Heat Exchangers - Copy

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Heat Exchangers


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Heat Exchangers

Shell and Tube Heat Exchanger

U-Tube Heat Exchanger

Double-Pipe Heat Exchanger Counter current

Cocurrent

Cross current

Flat-Plate Heat Exchanger

Spiral-Tube Heat Exchanger


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Counter Flow

In a counter-flow heat exchanger fluids flow in the

opposite direction


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Counter Flow


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Parallel Flow


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Parallel Flow


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Cross Flow Air to Liquid Heat

Exchanger


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Cross Flow Heat Exchanger


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Finned-Both Fluids

Unmixed

Unfinned-One Fluid Mixed

the Other Unmixed


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Cross Flow


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Shell and Tube Heat Exchanger

Provides a comparatively large ratio of heat transferarea to volume and weight

It provides this surface in a form which is relativelyeasy to construct in wide range of sizes

It is mechanically rugged enough to withstandnormal shop fabrication stresses, field erectionstresses, and normal operating conditions

They can be reasonably cleaned, and thosecomponents subject to failure-gaskets and tubes-

can be easily replaced Good design method exists

Expertise and shop facilities for successful designand construction are available throughout the world.


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Single Pass Heat Exchanger


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Shell and Tube with one shell

and One tube pass


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Basic Components of Shell and

Tube Heat Exchangers Tubes

Tube Sheet

Shell and Shell Side Nozzles Tube side nozzles

Channel Cover

Pass Divider

Baffles

Tie rods


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Baffles in Shells: to give high

enough shell-side velocities


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Baffles


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Baffles

Single-segmental baffles act as barriers, thus forcing the shell flow to take a

serpentine path through the shell. In this way that fluid (in this case a

condensing refrigerant) flows at nearly right angles to the tubes and the

resulting turbulence increases the shell-side heat transfer coefficient.


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Tube sheet


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Tube

Bundle


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Tube Pitch

Square pitch has advantage of easier cleaningTriangular pitch allows more tubes in a given

shell diameter


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Tube Pass


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Range of Shell and Tube Heat

Exchangers


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One Shell Pass,

Two Tube Passes

Two Shell Passes,

Four Tube Passes


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U-Tube Heat Exchangers


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U-Tube Heat Exchangers


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Spiral Heat Exchanger


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Working of Spiral Heat ExchangerA spiral heat exchanger is composed of two long flat plates, which arewrapped around a center tube, to form two concentric spiral channels.

The hot flow enters the heat exchanger in the center, spirals outwardsthrough the long flat channel to leave the exchanger in tangentialdirection. The cold fluid enters tangential, flowing through the long flatchannel to leave in axial direction. This way a counterflow is created,which maximizes heat transfer.

good alternative for ordinary shell-and-tube heat exchangers when itcomes to fouling

the curved form of the channels will create turbulence at any point inthe flow, even with low velocities. The same curved form causes highshear rates at the walls. These two effects can prevent particles from

clinging to the wall. In spiral heat exchangers, there is only one channel, so when some

foulant does attach, the flow still has to go through. The velocity willlocally increase, as will the shear rate, thereby removing the foulantagain.


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Plate and Frame Heat Exchanger


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Plate and Frame Heat Exchanger

consists of plates instead of tubes to separate the hotand cold fluids. The hot and cold fluids alternatebetween each of the plates.

Baffles direct the flow of fluid between plates. Because each of the plates has a very large surface

area, the plates provide each of the fluids with anextremely large heat transfer area.

a plate type heat exchanger, as compared to a similarly sized tube and shell heat exchanger, is capable oftransferring much more heat. This isdue to the larger area the plates provide over tube.


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Plate and Frame heat Exchanger

Due to the high heat transfer efficiency of the plates, plate typeheat exchangers are usually very small when compared to a tubeand shell type heat exchanger with the same heat transfercapacity.

Plate typeheat exchangers are not widely used because of the inability to reliably seal the large gaskets between each of theplates. Because of this problem, plate type heat exchangershave only been used in small, low pressure applications suchas on oil coolers for engines.

However, new improvements in gasket design and overall heatexchanger design have allowed some large scale applications of

the plate type heat exchanger. As older facilities are upgraded ornewly designed facilities are built, large plate type heat exchangersare replacing tube and shell heat exchangers and becoming morecommon.

LMTD Method

A M th d l f H t E h


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A Methodology for Heat Exchanger

Design Calculations- The Log Mean Temperature Difference (LMTD) Method -

A form of Newtons Law of Cooling may be applied to heat exchangers byusing a log-mean value of the temperature difference between the two fluids:

1mq U A T

1 2

1

1 21n /m

T TT

T T

Evaluation of depends on the heat exchanger type.1 2andT T

Counter-Flow Heat Exchanger:

1 ,1 ,1

, ,

h c

h i c o

T T T

T T

2 ,2 ,2

, ,

h c

h o c i

T T T

T T

LMTD Method (cont.)


