flow arrangement

8/12/2019 Flow Arrangement

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Flow arrangement

Countercurrent (A) and parallel (B) flows

Fig. 1:Shell and tube heat exchanger single pass (1!1 parallel flow)

Fig. ": Shell and tube heat exchanger "#pass tube side (1!" crossflow)

Fig. $: Shell and tube heat exchanger "#pass shell side "#pass tube side ("#" countercurrent)
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%here are three primar& classifications of heat exchangers according to their flow

arrangement. 'nparallel-flowheat exchangers the two fluids enter the exchanger at the same

end and traelin parallel to one another to the other side. 'ncounter-flowheat exchangers the

fluids enter the exchanger from opposite ends. %he counter current design is the most

efficient in that it can transfer the most heat from the heat (transfer) medium due to the factthat the aerage temperature difference along an& unit length is greater. See countercurrent

exchange.'n a cross-flowheat exchanger the fluids trael roughl& perpendicular to one

another through the exchanger.

For efficienc& heat exchangers are designed to maximie the surface area of the wall

between the two fluids while minimiing resistance to fluid flow through the exchanger. %he

exchanger*s performance can also be affected b& the addition of fins or corrugations in one or

both directions which increase surface area and ma& channel fluid flow or induce turbulence.

%he driing temperature across the heat transfer surface aries with position but an

appropriate mean temperature can be defined. 'n most simple s&stems this is the +log mean

temperature difference+ (,-%). Sometimes direct /nowledge of the ,-% is

not aailableand the 0% methodis used.

2edit3%&pes of heat exchangers

2edit3Shell and tube heat exchanger

A Shell and %ube heat exchanger

Main article: Shell and tube heat exchanger

Shell and tube heat exchangers consist of a series of tubes. 4ne set of these tubes containsthe fluid that must be either heated or cooled. %he second fluid runs oer the tubes that are

being heated or cooled so that it can either proide the heat or absorb the heat re5uired. A set

of tubes is called the tube bundle and can be made up of seeral t&pes of tubes: plain

longitudinall& finned etc. Shell and tube heat exchangers are t&picall& used for high#pressure

applications (with pressures greater than $6 bar and temperatures greater than "76 8C).

2"3%his is because the shell and tube heat exchangers are robust due to their shape.

Seeral thermal design features must be considered when designing the tubes in the shell

and tube heat exchangers:
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%ube diameter: sing a small tube diameter ma/es the heat exchanger both

economical and compact. 9oweer it is more li/el& for the heat exchanger to foul up

faster and the small sie ma/es mechanical cleaning of the fouling difficult. %o preail

oer the fouling and cleaning problems larger tube diameters can be used. %hus to

determine the tube diameter the aailable space cost and the fouling nature of the fluidsmust be considered.

%ube thic/ness: %he thic/ness of the wall of the tubes is usuall& determined to

ensure:

%here is enough room for corrosion

%hat flow#induced ibration has resistance

Axial strength

Aailabilit& of spare parts

9oop strength (to withstand internal tube pressure)

Buc/ling strength (to withstand oerpressure in the shell)

%ube length: heat exchangers are usuall& cheaper when the& hae a smaller shell

diameter and a long tube length. %hus t&picall& there is an aim to ma/e the heat

exchanger as long as ph&sicall& possible whilst not exceeding production capabilities.

9oweer there are man& limitations for this including space aailable at the installation

site and the need to ensure tubes are aailable in lengths that are twice the re5uired

length (so the& can be withdrawn and replaced). Also long thin tubes are difficult to ta/e

out and replace.

%ube pitch: when designing the tubes it is practical to ensure that the tube pitch (i.e.

the centre#centre distance of adoining tubes) is not less than 1."; times the tubes*

outside diameter. A larger tube pitch leads to a larger oerall shell diameter which leadsto a more expensie heat exchanger.

%ube corrugation: this t&pe of tubes mainl& used for the inner tubes increases the

turbulence of the fluids and the effect is er& important in the heat transfer giing a better

performance.

