This document discusses the use of water as a cooling fluid in industrial plants, emphasizing its thermodynamic properties and the presence of impurities that can cause issues like scale, fouling, and corrosion. It covers the basics of heat transfer mechanisms including conduction, convection, and radiation, as well as the specific characteristics and parameters of water that impact its cooling efficiency. Additionally, it details the implications of water quality, including pH levels, hardness, and the presence of dissolved gases and organic materials on the performance of cooling systems.

1. WATER AS
COOLING
FLUID IN
INDUSTRIAL
PLANTS
1

2. Water as cooling fluid
• To have good heat transfer, the cooling fluid
must be convenient from thermodinamic and cost
point of view
• High Thermal capacity
• Low specific volume
• Temperature sufficiently lower then that of the
process fluid
• High availability
• Low cost
2

3. Water as cooling fluid
• SPECIFIC HEAT
• is the measure of how well a substance can absorb
heat
• 1 Kcal/kg°C or 4,18 KJ/kg°C
• VAPORIZATION HEAT
• Energy to transform liquid to gaseous
• 540 Kcal/kg at 100°C
• 575 Kcal/kg at 40°C
3

4. Water as cooling fluid
• Main water characteristics
Water Air Glycerin
- ka (Kcal/hm °C) 0,541 0,023 0,243
- cp (kcal/kg °C) 0,998 0,24 0,6
- d (kg/m3
) 992 1,136 1250
- u - 104 (ns/m2
) 0,81 0,191 2800
- v . 106 (m2
/s) 0,686 16,8 2200
- Pr (cp.u/ka) 4,52 0,72 0,0025
- a . 104 (m2
/h) 5,46 841 3,2
ka = thermal conductivity
cp = specific heat
d = density
u = dynamic viscosity
v = cinematic viscosity
Pr = Prandtl number
a = thermal diffusivity

5. Water as cooling fluid
• However, due to the presence of
sevaral impurities, water can cause
severe problems to the heat transfer
equipment:
• HEAT TRANSFER BASICS
• WATER CHARACTERISTICS
• SCALE
• FOULING
• BIOLOGICAL FOULING
• CORROSION
5

6. Basics of heat
transfer
6

7. Basics of heat transfer
• To understand the impact of water
characteristics, is useful to remind the heat
transfer laws that help us to recognize the
importance of a right treatment
• CONDUCTION
• CONVECTION
• RADIATION
13/06/2024 7

8. Basics of heat transfer
• Conduction
• It is the transfer of heat between two
areas at different temperature of same
material (or different material in
contact), due to the heat flux between the
molecules without these move from their
average position
• Heat transfer capacity of a material,
called thermal conductivity, determine the
gradient of temperature accross the
material
• The foundamental law of the conduction is
the Fourier law:
where
13/06/2024 8
qk = ka (t2 - t1) / Lc
qk = heat flux, kcal/h m2
ka = thermal conductivity, kcal/m h°C
(t2-t1) = gradient of temperature through a layer of material, °C
Lc = thickness across which the heat transfer occurs, m

9. Basics of heat transfer
• Convection
• It is the heat transfer associated to the transport
of mass and energy beteen two points of a fluid or a
fluid with a solid
13/06/2024 9
Where:
qc = hc A (t2-t1)
qc = convection heat transfer, kcal/h
hc = convection heat transfer coefficient, kcal/hm2°C
A = heat transfer surface, m2
(t2-t1) = difference of temperature across a fluid film, °C

10. Basics of heat transfer
• Convection
• To get high heat transfer value it is
needed to reduce as much as possible the
limit film layer
• This can be done by increasing the flow
velocity and/or the turbolence of the
fluid
• The ratio between the gradient of
temperature inside the fluid in contact
with the surface and a reference gradient
of temperature is given by the Nusselt
Number (Nu)
• The Nu correlates the convection
coefficient h, the diameter D and the
13/06/2024 10
Nu = hc D / ka

11. Basics of heat transfer
• Convection
• Reynolds Number (Re) indicate if a fluid
is moving in the laminar or turbolence
region
• The Re is adimensional and it is
proportional directly to the diameter
(D)of the tube, the density (d) and
velocity (V) of the fluid and inverse to
the viscosity of the fluid (u)
• Approximatively, for clean tubes
Re = D d V / u
Re < 2100 laminar motion
Re > 2400 turbulent motion

12. Basics of heat transfer
• Convection
• According to the convection coefficient (hc), Nusselt
number is function of Reynolds number (Re) and Prantl
number (Pr)
• Where Pr is adimensional and it is given by the
relationship
• A realshionship to calculate the convection
coefficient, valid for turbulent region, is given by
Dittus and Boelter
13/06/2024 12
Nu = f(Re,Pr)
Pr = cp u / ka
hc D / ka = 0.023 Re0.6 Pr1/3 u/uw
uw = Skin temperature viscosity

13. Basics of heat transfer
• Radiation
• It is the heat transfer due to the radiation effect,
even in presence of material, between two bodies at
different temperature
• The radiation law is given by Stefan-Boltzman
(generalized)
13/06/2024 13
qi = ds e A1 (T4
1 - T4
2)
where: qi = thermal power of radiation, kcal/h
ds = Stefan-Boltzman’s constant, 4,88.10-8 kcal/m2h°K4
e = emissivity of the mateiral
A1 = surface, m2
(T1-T2) = differential of temperature across a fluid film, °K

14. Basics of heat transfer
• Heat transfer in the industrial equipments
• In the shell&tubes units the heat transfer occurs
mainly by conduction and convection
• In the furnaces and heaters the heat transfer occurs
by convection and radiation
13/06/2024 14
Hot Process In
Hot Cooling
Water Out
Cooled Process Out
Cold Cooling
Water In
Kcal's
Kcal's
SIMPLE HEAT TRANSFER

15. Basics of heat transfer
• Shell&tubes heat transfer – Water shell side
13/06/2024 15
External
deposit
External
Fluid film
Internal
deposit
Internal
Fluid film
Wall of tube
T1
T3
T2
T4
T5 T6
T8
T7
Cold
fluid
Hot
fluid
Laminar layer
Transition zone
Turbulent zone
Fluid
Outside
tube
Fluid
Inside
tube
Conduction:
T4 - T5
T3 - T4
T5 - T6
Convection:
T1 - T3
T6 - T8

