Biofouling is a costly problem in heat exchangers. Biocides can be used to minimize the formation of biofilms, but they are not always effective and, moreover, they are generally deleterious to the environment. The use of proper liquid velocities or of water jets in the exchanger tubes is also a means to prevent the build up of fouling deposits or to clean the surface once they are formed. Often, biofilms incorporate inorganic particles which modify the physical properties of the deposit and, thus, affect the effectiveness of anti-fouling measures. This paper presents experimental data that show the effects of the water velocity and of the presence of clay particles on the accumulation of biofilms and on their mechanical resistance to detachment caused by hydrodynamic forces. The results indicate that the fraction of dry biomass (micro-organisms plus extracellular biopolymers) in biofilms increases with the liquid velocity and that the deposits formed under higher hydrodynamic forces are more resistant to detachment. The resistance to detachment is even greater when the biofilms incorporate small (20 micrometer) clay particles

Melo, L. F.

Vieira, M. J.

Universidade do Minho: RepositoriUM

I ' J . ' j Experimental Hear Transfer, Flui.d .Vechanics tlnd. Thermodynamics 1997 M. Giot. F, Mayinger. G.P. Cclata (Editors) 0 1997 Edizioni ETS, All rights reserved, PREVENTING BIOFOULING IN HEAT EXCHANGERS: AN EXPERIMENTAL ASSESSME~"T OF THE EFFECTS OF WATER VELOCITY AND INORGANIC PARTICLES ON DEPOSIT DETACHMENT M.J. Vieira and L. F. Melo'* University of Minho, Deparunent of Biological Engineering - 4700 Braga. Portugal *E-mail: lmelo@deb,umlnho.pt ABSTRACT Biofouling is a cru;Uy problem in heat exchangers, BlPcides can be used to mlnirnlze the fonnation of biofi!ms, hut they ar~ no! always effective and, moreover. they are generally deleterious 10 the environment. The use of proper liquid w:locities or of water jets in the exchanger tubes is also a means 10 prevent the build up of fouling deposits or tv clean the surface once they are formed. Often, biofilms incorporate inorganic particles \\>hich modify the physical propenies of the deposit and. th\1$, affect the effectiveness uf anti-fouling me3llUtes. This paper presents experimental data i:hat mow the e.~u; -Of the water velocity and of the ~ce of clay particles on the accumulation of biofilms and on their mechanical resistance to detachment caused by hydrOOynamk forces. The results indicate that the fraction of dry biomass (mi~isms plus extracellular biopolymers) in biofilms increases w;th the liquid velocity and t.l;at the deposits fonncd under higher hydrodynamic forces are more :resistant to detachment. The resistance to detaehmcnt is even greater when the btotilms incorponue small (20 micrometer) clay particles. l. INTRODUCTIO!\' Cooling water, coming from rivers, I~ born holes. e«:,. is used in a number of industrial processes where hotter fluids must be cooled, or a phase change is needed such as in power plant condensers. Pre-treatment technologies do not eliminate all sources of organic and inorganic matter earned by the water into the heat exchanger tubes, These minor components are potential contributors to the growth of undesirable micrOOial films on the pipe walls (biofooHng), that cause an increase in the thumal resistance and in the pressure drop in the heat exchanger, More often than desired. the operation of the industrial plant may have to be stopped in order to clean the equipment A "biofilm" is a matrix composed mainly by water (often, more than 90%), micro-organisms and extracellular polymers excreted by the microbial species, Biocidcs are frequently used to combat this unwanted biological growth, but they are not always efficient or acceptable from an environmental standpoint. Minimizatlon of biofouling effects can also be achieved rhrough good design and operating procedures, where fluid velocity stands as a kt:y parameter. This physical variable has contradictory effects on biofilm development : in fact, an increase of the velocity enhances the transport of microorganisms and of nutrients to the Jep0sition surface but, at the same time, it increases the hydrodynamic forces that contribute to the detachment of the biological layer. Some authors. (Bott 1995) have reported that lm:reasing the water velocity above I mis reduces: biofilrn build up, although it was foWld that dlls reduction occurred also in the lower range of fluid velocities (Bott 1995, Characklis 1990, Pinheiro