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COOLING TOWER
8. COOLING TOWER
• Purpose of a cooling tower is to provide cool water at a certain temperature.
• The water used in a cooling tower is cooled by evaporation and is reused many times.
Cooling Water
• The original method of cooling Cooling Water system is what is called straight pond type.
• By 1920 the spray pond was the system generally in use.The water was sprayed into the air to break it up into smaller
particles, by discharging it through spray nozzlesThe water issuing from the nozzle created a draft which ,
aided by natural breeze, effected the necessary evaporationThe objections to the method were its low cooling capacity
and high water losses
Cooling Tower
The lost of water blown away in droplet form amount to 5 to 10 percent of the water circulated.
This is expensive and often damaging to nearby property and equipment
The result was installation of fences around pond.
thus , the spray type cooler was developed.
Cooling Tower
• The first natural draft type of cooling tower were built in 1920-30. These cooling towers were often called atmospheric type of towers.
• After a time the forced draft and induced draft towers were developed.
Cooling Tower
Advantage• Corrosion and scaling of cooling equipment is not as severe as with
untreated river or well water
• Cost of water is comparatively lower since it is reused many times.
• The large lines, pumps, and sewer used to transfer water from river to a unit and back to the river would very expensive
Fundamentals Governing Cooling
• Fluids require heat to change from a liquid state to a gaseous state and they give up heat in changing from a gas to a liquid. (When water is vaporized the liquid is cooled)
• The temperature at which a change of state occurs is constant during the change, but this temperature will vary with pressure. [Increase in pressure requires increase in temperature to change water to vapor].
• Heat always will flow from a body at a higher temperature to a body at a
lower temperature.
Evaporation
A large factor that has decided effect on cooling towerevaporation is the process of evaporation.
Laws of evaporations are reviewed here:• Evaporation increases as temperature increases.• Evaporation increases with the extent of exposed surface• Evaporation is much greater in dry air than in air containing vapor.• Evaporation increases as the vapor is removed from the surface of the
liquid.• The rate of evaporation is determined by the pressure on the exposed
surface (atmospheric pressure).
Type of Cooling Towers
1. Natural draft Also called atmospheric or open type, is one in which the air
movement through the tower is dependent only upon atmospheric condition
2. Forced draft A mechanical-draft tower in which the fans located at the inlet or
base of the tower and is forced upward through the top at lower velocity
3. Induced draft A mechanical draft cooling tower is a mechanical draft tower having
one or more fans installed at the air outlet of the tower
Cooling Tower: Mechanism of Heat Transfer
The cooling medium used in cooling towers is air. The amount of moisture that air can absorb varies with its temperature and increases greatly with a rise in temperature. The function of a cooling tower is to bring air and water into intimate contact so that the water will be cooled by the air.
This cooling or interchange of heat is accomplished by convection and evaporation. In a tower, convection is taking place between the air and the water. Heat is also being dissipated through evaporation.
For any liquid to evaporate, it must gain heat from some source. The relatively dry air is becoming laden with moisture from the evaporating water. The water that is now vapor has taken heat from the liquid water and, in turn, reduced the temperature of the liquid.
Cooling Tower
COOLING TOWER
COOLING TOWER
COOLING TOWER
Cooling Tower
Cooling Tower
Previous figure provides illustration of a mechanical draft type
cooling tower.
Advantage of mechanical draft tower over natural draft:
1. Require less space
2. Require less piping
3. Require only about half the pumping head, which varies from 10 to 30 ft. Height of mechanical draft tower does not have to be as great as a natural draft tower.
4. Realize improved operations due to colder water temperature
5. Are independent of wind velocity, hence, they can be designed for more exacting performance.
Mechanical Draft Tower
The primary features of this tower are:
1. Fan
2. Water return pipes
3. Water distribution box
4. Hot water basin
5. Fill
6. Cold water basin
7. Drift eliminator
Mechanical Draft Tower
Operations1. Air flow
• Air flow is created by use of propeller located in the top of the tower
• Air is drawn in through slats on the sides of the tower and discharged to the atmosphere from the top of the tower (Induced draft)
2. Water flow
• Water is pumped through plant cooling equipment• The returning hot water flows to the top of the tower through
the return system piping• The hot water flows into the hot water basin through the
distribution box at the top of the tower. The hot water basin is perforated to allow the water to drip through the tower in many small flow streams
Mechanical Draft Tower
• The space below the hot water basin is packed with “fill” which consists of staggered rows of perforated plates. The purpose of the fill is to expose as much water surface to the cooling action of the air flow as possible.