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Parallel-Flow Heat Exchanger:

1 ,1 ,1

, ,

h c

h i c i

T T T

T T

2 ,2 ,2

, ,

h c

h o c o

T T TT T

Note that Tc,ocan not exceed Th,ofor a PF HX, but can do so for a CF HX.

For equivalent values of UAand inlet temperatures,

1 , 1 ,m CF m PF T T

Shell-and-Tube and Cross-Flow Heat Exchangers:

1 1 ,m m CF T F T

Figures 11.10 - 11.13F


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LMTD correction factor

ChangeeTemperaturMaximum

FluidTubesideofchangeTemp

tT

ttP

_

11

12

sideshell

sidetube

C

C

tt

TTR

_

_

12

21

P represents the thermal effectiveness of tube side fluid and varies from 0 to 1

R varies from 0 to . When R=0, it means that the fluid on shell side is

undergoing phase change, and when R= , the tube side fluid is undergoing a

phase change. When R=0 or infinity, the correction factor is 1.


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LMTD Correction Factor


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Typical values of heat transfer coefficients

Type Application and Conditions UW/(m2K)

U

Btu/(ft2F

h)

Tubular, heating

or cooling

Gases at atmospheric pressure inside and outside

tubes5 - 35 1 - 6

Gases at high pressure inside and outside tubes 150 - 500 25 - 90

Liquid outside (inside) and gas at atmospheric

pressure inside (outside) tubes15 - 70 3 - 15

Gas at high pressure inside and liquid outside tubes 200 - 400 35 - 70

Liquids inside and outside tubes 150 - 1200 25 - 200

Steam outside and liquid inside tubes 300 - 1200 50 - 200

Tubular,

condensationSteam outside and cooling water inside tubes 1500 - 4000 250 - 700

Organic vapors or ammonia outside and cooling

water inside tubes

300 - 1200 50 - 200


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Tubular,

evaporation

steam outside and high-viscous liquid inside tubes,

natural circulation300 - 900 50 - 150

steam outside and low-viscous liquid inside tubes,

natural circulation600 - 1700 100 - 300

steam outside and liquid inside tubes, forced

circulation900 - 3000 150 - 500

Air-cooled heat

exchangers2)

Cooling of water 600 - 750 100 - 130

Cooling of liquid light hydrocarbons 400 - 550 70 - 95

Cooling of tar 30 - 60 5 - 10

Cooling of air or flue gas 60 - 180 10 - 30

Cooling of hydrocarbon gas 200 - 450 35 - 80

Condensation of low pressure steam 700 - 850 125 - 150

Condensation of organic vapors 350 - 500 65 - 90


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Plate heat exchanger liquid to liquid 1000 - 4000 150 - 700

Spiral heat exchanger liquid to liquid 700 - 2500 125 - 500

condensing vapor to liquid 900 - 3500 150 - 700


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According to Garrett-Price (1985) fouling is generally defined as

the forming of deposits on heat transfer surfaces, which interfereswith heat transfer and/or fluid flow. In other words, by using aheat exchanger small layers of insulating material will be formedon the heat transferring surfaces of that heat exchanger. Theinfluence of this layer is two-sided:

1) The layer has a high thermal resistance, higher then any otherpart of the heat exchanger, thereby increasing the total thermalresistance. This will decrease the amount of heat transferredthrough the surfaces and reduces the efficiency of the heatexchanger.

2) The presence of a layer will decrease cross-sectional flow area of

the medium. To achieve the same throughput through this smallerarea, theres a bigger pressure drop needed. Additional pumpingis needed, increasing to total amount of energy added to thesystem, decreasing the efficiency.

So fouling is a absolutely not-wanted phenomenon.

What exactly is fouling?


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Fouling in Heat Exchangers

Types of Fouling

Precipitation fouling

Particulate fouling

Chemical reaction fouling

Corrosion fouling Solidification fouling

Biological fouling


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Precipitation fouling


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Corrosion fouling


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Biological fouling


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Economic Considerations in Heat

Exchanger Selection

Temperatures of heating and cooling media:Ahigher heating media temperature results in a smallerheat exchanger for a given heating load. Limitations ofmaterials must be kept in mind here.

Pressure drops permitted by the system affect heatexchanger size The highest allowable pressure dropwill result in substantial savings in heat exchanger size.

Length restriction sometimes affect heat exchangercosts However, there are so many exceptions and

limiting conditions that we cannot simply say the longerthe cheaper.

Materials of construction: Corrosive tendencies andpurity requirements of fluids being handled. Often, thechoice here is based on reliable data and experience


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Heat Exchanger Design

Calculation of Heat Duty

Calculation of Mean Temperature Difference

Configuration Correction Factors for LMTD

Assume a Overall Heat Transfer Coefficient andcalculate area A.

Assume a specific number of tube and shell passes anddefine the size of tube 1 or outside diameter. Selecta tube pitch and baffle spacing.

