HEAT EXCHANGER Nitesh Dattaram Kamerkar

INTRODUCTION

FUNCTIONING

CLASSIFICATIONS

APPLICATIONS

CHALLENGES

Overview of

Heat

Exchanger

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HEAT TRASFER o People have always understood that something flows from hot objects to cold ones.

We call that flow “heat”. o The flow of the heat is all-pervasive. It is active to some degree or another in

everything. Heat flows constantly from our bloodstream to the air around us. Such

processes go on in all plant, animal life and in the air around us. They occur

throughout the earth, which is hot at its core and cooled around its surface. o Heat transfer describes the exchange of thermal energy, between physical systems

depending on the temperature and pressure, by dissipating heat.

o The exchange of kinetic energy of particles through the boundary between two

systems which are at different temperatures from each other or from their

surroundings. Heat transfer always occurs from a region of high temperature to

another region of lower temperature.

o Heat transfer changes the internal energy of both systems involved according to the

First Law of Thermodynamics. The Second Law of Thermodynamics defines the

concept of thermodynamic entropy, by measurable heat transfer.

o Thermal equilibrium is reached when all involved bodies and the surroundings reach

the same temperature. Thermal expansion is the tendency of matter to change in

volume in response to a change in temperature.

o The fundamental modes of heat transfer are conduction or diffusion, convection and

radiation.

VARIOUS MODES OF HEAT TRANSFER ARE AS FOLLOWS:-

CONDUCTION

o Conduction through a medium

- Solid, like aluminum or steel

- Gas, like still air or water.

o Occurs in fins and tubes of heat exchangers.

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CONVECTION

o From flowing fluid to a surface

- Flow may be due to pump, fan and motion of vehicle or

buoyancy driven. - Convection coefficients determined by analysis for simple

geometries or by test for most applications.

o Occurs from the fluid to the fins and tubes of heat

exchangers.

RADIATION

o From one surface to another

- Radiation in infrared wavelengths.

- Highly dependent on surface properties

o Generally small (ignored) in most heat exchanger applications.

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FUNCTION OF HEAT EXCHANGER o A heat exchanger is a device that is used to transfer thermal energy (enthalpy)

between two or more fluids, between a solid surface and a fluid, or between solid

particulates and a fluid, at different temperatures and in thermal contact.

o In heat exchangers, there are usually no external heat and work interactions. Typical

applications involve heating or cooling of a fluid stream of concern and evaporation

or condensation of single- or multi component fluid streams.

o In other applications, the objective may be to recover or reject heat, or sterilize,

pasteurize, fractionate, distill, concentrate, crystallize, or control a process fluid.

o In a few heat exchangers, the fluids exchanging heat are in direct contact.

o In most heat exchangers, heat transfer between fluids takes place through a

separating wall or into and out of a wall in a transient manner.

o In many heat exchangers, the fluids are separated by a heat transfer surface, and

ideally they do not mix or leak. Such exchangers are referred to as direct transfer

type, or simply recuperators.

o In contrast, exchangers in which there is intermittent heat exchange between the hot

and cold fluids—via thermal energy storage and release through the exchanger

surface or matrix—are referred to as indirect transfer type, or simply regenerators.

Such exchangers usually have fluid leakage from one fluid stream to the other, due

to pressure differences and matrix rotation/valve switching. Common examples of

heat exchangers are shell-and tube exchangers, automobile radiators, condensers,

evaporators, air pre-heaters, and cooling towers.

o If no phase change occurs in any of the fluids in the exchanger, it is sometimes

referred to as a sensible heat exchanger. There could be internal thermal energy

sources in the exchangers, such as in electric heaters and nuclear fuel elements.

o Combustion and chemical reaction may take place within the exchanger, such as in

boilers, fired heaters, and fluidized-bed exchangers.

o Mechanical devices may be used in some exchangers such as in scraped surface

exchangers, agitated vessels, and stirred tank reactors. Heat transfer in the

separating wall of a recuperator generally takes place by conduction.

o However, in a heat pipe heat exchanger, the heat pipe not only acts as a separating

wall, but also facilitates the transfer of heat by condensation, evaporation, and

conduction of the working fluid inside the heat pipe. In general, if the fluids are

immiscible, the separating wall may be eliminated, and the interface between the

fluids replaces a heat transfer surface, as in a direct-contact heat exchanger.

o Heat Exchangers are widely used in space heating, refrigeration, air

conditioning, power plants, chemical plants, petrochemical plants, petroleum

refineries, natural gas processing, and sewage treatment.

