chapter 1 introduction and basic concepts · • heat is first transferred from the hot fluid to...

HEAT EXCHANGERS

Heat Exchanger

Heat exchanger is an apparatus or an equipment in

which the process of heating or cooling occurs.

The heat is transferred from one fluid being heated

to another fluid being cooled.

OR

Heat exchanger is process equipment designed for

the effective transfer of heat energy between two

fluids; a hot fluid and a coolant.

The purpose may be either to remove heat from a

fluid or to add heat to a fluid.

Heat Exchanger

Examples of Heat exchangers are:

• Boilers, Evaporators, Heaters and

Condensers

Boilers

Evaporators Condensers

Heat Exchanger


• Automobile radiators and oil coolers of heat

engines

Automobile radiator Oil Coolers

Heat Exchanger


• Condensers and evaporators in refrigeration

units

• Water and air heaters or coolers

Air Heaters

Heat Exchanger

Heat transferred in the heat exchanger

may be in the form of latent heat (e.g. in

boilers and condensers) or sensible heat

(e.g. in heaters and coolers)

Classification of Heat Exchangers

1. Nature of heat exchange process

• Direct contact heat exchanger

• Indirect contact heat exchanger

Regenerator

Recuperator

2. Relative direction of motion of fluid

• Co-current (Parallel) flow

• Counter-current (Counter) flow

• Cross flow


3. Design and constructional features (Mechanical

design of heat exchange surface)

• Concentric tubes

• Shell and tube

• Multiple shell and tube passes

• Compact heat exchangers

4. Physical state of heat exchanging fluids

(condensation and evaporation)

• Condenser

• Evaporator


1. Nature of heat exchange

process

• Direct contact heat

exchanger

In a direct contact or

open heat exchanger

the exchange of heat

takes place by direct

mixing of hot and cold

fluids and transfer of

heat and mass take

place simultaneously.Direct contact or open

heat exchanger


The use of such units is made under conditions

where mixing of two fluids is either harmless or

desirable.

Examples: Cooing towers, Jet condensers,

direct contact feed heaters




In this type of heat exchanger, the heat

transfer between two fluids could be

carried out by transmission through wall

which separates the two fluids. This type

includes the following:

(a) Regenerator

(b) Recuperators or surface exchangers

Classification of Heat Exchangers1. Nature of heat exchange process


(a) Regenerator

The regenerator are those devices in which hot

and cold fluids alternately flow over the surface

(through a space containing solid particles

(matrix)).

The heat carried by the hot fluid is accumulated in

the walls of the equipment and is then transferred

to the cold fluid when it passes over the surface

next.

Regenerator

Fig shows a cylinder

containing a matrix that

rotates in such a way

that it passes alternately

through cold and hot gas

streams which are

sealed from each other.

Rotating Matrix Regenerator

Regenerator

Fig. shows a stationary

matrix regenerator in

which hot and cold

gases flow through them

alternately.

Stationary Matrix Regenerator

Regenerator

Examples: I.C. engines and gas turbines,

Open hearth and glass melting furnace, air

heaters of blast furnace

A regenerator generally operates periodically

(wall or solid matrix alternately stores heat

extracted from the hot fluid and then delivers it

to the cold fluid).

In some regenerators the matrix is made

rotate through the fluid passages arranged

side by side which makes the heat exchange

process continuous.

Regenerator

The performance of these regenerators is

affected by

• Heat capacity of regenerating material

• The rate of absorption

• The release of heat

Regenerator

Advantages of Regenerators

• Higher heat transfer coefficient

• Less weight per kW of the plant

• Minimum pressure drop

• Quick response to load variation

• Small bulk weight

• Efficiency quite high

Regenerator

Dis-advantages of Regenerators

• Costlier compared to recuperative heat

exchanger

• Leakage is the main trouble, therefore

perfect setting is required.




(b) Recuperator

Most common heat exchangers are the

recuperators in which both hot and cold fluids

separated from each other by a wall, flow

through the exchanger at the same time.

