Heat Exchanger lecture - 140.138.140.5

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Heat Transfer/Heat Exchanger• How is the heat transfer? • Mechanism of Convection• Applications . • Mean fluid Velocity and Boundary and their

effect on the rate of heat transfer.• Fundamental equation of heat transfer• Logarithmic-mean temperature difference.Logarithmic mean temperature difference.• Heat transfer Coefficients.• Heat flux and Nusselt correlation • Simulation program for Heat Exchanger

How is the heat transfer?

• Heat can transfer between the surface of a solid conductor and the surrounding medium whenever temperature gradient exists.ConductionConvection

N l iNatural convection Forced Convection

Natural and forced Convection

Natural convection occurs whenever heat flows between a solid and fluid, or between fluid layers.

As a result of heat exchange

Change in density of effective fluid layers taken place, which causes upward flow of heated fluid.

If this motion is associated with heat transfer mechanism only, then it is called Natural Convection

Forced Convection

If this motion is associated by mechanical means such as pumps, gravity or fans, the movement of the fluid is enforced.

And in this case, we then speak of Forced convection.

Heat Exchangers

• Heaters (sensible heat changes)

• Coolers (sensible heat changes)

• Condensers (also change of state, V to L)

• Evaporators (also change of state L to V)

Paul Ashall, 2008

• Evaporators (also change of state, L to V)

Principle of heat exchanger

6

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Condensers and boilers

7

Types of Heat Exchanger

• Shell and tube• Double pipe• Plate• Finned tubes/gas heaters• spiral

Paul Ashall, 2008

spiral• Vessel jackets• Reboilers and vapourisers/evaporatorsEtc• Direct/indirect

Uses in chemical processes

• Chemical reactors (jackets, internal heat exchangers/calandria)• Preheating feeds• Distillation column reboilers• Distillation column condensers• Air heaters for driers• Double cone driers• evaporators

Paul Ashall, 2008

• evaporators• Crystallisers• Dissolving solids/solution• Production support services – HVAC, etc• Heat transfer fluids• etc

continued

Important consideration in:

• Scale-up i.e. laboratory scale (‘kilo lab’) to pilot plant scale (250 litres) to full plant scale operation (10000 litres)

• Energy usage and energy costs

• Process design and development

Paul Ashall, 2008

Process design and development

Heat transfer fluids

• Steam (available at various temperatures and pressures)• Cooling water (15oC)• Chilled water (5oC)• Brines (calcium chloride/water fp. -18 deg cent. at 20% by

mass; sodium chloride/water fp. -16.5 deg cent. at 20 % by mass)

Paul Ashall, 2008

)• Methanol/water mixtures• Ethylene glycol/water mixtures• Propylene glycol/water mixtures (fp. -22 deg cent at a

concentration of 40% by mass)• Silicone oils (‘syltherm’)

Low temperature heat transfer fluids

Considerations:

• Temperature(s) required

• Freezing point

• Viscosity

• Specific heat

Paul Ashall, 2008

• Specific heat

• Density

• Hazardous properties

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Mechanisms for heat transfer

• Conduction• Convection• Radiation

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Driving force for heat transfer is temperature difference. Heat will only flow from a hotter to a colder part of a system.

Heat transfer by conduction

Fouriers law

dQ/dt = -kA(dT/dx)

dQ/dt – rate of heat transfer

k – thermal conductivity

A area perpendicular to direction of heat transfer x

Paul Ashall, 2008

A – area perpendicular to direction of heat transfer, x

dT/dx – temperature gradient in direction x

Heat transfer by conduction (steady state)

Q = kmA (ΔT/x)or q = km (ΔT/x)

h fl J/ 2

Paul Ashall, 2008

q – heat flux, J/s m-2

Q - rate of heat transfer, J/skm – mean thermal conductivity,A – area perpendicular to the direction of heat

transfer, m2

ΔT – temperature change (T1 – T2), Kx – length, m

Thermal conductivity, k

k (300K), Wm-1 K-1

Copper 400

Paul Ashall, 2008

pp

Water 0.6

air 0.03

Heat Exchangers• A device whose primary purpose is the transfer of energy

between two fluids is named a Heat Exchanger.

Applications of Heat Exchangers

Heat Exchangers prevent car engine

overheating and increase efficiency

Heat exchangers are used in Industry for

heat transfer

Heat exchangers are used in AC and

furnaces

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• The closed-type exchanger is the most popular one.

• One example of this type is the Double pipe exchanger.

• In this type, the hot and cold fluid streams do not come into direct contact with each other. They are separated by a tube wall or flat plate.

Advantages/disadvantages of double‐pipe HE

• Advantages

– Easy to obtain counter‐current flow– Can handle high pressure– Modular construction– Easy to maintain and repair– Many suppliers

• Disadvantage– Become expensive for large duties (above 1MW)

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Scope of double pipe HE

• Maximum pressure

– 300 bar(abs) (4500 psia) on shell side

– 1400 bar(abs) (21000 psia) on tubeside

• Temperature range

– ‐100 to 600oC (‐150 to 1100oF)

– possibly wider with special materials p y p

• Fluid limitations

– Few since can be built of many metals

• Maximum ε = 0.9

• Minimum ΔT = 5 K


Shell and tube heat exchanger

• Size per unit 100 ‐ 10000 ft2 (10 ‐ 1000 m2)

• Easy to build multiple units

• Made of carbon steel where possible


Advantages/disadvantages of S&T

• Advantages

– Extremely flexible and robust design

– Easy to maintain and repair

– Can be designed to be dismantled for cleaning

– Very many suppliers world‐wide

• Disadvantages

– Require large plot (footprint) area ‐ often need extra space to remove the bundle

– Plate may be cheaper for pressure below 16 bar (240 psia) and temps. below 200oC (400oF)


Scope of shell and tube(Essentially the same as a double pipe)


– 300 bar(abs) (4500 psia) on shell side

– 1400 bar(abs) (21000 psia) on tubeside• Temperature range

– ‐100 to 600oC (‐150 to 1100oF)

– possibly wider with special materials


– Few since can be built of many metals

• Maximum ε = 0.9 (less with multipass)



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Plate and frame heat exchanger

• Plates pressed from stainless steel or higher grade material

– titanium

– incoloy

– hastalloy

• Gaskets are the weak point.Made of

– nitrile rubber

– hypalon

– viton

– neoprene


Advantages of plate and frame HE

• High heat transfer ‐ turbulence on both sides

• High thermal effectiveness ‐ 0.9 ‐ 0.95 possible

• Low ΔT ‐‐ down to 1K

• Compact ‐ compared with a S&T

• Cost low because plates are thin• Cost ‐ low because plates are thin

• Accessibility ‐ can easily be opened up for inspectionand cleaning

• Flexibility ‐ Extra plates can be added

• Short retention time with low liquid inventory hencegood for heat sensitive or expensive liquids

• Less fouling ‐ low r values often possible


Disadvantages of plate & frame HE

• Pressure ‐ maximum value limited by the sealing ofthe gaskets and the construction of the frame.

• Temperature ‐ limited by the gasket material.

• Capacity ‐ limited by the size of the ports

• Block easily when solids in suspension unless specialy p pwide gap plates are used

• Corrosion ‐ Plates good but the gaskets may not besuitable for organic solvents

• Leakage ‐ Gaskets always increase the risk

• Fire resistance ‐‐ Cannot withstand prolonged fire(usually not considered for refinery duties)


Scope of plate & frame HE


– 25 bar (abs) normal (375 psia)

– 40 bar (abs) with special designs (600 psia)• Temperature range

– ‐25 to +1750C normal (‐13 to +3500F)

– ‐40 t0 +2000C special (‐40 to +3900F)

• Flow rates• Flow ratesup to 3,500 m3/hour can be accommodated in standard units


– Mainly limited by gasket

• Maximum ε = 0.95


Principal Applications

Gasketed plate and frame heat exchangers have alarge range of applications typically classified interms of the nature of the streams to be heated/cooledas follows:

Liquid‐liquid. Condensing duties. Evaporating duties.

