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Essential Expertise for Water, Energy and Air
SM
MOC Approach forOpen Cooling Water System
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Agenda
Mechanical Cooling tower
Heat exchanger
Metallurgy
Operation
pH
Cycle
Chemical Makeup water chemistry
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Essential Expertise for Water, Energy and AirSM
Cooling Tower
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Cooling tower performance provide significantcontribution to the plant performance and efficiency
Poor cooling tower performance could limit the plant
production
Why cooling is important?
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Impact of Cooling Water Temperature
A 3oC (5.4
oF) increase in
approach temperature (on all
stages) is equivalent to a 1%increase in energyconsumption
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Function of Cooling Towers
Remove heat from process operation
Mostly by evaporation (80%)
partially by sensible heat loss (20%) contact of hot water with cooler air
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
Obt i d f EDMS A d El t i C [P i t d 08 J l 2011]
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Cooling Tower Process
As ambient air is drawn past aflow of water, a small portion of
the water evaporate. The energy
released by water for
evaporation reduces the
remaining water temperature
Evaporation results in saturated
air conditions and lowers the
temperature of the water to a
value close to wet bulb air
temperature
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
Obtained from EDMS Approved Electronic Copy [Printed 08 Jul 2011]
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Types of Cooling Towers
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
Obtained from EDMS Approved Electronic Copy [Printed 08 Jul 2011]
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Crossflow Cooling Tower
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Counterflow Cooling Tower
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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12
Cooling Tower Fill
Purposes of Tower Fills
Increases the water surface
area for air contact
Increases contact time
Main Types of Fill Splash Fill
Film Fill
Tower Fill
pp py [ ]
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Fill Characteristics
SPLASH FILM
EFFICIENCY Medium High
DURABILITY Medium Low/Medium
FOULING Low High
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Problems with Film Fill
Easily fouled with
microbio and solids
Hard to clean once
fouled
Quick loss of
efficiency
Tower Fill
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Factors Affecting Cooling Tower Performance
Wet bulb temperature
Dry bulb temperature
Plant (heat) load
Cooling waterT
Cooling water flow No of pump running
Air flow
Fan (tip, angle, motor; etc)
Cooling water distribution
Distribution system Fill cleanliness
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Wet Bulb Temperature Measurement
A measurement of the lowest temperature that can
be achieved by evaporative cooling of a water
wetted ventilated surface.
Sling Psychrometer
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Liquid to Gas Ratio
TYPICAL L/G
L/G = 2.0 PERFORMANCE
CURVES
L/G = 1.5
L/G < 1, TOWER OVERSIZED
L/G = 1.0 - TURN OFF FANS OR DOWNSIZE
L/G > 2.5, TOWER UNDERSIZED
AND WILL NOT PROVIDE LOW
APPROACH TEMPERATURES
WET BULB TEMPERATURE, OC
14 16 18 20 22 24 26
27
37
25
29
31
33
35
COLDWELLTEMPERATURE,
OC
12
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Air Flow Issue
18
RO TA T I O N
Air Recycling(Incorrect Tip)
Fan Stall
(Incorrect Pitch)
Air In-leakage
From Fan Shaft
Air flow is a critical component to
ensuring the proper heat
rejection from a cooling system
Many cooling tower performance
issues may well be airflow
problems
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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What is the Correct Way toEvaluate Cooling Tower Performance ?
1) CW temperature range (T)
2) Cooling tower heat load
M x Cp x T
3) CW supply temperature
4) Approach temperature
5) Evaporation rate
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Performance Monitoring Cooling Tower
GPSA Nomograph:
(require T& wet bulb
temperatures)
Performance Factor proportional to:
L/G ratio (L = CW flow, G = air flow
both in kg/hr)
RR (or L)
1/(Fan Power)1/3
Performance Factor > design means
performance deteriorated
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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21Essential Expertise for Water, Energy and AirSM
Essential Expertise for Water, Energy and AirSM
Heat Exchanger
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Why is This Important?
A critical component of the cooling system
Affect efficiency
Fouled exchangers also decrease throughput and capacity
Leaking exchangers can force unscheduled shutdown
Also affect water treatment options
product selection and control limits
Water Distribution System
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Heat ExchangerWhat We Need to Know ?
