heat excahnger

8/8/2019 Heat Excahnger

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HEAT EXCHANGER

A heat exchanger may be defined as equipment which transfer energy from

a hot fluid to a cold fluid with maximum rate and minimum investment and

running costs.

In heat exchanger the temperature of each fluid changes as it passes through the

heat exchanger ad hence the temperature of the dividing wall between the fluids

also changes along the length of the heat exchanger.

CLASSIFICATION OF HEAT EXCHANGER:

Heat exchangers are designed in so many sizes, types, configurations and

flow arrangements and used for so many purposes. These are classified

according to heat transfer process, flow arrangement and type of construction.


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According to Heat Transfer Process:

(i) Direct contact type.

In this type of heat exchanger, the two immiscible fluids atdifferent

temperatures are come in direct contact.

For the heat exchange between two fluids, one fluid is sprayed through the

other. It cannot be used for transferring heat between two gases or

between two miscible liquids.

Cooling towers, jet condensers, desuperheaters, open feed water heaters

and -scrubbers are the best examples of such heat exchangers.

A direct contact type heat exchanger (cooling tower) is shown in Figure

(i) In direct contact type.

Here the heat transfer occurs between the two fluid stream that do not mix

and are usually separated by a metallic wall in the form of pipes aor tube

Example: regenerator,recuperator or surface exchanger


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According to Flow Arrangement:

(i) Parallel flow :

The hot and cold fluids enter at same end of the heat exchanger,

flow through in same direction and leave at other end.

It is also called the concurrent heat exchanger Figure.

Example : oil coolers, oil heaters ,water heaters.

(ii) Counter flow:

The hot and cold fluids enter at the opposite ends of heat exchangers,

flow through in opposite direction and leave at opposite ends Figure

This types of heat exchanger give a maximum rate of heat transfer fora given surface area.

Hence such heat exchangers are favored for heating and cooling of

fluids.


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(iii) Cross flow:

o The two fluids flow at right angle to each other.

o The cross flow heat exchanger is further classified as unmixed flow and

mixed flow depending on the flow configuration.

o If both the fluids flow through individual channels and are not free to

move in transverse direction, the arrangement is called unmixed as

shown in Figure a.

o if any fluid flows on the surface and free to move in transverse direction,

then this fluid stream is said to be mixed as shown in Figureb.


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According to Physical states of the fluid

Depending upon the physical state of the fluids, the heat exchangers are

classified as follows

(i) Condensers

In a condenser, the condensing fluid remands at constant temperature

throughout the exchanger while the temperature of the cooler fluid

gradually increases from the inlet to outlet.

The hot fluid loses latent part of heat which is absorbed by the cold fluid.

(ii) Evaporators:

In this case, the boiling fluid (cold fluid) remains at constant temperature

while the temperature of the hot fluid gradually decreases from inlet to

outlet


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Logarithmic mean temperature difference


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Logarithmic mean temperature difference (LMTD) is defined s that temperature

difference which, if constant, would give the same rate of heat transfer as actually

occurs under variable condition of temperature difference.

In order to derive the expression for LMTD for various types of heat exchanger ,the

following assumption are made

1. The overall heat transfer coefficient U is constant.

2. The flow condition is steady.

3. The specific heat and mass flow rate of both fluids are constant.

4. There is no loss of heat to the surrounding, due to heat exchanger being perfectly

insulated.

5. There is no change of phase either of the fluid during the heat transfer.

6. The change in potential energy and kinetic energy are negligible.

7. Axial conduction along the tubes of the heat exchanger is negligible.


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Logarithmic mean temperature difference for PARALLEL FLOW heat

exchanger.

Figure a and b shows the arrangement and distribution of temperature in single

pass parallel flow heat exchanger respectively.


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Let us consider an elementary area dA of the heat exchanger.

The rate of flow of heat through these elementary areas is given by

As a result of heat transfer dQ through the area dA, the hot fluid is cooled by dh,

whereas the cold fluid is heated up by dtc.

The energy balance over the differential area dA may be written as


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Substituting the values of dQ from equation (4) the above equation becomes

Integrating between the inlet and outlet condition (i.e. from A=0 to A=A ) we get

Now the total heat transfer rate between the two fluids is given by

Or

Or


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The above equation can be written as as


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Logarithmic mean temperature difference for COUNTER FLOW heat

exchanger.

Figure a and b shows the arrangement and distribution of temperature in single

pass COUNTER flow heat exchanger respectively.


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Let us consider an elementary area dA of the heat exchanger.

The rate of flow of heat through these elementary areas is given by

In this case also, due to the heat transfer dQ through the area dA, the hot fluid is

cooled down by dth whereas the cold fluid is heated by dtc.