3. Cooling tower• Illustrated tower design is called “cross flow” because the air
crosses the path of the water• Water cascades down through the fill section• Air is pulled across the flowing water and exhausted out the
top of the tower.• The water is cooled by contacting with the air
Cooling is accomplished primarily by evaporation of a small portion (2/3 % of the circulating water)
Temperature of water reduced by 25/30 oFEvaporation loss account to about 75 % of make up
water.
Cooling Water Management Fundamentals
1. Cycles of Concentration
2. Make Up - The loss of water by evaporation, blowdown, and windage requires that water be added to the system
3. Blowdown - Because of the loss of water from evaporation, the dissolved solids in the water are concentrated. - This would cause mineral deposits in the system if a portion of the water were not drained off and replaced with fresh water. This drain off is called blowdown. - This blowdown prevents the solids from building up in the water
and coating or fouling the cooler surface
Cooling Water System Problems
1. Scale – A dense adherent layer of minerals tightly bound to itself and to metal surface
2. Corrosion – A natural process converting processed metals to their native state.
3. Fouling – Loose non-adherent deposits made up of insoluble particulates present in the make up water or introduced to the cooling system by process leaks, wind or microbial growth.
Deposit Control
Deposits are conglomerates that accumulate on water wetted
surfaces and interfere with system performance, either by gradually
restricting flow or by interfering with heat transfer.
– Deposits include scale, foulants, or combination of the two.
– Scale forms when the concentration of a dissolved mineral exceeds its solubility limit and the mineral precipitates
- Langelier index or Stability index indicates a condition of CaCO3 supersuturation.
Deposit Control (con’t)– Foulant is any substance present in the water in an insoluble
form, such as:
1. silt
2. oil
3. process contamination
4. biological masses.
– Deposits are most often an accumulation of sediments or settled solids that drop out of at some point in a system where the water velocity falls to a level too low to support the material in the stream.
Deposit Control
Deposition: Likely order of events
1. - Silt from make up water may begin to deposit in a low flow
section of a heat exchanger
- If water is on the border of CaCO3 instability, the settling solids
may act as the initiator for the scaling mechanism, further
obstructing flow, and deposit (silt and scale) will form
- Dormant microbial organisms may become active, and if
the deposit were analyzed, all 3 constituents would be found.
2. - The sequence could have started with microbial activity
blocking the flow, causing the codeposition of the other 2
constituents
Deposit Control
Deposit Sources
The sources of potential depositing material may be:
1. External to the system
1. Water supply itself
- suspended solid such as silt
- soluble or precipitated iron
- manganese
- carryover from clarifier or other pretreatment unit
2. Air particularly in an open recirculating cooling system with cooling tower. Cooling towers act as large air scrubbers capturing
- dust
- microbes
- debris
Deposit Control
Deposit Sources (con’t)
external to the system con’t:
3. Industrial gases
- ammonia
- hydrogen sulfide
- sulfur dioxide etc
they react chemically, changing the water characteristics. Sometimes this change can be so dramatic that scaling, corrosion, or microbial masses may suddenly obstruct a system in a matter of days.
4. Leakage of process fluids into a water stream.
This leakage may contribute directly or indirectly to the deposits. Most common effect is to provide food for microbial growth.
- organic such as oil or food substances
Deposit Control
Deposit Sources
external to the system con’t:
5. Miscellaneous external sources
- water used in pumps
- lubricant applied to valves, pump glands and bearing
that leak into system.
Deposit Control
Deposit Sources (con’t)2. Internal to the system (originating from circulating system) 1. chemical precipitation 2. formation of corrosion products 3. polymerization 4. biological growth Chemical precipitation - is usually induced by temp change or disturbance in the equilibrium of system
Polymerization of organic – example is coagulation of proteins w/c occurs when water temp reaches 60 to 65 oC Biological activity - may be encouraged by nutrients and food substances present in the water.