er al. 1988). Such results were explained by the differences in the concentration of soluble nutrients, which caused the formation of biofilms w1th different characteristiCs ; low concentrations iead to thin and fmnly attached biofouling layers that are more difficult to remove from the surface than thicker and "fluffier" biofilms, Frequently, the waler contains inorganic particles in suspension that become incorporated in the biological deposit; it is therefore important to determine 2087 2088 • the changes brought about by the presence of these particles: as regards the detachment process. The experimental work here reported was carried out with the purpose of assessing the effects of the water velocity and of the presence of clay particles on the development of microbial films in a lab-sacie heat exchanger, and al.so on the resistance of these deposits to detachment l. MATERIALS AND METHODS A simulated vertical heat exchanger composed of two adjacent haJf .. tubes separated by a wall was used to monitor the b1.1ild up of the biofilm on aluminium surfaces. Two schematic views of this test apparatus {perpendicular and parallel to the flow directlon) are shown on Figure 1, The hydraulic diameter of the half tube is 2.02 cm. Water at 27 "C and pH 7, containing a diluted suspension of bacteria (6.107 cells.ml~ l of Pseudcmonas fl«onrscem) and nutrients (basically, glucose at the concentration of 20 mg.i-1 ), clrcuiated through one of the half-tubes of the test section (indicated as me ~test fluid# side), separated from the other half~tube by the aluminium plate (50 cm long) and a perspex wall ("water" side). The "test fluid" was heated by water at 60 "C flowing in the "water side"'. The heat flux could be calculated from the measurement of three temperatmes {Tl• T 2 and T 3} in each of the three 1>«tions indicated by A, B and C. Overall beat transfer coefficients were then calculated at different times during the growth of the biological deposit and, finally, the heat transfer resistances caused by lhe biofilm could be estimated at any instant of time. The results presented below refer always to the average values obtained in points A, B and C. More detailed information on this test section and on the method used to evaluate the fouling resistances of the biofilm can be found elsewhere (Vieira tt al 1993, Vieira and Melo 1995). Some runs were carried out using a mixed su~ion of ~teria (~rem fl~id") and 150 mg.J· of clay (kaohn) p.arttcles which had an equivalent diameter of about 20 micrometer. Different biofilms were formed (with and without clay particles) with the fluid circulating at velocities between 0.35 m.s·1 and l,2 m.s-l, wbK:b correspond to Reynolds numbers between 4 200 mu! 15 000. \--...'...,_~ ~ -D IA Test fluid ~' -Hot water~~~;;_ ________ _ Hotwoter Figure 1 • Simulated heat exchanger used in the blofou!ing tests 3. RESULTS AND DISCUSSION Figure 2 shows an example of a typical biofouling curve (fouling resistance as a function of time), while Figure 3 is a piot of the maximum (asymptotic) thermal resistance of the biofLim versus the Reynolds number. The latter confll'ITlS that the fouling resistance decreases as the Reyoolds no.mbet or the flow velocity increases (in this work. two identical test sections \Vtte used in all runs). A similar trend was observed as regards the thickness of the deposits. This kind of result is usually found in many types of fouling (Bott 1995). =or_ 60 S-0 • ~~ ~ ;!i~vr_, • I ! 0 4 8 12 Time (days) Figure 2 - Biofouling curve M6 0.05 0.04 1 ~.O'J :Se • 1 • < 0.02 • 0.01 o.oo 0 15000 Figure 3 - .~symptotic thermal resistance as a function of fluid velocity The amount of dry biomass (bacteria plus extracellular polymers excreted by them) was measured In blofilms formed under different velocities (Table l ), The data of 'fable l show that higher fluid velocities, at least in turbulent flow conditions, result in more compact biofilms. containing higher percentages of dry matter. The behaviour of the biofilms as regards their detachment from the surface was also different according to their previous .. history" (Table 2): the deposits that were formed under lower hydnxiynamic forces (Reynolds number :::::: 9 200) were partially removed from the aluminium plate when the Reynolds number was increased up to 25 500; the films formed under higher hydrodynamic forces (Re= lS 500) were not disturbed when an identical experiment was perfmmcd. Tests were also carried out t-0 assess the effect of clay particles