Taking either 𝐶𝑐 𝑜𝑟 𝐶ℎ to be 𝐶 𝑚 (both approaches give the same result), the relation above can be expressed more conveniently as for Cmin=Cc (26) (27) (28)
  • 42. 42 Effectiveness relations of the heat exchangers typically involve the dimensionless group UAs /Cmin. This quantity is called the number of transfer units NTU. For specified values of U and Cmin, the value of NTU is a measure of the surface area As. Thus, the larger the NTU, the larger the heat exchanger. Capacity ratio It can be shown that effectiveness of a heat exchanger is a function of the number of transfer units NTU and the capacity ratio c. 7.8.1 Numbers of transfer unit: NTU In heat exchanger analysis, it is also convenient to define another dimensionless quantity called the capacity ratio c as
  • 43. 43  Effectiveness relations have been developed for a large number of heat exchangers, and the results are given in Table.  The effectivenesses of some common types of heat exchangers are also plotted in Figure.
  • 44. 44  When all the inlet and outlet temperatures are specified, the size of the heat exchanger can easily be determined using the LMTD method.  Alternatively, it can be determined from the effectiveness–NTU method by first evaluating the effectiveness ε from its definition and then the NTU from the appropriate NTU relation.
  • 46. 46  The value of the effectiveness ranges from 0 to 1. It increases rapidly with NTU for small values (up to about NTU = 1.5) but rather slowly for larger values. Therefore, the use of a heat exchanger with a large NTU (usually larger than 3) and thus a large size cannot be justified economically, since a large increase in NTU in this case corresponds to a small increase in effectiveness.  For a given NTU and capacity ratio c = Cmin /Cmax, the counter-flow heat exchanger has the highest effectiveness, followed closely by the cross-flow heat exchangers with both fluids unmixed. The lowest effectiveness values are encountered in parallel-flow heat exchangers.  The effectiveness of a heat exchanger is independent of the capacity ratio c for NTU values of less than about 0.3. (NTU<0.3)  The value of the capacity ratio c ranges between 0 and 1. For a given NTU, the effectiveness becomes a maximum for c = 0 (e.g., boiler, condenser) and a minimum for c = 1 (when the heat capacity rates of the two fluids are equal). Observations from the effectiveness relations and charts
  • 47. 47 7.9 Selection of Heat Exchangers  The uncertainty in the predicted value of U can exceed 30 percent. Thus, it is natural to tend to overdesign the heat exchangers.  Heat transfer enhancement in heat exchangers is usually accompanied by increased pressure drop, and thus higher pumping power.  Therefore, any gain from the enhancement in heat transfer should be weighed against the cost of the accompanying pressure drop.  Usually, the more viscous fluid is more suitable for the shell side (larger passage area and thus lower pressure drop) and the fluid with the higher pressure for the tube side.  The proper selection of a heat exchanger depends on several factors: •Heat transfer rate •Cost •Pumping power •Size and weight •Type and Materials Annual cost of electricity associated with the operation of the pumps and fans: The rate of heat transfer in the prospective heat exchanger is
  • 48. 48
  • 49. 49
  • 50. 1/27/2020Heat Transfer 50 A shell-and-tube heat exchanger with 1-shell pass and 14-tube passes is used to heat water in the tubes with geothermal steam condensing at 120ºC (ℎ 𝑓𝑔 = 2203 kJ/kg) on the shell side. The tubes are thin-walled and have a diameter of 2.4 cm and length of 3.2 m per pass. Water (𝐶 𝑝 = 4180 J/kg·ºC) enters the tubes at 22ºC at a rate of 3.9 kg/s. If the temperature difference between the two fluids at the exit is 46ºC, determine (a) the rate of heat transfer, (b) the rate of condensation of steam, and (c) the overall heat transfer coefficient. Example 7.4 Assumptions 1 Steady operating conditions exist. 2 The heat exchanger is well-insulated so that heat loss to the surroundings is negligible and thus heat transfer from the hot fluid is equal to the heat transfer to the cold fluid. 3 Changes in the kinetic and potential energies of fluid streams are negligible. 4 Fluid properties are constant.
  • 51. 1/27/2020Heat Transfer 51 Example 7.5 Hot oil (𝐶 𝑝 = 2200 𝐽/𝐾𝑔℃) is to be cooled by water (Cp = 4180 J/kg · °C) in a 2-shell-pass and 12-tube-pass heat exchanger. The tubes are thin-walled and are made of copper with a diameter of 1.8 cm. The length of each tube pass in the heat exchanger is 3 m, and the overall heat transfer coefficient is 340 W/m · °C. Water flows through the tubes at a total rate of 0.1 kg/s, and the oil through the shell at a rate of 0.2 kg/s. The water and the oil enter at temperatures 18°C and 160°C, respectively. Determine, a) the rate of heat transfer in the heat exchanger and, b) the outlet temperatures of the water and the oil.
  • 52. 1/27/2020Heat Transfer 52 Consider a water-to-water counter-flow heat exchanger with these specifications. Hot water enters at 95ºC while cold water enters at 20ºC. The exit temperature of hot water is 15ºC greater than that of cold water, and the mass flow rate of hot water is 50 percent greater than that of cold water. The product of heat transfer surface area and the overall heat transfer coefficient is 1400 W/𝑚2· ºC. Taking the specific heat of both cold and hot water to be Cp= 4180 J/kg · ºC, determine (a) the outlet temperature of the cold water, (b) the effectiveness of the heat exchanger, (c) the mass flow rate of the cold water, and (d) the heat transfer rate. Example 7.6 Assumptions 1 Steady operating conditions exist. 2 The heat exchanger is well-insulated so that heat loss to the surroundings is negligible and thus heat transfer from the hot fluid is equal to the heat transfer to the cold fluid. 3 Changes in the kinetic and potential energies of fluid streams are negligible. 4 The overall heat transfer coefficient is constant and uniform.
  • 53. 53 Summary  Types of Heat Exchangers  The Overall Heat Transfer Coefficient  Fouling Factor  Analysis of Heat Exchangers  The Log Mean Temperature Difference Method  Counter-Flow Heat Exchangers  Multipass and Cross-Flow Heat Exchangers: Use of a Correction Factor  The Effectiveness-NTU Method  Selection of Heat Exchangers
  • 54. 1/27/2020Heat Transfer 54 Reference This heat transfer lecture power point adapted from 1. Yunus Cengel, Heat and Mass Transfer A Practical Approach, 3rd edition 2. Frank P. Incropera, Theodore l. Bergman, Adrienne S. Lavine, and David P Dewitt, fundamental of Heat and Mass Transfer, 7th edition 3. Lecture power point of heat transfer by Mehmet Kanoglu University of Gaziantep. You can download the previous lecture slide by using below link: https://www.slideshare.net/TarikuNegash/chapter-4-transient-heat- condution?qid=06cf696e-622b-45ac-ae41-cc4aa8067154&v=&b=&from_search=1 https://www.slideshare.net/TarikuNegash/two-dimensional-steady-state-heat-conduction https://www.slideshare.net/TarikuNegash/heat-transfer-chapter-one-and-two-81503043