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

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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

36

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)

44

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:

 

45

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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