introduction to heat ex changers

Introduction to Heat ExchangersCourse objectives

What are exchangers for?

Exchanger types

How are they specified?

The design task

ObjectivesBy the end of the course you will• be familiar with the main exchanger types• know which is likely to be the best type for a given

application• understand what are the key factors in exchanger

design• be able to estimate the size and cost of key exchanger

types• have the background necessary to start using

commercial exchanger design software• be an informed purchaser of heat exchangers

Lecture series

• Introduction to heat exchangers

• Selection of the best type for a given application

• Selection of right shell and tube

• Design of shell and tube

Q = U A T

Contents• Why we need heat exchangers

• The basics of their design

• Some general features of exchangers

• Different types of exchanger

• The design process

Example of an exchanger

Bundle for shell-and-tube exchanger

What are heat exchangers for?

• To get fluid streams to the right temperature for the next process– reactions often require feeds at high temp.

• To condense vapours

• To evaporate liquids

• To recover heat to use elsewhere

• To reject low-grade heat

• To drive a power cycle

Feed-effluent exchanger

Feed-effluentexchanger Exothermic reaction

Heat recovery

Distillation

Bottom product

Top product

Reflux condenser

Reboiler

Column

Typical crude oil distillation

Storage

Kerosene

Desalter

Top pumparound

Naphthaand gases

Kerosene

Furnace

Reduced crude

Lightgas oil

Heavygas oil

Reducedcrude

Heavy gas oil

Light gas oil

Bottom pumparound

Bottompumparound

Power cycle

Boiler Condenser

Steam turbine

Feedwaterheater

Q = U A T

We have thermal resistances in series

coldcold

Heat utilities

• Hot utilities– Boiler generating service steam (maybe a

combined heat and power plant)– Direct fired heaters (furnace)– Electric heaters

• Cold utilities– Cooling tower (wet or dry) providing service

cooling water– Direct air-cooled heat exchanger

Thermal integrationor process integration

• Reducing the hot and cold utility needs by interchanging heat between process streams

• If the plant needs are primarily heat, thermal integration is usually by “pinch technology” - Software HX-Net

• If the plant is concerned with heat and work, pinch technology is supplemented with “exergy analysis”

Local and mean values• “Overall” means from the hot side to the cold

side including all resistances

• However it is still at a particular point in the exchanger: i.e. it is local

• Hence you can have a local, overall coefficient

LOCALLY

FOR WHOLE EXCHANGER

mTmT TAUQ

Integrating over the exchanger area

Local equation

Rearranging

and integrating

Q AT T

Total area AT

Definitions of mean values From previous slides

Comparing the two sides

Q AT T

Tm T Q

UdAmT AT

Special case where Ts are linear with Q

• Eqn. integrates to give log. mean temperature difference - LMTD Ta

T TT T

T Tm LMa b

ln( / ) Tb

Multipass exchangers

• For single-phase duties, theoretical correction factors, FT, have been derived

• FT values are less than 1

• Do not design for FT less than 0.8

T F Tm T LM

Typical FT correction factor curvesFor shell and tube with 2 or more tube-side passes

T, t = Shell / tube side 1, 2 = inlet / outletP

Curves are for different values of R

Thermal effectiveness

T Tin out

Stream temperature rise divided by the theoretically maximum possible temperature rise

T1,in T1,out

T2,outT2,in

Compactness

• Can be measured by the heat-transfer area per unit volume or by channel size

• Conventional exchangers (shell and tube) have channel size of 10 to 30 mm giving about 100m2/m3

• Plate-type exchangers have typically 5mm channel size with more than 200m2/m3

• More compact types available

Compactness

m2/m3100 1000 10 000

Hydraulic diameter, mm60 10 1 0.1

Shell-&-tube

Plate fin

Car radiator

Special

Human lungs

Main categories of exchanger

Heat exchangers

Recuperators Regenerators

Wall separating streamsWall separating streams Direct contact

Most heat exchangers have two streams, hot and cold, but some have more than two

Recuperators/regenerators

RecuperativeRecuperative

Has separate flow paths for each fluid which flow simultaneously through the exchanger transferring heat between the streams

RegenerativeRegenerative

Has a single flow path which the hot and cold fluids alternately pass through.