Type of heat exchangers
Hydraulic, mechanical and
metallurgical
Mechanical stress to cooling water
treatment Performance monitoring
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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24
There are several general types of heat exchangers:
Shell-and-tube
Plate-and-frame
Spiral flow
Type of Heat Exchangers
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Shell and Tube Heat Exchanger (Water in Tube)
Most common heat
exchanger type found
in cooling water
systems
Typically has
adequate velocity
Throttle and spatial
relationship could
create problem
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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A shell side heat exchanger offers challenges
Low velocity
High skin temperatures
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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27
Mechanical Stresses
Three main mechanical stress factors that impact theefficiency of heat transfer in heat exchanger are:
1)Cooling Water Velocity
2)Skin Temperature
3)Heat Flux
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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28
Cooling Water Velocity Stress Range
Scale and fouling deposition is more prone to occur at low
velocity
Lower flow rate, lower turbulence that can lead to static
thicker film of water at metal surfaces reduces heat
transfer
Lower velocity = higher skin temperature
> 1 m/s
0.6-1 m/s0.3-0.6 m/s
< 0.3 m/s
Mild Stress Minimal effect of reliability
Moderate Stress Needs to be consideredHigh Stress Will typically be a problem
Severe Stress Typically a problem
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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29
Heat Flux Stress Range
High heat flux will lead to high skin temperature
Typically
v 3.1515.8 kW/ M2 : mild steel
v 15.831.5 kW/ M2 : alloy
v 31.563.1 kW/M2 : copper or stainless steel
< 7.5 kW/m2
7.5-25 kW/m2
25-40 kW/m2
> 40 kW/m2
Moderate Stress Needs to be considered
High Stress Will typically be a problem
Severe Stress Typically a problem
Mild Stress Minimal effect of reliability
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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30
Skin Temperature Stress Range
Reasons:
Corrosion rates ~double for each 10C increase in metaltemperature
Scaling tendencies become much more pronounced at high
temperature
Film boiling may occur as skin temperature
Some treatment chemicals break down at high temperature
< 500
C50 - 600C
60 - 700C
> 700C
High Stress
Severe Stress
Minimal effect of reliabilityNeeds to be considered
Will typically be a problem
Typically a problem
Mild StressModerate Stress
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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31
Other Important Mechanical Factors
1) Spatial Relationship
2) Metallurgy
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Spatial Relationship and CW Distribution
127C
124C
101JC 2002JC
116C
128C
1st
2nd
3rd
101JCoolers
108C 146C115C
Primary Cooling Water Header Tertiary Cooling Water HeaderSecondary Cooling Water Header Cooling Water Return Header
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Metallurgy
Copper alloys
High thermal conductivity
Sensitive to ammonia and oxidant
Need film forming inhibitor
Titanium
Self passivating metal
Extremely brittle with very thin walls
Leaks common (especially during initial
commissioning and startup) ANY mechanical impingement can cause leaks
Stainless Steel
Self passivating metal
Subject to chloride stress corrosion
Must ensure cooling water chloride level
remains within limits
Under-deposit corrosion, MIC issues
Ammonia
Grooving
Under Deposit
Corrosion / MIC
Pitting / MIC
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Chloride and Stainless Steel
PLANT DATA
Duty 3,286,000 Btu/hr
Area 424 ft2
U 91.2Btu/hr.ft2.C
Heat Flux 7750Btu/hr.ft2
Velocity 6.111 ft/s
T CW in 91F
T CW Out 100F
Process In 637F
Process Out 300F
CALCULATION
T Skin 106.34F
41.3C
Max T Skin 146F
62C
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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35
Importance of Heat Exchanger Monitoring
Prevent the loss of cooling
Predict Onset of Problems
Potential Surprises - scaled exchanger
Loss of production
Useful to troubleshoot problems
Methodology using appropriate data
Documentation
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Heat Exchanger Monitoring
Approach Temperature Generally could represent HE performance
Approach = T Process Exit (air) T Cooling Water Inlet(counter current)
Others: C Factor, U Value, Fouling Factor; etc
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Cooling Water OperationWhat We Need to Know ?
