Fouling of tube bundles under pool boiling conditions

Chemical Engineering Science60(2005)1503–

1513

https://www.360docs.net/doc/932351572.html,/locate/ces

Fouling oftube bundles under pool boiling conditions

M.R.Malayeri a,?,H.Müller-Steinhagen b,T.H.Bartlett c

a Institute of Technical Thermodynamics,German Aerospace Centre,Pfaffenwaldring38-40,D-70569Stuttgart,Germany

b Institute of Technical Thermodynamics,German Aerospace Centre,Institute for Thermodynamics and Thermal Engineering,

University of Stuttgart Pfaffenwaldring38-40,D-70569Stuttgart,Germany

c Department of Chemical an

d Materials Engineering,University of Auckland,New Zealand

Received19July2004;received in revised form14October2004;accepted31October2004

Available online30December2004

Abstract

This paper aims to provide experimental heat transfer results for boiling of CaSO4solutions on the outside oftube bundles,and also to shed some light into the mechanisms which in?uence heat transfer fouling under pool boiling conditions.Heat transfer coef?cients for three heater rods with an identical diameter of10.67mm on a35mm vertical pitch have been measured.The independent variables bulk concentration and heat?ux have been varied from0.8to1.6g/L and33to300kW/m2,respectively.The experimental results show that the mechanisms of fouling on the middle and top heater substantially differ from those at the bottom heater,due to the dominant effects of bubble impingement on the heat transfer surfaces.In particular,the competing effects on the degree of wall superheat and the supersaturation of the boiling liquid play a signi?cant role.Spalling effects are more pronounced on the upper heaters where thin,friable deposits were found with low density and adherence,particularly at higher heat?uxes and lower concentrations.In terms of tube bundle ef?ciency,the best results were found for lower concentrations.

?2004Elsevier Ltd.All rights reserved.

Keywords:Fouling;Heat transfer;Crystallisation;Pool boiling;Tube bundle;Heat exchangers

1.Introduction

Heat transfer fouling is an undesirable process in which unwanted materials with low thermal conductivity deposit on heat transfer surfaces.Several forms of fouling exist based on the chemical/physical conditions under which the deposit layer forms.One of the most severe forms of foul-ing occurs during pool boiling heat transfer(Jamialahmadi et al.,1989).Such conditions arise in many heat exchangers in which nucleate boiling is the dominant process;perhaps kettle reboilers and steam generators provide the most spec-tacular examples.In such devices,fouling of heat transfer surfaces causes signi?cant cost penalties,due to both re-duction in heat transfer coef?cient and increases in boiler metal wall temperature in order to provide the increased temperature differential necessary to overcome the fouling ?Corresponding author.Tel.:+49471168628365;

fax:+497116862712.

E-mail address:m.malayeri@dlr.de(M.R.Malayeri).

0009-2509/$-see front matter?2004Elsevier Ltd.All rights reserved. doi:10.1016/j.ces.2004.10.017resistance.Increasing boiler metal temperature also results in a rapid utilisation ofthe lif e ofthe components with possible creep rupture,ifthe temperature increase is too high.As a result,such devices often require costly shutdowns for inspection and removal off ouling deposits.

Several investigators have studied the mechanisms of fouling on single tubes during nucleate boiling(Partridge and White,1929;Freeborn and Lewis,1962;Hospeti and Mesler,1965;Palen and Westwater,1966;Jamialahmadi et al.,1989,1993;Bornhorst et al.,1999).Jamialahmadi and Müller-Steinhagen(1993),in a review paper,reported that the primary causes of fouling for solutions with inverse solubility are supersaturation as well as bubble formation behaviour.It is also stated that the following steps take place when a deposit layer forms on the surface of a single tube during pool boiling:

?Bubble nuclei form just above the nucleation sites that are available on the surface.Bubble density is directly

1504M.R.Malayeri et al./Chemical Engineering Science60(2005)1503–1513

proportional to the number ofnucleation sites,and hence the surface material and roughness are of prime importance.

?Bubble agitation initially produces considerable turbu-lence that enhances heat transfer.In this phase,bubble characteristics such as size,shape and rise velocity are af-fected by the physical/chemical properties of the boiling liquid,notably surface tension.

?Beneath the bubbles,a microlayer exists,in which the concentration ofthe dissolved salt is increased due to the pref erential evaporation ofthe solvent.Ifthe dissolved salt has an inverse solubility relationship with temperature, the considerable temperature gradient in the microlayer leads to further supersaturation of the solution.Under such circumstances,crystal nuclei will begin to precipitate on the surface and a deposit layer builds up,initially in the form of concentric rings around the nucleation sites.Both surface temperature and bulk concentration dominate the formation and thickness of the fouling layer in this phase. Knowledge oflocations in tube banks,where deposit has accumulated can aid in planning and targeting locations for cleaning and removal ofdeposits.Such inf ormation is linked to the understanding ofthe mechanisms ofthe f ouling pro-cess on tube bundles during pool boiling.Nevertheless,pub-lished studies to-date have been undertaken for single tubes or wires,while the authors are not aware ofany systematic information on fouling of tube bundles.The present study aims to investigate experimentally mechanisms off ouling under pool boiling conditions for inverse solubility salts, such as CaSO4.The experiments have been performed with three test heater rods arranged vertically above each other in a pool ofliquid,f or various bulk concentrations and heat ?uxes.One ofthe main aims ofthe investigation was the determination of the change in fouling from the bottom to the top heater.Ofparticular importance f or design and oper-ation is information about the interaction between the heat transfer mechanisms on the different tubes,which is dis-cussed in terms ofbundle ef?ciency.Finally,Scanning Elec-tron Microscope(SEM)analysis is provided to discern the structure of deposit for different bulk concentrations and heat?uxes.

