project report-ethe
TRANSCRIPT
A
PROJECT REPORT
ON
EARTH TUBE HEAT EXCHANGER
1
AcknowledgementWe take this opportunity to thank all the teachers of Mechanical Engineering
Department for allowing us to work on such an interesting & informative topic.
We are highly indebted to our project guide Mr. Kapil Chauhan Sir for his
guidance & words of wisdom. He always showed us the right direction during the
course of this project work. We are duly thankful to him to referring us to sites like
science direct, sciench tech & providing many research papers which had some
research work.
Success in such comprehensive report can’t be achieved single handed. It is the
team effort that sail the ship to the coast. So I would like to express my sincere
thanks to HOD of mechanical department Mr. Praveen Kumar Sir.
We worked as a team and saw ups and downs which are part of any project work.
But in the end it was their Guidance and my team work which made this project
possible. Last but not the least we would also like to thank all our teachers &
friends for their constructive criticism given in right spirit.
Mohit Singh Naula
(130970104032)
B.Tech
Mechanical Engineer
THDC-IHET
2
Certificate of Mentor
This is to certify that the project report on “EARTH TUBE HEAT
EXCHANGER” is a bonafide work carried out by Mr. SHUBHAM
SILSWAL student of Mechanical Engineering, THDC-IHET, B.Puram, NEW
TEHRI under my guidance and direction.
I wish him every success in his life.
SIGNATURE OF MENTOR
KAPIL CHAUHAN
(Assistant Professor)
THDC-IHET
3
Table of Content CONTENT PAGE NO.
1) Abstract
2) Introduction
3)Heat Exchanger
4) Flow Arrangement
5) Types of Heat Exchanger
6) Heat Exchanger Design Method
7) Monitoring and Maintenance
8) Selection of Heat Exchanger
9) Air Conditioning
10) Earth tube Heat Exchanger
11) ETHE analysis
12) Our Approach
12) Ground Temperature
13) Air Flow Calculation
14) Thermal COP value
15) Conclusion and Reference
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List of FiguresFigure Page no
Fig 1: Types of Flow Arrangement
Fig2: Schematic Diagram of Shell and Tube Heat Exchanger
Fig3: Shell and Tube heat exchanger
Fig4: Conceptual Diagram of Plate and Frame Heat Exchanger
Fig5: A Single Plate Heat Exchanger
Fig6: A heat exchanger in a steam power station contaminated with
Macrofouling
Fig7: Basic Refrigeration Cycle
Fig8: Earth Air Heat Exchanger
Fig9: Earth Tube Exchanger with Its Component
Fig10: Earth Air Tube Heat Exchanger System
Fig11: Closed Type ETHE
Fig12: Open Type ETHE
Fig13: Pipes Arrangement in Ground
Fig14: Earth Tube Heat Exchanger
Fig15: Variation of Temperature of Soil in May
Fig16: Variation of Temperature of Soil in January
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AbstractThe present scenario calls for a cheap, eco-friendly and efficient alternative to our
Existing air conditioning and heating systems. The average temperature below the
earth's Surface at a depth of 3-6 m is scientifically known to be in range of
10-28 °C against a much hotter or colder surface temperature (which is influenced
by pertaining weather conditions). Hence this temperature difference can be tapped
in a beneficial way to form a heating or cooling system depending upon our need.
This project focuses on Earth Air Heat Exchanger which is reducing energy
consumption in a building. The air is passing through the buried tubes and heat
exchange takes place between air and surrounding soil. This equipment helps to
reduce energy consumption of an air conditioning unit. This project analyze the
thermal performance of earth air heat exchanger.
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IntroductionEnergy is very much essential for existence of our society. It is important and
urgent to find alternative sources to replace conventional fuel or to reduce its
continuous consumption due to their limited reservoirs and bad impact on
environment. So, we have to find alternative source of energy. This energy should
be available in abundance on earth and it should be available at all parts of the
earth. Nowadays use of air conditioning is increasing in commercial as well as in
residential buildings. Vapor compression machines are used to achieve it. Vapor
compression machines are the source of chlorofluorocarbon (CFCs) gases which
are harmful for ozone layer depletion and also contributing to global warming. The
air conditioning is used in large scales across the world which is consuming large
portion of electrical energy. Electricity consumption reaches to peak value in
summer, requiring new power plants for electrical energy production as well as
increasing the cost of peak electricity. In addition, entire world is also concerned
about climate change and trying to find alternative clean and green sources of
energy. As a matter of fact, among the various energy sources, electricity is
characterized by the highest GHG emission factor. Many alternative techniques are
used to reduce high grade energy consumptions. One such method is earth air heat
exchanger.
Earth air heat exchanger exchanges heat with underground soil. It uses earth’s
constant underground soil temperature and it is used to heat or cool air or other
fluids for commercial or residential purposes. It comprises of long tubes that are
buried into the ground, through which air is passed. Because of high thermal inertia
of the ground, the temperature of underground soil remains almost unchanged as
compared to ground surface. Time lag also occurs between the temperature
fluctuations in the underground soil and at the surface. So at certain depth from
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upper ground surface, underground soil temperature is lower than outside air
temperature in summer and higher in winter. The fresh air can be cooled by
passing through the earth air heat exchanger and can be supplied to air
conditioning unit to reduce energy consumption. The effectiveness of earth air heat
exchanger depends upon material of tube, air inlet temperature, soil temperature,
depth, arrangement of pipe etc.