%ube ,a&out: refers to how tubes are positioned within the shell. %here are four main

t&pes of tube la&out which are triangular ($68) rotated triangular (768) s5uare (6 degrees to the adacent baffles forcing the fluid to

flow upward and downwards between the tube bundle. Baffle spacing is of large

thermod&namic concern when designing shell and tube heat exchangers. Baffles must be

spaced with consideration for the conersion of pressure drop and heat transfer. For

thermo economic optimiation it is suggested that the baffles be spaced no closer than

"6? of the shell@s inner diameter. 9aing baffles spaced too closel& causes a greater
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An interchangeable plate heat exchanger applied to the s&stem of a swimming pool

2edit3Plate heat exchanger

Main article: Plate heat exchanger

Another t&pe of heat exchanger is the plate heat exchanger.4ne is composed of multiple

thin slightl& separated plates that hae er& large surface areas and fluid flow passages for

heat transfer. %his stac/ed#plate arrangement can be more effectie in a gien space than

the shell and tube heat exchanger. Adances in gas/etand braingtechnolog& hae made

the plate#t&pe heat exchanger increasingl& practical. 'n9ACapplications large heat

exchangers of this t&pe are calledplate-and-frame when used in open loops these heat

exchangers are normall& of the gas/et t&pe to allow periodic disassembl& cleaning and

inspection. %here are man& t&pes of permanentl& bonded plate heat exchangers such as dip#

braed acuum#braed and welded plate arieties and the& are often specified for closed#

loop applications such asrefrigeration. late heat exchangers also differ in the t&pes of plates

that are used and in the configurations of those plates. Some plates ma& be stamped with

+cheron+ dimpled or other patterns where others ma& hae machined fins andDor grooes.

2edit3Plate and shell heat exchanger

A third t&pe of heat exchanger is a plate and shell heat exchanger which combines plate heat

exchanger with shell and tube heat exchanger technologies. %he heart of the heat exchanger

contains a full& welded circular plate pac/ made b& pressing and cutting round plates and

welding them together. 0oles carr& flow in and out of the platepac/ (the *late side*

flowpath).%he full& welded platepac/ is assembled into an outer shell that creates a second

flowpath ( the *Shell side*). late and shell technolog& offers high heat transfer high pressure

high operating temperature compact sie low fouling and close approach temperature. 'n

particular it does completel& without gas/ets which proides securit& against lea/age at high

pressures and temperatures.

2edit3Adiabatic wheel heat exchanger
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A fourth t&pe of heat exchanger uses an intermediate fluid or solid store to hold heat which is

then moed to the other side of the heat exchanger to be released. %wo examples of this are

adiabatic wheels which consist of a large wheel with fine threads rotating through the hot and

cold fluids and fluid heat exchangers.

2edit3Plate fin heat exchanger

Main article: Plate fin heat exchanger

%his t&pe of heat exchanger uses +sandwiched+ passages containing fins to increase the

effectiit& of the unit. %he designs include crossflow and counterflow coupled with arious fin

configurations such as straight fins offset fins and wa& fins.

late and fin heat exchangers are usuall& made of aluminium allo&s which proide high heat

transfer efficienc&. %he material enables the s&stem to operate at a lower temperature and

reduce the weight of the e5uipment. late and fin heat exchangers are mostl& used for low

temperature serices such as natural gasheliumand ox&genli5uefaction plants airseparation plants and transport industries such as motor and aircraft engines.

Adantages of plate and fin heat exchangers:

9igh heat transfer efficienc& especiall& in gas treatment

,arger heat transfer area

Approximatel& ; times lighter in weight than that of shell and tube heat exchanger.

Able to withstand high pressure

isadantages of plate and fin heat exchangers:

-ight cause clogging as the pathwa&s are er& narrow

ifficult to clean the pathwa&s

Aluminum allo&s are susceptible to -ercur& ,i5uid EmbrittlementFailure

2edit3Pillow plate heat exchanger

A pillow plate exchanger is commonl& used in the dair& industr& for cooling mil/ in large

direct#expansion stainless steel bul/ tan/s. %he pillow plate allows for cooling across nearl&

the entire surface area of the tan/ without gaps that would occur between pipes welded to

the exterior of the tan/.

%he pillow plate is constructed using a thin sheet of metal spot#welded to the surface of

another thic/er sheet of metal. %he thin plate is welded in a regular pattern of dots or with a

serpentine pattern of weld lines. After welding the enclosed space is pressuried with

sufficient force to cause the thin metal to bulge out around the welds proiding a space for

heat exchanger li5uids to flow and creating a characteristic appearance of a swelled pillow

formed out of metal.

2edit3Fluid heat exchangers

%his is a heat exchanger with a gas passing upwards through a shower of fluid (often water)and the fluid is then ta/en elsewhere before being cooled. %his is commonl& used for cooling
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gases whilst also remoing certain impurities thus soling two problems at once. 't is widel&

used in espresso machines as an energsaing method of cooling super#heated water to use

in the extraction of espresso.