2.Experimental set-up and procedure

2.1.Equipment and data acquisition

A schematic representation ofthe pool boiling test f acil-ity is shown in Fig.1.The fully integrated rig consists of a boiling vessel with a resistance band heater[1],an enclosed ?ow loop including a co-current condenser[2],a preheater [3],three heaters arranged vertically above each other(bot-tom[B],middle[M],and top[T])with power control units [4]and a data acquisition system[5]connected to a com-puter[6].

The boiling section is a cylindrical stainless steel vessel [1],with an internal diameter of30.4cm and30L capac-ity.Two glass view ports are provided at each end ofthe vessel to allow visual observation.The vessel is heated ex-ternally by a resistance band heater regulated by a variable a.c.power supply(Variac).In order to maintain the test so-lution at saturation temperature during the experiments,the power supply to preheater and tank heater is regulated by an electronic temperature controller and a variable transformer. Output from a thermocouple?tted inside the vessel is used to trip the Variac unit ifthe solution temperature exceeds a set limit.A saf ety reliefvalve is mounted at the top ofthe ?ow circuit to relieve the pressure in the event ofa pressure excursion due to a failure in cooling or failure of the Variac controllers.A condenser[2]was used to condense and return the evaporated liquid to the vessel.The condenser is a co-current three stage single tube heat exchanger with cooling water?ow being regulated by means ofa rotameter and man-ual?ow control valve.Following condensation,the water is reheated prior to returning to the vessel by an electric resis-tance heater which is manually controlled through a Variac power supply.All parts ofthe rig are insulated to minimise thermal losses.It has been estimated that heat losses from the system are in the order of10?2W/m2,which is neg-ligible.Vacuum is drawn using a domestic water powered eductor connected to the rig through a water trap.

The test heater section[4]consists ofa column ofthree precision electric resistance heat rods on a35mm vertical pitch.The heat rods are manufactured by Ashland Chemi-cal Company to an HTRI design and have four type E ther-mocouples embedded inside the heater as shown in Fig.2a. The heated zone is99.1mm long and the heat?ux ofeach test heater can be varied up to300kW/m2.Three ofthe em-bedded thermocouples are used for data acquisition and the fourth is used as a trip input for the test heater Variac power supply,should the internal temperature ofthe heater exceed a set limit.The orientation oftest heaters in the test vessel is depicted in Fig.2b.The middle heater[M]is positioned at the centreline ofthe test vessel.

Readings were initially taken at a time interval of30s, gradually increasing to about300s as the asymptotic region was approached.The average of?ve readings was used to calculate the surface temperature of the test heater(T s)from the measured values(T T C,see Fig.2a)for each thermo-couple as:

T s=T T C?

s

˙q,(1)

where“s”is the distance ofthe thermocouple f rom the sur-face of the heater and“ ”is the thermal conductivity ofthe material(copper)between the thermocouple and the surface ofthe heater.The value of“s/ ”was determined for each thermocouple by calibration ofthe test heater using the Wilson Plot technique as for example described by Wenzel (1992).The three individual thermocouple temperature

M.R.Malayeri et al./Chemical Engineering Science60(2005)1503–15131505

Band heater

Data recorder (D)

[1]

[5]

[6]

Fig.1.Schematic representation oftest rig.

Heater

element

T TC

(b)

Fig.2.(a)HTRI test heater,(b)schematic drawing oftest heater orientation

in the vessel.

differences(see Fig.2a)were averaged to give the average

surface temperature using

T s=1

3

3

i=1

T T C,i?

s

i

˙q

.(2)

The average heat transfer coef?cient was then determined

using

=

˙q

s b

,(3)

where T b is the bulk temperature.The fouling resistance

due to deposition ofa f ouling layer is then calculated as a

function of time from

R f=

T s?T b

˙q

t

?

T s?T b

˙q

.(4)

The subscripts“t”and“0”denote conditions at any time and

at the beginning ofthe experiment when the heat exchanger

is considered to be clean,respectively.

2.2.Preparation of test solutions

Test solutions ofcalcium sulphate were prepared by dis-

solving calcium sulphate hemi-hydrate(CaSO4.1

2

H2O)in

a separate container.In all cases,distilled water was used

as solvent.The calcium sulphate was mixed sparingly with

water and washed into the plastic container.Ifsolid lumps

ofcalcium sulphate were f ound these were broken up and

added to the container again.The1400rpm stirrer was then

switched on for about24h.Following mixing the calcium

sulphate solution was left for at least24h,so that any undis-

solved matter could settle out.The concentration was mea-

sured through a sample line,just before applying power to

the test heaters and also after each run.