Heat ExchangerA heat exchanger is a device that is used for transfer of thermal energy (enthalpy)
between two or more fluids, between a solid surface and a fluid, or between solid
particulates and a fluid, at differing temperatures and in thermal contact, usually
without external heat and work interactions. The fluids may be single compounds
or mixtures. Typical applications involve heating or cooling of a fluid stream of
concern, evaporation or condensation of a single or multicomponent fluid stream,
and heat recovery or heat rejection from a system. In other applications, the
objective may be to sterilize, pasteurize, fractionate, distill, concentrate, crystallize,
or control process fluid. In some heat exchangers, the fluids exchanging heat are in
direct contact. In other heat exchangers, heat transfer between fluids takes place
through a separating wall or into and out of a wall in a transient manner. In most
heat exchangers, the fluids are separated by a heat transfer surface, and ideally they
do not mix. Such exchangers are referred to as the direct transfer type, or simply
recuperates. In contrast, exchangers in which there is an intermittent heat exchange
between the hot and cold fluid via thermal energy storage and rejection through the
exchanger surface or matrix are referred to as the indirect transfer type or storage
type, or simply regenerators. Such exchangers usually have leakage and fluid
carryover from one stream to the other. A heat exchanger consists of heat
exchanging elements such as a core or a matrix containing the heat transfer
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surface, and fluid distribution elements such as headers, manifolds, tanks, inlet and
outlet nozzles or pipes, or seals. Usually there are no moving parts in a heat
exchanger; however, there are exceptions such as a rotary regenerator (in which the
matrix is mechanically driven to rotate at some design speed), a scraped surface
heat exchanger, agitated vessels, and stirred tank reactors. 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. The portion of the surface that also
separates the fluids is referred to as the primary or direct surface. To increase heat
transfer area, appendages known as fins may be intimately connected to the
primary surface to provide extended, secondary, or indirect surface. Thus, the
addition of fins reduces the thermal resistance on that side and thereby increases
the net heat transfer from/to the surface for the same temperature difference. The
heat transfer coefficient can also be higher for fins.
Flow ArrangementThere are three primary classifications of heat exchangers according to their flow
arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger
at the same end, and travel in parallel to one another to the other side. In counter-
flow heat exchangers the fluids enter the exchanger from opposite ends. The
counter current design is the most efficient, in that it can transfer the most heat
from the heat (transfer) medium per unit mass due to the fact that the average
temperature difference along any unit length is higher. See countercurrent
exchange. In a cross-flow heat exchanger, the fluids travel roughly perpendicular to
one another through the exchanger.
There are three primary classifications of heat exchangers according to their flow
arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger
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at the same end, and travel in parallel to one another to the other side. In counter-
flow heat exchangers the fluids enter the exchanger from opposite ends. The
counter current design is the most efficient, in that it can transfer the most heat
from the heat (transfer) medium per unit mass due to the fact that the average
temperature difference along any unit length is higher. See countercurrent
exchange. In a cross-flow heat exchanger, the fluids travel roughly perpendicular to
one another through the exchanger.
The driving temperature across the heat transfer surface varies with position, but an
appropriate mean temperature can be defined. In most simple systems this is the
"log mean temperature difference” (LMTD). Sometimes direct knowledge of the
LMTD is not available and the NTU method is used.
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Fig 1: Types of Flow Arrangement
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Types of Heat Exchanger
Heat exchangers are off-the-shelf equipment targeted to the efficient transfer of
heat from a hot fluid flow to a cold fluid flow, in most cases through an
intermediate metallic wall and without moving parts. We here focus on the thermal
analysis of heat exchangers, but proper design and use requires additional fluid
dynamic analysis (for each fluid flow), mechanical analysis (for closure and
resistance), materials compatibility, and so on. Heat losses or gains of a whole heat
exchanger with the environment can be neglected in comparison with the heat flow
between both fluid flows; i.e. a heat exchanger can be assumed globally adiabatic.
Thermal inertia of a heat exchanger is often negligible too (except in special cases
when a massive porous solid is used as intermediate medium), and steady state can
be assumed, reducing the generic energy balance to:
Where the total enthalpy ht has been approximated by enthalpy (i.e. negligible
mechanical energy against thermal energy), and means output minus input.
Although heat flows from hot fluid to cold fluid by thermal conduction through
the separating wall (except in direct-contact types), heat exchangers are basically
heat convection equipment, since it is the convective transfer what governs its
performance. Convection within a heat exchanger is always forced, and may be
with or without phase change of one or both fluids.
When one just relies in natural convection to the environment, like in the space-
heating hot-water home radiator, or the domestic fridge back-radiator, they are
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termed 'radiators' (in spite of convection being dominant), and not heat exchangers.
When a fan is used to force the flow of ambient air (or when natural or artificial
wind applies, like for car radiator) the name heat exchanger is often reserved for
the case where the ambient fluid is ducted. Other names are used for special cases,
like ‘condenser’ for the case when one fluid flow changes from vapour to liquid,
‘vaporizer’ (or evaporator, or boiler) when a fluid changes from liquid to vapour,
or the ‘cooling tower’ dealt with below.
Devices with just one fluid flow (like a solar collector, a spacecraft radiator, a
submerged electrical heater, or a simple pipe with heat exchange with the
environment) are never named heat exchangers.