Gasketed units may be used in refrigeration heat pump plants and extensively used in the processing of food and

drinks.


Comparison with Shell and Tube Heat Exchangers

In quantitative terms, 200 m2 of heat transfer surfacerequires a plate and frame heat exchanger approximately

3 metres long,

2 metres high and

1 meter wide.1 meter wide.

For a tubular heat exchanger achieving the same effect, some 600 m2 of surface would be required in a shell

5 metres long and

1.8 metre in diameter,

plus the extra length

needed for tube bundle removal.


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Welded plates heat exchanger

• Wide variety of proprietary types each with one ortwo manufactures

• Overcomes the gasket problem but then cannot beopened up

• Pairs of plates can be welded and stacked inconventional frame

• Conventional plate and frame types with all‐welded(using lasers) construction have been developed

• Many other proprietary types have been developed

• Tend to be used in niche markets as replacement toshell‐and‐tube



• As for gasketed plate and frame heat exchanger, but extended to include more aggressive media.

• Welded plate heat exchangers are used for the evaporation and condensation of refrigerants such as p gammonia and hydrochlorofluorocarbons (HCFCs), and for different chemicals.


Comparison with Shell and Tube Heat Exchanger

• As for gasketed plate and frame units.


Plate Fin Exchangers

• Formed by vacuum brazingaluminium plates separatedby sheets of finning

• Noted for small size andweight Typically 500 m2/m3weight. Typically, 500 m2/m3

of volume but can be 1800m2/m3

• Main use in cryogenicapplications (air liquifaction)

• Also in stainless steel engineering-resource.com

Scope of plate‐fin exchanger

• Max. Pressure 90 bar (size dependent)• Temperatures ‐200 to 150oC in Al

Up to 600 with stainless• Fluids Limited by material • Duties Single and two phase• Flow configuration Cross flow Counter flow• Flow configuration Cross flow, Counter flow• Multistream Up to 12 streams (7 normal)• Low ΔT Down to 0.1oC• Maximum ΔT 50oC typical• High ε Up to 0.98

use only with clean fluidsuse only with clean fluids



The plate‐fin heat exchanger is suitable for use over a wide range of temperatures and pressures for

• gas‐gas,

• gas‐liquid and

• multi‐phase duties.

Typically, these involve

• Chemical and petrochemical plant:

• Hydrocarbon off‐shore applications:

• Miscellaneous applications:


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• A plate‐fin heat exchanger with 6 fins/cm providesapproximately 1,300 m2 of surface per m3 of volume.This heat exchanger would be approximately 10% of thevolume of an equivalent shell and tube heat exchangerq gwith 19 mm tubes.


Spiral heat exchangers

• The classic design of a spiralheat exchanger is simple

• the basic spiral element isconstructed of two metal stripsrolled around a central coreforming two concentric spiralchannels.

• Normally these channels arealternately welded, ensuringthat the hot and cold fluidscannot intermix

Operating Limits

Maximum design temperature is 400oC set by thelimits of the gasket material.

Special designs without gaskets can operate withtemperatures up to 850oC.p p

Maximum design pressure is usually 15 bar, withpressures up to 30 bar attainable with specialdesigns.


Applications

• It is ideal for use in the food industry as well as in brewing and wine making.

• Spiral heat exchangers have many applications in the chemical industry including TiCl4 cooling, PVC y g g,slurry duties, oleum processing and heat recovery from many industrial effluents.

• Spiral heat exchangers also provide temperature control of sewage sludge.



• Spiral designs have a number of advantages compared toshell and tube heat exchangers:

• Optimum flow conditions on both sides of the exchanger.

• An even velocity distribution, with no dead‐spots.

A t t di t ib ti ith h t ld• An even temperature distribution, with no hot or cold‐spots.

• More thermally efficient with higher heat transfercoefficients.

• Small hold up times and volumes.

• Removal of one cover exposes the total surface area of onechannel providing easy inspection cleaning andmaintenance. engineering-resource.com

PLATE AND SHELL HEAT EXCHANGERS

• The plate and shell heat exchanger combines themerits of shell and tube with plate heat exchangers

• Current plate and shell heat exchanger modelsaccommodate up to 600 plates in a shell 2.5 m longp p gwith a 1 m diameter


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Operating Limits

• The maximum operating temperature of a plate and shell heat exchanger is 900oC

• maximum working pressure is 100 bar

• handle flow rates of 11 litres per second on the shell side.



• The principal applications for plate and shell heat exchangers are:

• · Heating including district heating.

• · Cooling including cryogenic applications.

• · Heat recovery Heat recovery.

• · Combined exchanger/reactors vessels.

• · Condensation/evaporation



• For heat exchangers of equivalent area and capacity, plate and shell designs are smaller due to the higher ratio of heat transfer area and specific volume. It is claimed that the plate and shell heat exchanger will occupy only 20 to 30% of the footprint of equivalent

it h ll d t b tcapacity shell and tube types.

• The maximum operating pressure of the plate and shell unit will also be higher.


Stream Location(Rules of thumb)

• more corrosive fluid goes tube‐side

– saves costs when using alloys, cheaper to construct tubes from alloys rather than the shell and tubesheet

• higher pressure stream goes tube‐side

– small diameter tubes handle stress better than large di t h lldiameter shells.

• more severely fouling fluid goes tube‐side

– easier to clean tube‐side using high pressure water lance, brushing, chemical cleaning, etc.

• fluid with lower film coefficient goes shell‐side

– allows use of finned tubing to increase Aoho

• fluid with low ΔPmax goes shell sideengineering-resource.com

Principle of Heat Exchanger• First Law of Thermodynamic: “Energy is conserved.”

generatedsin out

outin ewqhmhmdt

dE

ˆ.ˆ.

hh ˆˆ h

hphh TCmAQ ...

0 0 0 0

outin

hmhm ..

ccpcc TCmAQ ...

•Control Volume

Cross Section Area

HOT

COLD

Thermal Boundary Layer

Th Ti,wall

To,wall

Region III: Solid –Cold Liquid Convection

NEWTON’S LAW OF CCOLING

dqx hc . Tow Tc .dA

THERMAL

BOUNDARY LAYER

Energy moves from hot fluid to a surface by convection, through the wall by conduction, and then by convection from the surface to the cold fluid.

Q hot Q cold

Tc

Region I : Hot Liquid-Solid Convection

NEWTON’S LAW OF CCOLING

dqx hh . Th Tiw .dA Region II : Conduction Across Copper Wall

FOURIER’S LAWdqx k.

dT

dr

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• Velocity distribution and boundary layer

When fluid flow through a circular tube of uniform cross-suction and fully developed,

The velocity distribution depend on the type of the flow.

In laminar flow the volumetric flowrate is a function of the radius.

V 2 dr D / 2

V u2rdrr 0

V = volumetric flowrate

u = average mean velocity

In turbulent flow, there is no such distribution.

• The molecule of the flowing fluid which adjacent to the surface have zero velocity because of mass-attractive forces. Other fluid particles in the vicinity of this layer, when attempting to slid over it, are slow down by viscous forces.

r

Boundary layer

• Accordingly the temperature gradient is larger at the wall and through the viscous sub-layer, and small in the turbulent core.

heating

cooling

Tube wall

Twh

Twc

Tc

Metalwall

Warm fluid

cold fluid

qx hAT

qx hA(Tw T)

qx k

A(Tw T)

h

• The reason for this is 1) Heat must transfer through the boundary layer by conduction.2) Most of the fluid have a low thermal conductivity (k)3) While in the turbulent core there are a rapid moving eddies, which they are equalizing the temperature.