2.3.Experimental procedure

The test vessel was cleaned prior to each run by removing

the front view port and scraping any scale from the vessel

1506M.R.Malayeri et al./Chemical Engineering Science60(2005)1503–1513 Table1

Range ofoperating parameters

CaSO4concentration[g/L]

1.6 1.20.8

Heat?ux,(kW/m2)

33+

55+

76+

105++

145+++

200++

300+++

walls.Each run starts with introducing the test solution to

the vessel once the vacuum pump had reduced the system

pressure to approximately10kPa.The principal reason for

this was to give a measure ofdegassing ofthe test solution

during?lling ofthe vessel.The vessel band heater was then

turned on to preheat test rig and solution to the saturation

temperature.Meanwhile,the condenser had also to be turned

on to establish steady-state conditions.The vacuum pump

was used at regular intervals to maintain the pressure and

to remove residual gases which desorbed from the solution.

After approximately3h preheating,the test solution reached

the saturation temperature.Then,the power supply to the

test heaters was turned on and set to a de?ned value.At

the same time,the data acquisition system was started to

record temperatures,pressure and heat?uxes.The range of

operating parameters used in this investigation is given in

Table1.

It should be noted that the saturation concentration of

CaSO4.1

2H2O at100?C is1.6g/L.Any deposition occurring

at lower bulk concentrations has to be associated with local concentration effects in the vicinity of the growing bubbles.

2.4.Error analysis

The systematic experimental errors for the determination of heat transfer coef?cients and fouling resistances are due to:

?errors ofapproximately±0.25K in temperature mea-surement

?an error ofabout±2%for determination of heat?uxes as a result ofsystematic errors in reading electrical volt-age and current.

Referring to Eq.(3),the largest error for the clean heat trans-fer coef?cients occurs for the smallest temperature differ-ence between surface and bulk.Based on the instructions in the ASME Journal ofHeat Transf er(Jamialahmadi and Müller-Steinhagen,1993),uncertainty with95%con?dence is estimated to be±9.65%.However,for fouling resistance measurements the uncertainty changes as a function of time due to changes in surface temperature when deposits start

to

02004006008001000120014001600

Time, [min]

H

e

a

t

t

r

a

n

s

f

e

r

c

o

e

f

f

i

c

i

e

n

t

,

[

W

/

m

2

K

]

Initial time, [min]

I

n

i

t

i

a

l

h

e

a

t

t

r

a

n

s

f

e

r

05101520

Fig.3.Typical variation of heat transfer coef?cient as a function of time (solid and open symbols correspond to complete run and initial period of 100min).

form on the surface.The maximum calculated uncertainty occurs at the start ofall runs when the temperature dif f er-ence is small.For instance,the uncertainty for the heat trans-fer measurements at the bottom heater for a typical con-centration of1.6g/L CaSO4and200kW/m2changes from 43.7%to10.35%when the fouling resistance increases from 0.008m2K/kW to0.16m2K/kW.

3.Results and discussion

3.1.Heat transfer fouling on a single test heater

Calcium sulphate has an inverse solubility and thus with increasing temperature,the solubility decreases.The satura-tion concentration ofcalcium sulphate at100?C is approx-imately1.6g/L.With increasing surface temperature as the result ofthe adjusted heat?ux,the solution becomes su-persaturated.Due to local evaporation effects during bubble formation,a further increase in concentration by up to a fac-tor of3–4may occur(Jamialahmadi and Müller-Steinhagen, 2004a,b).Under such supersaturated conditions,the dis-solved salt will start to precipitate at the heat transfer surface after an initial surface conditioning period.

Fig.3illustrates the typical variation ofheat transf er coef?cient as a function of time for a single heater(bot-tom position),a concentration of1.2g/L and a heat?ux of 300kW/m2.Open circles represent data for the initial period up to20min which are dif?cult to identify in the complete experimental run,which is shown as solid circles.It is evi-dent that the heat transfer coef?cient drops rapidly[region I]followed by a gradual recovery to a secondary maximum [region II]which in turn is followed by a more gradual de-cline towards an asymptotic value ofabout6000W/m2K [region III].

The fouling process during nucleate boiling strongly depends on bubble characteristics such as size,shape,

M.R.Malayeri et al./Chemical Engineering Science60(2005)1503–15131507 Table2

Thermal conductivity(wet)and density(dry)ofdeposit layer f rom a

CaSO4concentration of1.2g/L on a single heater(Jamialahmadi and

Müller-Steinhagen,1993)

Heat?ux(W/m2)Thermal conductivity(W/m K)Density(kg/m3)

96310.751270

192610.851870

28892 1.092120

77045 2.12420

249964 2.092429

301204 2.22445

departure frequency and bubble rise velocity.In the initial period[region I]hemispherical bubbles with a large con-tact area are clearly visible.The bubble size during this time ranged from10to20mm in diameter with a bubble frequency of5–7bubbles per second.In regions II and III, bubbles were spherical with a uniform diameter of2mm, and the bubble departure frequency is much higher than that ofregion I.Jamialahmadi and Müller-Steinhagen(1993) have reported the same phenomena and found that changes in bubble growth mechanisms were related to the calcium sulphate deposition.The highest deposition rate occurs in the microlayer beneath the bubble.Deposits under the bub-bles have high density and adherence and tend to reduce local heat transfer,leading to the initial rapid drop in heat transfer coef?cient.Subsequently,the porous deposits form-ing around the original nucleation sites provide additional bubble nucleation sites which give rise to the observed recovery ofheat transf er coef?cient in region II.Bubbles that are generated above these new nucleation sites and the associated additional turbulence caused by these smaller bubbles improve the heat transfer coef?cient further.