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CLASSIFICATIONS OF HEAT EXCHANGER1

CLASSIFICATION ACCORDING TO TRANSFER PROCESS :-

- Indirect contact type

Direct transfer type

1. Single phase

2. Multiphase

Storage type

Fluidized Bed

- Direct contact type

Immiscible fluids

Gas - liquid

Liquid – vapor

CLASSIFICATION ACCORDING TO NUMBER OF FLUIDS:-

- Two – fluids

- Three – fluids

- N – fluids (N > 3)

CLASSIFICATION ACCORDING TO SURFACE COMPACTNESS :-

- Gas –to- fluid

1. Compact (β ≥ 700 m2/m3)

2. Non compact (β < 700 m2/m3)

- Liquid –to- liquid and phase change

1. Compact (β ≥ 400 m2/m3)

2. Non compact (β < 400 m2/m3)

CLASSIFICATION ACCORDING TO CONSTRUCTION :-

- TUBULAR

Double-pipe

Shell-and-tube 1. Cross flow to tubes

2. Parallel flow to tubes

1Classification of heat exchangers (Shah, 1981)

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Spiral Tube

Pipe coils

- PLATE TYPE

PHE (Plate Heat Exchanger)

1. Gasketed 2. Welded

3. Brazed

Spiral

Plate coil

Printed Circuit

- EXTENDED SURFACE

Plate – fin

Tube – fin 1. Ordinary separating wall

2. Heat – pipe wall

- REGENERATIVE

Rotary

Fixed – matrix

Rotary hoods

CLASSIFICATION ACCORDING TO FLOW ARRANGEMENTS:-

- Single – pass

1. Counter flow

2. Parallel flow

3. Cross flow

4. Split-flow

5. Divided-flow

- Multipass

Extended surface

1. Cross- counter flow

2. Cross- parallel flow

3. Compound flow

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Shell-and-tube

1. Parallel counter flow

a. m- shell passes

b. n- tube passes

2. Split- flow

3. Divided- flow

- Plate

Fluid 1 m passes

Fluid 2 n passes

CLASSIFICATION ACCORDING TO HEAT TRANSFER MECHANISMS:-

Single- phase convection on both sides

Single- phase convection on one side, two- phase convection on other side

Two- phase convection on both sides

Combined convection and radiative heat transfer

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TERMINOLOGY USED IN HEAT EXCHANGERS

TERMINOLOGY DEFINITION UNIT

Capacity Ratio Ratio of the products of mass flow rate and specific heat capacity

of the cold fluid to that of the hot fluid. Also computed by the ratio

of temperature range of the hot fluid to that of the cold fluid.

Higher the ratio greater will be size of the exchanger

Density It is the mass per unit volume of a material kg/m3

Effectiveness Ratio of the cold fluid temperature range to that of the inlet

temperature difference of the hot and cold fluid. Higher the ratio

lesser will be requirement of heat transfer surface

Fouling The phenomenon of formation and development of scales and

deposits over the heat transfer surface diminishing the heat flux.

The process of fouling will get indicated by the increase in pressure

drop

Fouling Factor The reciprocal of heat transfer coefficient of the dirt formed in the

heat exchange process. Higher the factor lesser will be the overall

heat transfer coefficient.

(m2.K)/W

Heat Duty The capacity of the heat exchanger equipment expressed in terms

of heat transfer rate, viz. magnitude of energy or heat transferred

per time. It means the exchanger is capable of performing at this

capacity in the given system

W

Heat exchanger Refers to the nomenclature of equipment designed and

constructed to transmit heat content (enthalpy or energy) of a

comparatively high temperature hot fluid to a lower temperature

cold fluid wherein the temperature of the hot fluid

decreases (or remain constant in case of losing latent heat of

condensation) and the temperature of the cold fluid increases (or

remain constant in case of gaining latent heat of vaporization). A

heat exchanger will normally provide indirect contact heating. E.g.