Treatment to Control System Deposits
Chemical treatments to control system deposits1. Threshold inhibitors2. Dispersants3. Surface active agents4. Crystal modifier
1. Threshold InhibitorsThis includes sequestering agents such as polyphosphates,organophosphorous compounds, and polymers (ie polyacrylates)These exerts a “threshold effect” reducing the potential for precipitation of calcium compounds, iron and maganese. Threshold inhibition causesa delay in precipitation by application of substoichiometric amount ofInhibitor. This threshold dosage is possible because chemical adsorbs Only on the surface of the incipient precipitate, so that only a smallFraction of the precipitating material consumes the active inhibitor.
(con’t)
2. Dispersants
Organic dispersants include organophosphorous compound and
Polyelectrolytes. Polyelectrolytes will disperse suspended solids by
Adsorbing to their surfaces, adding electrostatic charge to each
particle, causing mutual repulsion.
Other dispersants condition the surfaces of the suspended solids in
other ways to keep them from coagulating and settling.
3. Surfactants
They are surface active chemicals. Those that penetrate and dispere
Biomasses are called biodispersants. Some are surface active agents
are effective wetting agents and anti foulants which help fluidize solids
and keep them moving with the flowing water. Others emulsify
hydrocarbon and removed by blowdown.
(con’t)
4. Crystal modifiers
Presence of particulates induces precipitation of scale from
supersaturated solution. Scale is often one of the fraction of a deposit.
Although precipitation is not prevented by crystal modifier, the resultant
material that precipitates is structurally weak – more like a foulant than
scale.
Corrosion
Corrosion is the deterioration of a substance (usually meta) or its properties by either chemical or electro-chemical reaction with a given environment.
Why metals corrode.
Most metals are found in nature as “ores” which are metallic oxide. The most common form of iron ore is an oxide called hematite (Fe2O3). Rust is converted to iron by the addition of energy (during refining) and this same energy is expended when the iron converts back to rust due to corrosion.
It is the energy stored in the metal during the refining process which makes corrosion possible. This energy supplies the driving force for corrosion.
Corrosion
Nature of corrosion reaction• Nearly all corrosion problems are due to the presence of water.
Corrosion in the presence of water is an electrochemical process.• Electrical current flows during the corrosion process.• In order for current to flow, there must be a driving force, or a voltage
source, and a complete electrical circuit.
Corrosion
Voltage source• In order for current to flow, there must be a driving force, or a voltage
source, and a complete electrical circuit.
Corrosion
Corrosion is nature’s way of returning processed metals, such assteel, copper, and zinc to their native states as chemical compounds orminerals. For example iron in its natural state is an oxidized compound
(ie Fe2O3, FeO, Fe3O4), but when processed into iron and steel it losesoxygen and becomes elemental iron (Fe)
In the presence of water and oxygen, it revert back to an oxide (Fe2O3
And Fe3O4) Corrosion reaction1. Loss occurs from that part of the metal called the anodic area
(anode). In this case, iron (Fe0) is lost to the water solution and becomes oxidized to Fe2+ ion.
2. As a result of the formation of Fe2+, two electrons are released to flow through the steel to the cathodic area (cathode).
3. Oxygen (O2) in the water solution moves to the cathode and completes the electrical circuit by using the electrons that flow to the cathode to form hydroxyl ions (OH-) at the surface of the metal.
(con’t)
Chemically, the reactions are as follows:
Anodic reaction: Fe0 Fe2+ + 2e-
Cathodic reaction: ½ O2 + H2O + 2e- 2(OH-)
In the absence of oxygen, hydrogen ion (H+) participates in the reaction at the cathode instead of oxygen, and completes the electrical circuit as follows:
2H+ + 2e- H2
anode cathode
Fe2+
2e-
O2OH-
Corrosion Rate
• As noted before, 3 basic steps are necessary for corrosion to proceed. If any step is prevented from occurring, then corrosion stops. The slowest of the 3 steps determines the rate of the overall corrosion process. The cathodic reaction (step 3) is the slowest, so it determines rate of corrosion. This is due to difficulty of oxygen encounters in diffusing through water.
• One factor in increasing corrosion then is increasing water temperature, which reduce its viscosity and speed the diffusion of oxygen.