Region I : Hot Liquid –Solid Convection

Th Tiw q x

h h .A i

qx hhot. ThTiw .A

Region II : Conduction Across Copper Wall

qx kcopper .2L

lnro

ri

To,wallTi,wall qx .ln

ro

ri

kcopper.2L

Region III : Solid –Cold Liquid Convection

To,wall Tc qx

hc.Aoqx hc To,wallTc Ao

+

U = The Overall Heat Transfer Coefficient [W/m.K]

+

Th Tc qx

1

hh .Ai

ln

ro

ri

kcopper.2L

1

hc .Ao

qx U.A. Th Tc

1

1

.

ln.

.

coldicopper

i

oo

ihot

o

hrk

rr

r

rh

rU

Th Tc qx

R1 R2 R3

U 1

A .R

ro

ri

Experimental Equations:

Q = Uo A Tlm

Qh = Fh Cp T

Qc = Fc Cp T

53

Qc Fc Cp T

Q – heat loadUo – Overall heat transfer coefficient

A – heat exchanger areaTlm – log mean temperature differenceF – water flow rate

Cp – water heat capacity

Theoretical Equations

hoAokL

DiDo

hiAiUA

1

2

)/ln(11 • h – convective heat

transfer coefficient

• D –tube diameter

• A – cross sectional area

54

• k – thermal conductivity– o outer tube

– i inner tube

D

kNuh

CNu

kCp

VD

m

36.0PrRe

Pr

Re

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Uo=aVb

a b a bCounter-Current Laminar Max Cold Flowrate 9200 0.32 59000 0.54

Min Cold Flowrate 13000 0.44 9900 0.35

Shell and Tube Double Pipe

a & b Coefficients

55

Counter-Current Turbulent Max Cold Flowrate 30000 0.51 200000 1.08Min Cold Flowrate 28 -0.49 130000 0.7

Co-Current Laminar Max Cold Flowrate 13000 0.37 1200 0.04Min Cold Flowrate 6600 0.32 1900 0.15

Co-CurrentTurbulent Max Cold Flowrate 300000 0.85 40000 0.47Min Cold Flowrate 810000 1.1 63000 0.58

• In general Heat Transfer Coefficient for the Double-Pipe is higher than for the Shell and Tube

65000<Uo< 15000 [W/m2C]

56

• Shell and Tube 6000<Uo<1500 [W/m2C]

Calculating U using Log Mean Temperature

coldhot dqdqdq

ch TTT ch dTdTTd )(

hhphh dTCmdq ..

ccpcc dTCmdq ..

Hot Stream :

Cold Stream:

cpc

chph

h

Cm

dq

Cm

dqTd

..)(

dATUdq ..

cpc

hph CmCm

dATUTd.

1

.

1...)(

2

1

2

1

..)( A

A

c

h

hT

TdA

q

T

q

TU

T

Td

2

1

2

1

..

1

.

1.

)( A

Acpc

hph

T

TdA

CmCmU

T

Td

outc

inc

outh

inhch TTTT

q

AUTT

q

AU

T

T

...

ln1

2

11ch qqT

1

2

12

ln

.

TT

TTAUq

Log Mean Temperature

CON CURRENT FLOW

1

2

12

lnT

T

TTTLn

COUNTER CURRENT FLOW

Ln

cpc

Ln

hph

TA

TTCm

TA

TTCmU

.

..

.

.. 10763

Log Mean Temperature evaluation

T1

1 2

T2

T3

T6

T4 T6

T7 T8

Wall∆T1

∆T2

1 2

731 TTTTT inc

inh

1062 TTTTT outc

outh

1062 TTTTT inc

outh

731 TTTTT outc

inh

T1T2

T4 T5

T3

T7 T8 T9

T10

T6

Counter - Current Flow

T1 T2T4 T5

T6T3

T7T8 T9

T10

Parallel Flow

A

T9

T10∆ A

A

T1

1 2

T2

T3

T6

T4 T6

T7 T8

T9

T10

Wall

q hh Ai Tlm

Tlm (T3 T1) (T6 T2)

ln(T3 T1)

(T6 T2)

Aq hc Ao Tlm

Tlm (T1 T7) (T2 T10)

ln(T1 T7)

(T2 T10)

Nu f (Re, Pr, L / D , b / o )

DIMENSIONLESS ANALYSIS TO CHARACTERIZE A HEAT EXCHANGER

..Dv

k

Cp .k

Dh.

Nu a.Reb .Pr c•Further Simplification:

DCan Be Obtained from 2 set of experiments

One set, run for constant Pr

And second set, run for constant Re

q k

A(Tw T )

h

Nu D

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•For laminar flow

Nu = 1.62 (Re*Pr*L/D)

•Empirical Correlation

14.0

3/18.0 .Pr.Re.026.0

o

bLnNu

•For turbulent flow

o

•Good To Predict within 20%•Conditions: L/D > 10

0.6 < Pr < 16,700Re > 20,000

ExperimentalApparatus

Hot Flow Rotameters

TemperatureIndicator

Cold Flow rotameter

Switch for concurrent and countercurrent flow

• Two copper concentric pipes•Inner pipe (ID = 7.9 mm, OD = 9.5 mm, L = 1.05 m)

•Outer pipe (ID = 11.1 mm, OD = 12.7 mm)

•Thermocouples placed at 10 locations along exchanger, T1 through T10

Heat Controller

Temperature Controller

150

200

250

Nu

s

Examples of Exp. Results

2

2.5

3

3.5

4

4.5

5

5.5

6

9.8 10 10.2 10.4 10.6 10.8 11

ln (

Nu

)

ln (Re)

Theoretical trend

y = 0.8002x – 3.0841

Experimental trend

y = 0.7966x – 3.5415

Theoretical trend

y = 0.026x

Experimental trend

y = 0.0175x – 4.049

0

50

100

150 2150 4150 6150 8150 10150 12150N

Pr^X Re^Y

4

4.2

4.4

4.6

4.8

0.6 0.8 1 1.2 1.4

ln (

Nu

)

ln (Pr)

Theoretical trend

y = 0.3317x + 4.2533

Experimental trend

y = 0.4622x – 3.8097

Experimental Nu = 0.0175Re0.7966Pr0.4622

Theoretical Nu = 0.026Re0.8Pr0.33

15000

20000

25000

30000

35000

Coe

ffic

ien

t W

m-2

K-1

hi (W/m2K)

ho (W/m2K)

U (W/m2K)

Effect of core tube velocity on the local and over all Heat Transfer coefficients

0

5000

10000

15000

0 1 2 3 4

Hea

t Tr

ansf

er C

Velocity in the core tube (ms-1)

Heat Exchanger Analysis• Expression for convection heat transfer for flow of a fluid inside

a tube:)( ,, imompconv TTcmq

• For case 3 involving constant surrounding fluid temperature:

lms TAUq )/l (

iolm TT

TTT

Heat Exchangers Chee 318 66

lms )/ln( io TT

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Heat Exchanger Analysis

In a two-fluid heat exchanger consider the hot and cold fluids


In a two fluid heat exchanger, consider the hot and cold fluids separately:

)(

)(

,,,

,,,

icoccpcc

ohihhphh

TTcmq

TTcmq

lmTUAq and

The usual design goal is to determine the required area A for a heating duty q

Combine eqs. (11.1) and (11.2) and solve for A

Need to determine U and Tlm

Tlm: 1. Parallel-Flow Heat Exchangers

lmTUAq

)/ln( 12

12

TT

TTTlm

T1 T2


where

ocoh

icih

TTT

TTT

,,2

,,1

Tlm: 2. Counter-Flow Heat Exchangers

lmTUAq

)/ln( 12

12

TT

TTTlm

T1 T2


where

icoh

ocih

TTT

TTT

,,2

,,1

Overall Heat Transfer Coefficient

• For tubular heat exchangers we must take into account the conduction resistance in the wall and convection resistances of the fluids at the inner and outer tube


the wall and convection resistances of the fluids at the inner and outer tube surfaces.

kL

DDR

AhR

AhUA

iocond

oocond

ii

2

111

)/ln(

where inner tube surface

outer tube surface LDA

LDA

oo

ii

(11.3)

ooii AUAUUA

111

Types (cont.)