within the biofihn matrix. For this purpose, biofilms were formed with and without clay particles., under different velocities. After the deposits had reached their asymptotic thermal resistance (steady-state), gJucose was removed from the test fluid that was flo..,.ing through the experimental heat exchanger. Some time after the main nutrient was surpressed, the biofouHng thermal resistance started to decrease and part of the btofilm was removed and carried away by the fluid, 1be response of the various biofilms to these drastic conditions was different depending on clay particles: being incorporated in the deposit or not. as can be seen on Table 3. Table 1 - Dry biomass (bacteria+ polymers) per unit volume of wet biofilm (kg.m-3} Fluid veJocity {m.s· l} 0.37 0.49 0.63 Reynolds number 4570 6100 7 800 Dry biomass per unit vu1nme nf wet biofilm (kg.m-3) 14 20 2& 2089 2090 • Table 2 - Effect of increasing the Reynolds number on two biofilms forme.d under different hydro:iynamic conditions Steady-stale thermal Steady-state thermal %of resistance under resistance after increasing biofilm initial Reynolds :'.'Co. the Reynolds No. to 25 500 removed (m2.K.W"l) (m2.K.w-I) Biofilm formed under fluid R~·nolds number of 9 200 2s.10-4 15.lrr' 40% Biofilm formed under fluid Revnolds number of IS SOO 16.104 !6.J0-4 0% Table 3 • Loss of biofilm mass after glucose was suppressed from the test fluid Velocity of the fluid in contact % of biofilm tnass lost after with the biofilm {m.s-l) Biofilms without 0.34 cla·· -·--'cles 0.72 B iofilms with 0.34 clav --~iclcs 0.73 Although the results are not vcry expressive, it seems that the microbial films with clay particles were more resistant to removal when the nutrient supply was cut of[ Table 3 also shows that the % of biomass removed was higher when the biofilms were formed under higher velocittes. This may appear to be in contradiction with the results of Table 2. However. Table 2 describes a physical effect and Table 3 a metabolical one. In fact. since the biofilms formed at higher vclocities are thinner, the nutrients are able to penetrate more deeply than in thicker biofilms. Thus, a higher fraction of the thinner biofilms is more dependent on the availability of nutrients and detachs from the deposit after the nutrient concentration ls tedliCed to (practically) zero . . A.nother method, independent of the experiments reported in Table 3, was used to compare the characteristics of the bioflims with and without clay particles. The method relies upon the use of a simple and well known model that describes the build up of fouling layers. (Kem and Seaton 1959). The model assumes that the fouling rate depends on the competition between the rates of two simultaneous phenomena: deposition and removal The former sunnressinl! the t'!lucose su--!v 21% 91% 15% S3% is assumed to be constant with time, while the removal or detachment rate ('l>r) is considered ro increase proportionally to the thickness of the attached layer (here represented by its thermal resistance, Rf) : (1) 11\e higher the value of b, the easier will be the detachment of the deposit. 'Therefore, I lb can be interpreted as a "property" of the bioftlm that ls proportional to the "mechanical resistance" offered by the deposit to tbe removal forces of the fluid, The removal and deposition rates equal each other when the deposit reaches its steady state. In terms of thenna1 resistances, the integrated equation ls (Kern and Seaton 1959): Rf= Rf' [1- oxp(·b.t)] (2) where Rf is tbe thermal resistance of the fouling layer at any time t, and Rf is the asymptotic {steady-state} thermal resistance of this fouling layer, Values of lib for biofilms with and without clay particles were estimated by fitting Eq, (2) to biofouling curves obtained in different operating conditions (Table 4). ' Ta~le 4 Resistance of b1ofilms to detachment {lib} Fluid velocity {m.s· 1) and l/h, "resistance to detachment~ Revnolds number (sl Biofilms without 0.62 (Re= 7 620) l.&xlo-5 clav ....... k:Jes 0.9'7 (Re = l2 000) 2.sx.10-5 Bioftlms with 0.59 (Re= 7 505) 2.2x10·5 clav n~tticles 0.95 (Re= 11 7631 4.2xlo-5 The results confirm that the presence of kaolin particles in the deposits makes them more resistant to detactunent (higher lib, i,e., higher residence time of a given portion of matter in the deposit) when subject to identical hydrodynamic conditions. The data of Table 4 are also in accordance with Table 2. since the value -of the "mechanical resistance" parameter (lib) is higher for biofilms formed under higher fluid velocities than in the ca.re of biofilms subject to lower hydrodynamic stresses. 