عدد المساهمات : 3509
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: Cooling Water Problems and Solutions /مشاكل المياه المبردة وحلولها2(القشور والاملاح الإثنين يوليو 30, 2012 2:55 am|| |
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.
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)] of copper is 2674, while the common cooling water scale calcium carbonate has a thermal conductivity of 6.4 BTU/ [hr (ft2) (F/in)].
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.
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.
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.
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.
4. Chemical dosage - Apply scale inhibitors and conditioners in circulating water.
5. Physical water treatment methods – Filtration, magnetic and de-scaling devices
The quality of the makeup water can be adjusted by a water softener, a dealkalizer, or an ion exchange unit.
In areas where only hardness reduction is required, a water softener is used and where only alkalinity reduction is required, a dealkalizer is used.
In areas where combined treatment is required, an ion exchange unit is used.
Water softeners replace unwanted magnesium and calcium ions with sodium ions, which have none of the negative effects of hard water.
To do this, hard water runs through a bed of small plastic beads that have sodium ions attached to them.
As the water flows through, the sodium ions—which also occur naturally in water—are released into the water, and are replaced with the magnesium and calcium ions on the resin beads.
Eventually, the beads in a water softener contain nothing but calcium and magnesium ions and stop being effective.
To refresh the system, water softeners periodically go through a process known as regeneration.
In regeneration, a brine solution of sodium chloride—made from salt pellets or block salt—is flushed through the system, which replaces all calcium and magnesium in the system with sodium.
The calcium and magnesium, along with the remaining brine, is then drained into the wastewater system. A water softener is easy to operate and maintain.
These are also called single bed ion-exchange unit.
Dealkalizer units operate the same as water softeners, but use different resin bed materials and require strong caustic or acid regeneration.
The makeup water is passed through a treated resin bed where the contaminants in the water are collected through a chemical exchange process.
When the bed becomes saturated with contaminants, the bed is backwashed, treated with a concentrated electrolyte, rinsed, and placed back in service.
For critical or continuous operations, treatment units may be dual-column units that allow switching from a saturated column to a regenerated standby column so that service is not interrupted for routine column regeneration.
The ion exchange process is to remove calcium and magnesium ions by replacing them with an equivalent amount of sodium ions.
Unlike simple water softener, these are mixed bed ion-exchange unit consisting of cation and anion exchanger.
The cation exchanger section removes metals, such as calcium and magnesium (hardness), and the anion exchanger section controls alkalinity and may remove bicarbonates (corrosion and embrittlement), sulfates (hard scale), chlorides (foaming), and soluble silica (hard scale).
Control of scale with pH adjustment by acid addition is a simple and cost effective way to reduce the scaling potential.
It functions via chemical conversion of the scale forming materials to more soluble forms - calcium carbonate is converted to calcium sulfate (using sulfuric acid), a material several times more soluble.
Sulfuric acid (H2SO4) and hydrochloric acid (HCl) are the most common additives used for controlling the formation of calcium carbonate scale.
The acid is fed by variable stroke pumps that are controlled either by automatically adjusting the pump stroke based on circulating water pH, or by automatically starting and stopping manual stroke adjustment metering pumps based on circulating water pH.
Sulfuric acid is normally used at a concentration of 93 to 98 percent (66° Bé). Hydrochloric acid is normally used at a concentration of 28 to 36 percent (18° Bé to 22° Bé).
The reaction of the acid with calcium bicarbonate is:
H2SO4 + Ca (HCO3)2 = CaSO4 + 2H2O + 2CO2
2HCl + Ca (HCO3)2 = CaCl2 + 2H2O + 2CO2
The Langlier Saturation Index (LSI) and the Ryznar Saturation Index (RSI) are utilized for system setup when pH adjustment by acid addition is used for scale control.
Both indexes are merely convenient means of reducing the integrated parameters of calcium, alkalinity, pH, dissolved solids, and temperature to a single value, which indicates the tendency of water to form a calcium scale or promote corrosion.
A positive LSI number (RSI less than 5.0) indicates a scale forming water while a negative LSI number (RSI greater than 7.0) indicates a scale dissolving, or corrosive, water.
Normal practice is to maintain a slightly positive LSI number, +.2 to +.5, (RSI between 5.0 and 6.0) when utilizing pH adjustment by acid addition and add some chemical scale inhibitor to cope with the resultant slight tendency to scale.
Caution - Addition of excessive acid to the cooling water results in depressed pH values and extremely rapid corrosion of all system metals.
Therefore, proper pH control is required to provide a suitable environment for both scale and corrosion inhibitors work effectively.
The list that follows includes generic or families of chemicals which may be used to condition cooling water stream.
The specific name of the treatment product containing the listed chemical and the form of the chemical used will depend on the manufacturer.
Most chemical treatment manufacturers have developed proprietary "brand names" which combine a number of the chemicals illustrated below and include other agents to enhance the performance of the product.
Facilities should consult chemical suppliers to tailor the facilities’ chemical treatment needs to local conditions, and to establish procedures for safe chemical storage and handling.
1. Polymers (Polyacrylate, etc) - Disperse sludge and distort crystal structure of calcium deposits. Prevent fouling due to corrosion products. Commonly used, cost effective for calcium scale at 5 to 15 mg/l.
