Heat Exchangers

Process Design CEN 574

Spring 2004

Chemical Engineering and Materials Science

Syracuse University

Shell and Tube Heat Exchangers

Aspects of shell and tube heat exchangers to specify for design

Tube sizeTube bundle layoutShell size and type

Baffle type and geometryMaterials of construction

Heat transfer fluid

Tube Size• Available tube sizes 5/8 -1½ inch diameter

• ¾ - 1 inch most common

• As tube diameter increases

– Exchanger volume increases (more expensive)

– Film heat transfer coefficient decreases

– Overall heat transfer coefficient decreases

– Tube side P decreases

• Start with ¾ inch, increase if P constraints encountered

Tube Bundle

Tube Bundle LayoutPitch

• shortest center to center distance between two tubes. Average pitch is 1.25-1.5 times the tube diameter.

• Select 1.25 times the tube diameter, increase in shell side P constraints are not met.

Layout• Square (in-line): can be mechanically cleaned –

use 45o tube layout• Triangular (staggered): must use chemical

cleaning, but higher number of tubes per shell – use 30o tube layout

Tube Layout

Tube Layout

TEMA

Tubular

Exchanger

Manufacturers

Association

Shell Type and Size• Shell geometries indicated by

TEMA designation.• TEMA E: one pass shell• TEMA F: two pass shell with

longitudinal baffle• TEMA G, H, J: different flow

regimes

Shell Geometries

Baffle Type and Geometry

• Select single, segmented baffle: simple and proven.

• If shell side P too great, select double or triple cut segmented baffle.

Baffles

Segmented Baffles

Baffle Cut

Disk and Doughnut Baffles

Orifice Baffle

Flow Around Baffles

Optimum Baffle Spacing

Example

HEAT

EXCHANGER

1000 kmol BZ/hr

Cp=80 J/mol oC

Cp(H20) = 74.5 J/mol oC

¾ in = 0.02 m

112oC

50oC

38oC

28oC

Cooling water

Calculate heat lost by the process streamQprocess=[mCpT]process

Assume an inlet and outlet cw T based on heuristics

Calculate approximate heat transfer areaQ=UATLM

Calculate mcw

Qcw=Qprocess=[mCpT]cw

Is A>typical double pipe area?

NODesign double pipe

YESCalculate minimum and

maximum # tubes based on tube side velocity constraints

vmin,max=V/(Acsnmin,max)Calculate # tubes (nA) based on 16 ft length, single pass

A=CLnA

If nA between nmin and nmax

If nA>nmax

Single pass ConsiderExchangers in seriesMultiple passes

Shell and Tube Heat Exchanger Design Approach

Example1. Find heat transferred from process stream.

Q = mCpT

Q =(1000kmol/hr)(80J/moloC)(112-38)oC(1000mol/kmol)

Q = 5.9x109 J/hr

2. Find cooling water flow rate.

Qprocess=Qcw= 5.9x109 J/hr=(mCpT)cw

mcw= (5.9x109 J/hr)(moloC/74.5J)(1/50-28)oC

mcw= 3.6x103 kmol/hr

3. Find maximum # tubes (nmax) based on minimum velocity constraint for turbulent flow.

vmin = V/(Acs,tube)(nmax)

V=(1000kmol/hr)(78kg/kmol)(m3/992kg)(hr/3600s)

V = 0.0218 m3/s

Solving for nmax

nmax=(0.0218m3/s)/[(3.16x10-4m2)(1m/s)]

nmax = 69 tubes

4. Calculate the heat transfer area.

Qtransferred = UA TLM

U 850 J/s m2 oC from heuristics

T1=112-50 = 62oC

T2 = 38-28 = 10oC

Solving for AA=5.9x109J/hr)(sm2oC/850J)(hr/3600s)(ln[62/10]/(62-10)oC

A = 67.7 m2

shell Length (ft)diameter 8 10 12 16 20(in) # tubes Exchanger Heat Transfer Area (m2)

8 37 6 7 9 11 1412 92 14 18 21 28 35

15.25 151 23 29 35 46 58 ONE21.25 316 49 61 73 97 121 PASS

25 470 72 90 108 144 18131 745 114 143 172 229 28637 1074 165 206 247 330 4128 30 5 6 7 9 12

12 82 13 16 19 25 3115.25 138 21 26 32 42 53 TWO21.25 302 46 58 70 93 116 PASS

25 452 69 87 104 139 17431 728 112 140 168 224 28037 1044 160 200 241 321 4018 24 4 5 6 7 9

12 76 12 15 18 23 2915.25 122 19 23 28 37 47 FOUR21.25 278 43 53 64 85 107 PASS

25 422 65 81 97 130 16231 678 104 130 156 208 26037 1012 155 194 233 311 389

69 tubes

138 tubes

276 tubes

What is driving the multiple pass?