A cooling tower cannot be called a heat exchanger where water

is cooled by direct contact with air

Heat Flux The rate of heat transfer per unit surface of a heat

Exchanger

W/ m2

Individual Heat

transfer

Coefficient

The heat flux per unit temperature difference across Heat transfer

boundary layer of the hot / cold fluid film formed at the heat

transfer surface. The magnitude of heat transfer coefficient

indicates the ability of heat conductivity of the given fluid. It

increases with increase in density, velocity, specific heat, geometry

of the film forming surface

W/( m2.K)

LMTD

Correction

factor

Calculated considering the Capacity and effectiveness of a heat

exchanging process. When multiplied with LMTD gives the

corrected LMTD thus accounting for the temperature driving force

for the cross flow pattern as applicable inside the exchanger

Logarithmic

Mean

Temperature

difference

(LMTD)

The logarithmic average of the terminal temperature approaches

across a heat exchanger

°C

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Overall Heat

transfer

Coefficient

The ratio of heat flux per unit difference in approach across a heat

exchange equipment considering the individual coefficient and

heat exchanger metal surface conductivity. The magnitude

indicates the ability of heat transfer for a given surface. Higher the

coefficient lesser will be the heat transfer surface requirement

W/(m2.K)

Pressure drop The difference in pressure between the inlet and

outlet of a heat exchanger

Bar

Specific heat

capacity

The heat content per unit weight of any material per degree

raise/fall in temperature

J/(kg.K)

Temperature

Approach

The difference in the temperature between the hot and cold fluids

at the inlet / outlet of the heat exchanger. The greater the

difference greater will be heat transfer flux

°C

Thermal

Conductivity

The rate of heat transfer by conduction though any substance

across a distance per unit temperature difference

W/(m2.K)

Viscosity The force on unit volume of any material that will cause per

velocity

Pa (Pascal)

TEMA DESIGNATIONS OF HEAT EXCHANGERS

Because of the number of variations in mechanical designs for front and rear heads and

shells, and for commercial reasons,The Tubular Exchanger Manufacturers

Association(TEMA) has designated a system of notations that correspond to each major

type of front head, shell style and rear head. The first letter identifies the front head, the

second letter identifies the shell type and the third letter identifies the rear head type.

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SIGNIFICANT INDUSTRIES IN WHICH HEAT EXCHANGERS ARE USED

EXTENSIVE HVACR { Heating- Ventilation- Air Conditioning- Refrigeration}

Heat transfer is one of the most important industrial processes. Throughout any industrial

facility, heat must be efficiently added, removed or moved from one process stream to

another. In massive organizations human comfort is the big thing to tackle with in order to

obtain maximum output from their employees. For employee’s comfort all industries no

matter whether it’s small or big, install HVACR (Heat- Ventilation- Air Conditioning-

Refrigeration).

Typical Applications are:-

o Community heating

o District heating

o Geothermal heating

o Solar heating

o Steam heating

o Swimming pool heating

o Tap water heating

o Condenser protection

o District cooling

o Free cooling

o Glycol saving

o Pressure breaker

o Thermal storage

POWER INDUSTRY

As we already mentioned Heat transfer is one of the most important industrial processes

so be it power generation from coal power plant, oil power plant or gas power plant. In

power industry we burn the fuel- coal, oil or gas in order to generate heat and then that

heat is used to run the turbine and power our houses. During the whole process a lot of

heat losses occur. To minimize that heat loss & in other way to optimize great yield

output, heat exchangers plays major role at various stages of energy conversion process.

In fact Boilers are one kind of heat exchanger. Apart from Boilers many more type of

heat exchangers are used in Power Industry.

FOOD & AGRICULTURE

Heat exchangers are extensively used for drying process involved in Food & Agriculture.

Achieving the desired taste, a stable shelf life and commercial sterility of food products is

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reliant upon tight control of process timing and temperatures as well as careful and

correct raw ingredients and materials.