The heat transfer process consists of convection

between the fluid and wall, conduction through

the wall and convection between the wall and

other fluid.

In case the ∆T between wall and fluid is large,

radiation heat exchange may also occur.

Recuperator

Such exchangers are used where the cooling

and heating fluids can not be allowed to mix.

Examples:

Automobile radiators

Oil coolers, intercoolers, air preheaters,

economizers, super heaters, condensers and

surface feed heaters of a steam power plant

Milk chiller of pasteurizing plant

Evaporator of ice plant

Recuperator

Advantages of Recuperators

• Easy construction

• More economical

• More surface area for heat transfer

• Much suitable for stationary plants

Recuperator

Dis-advantages of Recuperators

• Less heat transfer coefficient

• Less generating capacity


2. Relative direction of motion of

fluid

• Co-current (Parallel) flow

(uni-direction flow)

In a parallel flow heat

exchanger, as the name

suggests, the two fluid

stream (hot and cold) travel

in the same direction.

The two stream enters at one

end and leaves at the other

end.

∆T between the hot and

cold fluids goes on

decreasing from inlet to

outlet

Parallel flow Heat Exchanger

Co-current (Parallel) flow

Since this type of heat exchanger needs a

large area of heat transfer, therefore, it is

rarely used in practice.

Examples: Oil coolers, oil heaters, water

heaters

As the two fluids are separated by a wall, this

type of heat exchanger may be called

parallel flow recuperator or surface heat

exchanger


2. Relative direction of

motion of fluid

• Counter flow

In counter flow heat

exchanger, the two fluids

flow in opposite

directions.

The hot and cold fluids

enter at the opposite

ends.∆T between the two fluids

remains more or less

nearly constant.

Counter flow Heat Exchanger

Counter flow

This type of heat exchanger, due to counter

flow, gives maximum rate of heat transfer for

a given surface areas.

Hence such heat exchangers are most

favored for heating and cooling of fluids.


2. Relative direction of

motion of fluid

• Cross flow

In cross flow heat

exchangers, the two

fluids (hot and cold)

cross one another in

space usually at right

angles.

Different flow configurations in cross-flow

heat exchangers.

Cross flow

Hot fluid flows in the

separate tubes and there

is no mixing of the fluid

streams. The cold fluid is

perfectly mixed as it flows

through exchanger. The

temperature of this mixed

fluid will be uniform across

any section and will vary

only in the direction of flow.

Examples: the cooling unit

of refrigeration system


In this case each of the

fluids follows a

prescribed path and is

unmixed as it flows

through heat

exchanger. Hence the

temperature of the fluid

leaving the heater

section is not uniform.

Examples: Automobile

radiator

Cross flow

In yet another arrangement, both the

fluids are mixed while they travel through

the exchanger; consequently the

temperature of both the fluids is uniform

across the section and varies only in the

direction in which flow takes place.




• Concentric tubes

In this type, two concentric tubes are used,

each carrying one of the fluids.

The direction of flow may be parallel or counter

The effectiveness of the heat exchanger is

increased by using swirling flow.




• Shell and tube

In this type of heat exchanger one of the fluids

flows through a bundle of tubes enclosed by a

shell.

The other fluid is forced through the shell and it

flows over the outside surface of the tubes.

Such an arrangement is employed where

reliability and heat transfer effectiveness are

important

35

Shell-and-tube heat exchanger: The most common type of heat

exchanger in industrial applications.

They contain a large number of tubes (sometimes several hundred)

packed in a shell with their axes parallel to that of the shell. Heat transfer

takes place as one fluid flows inside the tubes while the other fluid flows

outside the tubes through the shell.

Shell-and-tube heat exchangers are further classified according to the

number of shell and tube passes involved.

The schematic of a shell and tube heat exchanger (one shell pass and one

tube pass)




• Multiple shell and tube passes

Multiple shell and tube passes are used for

enhancing the overall heat transfer. Multiple

shell pass is possible where the fluid flowing

through the shell is re-routed. The shell side

fluid is forced to flow back and forth across the

tubes by baffles. Multiple tube pass

exchangers are those which re-route the fluid

through tubes in the opposite direction.