• Required area is forcing the large number of tubes.

• Heat exchangers in series (same number of tubes, ½ the area for each).

Form groups of two

Find the type of heat exchanger (passes, shell ID, # tubes) that

could be used if the benzene flow rate is 1500 kmol/hr and the

water outlet temperature is 60oC

shell Length (ft)diameter 8 10 12 16 20(in) # tubes Exchanger Heat Transfer Area (m2)

8 37 6 7 9 11 1412 92 14 18 21 28 35

15.25 151 23 29 35 46 58 ONE21.25 316 49 61 73 97 121 PASS

25 470 72 90 108 144 18131 745 114 143 172 229 28637 1074 165 206 247 330 4128 30 5 6 7 9 12

12 82 13 16 19 25 3115.25 138 21 26 32 42 53 TWO21.25 302 46 58 70 93 116 PASS

25 452 69 87 104 139 17431 728 112 140 168 224 28037 1044 160 200 241 321 4018 24 4 5 6 7 9

12 76 12 15 18 23 2915.25 122 19 23 28 37 47 FOUR21.25 278 43 53 64 85 107 PASS

25 422 65 81 97 130 16231 678 104 130 156 208 26037 1012 155 194 233 311 389

484 tubes

121 tubes

242 tubes

Detailed Calculation of Overall Heat Transfer Coefficient

fiimw

iw

o

i

oo

ifo

i

Rh1

Ak

At

AA

h1

AA

R

1U

overall inner heat transfer coefficient

)/Dln(D

)DπL(DA

io

iom

Parameters

tw = wall thickness

kw=thermal conductivity of the wall

ho=individual outside film heat trans. coeff.

hi= individual inside film heat trans. coeff

Ai=DiL

Ao= DoL

Rfo=outside fouling factor

Rfi=inside fouling factor

hi: tubeside individual heat transfer coefficient

2/3

i2/3PrD

PrReD

b

ii

L

D1

1)(N/8f12.71

)1000)(N/8)(N(f

k

DhNu

fD=(1.82log10(NRe)-1.64)-2

NRe=DiG/

G=flow rate/cross sectional area

NPr=Cp /k

Evaluated at tube bulk conditions

hi: shellside individual heat transfer coefficient

0.14

w

b

1/3

b

bpb

n

bb

o

μ

μ

k

μC

μ

DGC

k

DhNu

Kern Method:

D = hydraulic diameter

G = mass velocity normal to tubes closest to centerline

n = 0.55, C = 0.36

Heat Exchanger DesignIf I ask you to design a shell and tube heat

exchanger, what will you specify?

Tube size (diameter, length)

# tube passes

Tube layout

Shell size and type

Baffle type and geometry

Materials of construction

What are the constraints we have discussed in heat exchanger design?

• Minimum velocity in the tubes to maintain turbulent flow because the heat transfer coefficient is much higher for turbulent flow than for laminar flow.

Sets the maximum number of tubes.• Maximum velocity based on erosion and

efficiency.sets the minimum # of tubes.

• Required area for heat transfer based on

Qtransferred = UA TLM Sets the length of the tubes and # passes.

What other constraints should/could there be?

Tube and shell side pressure drop constraints.

Tube Side Pressure Drop

• Remember

So, the heat transfer coefficient is proportional to velocity to the 0.8 power

n54

Db

ii Pr0.023Rek

DhNu

0.8vh

What is the relationship between pressure drop and velocity?

Remember Bernoulli's Equation. Pressure drop is proportional to the velocity squared.

So, increasing the velocity, the pressure drop increases more rapidly than the heat transfer coefficient. Also as velocity increases, erosion increases.

This is where the recommended maximum velocity constraints were derived.

Allowable Pressure Drop• The allowable pressure drop is set by the

process and is based on economics. A high allowable pressure drop would cost more in pumping power, but would require a smaller exchanger due to increased heat transfer coefficients.

• Typically values are 7-10 psi (0.5-0.7 kg/cm2) for and tube side.

Poddar Plot

Method to visualize the constraints driving heat

exchanger design

5 Lines on a Poddar Plot1. Maximum tube count – based on

minimum tube side velocity constraint.

2. Minimum tube count – based on maximum tube side velocity constraint

3. Line for Qrequired/transferred

4. Allowable tube side P5. Allowable shell side P

Poddar Plot1. Maximum tube count based on minimum

tube side velocity constraint. Minimum velocity recommended = 1.0 m/s found from laminar to turbulent flow transition.

2. Minimum tube count based on maximum tube side velocity constraint. Maximum recommended liquid velocity – 2.0-3.0 m/s found from experience.

These two horizontal lines can be plotted on a plot of tube count verses tube length.

Tube Length

Tube

Cou

nt

maximum # of tubes

minimum # of tubes

Poddar Plot3. Line for Qrequired/transferred.

