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TO / pidillite COMPANY FOR RESIN ADHESIVES- INDUSTRIAL ZONE-SADAT CITY
REFFERED TO / GENERAL MANAGER
DATE / 5/4/2017
SUBJECT / COOLING WATER SYSTEM TREATMENT
A brief overview
Cooling Towers and Cooling Systems Treatment
Introduction
– The basics
Water is widely used as a coolant with heat being transferred from hot process fluids into cooling water through a heat exchange surface.
This cooling water then heats up but in evaporative cooling towers, the evaporation of a small percentage of the water reduces the temperature of the rest allowing it to be used again.
The evaporated water is replaced with make up water.
Evaporative condensers are often used to cool closed systems whereby pipework containing hot process fluids is sprayed with water to remove heat from the system.
If cooling systems are to operate effectively, their design and water treatment should ensure that they are safe and reliable and that they minimise the use of energy and water.
Water treatment programmes aid these objectives by protecting the structural integrity of the system, through maintaining efficient heat transfer and by controlling microbiological contamination.
Cooling Water
The term cooling water refers to water that is circulated around or through a particular process in order to remove heat from that process
The water that is used in cooling water applications contains impurities.
These impurities can result in a variety of problems within the cooling water system.
The following are the types of impurities found in cooling water supplies, and the types of
problems these impurities can cause
COOLING WATER IMPURITIES
SOLIDS (Suspended and Dissolved)
Impurities other than gases that are found in water are termed solids.
These solids can be in one of the two forms.
They can be either suspended or dissolved. Suspended solids include sand, silt, or particles of organic matter.
Suspended solids are those impurities that will settle to the bottom of a container of water if it is left undisturbed Any impurity that is dissolved in the water is referred to as a dissolved solid. supply
These are impurities that cannot be seen and will not settle to the bottom of an undisturbed container. Dissolved solids are composed mostly of mineral matter.
The measurement of all impurities that are dissolved in a given water supply is the total dissolved solids( TDS)
HARDNESS
Hardness is a term, which refers to the concentration of calcium and magnesium in water.
Calcium and magnesium compounds become insoluble as the water is heated and temperature increases.
These compounds are primary source of scale in cooling water systems
ALKALINITY
Alkalinity is a measure of bicarbonate, carbonate and hydroxyl ions in water. It is possible for all three of these ions to exist simultaneously.
There are many other impurities found in natural water supplies.
The following are some of the most common ones
Chloride
Sulfate
Ferrous Iron
Ferric Iron
Silica
Sodium
Manganese
pH
pH is the measure of the relative acidity or basicity of water.
The pH scale ranges from to 14 with 0 representing maximum acidity and 14 representing maximum basicity0
Control of pH is critical in the majority of cooling water systems. 7 is neutral, not acidic or basic (alkaline).
It is important to know that the pH scale is logarithmic.
The pH of 2 is ten times more acid than pH of 3 and 3 is ten times acid than pH of 4 (2 is 100 times more acid than 4)
CYCLES OF CONCENTRATION
The accumulation of the impurities in a water supply is referred to as the cycles of concentration.
As water is evaporated, the impurities remain behind.
In a cooling tower system, fresh water is brought into the system to maintain a constant water level
This fresh water brings in more impurities that remain in the tower this water is evaporated
if the cooling tower water contains twice the level of impurities as the fresh water supply, it is said to have two cycles of concentration.
If left unattended, the impurities in a cooling tower water system increase indefinitely.
The cycles of concentration of a cooling water system can be controlled by removing water with a high level of impurity from the system and replacing it with fresh water containing only the original level of impurities, the process of removing water from the system is called ‘bleed off’
COOLING WATER PROBLEMS
Cooling water system problems can be divided into four main categories
. Scale Deposition
. Corrosion
. Microbiological Growth
. Sludge formation & Fouling
These problems can occur in the cooling system heat exchangers, water carrying lines cooling tower, and other system components.
Scale deposition results when the concentration of impurities in the cooling water reaches the level where the water can no longer keep them dissolved.
This point is called the saturation point.
The saturation point can change with temperature and concentration of scale forming constituents Calcium carbonate CaCO3 is the most common scale that may form in cooling water systems.
In the water, calcium ions will combine with bicarbonate to form calcium bicarbonate.
As the temperature of the system increases, calcium bicarbonate is converted to calcium carbonate
SCALE DEPOSITS
Scale deposits interfere with heat exchangers, reducing their efficiency by insulating the heat transfer surfaces.
If scale deposits accumulate in enough quantity, restriction of water flow and plugging of piping and heat exchanger tubes may occur.
Even a small amount of scale will cost greatly and increase energy costs of cooling
What is scale?
Scale is a hard deposit of predominantly inorganic material on heating transfer surfaces caused by the precipitation of mineral particles in water.
As water evaporates in a cooling tower or an evaporative condenser, pure vapor is lost and the dissolved solids concentrate in the remaining water.
If this concentration cycle is allowed to continue, the solubility of various solids will eventually be exceeded.