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1) Shell and Tube Heat Exchanger : Shell and tube heat exchangers
consist of series of tubes. One set of these tubes contains the fluid that must
be either heated or cooled. The second fluid runs over the tubes that are
being heated or cooled so that it can either provide the heat or absorb the
heat required. A set of tubes is called the tube bundle and can be made up of
several types of tubes: plain, longitudinally finned, etc. Shell and tube heat
exchangers are typically used for high-pressure applications (with pressures
greater than 30 bar and temperatures greater than 260 °C).
Several thermal design features must be considered when designing the
tubes in the shell and tube heat exchangers: There can be many variations on
the shell and tube design. Typically, the ends of each tube are connected to
plenums (sometimes called water boxes) through holes in tube sheets. The
tubes may be straight or bent in the shape of a U, called U-tubes.
Fig2: Schematic Diagram Of Shell And Tube Heat Exchanger
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a) Tube diameter: Using a small tube diameter makes the heat exchanger both
economical and compact. However, it is more likely for the heat exchanger to foul
up faster and the small size makes mechanical cleaning of the fouling difficult. To
prevail over the fouling and cleaning problems, larger tube diameters can be used.
Thus to determine the tube diameter, the available space, cost and fouling nature of
the fluids must be considered.
b) Tube thickness: The thickness of the wall of the tubes is usually determined to ensure:
1) There is enough room for corrosion
2) That flow-induced vibration has resistance
3) Axial strength
4) Availability of spare parts
5) Hoop strength (to withstand internal tube pressure)
6) Buckling strength (to withstand overpressure in the shell)
c) Tube length: heat exchangers are usually cheaper when they have a smaller
shell diameter and a long tube length. Thus, typically there is an aim to make the
heat exchanger as long as physically possible whilst not exceeding production
capabilities. However, there are many limitations for this, including space available
at the installation site and the need to ensure tubes are available in lengths that are
twice the required length (so they can be withdrawn and replaced). Also, long, thin
tubes are difficult to take out and replace.
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d) Tube Pitch: when designing the tubes, it is practical to ensure that the tube
pitch (i.e., the centre-centre distance of adjoining tubes) is not less than 1.25 times
the tubes' outside diameter. A larger tube pitch leads to a larger overall shell
diameter, which leads to a more expensive heat exchanger.
e) Tube Corrugation: this type of tubes, mainly used for the inner tubes,
increases the turbulence of the fluids and the effect is very important in the heat
transfer giving a better performance.
f) Tube Layout: refers to how tubes are positioned within the shell. There are
four main types of tube layout, which are, triangular (30°), rotated triangular (60°),
square (90°) and rotated square (45°). The triangular patterns are employed to give
greater heat transfer as they force the fluid to flow in a more turbulent fashion
around the piping. Square patterns are employed where high fouling is experienced
and cleaning is more regular.
g) Baffle Design: Baffles are used in shell and tube heat exchangers to direct
fluid across the tube bundle. They run perpendicularly to the shell and hold the
bundle, preventing the tubes from sagging over a long length. They can also
prevent the tubes from vibrating. The most common type of baffle is the segmental
baffle. The semicircular segmental baffles are oriented at 180 degrees to the
adjacent baffles forcing the fluid to flow upward and downwards between the tube
bundles. Baffle spacing is of large thermodynamic concern when designing shell
and tube heat exchangers. Baffles must be spaced with consideration for the
conversion of pressure drop and heat transfer. For thermo economic optimization it
is suggested that the baffles be spaced no closer than 20% of the shell’s inner
diameter. Having baffles spaced too closely causes a greater pressure drop because
of flow redirection. Consequently, having the baffles spaced too far apart means
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that there may be cooler spots in the corners between baffles. It is also important to
ensure the baffles are spaced close enough that the tubes do not sag. The other
main type of baffle is the disc and doughnut baffle, which consists of two
concentric baffles. An outer, wider baffle looks like a doughnut, whilst the inner
baffle is shaped like a disk. This type of baffle forces the fluid to pass around each
side of the disk then through the doughnut baffle generating a different type of
fluid flow.
Fig3: Shell and Tube heat exchanger
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2) Plate Heat Exchangers: Another type of heat exchanger is the plate
heat exchanger. These exchangers are composed of many thin, slightly
separated plates that have very large surface areas and small fluid flow
passages for heat transfer. Advances in gasket and brazing technology have
made the plate-type heat exchanger increasingly practical.
In HVAC applications, large heat exchangers of this type are called plate-
and-frame; when used in open loops, these heat exchangers are normally of
the gasket type to allow periodic disassembly, cleaning, and inspection.
There are many types of permanently bonded plate heat exchangers, such as
dip-brazed, vacuum-brazed, and welded plate varieties, and they are often
specified for closed-loop applications such as refrigeration. Plate heat
exchangers also differ in the types of plates that are used, and in the
configurations of those plates. Some plates may be stamped with "chevron",
dimpled, or other patterns, where others may have machined fins and/or
grooves.
When compared to shell and tube exchangers, the stacked-plate arrangement
typically has lower volume and cost. Another difference between the two is
that plate exchangers typically serve low to medium pressure fluids,
compared to medium and high pressures of shell and tube. A third and
important difference is that plate exchangers employ more countercurrent
flow rather than cross current flow, which allows lower approach
temperature differences, high temperature changes, and increased
efficiencies.
