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Orthophosphates versus Polyphosphates
The selection of a phosphate water treatment chemical additive can be one of the most difficult chemical treatment decisions that many public water systems will make. This is particularly true because the chemistry of orthophosphate and polyphosphate chemical additives is complex, and phosphate water treatment chemical additives are commercially available in an overwhelming number of chemical blends.
Orthophosphates and polyphosphates are salts derived from two different forms of phosphoric acid. Orthophosphates are small molecules, formed from the smallest and most basic form of phosphoric acid. Polyphosphates are larger molecules, formed from a longer chain version of phosphoric acid.
Even though the words orthophosphate and polyphosphate contain the word "phosphate," these two chemical compounds serve radically different water treatment purposes. A public utility system's failure to understand the significant differences between these two treatment compounds could result in serious water-quality problems and possible MCL violations. An incorrect selection of phosphate chemical blends by a public utility system could even create serious public health problems.
In public water systems, orthophosphates are used for lead and copper corrosion-control purposes. Orthophosphates chemically react with lead and copper atoms that have leached off of piping and have entered into the surrounding water. This chemical reaction of orthophosphates with lead and copper atoms forms lead and copper phosphate. The lead and copper phosphate is then electrochemically drawn back down onto the piping surface, where it forms a tough, water-resistant coating on the piping. This tough, water-resistant coating helps prevent further leaching off of lead and copper atoms into the surrounding water. Most public utility systems have experienced far greater success with orthophosphate lead corrosion control than they have experienced with orthophosphate copper corrosion control.
Polyphosphates are sequestering agents that are virtually ineffective against lead and copper corrosion. When a jury in a criminal trial is sequestered, that jury is "held in seclusion." A chemical sequestering agent is a chemical agent that surrounds another molecule or atom and holds that other molecule or atom "in seclusion." By surrounding the other molecule or atom and holding it in seclusion, the chemical sequestering agent hides the molecule or atom from sight and prevents it from entering into various chemical reactions. As a sequestering agent, polyphosphates will only sequester soluble "invisible-in-water" metals that have not been oxidized into their insoluble forms. Polyphosphate applied to water before the water is chlorinated will prevent invisible iron and manganese from becoming visible after the water is chlorinated.
As a sequestering agent, water treatment polyphosphate is used to sequester soluble iron atoms that remain in settled water before it is chlorinated or that leach off of iron piping in water distribution systems. By surrounding and sequestering these soluble iron atoms, they are prevented from displaying the typical reddish colors associated with iron oxides and iron hydroxides. Water treatment polyphosphates also interfere with the crystallization of and formation of calcium and magnesium carbonate scales, but not with the cystallization of and formation of magnesium hydroxide scales. If any soluble manganese atoms are still present in water after the floc has settled out, polyphosphates will also serve to sequester these soluble manganese atoms, preventing them from displaying the typical dark manganese dioxide color.
A gross misconception about polyphosphate is the belief that the use of polyphosphate sequestrants to hide iron and manganese is a casual, routine treatment technique for the removal of excess iron and manganese that was not removed during a water treatment plant's sedimentation and filtration processes. In reality, the use of polyphosphate to sequester iron and manganese that a plant failed to remove during the sedimentation and filtration processes is a desperation maneuver. Undesirable quantites of iron and manganese in raw water should be properly oxidized by aeration, permanganate, or ozone and should be deposited in sedimentation basins as part of the floc. Polyphosphates, which sequester iron and manganese for only a limited period of time, are not the ideal or the preferred solution for any water treatment plant's iron and manganese problems. Polyphosphates should only be used to catch the few particles of iron and manganese that were missed during the initial aeration and oxidation process.
Most phosphate treatment compounds used by public water treatment systems are actually blends of polyphosphates and orthophosphates. Polyphosphates are usually added in their sodium or potassium polyphosphate form. Orthophosphates are added in the sodium or potassium orthophosphate form or in an orthophosphate form that is mixed with zinc chloride or zinc sulfate. The zinc in the mixture plays no role in forming the coatings that prevent lead and copper corrosion. Instead, the zinc plays a significant role in protecting galvinized surfaces (galvanized means "zinc-coated") and in preventing asbestos fibers from eroding off of asbestos-cement piping. It should however be noted, some phosphate blends may also contain zinc polyphosphates, but zinc orthophosphate formulations are much more commonly used in public water treatment operations.
Phosphate water treatment mixtures are available in dozens of different blends. There is no perfect blend that is universally usable in all situations. If a water treatment system uses a blend with more polyphosphate than their system needs, the coatings laid down by the orthophosphate can be stripped away. If a water treatment system uses a phosphate blend with more orthophosphate than their system needs, iron can be stripped away from iron piping. Water treatment plants need to regularly assess the lead, copper, iron, manganese, and asbestos levels in their water and consult with a professional phosphate specialist whenever a change in phosphate blends seems to be warranted.