The solids will then settle in pipelines or on heat exchange surfaces, where it frequently solidifies into a relatively soft, amorphous scale.
Problems Scale, in addition to causing physical blockage of piping, equipment, and the cooling tower, also reduces heat transfer and increases the energy use.
For example, the thermal conductivity BTU/ [hr"(ft2) (F/in)"]
[size] of copper is 2674, while the common cooling water scale calcium carbonate has a thermal conductivity of 6.4 BTU/ [/size][hr"(ft2) (F/in)"]
[size]. [/size]
A calcium carbonate scale of just 1.5 mil thickness is estimated to decrease thermal efficiency by 12.5 %.
In compression refrigeration systems, scale translates into higher head pressures, hence an increase in power requirements and costs.
For example, 1/8" of scale in a 100 ton refrigeration unit represents an increase of 22% in electrical energy compared to the same size unit free of scale.
Factors
The principle factors responsible for scale formation are:
1. As alkalinity increases, calcium carbonate- the most common scale constituent in cooling systems
- decreases in solubility and deposits.
2. The second—more significant—mechanism for scale formation is the in-situ crystallization of
sparingly soluble salts as the result of elevated temperatures and/or low flow velocity. Most salts become more soluble as temperature increases, however, some salts, such as calcium carbonate, become less soluble as temperature increases. Therefore they often cause deposits at higher temperatures.
3. High TDS water will have greater potential for scale formation.
Types
Typical scales that occur in cooling water systems are:
1. Calcium carbonate scale - Results primarily from localized heating of water containing calcium bicarbonate. Calcium carbonate scale formation can be controlled by pH adjustment and is frequently coupled with the judicious use of scale inhibiting chemicals.
2. Calcium sulfate scale - Usually forms as gypsum is more than 100 times as soluble as calcium carbonate at normal cooling water temperatures. It can usually be avoided by appropriate blowdown rates or chemical treatment.
3. Calcium and magnesium silicate scale - Both can form in cooling water systems. This scale formation can normally be avoided by limiting calcium, magnesium, and silica concentrations through chemical treatment or blowdown.
4. Calcium phosphate scale - Results from a reaction between calcium salts and orthophosphate, which may be introduced into the system via inadequately treated wastewater or inadvertent reversion of polyphosphate inhibitors present in recycled water.
The most common type of scaling is formed by carbonates and bicarbonates of calcium and magnesium, as well as iron salts in water. Calcium dominates in fresh water while magnesium dominates in seawater.
Control
Scale can be controlled or eliminated by application of one or more proven techniques:
1. Water softening equipment – Water softener, dealkalizer, ion exchange to remove scale forming minerals from make up water.
2. Adjusting pH to lower values - Scale forming potential is minimized in acidic environment i.e. lower pH.
3. Controlling cycles of concentration - Limit the concentration of scale forming minerals by controlling cycles of concentration. This is achieved by intermittent or continuous blowdown process, where a part of water is purposely drained off to prevent minerals built up.
CORROSION
The destructive attack of a metal by an electrochemical reaction with it’s environment is the definition of corrosion.
The deterioration and possible failure of heat exchanger tubes in cooling systems can result in a loss of efficiency or process contamination.
Corrosion in cooling systems can occur in a number of ways
General corrosion is a uniform attack of a metal surface; galvanic corrosion occurs when two dissimilar metals or alloys are connected by a conductive path forming a galvanic corrosion cell.
Pitting corrosion is a randomly occurring, highly localized form of attack on the metal surface.
Pitting is one of the most destructive forms of corrosion
Erosion
corrosion occurs when flow rates are excessive and the metal surface is actual worn away
Corrosion is defined as the destruction or loss of metal through chemical or electrochemical reaction with its surrounding environment. Mild steel is a commonly used metal in the cooling water system that is most susceptible to corrosion. Other metals in general, such as copper, stainless steel, aluminum alloys also do corrode but the process is slow. However in some waters and in presence of dissolved gases, such as H2S or NH3, the corrosion to these metals is more severe & destructive than to mild steel.
What causes corrosion?
Corrosion is a three step electrochemical reaction in which free oxygen in the water passes into a metal surface a one point (referred to as the cathode) and reacts with water and electrons, which have been liberated by the oxidation of metal at the anode portion of the reaction at another spot on the metal surface.
The combination of free electrons, oxygen and water forms hydroxide ions.
The hydroxide ions then combine with the metal ions, which were liberated at the anode as part of the oxidation reaction, to form an insoluble metal hydroxide.
The result of this activity is the loss of metal and often the formation of a deposit.
Corrosion Problems
Common problems arising from corrosion are reduction in heat transfer and water flow resulting from a partial or complete blockage of pipes, valves, strainers, etc.
Also, excessive wear of moving parts, such as pump, shaft, impeller and mechanical seal, etc. may resist the movement of the equipment. Hence, thermal and energy performance of heat exchange may degrade.
Factors
Many factors affect the corrosion rates in a given cooling water system.