4. CONCLUSIONS Proper choice of the fluid v-elooity in heat exchangers is of the utmost importaoce as a means of minimizing the build up of biofouling layers and of contribi.rting to more efficient cleaning procedures. In fact, although higher velocities result in thinner biologkal deposits, these are more compact, more firmly attached and more difficult to detach from the surface by means of hydrodynamic forces, Obviously, a balance must be sought between using higher or lower velocities, The standard liquid velocities in heat exchanger design are 1.5-2 ml~ but designers should take into account the fact that the properties of the biological layer may determine the efficacy of cleaning proi;;edures that often rely upon the use of water flows {containing detergents or other chemicals} to remove the deposit from the surface. FurthefmQre, when using biocldes to reduce biofouJing, it should be con.-;,idered that thinner but more compact biofilms may not allow the complete penetration of biocides : in this case, a residual and "hard'' layer can remain on the surface, which v.111 eventually promote the rapid re-growth of the microbial film. The "history" of the biofilm (the conditions under which it was formed) should thus be known in order to choose the most adequate anti-fouling measures. The results also indicated that the resistance to detachment is even greater when the biofilms incorporate very small clay particles. Therefore, elimination of these particles from the water can play a significant rote in the prevention ofbiofou1ing effects. ACKNOWLEDGEMENT The finaneial support of PRAXIS XXl programme through proje.ct no. 212. l 1810/37194 is gratefully acknowledged. NOl\IENCLA TURE b Re Rf t <I>, parameter inversely proportional to the mechanical strength of the deposit, s- l Reynolds number (dimensionless} thermal resistance of the deposit at any timet,m2.Kw-l asymptotic therma1 resistance of the deposit, m2 ,K. w· 1 time, s detachment or removal rate. m2.K.w~l.s-1 2091 

Preventing biofouling in heat exchangers: an experimental assessment of the effects of water velocity and inorganic particles on deposit detachment

I 
' 
J 
. ' 
j 
Experimental Hear Transfer, Flui.d .Vechanics tlnd. Thermodynamics 1997 
M. Giot. F, Mayinger. G.P. Cclata (Editors) 
0 1997 Edizioni ETS, All rights reserved, 
PREVENTING BIOFOULING IN HEAT EXCHANGERS: AN EXPERIMENTAL 
ASSESSME~"T OF THE EFFECTS OF WATER VELOCITY AND INORGANIC 
PARTICLES ON DEPOSIT DETACHMENT 
M.J. Vieira and L. F. Melo'* 
University of Minho, Deparunent of Biological Engineering - 4700 Braga. Portugal 
*E-mail: lmelo@deb,umlnho.pt 
ABSTRACT 
Biofouling is a cru;Uy problem in heat exchangers, BlPcides can be used to mlnirnlze the fonnation of biofi!ms, hut they 
ar~ no! always effective and, moreover. they are generally deleterious 10 the environment. The use of proper liquid 
w:locities or of water jets in the exchanger tubes is also a means 10 prevent the build up of fouling deposits or tv clean 
the surface once they are formed. Often, biofilms incorporate inorganic particles \\>hich modify the physical propenies 
of the deposit and. th\1$, affect the effectiveness uf anti-fouling me3llUtes. This paper presents experimental data i:hat 
mow the e.~u; -Of the water velocity and of the ~ce of clay particles on the accumulation of biofilms and on their 
mechanical resistance to detachment caused by hydrOOynamk forces. The results indicate that the fraction of dry 
biomass (mi~isms plus extracellular biopolymers) in biofilms increases w;th the liquid velocity and t.l;at the 
deposits fonncd under higher hydrodynamic forces are more :resistant to detachment. The resistance to detaehmcnt is 
even greater when the btotilms incorponue small (20 micrometer) clay particles. 
l. INTRODUCTIO!\' 
Cooling water, coming from rivers, 
I~ born holes. e«:,. is used in a number of 
industrial processes where hotter fluids must be 
cooled, or a phase change is needed such as in 
power plant condensers. Pre-treatment 
technologies do not eliminate all sources of 
organic and inorganic matter earned by the 
water into the heat exchanger tubes, These 
minor components are potential contributors to 
the growth of undesirable micrOOial films on the 
pipe walls (biofooHng), that cause an increase in 
the thumal resistance and in the pressure drop 
in the heat exchanger, More often than desired. 