COMMON POLYPHOSPHATE SALTS: Sodium acid pyrophosphate, Tetrasodium pyrophosphate, Tetrapotassium pyrophosphate, Sodium tripolyphosphate, Potassium tripolyphosphate, Sodium trimetaphosphate, Sodium hexametaphosphate (glassy)
COMMON ORTHOPHOSPHATE SALTS: Monosodium orthophosphate, Monopotassium orthophosphate, Disodium orthophosphate, Dipotassium orthophosphate, Trisodium orthophosphate, Tripotassium orthophosphate, Zinc orthophosphate
Cooling Product Testing and Control
With the extremely high cost of molybdenum in recent years, its use as a corrosion inhibitor or tracing agent in cooling water products, where product consumption is significant, has become essentially cost prohibitive. Other corrosion inhibitors such as phosphates, zinc, silicates, and organo- phosphorous compounds are now used largely in the absence of molybdates. Also, the use of molybdenum has been restricted in some areas because of environmental concerns, mostly centered around concentration limitations in municipally generated sludges.
Phosphates
Where orthophosphate or polyphosphates are in use, testing for the phosphate is a good and accurate test. There are a number of phosphate procedures, but all tests determine orthophosphate. Other forms of phosphate such as polyphosphate or organo-phosphates must first be converted to orthophosphate to determine their concentrations with a phosphate test procedure.
Control can become more complicated when there is phosphate in the makeup water. The form of the phosphate (orthophosphate, polyphosphate, or both) and the concentration range needs to be known so that it is accounted for in the cycled cooling water.
Example
Makeup water contains 0.5 ppm of orthophosphate and 0.4 ppm of polyphosphate as PO4. The cooling tower is operated at five cycles of concentration and a cooling water product that contains 4 % of orthophosphate is being applied. The desired inhibitor product dosage is 100 ppm.
At five cycles, there will be 2.5 ppm of orthophosphate from the makeup water orthophosphate, and 2.0 ppm of polyphosphate applied from the makeup water, but some of it will have reverted to orthophosphate. You should test for polyphosphate in the tower water initially and then periodically to determine the reversion rate for your system. Typically, we assume about a 50% reversion rate. The actual reversion rate will depend upon pH and retention time, and the specific type of polyphosphate.
If when tested the polyphosphate showed to be 1 ppm in the cycled tower water, then the total orthophosphate from the makeup would be 3.5 ppm. 100 ppm of the inhibitor product would add 4 ppm orthophosphate, so a tested residual of 7.5 ppm or orthophosphate would indicate that 100 ppm of the product was in the system.
Table 1: Phosphate Summary
|
Phosphate Concentrations |
Orthophosphate (ppm) |
Polyphosphate (ppm) |
|
Makeup Water |
0.5 |
0.4 as PO4 |
|
Tower Water, |
2.5 |
2.0 |
|
Tower Water, 5 Cycles After Reversion |
3.5 |
1.0 |
|
Orthophosphate From Product |
4.0 |
|
|
Total in Cycled Tower Water |
7.5 |
1.0 |
Phosphonates
Most all cooling tower products contain one or more phosphonates that are used for scale inhibition, corrosion inhibition, or both. Phosphonate testing is not as accurate as phosphate testing, but they can be used for controlling product feed. Phosphonates are subject to oxidation to orthophosphate by chlorine or bromine and are lost to precipitation with cations such as calcium. If the system is chlorinated or brominated, assume a 20 – 30% degradation to phosphate. The actual amount can be determined by testing for residual phosphonates and phosphate.
There are several phosphonates tests that can be used:
• Hach UV digestion, then phosphate test.
• Boiling with acid and persulfate, followed by phosphate test. • Palintest drop test.
• Taylor drop test.
UV Digestion
The test procedure is the most accurate and has a reproducibility of about ± 10%. A persulfate reagent is used along with a UV light to decompose the organo-phosphate (phosphonate) to orthophosphate. An orthophosphate test procedure then determines the amount of phosphate contributed by the phosphonates. Any orthophosphate already present before the digestion is subtracted from the total orthophosphate after digestion. This can be done by adding reagent to the tower water that has not had the digestion and use this as the blank, or actually determine orthophosphate in the tower water and subtract it from the total orthophosphate determined after the persulfate digestion.
The amount of phosphorus in each specific phosphonate molecule varies, so there is a specific conversion factor from orthophosphate to phosphonate. Each ppm of orthophosphate created by HEDP digestion = 1.085 ppm HEDP. The phosphorus content of PBTC is much lower. Each ppm of orthophosphate created from the digestion of PBTC = 2.84 ppm of the PBTC molecule.
Phosphonate Test Example:
The cycled tower water has 6 ppm orthophosphate and a cooling water product that contains 2.5% PBTC and 1.8% HEDP is being applied at a desired dosage of 120 ppm.
Assuming all of the phosphonates remain as phosphonates and have not been oxidized in the cooling tower by bromine or chlorine and assuming it has not been lost to precipitation, you should get 3.05 ppm of orthophosphate from the phosphonates after a persulfate / UV digestion.