Multi pass flow arrangements in

shell and tube heat exchangers




• Compact heat exchangers

There are special purpose heat exchangers

and have a very large surface area per unit

volume of the exchanger. They are generally

employed when convective heat transfer

coefficient associated with one of the fluids is

much smaller than that associated with the

other fluid.

Example: Plate fin, flattened fin tube

exchangers

Plate and frame (or just plate) heat exchanger: Consists of a series of

plates with corrugated flat flow passages. The hot and cold fluids flow in

alternate passages, and thus each cold fluid stream is surrounded by two

hot fluid streams, resulting in very effective heat transfer. Well suited for

liquid-to-liquid applications.

A plate-and-frame

liquid-to-liquid heat

exchanger.


4. Physical state of heat

exchanging fluids (condensation

and evaporation)

• Condenser

In condenser, the condensing

fluid remains at constant

temperature throughout the

exchanger while the

temperature of the colder fluid

gradually increases from inlet

to outlet. The hot fluid loses

latent part of heat which is

accepted by the cold fluid.


4. Physical state of heat

exchanging fluids

(condensation and

evaporation)

• Evaporator

In this case, the boiling

fluid (cold fluid) remains

at constant temperature

while the temperature of

hot fluid gradually

decreases from inlet to

outlet.

42

THE OVERALL HEAT TRANSFER COEFFICIENT

• A heat exchanger typically involves two

flowing fluids separated by a solid wall.

• Heat is first transferred from the hot fluid to

the wall by convection, through the wall by

conduction, and from the wall to the cold

fluid again by convection.

• Any radiation effects are usually included in

the convection heat transfer coefficients.

Thermal resistance network

associated with heat transfer in

a double-pipe heat exchanger.

43

U the overall heat transfer

coefficient, W/m2C

When

The overall heat transfer coefficient U is dominated by the smaller convection

coefficient. When one of the convection coefficients is much smaller than the other

(say, hi << ho), we have 1/hi >> 1/ho, and thus U hi. This situation arises frequently

when one of the fluids is a gas and the other is a liquid. In such cases, fins are

commonly used on the gas side to enhance the product UA and thus the heat

transfer on that side.

The two heat transfer surface

areas associated with a double

pipe heat exchanger (for thin

tubes, Di≈Do and thus Ai ≈Ao

44

The overall heat transfer coefficient

ranges from about 10 W/m2C for

gas-to-gas heat exchangers to about

10,000 W/m2C for heat exchangers

that involve phase changes.

For short fins of high

thermal conductivity, we

can use this total area in

the convection

resistance relation

Rconv = 1/hAs

To account for fin efficiency

When the tube is finned on one

side to enhance heat transfer, the

total heat transfer surface area on

the finned side is

45

Fouling Factor

The performance of heat exchangers usually deteriorates with time as a result of

accumulation of deposits on heat transfer surfaces. The layer of deposits represents

additional resistance to heat transfer. This is represented by a fouling factor Rf.

The fouling factor increases with the operating temperature and the length of

service and decreases with the velocity of the fluids.

Precipitation fouling of ash

particles of superheater tubes

Points worth noting

1. The overall heat transfer coefficient depends

upon the following factors:

• The flow rate

• The properties of the fluid

• The thickness of material

• The surface condition of the tubes and

• The geometric configuration of the heat

exchanger

Points worth noting

2. The overall heat transfer coefficient U will

generally decrease when any of the fluids (e.g.

tars, oils or any of the gases) having low values

of heat transfer coefficient, h flows on one side of

the exchanger.

3. The highly conducting liquids such as water and

liquid metals give much higher values of heat

transfer coefficient, h and overall heat transfer

coefficient, U. In case of boiling and

condensation processes also, the value of U are

high.

Points worth noting

4. All the thermal resistances in the heat exchanger

must be low for its efficient and effective design.