Basic mass balance calculation
Cycle management
Operational stress factor
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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39
EvaporationDrift
Windage Windage
Blowdown
LossLeaks
Makeup Cycled Water
Cooling Tower Mass Balance
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Blowdown to Reduce Cycles
500 ml
150 umhos
500 ml150 umhos 500 ml100 umhos 1000 ml125 umhos
1000 ml
150 umhos
Cycles = 125/100
= 1.25 Cycles
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Blow Down and Cycle
Blow down portion of the water ejected or drained from a
system to control solids in cooling water
Cycle number of times that dissolved minerals in cooling
water are allowed to concentrate. Cycle is controlled by
blowdown
Cycle = Dissolved Solids Concentration in Cooling Water
Dissolved Solids Concentration in M-Up
In general, the cycle of concentration allowed depends on both
the level of solids in the make-up and the level of solids that
can be tolerated in cooling water
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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42
Basic Cooling Tower Calculations
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Holding Time Index (HTI)
HTI is indicate the time required to reduce the
chemical added to the system to 50% of its original
concentration
HTI = 0.693 x Holding Volume
Blowdown Rate
It is important to select the right chemical program
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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44
Operational Stresses
Variation of operational and control parameters cause
stress in cooling water systems. The key examples of
operational stresses as follows
1)Variation of cycle
2)Variation of pH
3)Variation of HTI
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Variation of Cycle
80%
Mild Stress Minimal effect of reliability
Severe Stress Typically a problem
Moderate Stress Needs to be consideredHigh Stress Will typically be a problem
Low High Total
17.7% 5.3% 23.0%
% Out of Spec
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Variation of pH
+ 0.1
+ 0.2+ 0.4
+ 0.6
Mild Stress Minimal effect of reliability
Severe Stress Typically a problem
Moderate Stress Needs to be consideredHigh Stress Will typically be a problem
Control Lower Upper
Parameter Spec Spec LCL Mean UCL STD
pH 7.50 8.00 7.16 7.72 8.28 0.19
Statistical Calculations
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Variation of HTI
< 40 hrs40 - 100 hrs
100 - 200 hrs
> 200 hrs
Mild Stress Minimal effect of reliability
Severe Stress Typically a problem
Moderate Stress Needs to be considered
High Stress Will typically be a problem
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Cycle and pH Operating Window
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Cycle Management
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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50Essential Expertise for Water, Energy and AirSM
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Main Problems in Cooling Water OperationRelated to Water Treatment
There are basically four inter-related problems in the operation of acooling water system as depicted by the Cooling Water Treatment
Triangle below. Each problem affects and is affected by the other
problems.
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Corrosion
A natural process convertingprocessed metals to their
native states
Factors affecting:
- Water chemistry
- Physical environment
(temperature, velocity,
hydrodynamic)
- Dissolved gases
- Halogen or other oxidizers
- Deposit
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Scale
A dense, adherent layer ofminerals tightly bound to
itself and to metal surface
Factors influences
- pH
- Minerals concentration (Ca,
Mg, SiO2, Alkalinity;etc)
- Temperature
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Fouling
Deposits that formed frommaterial suspended in water
(clay, silt, iron, manganese,
microbiological)
Factors affecting
Suspended solids
concentration
Hydraulic and flow velocity
Spatial relationship
Scale, corrosion and
microbiological
Mi bi F li
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Microbio Fouling
The most prevalent problems inindustrial cooling water systems
A good control of microbial
growth is essential if we are to
control the other water
chemistry problems
Factors affecting:
Suspended solids
Contaminants (organics,
ammonia, phosphate; etc)
Physical factor (velocity and
hydrodynamic) pH
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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General Methods forCorrosion Inhibition
Use Corrosion Resistant Materials
Apply Inert Barrier or Coating
Use Cathodic Protection
Adjustments to Water Chemistry
Application of Corrosion Inhibitors
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Corrosion Inhibitors Cathodic & Anodic
Combinations of both anodic and cathodic inhibitors generally provide the best
protection.
These combinations are called synergistic because the combination provideslower corrosion rates than either inhibitor could alone,
Cathodic inhibitionPrevents the transfer of electrons
prevent the reduction of oxygen.
prevents the dissolution of the base metal,
iron.