• Shell-and-Tube Heat Exchangers

One Shell Pass and One Tube Pass

Baffles are used to establish a cross-flow and to induce turbulent mixing of the shell-side fluid, both of which enhance convection.

The number of tube and shell passes may be varied, e.g.:

One Shell Pass,Two Tube Passes

Two Shell Passes,Four Tube Passes

Types (cont.)

• Cross-flow Heat Exchangers

Finned-Both FluidsUnmixed

Unfinned-One Fluid Mixedthe Other Unmixed

For cross-flow over the tubes, fluid motion, and hence mixing, in the transverse direction (y) is prevented for the finned tubes, but occurs for the unfinned condition. Heat exchanger performance is influenced by mixing.

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• Compact Heat Exchangers Widely used to achieve large heat rates per unit volume, particularly when

one or both fluids is a gas. Characterized by large heat transfer surface areas per unit volume, small flow passages, and laminar flow.

(a) Fin-tube (flat tubes, continuous plate fins)(b) Fin-tube (circular tubes, continuous plate fins)(c) Fin-tube (circular tubes, circular fins)(d) Plate-fin (single pass)(e) Plate-fin (multipass)

Overall Heat Transfer Coefficient

• An essential requirement for heat exchanger design or performance calculations.

• Contributing factors include convection and conductionassociated with the two fluids and the intermediate solid, as well as the potential use of fins on both sides and the effects of time-dependent surface fouling.

• With subscripts c and h used to designate the hot and coldWith subscripts c and h used to designate the hot and coldfluids, respectively, the most general expression for the overall coefficient is:

, ,

1 1 1

1 1

c h

f c f hw

o o o oc c h h

UA UA UA

R RR

hA A A hA

o,

Overall surface efficiency of fin array (Section 3.6.5)

1 1

o

fc or h f

c or h

A

A

total surface area (fins and exposed base)surface area of fins only

tA AA

2 for a unit surfFouling fact ace area (m W)or K/fR

Table 11.1

conduction resistan Wall (K/Wce )wR

surface area of fins onlyfA

Assuming an adiabatic tip, the fin efficiency is

,

tanhf c or h

c or h

mL

mL

2 /c or h p w c or hm U k t

, partial overall coe1

fficientp c or hf c or h

hUhR

Shell-and-Tube Heat Exchangers• Why shell-and-tube?

• Scope of shell-and-tube

• Construction

• TEMA standards

• Choice of TEMA type

• Fluid allocationFluid allocation

• Design problems

• Enhancement

• Improved designs

Why shell-and-tube?• Can be designed for almost any duty with a very

wide range of temperatures and pressures

• Can be built in many materials

• Many suppliers

• Repair can be by non-specialists

D i th d d h i l d h b• Design methods and mechanical codes have been established from many years of experience

Scope of shell-and-tube• Maximum pressure

– Shell 300 bar (4500 psia)

– Tube 1400 bar (20000 psia)

• Temperature range

– Maximum 600oC (1100oF) or even 650oC

– Minimum -100oC (-150oF)( )

• Fluids

– Subject to materials

– Available in a wide range of materials

• Size per unit 100 - 10000 ft2 (10 - 1000 m2)

Can be extended with special designs/materialsCan be extended with special designs/materials

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Construction• Bundle of tubes in large cylindrical shell

• Baffles used both to support the tubes and to direct into multiple cross flow

• Gaps or clearances must be left between the baffle and the shell and between the tubes and the baffle to enable assembly

Shell

Tubes

Baffle

Shell-side flow

U-Tube Heat Exchanger Straight-Tube ( 1-Pass )

Straight-Tube ( 2-Pass )

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Heat Exchangers - Types

1-2 Heat Exchanger: 1 shell pass, 2 tube passes.


2-4 Heat Exchanger: 2 shell passes, 4 tube passes.

Heat Exchangers - Temperatures Heat Exchangers - Types

1 shell pass

2 tube passes

Shell and Tube Heat Exchanger cutaway in computer lab


End detail in heat exchanger


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Tube layouts

pitchTriangular30o

Rotatedt i l

SquareRotatedsquare

• Typically, 1 in tubes on a 1.25 in pitch or 0.75 in tubes on a 1 in pitch

• Triangular layouts give more tubes in a given shell

• Square layouts give cleaning lanes with close pitch

30o triangular60o

q90o

square45o

Front head type• A-type is standard for dirty tube side

• B-type for clean tube side duties. Use if possible since cheap and simple.

BA

Channel and removable cover Bonnet (integral cover)

More front-end head types• C-type with removable shell for hazardous tube-

side fluids, heavy bundles or services that need frequent shell-side cleaning

• N-type for fixed for hazardous fluids on shell side

• D-type or welded to tube sheet bonnet for high ( 150 b )pressure (over 150 bar)

B N D

Shell type• E-type shell should be used if possible but

• F shell gives pure counter-current flow with two tube passes (avoids very long exchangers)

E FLongitudinal baffle

One-pass shell Two-pass shell

Note, longitudinal baffles are difficult to seal withNote, longitudinal baffles are difficult to seal with

the shell especially when reinserting the shell afterthe shell especially when reinserting the shell after

maintenancemaintenance

More shell types• G and H shells normally only used for horizontal

thermosyphon reboilers

• J and X shells if allowable pressure drop can not be achieved in an E shell

HG

J

H

X

Split flow Double split flow

Divided flow Cross flow

Longitudinalbaffles

Rear head typeThese fall into three general types

• fixed tube sheet (L, M, N)

• U-tube

• floating head (P, S, T, W)Use fixed tube sheet if T below 50oC, otherwise use other

t t ll f diff ti l th l itypes to allow for differential thermal expansion

You can use bellows in shell to allow for expansion but these are special items which have pressure limitations (max. 35 bar)

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Fixed rear head typesL

Fixed tube sheet

• L is a mirror of the A front end head

• M is a mirror of the bonnet (B) front end

• N is the mirror of the N front end

Floating heads and U tube

Allow bundle removal and mechanical cleaning on the shell side

• U tube is simple design but it is difficult to clean the tube side round the bend

Floating headsT S

Pull through floating headNote large shell/bundle gap

Similar to T but with smaller shellbundle gapNote large shell/bundle gap bundle gap

Split backing ring

Other floating heads

• Not used often and then with small exchangers

P W

Outside packing to give smaller shell/bundle gap

Externally sealed floating tube sheetmaximum of 2 tube passes

Shell-to-bundle clearance (on diameter)

mm

150

100

P and S

T

0.5 1.0 1.5 2.0 2.50

Shell diameter, m

Cle

aran

ce,

0

50

Fixed and U-tube

P and S

Allocation of fluids• Put dirty stream on the tube side - easier to clean

inside the tubes

• Put high pressure stream in the tubes to avoid thick, expensive shell

• When special materials required for one stream, put that one in the tubes to avoid expensive shell