The energy balance over a differential area dA may be written as as

In a counter flow heat system ,the temperature of both the fluids decreases in the

direction of the heat exchanger length ,hence the negative signs

Substituting the values of dQ from equation (4) the above equation becomes


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Integrating between the inlet and outlet condition (i.e. from A=0 to A=A ) we get

Now the total heat transfer rate between the two fluids is given by

Or

Or


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The above equation can be written as as

Arithmetic temperature difference (AMTD)

When the temperature of the fluids are relatively small ,then the

temperature variation curves are approximately straight lines as

shown in the figure and adequately accurate results are obtained

by taking AMTD


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However, practical consideration suggest that the LMTD should be invariably used when

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Consideration of fouling or scaling

1. In a heat exchanger, during normal operation the tube surface get covered by

deposits of ash, soot ,dirt and scale etc

2. This phenomenon of rust formation and deposits of fluid impurities is called

fouling. Due to these surface deposits the thermal resistance is increases and

eventually the performance of the heat exchanger lowers.

3. Since it is difficult to ascertain the thickness and thermal conductivity of the

scale deposits ,the effect of scale on heat flow is considered by specifying an

equivalent scale heat transfer coefficient.hs

4. If hs1 and hs0 be the heat transfer coefficients for the scale deposited on the

inside and outsides surface respectively ,then the thermal resistance to scale

formation on the inside surface ( Rs1) and outside surface (Rs0) is given by

5. The reciprocal of scale heat transfer coefficient hS ,is called the fouling factor,

(Rf) or (F)


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6. Fouling factors are determined experimentally by testing the heat exchanger

in both the clean and dirty conditions. The fouling factor R f is thus defined as

Fouling processes

1. Precipitation or crystallization fouling.

2. Sedimentation or paniculate fouling

3. Chemical reaction fouling or polymerization

4. Corrosion fouling

5. Biological fouling

6. Freeze fouling

Parameter affecting fouling

1. Velocity

2. Temperature

3. Water chemistry


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4. Tube material

Prevention of fouling

The following method may be used to keep the fouling minimum

1. Design of heat exchanger

2. Treatment of process system

3. By using the cleaning system

Derivation for overall heat transfer coefficient considering

In case of heat exchanger heat transfer is given by


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Overall heat transfer coefficient based on inner surface area

Overall heat transfer coefficient based on outer surface area

Heat exchanger efficiency () and number of transfer units ( NTU)

A heat exchanger can be designed by the LMTD when the inlet and outlet

conditions are specified.

However, when the problem is to determine the inlet or exit temperature for

particular heat exchanger,the analysis is performed more easily ,by using amethod based on effectiveness of heat exchanger (concept first proposed by

Nusselt) and number of transfer units( NTU).the heat exchanger effectiveness is

defined as ratio of actual heat transfer to the maximum possible heat transfer.

Thus,


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The actual heat transfer rate Q can be determined by writing an energy balance

over either side of the heat exchanger

The product of mass flow rate and the specific heat, as a matter of convenience is

defines as the fluid capacity rate c:

The maximum rate of heat transfer for parallel and counter floe heat exchanger

would occur if the outlet temperature of the fluid with smaller value of C h or CC i.e.

Cmin were to be equal to the temperature of the other fluid.

The maximum possible temperature changes can be achieved by only one of the

fluids depending upon their rates, this maximum change cannot be obtained byboth the fluids except in the very special cases of equal heat capacity rates.

Qmax is the minimum of these two values

Once the effectiveness is known, the heat transfer rate can be very easily

calculated by using this equation


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Number of transfer units method (NTU method)

It is obvious form the above equation that effectiveness is a function of

several variables as such it is inconvenient to combine them in graphical or

tabular form

Also in calculation of effectiveness we required three temperatures, two out

of which are inlet temperature and one is outlet temperature. It is difficult to

predict outlet temperature of fluid before installing a heat exchanger.

However by compiling a non dimensional grouping, can be expressed as

a function of three dimensional parameter, this method is known as NTU

method. Here can be determined with the help of only inlet temperature.

This method also facilitates the comparison between the various types of

heat exchanger which may be used for a particular application.

The effectiveness expressions for the parallel flow and counter flow cases

can be derived as follows

1. Effectiveness of a heat exchanger is a function of several variable

and as such it is not convenient to combine them in graphical or

tabular form

2. However by compiling a non dimensional grouping, can be

expressed as a function of three dimensional parameter, this method

is known as NTU method.


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3. This method facilitated the comparison between the various type of

heat exchanger which may be used for particular application

Effectiveness for the parallel flow heat exchanger by NTU method

Consider an elemental layer at a distance of x having a thickness dx.

Let the temperature of the start of the elemental layer is Tn and Tc for the hot and

cold fluid.

We know,

Solving (1) and (2) we get


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Effectiveness for the counter flow heat exchanger by NTU method


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We know,

Solving (1) and (2) we get


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Prove that the effectiveness for condenser and evaporator

based on NTU method is given by

=1-e-NTU

Solution


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Derive any equation for effectiveness (parallel flow or counter

flow) then condenser for evaporator and condenser the ratio of

heat capacity.

