Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
HEAT EXCHANGERS
Heat Exchanger
Heat exchanger is an apparatus or an equipment in
which the process of heating or cooling occurs.
The heat is transferred from one fluid being heated
to another fluid being cooled.
OR
Heat exchanger is process equipment designed for
the effective transfer of heat energy between two
fluids; a hot fluid and a coolant.
The purpose may be either to remove heat from a
fluid or to add heat to a fluid.
Heat Exchanger
Examples of Heat exchangers are:
• Boilers (evaporators), super
heaters and condensers of a
power plantBoilers
Evaporators Condensers
Heat Exchanger
Examples of Heat exchangers are:
• Automobile radiators and oil coolers of heat
engines
Automobile radiator Oil Coolers
Heat Exchanger
Examples of Heat exchangers are:
• Condensers and evaporators in refrigeration
units
• Water and air heaters or coolers
Air Heaters
Heat Exchanger
Heat transferred in the heat exchanger
may be in the form of latent heat (e.g. in
boilers and condensers) or sensible heat
(e.g. in heaters and coolers)
Classification of Heat Exchangers
1. Nature of heat exchange process
• Direct contact heat exchanger
• Indirect contact heat exchanger
Regenerator
Recuperator
2. Relative direction of motion of fluid
• Co-current (Parallel) flow
• Counter-current (Counter) flow
• Cross flow
Classification of Heat Exchangers
3. Design and constructional features (Mechanical
design of heat exchange surface)
• Concentric tubes
• Shell and tube
• Multiple shell and tube passes
• Compact heat exchangers
4. Physical state of heat exchanging fluids
(condensation and evaporation)
• Condenser
• Evaporator
Classification of Heat Exchangers
1. Nature of heat exchange
process
• Direct contact heat
exchanger
In a direct contact or
open heat exchanger
the exchange of heat
takes place by direct
mixing of hot and cold
fluids and transfer of
heat and mass take
place simultaneously.Direct contact or open
heat exchanger
Classification of Heat Exchangers
The use of such units is made under conditions
where mixing of two fluids is either harmless or
desirable.
Examples: Cooing towers, Jet condensers,
direct contact feed heaters
Classification of Heat Exchangers
1. Nature of heat exchange process
• Indirect contact heat exchanger
In this type of heat exchanger, the heat
transfer between two fluids could be
carried out by transmission through wall
which separates the two fluids. This type
includes the following:
(a) Regenerator
(b) Recuperators or surface exchangers
Classification of Heat Exchangers1. Nature of heat exchange process
• Indirect contact heat exchanger
(a) Regenerator
The regenerator are those devices in which hot
and cold fluids alternately flow over the surface
(through a space containing solid particles
(matrix)).
The heat carried by the hot fluid is accumulated in
the walls of the equipment and is then transferred
to the cold fluid when it passes over the surface
next.
Regenerator
Fig shows a cylinder
containing a matrix that
rotates in such a way
that it passes alternately
through cold and hot gas
streams which are
sealed from each other.
Rotating Matrix Regenerator
Regenerator
Fig. shows a stationary
matrix regenerator in
which hot and cold
gases flow through them
alternately.
Rotating Matrix Regenerator
Regenerator
Examples: I.C. engines and gas turbines,
Open hearth and glass melting furnace, air
heaters of blast furnace
A regenerator generally operates periodically
(wall or solid matrix alternately stores heat
extracted from the hot fluid and then delivers it
to the cold fluid).
In some regenerators the matrix is made
rotate through the fluid passages arranged
side by side which makes the heat exchange
process continuous.
Regenerator
The performance of these regenerators is
affected by
• Heat capacity of regenerating material
• The rate of absorption
• The release of heat
Regenerator
Advantages of Regenerators
• Higher heat transfer coefficient
• Less weight per kW of the plant
• Minimum pressure drop
• Quick response to load variation
• Small bulk weight
• Efficiency quite high
Regenerator
Dis-advantages of Regenerators
• Costlier compared to recuperative heat
exchanger
• Leakage is the main trouble, therefore
perfect setting is required.