• Cross flow gives higher coefficients than in plane tubes, hence put fluid with lowest coefficient on the shell side

• If no obvious benefit, try streams both ways and see which gives best design

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Example 1

Debutaniser overhead condenser

Hot side Cold side

Fluid Light hydrocarbon Cooling waterwater

Corrosive No No

Pressure(bar) 4.9 5.0

Temp. In/Out (oC) 46 / 42 20 / 30

Vap. fract. In/Out 1 / 0 0 / 0

Fouling res. (m2K/W) 0.00009 0.00018

Example 2

Crude tank outlet heater

Cold side Hot side

Fluid Crude oil Steam

Corrosive No NoCorrosive No No

Pressure(bar) 2.0 10

Temp. In/Out (oC) 10 / 75 180 / 180

Vap. fract. In/Out 0 / 0 1 / 0

Fouling res. (m2K/W) 0.0005 0.0001

Rule of thumb on costing• Price increases strongly with shell diameter/number

of tubes because of shell thickness and tube/tube-sheet fixing

• Price increases little with tube length

• Hence, long thin exchangers are usually best

• Consider two exchangers with the same area: fixed tubesheet, 30 bar both side, carbon steel, area 6060 ft2 (564 m2), 3/4 in (19 mm) tubes

Length Diameter Tubes Cost

10ft 60 in 3139 $112k (£70k)

60ft 25 in 523 $54k (£34k)

Shell thickness

p is the guage pressure in the shell

pDs

t

p

t

t is the shell wall thickness

is the stress in the shell

From a force balance

2t pDs tpDs2

hence

Typical maximum exchanger sizes

Floating Head Fixed head & U tube

Diameter 60 in (1524 mm) 80 in (2000 mm)

Length 30 ft (9 m) 40 ft (12 m)

Area 13 650 ft2 (1270 m2) 46 400 ft2 (4310 m2)

Note that, to remove bundle, you need to allow at Note that, to remove bundle, you need to allow at least as much length as the length of the bundleleast as much length as the length of the bundle

FoulingShell and tubes can handle fouling but it can be reduced by

• keeping velocities sufficiently high to avoid deposits

• avoiding stagnant regions where dirt will collect

• avoiding hot spots where coking or scaling might occur

• avoiding cold spots where liquids might freeze or where corrosive products may condense for gasesproducts may condense for gases

High fouling resistances are a selfHigh fouling resistances are a self--fulfilling prophecyfulfilling prophecy

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Flow-induced vibrationTwo types - RESONANCE and INSTABILITY

• Resonance occurs when the natural frequency coincides with a resonant frequency

• Fluid elastic instability

Both depend on span length and velocity

-

Velocity Velocity

Resonance Instability

Tu

be

dis

pla

cem

ent

Avoiding vibration• Inlet support baffles - partial baffles in first few

tube rows under the nozzles

• Double segmental baffles - approximately halve cross flow velocity but also reduce heat transfer coefficients

• Patent tube-support devices

• No tubes in the window (with intermediate support baffles)

• J-Shell - velocity is halved for same baffle spacing as an E shell but decreased heat transfer coefficients

Avoiding vibration (cont.)

Inlet support Double-segmental bafflesppbaffles

Double segmental baffles

No tubes in the window - with intermediate support baffles

TubesWindows with no tubes

Intermediate baffles

Shell-side enhancement• Usually done with integral, low-fin tubes

– 11 to 40 fpi (fins per inch). High end for condensation

– fin heights 0.8 to 1.5 mm• Designed with o.d. (over the fin) to fit into the a standard g ( )

shell-and-tube

• The enhancement for single phase arises from the extra surface area (50 to 250% extra area)

• Special surfaces have been developed for boiling and condensation

Low-finned Tubes• Flat end to go into tube sheet and intermediate flat

portions for baffle locations

• Available in variety of metals including stainless steel, titanium and inconels

Problems of Conventional S & T

Zigzag path on shell side leads to

• Poor use of shell-side pressure drop

• Possible vibration from cross flow

• Dead spots– Poor heat transfer

– Allows fouling

• Recirculation zones– Poor thermal effectiveness,

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Conventional Shell-side FlowShell-side axial flow

Some problems can be overcome by having axial flow

• Good heat transfer per unit pressure drop but – for a given duty may get very long thin units

– problems in supporting the tube

RODbaffles (Phillips petroleum)

• introduced to avoid vibrations by providing additional support for the tubes

• also found other advantages– low pressure drop

– low fouling and easy to clean

– high thermal effectiveness

RODbafflesTend to be about 10% more expensive for the same shell

diameter

Twisted tube (Brown Fintube)

• Tubes support each other

• Used for single phase and condensing duties in the power, chemical and pulp and paper industries

Shell-side helical flow (ABB Lummus)

Independently developed by two groups in Norway and Czech Republic

Comparison of shell side geometriesTwisted

tubeSegmental

bafflesHelicalbaffles

RODbaffles

Good p Y N Y YHigh shell N Y Y NLow fouling Y N Y YEasycleaning

Y With squarepitch

With squarepitch

Ycleaning pitch pitchTube-sideenhance.

Included With inserts With inserts With inserts

Can givehigh

Y N N Y

Lowvibration

Y With specialdesigns

With doublehelix

Y

2011/3/17

21

Table 8.2


Shell-and-Tube Heat Exchangers

Baffles are used to establish a cross-flow and to induce turbulent mixing of the shell-side fluid, both of which enhance convection.


The number of tube and shell passes may be varied

One Shell Pass and One Tube Pass

One Shell Pass,Two Tube Passes

Two Shell Passes,Four Tube Passes

The correction factor F for multi-pass and cross-flow

• The standard lmtd formulation is limited to the simple cases of parallel and counter flow configurations.

• In more complex cases as cross flow and multi-pass the correction factor F has to be considered.

123

F factors for various flow configurations

124

Example A shell-and-tube heat exchanger must be designed to heat 2.5 kg/s of water from 15 to 85°C. The heating is to be accomplished by passing hot engine oil, which is available at 160°C, through the shell side of the exchanger. The oil is known to provide an average convection coefficient of ho=400 W/m2.K on the outside of the tubes. Ten tubes pass the water through the shell. Each tube is thin walled, of diameter D 25 d k i ht th h th h ll If th il l

125

D=25 mm, and makes eight passes through the shell. If the oil leaves the exchanger at 100°C, what is the flow rate? How long must the tubes be to accomplish the desired heating?

Fouling

• Heat exchanger surfaces are subject to fouling by fluid impurities, rust formation, or other reactions between the fluid and the wall material. The subsequent deposition of a film or scale on the surface can greatly increase the resistance to heat transfer between the fluids.

• An additional thermal resistance can be introduced: The


• An additional thermal resistance, can be introduced: The Fouling factor, Rf.

Depends on operating temperature, fluid velocity and length of service of heat exchanger. It is variable during heat exchanger operation.

• Fouling factors can be found in Table 11.1 textbook (SI units) or p. 51 heat exchanger lecture notes (EE units)

2011/3/17

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NTU-method.NTU –Number of transfer units

• An additional and often used method for the calculations of heat exchangers is the NTU method.

• The NTU method gives similar results to the lmtd method. However it can in some cases be more beneficial to apply to a given problem

Th NTU i d fi d

127

• The NTU is defined as:

• And C_min found from: – Cc=cold massflow x cold heat capacity (heat capacity stream)– Ch=hot massflow x hot heat capacity

NTU

• The actual heat transfer can now be found from

128

• Where ε is the effectiveness of the heat exchanger and is a function of NTU and C .