As the deposition process continues,the deposited layer increases in thickness and compactness,causing deactivation ofnucleation sites.Jamialahmadi and Müller-Steinhagen (1993)experimentally measured the density and thermal conductivity of the fouling layer for different heat?uxes dur-ing boiling on a single heater.Table2summarises these mea-surements,which show that the deposit layer is more dense at higher heat?uxes which is the case in Fig.3(300kW/m2). Furthermore,bubbles start to form at the interface between the deposit and solution rather than at the metal heat transfer surface.Since the interfacial tension between the calcium sulphate deposit and the solution(≈0.045N/m)is much lower than that between metal and solution(≈0.069N/m), smaller bubbles are formed.The heat transfer coef?cient de-creases due to the combined ef f ect ofthe growth ofa ho-mogeneous deposit layer and the deactivation ofnucleation sites,until?nally a constant asymptotic value is reached.

3.2.Heat transfer fouling on the test tube bundle Fouling processes on tube bundles are more complex than those on single tubes,due to the substantial turbulence

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0100020003000400050006000

Time, [min]

H

e

a

t

t

r

a

n

s

f

e

r

c

o

e

f

f

i

c

i

e

n

t

,

[

W

/

m

2

K

] C = 6.1g/L

q = 33 kW/ m2Bottom(1) Middle (2)Top (3)

(1)

(3)

(2)

.

-2.5E-04

-2.0E-04

-1.5E-04

-1.0E-04

-5.0E-05

5.0E-05

1.0E-04

1.5E-04

2.0E-04

0100020003000400050006000

Time, [min]

F

o

u

l

i

n

g

r

e

s

i

s

t

a

n

c

e

,

[

m

2

K

/

W

]

(a)

(b)

Fig.4.(a)Heat transfer coef?cient versus time,1.6g/L CaSO4,33kW/m2.

(b)Fouling resistance versus time,1.6g/L CaSO4,33kW/m2.

caused by bubbles rising from the lower heat transfer sur-faces,which impinge on and slide around the upper tubes. This will result in substantial changes in the fouling mech-anisms due to the distribution ofwall shear stresses.

For heat?uxes of105kW/m2(T s=107.4?C)and below, bubbles from the bottom heater enhanced the heat transfer coef?cients at the two upper test heaters,which additionally did not show any dense and adherent scale.Fig.4a shows parallel measurements of the heat transfer coef?cients for all three test heaters,as a function of time for a heat?ux of33kW/m2and a salt concentration of1.6g/L CaSO4.All three heaters have the same surface characteristics,such as material and mean surface roughness of0.407 m.It is ev-ident in this?gure that the middle[M]and top[T]heaters show an increase in heat transfer coef?cient,while the bot-tom heater[B]exhibits only a marginal decrease in heat transfer coef?cient.The heat transfer enhancement is partic-ularly pronounced for the top heater,where the heat transfer coef?cients are twice as high as for the middle heater.Fig. 4b demonstrates that the thermal fouling resistances either remained around0m2K/W(bottom heater)or even became negative(middle and top heaters),the latter being an arti-fact of the de?nition of R f according to Eq.(4)and the

1508M.R.Malayeri et al./Chemical Engineering Science 60(2005)1503–1513

H e a t t r a n s f e r c o e f f i c i e n t , [W /m 2K ]0

1.0E-05

2.0E-05

3.0E-05

4.0E-05

5.0E-05

6.0E-05

7.0E-05Time, [min]

F o u l i n g r e s i s t a n c e , [m 2K /W ]

(a)

(b)

Fig. 5.(a)Heat transfer coef?cient versus time,0.8g/L CaSO 4,300kW/m 2.(b)Fouling resistance versus time,0.8g/L CaSO 4,300kW/m 2.

bubble-related improvement in heat transfer.This is an ef-fect of the signi?cant shear forces generated by the bubbles from the lower heaters ?owing across the upper test heaters.Observations showed that any scale formed on the middle and top heaters subsequently re-dissolved in the test solu-tion.The very thin deposits on the bottom heater were ini-tially white and gradually developed into yellowy brown.At heat ?uxes above 105kW/m 2,the fouling resistance increased also for the middle and top heaters and,unless the scale deposits broke off the heater surfaces,dense and ad-herent deposits formed.Figs.5a and b depict the heat trans-fer coef?cients and fouling resistances as a function of time for 300kW/m 2and a sub-saturated concentration of0.8g/L.Fouling started at the bottom ofthe lowest heater where only relatively few bubble nucleation sites and large bubbles were seen.It then developed at the upper side ofthe middle heater and ?nally a slight amount ofscale also f ormed on the top heater.The middle heater had a thin,tenacious and almost transparent scale on the top which was comparatively rough.This rough surface would have provided more bubble nu-cleation sites,hence generating more bubbles which would have led to heat transfer enhancement at the top heater.From visual observations,it was found that the upper side of the

005001000150020002500300035004000

Time, [min]

H e a t t r a n s f e r c o e f f i c i e n t , [W /m 2K ]

Fig.6.Heat transfer coef?cient as a function of time for different con-centrations and 300kW/m 2.

middle heater was covered with circular scale rings,which had formed around the sites where bubbles had nucleated.The top heater had small circular scale rings on top,uneven scale in the middle section and very thin scale at the bot-tom.The formation of circular scale rings is consistent with observations of Freeborn and Lewis (1962).