Find out the number of tubes by

A = ndL


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Rules of Thumb for Heat

Exchangers

Take countercurrent shell-and-tube heat exchanger as abasis.

Standard tubes are OD, triangular spacing, 16ft long.

A 1 ft OD shell accommodates 100 ft2, 2 ft OD-400 ft2, 3ft OD-1100 ft2.

Pressure drops are 1.5 psi for boiling services and 3-9psi for other services.

Minimum temperature approach is 20 F with normalcoolants, 5-10 F with refrigerants.

Water inlet temperature is 90 F, maximum outlet 120 F.


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Calculate the tube side flow rate based

upon the assumed number of tubes/pass

and the heat balance. The velocity should

be between 3-8 ft/s

Compute tube side coefficient (hi) using

Dittus Boelter correlation

n

k

CpDv

k

hiD

8.0

023.0


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Determine the shell side coefficient for an

assumed baffle spacing by using equation

14.03/155.0

36.0

w

seo

k

CpDG

k

Dh

Where Gs is mass velocity based on the minimum area

Bspt

doptDA

s

)(min

Ds= shell diameterpt = tube pitch

Do= outside diameter

Bs = Baffle spacing


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Determine the tube side and shell side fouling factors. Ifnothing is given assume a value of 1000 BTU/ hr ft2oF.(1 Btu/(ft2 F h) = 5.6785 W/(m2 K))

Calculate Uo and Ui Check with assumed value

Estimate area by using equation

q= UATlm

If this area is 10-20% excess than the assumed one thanassumed one is acceptable otherwise go to step 5(no. of tube passes)

Calculate number of tubes / pass by using

A = ndo L

If not satisfactory go to step 5 and recalculate bychanging baffle spacing etc.

Calculate shell and tube side pressure pressure drop. Ifpressure drop is too high reassume the unit in step 5.


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Allocation of Streams

more corrosive fluid goes tube-side because it ischeaper to construct tubes from alloys rather thanthe shell and tubesheet

higher pressure stream goes tube-side because

small diameter tubes handle stress better more severely fouling fluid goes tube-side because it

is easier to clean tube-side

fluid with lower film coefficient goes shell-side

because it allows use of finned tubing to increaseAoho

fluid with low Pmax goes shell side

viscous fluid goes shell-side


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Tube Size and Pitch

The standard length of tubes in a shell-and - tubeexchanger are 8, 12, 16, and 20 ft.

Tube IDs range from 5/8 to 1 1/2 exchangers withsmall-diameter tubes are less expensive than thosewith large diameter tubes. However smaller tubesare more difficult to clean

Tubes are usually laid out in a square or a triangularpattern (1.25 O.D for triangular pitch and min. forsquare pitch)

Finned tubes are preffered in condensers and whilehandling gases (5/8- O.D are preffered)


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Baffles

Baffling the shell-side increases the pressuredrop and pumping costs

These disadvantages are usually more thanoffset by increases fluid mixing and improvedheat transfer coefficients

Baffle spacing typically lies between 1/5 of theshell diameter and 1 shell diameter

Baffles are provided with 20-50% cut (25%common)


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Tie rods

For

6 15 shell 4 rods diameter

16 27 shell 6 rods 3/8 diameter

28 33 shell 6 rods diameter

34 48 shell 8 rods of diameter


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Pressure Drops

There are recommendations based on

velocity in case of liquids and operating

pressure in case of gasesWith liquids of 1Cp = 5 lb/in2

1 - 10Cp = 7 lb/in2

> 10 Cp = 10 lb/in2


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Applications of Heat Exchanger

Oil Coolers

Fuel Oil Pre-Heaters

Aftercoolers Steam Converters

Condensers

Thermo-Syphon Reboilers

Falling Film Evaporators


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Single Pass Condenser

Multi Pass Condenser


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Multi Pass Condenser


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Evaporation

Evaporation is the process of removal of volatile

solvent by vaporization, from an aqueous solution

consisting of non-volatile solute. In majority of the

cases of evaporation, the solvent is water.

Evaporation plays a major role in the manufacture of

a variety of products in chemical process industries,

including food processing, pulp & paper,pharmaceuticals, fertilisers etc.


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Evaporation


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Natural Convection Horizontal

Short Tube Evaporator


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Forced Circulation Evaporators


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A: Product 1: Head

B: Vapor 2: Calandria

C: Concentrate3: Calandria,

Lower part

D: Heating Steam4: Mixing

Channel

E: Condensate5: Vapor

Separator


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Climbing Film

Evaporator

A : Product

B : VaporC: Concentrate


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Climbing Film Evaporator


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Evaporator Calandria

F d Ci l i


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Forced Circulation

Evaporators

F d Ci l ti


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Forced Circulation

Evaporators



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A Product

B Vapour

C Concentrate

D Heating steam

E Condensate

Heat Exchangers - Copy

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