ME421Lec5

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Fouling

Accumulation of undesirable deposit on a HEX surface.

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Fouling occurs in natural systems as well.

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Introduction• HEX are designed for thermal processes to work within

allowable pressure drop limits and for a specified timeperiod.

• Fouling results in higher thermal resistance and higher

pressure drop, thus pumping power.• Excessive fouling causes HEX cleaning or replacement.

• Fouling affects the initial cost

(more material may be needed),operating cost (cleaning is very

expensive!), and HEX

performance significantly.

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Basic Considerations

• Remember rate of heat transfer (general form)

• U will be different for clean HEX and fouled HEX surfaces. They can be related by

Rft: total fouling

resistance

• Heat transfer rate under fouled conditions is

• Specified processes fix Q and ∆ Tm under both conditions, so

• UcRft : Additional surface area required due to fouling

m TAUQ ∆=

fo

i

fioftft

cf

RA

RAR,R

U

1

U

1+=+=

mf f f f TAUQ ∆=

ftcc

f RU1A

A+=

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Figure 5.2 Effect of Fouling on Surface Area

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Effects of Fouling

Effect of fouling on heat transfer

• Overall heat transfer coefficient decreases

• Average Rft

values specified in the design of ~750shell-and-tube HEX are given in Table 5.1

o

foioo

fi

i

o

ii

of

h

1R

kL2

)d/dln(AR

A

A

hA

A

1

U ++π

++=

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• Using Table 5.1 and typical h values for boiling, condensation,and gas flow, Table 5.2 is prepared.

• Table 5.2 shows % increase in area for shell-and-tube boiler,evaporator, and condensers; single-phase flow on tube side.

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Effect of fouling on pressure drop

• The change in flow geometry due to fouling affects the flow

field and pressure drop, thus pumping power.• Remember pressure drop

• Assuming that mass flow rate is the same under clean andfouled conditions,

• Inner diameter under fouled conditions and fouling thickness

• Table 5.3 lists kf , tf , % area remaining, and % increase in ∆p

of typical fouling materials• ∆p increase up to 70% (assuming f f = f c)

2

u

d

L4f p

2m

i

ρ=∆

5

f

c

c

f

c

f

dd

f f

pp ⎟⎟

⎠ ⎞⎜⎜

⎝ ⎛ =∆∆

⎟⎟ ⎠ ⎞⎜⎜

⎝ ⎛ −=

c

f f cf

dR2kxpedd ⎥

⎦⎤⎢

⎣⎡ ⎟⎟

⎠ ⎞⎜⎜

⎝ ⎛ −=

c

f f cf

dR2kxpe-1d5.0t

Note correctionson formulas

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Effect of fouling on cost

• Increased capital expenditure

– Heat transfer area is increased

– Pumps and fans are oversized

– May need duplicate HEX during cleaning• Increased maintenance cost due to on-line and off-line

cleaning

• Loss of production due to operation at reduced capacity

• Energy losses

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Fouling is named as the major unresolved problem in heattransfer.

Aspects of Fouling (read Section 5.4 for detail)

Categories of fouling (classified according to principalprocesses that cause it)

• Particulate (solid particle accumulation on surface)

• Crystallization (mainly due to dissolved inorganic salts)

• Corrosion (corrosive fluids or impurities)• Biofouling (material of biological origin; bacteria, mold,

seaweed)

• Chemical reaction (polymerization, coking of hydrocarbons)

Different types of fouling can occur simultaneously, orcomplement each other.

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Fundamental processes of fouling (fouling mechanisms)

• Initiation (surface conditioning – temperature, material,roughness, etc.)

• Transport (diffusion, sedimentation, and thermophoresis)

• Attachment

• Removal (due to shear forces; dissolution, erosion,spalling)

• Aging (property change)

P di i f f li (f li h i )

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Prediction of fouling (fouling mechanisms)

• We must know how fouling progresses with time

• The constant value of Rf used is the value reached beforecleaning

• Variation of fouling with time

φd : deposit rateφr : removal rate

tD : delay time

rdf

dt

dRφ−φ=

Figure 5.3 Typical Rf – time curve

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Figure 5.3 continued

• Linear (A)

– Constant φd + negligible φr (gives Rf =φf t), or constant φd - φr

– Tough, hard, adherent deposits

– Fouling increases until cleaning• Asymptotic (B)

– Constant φd and φr proportional to fouling layer thickness

– Stability of the layer deteriorates

– Soft, flaky deposits

• Falling-rate fouling (C)– φd inversely proportional to fouling layer thickness

• Saw tooth (D)

– Periodic change in operating conditions

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Design of HEX Subject to Fouling

• Fouling provisions must be made during design stage.

• Operating characteristics and cleaning schedulesdepend on the design fouling factor.

• Several approaches to provide allowance for fouling,all result in excess heat transfer surface area.

• We will cover three methods:

- Specifying fouling resistances- The cleanliness factor (CF)

- Percent over surface (OS)

Fouling Resistance

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Fouling Resistance

• Specify Rf initially → U ↓ → Excess surface area

• HEX will perform until Rf is reached, then it must be cleaned.• TEMA Tables 5.5 - 5.11 provide Rf for various (but limited)

process fluids, flow conditions, and HEX configurations.

Cleanliness Factor (developed for the steam power industry)

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Cleanliness Factor (developed for the steam power industry)

• Figure 5.4 gives Rft based on CF for Uc values

• Low velocities, thus low

Uc increase fouling.• Typical value for CF

is 0.85

ftcf

c

f

RU

1

U

1

U

UCF

+=

=

cftcft

UR11CFor

CFUCF1R +=−=⇒

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Percent Over Surface

• In design, add a % of clean surface area to account for

fouling

• Taking Q and ∆ Tm same under both conditions,

• Typical value for OS is 25%

• e.g. in shell-and-tube HEX, provide additional surface byincreasing tube length or number of tubes (thus, shell

diameter)

• The changes will affect design conditions (flowvelocities, number of passes, baffle spacing, etc.)

ftcc

f RU1001A

A100OS =⎟⎟

⎠ ⎞

⎜⎜⎝ ⎛ −=

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Relationship between Rf , CF, and OS(Table 5.12, for Example 5.1 in book)

Rft (m2K/W) CF OS (%)

0.00005 0.85 17.7

0.00010 0.74 35.3

0.00015 0.65 53.0

0.00020 0.59 70.7

0.00025 0.53 88.4

0.00030 0.49 106.0

0.00035 0.45 123.70.00040 0.41 141.4

Review Examples 5.1-5.3 in book.Example 5.3 discusses the operation of a HEX with fouling

T h i t C t l F li

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Techniques to Control Fouling

• Surface cleaning techniques (on-line or off-line)- Continuous cleaning (installation of cleaning materials

into the system)

- Periodic cleaning (mechanical or chemical removal of fouling)

• Chemical additives

- Crystallization fouling: acids and polyphosphatesadded to increase solubility (easier removal) of foulingdeposits

- Particulate fouling: flocculants (coagulants) used to aid

filtration of particles, dispersants to keep particles insuspension

- Biological fouling: chlorine or other biocides

- Corrosion fouling: additives to produce protective filmson the surface

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ME421Lec5