2edit3Waste heatrecovery units

Aaste 9eat Gecoer& nit(9G) is a heat exchanger that recoers heat from a hot gas

stream while transferring it to a wor/ing medium t&picall& water or oils. %he hot gas stream

can be the exhaust gas from a gas turbine or a diesel engine or a waste gas from industr& or

refiner&.

2edit3Dynamic scraped surface heat exchanger

Another t&pe of heat exchanger is called +(d&namic) scraped surface heat exchanger+. %his is

mainl& used for heating or cooling with high#

iscosit&productscr&stalliationprocesses eaporationand high#foulingapplications. ,ong

running times are achieed due to the continuous scraping of the surface thus aoidingfouling and achieing a sustainable heat transfer rate during the process.

2edit3Phase-change heat exchangers

%&pical /ettle reboiler used for industrial disti llation towers

%&pical water#cooled surface condenser

'n addition to heating up or cooling down fluids in ust a single phase heat exchangers can be

used either to heat a li5uidto eaporate (or boil) it or used ascondensersto cool

aaporand condenseit to a li5uid. 'n chemical plantsand refineriesreboilersused to heat

incoming feed fordistillationtowers are often heat exchangers.2$32=3
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istillation set#ups t&picall& use condensers to condense distillate apors bac/ into li5uid.

ower plantsthat use steam#drienturbinescommonl& use heat exchangers to

boilwaterintosteam. 9eat exchangers or similar units for producing steam from water are

often called boilersor steam generators.

'n the nuclear power plants called pressuried water reactorsspecial large heat exchangers

pass heat from the primar& (reactor plant) s&stem to the secondar& (steam plant) s&stem

producing steam from water in the process. %hese are called steam generators. All fossil#

fueled and nuclear power plants using steam#drien turbines haesurface condensersto

conert the exhaust steam from the turbines into condensate (water) for re#use. 2;3273

%o consere energ& andcooling capacit&in chemical and other plants regeneratie heat

exchangers can transfer heat from a stream that must be cooled to another stream that must

be heated such as distillate cooling and reboiler feed pre#heating.

%his term can also refer to heat exchangers that contain a material within their structure thathas a change of phase. %his is usuall& a solid to li5uid phase due to the small olume

difference between these states. %his change of phase effectiel& acts as a buffer because it

occurs at a constant temperature but still allows for the heat exchanger to accept additional

heat. 4ne example where this has been inestigated is for use in high power aircraft

electronics.

2edit3irect contact heat exchangers

irect contact heat exchangers inole heat transfer between hot and cold streams of two

phases in the absence of a separating wall.2H3%hus such heat exchangers can be classified

as:

Ias ! li5uid

'mmiscible li5uid ! li5uid

Solid#li5uid or solid ! gas

-ost direct contact heat exchangers fall under the Ias# ,i5uid categor& where heat is

transferred between a gas and li5uid in the form of drops films or spra&s. 2"3

Such t&pes of heat exchangers are used predominantl& in air conditioning humidification

industrial hot water heating water coolingand condensing plants.2>3

Phases2


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Jes Cooling towers falling droplet

eaporators

Forced 0o Spra& coolersD5uenchers

,i5uid flow Jes Spra& condensersDeaporation et

condensers

,i5uid Irait& 0o Bubble columns perforated tra&

columns

Jes Bubble column condensers

Forced 0o Ias spargers

Ias flow Jes irect contact eaporators submerged

combustion

2edit39AC air coils

4ne of the widest uses of heat exchangers is for air conditioningof buildings and ehicles.

%his class of heat exchangers is commonl& called air coils or ust coilsdue to their often#

serpentine internal tubing. ,i5uid#to#air or air#to#li5uid9ACcoils are t&picall& of modified

crossflow arrangement. 'n ehicles heat coils are often calledheater cores.

4n the li5uid side of these heat exchangers the common fluids are water a water#gl&col

solution steam or a refrigerant.For heating coils hot water and steam are the mostcommon and this heated fluid is supplied b&boilers for example. For cooling coils chilled

water and refrigerant are most common. Chilled water is supplied from a chillerthat is

potentiall& located er& far awa& but refrigerant must come from a nearb& condensing unit.

hen a refrigerant is used the cooling coil is theeaporatorin the apor#compression

refrigerationc&cle. 9AC coils that use this direct#expansion of refrigerants are commonl&

called DX coils.