3.2.1.Effect of CaSO 4concentration

The movement of bubbles after detachment from the heat transfer surface generates considerable agitation in the sur-rounding liquid.This promotes convection and decreases the thickness ofthe thermal boundary layer.Najibi et al.(1996)investigated the effect of CaSO 4concentration on heat trans-fer coef?cients for clean surfaces.They observed a relatively uniform bubble diameter of around 2mm and a low bub-ble departure frequency during pool boiling of distilled wa-ter at saturation temperature for low heat ?uxes (38kW/m 2which corresponds to a surface temperature of 105.7?C).As the concentration ofsalts with inverse solubility (such as CaSO 4)was increased from 0.253to 1.5g/L,the number of active bubble nucleation sites decreased and the bubble de-parture diameter increased from 2.1mm to 3.2mm.There-fore,the increasing CaSO 4concentration results in lower heat transfer coef?cients due to changes in the mechanisms ofbubble f ormation associated with the changing physical properties ofthe solution,particularly surf ace tension.At

high heat ?uxes (300kW/m 2which corresponds to a surface

temperature of114.1?C),however,bubble breakup occurs,giving rise to small and almost undistinguishable individual rigid bubbles.Formation ofthese small bubbles and their subsequent release into the surrounding liquid enhances con-vection currents around the heater and as a result the heat transfer coef?cient.

Fig.6illustrates the deposition-related changes in heat transfer coef?cient as a function of CaSO 4concentration,for a constant heat ?ux of 300kW/m 2.This ?gure only depicts experimental data for the top and bottom heaters,as examples for operating conditions with and without effects

M.R.Malayeri et al./Chemical Engineering Science 60(2005)1503–15131509

02000

40006000800010000120000

1000

2000

300040005000

6000

Time, [min]

H e a t t r a n s f e r c o e f f i c i e n t , [W /m 2K ]

Fig.7.Heat transfer coef?cient of bottom heater as a function of time for different heat ?uxes and 1.6g/L.

ofupward bubble ?ow on the f ouling process.For both heaters,the heat transfer coef?cients decreases more rapidly as the CaSO 4concentration was increased.This is at-tributed to the increased supersaturation ofthe salt solution at the triple interface of vapour,liquid and heat transfer surface.The measurements also con?rmed the observations of Najibiet al.(1996),i.e.,that an increase in CaSO 4con-centration results in bigger bubbles.Interestingly,the effect ofCaSO 4concentration on the deposition-related drop in heat transfer coef?cient is more pronounced for the bottom heater,while only a small difference between the concen-trations of0.8and 1.2g/L can be seen f or the top heater.At the saturation concentration of1.6g/L,the power input to the heaters had to be terminated fairly early,due to an excessive increase in temperature caused by the rapid for-mation ofdeposits on the surf aces.For the 1.2g/L solution,this was required only after 2500min.

3.2.2.Effect of heat ?ux

The variation of heat transfer coef?cient for different heat ?uxes and a constant salt concentration of1.6g/L is illus-trated in Fig.7for the bottom heater.All ?ve data sets ex-hibit the same trend as already shown in Fig.3,i.e.,three regions starting with a rapid decrease,followed by a recov-ery to a plateau (both regions together may be termed initial period or delay time)and ?nally a gradual reduction towards an asymptotic value.In the initial period,heat transfer coef-?cients depend strongly on the heat ?ux.Furthermore,the initial time decreases as the heat ?ux increases;for this rea-son,regions I and II cannot clearly be distinguished in Fig.7.For example,for 33kW/m 2it is about 1300min,while it is only 20min for 300kW/m 2.The dependency ofheat transfer coef?cient on heat ?ux is less important as time goes on,in particular for heat ?uxes below 105kW/m 2,due to the dominating in?uence ofthe thermal resistance ofthe deposit layer.The examination ofother data-sets f or lower concentrations did not show the same trend.The effect of

02000

40006000

800010000120000

1000

2000

300040005000

6000Time, [min]

H e a t t r a n s f e r c o e f f i c i e n t , [W /m 2K ]

Fig.8.Heat transfer coef?cient of top heater as a function of time for different heat ?uxes and 1.6g/L.

heat ?ux may be explained by analysing the mechanisms of bubble nucleation.

Pool boiling can only occur when the heated surface con-tains very small pits and cracks known as “cavities”,that are required for bubble nucleation.A cavity can ideally be imagined as being conical and having a circular opening.For non-fouling pool boiling on clean metal surfaces,Whalley (1996)proposed the following equation for the prediction ofthe cavity radius:R =

2 T sat

g h v T superheat

.

(5)

In this equation,R is the cavity radius, the surface ten-sion, g the density ofthe vapour and h v the latent heat ofevaporation.The equation indicates that as the wall su-perheat increases,more cavities ofsmaller radius are able to become active,giving rise to a higher heat transfer rate.The same mechanism applies at the start off ouling process when both degree ofwall superheat and number ofnucle-ation sites dominate the development ofthe additional ther-mal resistance.