Table 2: Phosphate Summary
|
From PBTC: 120 ppm x 2.5% = 3 ppm |
1.06 ppm orthophosphate |
|
From HEDP: 120 ppm x 1.8% = 2.16 ppm 2.16 HEDP ÷ 1.085 ppm HEDP per ppm PO4 = |
1.99 ppm orthophosphate |
|
From orthophosphate in the tower water: |
6 ppm |
|
Total orthophosphate in sample after digestion: |
9.05 ppm |
|
Orthophosphate from phosphonate digestion: |
3.05 ppm |
Boiling With Acid and Persulfate
A digestion can also be accomplished by adding acid and persulfate, then boiling for about 30 minutes. If just acid were used, only polyphosphate would be hydrolyzed or reverted to orthophosphate. It persulfate is also added, the organo-phosphates and polyphosphates will be digested to orthophosphate. This test would be more applicable for samples that do not have polyphosphates, since the test will not distinguish between orthophosphate developed from phosphonates or polyphosphates.
Phosphonate Drop Counts
We recommend the Palintest procedure. This procedure is less accurate and subject to interferences. It is best to determine the number of drops on a known product concentration and relate the number of drops to that concentration. It is advisable to also compare these results initially and periodically to the digestion method.
Where PBTC is in use, the Palintest method is preferred. The procedure buffers the pH to around 3.0 and is more effective at detecting the PBTC along with the HEDP and AMP.
On the Palintest method, each 0.7 ppm of HEDP or AMP in the water should require one drop of titrant, and each 2.0 ppm of PBTC should require one drop.
Polyphosphate and some organics will interfere with the test and show up as phosphonates. To account for this, a blank is run on the makeup water. If it takes two drops for the color change on the blank, then those two drops are subtracted from the test results of the treated water. Note that the blank results are not cycled up by the tower cycles. Polyphosphates revert to orthophosphate which does not interfere and experience has shown that cycling the blank should not be done. If the product contains polyphosphate and a residual in the cycled water, it will increase the number of drops required.
If fluorides are in the cycled tested water at > 1.0 ppm, this causes a substantial interference that may disqualify the drop test procedure from being usable. It is advisable to check with the city supplier to see if they add fluorides and at what level. If high fluorides are present, an idea that may work is to first run the drop test procedure on the tower water to get a baseline number. Then take a sample of the cycled tower water and add 100 ppm of product and see how many drops are required. Subtract the number of drops used for the baseline from the drops required for the 100 ppm sample to determine how many drops represent 100 ppm of product as a basis for setting control limits.
The Palintest end point is the drop when the color change from green/gray to blue/purple first occurs.
Palintest Phosphonate Drop Count Example:
The cooling water is treated with 140 ppm of a product that contains 2.5% PBTC and 1.8% HEDP. The product has a specific gravity of 1.16. There is no fluoride in the water.
First, determine the interferences in the makeup water by running the test procedure on an untreated sample. On this example assume it took two drops.
Next, make a 100 ppm solution. To do this add 1 gram or 0.86 mL (1mL/1.16 gram/mL) of the chemical product to 99 grams (99 mL) of makeup water. Mix this up well, then add 1 gram (1 mL) of this 1% solution to 99 grams (99 mLs) of makeup water. This is now a 0.01% solution or 100 ppm of the product. This would place 1.8 ppm of HEDP and 2.5 ppm of PBTC in the solution. Run the phosphonates test on this solution, and for this example it required the theoretical number of drops of about 6.
Table 3: Theoretical Phosphonate Titrant Usage
|
From HEDP: 1.8 ppm ÷ 0.7 ppm HEDP / Drop |
2.5 Drops |
|
From PBTC: 2.5 ppm ÷ 2.0 ppm PBTC / Drop |
1.25 Drops |
|
From Blank: |
2 Drops |
|
Total Drops: |
5.75 drops, which will require 6 drops to see the color change. |
140 ppm of product would be about (140 ÷ 100) x 4 drops = 5.6 drops or 5-6 drops + 2 drops for the blank = 8 drops. This can be confirmed by making a 140 ppm solution and testing it.
Azole, Zinc, or Silica Tests
The Hach test procedures for azole, zinc, or silica can be used to check product dosage if the specific ingredient is in the applied product. Remember, as with phosphonates, the applied concentrations and actual residuals can be different. Azole residuals decrease as they film with copper. Zinc is lost as it precipitates at the cathode or in the bulk water. Silica is lost as it films metal surfaces. In establishing control ranges and dosages, take into account some of this loss. For example, we may apply azole at 2 ppm, but have a desired residual in the water of only 1 ppm.
Mass Balance
Chemical dosages should be confirmed by mass balances and compared to chemical testing. Mechanisms should be set up on each system to conveniently determine water makeup, cycles, water loss, and chemical consumption. The concentration in the recirculating water should be calculated from the actual product usage and blowdown or water loss.
Mass Balance Example:
The cooling tower is operating at five cycles of concentration. The makeup meter shows 120,000 gpd makeup. At five cycles, this is a water loss of 24,000 gpd. The product being fed contains 1.8% HEDP, 2.5% PBTC, 1.5% BZT, and 1% zinc; and the desired dosage is 100 ppm.