16. Water
Characteristi
cs
6/13/2024 16

17. Impact of water
characteristics
• Industrial plants take advantage from the
thermodinamic properties of the water as cooling
fluid
• The impurities present in the water can cause
severe problems to the heat transfer units
• It is important to control the operation of heat
transfer equipments within the allowed limits of
the chemical-physical parameters
• Water, that become warmer due to the heat
transfer, couid cause problems due to the
original impurities, their concentration in
evaporative units and absorption of pollutants
from processes and atmosphere
13/06/2024 17
• Scale
• Deposit
• Corrosion
• Fouling

18. Impact of water
characteristics
• Aspect
• Can be observed materials suspended that could
potentially settled on the heat transfer surface
• These materials could cause deposit, scale,
corrosion, erosion, biological foulinf and foam
13/06/2024 18

19. Impact of water characteristics
• Temperature
• Specially the skin temperature, represents a
significative index of the potential problems that
can occurs (scale, corrosion, fouling)
• The solubility of scaling salts decrease with the
increase of the temperature. So, where this is
higher the potential precipitation is more
critical.
• Higher potential of scale formation occurs in the
hottest areas of the heat transfer unitse (i.e. The
inlet of the process where the temperature is
higher)
13/06/2024 19

20. Impact of water
characteristics
• pH
• pH units give the degree of acidity or basicity of
water
• pH represents a very important factor on the
development and entity of scale, corrosion and
biological fouling phenomena
• pH is the basic parameter to calssify the
corrosiveness of waters
13/06/2024 20

21. Impact of water
characteristics
• Electrical conductivity
• Electrical conductivity represents the reciprocal
of the resistivity to the flowing of the electrical
current
• It is typical for each electrolyte, function of the
ion concentration
• It is an indirect measurement of the dissociated
matters present in the water
• It is an index of the purity and salinity
• Higher ion concentration increase generally the
solubility of the scaling salts
• Lower resistance increases the aggressivity of the
water
13/06/2024 21

22. Impact of water
characteristics
• Total solids (TS)
• TS represents all the solids present in the water as
soluble, colloidal and suspended
• It is expressed as mg/liter and is determined by
evaporation at 180°C
• High ST value indicates normally poor water quality
13/06/2024 22

23. Impact of water
characteristics
• Total dissolved solids (TDS)
• TDS are the total dissolved solids in the water
• It is expressed as mg/liter and are determined by
evaporation at 180°C, after filtration at 0.45
micron
• High TDS value promote the scale and corrosion
phenomena
13/06/2024 23

24. Impact of water
characteristics
• Total hardness
• It represents the amount of alkaline salts, mainly
calcium and magnesium salts, that are solubilized in
the water
• It is expressed as mg/liter of Calcium carbonate
• The presence of hardness cause scale if no tretment
is applied to inhibit the precipitation of the
scaling salts
13/06/2024 24

25. Impact of water
characteristics
• Temporary hardness
• It represents the amount of earth-alkaline
bicarbonates (calcium and magnesium) in the water
• It is expressed as mg/liter of Calcium carbonate
• It is directly responsible for scale formation
13/06/2024 25

26. 13/06/2024 26
Impact of water
characteristics
• Calcium hardness
• It represents the total content of calcium as
bicarbonate, sulfate and possible chloride present
in the water
UNITS OF HARDNESS
RELATION
Hardness, °d French, °d German, °D English, °D USA, °d mVal ppm
French 1.00 0.56 0.70 0.58 0.20 10.00
German 1.79 1.00 1.25 1.05 0.36 17.85
English 1.43 0.80 1.00 0.84 0.29 14.30
USA 1.71 0.96 1.20 1.00 0.34 17.10

27. Impact of water
characteristics
• Water hardness
13/06/2024 27
Temporary Hardness (carbonatic) Permanent Hardness (non-carbonatic
Calcium carbonate [CaCO 3] Calcium sulfate [CaSO 4
]
Magnesium carbonate[MgCO 3] Magnesium silicate [MgSiO3]
Calcium bicarbonate[Ca(HCO3)
2] Calcium chloride [CaCl
2]
Magnesium bicarbonate
[Mg(HCO 3)
2] Magnesium sulfate
Silicates
[MgSO 4]

28. Impact of water
characteristics
• Total alkalinity
• It represents the total alkaline salts present
in the water (bicarbonates, carbonates,
hydrates, alkaline phosphates)
• It is expressed as mg/liter of Calcium
carbonate
• Alkalinity < hardness = total calcium and
magnesium bicarbonate
• temporary hardness / carbonatic hardness
• Hardness - Alkalinity = non carbonatic
alkalinity
• permanent hardness
13/06/2024 28

29. Impact of water
characteristics
13/06/2024 29
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
pH
Total Alkalinity of recirculating water, ppm CaCO3
pH/Alkalinity Relationship
(Experimental values from operating cooling towers)

30. Impact of water
characteristics
13/06/2024 30
7
7.2
7.4
7.6
7.8
8
8.2
8.4
8.6
8.8
9
0
30
60
90
120
150
180
210
240
270
300
pH
Acid added
Effect of acid addition

31. Impact of water
characteristics
• Effect of acids on Malkalinity
13/06/2024 31
Acid Degrees Bè %Active ppm required to
reduce 1 ppm
“M” Alkalinity
H2SO4 (Sulfuric) 66 93.35 1.05
NH2SO3H (Sulfamic) Crystals 99.6 1.95
Granular 93 2.06
HCI (Muriatic) 18 30 2.43
36.5 2.00
HNO3 (Nitric) 36 52.3 2.41
38 56.5 2.23
40 61.4 2.05
42 67.2 1.87
HC2H3O2 (Acetic) 12 99.5 1.20
H3C6H4O7 (Citric) Crystals 99.5 1.28
HCHO2 (Formic) - - - 99.5 0.92
H2C2O4 2H2O (Oxalic) Crystals 99.0 0.91
ppm Acid = 1ppm Alk / PE CaCO3 * PE acido/ %activ acid