Classification of Heat Exchangers
1. Nature of heat exchange process
• Indirect contact heat exchanger
(b) Recuperator
Most common heat exchangers are the
recuperators in which both hot and cold fluids
separated from each other by a wall, flow
through the exchanger at the same time.
The heat transfer process consists of convection
between the fluid and wall, conduction through
the wall and convection between the wall and
other fluid.
In case the ∆T between wall and fluid is large,
radiation heat exchange may also occur.
Recuperator
Such exchangers are used where the cooling
and heating fluids can not be allowed to mix.
Examples:
Automobile radiators
Oil coolers, intercoolers, air preheaters,
economizers, super heaters, condensers and
surface feed heaters of a steam power plant
Milk chiller of pasteurizing plant
Evaporator of ice plant
Recuperator
Advantages of Recuperators
• Easy construction
• More economical
• More surface area for heat transfer
• Much suitable for stationary plants
Recuperator
Dis-advantages of Recuperators
• Less heat transfer coefficient
• Less generating capacity
• Sooting problems
Classification of Heat Exchangers
2. Relative direction of motion of
fluid
• Co-current (Parallel) flow
(uni-direction flow)
In a parallel flow heat
exchanger, as the name
suggests, the two fluid
stream (hot and cold) travel
in the same direction.
The two stream enters at one
end and leaves at the other
end.
∆T between the hot and
cold fluids goes on
decreasing from inlet to
outlet
Parallel flow Heat Exchanger
Co-current (Parallel) flow
Since this type of heat exchanger needs a
large area of heat transfer, therefore, it is
rarely used in practice.
Examples: Oil coolers, oil heaters, water
heaters
As the two fluids are separated by a wall, this
type of heat exchanger may be called
parallel flow recuperator or surface heat
exchanger
Classification of Heat Exchangers
2. Relative direction of
motion of fluid
• Counter flow
In counter flow heat
exchanger, the two fluids
flow in opposite
directions.
The hot and cold fluids
enter at the opposite
ends.∆T between the two fluids
remains more or less
nearly constant.
Counter flow Heat Exchanger
Counter flow
This type of heat exchanger, due to counter
flow, gives maximum rate of heat transfer for
a given surface areas.
Hence such heat exchangers are most
favored for heating and cooling of fluids.
Classification of Heat Exchangers
2. Relative direction of
motion of fluid
• Cross flow
In cross flow heat
exchangers, the two
fluids (hot and cold)
cross one another in
space usually at right
angles.
Different flow configurations in cross-flow
heat exchangers.
Cross flow
Hot fluid flows in the
separate tubes and there
is no mixing of the fluid
streams. The cold fluid is
perfectly mixed as it flows
through exchanger. The
temperature of this mixed
fluid will be uniform across
any section and will vary
only in the direction of flow.
Examples: the cooling unit
of refrigeration system
Classification of Heat Exchangers
In this case each of the
fluids follows a
prescribed path and is
unmixed as it flows
through heat
exchanger. Hence the
temperature of the fluid
leaving the heater
section is not uniform.
Examples: Automobile
radiator
Cross flow
In yet another arrangement, both the
fluids are mixed while they travel through
the exchanger; consequently the
temperature of both the fluids is uniform
across the section and varies only in the
direction in which flow takes place.
Compact heat exchanger: It has a large heat
transfer surface area per unit volume (e.g., car
radiator, human lung). A heat exchanger with the
area density > 700 m2/m3 is classified as being
compact.
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.
A gas to liquid compact heat exchanger for a
residential air conditioning system
Classification of Heat Exchangers
3. Design and constructional features (Mechanical
design of heat exchange surface)
• Concentric tubes
In this type, two concentric tubes are used,
each carrying one of the fluids.