Daily product use determined by drum level and confirmed with drawdown cylinder testing is 28 lbs per day. This is a calculated applied dosage of 140 ppm of product in the cycled cooling tower water (140/120 x 24,000/1000 = 28 lbs).
Chemical testing showed 4 drops of phosphonates (6 drops from the test – 2 drops for the blank), which was previously determined to represent 100 ppm product. Testing also revealed 1.5 ppm BZT and 0.8 ppm of zinc residuals in the water. All of the chemical tests show that some portion of the active component has been consumed or residuals would have been higher at 140 ppm of applied product.
|
Product Component |
Expected Residuals with No Loss When Applied at 140 ppm |
Calculated Dosage Based on Actual Residual |
Product Loss to System Reactions |
|
Phosphonate |
8 drops |
6 drops = 100 ppm Product |
40 ppm Product |
|
BZT |
2.1 ppm |
1.5 ppm BZT = 100 ppm Product |
40 ppm product |
|
Zinc |
1.4 ppm |
0.8 ppm Zinc = 80 ppm Product |
60 ppm Product |
Conclusion
Mass balance is the most accurate way to determine applied dosage. If the product dosage was projected to be effective at 100 ppm, it is likely that this product is being overfed by 40%. Chemical testing suggests that there is more than sufficient residual of active components even after some loss to the system, so product dosage can be lowered and results monitored to confirm that desired results are maintained. There is expected to be some loss of active components as they react with the materials in the system and the impurities in the water.
Where molybdate is used or has been used as a monitoring method for product control and consumption, generally its loss to the system is minimal. That means that if the product shown above contained 1% molybdate as Mo,
it is likely that the test results would have been very close to 1.4 ppm Mo and the product dosage would have been decreased to 100 ppm to lower Mo to 1.0 ppm. Molybdate used as a tracer, then, would commonly yield a lower product usage rate because the other active components would not ordinarily be used to control the dosage.
HOSPITAL REDUCES WATER USAGE IN COOLING TOWERS WITH AUTOMATION
With the state's initiative to reduce water usage by 20 percent by the year 2020, many plants in California are striving to become more environmentally friendly. One such facility includes a leading California hospital that sought to reduce water treatment costs for its HVAC system. The hospital has three individual cooling tower systems that service three centrifugal chillers, with a combined total of 2,800 tons of capacity.
The water treatment program currently in use at the facility was operating at 2.8 cycles of concentration, resulting in 35.7 percent of the tower water makeup being bled to the sewer by the current treatment provider. Given the water quality in the area, this was the maximum cycles of concentration that could be achieved without employing the use of acid or water softening.
The savings that the hospital sought were realized by reconsidering various ways to optimize the water treatment program. Working closely with the Los Angeles Department of Water and Power (LADWP), it was revealed that by introducing a water conservation program to reduce water use through increased cycles of concentration, the facility would actually save more money than it would spend to alter the program, making the proposed project sustainable.
Through testing and lab analysis, the team was able to conclude that six cycles of concentration could be attained, resulting in only 16.7 percent of the tower makeup water being bled into the sewer treatment system. This could be achieved through the introduction of a safe acid feed system that would minimize scale, corrosion and microbiological fouling to enable the increase in cycles of concentration while also protecting facility staff from coming into contact with the chemicals.
The evaporation of the cooling tower remained the same, but U.S. Water was able to reduce blowdown, cutting water usage by an estimated 3.6 million gallons per year and decreasing water and sewage costs. The plant was able to save over $76,000 (see Fig. 1).
Anytime chemistry in the cooling tower is stressed by adding more cycles, tight control of the chemistry is required to prevent scale formation. This led to the introduction of U.S. Water's advanced automation controls. The advanced automation program included wireless monitoring and alarm notifications to manage the overall program performance, and the equipment monitored conductivity, pH, scale inhibitor levels, tower makeup usage, and tower bleed usage.
At any given time, designated hospital personnel and U.S. Water representatives, using various levels of password-protected security outlined by the facility, can securely access the data for review and online adjustment. If designated parameters fell above or below the specified range, a U.S. Water representative was alerted for quick response (see Fig. 3).
Second to irrigation, cooling towers offer the largest potential for water savings in California. As an added incentive, the state of California has put programs in place to rebate facilities for the cost of automating their systems. LADWP and the Metropolitan Water District (MWD), for example, offer three programs that finance automation for cooling towers due to their ability to increase cycles of concentration, which reduces water use.
This financing allowed U.S. Water to implement the $34,000 advanced automation program to monitor and control the water treatment program for this hospital at no cost to the hospital.
Results to date for the facility include significant reduction in water usage, lower water and sewage bills and more efficient monitoring due to the installed automation software to protect the equipment assets.