the operation of the industrial plant may have to 
be stopped in order to clean the equipment 
A "biofilm" is a matrix composed 
mainly by water (often, more than 90%), micro-
organisms and extracellular polymers excreted 
by the microbial species, Biocidcs are 
frequently used to combat this unwanted 
biological growth, but they are not always 
efficient or acceptable from an environmental 
standpoint. Minimizatlon of biofouling effects 
can also be achieved rhrough good design and 
operating procedures, where fluid velocity 
stands as a kt:y parameter. This physical variable 
has contradictory effects on biofilm 
development : in fact, an increase of the velocity 
enhances the transport of microorganisms and of 
nutrients to the Jep0sition surface but, at the 
same time, it increases the hydrodynamic forces 
that contribute to the detachment of the 
biological layer. Some authors. (Bott 1995) have 
reported that lm:reasing the water velocity above 
I mis reduces: biofilrn build up, although it was 
foWld that dlls reduction occurred also in the 
lower range of fluid velocities (Bott 1995, 
Characklis 1990, Pinheiro er al. 1988). Such 
results were explained by the differences in the 
concentration of soluble nutrients, which caused 
the formation of biofilms w1th different 
characteristiCs ; low concentrations iead to thin 
and fmnly attached biofouling layers that are 
more difficult to remove from the surface than 
thicker and "fluffier" biofilms, Frequently, the 
waler contains inorganic particles in suspension 
that become incorporated in the biological 
deposit; it is therefore important to determine 
2087 
2088 
• the changes brought about by the presence of 
these particles: as regards the detachment 
process. 
The experimental work here reported 
was carried out with the purpose of assessing the 
effects of the water velocity and of the presence 
of clay particles on the development of 
microbial films in a lab-sacie heat exchanger, 
and al.so on the resistance of these deposits to 
detachment 
l. MATERIALS AND METHODS 
A simulated vertical heat exchanger 
composed of two adjacent haJf .. tubes separated 
by a wall was used to monitor the b1.1ild up of the 
biofilm on aluminium surfaces. Two schematic 
views of this test apparatus {perpendicular and 
parallel to the flow directlon) are shown on 
Figure 1, The hydraulic diameter of the half tube 
is 2.02 cm. Water at 27 "C and pH 7, containing 
a diluted suspension of bacteria (6.107 cells.ml~ 
l of Pseudcmonas fl«onrscem) and nutrients 
(basically, glucose at the concentration of 20 
mg.i-1 ), clrcuiated through one of the half-tubes 
of the test section (indicated as me ~test fluid# 
side), separated from the other half~tube by the 
aluminium plate (50 cm long) and a perspex 
wall ("water" side). The "test fluid" was heated 
by water at 60 "C flowing in the "water side"'. 
The heat flux could be calculated from the 
measurement of three temperatmes {Tl• T 2 and 
T 3} in each of the three 1>«tions indicated by A, 
B and C. Overall beat transfer coefficients were 
then calculated at different times during the 
growth of the biological deposit and, finally, the 
heat transfer resistances caused by lhe biofilm 
could be estimated at any instant of time. The 
results presented below refer always to the 
average values obtained in points A, B and C. 
More detailed information on this test 
section and on the method used to evaluate the 
fouling resistances of the biofilm can be found 
elsewhere (Vieira tt al 1993, Vieira and Melo 
1995). 
Some runs were carried out using a 
mixed su~ion of ~teria (~rem fl~id") and 
150 mg.J· of clay (kaohn) p.arttcles which had 
an equivalent diameter of about 20 micrometer. 
Different biofilms were formed (with and 
without clay particles) with the fluid circulating 
at velocities between 0.35 m.s·1 and l,2 m.s-l, 
wbK:b correspond to Reynolds numbers between 
4 200 mu! 15 000. 
\--...'...,_~ ~ 
-D IA 
Test fluid ~' -
Hot water~~~;;_ ________ _ 
Hotwoter 
Figure 1 • Simulated heat exchanger used in the 
blofou!ing tests 
3. RESULTS AND DISCUSSION 
Figure 2 shows an example of a typical 
biofouling curve (fouling resistance as a 
function of time), while Figure 3 is a piot of the 
maximum (asymptotic) thermal resistance of the 
biofLim versus the Reynolds number. The latter 
confll'ITlS that the fouling resistance decreases as 
the Reyoolds no.mbet or the flow velocity 
increases (in this work. two identical test 
sections \Vtte used in all runs). A similar trend 
was observed as regards the thickness of the 
deposits. This kind of result is usually found in 
many types of fouling (Bott 1995). 
