1 418 Heat Exchangers

8/12/2019 1 418 Heat Exchangers

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HEAT EXCHANGERS

Dr. Ali K. Abdel-RahmanM echani cal Engineer ing Depar tment

F acul ty of Engineer ing

Assiut Universi ty


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OUTLINE

Definition & classification of heat exchangers

Heat exchangers according to construction Plate heat exchangers Extended surface heat exchangers Tubular heat exchangers

Heat exchangers according to phase change Condensers Evaporators


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WHAT IS A HEAT EXCHANGER?

They are devices specially designed for the efficienttransfer of heat from one fluid to another fluid over

a solid surface.


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WHAT ARE HEAT EXCHANGERSUSED FOR?

They have the function to transfer heat asefficiently as possible. Heat exchangersare widely used in :

I. refrigerationII. air conditioning

III. space heatingIV. electricity generationV. chemical processing


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CLASSIFICATION OF HEAT

EXCHANGERS

Heat exchangers may be classifiedaccording to the following main criteria:

Recuperators and regenerators Transfer processes: direct contact and indirect

contact Geometry of constructions: tubes, plates and

extended surfaces Phase change mechanisms: condensers andevaporators

Flow arrangements: parallel, counter and cross flow


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RECUPERATORS

The conventional heat exchangers with heattransfer between two fluids.

Hot stream A recovers some of the heat from

stream B.


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REGENERATORS

Storage type heat exchangers.The same flow passage (matrix) is alternatelyoccupied by one of the two fluids.

Thermal energy is not transfered through the wall.


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TRANSFER PROCESSES

Heat transfer between the cold

and hot fluidsthrough a directcontact betweenthese fluids.

Examples: Spray andtray condensers,cooling towers.

1. Direct contact type heat exchangers:


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2. I ndirect contact type heat exchangers :

Heat energy isexchanged between

hot and cold fluidsthrough a heattransfer surface.

The fluids are notmixed.


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FLOW ARRANGEMENTS

1. Paralel F low H eat Exchangers:

Two fluid streams enter together at one end,flow through in the same direction, and leavethrough at the other end.


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2. Counter F low H eat Exchangers:

Two fluid streams flow in opposite directions.


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3. Cross Flow Heat Exchangers:

The direction of fluids are perpendicular to eachother.


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BASIC CRITERIAS FOR THE

SELECTION OF HEAT EXCHANGERS Process specificationsService conditions of the plant environment,resistance to corrosion by the processMaintenance, permission to cleaning andreplacement of componentsCost- EffectivenessSite requirements, lifting, servicing,capabilities


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PLATE HEAT

EXCHANCERS

GASKETED PLATE SPIRAL PLATE LAMELLA


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Limited to below 25 bar

and 250C. Plate heat exchangers

have three main types :gasketed, spiral heatexchangers and lamella.

The most common of the plate-type heatexchangers is thegasketed plate heatexchanger.


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GASKETED PLATE HEAT

EXCHANGER The most common of

the plate-type heatexchangers is thegasketed plate heat

exchanger.


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SPIRAL PLATE HEAT

EXCHANGER

Ideal flow conditions and the smallest possible heating surface.


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LAMELLA

Consisting of cylindrical

shell surrounding a numberof heat transfering lamellas. Similar to tubular heat

exchanger.


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ADVANTAGES

Plate heat exchangers yield heat transfer ratesthree to five times greater than other types of heatexchangers.

The design of the plate heat exchanger allows toadd or remove plates to optimize performance, orto allow for cleaning, service, or maintenancewith a minimum of downtime.

Plate exchangers offer the highest efficiencymechanism for heat transfer available in industry.


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DISADVANTAGES

Plate exchangers are limited when high pressures, high temperatures, or aggressivefluids are present.

Because of this problem these type of heatexchangers have only been used in small, low

pressure applications such as on oil coolers forengines.


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2. EXTENDED SURFACE

HEAT EXCHANGERS

- PLATE FIN HEAT EXCHANGER- TUBE FIN HEAT EXCHANGER


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PLATE FIN HEAT

EXCHANGER For gas to gasapplications.

Widely used incryogenic, energyrecovery, processindustry, refrigerationand air coditioningsystems.


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TUBE FIN HEATEXCAHNGER

For gas to liquid heatexchangers.