32. Impact of water
characteristics
• Alkalinity
13/06/2024 32

33. Impact of water
characteristics
• Alkalinity
13/06/2024 33
CONCENTRATION BASE ON ALKALINITY
M (methylorange) e P (phenolphtalein)
(HCO 3
-) (CO 3
-) (OH -),
P = 0 M 0 0
2P < M M-2P 2P 0
2P = M 0 M 0
2P > M > P 0 2(M-P) 2P - m
P = M 0 0 M

34. Impact of water
characteristics
• Alkalinity and acidity
13/06/2024 34

35. Impact of water
characteristics
• Chlorides
• High level of chlorides are corrosive to most of
metals, specially SS
• It is normally considered to calculate the
concentration ratio in the open systems compared to
that in the make-up water
• Can be used as indicator for a correct operating of
the cooling system
• It is expressed as mg/liter of Cl
13/06/2024 35

36. Impact of water
characteristics
• Silica
• Concentration of silica must be under control to
avoid severe scaling problem
• It can precipitate as silica or as magnesium
silicate
• Since the resistivity is high, the impact on heat
transfer is very critical
• Cleaning of silica deposits is very difficult
• It is expressed as mg/liter of SiO2
13/06/2024 36

37. Impact of water
characteristics
• Iron
• It can cause indirect deposition and corrosion
• High value of iron requires a specific pre-treatment
of the water
• Presence of iron at > 1ppm has significative impact
on the scale and corrosion inhibitor by incresing
the consumption by adsorption and chelation
phenomena
• It is expressed as mg/liter of Fe
13/06/2024 37

38. Impact of water
characteristics
• Organic matter
• It could be present in the make-up or adsorbed in
the circulating water from the atmosphere or leak
process or bacteria proliferation
• Presence of organic matter could cause foaming,
bacteria growth, which can initiate corrosion
processes
• It is expressed as mg/liter of KMnO4 or mg/liter O2
13/06/2024 38

39. Impact of water
characteristics
• Dissolved gases (CO2 / O2 / H2S / NH3 / light
hydrocarbons)
• Enter in the system with make-up, the atmosphere anf
process leaks
• Could cause severe corrosion problems, high bacteria
growth, react with the normal chemical treatments
• It is expressed as mg/liter of the gas
13/06/2024 39

40. Impact of water characteristics
• CO2
• Relationship
between
temperature (°C),
pressure and the
solubility of
carbon dioxide
(CO2) in water
13/06/2024 40

41. Impact of water characteristics
• NH3
• Relationship
between percent
ammonium ion
(NH4+) versus
temperature and
pH.
• As the pH and the
temperature rise,
the fraction of
NH3 in the NH4
+
form increases.
13/06/2024 41

42. Impact of water
characteristics
• Ether extractable
• Determines the presence of Greases & Oils
• Could affect the cooling system by promoting the
bacteria growth
• Could create deposits with bacteria growth and
initiating corrosion phenomena
• It is expressed as mg/liter as it is
13/06/2024 42

43. Impact of water
characteristics
• Manganese
• This element can be normally present in deep waters
• It is easily oxidized to form deposits, specially
in high temperature areas
• Could initiate galvanic corrosion by deposition of
the oxide with high electrochemical potential
• Presence of Mn at a value exceeding the recommended
value, need demanganizing pretreatment
• It is expressed as mg/liter Mn
13/06/2024 43

44. Impact of water
characteristics
• Copper
• It is rare to find significative concentration of
this element in the make-up waters (< 10 ppb)
• The source in the circulating water is normally the
corrosion process
• Can be deposited on carbon steel surface to initiate
galvanic corrosion
• It is expressed as mg/liter Cu
13/06/2024 44

45. Impact of water
characteristics
• Microbiological formations
• Include several different species (bacteria, algae,
fungi, yeasts) the proliferate in the ideal cooling
water conditions
• It is expressed as units per milliliter
• Microbiological growth is one of the most critical
factor for the cooling water system
• Produce bio-fouling with decomposition products
• Fast reduction of water flow and reduction of the
heat transfer
• Metabolize acidic environment and induced corrosion
(MIC)
13/06/2024 45
Microfouling

46. Mineral Salt
Scale
6/13/2024 46

47. Mineral Salt Scale
• Topics to be covered:
• Effects of Scale on Process
• Mechanisms of Scale Formation
• Factors Affecting Scale Formation
• Types of Commonly Encountered Scales
• Prediction of scale potential
• Scale Inhibition and Prevention
• Chemical Scale Inhibitors
• Control of Scale Inhibition Programs
• Norms and Economics
13/06/2024 47

48. Scale
• Effect of Scale on Cooling Water Systems
• Reduces heat transfer efficiency
• Decreases unit or production capacity
• Can promote corrosion & microbial growth
• Increases pump pressure requirements
• Higher operating costs/decreased profits
13/06/2024 48

49. Scale
• Impact on heat transfer
• Scale due to the formation of coherent and hard
deposits is the predominant cause of the reduction
of heat transfer
• It is formed with the precipitation of the scaling
salts by the effect of temperature increase, high
concentration (absolute and relative), pH and
alkalinity increase, and critical flu-dynamic
conditions
13/06/2024 49

50. Scale
• Impact on heat transfer
• From conduction heat transfer equation, is
derived the negative impact that deposits have
on the efficiency of the heat transfer
• The thermal resistance is determined by the
relationship
• where
• The resistance of the water side scale (Rde)
reduces the efficiency of the heat transfer
equipment
13/06/2024 50
Rd = 1 / Ud - 1 / Uc
1
Ud = ____________________________________
1 /he + Rde + Rk + Rdi Ae /Ai + Ae/Ai hi

51. Scale
Impact on heat transfer
13/06/2024 51
Type of deposit Thermal Conductivity
(W/m°C)
Calcium Carbonate 2.25 - 2.94
Calcium Sulfate 2.25
Calcium Phosphate 2.6
Magnesium Phosphate 2.25
Iron oxide 2.94
Biofilm 0.69
Iron metal 61,6

52. Scale
• Impact on heat transfer
13/06/2024 52
Insulating effect of foulant deposited uniformly
on clean heat-transfer surface
0.001
0.01
0.1
0
10
20
30
40
50
60
70
80
90
100
Heat transfer reduction, %
Scale
thickness,
in
SiO2
Clay
CaSO4
CaCO3
Al2O3