Rotating wheel

Double PipeSimplest type has one tube inside another - inner

tube may have longitudinal fins on the outside

However, most have a number of tubes in the outer tube - can have very many tubes thus becoming a shell-and-tube

Shell and TubeTypical shell and tube exchanger as used in the process industry

Shell-side flow

Complete shell-and-tube

Plate and frame

• Plates hung vertically and clamped in a press or frame.

• Gaskets direct the streams between alternate plates and prevent external leakage

• Plates made of stainless steel or higher quality material

• Plates corrugated to give points of support and increase heat transfer

Chevron Washboard

Plate types

Corrugations on plateimprove heart transfergive rigidity

Many points ofcontact and atortuous flow path

General view of plate exchanger

“Plate exchanger” normally refers to a gasketted plate- and-frame exchanger

Flow Arrangement within a PHE

Alternate plates (often same plate types inverted)

Gasketsarranged foreach stream toflow betweenalternate plates

Air-cooled exchanger

• Air blown across finned tubes (forced draught type)Air blown across finned tubes (forced draught type)

• Can suck air across (induced draught)Can suck air across (induced draught)

Finned tubes

ACHE bundle

Plate-fin exchanger

• Made up of flat plates (parting sheets) and corrugated sheets which form fins

• Brazed by heating in vacuum furnace

Can have many streams

7 or more streams are typical

Typical plate-fin

Spiral (plate)

Good for streams with large solids

Cooling Towers• Large shell with packing at the bottom over which

water is sprayed

• Cooling by air flow and evaporation

• Air flow driven by forced or natural convection

• Need to continuously make up the cooling water lost by evaporation

Agitated Vessel• Used for batch heating or cooling of fluids

• An agitator and baffles promote mixing

• A range of agitators are used

• Often used for batch chemical reaction

Proprietary types

• Types described so far are generic types

• These can be made by any company with necessary skills (no real patent protection)

• There are now many special, proprietary exchangers made by one company or a small number of companies under licence

• One example is the “printed circuit exchanger” by Heatric

Printed circuit heat exchanger• Plates are etched to

give flow channels

• Stacked to form exchanger block

• Block diffusion welded under high pressure and temperature

• Bond formed is as strong as the metal itself

Printed circuit exchanger

Note that “compact” does notmean small but means largesurface area per unit volume

Distribution of typesin terms of market value in Europe

Shell & Tube42%

Other Tubular5%

Plate & Frame13%

Other Plate4%

Other Proprietary2%

Air Coolers10%

Cooling Towers9%

Waste Heat Boilers

Other Heat Recovery

Preliminary points on selection• Tubes and cylinders can withstand higher

pressures than plates

• If exchangers can be built with a variety of materials, then it is more likely that you can find a metal which will cope with extreme temperatures or corrosive fluids

• More specialist exchangers have fewer suppliers, longer delivery times and must be repaired by experts

• S&Ts cannot normally give high thermal effectiveness,

Design sequence

• Design the process flow flow-sheet

• Specify the heat exchanger requirements

• Select the best exchanger type for the job

• Thermal design of exchanger

• Mechanical design of exchanger

Looping back may be necessary at any stage but can be difficult because of the project timetable

Who does what?

• Design the process flow flow-sheet

• Specify the heat exchanger requirements

• Select the best exchanger type for the job

• Thermal design of exchanger

• Mechanical design of exchanger

Processor/end user

Contractor

Manufacturer

Exchanger specification• Heat load (duty) along with the terminal

temperatures of the streams• Maximum pressure drop each streams

– liquids - 0.5 bar

– gases/vapours below 2bar - 10% of inlet pressure

• Design pressures and temperatures• Size/weight constraints• Standards to apply

– General standards like ISO, TEMA, ASME etc

– Companies own standards

• Other requirements

The designer must supply an exchanger which

• Meets the stated specification

• Has reasonable initial costs and operating costs (most exchangers are bought on the basis of the cheapest tender)

• Has a reasonable lifetime– no damaging vibration– no thermal fatigue– no unexpected fouling or corrosion

introduction to heat ex changers

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