=or_ 
60 
S-0 
• 
~~ ~ 
;!i~vr_, 
• 
I 
! 
0 4 8 12 
Time (days) 
Figure 2 - Biofouling curve 
M6 
0.05 
0.04 
1 ~.O'J :Se • 
1 • < 
0.02 • 
0.01 
o.oo 
0 15000 
Figure 3 - .~symptotic thermal resistance 
as a function of fluid velocity 
The amount of dry biomass (bacteria 
plus extracellular polymers excreted by them) 
was measured In blofilms formed under different 
velocities (Table l ), 
The data of 'fable l show that higher 
fluid velocities, at least in turbulent flow 
conditions, result in more compact biofilms. 
containing higher percentages of dry matter. 
The behaviour of the biofilms as 
regards their detachment from the surface was 
also different according to their previous 
.. history" (Table 2): the deposits that were 
formed under lower hydnxiynamic forces 
(Reynolds number :::::: 9 200) were partially 
removed from the aluminium plate when the 
Reynolds number was increased up to 25 500; 
the films formed under higher hydrodynamic 
forces (Re= lS 500) were not disturbed when an 
identical experiment was perfmmcd. 
Tests were also carried out t-0 assess the 
effect of clay particles within the biofihn matrix. 
For this purpose, biofilms were formed with and 
without clay particles., under different velocities. 
After the deposits had reached their asymptotic 
thermal resistance (steady-state), gJucose was 
removed from the test fluid that was flo..,.ing 
through the experimental heat exchanger. Some 
time after the main nutrient was surpressed, the 
biofouHng thermal resistance started to decrease 
and part of the btofilm was removed and carried 
away by the fluid, 1be response of the various 
biofilms to these drastic conditions was different 
depending on clay particles: being incorporated 
in the deposit or not. as can be seen on Table 3. 
Table 1 - Dry biomass (bacteria+ polymers) per unit volume of wet biofilm (kg.m-3} 
Fluid veJocity {m.s· l} 0.37 0.49 0.63 
Reynolds number 4570 6100 7 800 
Dry biomass per unit vu1nme nf wet biofilm (kg.m-3) 14 20 2& 
2089 
2090 
• Table 2 - Effect of increasing the Reynolds number on two biofilms 
forme.d under different hydro:iynamic conditions 
Steady-stale thermal Steady-state thermal %of 
resistance under resistance after increasing biofilm 
initial Reynolds :'.'Co. the Reynolds No. to 25 500 removed 
(m2.K.W"l) (m2.K.w-I) 
Biofilm formed under fluid 
R~·nolds number of 9 200 2s.10-4 15.lrr' 40% 
Biofilm formed under fluid 
Revnolds number of IS SOO 16.104 !6.J0-4 0% 
Table 3 • Loss of biofilm mass after glucose was suppressed from the test fluid 
Velocity of the fluid in contact % of biofilm tnass lost after 
with the biofilm {m.s-l) 
Biofilms without 0.34 
cla·· -·--'cles 0.72 
B iofilms with 0.34 
clav --~iclcs 0.73 
Although the results are not vcry 
expressive, it seems that the microbial films with 
clay particles were more resistant to removal 
when the nutrient supply was cut of[ Table 3 
also shows that the % of biomass removed was 
higher when the biofilms were formed under 
higher velocittes. This may appear to be in 
contradiction with the results of Table 2. 
However. Table 2 describes a physical effect 
and Table 3 a metabolical one. In fact. since the 
biofilms formed at higher vclocities are thinner, 
the nutrients are able to penetrate more deeply 
than in thicker biofilms. Thus, a higher fraction 
of the thinner biofilms is more dependent on the 
availability of nutrients and detachs from the 
deposit after the nutrient concentration ls 
tedliCed to (practically) zero . 
. A.nother method, independent of the 
experiments reported in Table 3, was used to 
compare the characteristics of the bioflims with 
and without clay particles. The method relies 
upon the use of a simple and well known model 
that describes the build up of fouling layers. 