You will find the article at: http://www.waterworld.com/articles/iww/print/volume-14/issue-5/columns/case-study/hospital-reduces-water-usage-in-cooling-towers-with-automation.html
Cooling Tower Molybdate Disapearence
A customer had a cooling tower system with pH control, a molybdate traced scale/corrosion inhibitor, and bleach-based biocide program. The operators contacted their water management specialist with concerns about why their tested molybdate levels were so low. Their results did not correspond with the amount of scale/corrosion inhibitor being feeding into the system.
What factors could affect the tested molybdate levels in a cooling tower system? Take a few moments to consider the system diagram below and think of what could cause this.
City Makeup
Cooling Tower Blowdown Process
3005NSS Feed
Citric Acid Feed
Bleach Feed
Cooling Towers 279
Problems such as this can be very perplexing and can have several causes. Sometimes, you have to go the extra mile to get the answer. This is exactly what the Associate in charge of this account did.
Applied Chemistry
The chemistry applied to the cooling tower system was:
- 3005NSS - a molybdate-containing scale and corrosion inhibitor
- Citric acid - pH control
- Bleach - an oxidizing biocide used for microbiological control
Possible Causes of Low Molybdate Readings
- Low Cooling Tower Cycles: Running the cooling tower are lower cycles of concentration will result in lower molybdate levels even though the 3005NSS feedrate was the same. This was not the case in this situation.
- 3005NSS Underfeed: Underfeeding the 3005NSS would directly affect the tested molybdate residuals; however, the theoretical feed rate and actual feed rate compared closely to one another.
- Chemical Line Leak: A leak in the 3005NSS chemical line would obviously reduce the amount of chemical fed to the cooling tower, but no leak was found.
- Improper Product Blend: It is rare, but not impossible to get a product that has been blended improperly. Being able to do a dilution to test product concentrations is a skill all professional water treatment experts should possess. A sample of 3005NSS was diluted and tested for molybdate concentration. The tested molybdate level was very close to the theoretical level expected.
- Loss of Chemical Component in System: Some chemistries are naturally lost in the system due to evaporation or consumption.
Molybdate is nonvolatile and consumption to form a protective passivated layer on the metal surfaces should be minimal once the chemistry has been established. Molybdate was not being lost in the system.
- Test interference: An expert water treater should be aware of possible interferences to the tests he runs. The analysis procedures manuals for the colorimeters and spectrophotometers do a great job listing these interferences. An initial review of these interferences showed nothing in the system that should be interfering with the molybdate testing. Both the bleach feed and citric acid feed were Cooling Towers 280 new additions to the treatment program and corresponded to the approximate time that the operators started having problems with their molybdate testing. The analysis interference chart showed that it would take a chlorine reading of 7.5 ppm to cause interference. The free chlorine levels had never been anywhere near that high in the system. The analysis interference chart did not specifically list citric acid; however just because something isn't listed does not mean it is not an interference.
Checking for Citric Acid Interference
The water management associate telephoned the water analysis manyfacturer to ask if citric acid interfered with molybdate testing. They were unsure but recommended a test be conducted to determine if it did.
A dilution of 3005NSS was prepared. This dilution was divided into several containers and the pH was adjusted to various levels using citric acid and sulfuric acid (as a control). The results were as follows:
Table 1 - Citric Acid Interference Determination
Acid Used Sample pH Molybdate (ppm)
Citric 6.7 0.7
Citric 6.3 0.3
Citric 4.9 0.0
Sulfuric 7.0 1.6
Sulfuric 6.2 1.5
As Table 1 shows, the citric acid was indeed an interference.
Conclusion
Through the detective work of the water management associate, citric acid was determined to be the interference with the low range molybdate testing.
The next steps required were to:
- Ensure the theoretical 3005NSS dosage was being fed on a daily basis while citric acid was still being fed to the cooling tower.
- Discuss the pros and cons of sulfuric acid feed with the customer so the inhibitor levels can be properly measured
Treatment of Closed-Loop Cooling Water Systems
In part because they are closed-loop systems, it can be easy to ignore power plant cooling water systems that support the reliable functioning of everything from stator bars in the generator to critical pump bearings for feed pumps and heat exchangers on air compressors. Proper monitoring and maintenance of these water systems can help you avoid more-costly repairs to the mechanical systems they cool.
There may be multiple closed-loop cooling systems at your power plant. Chances are good that they cool or control temperature on some very critical components. The two that are most likely to exist are the so-called bearing cooling water system (which takes care of more than just bearings) and the stator cooling system, for those plants that have a water-cooled stator. Closed-loop cooling systems can also be found in air coolers on the intakes of combustion turbines.
By its very nature, when a closed-loop system remains closed and operates properly for an extended period of time, it is often forgotten—or at least neglected. Small changes in the chemistry or the flow rates and differential pressures throughout the system may not be noticed. However, once corrosion processes get a foothold in these systems, it can be very difficult to correct them. In the meantime, critical data equipment may be damaged to the point where it affects the ability of the plant to operate.
We begin with some general principles and practices for closed-loop cooling water systems before looking at the stator cooling water system, which is a special case.