53. Scale
• Impact on heat transfer
• Economic value
• The thickness of the scale can be monitored by
both the increse of the pressure drop and the
reduction of the heat transfer
• The entity of the scale can be measured by
periodic computation of the “Global Heat
transfer Coefficient (U)”
• Comparing the value of the actual U with the
design U can be valued the loss of efficiency
and the increase of energy needed
• Added costs are:
13/06/2024 53
• Higher maintenance frequency
• Lower shelf life of the equipments – Under deposit corrosion
• Production loss
• Out of spec production

54. Scale
• Impact on heat transfer
• Economic value – Example 1
13/06/2024 54
Tube diameter = 12.7 mm
Deposit thickness = 1.6 mm
Water flow rate reduction = 40-50%
1.6 mm 12.7 mm

55. Scale
• Impact on heat transfer
• Economic value – Example 2
• Having 1 mm thickness of calcium carbonate scale in the
inside surface of the tubes in a shell-tubes exchanger –
water tube side – with the following design operating
conditions:
13/06/2024 55
Temp. outlet, (T2, t2) 46 41 °C
Specific heat, (Cp, cp) 0.56 1 Kcal/kg°C
Uc kcal/m2
.h.°C
Rd m2
.h.°C/kcal
Ud kcal/m2
.h.°C
Approach temperature, °C
Heat load (Q) Q=Wp x Cp x (T2 - T1) 1,713,600 kcal/h
Cooling water flow rate (Wc) Wc=Q / ((t2 - t1) x cp) 81600 m3
/h
Mean log temperature (LMTD) ((T2-t1)-(T1-t2)) / (Ln(T2-t1)/(T1-t2)) 32.06 °C
5
580
0.0004
471

56. Scale
• Impact on heat transfer
• Economic value – Example 2 (cont’d)
• We can calculate the extra energy cost of pumping to
compensate the heat transfer loss:
13/06/2024 56
Heat load (Q) Q=Wp x Cp x (T2 - T1) 1,713,600 kcal/h
Cooling water flow rate (Wc) Wc=Q / ((t2 - t1) x cp) 81600 m3
/h
Mean log temperature (LMTD) ((T2-t1)-(T1-t2)) / (Ln(T2-t1)/(T1-t2)) 32.06 °C
Heat transfer surface Q / (Ud x LMTD) 113.53 m2

57. Scale
• Impact on heat transfer
• Economic value – Example 2 (cont’d)
• Calculation of extra water flow needed
13/06/2024 57
Resistivity Calcium Carbonate 0.516 m2
.h.°C/kcal.m
Resistivity for 1 mm CaCO3 Rd 0.000516
Ui 1 / (1/Uc + Rd) 446.4 kcal/ m2
.h.°C
LMTDi Q / (Ui x S) 33.81 °C
(t2 - t1)x (found with solver) Q / (Wc x cp) 16.95 °C
t2i t1 + ((t2 - t1)i 36.95 °C
(T2 - T1)i (Wc x cp x (t2 - t1)i/ (Wc x Cp)) 34.00 °C
T2i T1 - (T2 - T1)i 46.00 °C
Approach temp. (T2 - t2)i 9.05 °C
Wci Q / (cp x (t2 - t1)i) 101,116 m3
/h
Wci - Wc 19,516 m3
/h
Qi Ui x S x LMTDi 1,713,600 kcal/h
Qi Wci*(t2i-t1) *cp 1,713,600 kcal/h
Qi Wc*(T2i-T1) *Cp 1,713,600 kcal/h
Check

58. Scale
Chemistry
• Four Requirements for Scale Formation
• Ion Supersaturation
• A Nucleation Site
• Adequate Contact Time
• Dissolution & Precipitation
13/06/2024 58

59. Scale
Chemistry
• Factors Affecting Scaling Potential
• Ion Concentration
• pH
• Time
• Temperature
13/06/2024 59

60. Scale
Chemistry
• Contributing Factors:
• Hydraulics and Flow Velocities
• Surface Characteristics
• Corrosion
• Fouling
• Microbial activity
• System design and operation
13/06/2024 60

61. Scale
Chemistry
• Contributing Factors (cont’d):
• Presence of Competing ions
• Latent period of precipitation
• Total Dissolved Solids (TDS)
• Total Suspended Solids (TSS)
• Co-precipitation
• Post-precipitation
13/06/2024 61

62. Scale
Chemistry
• Commonly Encountered Cooling Water Scales
• Calcium carbonate
• Calcium phosphate
• Iron phosphate
• Iron oxides
• Silicates
13/06/2024 62

63. Scale
• Chemistry
• Precipitation and crystal growing
13/06/2024 63

64. Scale
Chemistry
The solubility of calcium carbonate, main
component of scale, is function of
temperature and partial pressure of carbon
dioxide
13/06/2024 64
SOLUBILITY OF CALCIUM CARBONATE AS
FUNCTION OF TEMPERATURE
T °C Solubility of CaCO3 (ppm) at the p.p. (atm) CO2
Partial pressure
CO2 ----
0.0003
(P atm)
0.001 0.01 1.00 10.00
0 95 1600
10 75 1250
20 59 1000
25 53 78 170 900 2250
30 47 800
50 32 550

65. Scale
Chemistry
• Calcium carbonate
• Solubility as
function of
temperature
13/06/2024 65

66. Scale
Chemistry
• Calcium sulfate
• Solubility in
high salinity
water
13/06/2024 66

67. Scale
Chemistry
•Other Scales found in Cooling
Water Systems
• Magnesium silicate
• Silica
• Calcium sulfate
• Zinc phosphate
• Aluminum phosphate
• Calcium fluoride
13/06/2024 67

68. Scale
• Chemistry
13/06/2024 68
Magnesium silicate
Max concentration
of Mg
and SiO2 in
cooling water
systems

69. Scale
• Chemistry
13/06/2024 69
Silica
Solubility of different
silica state as function
of temperature

70. Scale
Chemistry
13/06/2024 70
Evaporative sea water cooling tower
Saturation curves of problematic scaling salts
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
Temperature
Seawater
concentration
facror
Experimented
operating
conditions
CaSO4.2H2O
CaSO4
CaSO4.1/2H2O
Mg(OH)2 pH=8.5
Mg(OH)2 pH=8.0 Mg(OH)2 pH=7.5