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Fig4: Conceptual Diagram Of Plate And Frame Heat Exchanger
Fig5: A Single Plate Heat Exchanger
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3) Plate and Shell Heat Exchanger : A third type of heat exchanger is a plate
and shell heat exchanger, which combines plate heat exchanger with shell and tube
heat exchanger technologies. The heart of the heat exchanger contains a fully
welded circular plate pack made by pressing and cutting round plates and welding
them together. Nozzles carry flow in and out of the plate pack (the 'Plate side' flow
path). The fully welded plate pack is assembled into an outer shell that creates a
second flow path (the 'Shell side'). Plate and shell technology offers high heat
transfer, high pressure, high operating temperature and close approach
temperature. In particular, it does completely without gaskets, which provides
security against leakage at high pressures and temperatures.
4) Adiabatic Wheel Heat Exchanger: A fourth type of heat exchanger uses
an intermediate fluid or solid store to hold heat, which is then moved to the other
side of the heat exchanger to be released. Two examples of this are adiabatic
wheels, which consist of a large wheel with fine threads rotating through the hot
and cold fluids, and fluid heat exchangers.
5) Plate Fin Heat Exchanger: This type of heat exchanger uses "sandwiched"
passages containing fins to increase the effectiveness of the unit. The designs
include cross flow and counter flow coupled with various fin configurations such
as straight fins, offset fins and wavy fins.
Plate and fin heat exchangers are usually made of aluminum alloys, which provide
high heat transfer efficiency. The material enables the system to operate at a lower
temperature difference and reduce the weight of the equipment. Plate and fin heat
exchangers are mostly used for low temperature services such as natural
gas, helium and oxygen liquefaction plants, air separation plants and transport
industries such as motor and aircraft engines.
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Advantages of plate and fin heat exchangers:
a) High heat transfer efficiency especially in gas treatment
b) Larger heat transfer area
c) Approximately 5 times lighter in weight than that of shell and tube heat
exchanger.
d) Able to withstand high pressure
Disadvantages of plate and fin heat exchangers:
a) Might cause clogging as the pathways are very narrow
b) Difficult to clean the pathways
c) Aluminum alloys are susceptible to Mercury Liquid Embrittlement Failure.
6) Pillow Plate Heat Exchanger: A pillow plate exchanger is commonly used
in the dairy industry for cooling milk in large direct-expansion stainless steel bulk
tanks. The pillow plate allows for cooling across nearly the entire surface area of
the tank, without gaps that would occur between pipes welded to the exterior of the
tank.
The pillow plate is constructed using a thin sheet of metal spot-welded to the
surface of another thicker sheet of metal. The thin plate is welded in a regular
pattern of dots or with a serpentine pattern of weld lines. After welding the
enclosed space is pressurized with sufficient force to cause the thin metal to bulge
out around the welds, providing a space for heat exchanger liquids to flow, and
creating a characteristic appearance of a swelled pillow formed out of metal.
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7) Fluid Heat Exchangers: This is a heat exchanger with a gas passing
upwards through a shower of fluid (often water), and the fluid is then taken
elsewhere before being cooled. This is commonly used for cooling gases whilst
also removing certain impurities, thus solving two problems at once. It is widely
used in espresso machines as an energy-saving method of cooling super-heated
water to use in the extraction of espresso.
8) Dynamic Scraped Surface Heat Exchanger : Another type of heat
exchanger is called "(dynamic) scraped surface heat exchanger". This is mainly
used for heating or cooling with high-
viscosity products, crystallizationprocesses, evaporation and high-
fouling applications. Long running times are achieved due to the continuous
scraping of the surface, thus avoiding fouling and achieving a sustainable heat
transfer rate during the process.
9) Phase-change heat exchangers:In addition to heating up or cooling down
fluids in just a single phase, heat exchangers can be used either to heat a liquid to
evaporate (or boil) it or used as condensers to cool a vapor and condense it to a
liquid. In chemical plants and refineries, reboilersused to heat incoming feed
for distillation towers are often heat exchangers.
Distillation set-ups typically use condensers to condense distillate vapors back into
liquid.
Power plants that use steam-driven turbines commonly use heat exchangers to
boil water into steam. Heat exchangers or similar units for producing steam from
water are often called boilers or steam generators.
In the nuclear power plants called pressurized water reactors, special large heat
exchangers pass heat from the primary (reactor plant) system to the secondary
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(steam plant) system, producing steam from water in the process. These are
called steam generators. All fossil-fueled and nuclearpower plants using steam-
driven turbines have surface condensers to convert the exhaust steam from the
turbines into condensate (water) for re-use.
To conserve energy and cooling capacity in chemical and other plants, regenerative
heat exchangers can transfer heat from a stream that must be cooled to another
stream that must be heated, such as distillate cooling and reboiler feed pre-heating.
This term can also refer to heat exchangers that contain a material within their
structure that has a change of phase. This is usually a solid to liquid phase due to
the small volume difference between these states. This change of phase effectively
acts as a buffer because it occurs at a constant temperature but still allows for the
heat exchanger to accept additional heat. One example where this has been
investigated is for use in high power aircraft electronics.
Heat exchangers functioning in multiphase flow regimes may be subject to
the Leading instability.
10) Direct Contact Heat Exchangers: Direct contact heat exchangers
involve heat transfer between hot and cold streams of two phases in the absence of
a separating wall.[8] Thus such heat exchangers can be classified as:
a) Gas – liquid
b) Immiscible liquid – liquid
c) Solid-liquid or solid – gas
Most direct contact heat exchangers fall under the Gas – Liquid category, where
heat is transferred between a gas and liquid in the form of drops, films or sprays.[2]
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Such types of heat exchangers are used predominantly in air
conditioning, humidification, industrial hot water heating, water cooling and
condensing plants.