Understanding Closed-Loop Cooling Systems
Most power plants using closed-loop water cooling for mechanical systems (rather than for the steam cycle) have several subsystems. The bearing cooling water system generally provides cooling for critical pump bearings and seals, hydrogen coolers for the generator, lube oil, and air compressor coolers. Other closed-loop cooling systems can include chilled water systems for air chillers used at the air inlet to the gas turbines at a combined cycle power plant and the chemistry sample panel.
A closed-loop cooling system can exchange heat with the main cooling water system in conventional tube and shell heat exchangers or plate and frame heat exchangers. Chilled water systems (air chillers) exchange heat with the compressor, which in turn uses a cooling tower to throw heat back into the environment.
Generally, demineralized water is used for closed-loop cooling water makeup, but chemical treatments are required to prevent corrosion and, in some systems, freezing. Most commonly, the piping in a closed-loop system is carbon steel. Heat exchange surfaces, such as air chiller assemblies, may be copper or even aluminum. Plate and frame heat exchangers are often made of stainless steel plates. Care and keeping of these systems requires that you pay attention to all the metals.
In a closed-loop system, oxygen pitting is the most common type of corrosion (Figure 1). Symptoms of oxygen pitting may be rusty water or recurring maintenance on bearings due to the abrasion caused by the corrosion products against the seal surfaces.
In order for oxygen pitting to occur, there must first be a deposit that covers a portion of the metal surface, creating a differential between the oxygen content underneath the deposit and the oxygen content in the bulk water. The oxygen-deficient area underneath the deposit becomes the anode, and the area around the deposit that is exposed to the bulk water becomes the cathode. This “big cathode, little anode” configuration causes concentrated and accelerated pitting in a confined area, producing pinhole leaks.
If bacteria are allowed to propagate inside the closed-loop system, they can create a “living” deposit. The byproducts of bacterial respiration are often acidic, and respiration also consumes oxygen, causing the base of the biofilm to be conducive to corrosion of the base metal. This further encourages some types of bacteria, as they use the oxidized metal in their metabolism.
Chemical Treatments for Closed-Loop Water Cooling
When a closed-loop cooling system is tight—experiencing no water loss—the chemical treatment that is applied can last for weeks or months before it needs to be refreshed. This can lead to complacency. On the other hand, closed-loop cooling systems that have leakage—and which have significant water loss—can be nearly impossible (and sometimes very expensive) to maintain at the proper treatment levels. Improper treatment levels will always lead to corrosion of these systems.
Below we list of few options that you can successfully use for treating closed-loop cooling systems such as the bearing cooling water system or closed-loop air chiller system. Generally, you find a treatment program that works well for the various metals in your system and system requirements (for example, determine if you need freeze protection) and then stick with it.
Regardless of which of the three chemical treatments you choose, they are likely to also contain pH buffers (caustic and sodium borate are common) to maintain an alkaline pH, which is conducive to minimizing corrosion in carbon steel. If there is copper in the closed-loop system, an azole may be added to the treatment to maintain a protective chemical layer on top of the exposed copper metal surfaces.
Sodium Nitrite. Sodium nitrite has been in use for many years to prevent corrosion in a wide variety of closed-loop systems. Nitrite is an oxidizer and essentially stops corrosion by “corroding” everything evenly. This seems counterintuitive, but when everything becomes the cathode and there is no anode, corrosion stops.
A constant supply of nitrite in the system ensures that any bare spots that are created quickly become passivated. However, if there is insufficient nitrite in the chilled water loop, an anode can form in the piping, and again we have the big cathode/little anode corrosion cell. The general guidelines for nitrite-based treatments are for a minimum of 700 ppm of nitrite.
Nitrites are utilized by some bacteria as an energy source. If the closed-loop system becomes contaminated with these bacteria, the nitrite level can decrease rapidly. The bacteria also generate biofilms, which create deposits producing areas that are anodes to the rest of the piping. Adding more nitrite only further accelerates the reproduction of the bacteria, making the problem worse. Systems using nitrite should be regularly tested for the presence of bacteria. In some systems, nonoxidizing biocides such as glutaraldehyde or isothiazoline are added to the treatment to prevent bacterial growth.
Sodium Molybdate. Sodium molybdate is generally classified as an anodic oxidizing inhibitor. Molybdate works with the dissolved oxygen in the water to form a protective ferricmolybdate complex on the steel.
Molybdate treatment levels can be anywhere between 200 ppm and 800 ppm as molybdate. Closed-loop systems that use demineralized water makeup would tend to be on the lower end of this range. Unfortunately, the world supply of molybdate metal tends to be concentrated in areas of historical political unrest, and over the years, molybdate prices have varied dramatically. That price variability can make molybdate treatment competitive with nitrite—or far more expensive.
Ironically, in closed-loop systems that are very tight, dissolved oxygen levels can drop, and thus minimize the effectiveness of a molybdate treatment (which requires dissolved oxygen to form a passive layer). Experts recommend a minimum of 1 ppm of dissolved oxygen in molybdate-treated systems.