Few important factors are:
1. Dissolved Oxygen - Oxygen dissolved in water is essential for the cathodic reaction to take place.
2. Alkalinity & Acidity - Low alkalinity waters have little pH buffering capability. Consequently, this type of water can pick up acidic gases from the air and can
dissolve metal and the protective oxide film on metal surfaces. More alkaline water favors the formation of the protective oxide layer.
3. Total Dissolved Solids - Water containing a high concentration of total dissolved solids has a high
conductivity, which provides a considerable potential for galvanic attack.
Dissolved chlorides and sulphates are particularly corrosive.
4. Microbial Growth - Deposition of matter, either organic or inorganic, can cause differential aeration pitting (particularly of austenitic stainless steel) and erosion/corrosion of some alloys because of increased local turbulence.
Microbial growths promote the formation of corrosion cells in addition; the byproducts of some organisms, such as hydrogen sulphide from anaerobic corrosive bacteria are corrosive.
5. Water Velocity - High velocity water increases corrosion by transporting oxygen to the metal and carrying away the products of corrosion at a faster rate. When water velocity is low, deposition of suspended solids can establish localized corrosion cells, thereby increasing corrosion rates.
6. Temperature - Every 25-30°F increase in temperature causes corrosion rates to double. Above 160°F, additional temperature increases have relatively little effect on corrosion rates in cooling water system.
Some contaminants, such as hydrogen sulfide and ammonia, can produce corrosive waters even when total hardness and alkalinity are relatively high.
Corrosion Types
Many different type of corrosion exist, but the most common is often characterized as general, pitting and galvanic corrosion.
1. General attack: exists when the corrosion is uniformly distributed over the metal surface. The considerable amount of iron oxide produced contributes to fouling problems.
2. Pitting attack: exists when only small area of the metal corrodes. Pitting may perforate the metal in short time. The main source for pitting attack is dissolved oxygen.
3. Galvanic attack: can occur when two different metals are in contact. The more active metal corrodes rapidly. Common examples in water systems are steel & brass, aluminum & steel, Zinc & steel and zinc & brass. If galvanic attack occurs, the metal named first will corrode.
MICROBIOLOGICAL GROWTH
Microbiological growth is continually infecting cooling systems.
If not controlled microbes multiply rapidly in the warm cooling water environment.
The accumulation of microbiological growth can lead to fouled heat exchangers, metal deterioration through corrosion, and clogged filters and screens.
The most common types of microorganisms found in cooling water systems are bacteria algae, and fungus
FOULING
Fouling refers to the physical accumulation of suspended solids on the heat exchanger surfaces.
These suspended solids can be composed of microbiological growths or by products, organic material or precipitated inorganic matter.
Mechanical cleaning of cooling systems is very important to help control fouling and biological growth
Temperature Drop Across Tower (ΔT)
Water is cooled predominantly by evaporation. For each 10 oF (5.5 oC) the water is cooled, about 1% of the water is evaporated.
Determining the average ΔT can sometimes be very challenging.
Several ways are:
1. Actual Plant Records of Temperatures - These provide the very best estimate of the average ΔT across the tower. Some large plants will continuously monitor and record this information.
Beware: Many operators of cooling towers will know the design ΔT and give you this as the average ΔT.
Rarely does a tower operate full time at full load (i.e. maximum design ΔT).
2. Measurement of Temperatures - Obviously, measuring the temperature of the
water from the hot return to the tower and then the temperature of the water in the sump will provide the ΔT.
Unfortunately, this will only be the ΔT for that moment in time. Where cooling requirements in a system can vary significantly, multiple measurements over a long period of time are required.
3. Freon Air Conditioning Systems - Air conditioning systems are typically designed to provide sufficient cooling to maintain the temperature in the air conditioned space at 72 oF when the outside temperatures are at their highest and the maximum number of people (a heat source) are occupying the space being cooled.
Therefore, from noon to 5:00 P.M. on a hot summer day, the system will
have a ΔT of 10 oF (typical design).
The rest of the time, the system will be idling at less than maximum load.
Since the flow of water in the system is a constant, and since the heat removal
requirement is less, the temperature of the hot return water will be less and the ΔT therefore will be smaller
In recent times with the increased emphasis on energy conservation, many systems are being operated less than 24 hours/day (12 - 16 hours is typical).
Therefore, during the periods of operation, the average daytime heat load on the system is higher and the ΔT average is therefore higher.
For 12-hour operation, increase the average ΔT by 2 oF, for 16 hours, 1.5 oF. Also, when calculating average daily requirements, remember to reduce the
length of the operating day.
4. Steam Absorption Air Conditioning - System design characteristics vary with
respect to recirculation rates and the ΔT per ton of refrigeration.
However, the ΔT maximums may be reduced by the same percentages as indicated above for freon systems.
5. Evaporative Condensers - Measurement of the ΔT in an evaporative
condenser is not possible since the water is heated, evaporated and cooled in-situ
(all at the same time and place).
Units are usually rated in tons with 3 gpm and 10 oF ΔT typical per ton of cooling.
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