2. Polymethacrylate - Less common for calcium scale at 5 to 15 mg/l.
3. Polymaleic - Very effective for calcium scales at 10 to 25 mg/l, higher cost.
4. Phosphonates - Phosphonates are excellent calcium scale inhibitors at levels from 2 to 20 mg/l.
5. Sodium Phosphates (NaH2PO4, Na2HPO4, Na3PO4, NaPO3) - Precipitates calcium as hydroxyapatite (Ca10(OH)2(PO4)6). Stream pH must be kept high for this reaction to occur.
6. Sodium Aluminates (NaAl2O4) - Precipitates calcium and magnesium.
7. Chelants (EDTA, NTA) - Control scaling by forming heat-stable soluble complexes with calcium and magnesium.
8. Coploymers - These products commonly incorporate two active groups, such as a sulfonate and acrylate, to provide superior performance to a single group compound at use levels at 5 to 20 mg/l, higher cost.
9. Terpolymers - Like the co-polymers, only incorporate three active groups to give yet better performance under severe conditions at use levels of 5 to 20 mg/l, costly.
10. Polyphosphates - Fairly good calcium scale control under mild conditions. Caution - Polyphosphates are of some value for scale control but must be applied cautiously, because hydrolysis of the polyphosphate results in the formation of orthophosphate ions.
If this process is not properly controlled, calcium phosphate deposits may result.
11. Tannins, starches, glucose, and lignin derivatives - Prevent feed line deposits by coating scale crystals to produce sludge that will not readily adhere to heat exchanger surfaces.
As a general rule, common chemical scale inhibitors such as polyacrylate and phosphonate can be utilized if the Saturation Index (LSI) value of the cycled cooling water does not exceed 2.0. Cycled cooling water SI values up to 3.5 can be obtained by use of co- and terpolymers combined with surfactants.
Multiple water treatment firms have reported operation of cooling
systems with newer treatment chemistries scale free at cycled LSI values from 2.5 to 3.5 without pH adjustment.
عدد المساهمات : 3509
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: Cooling Water Problems and Solutions /مشاكل المياه المبردة وحلولها2(القشور والاملاح الإثنين يوليو 30, 2012 3:07 am|| |
Controlling Cycles of Concentration
To a great extent the quality of the inflow water determine the extent depositions on heat exchange surfaces.
The higher the levels of hardness and alkalinity in the inflow water, the greater will be the potential for scale accumulation.
Two key relationships - “cycles of concentration” and “bleed off water” are important.
Cycles of Concentration
The cycle of concentration (also referred to as concentration ratio) is defined as the ratio of the concentration of a specific dissolved constituent in the recirculated cooling water to the concentration of the same constituent in the makeup water.
This figure establishes the minimum blowdown rate that must be achieved.
The concentration ratio (C) is determined by the following equation:
C = [E / (B + D)] + 1…………. (1)
E = Evaporation rate
B = Blowdown
D = Drift
Ignoring insignificant drift loss (D), the equation can be simply put as:
C = M / B…………….. (2)
M = Make up water equal to E + B.
The equation (2) tells us that as long as the amount of blowdown water is proportional to the amount of water entering the system, the concentration ratio will remain constant irrespective of variations in the inflow water chemistry.
Blowdown or Bleed-off
Evaporative loss from a cooling tower system leads to an increased concentration of dissolved or suspended solids within the system water as compared to the make up water.
Over concentration of these impurities may lead to scale and corrosion formation, hence, fouling of the system.
Therefore, concentration of impurities must be controlled by removing system water (bleed-off) and replacing with make up water.
In order to control the total dissolved solids (TDS), bleed-off volume can be determined by the following formula:
• B – Blowdown rate (L/s)
• E – Design evaporative rate (L/s)
• C – Cycle of concentration
• D – Design drift loss rate (L/s)
The equation (3) tells us that it is practically impossible to ever achieve a concentration ratio of 1, because to do so would require an infinite amount of water.
It also shows that as the concentration ratio increases the blowdown requirement decreases.
As a rule of thumb, the minimum cycle of concentration shall be maintained at 5 to 6 for fresh water type cooling tower and 2 or less for seawater type cooling tower.
Increasing the blowdown is a simple way to reduce the levels of calcium and alkalinity in the water, but it is not in interests of water conservation.
Circulating Water System Design Data
Data affecting the circulating water cycles of concentration and the treatment equipment selection are:
1. Cooling tower evaporation rate - The evaporation rate is calculated based on the heat rejected by the cooling tower and site-specific conditions such as relative humidity and wet bulb temperature.
As a “Rule of thumb”, the evaporation rate is approximately 1.0 percent of the cooling tower recirculation rate for each 10 °F temperature drop across the cooling tower.
The percentage varies depending on the plant's geographic location.
The evaporation rate used for final design should come from the cooling tower supplier.
2. Cooling tower drift rate - The drift rate is a function of the type of tower (induced, forced, or natural draft) and the internal mist eliminator design.
3. Circulating water recirculation rate - The recirculation rate is determined by the heat balance for the cooling system.
4. Cooling tower, piping, and heat transfer equipment construction materials and linings (if used) - The material selection determines the need for corrosion inhibitors or for modification of chemical operating parameters for equipment protection.