71. Scale
• Chemistry
13/06/2024 71
Deposit composition in sea water evaporator
0
10
20
30
40
50
60
70
80
90
100
60 66 71 77 82 88 93 99 104
Evaporator water temperature, °C
Composition
of
scale
CaSO4
CaCO3
Mg(OH)2
Nominal temperature
difference - 27 °C
Concentration factor = 2

72. Scale
Prediction
• To predict the scaling potential and/or the
aggressivity of the waters have been elaborated
several indices
• These are derived from thermodinamic and practical
experiences on natural waters
• These indices have low meaning in case the ionic
equilibrium is changed or, in presence of internal
chemical treatments that modify the natural
precipitation
13/06/2024 72

73. Scale
• Prediction
• Calcium carbonate indices
13/06/2024 73
CALCIUM CARBONATE INDICES
Index SCALING NO-SCALING
LI (Langelier) > 0 < 0
RI (Ryznar) < 6 > 6
DFI (Driving force) > 1 < 1
AI (Aggressiviness) > 10 < 10
ME (Momentary excess) > 0 < 0
LBI (Larson & Buswell) > 0 < 0
SDI (Stiff & Davis) > 0 < 0
PSI (Puckorious) < 6 > 6
CCPP (Rossum & Merril) > 0 < 0

74. Scale
• Prediction
• Langelier index (LI), with qualitative meaning,
determines the equilibrium pH of saturated water
with calcium carbonate, called saturation pH (pHs)
• This index is expressed by the relationship
• where:
• pH = pH actual
• pHs = pH saturation.
13/06/2024 74
LI = pH - pHs

75. Scale
• Prediction
• Langelier index
• It is calculated with the relationship
• Where
• pK'2 = - log10 second apparent dissociation constant of
carbonic acid
• pK' sp = - log10 solubility product of calcium carbonate
• pCa = - log10 (Ca) in mols/liter
• pAlk = - log10 (alk) in equivalents/liter
13/06/2024 75
pHs = (pK'2 - pK' sp) + pCa + pAlk

76. Scale
• Prediction
• Ryznar index
• Proposed by Ryznar, this index, called “Stability index” is
computed with the relationship
• Where
• pHs = saturation pH
• pH = actual pH
13/06/2024 76
RI = 2pHs - pH

77. Scale
• Prediction
• Ryznar Index
13/06/2024 77
CHARACTERIZATION OF THE WATERS BY RYZNAR INDEX
Ryznar Index Water characterization
4 to 5 High scaling
5 to 6 scaling
6 to 7 light scaling and/or aggressive
7 to 7.5 aggressive
7.5 to 9 very aggressiva
> 9 estremely aggressive

78. Scale
• Prediction
• Mineral solubility as function of cycles and pH in a open
cooling water system without pH control
13/06/2024 78
9
8
7
6
5
4
3
2
1
Cycles
number
5.5 6.0 7.0
6.5 8.0
7.5 9.0
8.5 9.5 10.0 10.5 11.0
CaCO3
Ca3(PO4)2(OH)
MgSiO3
SiO2
FePO4.H2O
Soluble
area
Insol.
Sol.
Not soluble:
FeO(OH)
Fe2O3
Fe3O4
pH
Sol. Ins.

79. Scale
Inhibition
• Methods
• Limit the Concentration of Precipitating Ions
• Alkalinity Reduction
• pH Reduction
• Cation Reduction
13/06/2024 79

80. Scale
Inhibition
• Methods (cont’d)
• Alter system design/operation
Velocity
Air Rumble
Modify Design
Metallurgy
Lower Heat Flux
13/06/2024 80

81. Scale
Inhibition
• Methods (cont’d)
• Add Chemical Scale Inhibitors
• Crystal Modifiers
• Chelants and Sequestering Agents
• Conditioners and Dispersants
13/06/2024 81

82. Scale
Mechanism of Chemical Scale Inhibitors
• To control the deposition of the scaling
salts are added specific chemicals capable
to be absorbed on the surface of micro
crystals limiting the growing process
• The absorption of the ions of these
materials represents the most important
factor for the control of growing process
• The kinetic of this process is dependant
also from the degree of super saturation
of the solution
• where:
13/06/2024 82
S = (Ci - Cf) / Cf
S = super saturation
Ci = initial concentration of salt
Cf = final concentration of salt at the steady state

83. Scale
Mechanism of Chemical Scale Inhibitors
13/06/2024 83
Scale of macrocrystal
Microcrystal layer
(corrosion protective) With inhibitor
Stabilization
Growth suppression Dispersion
Crystal modification
A
B
C
Microcrystal
solubility
Macrocrystal
solubility
A
C
B

84. Scale
Mechanism of Chemical Scale Inhibitors
• Crystal modification (organophosphonates)
• Modifies the Nucleation Site, allowing the
formation of stable microcrystals
• Sequestration (polyphosphates, polymers,
chelants)
• Formation of soluble ion complexes, preventing
precipitation reactions
• Scale conditioners (dispersants, lignins,
tannins)
• Prevent precipitants from depositing
13/06/2024 84

85. Scale
• Crystal modification
13/06/2024 85

86. Fouling
6/13/2024 86

87. Fouling
Some common foulants are:
• Silt, Sand, Mud and Iron
• Dirt and Dust
• Process contaminants, e.g. Oils,
Corrosion Products, Microbio growth
13/06/2024 87

88. Fouling
Factors which influence fouling are:
• Water Characteristics
• Water Temperature
• Water Flow Velocity
• Microbiological Growth
• Corrosion
• Process Contamination
• Environmental (i.e. atmospheric
pollutants)
13/06/2024 88

89. Fouling
Three levels to address the effects of fouling:
• 1. Prevention
• 2. Reduction
• 3. Ongoing Control
• Chemical Treatment
• Charge Reinforcers
• Wetting Agents
13/06/2024 89

90. Fouling
• Charge reinforcement mechanism
13/06/2024 90
Slightly Anionic
Suspended Particle
Suspended Solid
Which Has Adsorbed
Highly Anionic Chemical
Highly
Anionic
Chemical