Qtransfered=UA TLM

iiAU

1

UA

1 )

ΔTΔT

ln(

ΔTΔTFΔT

2

1

21LM

A=(tube circumference)(length of tubes)(number of tubes)

A=(2r)(L)(n)

Poddar Plot – line 3

fiimw

iw

o

i

oo

ifo

i

Rh1

Ak

At

AA

h1

AA

R

1U

overall inner heat transfer coefficient

Ai=DiL

Ao= DiL

Rfo=outside fouling factor

Rfi=inside fouling factor

)/Dln(D

)DπL(DA

io

iom

Poddar Plot – line 3

tw = wall thickness

kw=thermal conductivity of the wall

ho=individual outside film heat trans. coeff.

hi= individual inside film heat trans. coeff

Poddar Plot – line 3hi: tubeside individual heat transfer coefficient

2/3

i2/3

D

PrReD

b

ii

L

D1

1)Pr(N/8f12.71

)1000)(N/8)(N(f

k

DhNu

fD=(1.82log10(NRe)-1.64)-2

NRe=DiG/

G=flow rate/cross sectional area

NPr=Cp /k

Evaluated at tube bulk conditions

Poddar Plot – line 3hi: shellside individual heat transfer coefficient

0.14

w

b

1/3

b

bpb

n

bb

o

μ

μ

k

μC

μ

DGC

k

DhNu

Kern Method:

D = hydraulic diameter

G = mass velocity normal to tubes closest to centerline

N = 0.55, C = 0.36

Method for Drawing Line 3.

• Calculate Qrequired from Q=mCp Tprocess fluid

• Set Qrequired = Qtransferred

• Qtransferred = UA FTLM

U = f(L, # of tubes), A = f(L, # tubes)

• Select L, by trial and error find # tubes that satisfies Qrequired=Qtransfered, obtain 1 (L, # tube) point, repeat

• Plot L, # tubes on Poddar plot.

Tube Length

Tube

Cou

nt

maximum # of tubes

minimum # of tubes

Q transferred

Poddar Plot: Allowable tube side pressure drop (line 4)

Handout from McCabe and Smith pg 652-659 (details in handout)

Journal article Ptube side=0.5-0.7 kg/cm2

iiic

p2'

ii

φDρg

LnG2fBΔP

allowable: Journal article uses up to 1.3 km/cm2

To calulate use McCabe and Smith handout.

Poddar Plot: Allowable shell side pressure drop (line 5)

oc

2sRo

ρg

GN2f'BΔP

Poddar Plot: Allowable pressure drop (line 4 and 5)

• Set the equations equal to the allowable pressure drop.

• Pick and L, calculate # tubes. Repeat and will have several points to plot a lines.

• Objective – change baffle cut until or tube passes until shell and tube side allowable pressure drop lines coincide – then fully utilizing the shell side pressure drop.

Tube Length

Tube

Cou

nt

maximum # of tubes

minimum # of tubes

Q transferred

shell side pressure drop

tube side pressure drop

Tube Length

Tube

Cou

nt

maximum # of tubes

minimum # of tubes

Q transferred

shell side pressure drop

tube side pressure drop

Optimum # tubes and length that satisfy the constraints

Thinnest exchanger is the most economical

Shell and Tube Heat Exchanger

Constraints

• Minimum velocity to operate in the turbulent region where heat transfer coefficients are high.

• Provide the area required for heat transfer.

• Operable and economic pressure drops.

Double Pipe Heat Exchangers

• Competitive with shell and tube up to 100 ft2 heat transfer area.

• Length limited to 20 ft.

• Not recommended for boiling service.

Double Pipe Heat Exchangers

Compact Heat Exchangers• Plate and Frame, spiral plate, and

plate-fin heat exchangers.• Smaller volume for the same heat

transfer area.• Less advantages for high pressure,

temperatures, and particulate containing streams than shell and tube heat exchangers.

Plate and Frame Heat Exchangers

Plate and frame heat exchangers consist of a series of gasketed, embossed metal plates bolted together between end frames to form channels through which hot and cold media flow.  Plate and frame design offers advantages over its shell and tube in that it is expandable, cleanable, compact and efficient.  The "U" values are up to five times greater than other exchangers.

Plate and Frame Heat Exchangers

• Recommended for fluids that must be kept clean – food, beverage, pharmaceutical industries.

• Low fouling, high heat transfer coefficients, easily cleaned, up to 16,000 ft2 of heat transfer area, no phase change.

• Can use very small approach temperatures.

• Ideal for viscous, corrosive fluids.

Spiral Plate

Plate-Fin

Spiral Plate and Plate-FinSpiral Plate

• High heat transfer coefficients.

• Counter current flow.• Viscous, corrosive,

fouling and scaling fluids.• Moderate temperatures.• Up to 2000 ft2 heat

transfer area.

Plate-Fin

• Gases.

• Area/volume high.