4n the air side of 9AC coils a significant difference exists between those used for heating

and those for cooling. ue tops&chrometrics air that is cooled often has moisture condensing

out of it except with extremel& dr& air flows. 9eating some air increases that airflow*s capacit&

to hold water. So heating coils need not consider moisture condensation on their air#side but
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cooling coils mustbe ade5uatel& designed and selected to handle their

particular latent(moisture) as well as the sensible(cooling) loads. %he water that is remoed

is called condensate.

For man& climates water or steam 9AC coils can be exposed to freeing conditions.

Because water expands upon freeing these somewhat expensie and difficult to replace

thin#walled heat exchangers can easil& be damaged or destro&ed b& ust one freee. As such

freee protection of coils is a maor concern of 9AC designers installers and operators.

%he introduction of indentations placed within the heat exchange fins controlled condensation

allowing water molecules to remain in the cooled air. %his inention allowed for refrigeration

without icing of the cooling mechanism.2163

%he heat exchangers in direct#combustion furnacest&pical in man& residences are not

*coils*. %he& are instead gas#to#air heat exchangers that are t&picall& made of stamped steel

sheet metal. %he combustion products pass on one side of these heat exchangers and air to

heat on the other. A cracked heat exchangeris therefore a dangerous situation that re5uires

immediate attention because combustion products ma& enter liing space.

2edit3Spiral heat exchangers

Schematic drawing of a spiral heat exchanger.

Aspiralheat exchanger (S9E) ma& refer to a helical(coiled) tube configuration more

generall& the term refers to a pair of flat surfaces that are coiled to form the two channels in a

counter#flow arrangement. Each of the two channels has one long cured path. A pair of fluidports are connected tangentiall&to the outer arms of the spiral and axial ports are common

but optional.2113

%he main adantage of the S9E is its highl& efficient use of space. %his attribute is often

leeraged and partiall& reallocated to gain other improements in performance according to

well /nown tradeoffs in heat exchanger design. (A notable tradeoff is capital cost s operating

cost.) A compact S9E ma& be used to hae a smaller footprint and thus lower all#around

capital costs or an oer#sied S9E ma& be used to hae lesspressuredrop less

pumping energ& higher thermal efficienc&and lower energ& costs.

2edit3Construction
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%he distance between the sheets in the spiral channels are maintained b& using spacer studs

that were welded prior to rolling. 4nce the main spiral pac/ has been rolled alternate top and

bottom edges are welded and each end closed b& a gas/eted flat or conical coer bolted to

the bod&. %his ensures no mixing of the two fluids occurs. An& lea/age is from the peripher&

coer to the atmosphere or to a passage that contains the same fluid. 21"3

2edit3Self cleaning

S9Es are often used in the heating of fluids that contain solids and thus tend to foul the inside

of the heat exchanger. %he low pressure drop lets the S9E handle fouling more easil&. %he

S9E uses a Kself cleaningL mechanism whereb& fouled surfaces cause a localied increase

in fluid elocit& thus increasing thedrag(or fluidfriction) on the fouled surface thus helping to

dislodge the bloc/age and /eep the heat exchanger clean. +%he internal walls that ma/e up

the heat transfer surface are often rather thic/ which ma/es the S9E er& robust and able to

last a long time in demanding enironments.+ %he& are also easil& cleaned opening out li/e

an oenwhere an& build up of foulant can be remoed b&pressure washing.

Self#Cleaning ater filters are used to /eep the s&stem clean and running without the need to

shut down or replace cartridges and bags.

2edit3Flow arrangements

Concurrent and countercurrent flow.

%here are three main t&pes of flows in a spiral heat exchanger:

1. Counter-current Flow: Fluids flow in opposite directions. %hese are used for li5uid#

li5uid condensing and gas cooling applications. nits are usuall& mounted erticall&

when condensing apour and mounted horiontall& when handling high

concentrations of solids.

". Spiral Flow!Cross Flow"4ne fluid is in spiral flow and the other in a cross flow.

Spiral flow passages are welded at each side for this t&pe of spiral heat exchanger.

%his t&pe of flow is suitable for handling low densit& gas which passes through the
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cross flow aoiding pressure loss. 't can be used for li5uid#li5uid applications if one

li5uid has a considerabl& greater flow rate than the other.

$. Distributed #apour!Spiral flow"%his design is that of a condenser and is usuall&

mounted erticall&. 't is designed to cater for the sub#cooling of both condensate and

non#condensables. %he coolant moes in a spiral and leaes ia the top. 9ot gasesthat enter leae as condensate ia the bottom outlet.