We know,Effectiveness of parallel flow heat exchanger is

Scaling and fouling

In heat exchanger ,during the normal operation the tube surface get

covered by deposits of soot, dirt and scale etc


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This phenomenon of rust formation and deposition of fluid impurities is

called Scaling and fouling

The scale formed reduces the effectiveness of the heat exchanger and

thus it is undesirable and thus the thickness and thermal conductivity of

scale deposits are difficult to ascertain ,the effect of a scale on heat flow is

consider by specifying an equivalent scale heat transfer coefficient .

If hsi and hso denote the heat transfer coefficient for the scale

formed on the inside and outside surface respectively then

Thermal resistance due to scale on the inside surface is

Thermal resistance due to scale on the outside surface is

Considering the effect of scaling, the thermal resistance for a

cylindrical separating wall is expressed as




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Fouling factor

The reciprocal of scale heal transfer coefficient (hs) is called the

fouling factor .

It is denoted by Rf

Thus

Fouling factor are determined experimentally by testing the heat

exchanger in both the clean and dirty condition

Design of heat exchanger


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In the design of heat exchanger, following consideration is important

1. Heat transfer requirement

2. Cost

3. Physical stock

4. Pressure drop characteristics

In order to increase the overall heat transfer coefficient, the fluid may be

forced at higher velocity but this higher velocity results in a larger pressure

drop through the heat exchanger (meaning larger pump cost)

If the pressure drop is to keep minimum ,the surface area of the exchanger will

be larger due to lower value of overall heat transfer coefficient, but there is a

limit of physical size that can be accommodated and moreover a ,larger

physical size results in higher costs.

This suggest that a correct comprise between all these conflicting factors will

lead the proper design of a heat exchanger.

Heat exchanger are generally designed in following types

(i) Tubular heat exchanger or Concentric tubes.

These are also called tube in tube orconcentric tube ordouble pipe heatexchangeras shown in Figure .

These are widely used in many sizes and different flow arrangements

and type.

The effectiveness of the heat exchanger is increased by using swirling

flow.


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(ii) Shell and tube type heat exchanger.

These are also called surface condensers and are most commonly used for

heating, cooling, condensation or evaporation applications.

It consists of a shell and a large number of parallel tubes housing in it. The

heat transfer takes place as one fluid flows through the tubes and other fluid

flows outside the tubes through the shell.

The baffles are commonly used on the shell to create turbulence and to keep

the uniform spacing between the tubes and thus to enhance the heat transfer

rate.

They are having large surface area in small volume. A typical shell and tubetype heat exchanger is shown in Figure


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The shell and tube type heat exchangers are further classified according to

number of shell and tube passes involved.

A heat exchanger with all tubes make one U turn in a shell is called one shell

pass and two tube pass heat exchanger.

Similarly, a heat exchanger that involves two passes in the shell and four passes

in the tubes is called a two shell pass and four tube pass heat exchanger as

shown in Fig.9

(iii)Finned tube type.

When a high operating pressure or an enhanced heat transfer rate is

required, the extended surfaces are used on one side of the heat

exchanger.

These heat exchangers are used for liquid to gas heat exchange.

Fins are always added on gas side.

The finned tubes are used in gas turbines, automobiles, aero planes,

heat pumps, refrigeration, electronics, cryogenics, air-conditioningsystems etc.

The radiator of an automobile is an example of such heat exchanger.


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(iv) Compact heat exchanger.

These are special class of heat exchangers in which the heat

transfer surface area per unit volume is very large.

The ratio of heat transfer surface area to the volume is calledarea density.

A heat exchanger with an area density greater than 700 m2/m3is

called compact heat exchanger.

The compact heat exchangers are usually cross flow, in which the

two fluids usually flow perpendicular to each other.

These heat exchangers have dense arrays of finned tubes or

plates, where at least one of the fluid used is gas.

For example, automobile radiators have an area density in order

of 1100 m2/m3.


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Overall heat transfer coefficient

A heat exchanger is a device in which heat is transfers from one fluid to another

across a good conducting solid wall.

Thus the rate of heat transfer (Q) is given by

When the fluid are separted by plane wall


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When the two fluids are separated by a cylindrical wall , the cross section area of

the heat flow path is not constant but varies with radius. Thus it becomes

necessary to specify area upon which overall heat transfer coefficient is based

Thermal resistance



If the resistance due to material is neglected them


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Further if wall thickness is very small then

All the above equation are valid only for clean and uniformsurfaces.


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Heat exchanger effectiveness

1. It is defines as the ratio of actual transfer to the maximum

possible heat transfer.

2. It is denoted by

3. Maximum heat transfer Qmax is the minimum out of


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