Polymer Treatments. Polymer treatments have been used for many years to prevent scale and corrosion product accumulations in open cooling towers. Similar polymers are also now sold for use in closed-loop systems. It appears that the polymer acts as a dispersant for any corrosion products or scale that might form, so it prevents corrosion by keeping the surface clean and ensuring that any dissolved oxygen in the water attacks all surfaces evenly. This produces a general, but overall low level of corrosion.
One of the advantages of this treatment is that it is thought to be very environmentally benign, although as long as the closed-loop system remains closed, there should be no impact on the environment.
Monitoring Closed-Loop Cooling Water
Key to keeping your closed-loop system functioning properly is regular monitoring. Whatever the active agent is in your treatment (nitrite, molybdate, or polymer) the concentration must be regularly monitored. Generally, weekly testing is sufficient unless the levels of the treatment are dropping. (You won’t know that if you are not monitoring regularly.) Because the carbon steel and copper corrosion treatment are typically blended into one product, low levels of treatment may affect more than just the carbon steel piping.
The pH of the water should also be tested regularly. Considering the amount of pH buffering in chemical treatment, the pH of the water should be rock solid. Drops in pH may indicate bacterial contamination, particularly with the nitrite-based treatments. Another thing that can drop the pH is leaks in the system, which bring in fresh demineralized water makeup.
Be on the lookout for other signs of bacterial contamination, such as slimy growth in any sightglass or flow indicators, or septic smells when the sample is collected. Plate and frame heat exchangers have a very large surface and small spacing for heat exchange between the plates. Bacterial contamination can not only seriously affect heat transfer, but it also can cause pinhole leaks in the stainless steel plates. Depending on the pressure of the closed-loop versus open-loop system at this point, the bearing cooling water may leak out, or the open cooling water may leak in.
Remember that it is much easier to prevent bacterial contamination than it is to try to recover from a system that is severely contaminated.
Read more at: http://www.powermag.com/monitoring-treatment-closed-loop-cooling-water-systems/?pagenum=1
Ozonation of Cooling Towers
The objective of ozone use with cooling towers is to maintain the highest purity of water with the least amount of water waste and chemical use. Chemical use in cooling towers leads to ever-increasing total dissolved solids (TDS), which must be reduced by eliminating water (blow down/bleed off) and then refilling with raw/lower TDS water. This is a vicious circle that will never end unless one of the TDS-increasing culprits (a.k.a. chemicals) is eliminated or reduced.
The Problem
Cooling tower water quality tends to be extremely poor. Cooling tower traditional treatment is based on extreme chemical use only. This means that you, the water treatment professional, have a chance to create an entirely new income base and aid in environmental integrity and responsibility. There are three main problems surrounding cooling towers.
* Water quality control is difficult due to
- Evaporation rate,
- Environmental contaminants, and
- Extreme chemical use.
* Chemical dependence is promoted by an industry that serves and maintains cooling towers. Most cooling tower manufacturers do nothing about recommending or selling treatment equipment along with the towers. In most cases, it is left up to the end users to set up the treatment method. The cost of chemicals is lower on the front end than water treatment equipment, but far higher based upon the ongoing nature of the use.
* The water waste issue. For example, it is not uncommon to see a 3,000-gallon cooling tower constantly draining water, then constantly replenishing raw water just to lower TDS. This ever-increasing TDS is contributed to a great degree by the chemicals that are used for treatment.
Not only is there an extreme amount of water being wasted on a daily basis, but the environmental impact from the chemical-laden wastewater is deplorable. This chemical-laden wastewater eventually will make its way into our lakes, streams, rivers and groundwater. That is why this wastewater is becoming the subject of more stringent U.S. Environmental Protection Agency regulations.
Cooling Towers and Ozone Primary Uses
Ozone is used in cooling tower treatment for
* Bacteria/virus elimination/prevention
* Organic build-up elimination/prevention
* Blow-down reduction/elimination
* Bleed-off reduction/elimination
* Improved clarity
* Scale reduction
* Cooler running temperatures where scale is inhibited or reduced
* Reduction or elimination chemicals needed for algae control
Principle of Operation
Ozone is injected into the water flow created by a separate circulation pump. This pump pulls the water from the tower's sump or basin and sends it to the ozone injector, contact tank and scale removal/filtration system. Lastly, the treated water returns back to the sump or basin. The principle is to treat the water and eliminate/reduce the following contaminants.
* Scale-forming minerals
* Organics
* Algae
* Harmful microbes
The clean water then is used to clean the entire sump, basin, pipes and peripheral equipment.
The ozone treatment system is simple and can be broken down into three easy steps.
* Ozone injection. Ozone is injected into the side stream flow. Oxidation starts to take place immediately on microbes, organics, bacteria and viruses.
* Contact/mixing. A contact tank helps to further the ozone's ability to oxidize particles allowing them time to react prior to returning to the system. As water flows down the off-gas tank, ozonated water rises and strips any gas in the incoming water. (The off gas tank is the same design as what was discussed in my column "Ozone Installation," February 2003, and "Well-Ozone Again," December 2002, Water Quality Products.)