Fig.8shows experimental data from the same run as shown in Fig.7,but this time for the top heater.Obviously,not all data sets follow the same trend as shown in Figs.3and 7.While comparable trends may be observed for the higher heat ?uxes of200and 300kW/m 2,for lower heat ?uxes,the top heater is experiencing a continuous heat transfer en-hancement.This implies that while both,the degree ofwall superheat and the number ofnucleation sites still play a role in the mechanism of heat transfer,the main parameters for the upper heaters are the ?ow ofdeparting bubbles f rom the lower heaters and the way they impinge on the surfaces of the upper heaters.The effect of the bubbles is three-fold:they cause more turbulence when impinging on the surface,they sweep away bubbles which otherwise would have a longer residence time at the surface,and they possibly scour the frail deposit layer on the upper heaters.These effects

1510M.R.Malayeri et al./Chemical Engineering Science 60(2005)1503–

1513

Fig.9.Bubble behaviour and impingement on the upper heaters during a typical fouling run,200kW/m 2and 1.2g/L CaSO 4.

50

100

150200250

300

350

Heat flux, [kW/m

2]F o u l i n g r a t e ×108, [m 2K /J ]

Fig.10.Fouling rate as a function of heat ?ux for a given concentration of1.6g/L f or bottom and top heaters,respectively.

are,however,only dominant for low heat ?uxes,which are characterized by low nucleation site densities and large bub-bles.For higher heat ?uxes with a high degree ofwall super-heat,the bubbles originating from the heat transfer surfaces are dominating the mechanisms ofheat transf er,and the heat transfer coef?cients follow the same trend as those for the bottom heaters.Fig.9puts the above-mentioned mech-anisms into perspective,which shows bubble behaviour in the vessel during a fouling run for a heat ?ux of 200kW/m 2and 1.2g/L CaSO 4.Apparently,bubble impingement on the top heater prevents formation of any deposit layer while a visible scale layer has formed on the bottom heater.

The experiments already shown in Figs.7and 8are pre-sented in Fig.10in terms ofa f ouling rate de?ned as ˙R

f =R f,t 2?R f,t 1

t 2?t 1

(6)

for the bottom and top heaters as a function of heat ?ux.In Eq.(6),t 1and t 2correspond to the time at the start and the end ofregion II (see Fig.3),i.e.,the time span over which

the main changes in heat transfer occur.Negative values of the fouling rate indicate conditions,where the measured heat transfer coef?cients actually increased due to the formation ofadditional bubble nucleation sites as a result ofsome mi-nor deposit.All fouling rates are determined from the exper-imental data for a saturation concentration of 1.6g/L.Both heaters exhibit similar trends,but the top clearly bene?ts more from the additional bubble nucleation site formation at low heat ?uxes and suffers less from severe scale formation at higher heat ?uxes.For very high heat ?uxes,both heaters seem to approach the same fouling rate,indicating the less important effect of bubble impingement on the upper tube and instead the dominant effect of higher wall superheat that leads to rapid formation of deposit layers.

3.2.3.Bundle ef?ciency

The design ofheat exchangers operating under pool boil-ing conditions requires information about the best way to arrange the tubes in the bundle,in order to achieve the max-imum heat transfer rate per unit area.The bundle effect,which is mainly used for clean heat exchangers,is de?ned as the ratio heat transfer from an upper tube to that from a lower tube,as ifthe lower tube was activated alone in the bundle,in the absence ofany possible ?ow turbulence caused by generation ofbubbles on the upper tubes.For clean pool boiling and at high heat ?uxes,Cornwell (1988)showed that convection effects as well as bubbles from lower tubes im-pinging on,and sliding around,upper tubes can account for bundle effects above one,and that bubble nucleation only oc-curs on the lower tubes.Memory et al.(1995)reported that the bundle effect is minimised by the achievement of maxi-mum turbulence in the bulk ?ow.With respect to the present investigations on scale formation,a bundle ef?ciency can be de?ned as the ratio of the heat transfer coef?cients for top tubes ofthe bundle to those f or the bottom tubes,once the bundle has reached its asymptotic fouling resistance.In this study,the bundle ef?ciency is used because information on activated single heaters,which is required for the calculation ofthe bundle ef f ect,is not available.

Fig.11shows the bundle ef?ciency as a function of heat ?ux for concentrations of 0.8,1.2,and 1.6g/L CaSO 4.This value is calculated using heat transfer coef?cients in the asymptotic region where the changes in fouling resis-tance with time are insigni?cant.For heat ?uxes above 150kW/m 2,the bundle ef?ciency decrease with increas-ing CaSO 4concentration.Interestingly,the curves for 1.2and 1.6g/L pass through a maximum which,however,is much more pronounced for the higher concentration.The curve for 0.8g/L does not display the same trend within the investigated range ofheat ?uxes,since no experiments were carried out for this concentration below 145kW/m 2due to the absence off ouling at such low heat ?uxes and low concentrations (Jamialahmadi et al.,1989).For the saturation concentration of1.6g/L and at low heat ?uxes the main effect of the generated bubbles is to increase heat

M.R.Malayeri et al./Chemical Engineering Science 60(2005)1503–15131511

1.5

2.5

3.50

50

100

150200250

300350

Heat flux, [kW/m 2]

αT / αB , [-]

Fig.11.Bundle ef?ciency as a function of heat ?ux for different concen-trations.

transfer through additional turbulence.As shown in Fig.10,fouling rates for these conditions are very small or even declining.With increasing heat ?ux,i.e.,increasing number ofbubbles,the bundle ef ?ciency continues to increase up to a maximum.The bundle ef?ciency then starts to decline for two reasons:(i)developed nucleate boiling at the upper heaters reduces the effect of rising bubbles and (ii)a sub-stantial deposit layer starts to form,initially at the lower heaters.This can be seen by comparison of Figs.7and 8for heat ?uxes below 105kW/m 2,where only the bottom heater experiences a deposit-related drop in heat transfer,whereas the values for the top heater remained virtually constant.Somewhere between 105and 150kW/m 2,the ef-fect of deposition also shows for the top heater,giving rise to an even accelerated drop in bundle ef?ciency.