• A large cathodic surface area relative to the anodic area allows more oxygen, water, and electrons to react. Increasing the flow of electrons from the anode to corrode it more rapidly.
• Conversely, as the cathodic area becomes smaller relative to the anodic area, the corrosion rate decreases.
Polarization/Depolarization
Polarization• As noted earlier, hydroxyl ions (OH-), hydrogen gas (H2) or both, are
produced at the cathode as a result of the corrosion reaction• If these products remain at the cathode they produce a barrier that
slows the movement of oxygen gas or hydrogen ions to the cathode• This barrier becomes a corrosion inhibitor because it insulates or
physically separates oxygen in the water and the electrons at the metal surface
Depolarization• The removal or disruption of this barrier exposes the cathode and
corrosion resumes
Polarization/Depolarization
(con’t)
Depolarization• Barrier removal is enhanced by two factors:
1. Lowering the ph of water. This increases the concentration of the hydrogen ions reacting with hydroxyl ions to form water, thereby eliminating the hydroxyl barrier
2. Increasing water velocity into the turbulent flow region tends to sweep away hydroxyl ions and hydrogen from the surface of the cathode, therby depolarizing it.
(con’t)
Metal surface is covered with innumerable small anodes and cathodes develop from:
1. Surface irregularities from forming, extruding and other metalwork
operations
2. Stresses from welding, forming, or other work
3. Compositional differences at the metal surface
(different microstructure)
Types/Form of Corrosion
1. Galvanic corrosion
2. Concentration cell corrosion
3. Stress corrosion cracking
4. Caustic embrittlerment
5. Chloride induced stress corrosion cracking
6. Corrosion fatique cracking
7. Tuberculation
8. Impingement attack
Type/Form of Corrosion
Galvanic corrosion• Two dissimilar metals are connected and exposed to water
environment• One metal becomes cathodic, other becomes anode
example – copper and steel, steel becomes the anode. It is said to be anodic to copper w/c is the cathode
• Fixed concentration of water
Type/Form of Corrosion
Concentration cell corrosion• Single metal exposed to different concentration (ionic strengths)
of water solution• Galvanic current – attack occurs at the anode• Take place in the concentrated solution (concentration cell)
Type/Form of Corrosion
Stress corrosion cracking• Corrosion environment• Tensile stress
1. external force which causes stretching, bending
2. internal stresses locked in metal during fabrication, rolling,
shaping, welding the metal
Caustic Embrittlement• Type of stress corrosion that sometimes accurs in boilers• Caused by high concentration of NaOH in boiler water• High stress such as where the boiler tubes are rolled into the
drum• Water must contain silica, which directs the attack to grain
boundaries leading to intercrystalline attack
Type/Form of Corrosion
Chloride induced stress corrosion cracking• Caused by chloride concentration and tensile stress focused
together to cause both inter-granular and trans-granular branch-type cracking
• Type of stress corrosion cracking induced by a chloride concentration cell
• Chloride level in water is not much of a factor• Main factor is the existence of conditions that allow chloride
concentration cells to develop
Type/Form of Corrosion
Tuberculation• Is the results of a series of circumstances that cause various
corrosion process to produce a unique nodule on steel surfaces
1. Initially, metal ions are produced at an anodic site
2. A high ph, caused by hydroxyl or carbonate ions, encourages
iron to redeposit adjacent to the anodic area
3. Mechanism continues until the original anodic area is pitted
from metal loss and the pit is filled with porous iron compounds
forming a mound
4. Within the tubercle, the aquatic environment is high in
chlorides and sulfates and low in passivating oxygen
5. As a result, both oxygen differential cells and concentration
cells forms
6. Advanced tubercles may contain sulfides or acids.
Types/forms of corrosion
Impingement attack• A form of selective corrosion involving both physical and chemical
conditions, which produce a high rate of metal loss and penetration in a localized area.
1. It occurs when a physical force is applied to the metal surface by suspended solids, gas bubbles, or the liquid itself, with sufficient force to wear away the natural or applied passivation film of the metal 2. Process occurs repeatedly and each occurrence results in the removal of successive metal oxidation layers 3. Cavitation is a form of impingement attack often found in pump impellers. This is caused by collapse of air or vapor bubbles on metal surface with sufficient force to produce rapid, local metal loss.