11) Microchannel Heat Exchanger : Micro heat exchangers, Micro-scale
heat exchangers, or micro structured heat exchangers are heat exchangers in which
(at least one) fluid flows in lateral confinements with typical dimensions below
1 mm. The most typical such confinement are micro channels, which are channels
with a hydraulic diameter below 1 mm. Micro channel heat exchangers can be
made from metal, ceramic, and even low-cost plastic. Micro channel heat
exchangers can be used for many applications including:
a) high-performance aircraft gas turbine engines.
b) Heat pumps.
c) Air conditioning
d) Heat recovery ventilators
Heat Exchanger Design MethodThe goal of heat exchanger design is to relate the inlet and outlet temperatures, the
overall heat transfer coefficient, and the geometry of the heat exchanger, to the rate
of heat transfer between the two fluids. The two most common heat exchanger
design problems are those of rating and sizing. We will limit ourselves to the
design of recuperators only. That is, the design of a two fluid heat exchanger used
for the purposes of recovering waste heat. We will begin first, by discussing the
basic principles of heat transfer for a heat exchanger. We may write the enthalpy
balance on either fluid stream to give:
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Qc = ˙mc(hc2 − hc1) and Qh = ˙mh(hh1 − hh2)
For constant specific heats with no change of phase, we may also write
Qc = ( ˙mcp)c(Tc2 − Tc1) and Qh = ( ˙mcp)h(Th1 − Th2).
Now from energy conservation we know that
Qc = Qh = Q
and that we may relate the heat transfer rate Q and the overall heat transfer
coefficient U, to the some mean temperature difference ∆Tm by means of
Q = UA∆Tm
where A is the total surface area for heat exchange that U is based upon.
Later we shall show that
∆Tm = f(Th1,Th2,Tc1,Tc2)
It is now clear that the problem of heat exchanger design comes down to obtaining
an expression for the mean temperature difference. Expressions for many flow
con- figurations, i.e. parallel flow, counter flow, and cross flow, have been
obtained in the heat transfer field. We will examine these basic expressions later.
Two approaches to heat exchanger design that will be discussed are the LMTD
method and the effectiveness - NTU method. Each of these methods has particular
advantages depending upon the nature of the problem specification.
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1) LMTD Method
The log mean temperature difference (LMTD) is derived in all basic heat transfer
texts. It may be written for a parallel flow or counterflow arrangement. The LMTD
has the form:
∆TLMTD = ∆T2 − ∆T1 / ln (∆T2 /∆T1)
where ∆T1 and ∆T2 represent the temperature difference at each end of the heat
exchanger, whether parallel flow or counterflow. The LMTD expression assumes
that the overall heat transfer coefficient is constant along the entire flow length of
the heat exchanger. If it is not, then an incremental analysis of the heat exchanger
is required. The LMTD method is also applicable to crossflow arrangements when
used with the crossflow correction factor. The heat transfer rate for a crossflow
heat exchanger may be written as:
Q = FUA∆TLMTD
where the factor F is a correction factor, and the log mean temperature difference is
based upon the counter flow heat exchanger arrangement. The LMTD method
assumes that both inlet and outlet temperatures are known. When this is not the
case, the solution to a heat exchanger problem becomes somewhat tedious. An
alternate method based upon heat exchanger effectiveness is more appropriate for
this type of analysis. If ∆T1 = ∆T2 = ∆T, then the expression for the LMTD
reduces simply to ∆T.
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2) € − NTU Method
The effectiveness / number of transfer units (NTU) method was developed to
simplify a number of heat exchanger design problems. The heat exchanger
effectiveness is defined as the ratio of the actual heat transfer rate to the maximum
possible heat transfer rate if there were infinite surface area. The heat exchanger
effectiveness depends upon whether the hot fluid or cold fluid is a minimum fluid.
That is the fluid which has the smaller capacity coefficient C = ˙mCp. If the cold
fluid is the minimum fluid then the effectiveness is defined as:
€= Cmax(TH,in − TH,out)/ Cmin(TH,in − TC,in)
otherwise, if the hot fluid is the minimum fluid, then the effectiveness is defined
as:
€ = Cmax(TC,out − TC,in)/ Cmin(TH,in − TC,in)
We may now define the heat transfer rate as:
Q = € Cmin(TH,in − TC,in)
It is now possible to develop expressions which relate the heat exchanger
effectiveness to another parameter referred to as the number of transfer units
(NTU). The value of NTU is defined as:
NTU = UA/ Cmin
€ = f(NTU,Cr)
Where Cr = Cmin/Cmax.
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Numerous expressions have been obtained which relate the heat exchanger
effectiveness to the number of transfer units. The handout summarizes a number of
these solutions and the special cases which may be derived from them. For
convenience the € − NTU relationships are given for a simple double pipe heat
exchanger for parallel flow and counter flow:
ParallelFlow € = 1 –exp[−NTU(1 + Cr)]/(1 + Cr)
Or
NTU = − ln[1 − € (1 + Cr)]/ (1 + Cr)
Counter Flow € = 1 − exp[−NTU(1 − Cr)]/[ 1 + Cr exp[−NTU(1 − Cr)], Cr <
1
and€ = NTU/( 1 + NTU) , Cr = 1
or NTU = 1 /Cr – 1[ ln{(€ − 1)/( € Cr – 1)} ] , Cr < 1 and
NTU = € /(1 − €), Cr = 1
For other configurations, the student is referred to the Heat Transfer course text, or
the handout. Often manufacturer’s choose to present heat exchanger performance
in terms of the inlet temperature difference ITD = (Th,i −Tc,i). This is usually
achieved by plotting the normalized parameter Q/ITD = Q/(Th,i − Tc,i). This is a
direct consequence of the € − NTU method.