Anodic inhibition
Saturation Index as a Tool for Scale Control
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Saturation Index as a Tool for Scale Control
WATER
CHEMISTRY
pH TEMPERATURE SYSTEMDESIGN
SATURATION INDEX
PROGRAM DESIGN
VARY
CYCLE VARY
pHVARY
TEMPERATURE
VARY
CHEMISTRY
Saturation Index Reference Chart
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Saturation Index Reference Chart
Calcite 6 80 140
Aragonite 7 80 140
Anhydrite 1.2 3 4
Gypsum 1.2 4 6
Tricalcium Phosphate 50 1000 1500
Hydoxyapetite Insufficent Data Insufficent Data Insufficent DataFlourite Insufficent Data Insufficent Data Insufficent Data
Silica 1.1 1.2 1.5
Brucite 1.1 1.2 1.5
Magnesium Silicate 6 7 8
Iron Use of SI not indicative of iron fouling problems
Mineral Scale Mild Stress ModerateStress
High Stress
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Performance Comparison Scale Inhibitors
Capability AMP PSO HEDP PBTC PAPEMP
Corrosion Inhibition 2 5 3 1 1
CaCO3 SI 105 80 100 180 200+
CaSO4 734 483 130 257 574+
Cl2 Stab 2 5 3 5 3
Thermal Stability 3 5 2 5 4
Note:
5 Best, 1 Worst
Nalco Chemical Programs for
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Nalco Chemical Programs forScale and Corrosion
Under-saturated(corrosive)
Over-saturated(Scale)
PSO-Phosphate
Calcium Carbonate(Calcite)Saturation
Stabilized Phosphate
All organic
pHreedom
Low pH
Low AlkalinityHigh environmental impact
High pHHigh Alkalinity
Least environmental impact
Alkaline Zinc
Function of Dispersant
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Function of Dispersant
Prevent deposition commonly found
minerals & inorganic particulates such as
Calcium phosphate
Iron,
Silt
Zinc
Keep corrosion inhibitor soluble To be able function the inhibitors must be
dispersed in the water otherwise localized
deposition will occur
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Mechanism of Chemical Scale & Fouling Inhibitors
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Mechanism of Chemical Scale & Fouling Inhibitors
Scale Inhibitor Mechanisms Treshold Inhibition & Crystal Modification
Sequestration
Scale conditioner (dispersant)
Fouling Inhibitor Mechanisms
Dispersant
Surfactants
Calcium Phosphate Crystal
Mechanism of Chemical Scale & Fouling Inhibitors
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Microbiological Control
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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The goal is not to STERILIZE the system, but
to MANAGE microbial fouling to a level that
minimizes mechanical, operational, and chemical
problems at an acceptable cost.
Microbiological Control
Bio-Control Practices in
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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67
Bio Control Practices inRecirculating Cooling Water
Oxidizers (are usually the primary biocide)
Chemically oxidize organiccomponents of the cell
Effective against nearly allmicroorganisms
Chlorine: as gas or bleach Bromine: as NaBr or other bromine
donors
Chlorine Dioxide
Bio-Control Practices in
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Recirculating Cooling Water
Non-Oxidizing biocides
Organic compounds, react
with cell components
Disrupt cell wall, metabolism,
or reproduction
Effective to control specificorganisms
Algae control
Biofouling control
Pathogen control
Improved Bio-Control / Bio-
Manage practices
Isothiazoline DBNPA
enyzm enyzm
Glut
amino
Quat
Bio-Control Practices in
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Recirculating Cooling Water
Biodispersant & Bio-detergentsDisperse deposited particles
Improve the efficacy of biocides
Foaming could limit its application
Before addition of biodetergent
After addition of biodetergent
Other Scale/Fouling Control Methods
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Mechanical
Providing more cooling water flow HE modification
Air rumbling, reverse flow and ball cleaning
Install side stream filter
Operational factor Cycle management
pH control
Summary
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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ASUs offers water treatment challenges High skin temperatures
Low flow
Control capability
Water chemistry
Contamination Program limitations
Understanding the MOC enables a complete
understanding of the system stresses that need to be
addressed
Summary
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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M knowing the system CT performance, H/E data, flows, temperatures, locations, etc.
Determine potential trouble spots.
Determine chemical program limitations.
O Operational variables
Control is very important for getting performance. Automationis best.
C Chemical variables Scale and corrosion modeling with programs available.
Determine control limits.
Obtained from EDMS, Approved Electronic Copy. [Printed 08 Jul . 2011]
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Thank You!
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