Typical Applications are:-

o Wide ranges of gums are used throughout the food industry. These can consist of

hydrocolloids, biopolymers and pectins. Heat exchangers work hard to tackle the

tough challenges of processing gums.

o The overall economy of sugar production is heavily reliant upon the cost of

energy. Plate heat exchanger and plate evaporator help sugar producers to

maximize product quality and minimize operating and energy costs.

o Purpose-built plate or tubular heat exchangers decrease energy consumption in

dairy processing. And whether it’s processing of milk, long-life products, cultured

products, ice cream, cheese or whey products the proven design of plate heat

exchangers offer superior hygienic reliability and simple cleaning-in-place (CIP)

routines.

CHEMICAL / PETROCHEMICAL

Plate heat exchangers have been successfully employed for decades in the chemical

industry in the most diverse sectors, such as the cooling and heating of base,

intermediate and final products, heat recovery or also the tempering of containers,

reactors and autoclaves.

Typical Applications are:-

o Cooling and heating of acids and caustic solutions

o Cooling of highly-viscous products (e.g. latex)

o Tempering and condensation of solvents (e.g. toluene)

o Cooling of water circuits

o Condensation of exhaust vapors, steam and multiple-material mixtures

o Secondary circuits with high levels of temperature similarity (∆t <2°C)

o Safety circuits to avoid contamination

PULP & PAPER

Certain industries benefit more than others from specific industrial products or machines.

Heat exchangers are pretty popular across the board, but are absolute essentials for the

paper industry. Without heat exchangers, paper mills would have exceedingly high

energy costs, which is bad for the environment as well as the purse size of the paper

companies. There are a couple ways that the paper and pulp industry utilize heat

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exchangers, both of which are financial boosts, eco-friendly, efficient and effective

methods.

In essence, the paper process, beginning with wood pulp processing and refining and

then going into the bleaching and cleaning before the paper formation, is all done with

heat exchanger help. Specifically, plate heat exchangers are used to heat the liquids

used to create pulp from wood, a process involving chemical compounds that pull apart

the wood structure, leaving a goopy like substance that can be formed into paper after

more processing. Secondary processes involve bleaching or dying the pulp, which is a

process also heated by plate or spiral heat exchangers. After the desired color is

achieved, the pulp becomes paper by way of a paper machine, which knits the pulp

into thin webs that form sheets. During which all the moisture is removed and some type

of blower is utilized to dry it out completely.

MARINE

When conditions are tough, crew and equipment are really put to the test. The main

engine oil cooler and central fresh water cooler simply have to work. There is no room for

compromise when the sea is rough and the harbour far away.

For many decades plate heat exchangers have proved to be the perfect solution for

various closed-circuit cooling systems at sea. They are also frequently found in other

applications on board, such as tap-water production systems and HVAC systems.

A plate heat exchanger offers many advantages compared with conventional shell-and-

tube exchangers

o Up to 50% more efficient

o Up to 90% more compact

o 3-5 times higher k-values

o Unique turbulent flow design

o Closer temperature approach – as low as 1K

o Far less material – less use of exotic alloys or titanium

Likewise heat exchangers are used in almost every industry for heat transfer and heat recovery

applications. Even for our residential uses we all uses numerous types of heat exchangers like

Air Conditioning, Refrigerator, Oven, Water Heater and many more to mention.

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CHALLENGES IN HEAT EXCHANGERS :-

Some of the major Challenges in Heat Exchangers are :-

o To minimize size and weight.

o To minimize pressure drop.

o To meet required life.

o To be resistant to fouling and contamination.

o To minimize cost.

Fouling

Fouling occurs when impurities deposit on the heat exchange surface. Deposition of

these impurities can decrease heat transfer effectiveness significantly over time and are

caused by:

o Low wall shear stress

o Low fluid velocities

o High fluid velocities

o Reaction product solid precipitation

o Precipitation of dissolved impurities due to elevated wall temperatures

Heat exchanger performance can deteriorate with time, off design operations and

other interferences such as fouling, scaling etc. It is necessary to assess periodically the

heat exchanger performance in order to maintain them at a high efficiency level.

A heat Exchanger in steam power station contaminated with fouling