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Monitoring and Maintenance
Online monitoring of commercial heat exchangers is done by tracking the overall heat transfer coefficient. The overall heat transfer coefficient tends to decline over time due to fouling.
U=Q/AΔTlm
By periodically calculating the overall heat transfer coefficient from exchanger
flow rates and temperatures, the owner of the heat exchanger can estimate when
cleaning the heat exchanger is economically attractive. Integrity inspection of plate
and tubular heat exchanger can be tested in situ by the conductivity or helium gas
methods. These methods confirm the integrity of the plates or tubes to prevent any
cross contamination and the condition of the gaskets.Mechanical integrity
monitoring of heat exchanger tubes may be conducted through Non destructive
methods such as eddy current testing.
Fouling: Fouling occurs when impurities deposit on the heat exchange surface.
Fig6: A heat exchanger in a steam power station contaminated with macrofouling.
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Deposition of these impurities can decrease heat transfer effectiveness significantly
over time and are caused by:
1) Low wall shear stress
2) Low fluid velocities
3) High fluid velocities
4) Reaction product solid precipitation
5) Precipitation of dissolved impurities due to elevated wall temperatures
The rate of heat exchanger fouling is determined by the rate of particle deposition
less re-entrainment/suppression. This model was originally proposed in 1959 by
Kern and Seaton.
Crude Oil Exchanger Fouling: In commercial crude oil refining, crude oil is
heated from 21 °C (70 °F) to 343 °C (649 °F) prior to entering the distillation
column. A series of shell and tube heat exchangers typically exchange heat
between crude oil and other oil streams to heat the crude to 260 °C (500 °F) prior
to heating in a furnace. Fouling occurs on the crude side of these exchangers due to
asphaltene insolubility. The nature of asphaltene solubility in crude oil was
successfully modeled by Wiehe and Kennedy. The precipitation of insoluble
asphaltenes in crude preheat trains has been successfully modeled as a first order
reaction by Ebert and Panchal who expanded on the work of Kern and Seaton.
Cooling Water Fouling: Cooling water systems are susceptible to fouling.
Cooling water typically has a high total dissolved solid content and suspended
colloidal solid. Localized precipitation of dissolved solids occurs at the heat
exchange surface due to wall temperatures higher than bulk fluid temperature. Low
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fluid velocities (less than 3 ft/s) allow suspended solids to settle on the heat
exchange surface. Cooling water is typically on the tube side of a shell and tube
exchanger because it's easy to clean. To prevent fouling, designers typically ensure
that cooling water velocity is greater than 0.9 m/s and bulk fluid temperature is
maintained less than 60 °C (140 °F). Other approaches to control fouling control
combine the “blind” application of biocides and anti-scale chemicals with periodic
lab testing.
MaintenancePlate and frame heat exchangers can be disassembled and cleaned periodically.
Tubular heat exchangers can be cleaned by such methods as acid
cleaning, sandblasting, high-pressure water jet, bullet cleaning, or drill rods.
In large-scale cooling water systems for heat exchangers, water treatment such as
purification, addition of chemicals, and testing, is used to minimize fouling of the
heat exchange equipment. Other water treatment is also used in steam systems for
power plants, etc. to minimize fouling and corrosion of the heat exchange and
other equipment.
A variety of companies have started using water borne oscillations technology to
prevent biofouling. Without the use of chemicals, this type of technology has
helped in providing a low-pressure drop in heat exchangers.
Selection Of Heat ExchangerDue to the many variables involved, selecting optimal heat exchangers is
challenging. Hand calculations are possible, but many iterations are typically
needed. As such, heat exchangers are most often selected via computer programs,
either by system designers, who are typically engineers, or by equipment vendors.
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To select an appropriate heat exchanger, the system designers (or equipment
vendors) would firstly consider the design limitations for each heat exchanger type.
Though cost is often the primary criterion, several other selection criteria are
important:
1. High/low pressure limits
2. Thermal performance
3. Temperature ranges
4. Product mix (liquid/liquid, particulates or high-solids liquid)
5. Pressure drops across the exchanger
6. Fluid flow capacity
7. Clean ability, maintenance and repair
8. Materials required for construction
9. Ability and ease of future expansion
10. Material selection, such as copper, aluminum, carbon steel, stainless
steel, nickel alloys, ceramic, polymer, and titanium.
Air ConditioningAir conditioning is a collective process that performs many functions
simultaneously. It conditions air, transports it, and introduce into the conditioned
space. It provides heating and cooling from its central plant or roof top units. It
also controls and maintains the temperature, humidity, air movement, air
cleanliness, sound level, and pressure discrepancy in a space within predetermined
limits for the comfort and health of the occupants of the conditioned space or for
the purpose of product processing. Air-conditioning systems is the largest energy
consumer that is the biggest challenge which arises now a days. This problem can
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be overcome by the use of ground coupled heat exchanger in air conditioning
system.