The direction of flow may be parallel or counter
The effectiveness of the heat exchanger is
increased by using swirling flow.
Classification of Heat Exchangers
3. Design and constructional features (Mechanical
design of heat exchange surface)
• Shell and tube
In this type of heat exchanger one of the fluids
flows through a bundle of tubes enclosed by a
shell.
The other fluid is forced through the shell and it
flows over the outside surface of the tubes.
Such an arrangement is employed where
reliability and heat transfer effectiveness are
important
36
Shell-and-tube heat exchanger: The most common type of heat
exchanger in industrial applications.
They contain a large number of tubes (sometimes several hundred)
packed in a shell with their axes parallel to that of the shell. Heat transfer
takes place as one fluid flows inside the tubes while the other fluid flows
outside the tubes through the shell.
Shell-and-tube heat exchangers are further classified according to the
number of shell and tube passes involved.
The schematic of a shell and tube heat exchanger (one shell pass and one
tube pass)
Classification of Heat Exchangers
3. Design and constructional features (Mechanical
design of heat exchange surface)
• Multiple shell and tube passes
Multiple shell and tube passes are used for
enhancing the overall heat transfer. Multiple
shell pass is possible where the fluid flowing
through the shell is re-routed. The shell side
fluid is forced to flow back and forth across the
tubes by baffles. Multiple tube pass
exchangers are those which re-route the fluid
through tubes in the opposite direction.
Multi pass flow arrangements in
shell and tube heat exchangers
Classification of Heat Exchangers
3. Design and constructional features (Mechanical
design of heat exchange surface)
• Compact heat exchangers
There are special purpose heat exchangers
and have a very large surface area per unit
volume of the exchanger. They are generally
employed when convective heat transfer
coefficient associated with one of the fluids is
much smaller than that associated with the
other fluid.
Example: Plate fin, flattened fin tube
exchangers
Plate and frame (or just plate) heat exchanger: Consists of a series of
plates with corrugated flat flow passages. The hot and cold fluids flow in
alternate passages, and thus each cold fluid stream is surrounded by two
hot fluid streams, resulting in very effective heat transfer. Well suited for
liquid-to-liquid applications.
A plate-and-frame
liquid-to-liquid heat
exchanger.
Classification of Heat Exchangers
4. Physical state of heat exchanging fluids
(condensation and evaporation)
• Condenser
In condenser, the condensing fluid remains at
constant temperature throughout the exchanger
while the temperature of the colder fluid
gradually increases from inlet to outlet. The hot
fluid loses latent part of heat which is accepted
by the cold fluid.
Classification of Heat Exchangers
4. Physical state of heat
exchanging fluids (condensation
and evaporation)
• Condenser
In condenser, the condensing
fluid remains at constant
temperature throughout the
exchanger while the
temperature of the colder fluid
gradually increases from inlet
to outlet. The hot fluid loses
latent part of heat which is
accepted by the cold fluid.
Classification of Heat Exchangers
4. Physical state of heat
exchanging fluids
(condensation and
evaporation)
• Evaporator
In this case, the boiling
fluid (cold fluid) remains
at constant temperature
while the temperature of
hot fluid gradually
decreases from inlet to
outlet.
44
THE OVERALL HEAT TRANSFER COEFFICIENT
• 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.
Thermal resistance network
associated with heat transfer in
a double-pipe heat exchanger.
45
U the overall heat transfer
coefficient, W/m2C
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. 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.
The two heat transfer surface
areas associated with a double
pipe heat exchanger (for thin
tubes, Di≈Do and thus Ai ≈Ao
46
The overall heat transfer coefficient
ranges from about 10 W/m2C for
gas-to-gas heat exchangers to about
10,000 W/m2C 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
To account for fin efficiency
When the tube is finned on one
side to enhance heat transfer, the
total heat transfer surface area on
the finned side is
47
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.