Common failures in heat exchanger

• Choking of tubes either expected or extraordinary

• Excessive transfer rates in heat exchanger

• Increasing the pump pressure to maintain throughout

• Failure to clean tubes at regularly scheduled intervals

• Excessive temperatures in heat exchangers

• Lack of control of heat exchangers atmosphere to retard

scaling

• Increased product temperature over a safe design limit.

• Unexpected radiation from refractory surfaces

• Unequal heating around the circumference or along the

length of tubes

ANALYSIS OF HEAT EXCHANGERSAn engineer often finds himself or herself in a position

1. to select a heat exchanger that will achieve a specified temperature

change in a fluid stream of known mass flow rate - the log mean

temperature difference (or LMTD) method.

2. to predict the outlet temperatures of the hot and cold fluid streams in

a specified heat exchanger - the effectiveness–NTU method.

The rate of heat transfer in heat exchanger (HE is insulated):

heat capacity rate

Two fluid

streams that

have the same

capacity rates

experience the

same

temperature

change in a well-

insulated heat

exchanger.

Variation of

fluid

temperatures

in a heat

exchanger

when one of

the fluids

condenses or

boils.

is the rate of evaporation or condensation of the fluid

hfg is the enthalpy of vaporization of the fluid at the specified temperature or pressure.

The heat capacity rate of a fluid during a phase-change process must approach

infinity since the temperature change is practically zero.

Tm an appropriate mean (average)

temperature difference between the two fluids

THE LOG MEAN TEMPERATURE DIFFERENCE

METHOD

Variation of the fluid

temperatures in a

parallel-flow double-pipe

heat exchanger.log mean

temperature

difference

The arithmetic mean temperature difference

The logarithmic mean temperature

difference Tlm is an exact representation

of the average temperature difference

between the hot and cold fluids.

Note that Tlm is always less than Tam.

Therefore, using Tam in calculations

instead of Tlm will overestimate the rate of

heat transfer in a heat exchanger between

the two fluids.

When T1 differs from T2 by no more than

40 percent, the error in using the arithmetic

mean temperature difference is less than 1

percent. But the error increases to

undesirable levels when T1 differs from

T2 by greater amounts.

Counter-Flow Heat Exchangers

In the limiting case, the cold fluid will be

heated to the inlet temperature of the hot

fluid.

However, the outlet temperature of the cold

fluid can never exceed the inlet

temperature of the hot fluid.

For specified inlet and outlet temperatures,

Tlm a counter-flow heat exchanger is

always greater than that for a parallel-flow

heat exchanger.

That is, Tlm, CF > Tlm, PF, and thus a

smaller surface area (and thus a smaller

heat exchanger) is needed to achieve a

specified heat transfer rate in a counter-

flow heat exchanger.

When the heat capacity rates

of the two fluids are equal

55

Multipass and Cross-Flow Heat Exchangers:

Use of a Correction Factor

F correction factor depends on the

geometry of the heat exchanger and the

inlet and outlet temperatures of the hot

and cold fluid streams.

F for common cross-flow and shell-and-

tube heat exchanger configurations is

given in the figure versus two

temperature ratios P and R defined as

1 and 2 inlet and outlet

T and t shell- and tube-side temperatures

F = 1 for a condenser or boiler

56

Correction factor

F charts for

common shell-

and-tube heat

exchangers.

57

Correction

factor F charts

for common

cross-flow heat

exchangers.

The LMTD method is very suitable for determining the size of a

heat exchanger to realize prescribed outlet temperatures

when the mass flow rates and the inlet and outlet

temperatures of the hot and cold fluids are specified.

With the LMTD method, the task is to select a heat exchanger

that will meet the prescribed heat transfer requirements. The

procedure to be followed by the selection process is:

1. Select the type of heat exchanger suitable for the application.

2. Determine any unknown inlet or outlet temperature and the

heat transfer rate using an energy balance.

3. Calculate the log mean temperature difference Tlm and the

correction factor F, if necessary.