91. Fouling
• Deposit break-up
13/06/2024 91

92. Bio Fouling
6/13/2024 92

93. Bio Fouling
Growth of microorganisms are affected by the
following
• Nutrients
• Temperature
• pH
• Location
• Atmosphere
13/06/2024 93

94. Bio Fouling
Types of Microorganism
• BACTERIA - need/ do not need Oxygen
• Aerobic - Slime and Spore former
• Anaerobic - SRB, Clostridia, etc.
• Iron bacteria – Gallionella
• Nitrification - Nitrosomas, Nitrobacter
• ALGAE - need light, food source
• FUNGI - destroys wood, reinforces deposits
• PROTOZOA - feed on bacteria/algae
13/06/2024 94

95. Bio Fouling
Microbiological Treatments
• Common Oxidizing Biocides
• Chlorine Gas
• Bleach – Hypochlorite
• Bromine Tablets
• Stabilized Chlorine and STABREX
• Ozone
• Non-oxidicing Biocides
• Aldehydes, Quats, Thiazolone, DBNPA
• Biodispersants
• do not kill but remove biofilm from surfaces
13/06/2024 95

96. Bio Fouling
• Mode of Biocidal Action
13/06/2024 96
Isothiazolone DBNPA
Glutaraldehyde
Quats
STABREX
Chlorine
Oxidants
enzyme

97. Bio Fouling
pH Effect on Chlorine/Bromine
13/06/2024 97
-
Cl + H O HOCl OCl + H
2 2
+
0
10
20
30
40
50
60
70
80
90
100
4 5 6 7 8 9 10 11 12
pH
HOCl
or
HOBr
in
%
0
10
20
30
40
50
60
70
80
90
100
OCl-
or
OBr-
in
%
HOCl
HOBr

98. Bio Fouling
Volatility of Common Oxidants
• Stabilized HOBr 0.1 X
• HOBr 1 X
• Bleach - HOCl 1
2 X
• Chlorine Dioxide 1
1,800 X
• Ozone 2
160,000 X
•
1
Blatchley et al., 1992
2
Montgomery, 1985
13/06/2024 98

99. Bio fouling
Impact on heat transfer
13/06/2024 99
0
10
20
30
40
50
60
70
80
0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5
Deposits in mm
Losses
in
%
Biofilm
Calcium carbonate

100. Corrosion
6/13/2024 100

101. Corrosion
• Basic discussion
• Closed systems
• Open systems
• Once through
13/06/2024 101

102. Corrosion
• An electrochemical process in which a
metal (i.e. iron, zinc, copper) in
it’s elemental form returns to it’s
native (i.e., oxidized) state.
• For example, iron is naturally present
as iron oxide (FeO, Fe3
O4
, Fe2
O3
). By
reduction process it is transformed to
native iron (Fe°)
• In presence of water, the elemental
iron tends naturally to transform
again to the oxide state, normally as
combination of Fe3
O4
and Fe2
O3
13/06/2024 102

103. Corrosion
• Principles of the corrosion cell
• The following elements are required for corrosion to
occur:
• a corrodible surface - one with electrons to lose (ANOD)
• a difference in potential - a driving force for the
electrons
• an electron acceptor - a place for the electrons to go
(CATHOD)
• an electrolyte, to close the circuit - conditions conducive
for electron flow
13/06/2024 103

104. Corrosion
• Principles of the corrosion cell
• If one of these factors is not available, the
corrosion does not occurs
• Main reactions in neutral-alkaline conditions, are
the oxidation of the metal at the anod and the
reduction of the oxygen at the cathod
• Basic principle of the corrosion phenomenon is that
the anodic corrosion rate is equal to the cathodic
corrosion rate
13/06/2024 104

105. Corrosion
• Principles of the corrosion cell
• Chemistry of the corrosion reactions:
• Anodic reaction
• Fe°--- Fe
+2
+ 2e
-
(oxidation)
• Cathodic reaction
• 1/2 O2+ H2O + 2e
-
--- 2OH
-
(reduction)
• At low pH value (<5) the hydrogen ion (H
+
) can take the
place of the oxygen and close the electric circuit
according to the following reaction
• 2H
+
+ 2e
-
--- H2 (reduction)
13/06/2024 105

106. Corrosion
Principles of the corrosion cell
13/06/2024 106

107. Corrosion
The Type of Corrosion is Determined by the
Environment at the Anode:
• Chemistry Anomalies
• Differential Ion Cells
• Differential Oxygen Cells
• Surface Anomalies
• Deposits
• Surface Imperfections
• Dissimilar Metals
13/06/2024 107

108. Corrosion
Principles of the corrosion cell
13/06/2024 108

109. Corrosion
Impact of corrosion
• Reduction of the heat transfer efficiency
• Higher frequency for maintenance and cleaning
• Equipment replacement
• Leaks on water and process side
• Unscheduled shut-down
13/06/2024 109

110. Corrosion
Principles of the corrosion cell
• Polarization
• Polarization of the anod occurs as the corrosion products
form a tick and uniform layer on the metal surface
• Polarization of the cathod occurs as the hydroxyl ions, the
hydrogen or a corrosion inhibitor, form a barrier that
impedes a further reduction of the such gases (O2)
• As these barriers are breaked (depolarization), the
corrosion starts again
13/06/2024 110

111. Corrosion
Principles of the corrosion cell
• Depolarization
• Main factors that can case the depolarization are:
• Water velocity (erosion, cavitation)
• pH
• High water velocity can remove the barrier causing the
depolarization
• As the pH drops, the concentration of hydrogen increses.
These ions will react with the hydroxyl ions at the cathod
that is then depolarized
• Since at low pH the solubility of the anodic corrosion
products increses, also the anod will be depolarized
13/06/2024 111

112. Corrosion
Principles of the corrosion cell
• Polarization
13/06/2024 112

113. Corrosion
Principles of the corrosion cell
• Impact of water velocity
13/06/2024 113

114. Corrosion
Principles of the corrosion cell
• Galvanic corrosion
• It is a particular type of corrosion that occurs when two
different metals are in contact between them and to the
electrolyte
• Is formed a corrosion cell where one metal is the anod and
the other is the cathod
• One example is the copper in contact with the carbon steel
where this become the anod because, being less noble, will
release more easily the electrons, and copper will be the
cathod. Loss of metal occurs at the anod side, then the
carbon steel will be corroded
13/06/2024 114