• ε =actual heat transfer area/maximum possible heat transfer area

NTU = UAs/Cmin

129 130

NTU can also be found if the efficiency is known

131

Overall Heat Transfer Coefficient•The overall heat transfer coefficient can be written:

ooo

ofcond

i

if

iiooii AhA

RR

A

R

AhAUAUUA

11111

",

", (11.4a)

1


oofcondo

i

ifo

ii

o

o

hRRA

A

RA

AhA

U1

1

",

", (11.4b)

Need to determine hi and ho

2011/3/17

23

Determination of ho• Approach 1: Using correlations from Chapter 7

• Approach 2: Using chart by Kern, p. 56 heat exchanger lecture notes

Heat Exchangers 133Typical values of baffle cuts 20-25% for liquids and 40-45% for vapour

Determination of tube side film coefficient, hi


Equations: Theoretical Values

• DiHus-Boelter Equation(Incorpera & DeWitt, p491)

• Nusslet Number(Incorpera & DeWitt, p486)

NuD 0 023 0 8 0 3. Re Pr. .

Nuh D

k

• Reynolds's Number(Incorpera & DeWitt, p467)

• Prandalt Number(Incorpera & DeWitt, p348)

k

Re v D

Pr

Equations: Theoretical Values

• Overall Heat Transfer Coefficient (Incorpera & DeWitt, p92)

UR A Lo

tot c

1 11 1

[( ) ( ) ( )]

• Hydraulic Diameter (Incorpera & DeWitt, p501)

h k hi o

[( ) ( ) ( )]

D D Dh o i

Determination of Conduction Resistance

• Recall that

)/ln(

)/ln(

ioo

condo

iocond

DDk

DRA

kL

DDR

2

2


• In EE units

)/ln( iow

ocondow DD

k

DRAr

24

Pressure Drop

In practice there can be a significant pressure drop along the pipes of a multipass heat exchanger.

Results in property changes

• Pressure drop must be accounted for in real design situations.

138

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Overall Energy Balance

• Assume negligible heat transfer between the exchanger and its surroundings and negligible potential and kinetic energy changes for each

• Application to the hot (h) and cold (c) fluids:

surroundings and negligible potential and kinetic energy changes for each fluid. , ,h i h ohq m i i

, ,c c o c iq m i i

fluid enthalpyi

• Assuming no l/v phase change and constant specific heats, , , ,p h h i h ohq m c T T

, ,h h i h oC T T

, , ,c p c c o c iq m c T T

, ,c c o c iC T T

, Heat capacity r s ateh cC C

Special Conditions

Special Operating Conditions

Case (a): C >>C or h is a condensing vapor hC Case (a): Ch>>Cc or h is a condensing vapor .hC

– Negligible or no change in , , .h h o h iT T T

Case (b): Cc>>Ch or c is an evaporating liquid .cC

– Negligible or no change in , , .c c o c iT T T

Case (c): Ch=Cc.1 2 1mT T T –

CostCost EstimationEstimation OfOf AA HeatHeatExchangerExchanger

Group Members

Chinthaka Perera

Chris Peng

Sandy Lee

Terry Peters

540:343 Engineering Economics Spring 1998

Outline

• Introduction and Problem Definition

• Designs of Heat Exchangers

• Base Cost Estimation

• Complete Cost Estimation

• Conclusions & Recommendation• Conclusions & Recommendation

• Questions


Problem Definition

•Heat cold water from 15 oC to 85 oC using the minimum resources

•Area Limited to 50 square meters

•Minimize the cost of operation and maintenance of the machine


Shell And Tube Heat Exchanger

Efficiency: 48%

Power: 7.317x10 5 Watts

Length of Pass: 4.7 meters, Life Cycle 5 yr.

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Counter Flow Heat Exchanger

Efficiency: 48%

Power: 7.317 x 105 Watts

Length 129.55 meters, 8 revolutions, Life Cycle: 5yr.540:343 Engineering Economics Spring 1998

Cross Flow Heat Exchanger

Efficiency: 61%

Power:2.92 x 105 Watts

Area: 28.4 square meters, Tubes: 7898, Life Cycle 25 yr.540:343 Engineering Economics Spring 1998

Summary of Designs

Power(Watts)

LifeCycle

Efficiency

Shell &Tube

7.137 E5 5 yrs 48 %

CounterFlow

7.137 E5 5 yrs 48 %

CrossFlow

2.92 E5 25 yrs 61 %


Base Cost Estimating Technique

Base Cost = Cost of Materials to Construct Heat ExchangerBase Cost Cost of Materials to Construct Heat Exchanger

+ Cost Of Installation of Heat Exchanger

Conversion of Base Cost to Annual Worth using MARR = 10%

AWHeat Exchanger = (Base Cost) x (A/P,10%,Life Cycle)


Base Cost Estimation Of Heat Exchanger Designs

Using Annual Worth and assuming repeatabilityUsing Annual Worth and assuming repeatability

AW $ 14400AWShell and Tube = - $ 14400

AWCounter flow = - $ 63400

AWCross flow = - $2200

Best Alternative for Heat Exchanger is

Cross Flow Heat ExchangerCross Flow Heat Exchanger540:343 Engineering Economics Spring 1998

Complete Cost Estimating Technique

Complete Cost = Base Cost + Operation and Maintenance

Conversion of Complete Cost to Annual Worth using MARR = 10%

AWHeat Exchanger = (Base Cost) x (A/P,10%,Life Cycle) +

(O & M) x (A/P,10%,Life Cycle)


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Complete Cost Estimation

B C t f C Fl H t E h $ 2200•Base Cost of Cross Flow Heat Exchanger - $ 2200

•Operation and Maintenance Cost + Cost of Gas - $ 2300


Total Cost Over Life Cycle - $ 4500

Conclusions & Recommendation

•Cross Flow Heat Exchanger is the most effective for the purpose

Cross Flow Heat Exchanger is the most Cost•Cross Flow Heat Exchanger is the most Cost EffectiveAside:

The Shell & Tube and Counter flow reused waste to heat water.


Recommendation : Cross Flow Heat ExchangerRecommendation : Cross Flow Heat Exchanger

Conventional heat exchanger (segmental baffles)

Helixchanger(helical baffles)

Compariso

IMPROVED PROCESS AND EQUIPMENT DESIGNIMPROVED PROCESS AND EQUIPMENT DESIGN(continued)(continued)

Example:p = 44 kPa

Comparison

(crude oil preheating,1 MW, 90t/hr)

p = 17 kPaResult: 60% reduction of operating cost

6.3% reduction of total cost

Optimization of Plate Type Heat Exchanger

Minimization of total cost(utilizing relation „p - h.t.c.”)

Obtaining optimum dimensions

IMPROVED PROCESS AND EQUIPMENT DESIGNIMPROVED PROCESS AND EQUIPMENT DESIGN(continued)(continued)

Example:Industrial unit for the thermal treatment of polluting hydrocarbonsof synthetic solvents containedin air (4.52 MW)Result: up to 14% reduction of annual

total cost can be achieved

Compact Heat Exchangers -- A New Approach

P.M.V Subbarao

Associate ProfessorMechanical Engineering Department

Indian Institute of Technology, Delhi

Introductory Remarks

• Power, Process, Refrigeration and A/c and Aerospace industries require small size and light weight heat exchanger devices.

• The size of heat exchanger is very large in those applications where gas is a medium of heat exchange.

• Continuous research is focused on development of Compact Heat Exchangers --- High rates of heat transfer per unit volume.

• The rate of heat exchange is proportional to– The value of Overall heat transfer coefficient.