4. Obtain (select or calculate) the value of the overall heat

transfer coefficient U.

5. Calculate the heat transfer surface area As .

The task is completed by selecting a heat exchanger that has a

heat transfer surface area equal to or larger than As.

THE EFFECTIVENESS–NTU METHODA second kind of problem encountered in heat exchanger analysis is the

determination of the heat transfer rate and the outlet temperatures of the hot and

cold fluids for prescribed fluid mass flow rates and inlet temperatures when the

type and size of the heat exchanger are specified.

Heat transfer effectiveness

the maximum possible heat transfer rate

Cmin is the smaller of Ch and Cc

Actual heat transfer rate

The effectiveness of a

heat exchanger depends

on the geometry of the

heat exchanger as well

as the flow arrangement.

Therefore, different types

of heat exchangers have

different effectiveness

relations.

We illustrate the

development of the

effectiveness e relation

for the double-pipe

parallel-flow heat

exchanger.

Effectiveness relations of the heat exchangers typically involve the

dimensionless group UAs /Cmin.

This quantity is called the number of transfer units NTU.

For specified values of U and Cmin, the value

of NTU is a measure of the surface area As.

Thus, the larger the NTU, the larger the heat

exchanger.capacity

ratio

The effectiveness of a heat exchanger is a function of the

number of transfer units NTU and the capacity ratio c.

Effectiveness

for heat

exchangers.

When all the inlet and outlet temperatures are specified, the size of

the heat exchanger can easily be determined using the LMTD

method. Alternatively, it can be determined from the effectiveness–

NTU method by first evaluating the effectiveness from its definition

and then the NTU from the appropriate NTU relation.

(e.g., boiler, condenser)

Observations from the effectiveness relations and charts

• The value of the effectiveness ranges from 0 to 1. It

increases rapidly with NTU for small values (up to about

NTU = 1.5) but rather slowly for larger values. Therefore,

the use of a heat exchanger with a large NTU (usually

larger than 3) and thus a large size cannot be justified

economically, since a large increase in NTU in this case

corresponds to a small increase in effectiveness.

• For a given NTU and capacity ratio c = Cmin /Cmax, the

counter-flow heat exchanger has the highest

effectiveness, followed closely by the cross-flow heat

exchangers with both fluids unmixed. The lowest

effectiveness values are encountered in parallel-flow heat

exchangers.

• The effectiveness of a heat exchanger is independent of

the capacity ratio c for NTU values of less than about 0.3.

• The value of the capacity ratio c ranges between 0 and 1.

For a given NTU, the effectiveness becomes a maximum

for c = 0 (e.g., boiler, condenser) and a minimum for c = 1

(when the heat capacity rates of the two fluids are equal).

SELECTION OF HEAT EXCHANGERSThe uncertainty in the predicted value of U can exceed 30 percent. Thus, it is

natural to tend to overdesign the heat exchangers.

Heat transfer enhancement in heat exchangers is usually accompanied by

increased pressure drop, and thus higher pumping power.

Therefore, any gain from the enhancement in heat transfer should be weighed

against the cost of the accompanying pressure drop.

Usually, the more viscous fluid is more suitable for the shell side (larger

passage area and thus lower pressure drop) and the fluid with the higher

pressure for the tube side.

The proper selection of

a heat exchanger depends

on several factors:

• Heat Transfer Rate

• Cost

• Pumping Power

• Size and Weight

• Type

• Materials

The annual cost of electricity associated with

the operation of the pumps and fans

The rate of heat transfer in the

prospective heat exchanger

Summary

• Types of Heat Exchangers

• The Overall Heat Transfer Coefficient

Fouling factor

• Analysis of Heat Exchangers

• The Log Mean Temperature DifferenceMethod

Counter-Flow Heat Exchangers

Multipass and Cross-Flow Heat Exchangers:Use of a Correction Factor

• The Effectiveness–NTU Method

• Selection of Heat Exchangers

chapter 1 introduction and basic concepts · • heat is first transferred from the hot fluid to...

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