For the lower concentration of1.2g/L,the maximum bun-dle ef?ciency at 145kW/m 2is far smaller than that for 1.6g/L.The ?at maximum implies that the effect of turbu-lence caused by the generated bubbles on the heat trans-fer coef?cients of the top heater is less pronounced and that there is no delayed appearance off ouling deposits on the top heaters.This is con?rmed by the experimental ob-servations,showing that the bubble departure diameters at 1.2g/L CaSO 4are considerably smaller than for 1.6g/L and that only the bottom heater displayed a thin and friable de-posit layer,whereas the other two heaters stayed essentially deposit-free.The curve for 1.2g/L CaSO 4shows an unex-pected behaviour at 105kW/m 2,where the bundle ef?ciency is smaller than that for 1.6g/L.An examination of the ex-periments for this heat ?ux disclosed that the top heater remained clean for both concentrations of 1.2and 1.6g/L.However,the bottom heater for 1.6g/L CaSO 4experienced a considerably thicker fouling layer and hence a lower heat transfer coef?cient,which resulted in a higher bundle ef-?ciency.Fig.12compares the heat transfer coef?cients of both,bottom and top heater,for the two concentrations of 1.2and 1.6g/L.

2000

400060008000100001200001000

200030004000

Time, [min]

H e a t t r a n s f e r c o e f f i c i e n t , []W /m 2K ]

https://www.360docs.net/doc/932351572.html,parison ofheat transf er coef ?cient between bottom and top heaters for two concentrations of 1.2and 1.6g/L and constant heat ?ux of105kW/m 2.

11.21.41.61.822.22.40.6

0.8

1 1.

2 1.4 1.6 1.8

CaSO 4 concentration, [g/L]

αT / αB , [ -]

Fig.13.Bundle ef?ciency as a function of concentration for different heat ?uxes.

Fig.13highlights the effect of CaSO 4concentration on the bundle ef?ciency,for two heat ?uxes of 145and 200kW/m 2.The ?gure indicates that CaSO 4concentration has only a marginal impact on bundle ef?ciency when it changes from 0.8to 1.2g/L,i.e.,for sub-saturation conditions.However,the bundle ef?ciency reduces strongly when the concentra-tion increases from 1.2to 1.6g/L.This behaviour is con-sistent for both heat ?uxes,but the bundle ef?ciency is somewhat higher for the lower heat ?ux of 145kW/m 2.In summary,experimental results in this section show con-vincingly that fouling resistances obtained from single-tube experiments cannot be applied to tube bundles,since this would lead to excessive over-design.Furthermore,the results may hint to innovative design approaches,where the lowest tubes in the bundle are,at a higher cost,manufactured with low fouling surfaces (Müller-Steinhagen and Zhao,1997).Trends reported in the present investigation may,with the

1512M.R.Malayeri et al./Chemical Engineering Science 60(2005)1503–

1513

Fig.14.SEM pictures of deposit structure from the bottom heater for different heat ?ux and concentrations.

appropriate caution,be extrapolated to similar operating con-ditions.There are no doubts,however,that a lot ofadditional work is necessary to understand the mechanisms off ouling with and without bubble formation.

3.2.

4.Deposit morphology

The deposit morphology was determined using a SEM (Hitachi S-3200N,Resolution 3.5nm and with a magni?ca-tion range between 20×and 200,000×).All samples were taken at the end ofthe respective experiments,with great care to avoid distortion ofthe deposit structure.For this purpose,vessel was cooled down and the test heaters re-moved carefully and placed temporarily into another vessel that contained distilled water at ambient temperature.The heater was then dried and with the use ofa sharp blade,the rigid deposit layer was carefully separated from the surface.It was found that the structure of calcium sulphate was in?uenced by both concentration and heat ?ux.Fig.14presents some typical SEM pictures for different concentra-tions and heat ?uxes for the bottom heater.For the CaSO 4concentration of1.2g/L at 105kW/m 2,the structure is rather porous with a large number ofdistorted dome-shaped crys-tals.This structure becomes much denser for the higher heat ?ux of300kW/m 2.There is no signi?cant difference in the range ofcrystal sizes f or the two heat ?uxes;the distinctive feature is the evolution of the crystal shape.

For a CaSO 4concentration of1.6g/L,the deposit struc-ture in terms ofshape,size and crystal arrangement is en-tirely different than for 1.2g/L.Crystals for 300kW/m 2are much smaller than those for 33kW/m 2and their orientation shifts from perpendicular to parallel with respect to the di-rection ofbubble growth.In general,crystals f or higher heat ?uxes are denser,harder,more compact and more adherent than those for lower heat ?uxes.