115. Corrosion
Principles of the corrosion cell
• Galvanic corrosion
13/06/2024 115
- Wider cathodic area >> higher corrosion rate
- Small cathodic area >> lower corrosion rate
- Wide anodic area >> generalized corrosion
- Small anodic area >> pitting corrosion

116. Corrosion
Principles of the corrosion cell
• Galvanic series in sea water
13/06/2024 116
Magnesium
Magnesium Alloys
Zinc
Galvanized Steel
Aluminum 1100
Aluminum 6053
Alclad
Cadmium
Aluminum 2024 (4.5 Cu, 1.5 Mg, 0.6 Mn)
Mild Steel
Wrought Iron
Cast Iron
13% Chromium Stainless Steel
Type 410 (Active)
18-8 Stainless Steel
Type 304 (Active)
18-12-3 Stainless Steel
Type 316 (Active)
Lead-Tin Solders
Lead
Tin
Muntz Metal
Manganese Bronze
Naval Brass
Nickel (Active)
76 Ni - 16 Cr - 7 Fe Alloy (Active)
Ni - 30 Mo - 6 Fe - 1 Mn
Yellow Brass
Admiralty Brass
Aluminum Brass
Red Brass
Copper
Silicon Bronze
70:30 Cupro Nickel
G-Bronze
M-Bronze
Silver Solder
Nickel (Passive)
76 Ni - 16 Cr - 7 Fe
Alloy (Passive)
67 Ni - 33 Cu Alloy (Monel)
13% Chromium Stainless Steel
Type 410 (Passive)
Titanium
18-8 Stainless Steel
Type 304 (Passive)
18-12-3 Stainless Steel
Type 316 (Passive)
Silver
Graphite
Gold
Platinum
Passive Passive Passive
Active Active Active

117. Corrosion
• Types of Cooling Water Corrosion
• General Etch
• Concentration Cell Corrosion
• Cracking
• Mechanical Damage
13/06/2024 117

118. Corrosion
Types of Cooling Water
Corrosion
•General Etch
• Metal loss in which a given area is alternately a cathode
and an anode. Metal loss occurs uniformly over the entire
surface
• This is the preferred type of corrosion.
13/06/2024 118

119. Corrosion
Types of Cooling Water
Corrosion
•Concentration Cell Corrosion
• A localized attack caused by a chemical anomaly
• Crevice Corrosion
 Under Deposit Corrosion
 Tuberculation
 Biologically Induced Corrosion
 Acid or Alkaline Corrosion
13/06/2024 119

120. Corrosion
Types of Cooling Water
Corrosion
• Tuberculation
• Highly structured mounds of corrosion products that cap
localized regions of metal loss
13/06/2024 120

121. Corrosion
Types of Cooling Water Corrosion
• Cracking
• Failures caused by the combined effects of
corrosion and metal stress. Initiate on
the surface exposed to the corrodant, and
propagate into the metal in response to
the stress state. The critical factors
are:
• Sufficient Tensile Stress
• A Specific Corrodant
13/06/2024 121

122. Corrosion
Types of Cooling Water
Corrosion
•Mechanical Damage
• Corrosion Fatigue
• Erosion – Corrosion
• Cavitation
• Dealloying
13/06/2024 122

123. Corrosion
Principles of the corrosion cell
• Localized corrosion cell
• When the metal is exposed to different concentration of a
species in solution
• If the concentration of the oxygen or chloride ion is
different close to two areas of the metal surface, can be
formed a corrosion cell
• This type of cell is called differential aeration cell or
differential concentration cell
13/06/2024 123

124. Corrosion
Principles of the corrosion cell
• Differential aeration cell
• Different oxygen concentration available for different
surface area of the metal
• Deposit formation create the ideal conditions to form the
differential aration cell by creating a barrier to the
homogeneus oxygen diffusion
13/06/2024 124

125. Corrosion
Principles of the corrosion cell
• Differential aeration cell
13/06/2024 125

126. Corrosion
Principles of the corrosion cell
• Differential Concentration cell
• Deposit formation can increase pH at the cathod and decrease pH
at the anod
• This phenomenon is called corrosion cell at different ions
concentration and occurs in a occluded cell
• In the occluded cell, a barrier is formed that is permeable only
to particular species such as the chlorides or hydrogen (H2)
• The negative ions of chlorides are accumulated to the anod to
balance the cationic Fe++ produced by meta oxidation
• In the rich hydrogen and chlorides environmental most of the
metal are more soluble
13/06/2024 126

127. Corrosion
Principles of the corrosion cell
• Differential ion concentration cell
13/06/2024 127

128. Corrosion
Principles of the corrosion cell
• Differential ion concentration cell
13/06/2024 128

129. Corrosion
Principles of the corrosion cell
• Differential ion concentration cell
13/06/2024 129

130. Corrosion
Principles of the corrosion cell
• Impact on corrosion - Dissolved solids
13/06/2024 130

131. Corrosion
Principles of the corrosion cell
• Impact on corrosion - pH
13/06/2024 131
100
10
0
5 6 7 8 9 10
Corrosion
Rate,
Relative
Units
pH

132. Corrosion
Principles of the corrosion cell
• Impact on corrosion - Dissolved gas
• Main interesting gases in industrial cooling water
systems are carbonic dioxide and oxygen
• As CO2 increases, pH will drop causing the
depolarization of the cathodic areas
•In low buffered water (i.e.
Condensate or demineralized
water), the pH is dropped more
easily than in buffered water
• Sulfuric acid is almost totally ionized as hydrogen
sulfate ion and bisulfate ion that cause the
depolarization of the anodic areas
• Ammonia increases the corrosion of copper and alloys
of copper by complexing the copper present in the
protective layer of copper oxide or copper carbonate
13/06/2024 132