Type/Form of Corrosion
Dezincification• Type of corrosion usually limited to brass• Two forms – general (large surface of affected) and plug type
(highly localized)• Occurs when:
1. Zinc and copper are solubilized at the liquid-metal interface
2. Zinc is carried off in the liquid medium, while copper replates
3. The replated copper is soft and lacks the mechanical strength
of the original metal
Corrosion Inhibition
• Complete corrosion protection of metal and alloys maybe impractical
• Goal is control corrosion to tolerable level
1. By good design
2. Selection of proper materials of construction
3. Effective water treatment
Note:
Level of corrosion = metal loss in mils per year
1 mil = 0.001 in = 0.0025 cm
1mpy = 0.025 mm/yr
In cooling system, an acceptable loss may as much as 10 to 15 mpy
In supercritical boiler it might be zero
Corrosion Inhibition
Material of construction• Use of corrosion- resistant materials such as copper, stainless steel,
copper-nickel alloy, concrete, and plastic may offer advantages over carbon steel
• Coating and lining• Use of insulation if joining of dissimilar metal which can lead to
galvanic corrosion can not be avoided
e.i. – insulating aluminum equipment from steel piping to prevent galvanic attack
Corrosion Inhibition Applied Chemical Inhibitors • Any chemical applied to the water to stop anodic reaction will stop
corrosion• Any material added to reduce the rate-determining cathodic reaction
will reduce corrosion.• Typical corrosion inhibitors
Anodic Cathodic Both anodic/cathodic
Chromate Calcium carbonate Organic filming amines
Nitrite Zinc Phosphonates
Orthophosphate Silicate
Polyphosphate
Monitoring Results
1. Corrosion coupon
- Pre-weighed metal specimens put into system for 30 to 90 days
- Following removal, they are cleaned, reweighed, and observed
- Metal loss and type of attack (general, pitting) is then determine
2. Corrosion nipples
- Similar to coupons in concept, but not preweighed and only
visually evaluated
3. Corrosion meter
- The meter works by measuring an electrical potential across
electrodes made of the metal being evaluated
Control of Microbial Activity
Planning an effective microbial control program for a specific water
treatment process requires an examination of:
1. The types of organisms present in the water system and the associated problems they can cause
2. The population of each type of organism that may be tolerated before causing a significant problem
Typical Microorganisms and their Associated Problems
Type of Organism
A. Bacteria1. Slime forming bacteria
2. Spore forming bacteria
Type of Problem
Form dense, sticky slime with subsequent fouling. Water flows can be impeded and promotion of other organism growth occurs.
Become inert when their environment becomes hostile to them. However, growth recurs whenever the environment becomes suitable again. Difficult to control if complete kill is required. However, most processes are not affected by spore formers when the organism is in the spore form.
Typical Microorganisms and their Associated Problems
Type of Organism
A. Bacteria3. Iron depositing bacteria
4. Nitrifying bacteria
5. Sulfate reducing
bacteria
Type of Problem
Cause the oxidation and subsequent deposition of insoluble iron from soluble iron
Generate nitric acid from ammonia contamination. Can cause severe corrosion.
Generate sulfides from sulfates and can cause serious localized corrosion.
Typical Microorganisms and their Associated ProblemsType of Organism
A. Bacteria6. Anaerobic corrosive
bacteria
B. Fungi 1. Yeasts and molds
Type of Problem
Create corrosive localized environments by secreting corrosive wastes. They are always found underneath other deposits in oxygen deficient locations.
Cause the degradation of wood in contact with the water system. Cause spots on paper products.
Typical Microorganisms and their Associated Problems
Type of Organism
C. Algae
D. Protozoa
E. Higher life forms
Type of Problem
Grow in sunlit areas in dense fibrous mats. Can cause plugging of distribution holes on cooling tower decks or dense growth on reservoirs and evaporating ponds.
Grow in almost any water which is contaminated with bacteria; indicate poor disinfection.
Clams and other shell fish plug inlet screen
Typical Microorganisms and their Associated Problems
Notes:
In water treatment bacteria is grouped according to its preferred environment
1. Aerobic bacteria – group that require oxygen. Found in aerated
water such as in cooling water basin.