Used as condersers inelectric power plant, asoil coolers in propulsive

power plants, as ircooled exchangers in

process and powerindustires.


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TUBULAR HEAT EXCHANGERS

SHELL AND TUBE

DOUBLE-PIPE


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SHELL AND TUBE HEATEXCHANGERS


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Are the most commonlyused heat exchangers in oilrefineries and other largechemical processes.

Are used when a process

requires large amounts offluid to be heated orcooled.


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Provide transfer of heatefficiently.

Use baffles on the shell-side fluid to accomplishedmixing or turbulence.


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SHELL AND TUBE HEAT EXCHANGERS

U - TUBE HEAT EXCHANGERS

FIXED TUBE HEAT EXCHANGERS

FLOATING HEAD HEAT EXCHANGERS


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U - TUBE HEAT EXCHANGERS Heat exchanger systems consisting of straight

length tubes bent into a U-shape surrounded by a shell.


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Both initial and maintenance costs are reduced by reducing the number of joints.

They have drawbacks like inability to replaceindividual tubes except in the outer row andinability to clean around the bend.


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Examples: reboilers, evaporators and Kettletype.

They have enlarged shell sections for vapor-liquid separation.


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Have straight tubes that are secured at bothends to tube sheets welded to the shell.


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They are the most economical type design. They have very popular version as the heads

can be removed to clean the inside of thetubes.

Cleaning the outside surface of the tubes is

impossible as these are inside the fixed part. Chemical cleaning can be used.


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FLOATING HEAD HEAT EXCHANGERS

Examples : kettle boilers which have dirtyheating medium.

They have the most highest construction cost of

all exchanger types.


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DOUBLE-PIPE HEAT EXCHANGERS

Utilize true counter-current flow which

maximizes the temperature differences

between the shell side and tube side fluids.


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When the process callsfor a temperature cross,

it is the most efficientdesign and will result infewer sections and lesssurface area.



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ADVANTAGES

Operates in true counter current flow permittingextreme temperature cross.

Economically adaptable to service differentials. Ideal for wide temperature ranges and

differentials. Provides shorter deliveries than shell and tube

due to standardization of design andconstruction.


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1)REBOILER to generate vapor to drive fractional distillationseparation

Most Common Reboiler s Types Kettle Reboilers Forced Recirculation ReboilersThermosiphon Reboiler


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Kettle Reboilers


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Major factors influence reboiler typeselection:

Plot space available

Total duty required

Fraction of tower liquid traffic vaporized

Fouling tendency

Temperature approach available

Temperature approach required


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Kettle Reboilers

AdvantagesInsensitive tohydrodynamicsHigh heat fluxes are

possibleCan handle highvaporization

Simple pipingUnlimited area

DisadvantagesAll the dirt collects andnon volatilesaccumulateShell side is difficult tocleanDifficult to determinethe degree of mixingOversize shell isexpensive


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Thermosiphon Reboiler


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Thermosiphon Reboiler

operate using natural circulation with processflow on the shell side

process flow on the tube or shell side invertical units.not require a pump for recirculationhave sensible heat transfer followed bynucleate boiling.


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F orced Recirculation Reboilers


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F orced Recirculation Reboilers

These reboiler types have two mechanismsof heat transfer: sensible heat transfer

followed by nucleate boiling.

Process flow is typically on the tube side of

a standard exchanger in the vertical position.


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2)CONDENSERS

a) Water-CooledCondensensersHorizontal shell andtubeVertical shell andtubeShell and coilDouble pipe

b) Air- CooledCondensers

Phases:1) de-super-heating

2) Condensing3) Subcooling


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Single-Pass Condenser


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SELECT AN WATER-COOLED

CONDENSER IF:

1. Adequate water supplies are available from tower, city or well sources.2. Water supply is of good quality.3. Heat recovery is not practical or unimportant.4. Plant ambient temperatures consistently exceed 95F.5. Ambient air is polluted with large dust and dirt particles.

ADVANTAGE & DISADVANTAGES

1. Offer lower capital investment.2. Operates more efficiently on hot summer days.3. Easier to operate.4. Does not offer summer ventilation.


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OUTLINE

Definition & classification of heat exchangers

Heat exchangers according to construction Plate heat exchangers Extended surface heat exchangers Tubular heat exchangers

Heat exchangers according to phase change Condensers Evaporators


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