Fig7: Basic Refrigeration Cycle
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Earth Tube Heat ExchangerThe idea of using earth as a heat sink was known in ancient times. In about 3000
B.C., IRANIAN ARCHITECTS used wind towers and underground air tunnels for
passive cooling. Underground air tunnels (UAT) systems, nowadays also known as
Earth to Air Heat Exchangers have been in use for years in developed countries
due to their higher energy utilization efficiencies compared to the conventional
heating and cooling system. Earth -air heat exchanger is a system of work that the
thermal inertia of the earth for heating / cooling use of buildings, offices,
residential, industrial, etc. or another word of earth-air heat exchangers are
effective as emphatic substitute for these rated can be used for heating / cooling the
building. This is a principally a series of metallic, plastic or concrete pipes
immerse below the earth at a particular depth. Energy savings of great thought is
everywhere a special challenge in the desert climate. The climate of the desert can
be classified as hot and dry and such a condition exists in a number of areas around
the world. In general, most people probably when the temperature is between 20 °
C and 26 ° C and a relative humidity is ranging from 40 to 60%.
Fig8: Earth Air Heat Exchanger
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Fig9: Earth Tube Exchanger With Its Component
These conditions are often achieved by the use of air conditioners. Air conditioning
is widely used for the comfort of the occupants and the industrial productions. It
can be effectively achieved by vapour compression machines, but to minimize due
to the depletion of ozone layer and global warming by chlorofluorocarbons and the
need for high-grade energy consumption various passive techniques are now
introduced a day, such a process is the ground coupled heat Exchanger.
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A ground-coupled heat exchanger is an underground heat exchanger that can
capture heat from and/or dissipate heat to the ground. They use the Earth's near
constant subterranean temperature to warm or cool air or other fluids for
residential, agricultural or industrial uses. If building air is blown through the heat
exchanger for heat recovery ventilation, they are called earth tubes (also known as
earth cooling tubes or earth warming tubes) in Europe or earth-air heat
exchangers (EAHE or EAHX) in North America. These systems are known by
several other names, including: air-to-soil heat exchanger, earth channels, earth
canals, earth-air tunnel systems, ground tube heat exchanger, hypocausts, subsoil
heat exchangers, thermal labyrinths, underground air pipes, and others.
Earth tubes are often a viable and economical alternative or supplement to
conventional central heating or air conditioning systems since there are no
compressors, chemicals or burners and only blowers are required to move the air.
These are used for either partial or full cooling and/or heating of facility ventilation
air. Their use can help buildings meet Passive House standards
or LEED certification.
Earth-air heat exchangers have been used in agricultural facilities (animal
buildings) and horticultural facilities (greenhouses) in the United States over the
past several decades and have been used in conjunction with solar chimneys in hot
arid areas for thousands of years, probably beginning in the Persian Empire.
Implementation of these systems in Austria, Denmark, Germany, and India has
become fairly common since the mid-1990s, and is slowly being adopted in North
America.
Ground-coupled heat exchanger may also use water or antifreeze as a heat transfer
fluid, often in conjunction with a geothermal heat pumpAn earthair heat exchanger
consist in one or more pipe/tubes below the earth about 2.5 to 3 m in order to cool
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in summer climates and pre-heat in winter climates air to be supplied in a building.
The physical phenomena of earth-air heat exchanger is simple the ground
temperature or undisturbed temperature of earth generally higher than the outdoor
air temperature in winter and lower in summer, so it makes the use of the earth
suitable as warm or cold sink respectively. Both of the above uses of earth air heat
exchanger can pass to reduction in energy consumption. Several researchers have
described the earth-to-air heat exchangers (EAHE) coupled with buildings as an
effective passive energy source for building space conditioning. An earth- to-air
heat exchanger system suitably meets heating and cooling energy loads of a
building. Its performance is based upon the seasonally varying inlet temperature,
and out let temperature which further depends on the ground temperature or
undisturbed temperature. The performance of the EAHE system depends on the
temperature and humidity distribution in the soil, as well as to the surface
conditions.
Working Principle:The principle of the basic inertia for heating and cooling
using is not a new concept, but a modified concept that goes back to the ancients.
This technology has been used throughout history by the ancient Greeks and
Persians in the pre-Christian era until recent history (Santamouris and
Asimakopoulos, 1996). For instance the Italians in the middle Ages used caves
called colvoli, to pre cool /pre heat the air before it entered the building. The
system, which is currently used, consists of a matrix of on buried pipelines,
through the air by a fan / blower. In summer, the supply of ambient air through the
tubes to the buildings is due to the fact, cooled, that the undisturbed temperature is
lower around the heat exchanger than the ambient temperature. Same as opposite
rule of winter climates, the undisturbed temperature is the greater than the ambient
temperature and the air gets preheated.
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Fig10: Earth Air Tube Heat Exchanger System
Types Of Earth Tube Heat Exchanger
There are two types of heat exchanger
A. Closed type Earth tube heat exchanger
B. Open type Earth tube heat exchanger
A. Closed type ETHE
Air from inside the home or structure is a U-shaped loop of typically 30 to 150 m
(100 to 500 ft) blown from tubes, where it will be hosted near ground temperature
before over in the house or the structure distribute air ducts returns. The closed
loop system may be more effective (while the air temperature extremes) as an open
system, as it cools and cools again, the same air. In this case heat exchangers are
arranged underground, either vertical or oblique position, and a heat transfer
medium in the heat exchanger circulates in horizontal to transfer the heat from the
soil to a heat pump, or vice versa.