(Kem and Seaton 1959). The model assumes 
that the fouling rate depends on the competition 
between the rates of two simultaneous 
phenomena: deposition and removal The former 
sunnressinl! the t'!lucose su--!v 
21% 
91% 
15% 
S3% 
is assumed to be constant with time, while the 
removal or detachment rate ('l>r) is considered ro 
increase proportionally to the thickness of the 
attached layer (here represented by its thermal 
resistance, Rf) : 
(1) 
11\e higher the value of b, the easier will be the 
detachment of the deposit. 'Therefore, I lb can be 
interpreted as a "property" of the bioftlm that ls 
proportional to the "mechanical resistance" 
offered by the deposit to tbe removal forces of 
the fluid, The removal and deposition rates 
equal each other when the deposit reaches its 
steady state. In terms of thenna1 resistances, the 
integrated equation ls (Kern and Seaton 1959): 
Rf= Rf' [1- oxp(·b.t)] (2) 
where Rf is tbe thermal resistance of the fouling 
layer at any time t, and Rf is the asymptotic 
{steady-state} thermal resistance of this fouling 
layer, Values of lib for biofilms with and 
without clay particles were estimated by fitting 
Eq, (2) to biofouling curves obtained in different 
operating conditions (Table 4). 
' 
Ta~le 4 Resistance of b1ofilms to detachment {lib} 
Fluid velocity {m.s· 1) and l/h, "resistance to detachment~ 
Revnolds number (sl 
Biofilms without 0.62 (Re= 7 620) l.&xlo-5 
clav ....... k:Jes 0.9'7 (Re = l2 000) 2.sx.10-5 
Bioftlms with 0.59 (Re= 7 505) 2.2x10·5 
clav n~tticles 0.95 (Re= 11 7631 4.2xlo-5 
The results confirm that the presence of 
kaolin particles in the deposits makes them more 
resistant to detactunent (higher lib, i,e., higher 
residence time of a given portion of matter in 
the deposit) when subject to identical 
hydrodynamic conditions. The data of Table 4 
are also in accordance with Table 2. since the 
value -of the "mechanical resistance" parameter 
(lib) is higher for biofilms formed under higher 
fluid velocities than in the ca.re of biofilms 
subject to lower hydrodynamic stresses. 
4. CONCLUSIONS 
Proper choice of the fluid v-elooity in 
heat exchangers is of the utmost importaoce as a 
means of minimizing the build up of biofouling 
layers and of contribi.rting to more efficient 
cleaning procedures. In fact, although higher 
velocities result in thinner biologkal deposits, 
these are more compact, more firmly attached 
and more difficult to detach from the surface by 
means of hydrodynamic forces, Obviously, a 
balance must be sought between using higher or 
lower velocities, The standard liquid velocities 
in heat exchanger design are 1.5-2 ml~ but 
designers should take into account the fact that 
the properties of the biological layer may 
determine the efficacy of cleaning proi;;edures 
that often rely upon the use of water flows 
{containing detergents or other chemicals} to 
remove the deposit from the surface. 
FurthefmQre, when using biocldes to reduce 
biofouJing, it should be con.-;,idered that thinner 
but more compact biofilms may not allow the 
complete penetration of biocides : in this case, a 
residual and "hard'' layer can remain on the 
surface, which v.111 eventually promote the rapid 
re-growth of the microbial film. The "history" of 
the biofilm (the conditions under which it was 
formed) should thus be known in order to 
choose the most adequate anti-fouling measures. 
The results also indicated that the 
resistance to detachment is even greater when 
the biofilms incorporate very small clay 
particles. Therefore, elimination of these 
particles from the water can play a significant 
rote in the prevention ofbiofou1ing effects. 
ACKNOWLEDGEMENT 
The finaneial support of PRAXIS XXl 
programme through proje.ct no. 
212. l 1810/37194 is gratefully acknowledged. 
NOl\IENCLA TURE 
b 
Re 
Rf 
t 
<I>, 
parameter inversely proportional to the 
mechanical strength of the deposit, s- l 
Reynolds number (dimensionless} 
thermal resistance of the deposit at any 
timet,m2.Kw-l 
asymptotic therma1 resistance of the 
deposit, m2 ,K. w· 1 
time, s 
detachment or removal rate. 
m2.K.w~l.s-1 
2091 


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Preventing biofouling in heat exchangers: an experimental assessment of the effects of water velocity and inorganic particles on deposit detachment

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Universidade do Minho: RepositoriUM

Universidade do Minho: RepositoriUM

Universidade do Minho: RepositoriUM