133. Corrosion
Principles of the corrosion cell
• Impact on corrosion - Dissolved gas
13/06/2024 133

134. Corrosion
Principles of the corrosion cell
• Impact on corrosion – Temperature
• As rule of thumb, each 10°C of water temperature increase,
corrosion rate will double
• This because, increasing the temperature, rises the
diffusion rate of the oxygen at the cathod and the kinetic
of the cathodic reaction of the oxygen reduction
• The oxygen corrosion on carbon steel is maximum at 80°C.
This beacuse the solubility of oxygen decreases with the
increment of the temperature that is in opposition with the
kinetic of reaction increment with the temperature
13/06/2024 134

135. Corrosion
• Principles of the corrosion cell
• Impact on corrosion – Water velocity
• If the water velocity is too high, the corrosion products
formed at the anod are removed from the metal surface and
the protective film cannot be formed
• In general, corrosion increases with the increment of the
water velocity
• However, more generalized and uniform corrosion occurs with
the increment of the water velocity
• Increment of the water velocity reduce the corrosion of
some metals, like the stainless steel and aluminum, that
need the oxygen to form the passive film
13/06/2024 135

136. 13/06/2024 136
Factors impact on metal failure
• Fluid velocity
• Recommended fluid velocity for tube
material in salt water (m/s)
< 0.8 0.9-1.5 1.5-1.8
Copper
Tube
metal
1.8-2.7 2.7-3.6 3.6
Admiralty
C70600
C71500
C72200
70-30 2Fe 2Mn
SS
= good performance
= may give good performance, but may require a
closer study of the conditions at the site and
relationships with other factors
= material not performed well

137. Corrosion
General Methods for Corrosion Inhibition
• Use Corrosion Resistant Materials
• Apply Inert Barrier or Coating
• Use Cathodic Protection
• Adjustments to Water Chemistry
• Application of Corrosion Inhibitors
13/06/2024 137

138. Corrosion
Chemical Corrosion Inhibitors
• Mechanism
• Principally Anodic
• Principally Cathodic
• Both Anodic and Cathodic
13/06/2024 138

139. Corrosion
Anodic Inhibitors
• Function by adjusting the chemistry at the anode
(point of high potential)
• Chromate
• Molybdate
• Nitrite
• Ortho Phosphate (High Dose)
• Silicate
13/06/2024 139

140. Corrosion
Cathodic Inhibitors
•Function via reactions at the
cathode (point of high pH)
13/06/2024 140
electrons
O2
OH- OH- OH-
Cathode
Zn++ + 2 OH- Zn(OH)2
Ca++ + OH- + HCO3
- CaCO3 + H2O
Ca++ + OH- + H2PO4
- CaHPO4 + H2O

141. Corrosion
Cathodic Inhibitors
• Zinc
• Ortho Phosphate (low dose)
• Polyphosphate
• Phosphonates
• Calcium Carbonate
13/06/2024 141

142. Corrosion
Both Anodic and Cathodic Inhibitors
• Soluble Oils
• Azole Filmers (copper alloys protection)
 Mercaptobenzothiazole (MBT)
 Benzotriazole (BZT)
 Tolytriazole (TT)
13/06/2024 142

143. Corrosion
Closed systems
13/06/2024 143

144. Corrosion
• Closed system
• Operation
• Medium/High heat flux
• Use of demineralized water
• High residence time
• Presence of multi-metals
• Low/no water consumption (low/no fresh water / treatments)
13/06/2024 144

145. Corrosion
• Closed system – Operation
• Water characteristics
• Desalinated water quality (demineralized, softened,
condensate) is normally used to avoid scale problems
• However, desalinated water, not buffered, is very
aggressive to the metals, specially carbon steel and copper
alloys
• Biological fouling must also be under control to prevent
Microbiological Induced Corrosion (MIC)
• Good metal passivation is absolutely necessary to prevent
the damage of the equipments
13/06/2024 145

146. Corrosion
• Closed system - Metal protection
• Protection of closed circulating water systems
generally features both high pH and high inhibitor
levels
• System pH is most often held at 8,5-9,5 – This
reduce the corrosion of mild steel substantially
13/06/2024 146

147. Corrosion
• Closed system – Metal Protection
• Treatment (anodic)
• Must be capable to create a very strong passive barrier by
modifying the corrosion potential in the anodic direction
13/06/2024 147
Corrosion current
Equilibrium
potential
Anodic
Cathodic
Passive
potential
E
Log(i)

148. Corrosion
• Closed system – Metal Protection
• Basic treatments
• Chromates (200 – 1500 ppm)
• Restricted by envirnmental laws
• Pump seal failure at high dosage
• Nitrites (300 – 1500 ppm)
• Excellent mild steel protection, even for
pre-corroded surface
• Use borax as buffering agent (pH > 8,5)
• Nitrite is a nutrient for bacteria growth
(Nitrification)
• Bacteria produce low pH and slime deposits
• Molybdates (100 – 200 ppm)
• Week anodic inhibitor
• Often combined with nitrites and other agents
• High risk in case of under-dosing
13/06/2024 148

149. Corrosion
• Guideline for assessing corrosion*
13/06/2024 149
Metal Corrosion
rate
( mils /year )
Comment
Carbon steel 0 -2 Excellent corrosion resistance
2 - 3 Generally acceptable for all systems
3 - 5 Fair corrosion resistance; acceptable with
iron fouling-corrosion program
5 - 10 Unacceptable corrosion resistance:
migratory corrosion products may cause
severe iron fouling
Admiralty
brass
0 - 0.2 Generally safe for heat exchanger tubing
and mild steel equipment
0.2 - 0.5 High corrosion rate may enhance
corrosion of mild steel
> 0.5 Unacceptable high rate for long term;
significantly affects mild-steel corrosion
Stainless steel 0 - 1 Acceptable
> 1 Unacceptable corrosion resistance
*Indicated rates apply to general system corrosion
mm/year
0 to 0.05
0.05 to
0.075
0.075 to
0.13
0.13 to
0.25
0 to 0.05
0.05 to
0.13
>0.13
0 to 0.03

150. Monitoring
Detection methods for water problems:
13/06/2024 150
Problem Monitoring Technique
Corrosion Corrosion coupons
Electrical probe
Deposit monitor
Chemical analysis
Scale Deposit monitor - fouling of heat-transfer surface
Chemical analysis - pH, system balance
Deposit analysis
Biofilm Total counts - Microbiological differential analysis
Fouling factor