– The surface area of heat transfer available.

– The mean temperature difference.

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Overall Heat Transfer Coefficient and Thermal Resistance

• In general the heat transfer coefficient of the gas may be 10 to 50 times smaller than that of the liquid.

• In phase-change heat exchangers, the air side heat transfer often limits the thermal performance of heat exchangers.

• The air side can comprise 75% of the thermal resistance in an pevaporator and 95% in a condenser used in typical refrigeration applications.

• In applications where the thermal resistance of one fluid dominates, significant cost reductions and energy savings can be achieved by using heat transfer augmentation devices or methods.

• For this reason development of high-performance surfaces for air side heat transfer augmentation is an important area of interest.

Large surface area Heat Exchangers

• The use of extended surfaces will reduce the gas side thermal

resistance.

• To reduce size and weight of heat exchangers, many compact heat exchangers with various fin patterns were developed to reduce the air side thermal resistance.

• Fins on the outside the tube may be categorized as• Fins on the outside the tube may be categorized as – 1) flat or continuos (plain, wavy or interrupted) external fins on arrays of

tubes,

– 2) Normal fins on individual tubes,

– 3) Longitudinal fins on individual tubes.

– Kays and London presented pressure drop and heat transfer characteristics of a wide variety of configurations of compact heat exchanger matrices.

Fin and Tube Heat Exchanger : A primitive Compact Heat Exchanger

Anatomy of Fin & Tube Heat Exchanger

Liquid Flow

Gas Flow

TubePlate

Two Dimensional Conduction In Cylindrical Coordinates System

Z

Xr d

r

(r+dr)d

dz

Conduction Equation

j

T

ri

r

Tkjqiqq rrr

1"""

Local Heat Flux Vector:

GDE for Steady State Temperature Distribution in Fin

TTkdz

hT

rr

T

rr

T 2112

2

22

2

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Variation of Heat Transfer CoefficientComments on Conventional Compact Heat

exchangers

• Conventional compact heat exchangers were developed by increasing heat transfer surface area per unit volume of heat exchanger.

• In spite of much larger heat transfer area the major part of thermal heat transfer resistance is due to gas side.

• The potential for increasing the fin area is limited by the fact that the increasing fin area leads to drop in fin efficiency. g p y

• Major part of the fin area is wetted by low heat transfer coefficient gas flow.

• large number of fins makes the system costlier, heavier, and spacious.

• An attempt for having a broader vision on the enhancement of heat transfer coefficient is yet to be achieved.

Enhancement of Heat Transfer Coefficient

• The magnitude of heat transfer coefficient is proportional to Reynolds number.

• Huge increase in heat transfer coefficient is seen in turbulent flows when compared to laminar flows.

• Turbulent flow is due to a combination of set of vortices (Eddies).

• Recent research findings have shown that the vortices play an important role in enhancing the heat transfer.

• At a given Reynolds number, the flow can be made more turbulent by introducing artificial vortices into the flow.

• Artificial introduction of vortices on the gas side can be another potential alternative for augmenting heat transfer in compact heat exchangers.

Compact Heat Exchangers -- A New Concept

• Creation of concept.

• Testing and understanding of concept.

• Explanation of the concept.

• Discovery of various performance parameters of the concept.

• Development of compact heat exchangers using the new concept.Development of compact heat exchangers using the new concept.

• Testing and performance evaluation.

• Generation of Design data.

VORTEX GENERATORS

Thursday, 29 May, 2008 Mechanical Department, IIT Delhi 167

•Wing type turbulators (a,b)•Winglet type turbulators (c,d)

WINGLET VORTEX GENERATORS


h: Height of trailing edge L: Base lengthβ: Angle of attackX and Y: position coordinates with respect to center of tubet: Thickness of wingletAR: 2h/L, Aspect ratio

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VORTICAL DESCRIPTION OF FLOW PAST A DELTA WINGLET


HEAT TRASNFER ENHANCEMENT BY WINGLET


COMMON FLOW DOWN AND COMMON FLOW UP


Top view

Flow(a)

Flow(b)

Fin-Tube Heat Exchangers with winglets

A Model Heat exchanger

Theory of circular fins

dr

dz

(r+dr)dr

z

r.d

TT

tkT

rr

T

rr

Trh

2

.11,

2

2

22

2

dr

x

y

Local heat Transfer Coefficient:

2011/3/17

30

Measurement of Isotherms

• Fifty thermocouples are embedded in central plate

• The cylinder is electrically heated.

• A special wind tunnel is designed developed and tested for this purpose.

• Steady state temperature distribution is transferred to computer.

Anatomy of Fin-Tube Heat Exchangers

Wind Tunnel Heat Transfer in Circular Fins

Positioning of Winglets around Tube

• Objective is to raise to promote heat transfer rate in downstream of a tube.

• High temperature gradients and heat transfer coefficients are to be created.

• A placement of winglet also leads to raise in pressure drop.

• An optimum location will produce a maximum heat transfer enhancement with low raise in pressure drop.

Heat Transfer in Circular Fins with winglets

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Effect of Winglet GeometryEffect of winglet Position

Main Observations

• Mechanism of Heat Transfer Enhancement by Winglet

• The winglet entrains more fluid into the downstream (wake) region and enhances fluid circulation.

• This leads to increase in the value of local heat transfer coefficient and hence enhancement in the heat transfer rates in wake regionhence enhancement in the heat transfer rates in wake region.

• Best winglet and best positions are those, which entrain more fluid into wake region.

A Prototype Compact Heat Exchanger

Performance of Prototype

The Compact Heat Exchanger

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32

Concluding Remarks

• The winglet with = 1.33, located at X= 0.5D and Y=0.5D is the the option

• The Result on prototype are encouraging.

• The enhancement in heat transfer coefficient shows a great promise in reducing gas side thermal resistance.

• Another effective alternate concept for construction of Compact HeatAnother effective alternate concept for construction of Compact Heat Exchangers.

• Testing of real equipment and generation of final design procedure is our final goal.

HEAT PIPE HEAT EXCHANGERS

Introduction to Heat PipesHeat Pipe :Heat pipe is a self-contained passive energy recovery device. Aheat pipe can transfer up to 1000 times more thermal energy, thancopper, the best known conductor; that too with less than 57°C/mtrtemperature drop. One of the amazing features of the heat pipes isthat they have no moving parts and hence require minimummaintenance. They are completely silent and reversible in operationand require no external energy other than the thermal energy theytransfer. Heat pipes are ruggedly built and can withstand a lot ofabuse.

What is Heat Pipe :

fA traditional heat pipe is a hollow cylinder filled with a vaporizableliquid. The Heat Pipe functions as follows :A - Heat is absorbed in the evaporating section.B - Fluid boils to vapor phase.C - Heat is released from the upper part of cylinder to the

environment; vapor condenses to liquid phase.D - Liquid returns by gravity to the lower part of cylinder

(evaporating section).

When heat is added to the evaporator section, the working fluid boils and converts into vapor absorbinglatent heat.After reaching the condenser section, due to partial pressure build up, the vapor transforms back intoliquid thus releasing latent heat. From the condenser section, heat is taken away by means of watercooling / air cooling with fins etc. The liquid condensate returns to the original position through thecapillary return mechanism, completing the cycle. Due to very high latent heat of vaporization a largequantity of heat can be transferred.

Hot Gas to Air Heat Pipe Heat Exchangers

Hot Gas to Air Heat Pipe Heat Exchangers :

Typical finned Hot Gas to airHeat Pipe heat exchangerscomprise of number of tubulargravity assisted finned HeatPi d i t dPipes arranged in staggeredpitch, depending upon theapplication. One of theadvantages of the Heat PipeHeat Exchanger is its ability tooperate without crosscontamination between the twostreams.