Precipitation fouling of ash
particles of superheater tubes
Points worth noting
1. The overall heat transfer coefficient depends
upon the following factors:
• The flow rate
• The properties of the fluid
• The thickness of material
• The surface condition of the tubes and
• The geometric configuration of the heat
exchanger
Points worth noting
2. The overall heat transfer coefficient U will
generally decrease when any of the fluids (e.g.
tars, oils or any of the gases) having low values
of heat transfer coefficient, h flows on one side of
the exchanger.
3. The highly conducting liquids such as water and
liquid metals give much higher values of heat
transfer coefficient, h and overall heat transfer
coefficient, U. In case of boiling and
condensation processes also, the value of U are
high.
Points worth noting
4. All the thermal resistances in the heat exchanger
must be low for its efficient and effective design.
Common failures in heat exchanger
• Choking of tubes either expected or extraordinary
• Excessive transfer rates in heat exchanger
• Increasing the pump pressure to maintain throughout
• Failure to clean tubes at regularly scheduled intervals
• Excessive temperatures in heat exchangers
• Lack of control of heat exchangers atmosphere to retard
scaling
• Increased product temperature over a safe design limit.
• Unexpected radiation from refractory surfaces
• Unequal heating around the circumference or along the
length of tubes
ANALYSIS OF HEAT EXCHANGERSAn 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.
The rate of heat transfer in heat exchanger (HE is insulated):
heat capacity rate
Two fluid
streams that
have the same
capacity rates
experience the
same
temperature
change in a well-
insulated heat
exchanger.
Variation of
fluid
temperatures
in a heat
exchanger
when one of
the fluids
condenses or
boils.
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.
The heat capacity rate of a fluid during a phase-change process must approach
infinity since the temperature change is practically zero.
Tm an appropriate mean (average)
temperature difference between the two fluids
THE LOG MEAN TEMPERATURE DIFFERENCE
METHOD
Variation of the fluid
temperatures in a
parallel-flow double-pipe
heat exchanger.log mean
temperature
difference
The arithmetic mean temperature difference
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.
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.
For specified inlet and outlet temperatures,
Tlm 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.
When the heat capacity rates
of the two fluids are equal
57
Multipass and Cross-Flow Heat Exchangers:
Use of a Correction Factor
F correction factor depends on the
geometry of the heat exchanger and the
inlet and outlet temperatures of the hot
and cold fluid streams.
F for common cross-flow and shell-and-
tube heat exchanger configurations is
given in the figure versus two
temperature ratios P and R defined as
1 and 2 inlet and outlet
T and t shell- and tube-side temperatures
F = 1 for a condenser or boiler
58
Correction factor
F charts for
common shell-
and-tube heat
exchangers.
59
Correction
factor F charts
for common
cross-flow heat
exchangers.
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.
THE EFFECTIVENESS–NTU METHODA 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
the maximum possible heat transfer rate
Cmin is the smaller of Ch and Cc
Actual heat transfer rate
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.
We illustrate the
development of the
effectiveness e relation
for the double-pipe
parallel-flow heat
exchanger.
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
The effectiveness of a heat exchanger is a function of the
number of transfer units NTU and the capacity ratio c.
Effectiveness
for heat
exchangers.
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 from its definition
and then the NTU from the appropriate NTU relation.
(e.g., boiler, condenser)
Observations from the effectiveness relations and charts
• 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.
• 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).
SELECTION OF HEAT EXCHANGERSThe 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
• Materials
The annual cost of electricity associated with
the operation of the pumps and fans
The rate of heat transfer in the
prospective heat exchanger
Summary
• Types of Heat Exchangers
• The Overall Heat Transfer Coefficient
Fouling factor
• Analysis of Heat Exchangers
• The Log Mean Temperature DifferenceMethod
Counter-Flow Heat Exchangers
Multipass and Cross-Flow Heat Exchangers:Use of a Correction Factor
• The Effectiveness–NTU Method
• Selection of Heat Exchangers