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Page 1: 10 L1 L2 Heat Exchange

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Heat Exchangers

Chapter 18

ChEN 4253Terry A. Ring

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Flow Patterns

• Parallel Flow

• Counter Current Flow

• Shell and Tube with baffles

• Cross Flow

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Temperature Profiles

ΔT = Approach Temperature 

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Heat

Exchanger Temperature

Profiles

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Flow Structure

Q=U A F ΔTlm-counter  

incold inhot 

incold out cold 

incold out cold 

out hot inhot 

T T 

T T S 

T T 

T T  R

 R RS 

 R RS  R

 RS 

S  R

 F 

112

112ln1

1

1ln1

2

2

2

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Overall Heat Transfer Coefficient

• Series of Resistances

• Basis

 – Inside

 – Outside

1

,,1)ln(21)1(

o f  oioowiioi

o

i f  oRh D D Dk h D D

 A

 A RU 

Inside

Tubes

R f =fouling factors, inside and outside

See table 18.5 for range of U values for different cases.

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Heat Transfer inside a tube

Turbulent000,10Re

700,16Pr 7.0

esmooth tub,60/

Pr Re027.0

14.0

3/18.0

 

 

 

 

 f  

 p

w

b

 f  

 D Lk 

hD Nu

 

 

 

Also other correlations valid over wider ranges

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Heat Transfer outside of Tube

000,100Re40

300Pr 67.0

2.525.0

Pr )Re06.0Re4.0(

25.0

4.03/25.0

 

 

 

 

 f  

 p

w

b

w

b

 f  

hD Nu

 

 

 

 

 

Also other correlations valid over wider ranges

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Thermal Conductivity

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What Temperature Approach

• Heuristic 26.

 –  Near-optimal minimum temperature approaches

in heat exchangers depend on the temperaturelevel as follows:

 –  10°F or less for temperatures below ambient,

 –  20°F for temperatures at or above ambient up to 300°F, 

 –  50°F for high temperatures, –  250 to 350°F in a furnace for flue gas temperature above

inlet process fluid temperature.

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Where are the Heat Exchangers?

What is happening in each

Octane Reaction

2C2H4 + C4H10  C8H18

P= 20 psia, T=93C,

X=98% Conversion

TBP C2H4  −103.7 °C

C4H10 +0.5 °C

C8H18 +125.52 °C

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Where are the Heat Exchangers?

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Heat Transfer With Phase

Change• Tricky Problems

 – Examples

• Reboiler on Distillation Unit

• Condenser on Distillation Unit

• Flash Units

• Boilers

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A Word About Steam

• Simulator Assumptions

 – Inlet – Saturated Vapor 

 –  Pressure –  100% Vapor 

 – Outlet – Saturated Liquid

• Liquid Only Leaves via steam trap

 –  Pressure = Pin- ΔP (1.5 psi, Heuristic-31)

 –  100% Liquid

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Where are the Tricky

Heat Exchangers?

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Condensation Heat Transfer 

• Drop Wise Condensation

 – Special Case

• Very High Heat Transfer 

• 5 to 10 x Film Condensation

• Film Condensation

 – Laminar 4/1

3

)(4

)(

 xT T 

k  H  g 

 xh Nu

wvl 

l vapvl l 

 x x

 

      

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Laminar to Turbulent Condensate

Flow

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Boiling Heat Transfer Coefficient

Various correlations depending upon boiling mechanism

Highest Heat

Transfer Coef.

But hard tocontrol HX

operating here

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Heuristic 28

• Boil a pure liquid or close-boiling liquid

mixture in a separate heat exchanger, using

a maximum overall temperature drivingforce of 45 F to ensure nucleate boiling and

avoid undesirable (low h) film boiling.

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Effective

Flow

Conditions

with

Boiling inThermo

siphon

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Kettle (Re)Boiler Design

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Aspen - Zone Analysis

ProMax – Heat Release Increments• Heuristic 29.

 –  When cooling and condensing a stream in a heat exchanger, a zone

analysis, described in Section 18.1, should be made to make sure

that the temperature difference between the hot stream and the coldstream is equal to or greater than the minimum approach

temperature at all locations in the heat exchanger. The zone

analysis is performed by dividing the heat exchanger into a number 

of segments and applying an energy balance to each segment to

determine corresponding stream inlet and outlet temperatures for the segment, taking into account any phase change. A process

simulation program conveniently accomplishes the zone analysis.

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Pressure Drop

& Flow Rate• Laminar vs. Turbulent

 – Heuristic 31.

• Estimate heat-exchanger pressure drops as follows: –  1.5 psi for boiling and condensing,

 –  3 psi for a gas,

 –  5 psi for a low-viscosity liquid,

 –  7-9 psi for a high-viscosity liquid, –  20 psi for a process fluid passing through a furnace.

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Controlling ΔP in Simulator 

• Shell side –  Nozzle diameter 

• Inlet and Outlet

 –  Number of Baffles –  Tubes

•  Number, diameter, pitch, No. passes

• Tube side

 –  Nozzle diameter • Inlet and Outlet

 –  Tubes•  Number, diameter, pitch, No. passes

 Note interactions!

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Shell Heads, Shell Type

• See ProMax Help/index “Shell, types” 

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HX Cost

• Size Factor HX Area

• CBase(6-2000)=exp[11.0545-0.9228*ln(A)+0.09861*ln(A)2]

• Purchase Price –  CP-fob=F

P(P)*F

Material(A)*FL(L)*CBase*(CPI/394)

 – CBM=FBM*CP-fob

 – CBM=3.17*CP-fob

• Cost depends on HX Area• Pumping Cost

 – Work = Q*ΔP

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Controlling A in Simulator 

• A = Ntubes π Dtubes Ltubes

• Shell

 –  Shell Diameter and pitch determines Ntubes 

• Tubes

 –  Dtubes

 –  Ltubes

 –  Tube pitch-The transverse pitch is the shortest distancefrom the center lines of two adjacent tubes.

 –  Tube pitch ratio 1.25 to 1.5 typically

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Controlling U in a Simulator 

• For a given heat duty and geometry - U determines the HXarea

• Steps

 –  Identify the controlling heat transfer resistance –  ho-Manipulate the shell side Reynolds number 

• Shell diameter 

• Tube pitch

•  Number of baffles

 –  hi-Manipulate the tube side Reynolds number 

• Tube diameter •  Number of tubes (shell diameter and tube pitch)

•  Number of passes

 –  If odd things happen check to see that you have the same controlling heattransfer resistance

 Note interactions!

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Other Issues 

• Materials of Construction

 – Strength at temperature, life time, heat conduction,

fouling

• Design layout

 – Tube pitch, baffles, tube and shell diameters

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Heat Exchanger 

Problems

• Temperatures Cross EachOther 

 –  Non-functioning Exchanger 

 –  To solve increase approach ΔT

• Condensation/Evaporation

 –  Heat transfer with multiple heattransfer coefficients in a single

apparatus• Various regimes of boiling

• Various regimes of condensation