2. Anaerobic bacteria – don’t use oxygen. Found in oxygen deficient
areas such as under deposits, in crevices
and in sludges.
Physical Factors Affecting Microbe Growth
1. Temperature• Species of microbes indigenous to soil, water and
vertebrates organisms thrive in broad temperature range of 10 to 45oC.
• But nature has also produced select organism that can live at temperature as low as 0oC and as high as 100 oC.
• Scientists also report finding life in hot springs and adjacent to ocean vents on the sea floor at temperature of over 200oC.
• Denaturation of protein, which causes coagulation within cell occurs below 70oC.
• Commercial pasteurization (a de-naturation process) of milk is done at 63oC holding temperature for 30 minutes or at 72oC, process is completed in only 15 seconds. This kills all disease producing (pathogenic) organism.
Physical Factors Affecting Microbe Growth
1. Temperature• Most actively growing microbes of interest in water
treatment technology are killed at 70 oC.• At 0 to 5oC organisms becomes dormant. Freezing kills
many cells, but those that survive are capable of complete recovery from the shock
• Dry heat results in dehydration of all cellular matter and oxidation of intracellular constituents
Physical Factors Affecting Microbe Growth
2. Moisture• moisture is required for microorganism to grow actively. Many species of pathogenic organisms are killed quickly by drying• Organisms in the spore or cyst state can survive low moisture environment. If transported to a location where moisture levels suit them, they survive and form new colonies.
3. Radiation• organisms containing chlorophyll are able to use the radiant
energy or artificial lighting to convert CO2 to carbohydrates, which they need for cell synthesis
• Not all radiant energy are useful to cells and certain frequencies of radiation are harmful. Radiation is therefore
one method of microbe control.
Physical Factors Affecting Microbe Growth
4. Osmosis• The diffusion of water through a semipermeable membrane
separating 2 solutions of different solute concentrations. The
water flows in a direction to equalize the concentration. • When microbes are placed in salt solution, the water inside
the cells is extracted by the surrounding medium. This
dehydrates the cell so they are unable to grow or are killed.
Technique is used to preserve food.
Chemical Factors Affecting Microbial Growth
• Microbes have been found to exist in the broad ph range of 1 to 13• Most common microbes associated with water – algae and bacteria
– usually maintain their internal ph at 7.• Generally yeasts and molds favor depressed ph in the range of 3 to4• Bacteria and fungi can both contribute to industrial problems over a
ph range of 5 to 10• One of the surprising facts of microbe life is that there is such a
profusion and variety of forms that some can almost always be found that will resist damage by , or even thrive on, chemicals that are toxic to animal and plant life.
example: Phenol was one chemical biocides used for sterilization, yet at low concentration, it is digested in activated sludge waste treatment. Similarly some bacteria thrive in wastewaters that contain herbicides, pesticides, and other toxic chemical.
Method for Controlling Microbial Activity
• Only limited use can be made of physical conditions that inhibit or destroy microbial life.
• Among chemical conditions that might be used for microbe control, ph is the only likely candidate for practical results. Even this is limited unless the system water can be kept at ph over 10.
• Since neither physical nor aquatic chemical conditions can be changed in a practical way to control microbial growth, toxic chemicals must be applied as biocides.
• The 2 commonly used types are oxidizing and nonoxidizing
Chick-Watson Law
Law showing the relationship for all chemical biocides that expresses effectiveness, measured as percent kill or inactivation, concentration of biocide applied to the water, and time of contact of the biocide with the organism or virus.
Chick – Watson Law: N/No = exp(- k’cnt)
where: No represents the bacteria population at the time zero
N is the reduced population at time t, after biocide application
exp = base of natural logarithm system 2.718
k’ is the rate constant
c is the concentration of biocide, mg/l
n is an empirical value
t is the time of contact, min
Oxidizing Biocide
• Chlorine gas, the chemical biocide most commonly used hydrolyzes rapidly when dissolved in water according to the following equation:
Cl2 + H2O H+ Cl- + HOCl
Hydrolysis occurs less than a second at 65oF. Hypochlorous acid (HOCl) is the active ingredient formed by this reaction.
This weak acid (HOCl) tends to undergo partial dissociation as follows:
HOCl H+ + OCl-
This reaction produces a hypochlorite ion and a hydrogen ion.