1) Increases efficiency.
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2) Reduce moisture problems inside tube condenses.
3) Domestic air circulates through the heat exchanger Earth air tube.
Fig11: Closed Type ETHE
B. Open type ETHE
Outside air is drawn from a filtered air inlet. The cooling tubes are typically 30 m
(100 ft) long straight pipes in the home. An open system with energy recovery
ventilation is combined, can be almost as effective (80-95%) as a closed loop and
ensures that fresh air enters, is filtered and tempered. In open systems environment
39
air passes through pipes buried in the ground for preheating or pre-cooling and
then the air is heated or cooled by a conventional air conditioning unit before
entering the building.
1) Outside air is drawn into the tubes and air handling units (AHUs) or directly
supplied to the inside of the building.
2) Hopefully ventilation ensures under cooling or the building interior heating.
3) Improves indoor air quality (IAQ).
Fig12: Open Type ETHE
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Fig113:Pipes Arrangement In Ground
41
ETHE AnalysisEarth tubes are low technology, sustainable passive cooling-heating systems
utilized mostly to preheat a dwelling's air intake. Air is either cooled or heated by
circulating underground in horizontally buried pipes at a specified depth.
Specifically air is sucked by means of a fan or a passive system providing adequate
pressure difference from the ambient which enters the building through the buried
pipes. Due to ground properties the air temperature at the pipe outlet maintains
moderate values all around the year. Temperature fluctuates with a time lag (from
some days to a couple of months) mainly relative to the depth considered.
Temperature values remain usually in the comfort level range (15-27 °C).
This technology is not recommended for cooling of hot humid climates due to
moisture reaching dew point and often remaining in the tubes. However there are
southern European coastal regions as in Greece where the climate remains hot and
dry. In such locations these systems could have impressive results.
The material of a pipe can be anything from thin wall 'sewer' plastic, metal or
concrete. However concrete should better be avoided in order not to be dependent
on carbon filtration UV sterilization for the musty air coming out of concrete earth
tubes.
The effectiveness of a buried pipe system is mainly related to the following
parameters:
1) Ground temp. at depth of the installed exchanger
2) Thermal diffusivity of soil
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3) Pipe length, width
4) Inlet air temp.
5) Thermal conductivity of pipes
6) Air velocity
Our Approach
Fig14: Earth Tube Heat Exchanger
In the model we have developed, we have considered an open loop earth to air heat
exchanger including a 60 m low conductivity pipe of 0.10m diameter, 3m
underground at a moderate air velocity of 11m/s provided by a fan (300W power
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consumption) for a grass soil.
Ground Temperature
The ground temperature was approximated to be 15-17 degrees (yearly variation)
based on parameters mentioned above and the fact that local maximum
temperatures in Palermo don't exceed 30°C so often. Usually those varied between
23-30°C for a summer period significantly lower than 23-38C Furthermore these
values were compared to Jacovide's article used also by the Hellenic national
observatory. In this case a grass surface's mean and summer value at 2 m depth for
Greece's higher temperatures were 18.5C and 23.5C respectively.
The ground's temperature monthly values are illustrated below in °C :
JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC
14.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 16.5 16.0 15.5 15.0
Temperature variation (in May)
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Fig15: Variation of Temperature of Soil in May
Temperature Variation (in Jan)
Fig16: Variation of Temperature of Soil in January
Air Flow CalculationIn order to calculate the mass flow rate of the model’s pipes we have used the equation:
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The power value used in the above equation is the max power required
instantaneously by the two zones in the period assessed as noticed in
ESP-r cooling results.
As the flow rate is v=0.087m3/s and the cross section area is equal to
0.0078m2 the required velocity of the air through the pipes is 11m/s.
For the estimation of the convection coefficient we have used the
following equation:
From the above the convection coefficient is: hc=39.76W/m2K.
The sizing of our system and optimum values regarding air flow rate
and velocity where influenced by similar cases in literature relating to
climatic conditions and system magnitude, power consumption and
thermal load output.
Thermal COP ValuesCoefficient of performance is a measure of heat exchanger efficiency. It
is defined as (ASHRAE 1985). COP values mentioned bellow regard
the energy output to input ratio. Input values are the energy consumed
by the blower (300W) and output energy is the cooling or heating
thermal energy introduced in a building.
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Where,
Ti-Temperature of air entering the pipe
To-Temperature of outlet air
Ti and To are treated as bulk temperatures, independent of radial variation as the pipe diameter is only 10cm
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ConclusionIn this project the performance of earth air tube heat exchanger system find out and
we have observed the following:
1) The increasing of pipe length, decreasing pipe diameter and decreasing mass
flow rate of flowing air inside the buried pipe and earth below the depth up to 4 m
then the performance of EATHE becomes better.
2) EATHE can be used with the conventional air conditioning system and make it
more efficient.
3) EATHE is the better result of summer as well as in winter
4) The design of earth air tube heat exchanger mainly depends on the heating /
cooling load requirement of a building to be conditioned.
5) After calculation of heating /cooling load, the design of the earth air tube heat
exchanger only depends on the geometrical constraints and cost analysis.
6) The pipe length, diameter of pipe and number of pipes are the main of
parameters to be investigated.
7) With an increase of pipe length then pressure drop and thermal performance
increase.
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