2edit3Applications

%he S9E is good for applications such as pasteuriation digester heating heat recoer& pre#

heating (see:recuperator) and effluent cooling. For sludge treatment S9Es are generall&

smaller than other t&pes of heat exchangers. 2citation needed3

2edit3Selection

ue to the man& ariables inoled selecting optimal heat exchangers is challenging. 9and

calculations are possible but man& iterations are t&picall& needed. As such heat exchangers

are most often selected ia computer programs either b& s&stem designers who are

t&picall&engineersor b& e5uipment endors.

%o select an appropriate heat exchanger the s&stem designers (or e5uipment endors) would

firstl& consider the design limitations for each heat exchanger t&pe. %hough cost is often the

primar& criterion seeral other selection criteria are important:

9ighDlow pressure limits

%hermal performance

%emperature ranges

roduct mix (li5uidDli5uid particulates or high#solids li5uid)

ressure drops across the exchanger

Fluid flow capacit&

Cleanabilit& maintenance and repair

-aterials re5uired for construction

Abilit& and ease of future expansion

Choosing the right heat exchanger (9M) re5uires some /nowledge of the different heatexchanger t&pes as well as the enironment where the unit must operate. %&picall& in the

manufacturing industr& seeral differing t&pes of heat exchangers are used for ust the one

process or s&stem to derie the final product. For example a /ettle 9M for pre#heating a

double pipe 9M for the Ncarrier@ fluid and a plate and frame 9M for final cooling. ith sufficient

/nowledge of heat exchanger t&pes and operating re5uirements an appropriate selection can

be made to optimise the process.21$3

2edit3-onitoring and maintenance
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13/18

4nline monitoring of commercial heat exchangers is done b& trac/ing the oerall heat transfer

coefficient. %he oerall heat transfer coefficient tends to decline oer time due to fouling.

OPDAQ%lm

B& periodicall& calculating the oerall heat transfer coefficient from exchanger flow rates andtemperatures the owner of the heat exchanger can estimate when cleaning the heat

exchanger is economicall& attractie.

'ntegrit& inspection of plate and tubular heat exchanger can be tested in situ b& the

conductiit& or helium gas methods. %hese methods confirm the integrit& of the plates or

tubes to preent an& cross contamination and the condition of the gas/ets.

-echanical integrit& monitoring of heat exchangertubesma& be conducted

through 0ondestructie methodssuch asedd& currenttesting.

2edit3Fouling

Main article: Fouling

A heat exchanger in a steam power station contaminated with macrofouling.

Foulingoccurs when impurities deposit on the heat exchange surface. eposition of

these impuritiescan be caused b&:

,ow wall shear stress

,ow fluid elocities

9igh fluid elocities

Geaction product solid precipitation

recipitation of dissoled impurities due to eleated wall temperatures

%he rate of heat exchanger fouling is determined b& the rate of particle deposition less re#

entrainmentDsuppression. %his model was originall& proposed in 1


14/18


15/18

Countercurrent exchange conseration circuit

Further information: Countercurrent exchange in biological sstems

+Countercurrent+ heat exchangers occur naturall& in the circulation s&stem of fishwhalesand

othermarine mammals.Arteries to the s/in carr&ing warm blood are intertwined with eins

from the s/in carr&ing cold blood causing the warm arterial blood to exchange heat with the

cold enous blood. %his reduces the oerall heat loss in cold waters. 9eat exchangers are

also present in the tongue of baleen whalesas large olumes of water flow through their

mouths.21>321


16/18


17/18

where is the +thermal mass flow rate+. %he differential

e5uations goerning the heat exchanger ma& now be written as:

0ote that since the s&stem is in a stead& state there are no partial

deriaties of temperature with respect to time and since there is

no heat transfer along the pipe there are no second deriaties

inxas is found in the heat e5uation. %hese two coupled first#

orderdifferential e5uationsma& be soled to &ield:

where and

"and #are two as &et undetermined constants ofintegration. ,et and be the temperatures at xO6

and let and be the temperatures at the end of

the pipe at xO,. efine the aerage temperatures in each

pipe as:

sing the solutions aboe these temperatures

are:
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18/18

Choosing an& two of the temperatures aboe

eliminates the constants of integration letting

us find the other four temperatures. e find

the total energ& transferred b& integrating the

expressions for the time rate of change of

internal energ& per unit length:

B& the conseration of energ& the

sum of the two energies is ero. %he

5uantit& is /nown as

the !og mean temperature

differenceand is a measure of the

effectieness of the heat exchanger

in transferring heat energ&.
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flow arrangement

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