Use of Gravity Assisted Heat Pipe complies orientationevaporatorabove condenser for the Heat Pipe Heat Exchanger.

Hot Gas to Liquid Heat Pipe Heat Exchangers

Hot Gas to LiquidHeat Pipe Heat Exchangers :This heat exchanger resemblesAir to Air unit, only difference isliquid/water tank is provided atthe condensation section topreheat liquid/water.Manor heat pipe heat exchanger

Manor heat pipe heat exchangers as standard are suitable for air-to air heatrecovery for a temperature range of - 10oC to + 260oC. With specialmaterials of construction for the tube, the exchanger can be designed toextend the range to 427oC to 482oC

can recover up to 85% ofexhausted thermal energy.However under ideal conditionsthe thermal efficiency of aneconomic system rangesbetween 55 to 70% and savesmillions of BTUs year after year.

Heat Pipe Panel Cooler ( HPPC )Heat Pipe Panel Cooler ( HPPC ) :Electrical and electronics control panels are normally made airtight to protect costly electronic and electrical components fromdust. Inside components get overheated which leads to failureor mal-function. Control panel coolers extract heat from insidethe panel without disturbing inside environment of panel.

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Heat Pipe Dehumidifier ( HPD )Heat Pipe Dehumidifiers ( HPD ) : In an air conditioning system, additionalmoisture is condensed out, as the air becomes colder and colder. The heatpipe is designed to have one section in the warm incoming stream and theother in the cold outgoing stream. By transferring the heat from the warmreturn/incoming air to the cold outgoing supply air, the heat pipes create thedouble effect of pre-cooling the air before it goes to the evaporator and thenre-heating it immediately. This lowers the cooling load, evaporatingtemperature and heating load.

Activated by temperaturedifference and thereforeconsuming no energy the heatconsuming no energy, the heatpipe, due to its pre-coolingeffect, allows the evaporator coilto operate at a lowertemperature, increasing themoisture removal capability ofthe air conditioning system by50-100%.

With lower relative humidity, indoor comfort can be achieved at higherthermostat settings, which results in net energy savings. Generally, for each 1°C rise in thermostat setting, there is a 3% savings in electricity cost. Inaddition, the pre-cooling effect of the heat pipe allows the use of a smallercompressor.

Heat Pipe Applications

Annealing furnaces

• Bakery Oven • Boiler • Brick Kiln • Chemical fluid bed dryer • Dehumidifier

Reverberatory furnace

Solvent boil off oven Spray dryer Textile cloth finishing oven Tanter frame textile drying oven T bl d• Dehumidifier

• Epoxy coating curing Oven • Foundry hot blast • Fume scrubber • Heater flue gas • Heat treatment furnace • Humber dryer • Plastic laminate dryer • Print dryer

Tumble dryer Veneer dryer etc.

Problem: Overall Heat Transfer Coefficient

Problem 11.5: Determination of heat transfer per unit length for heat recovery device involving hot flue gases and water.

KNOWN: Geometry of finned, annular heat exchanger. Gas-side temperature and convection coefficient. Water-side flowrate and temperature.

FIND: Heat rate per unit length.

SCHEMATIC:

Do = 60 mm Di,1 = 24 mm Di,2 = 30 mm t = 3 mm = 0.003m L = (60-30)/2 mm = 0.015m

Problem: Overall Heat Transfer Coefficient (cont.)

ASSUMPTIONS: (1) Steady-state conditions, (2) Constant properties, (3) One-dimensional conduction in strut, (4) Adiabatic outer surface conditions, (5) Negligible gas-side radiation, (6) Fully-developed internal flow, (7) Negligible fouling.

PROPERTIES: Table A-6, Water (300 K): k = 0.613 W/mK, Pr = 5.83, = 855 10-6 Ns/m2.

ANALYSIS: The heat rate is

where

m,h m,ccq UA T T

w oc c h1/ UA 1/ hA R 1/ hA

i,2 i,1 4w

ln D / D ln 30 / 24R 7.10 10 K / W.

2 kL 2 50 W / m K lm


the internal flow is turbulent and the Dittus-Boelter correlation gives

4 / 5 0.44 / 5 0.4 2c i,1 D

0.613 W / m Kh k / D 0.023Re Pr 0.023 9990 5.83 1883 W / m K

0.024m

11 2 3chA 1883 W / m K 0.024m 1m 7.043 10 K / W.

The overall fin efficiency is

f f1 A / A 1

With

D 6 2i,1

4m 4 0.161 kg / sRe 9990

D 0.024m 855 10 N s / m

o f f1 A / A 1

2fA 8 2 L w 8 2 0.015m 1m 0.24m

2 2f i,2A A D 8t w 0.24m 0.03m 8 0.003m 0.31m .

From Eq. 11.4,

ftanh mL

mL


Hence f 0.800 /1.10 0.907 o f f1 A / A 1 1 0.24 / 0.31 1 0.907 0.928

11 2 2

where

1/ 2 1/ 22 1m 2h / kt 2 100 W / m K / 50 W / m K 0.003m 36.5m

1/ 2 1mL 2h / kt L 36.5m 0.015m 0.55

1/ 2tanh 2h / kt L 0.499.

11 2 2o hhA 0.928 100 W / m K 0.31m 0.0347 K / W.

It follows that

1 3 4cUA 7.043 10 7.1 10 0.0347 K / W

cUA 23.6 W / K

and q 23.6 W / K 800 300 K 11,800 W < for a 1m long section.

2011/3/17

34


COMMENTS: (1) The gas-side resistance is substantially decreased by using the fins

,2f iA D and q is increased.

(2) Heat transfer enhancement by the fins could be increased further by using a material of larger k, but material selection would be limited by the large value of Tm,h.

Problem: Ocean Thermal Energy Conversion

Problem 11.47: Design of a two-pass, shell-and-tube heat exchanger to supply vapor for the turbine of an ocean thermal energy conversion system based on a standard (Rankine) power cycle. The powercycle is to generate 2 MWe at an efficiency of 3%. Ocean water enters the tubes of the exchanger at 300K, and its desiredoutlet temperature is 292K. The working fluid of the powercycle is evaporated in the tubes of the exchanger at itsphase change temperature of 290K, and the overall heat transfercoefficient is known.

FIND ( ) E t (b) W t fl tFIND: (a) Evaporator area, (b) Water flow rate.

SCHEMATIC:

Problem: Ocean ThermalEnergy Conversion (cont)

ASSUMPTIONS: (1) Negligible heat loss to surroundings, (2) Negligible kinetic and potential energy changes, (3) Constant properties.

PROPERTIES: Table A-6, Water ( mT = 296 K): cp = 4181 J/kgK.

ANALYSIS: (a) The efficiency is

W 2 MW

0.03.q q

Hence the required heat transfer rate is

2 MW

q 66.7 MW.0 03

0.03

Also

m,CF300 290 292 290 C

T 5 C300 290

n292 290

and, with P = 0 and R = , from Fig. 11.10 it follows that F = 1. Hence

7

2m,CF

q 6.67 10 WA

U F T 1200 W / m K 1 5 C

2A 11,100 m .

Problem: Ocean ThermalEnergy Conversion (cont)

b) The water flow rate through the evaporator is

7

hp,h h,i h,o

q 6.67 10 Wm

4181 J / kg K 300 292c T T

hm 1994 kg / s.

COMMENTS: (1) The required heat exchanger size is enormous due to the small temperature differences involved,

(2) The concept was considered during the energy crisis of the mid 1970s but has not since been implemented.

Heat Exchanger lecture - 140.138.140.5

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