4.Conclusions

Measurements presented in this paper highlight some im-portant features of scale formation processes in tube bundles.The heater at the lowest position in the tube arrangement ex-perienced a deposit layer in all experiments with CaSO 4con-centrations above or equal to 0.8g/L,but less signi?cantly for lower heat ?uxes and bulk concentrations.Deposits on the bottom heater in general were rather dense and adherent as shown in Table 2as well as Fig.14.For heat ?uxes of 105kW/m 2or below,the thermal fouling resistances for the top and middle heaters either remained zero or even negative (i.e.,an increase in heat transfer due to increased bubble for-mation on the bottom heater).In cases where a thin deposit layer was formed,it was very porous,friable and could be removed easily.For a CaSO 4concentration of1.6g/L and

M.R.Malayeri et al./Chemical Engineering Science60(2005)1503–15131513

heat?uxes above145kW/m2,the heat transfer coef?cient also decreased for the middle and top heaters.

The frequency,density and size of bubbles generated on the bottom heater has a substantial impact on the heat trans-fer enhancement of the middle and top heaters.Bubble ag-itation does not only increase heat transfer through higher local shear forces,but may also reduce the local supersat-uration in the vicinity ofbubbles nucleating on the upper heaters.Ifany deposition occurred on the top heaters,then the deposits were only formed on the upper side.The effect ofbubble agitation is considerably more signi?cant at lower concentrations and higher heat?uxes due to the larger num-ber ofbubbles that generate turbulence and the higher like-lihood that this may scour some ofthe f rail deposit layers on the upper tubes(Fig.13).The CaSO4concentration also affects substantially the mechanism of heat transfer fouling. Increasing the heat?ux and,therefore the wall superheat, has two competing effects on pool boiling heat transfer to scale forming solutions.The increased bubble formation ac-tivity tends to increase the bundle heat transfer coef?cients and the shear forces on tube walls,which both reduce the formation of deposits.At the same time,local concentration and saturation effects tend to increase the rate of deposit for-mation.As a result,a maximum bundle heat transfer coef?-cient was observed for an intermediate heat?ux of around 100kW/m2,for the investigated experimental conditions. Notation

C concentration,g L?1

˙q heat?ux,W m?2

R cavity radius,m

R f fouling resistance,m2K W?1

˙R f fouling rate,m2K J?1

s distance ofthe thermocouple f rom the surf ace ofthe heater,m

t time,s

T temperature,K

Greek letters

heat transfer coef?cient,W m?2K?1

h v latent heat ofevaporation,J kg?1

T superheat degree ofwall superheat=T s?T sat,K

thermal conductivity,W m?1K?1

density,kg m?3

g vapour density inside bubble nucleus,kg m?3 surface tension,N m?1Subscripts

b bulk

B bottom heater

s surface

sat saturation

T topheater

TC thermocouple

References

Bornhorst,A.,Zhao,Q.,Müller-Steinhagen,H.,1999.Reduction ofscale formation by ion implantation and magnetron sputtering on heat transfer surfaces.Heat Transfer Engineering20(2),6–14.

Cornwell,K.,1988.The in?uence ofbubbly?ow on boiling f rom a tube in a bundle.Advances in pool boiling heat transfer,Proceedings of Eurotherm Seminar8,Paderborn,pp.177–183.

Freeborn,J.,Lewis,D.,1962.Initiation ofboiler scale f ormation.Journal ofMechanical Science4,46–52.

Hospeti,N.B.,Mesler,R.B.,1965.Deposits formed beneath bubbles during nucleate boiling ofradioactive calcium sulphate solutions.

A.I.Ch.E.Journal11,662–665.

Jamialahmadi,M.,Müller-Steinhagen,H.,1993.Scale formation during nucleate boiling—A review.Corrosion Reviews11(1&2),25–54. Jamialahmadi,M.,Müller-Steinhagen,H.,2004a.A new model for the effect of calcium sulfate scale formation on pool boiling heat transfer.

ASME Journal ofHeat Transf er126,507–517.

Jamialahmadi,M.,Müller-Steinhagen,H.,2004b.Journal ofheat transf er policy on reporting uncertainties in experimental and results,1993.

ASME Journal ofHeat Transf er115,5–6.

Jamialahmadi,M.,Bl?chl,R.,Müller-Steinhagen,H.,1989.Bubble dynamics and scale formation during nucleate boiling of aqueous calcium sulphate solutions.Chemical Engineering and Processing26, 15–26.

Memory,S.B.,Akcasayar,N.,Eraydin,H.,Marto,P.J.,1995.Nucleate pool boiling ofR-114and R-114-oil mixtures f rom smooth and enhanced surfaces II.Tube bundles.International Journal of Heat and Mass Transfer38(9),1363–1376.

Müller-Steinhagen,H.,Zhao,Q.,1997.Investigation oflow f ouling surf ace alloys made by ion implantation technology.Chemical Engineering and Science52,3321–3332.

Najibi,S.H.,Müller-Steinhagen,H.,Jamialahmadi,M.,1996.Boiling and non-boiling heat transfer to electrolyte solutions.Heat Transfer Engineering17(4),46–63.

Palen,J.W.,Westwater,J.W.,1966.Heat transfer and fouling rates during pool boiling ofcalcium sulphate solutions.Chemical Engineering Progress Symposium Series62(64),77–86.

Partridge,E.P.,White,A.H.,1929.Mechanism off ormation ofcalcium sulphate boiler scale.Industrial Engineering Chemistry21,834–838. Wenzel,U.,1992.Saturated pool boiling and sub-cooled?ow boiling ofmixtures at atmospheric pressure.Ph.D.Thesis,The University of Auckland,New Zealand.

Whalley,P.B.,1996.Two-phase?ow and heat transfer.Oxford University Press,NY,USA.