* Filtration, scale control, particle removal. Possibly the most important aspect of any water treatment is the removal of the particles that have been oxidized. Without this step, all you have done with the ozone is change the structure of the particles by making them larger, insoluble and/or heavier. This step is necessary for systems that require scale control and particulate removal.
It is very important not to construct an ozone unit too large to handle the bacteria, scale and algae. The problem encountered at this point could be corrosion. If you carry an ozone residual too high to de-scale downline you stand a chance of creating a corrosive situation in the sump and its adjacent equipment. For this reason, it's important to utilize existing water treatment technology and equipment in conjunction with ozonation. The result is a system that works without high maintenance, dangerous chemicals, extreme water waste and costly corrosion.
Read this article at https://www.wqpmag.com/o-zone-todays-lesson-ozonation-cooling-towers
Utilize valvulas de control para optimizar su eficiencia
El gasto de tratamiento de agua para el uso en torres de refrigeración, calderas, y otras aplicaciones de la planta está aumentando rápidamente. Además de los altos costes, las plantas se enfrentan a menudo la necesidad de cumplir con los efluentes de las regulaciones que rigen, incluyendo el agua de enfriamiento, enviada a través de las instalaciones de tratamiento antes de su descarga.
funcionamiento óptimo de los sistemas de agua de refrigeración significa un uso mínimo de agua mientras se mantiene la temperatura adecuada para limitar el crecimiento de algas y enfriar todo el equipo correctamente. Una forma de ayudar a lograr estos objetivos al tiempo que reduce significativamente el consumo de energía es la instalación de válvulas de control de agua (CW) de refrigeración.
Un sistema bien equilibrado
Funcionamiento de un sistema de agua de refrigeración de manera eficiente requiere equilibrio. Un sistema bien equilibrado es una de la que se elimina los cortocircuitos. El cortocircuito se produce cuando el agua de refrigeración excesiva fluye a través de un enfriador causando flujo insuficiente a través de los otros. Esta morir de hambre a menudo se produce al final de un sistema o en unidades en las elevaciones más altas.
El logro de un sistema equilibrado es un proceso detallado y complicado. Las caídas de presión deben ser figurado para cada pieza de equipo y las tuberías correspondientes, y para cada rama del circuito de agua de refrigeración. Incluso si estos cálculos se realizan cuando una planta es nueva, las condiciones cambian con el tiempo. Depósitos se acumulan en las superficies que alteran el coeficiente de transferencia de calor y resistencia al flujo (caída de presión). Adición o eliminación de equipo del sistema también cambia el equilibrio y puede conducir a cortocircuitos.
equilibrio manual de un sistema de agua de refrigeración usando placas de orificios es difícil y consume tiempo. Las preocupaciones de seguridad a menudo dictan que un orificio estar dimensionado para la máxima demanda. Como resultado, la bomba de agua de refrigeración debe ser dimensionado para, y a menudo debe operar a, altas velocidades de flujo en exceso.
A veces se hacen intentos para equilibrar un sistema mediante el uso de una válvula de globo y estrangular manualmente el flujo. Por desgracia, este enfoque conduce a menudo a un operador de abrir la válvula completamente cuando se necesita flujo máximo, entonces no es el reajuste. De nuevo el resultado es alto flujo cuando el sistema requiere un flujo promedio o mínimo.
El exceso de flujo se indica por una temperatura de enfriamiento de salida de agua sólo unos pocos grados por encima de la entrada. Esta condición de flujo desequilibrado conduce a un mayor consumo de energía de bombeo y zonas distantes o elevadas que a menudo se ven privadas para el agua.
Superior de retorno de agua de refrigeración (salida) las temperaturas dan como resultado un menor consumo de agua de refrigeración. En estas condiciones, la temperatura del agua de refrigeración se debe aumentar hasta el máximo permitido por el proceso. Este incremento se logra reduciendo al mínimo el flujo. Pero antes de tomar esta acción, otras condiciones deben ser evaluados.
Las temperaturas más altas (por encima de 120 F) pueden hacer que el calcio para precipitar fuera del agua a una velocidad alta, resultando en escala y conduce a una mayor caída de presión y la transferencia de calor reducida. El aumento de las temperaturas también promueven el crecimiento de algas. La tasa varía con la calidad del agua y tipo de tratamiento.
La distribución de agua de refrigeración a lo largo de un sistema requiere controladores adecuados que mantienen las temperaturas de salida dentro de un rango especificado, incluso durante el enfriamiento parcial. Si las temperaturas de salida no se pueden aumentar, los controladores todavía pueden reducir el flujo cuando las necesidades de agua caen.
configuraciones de válvula de control
Las válvulas de control se aplican con éxito en una variedad de sistemas de refrigeración de agua. En la mayoría de sistemas, una válvula de control CW proporcional se puede instalar en la línea de retorno (Fig. 1). La válvula, que controla el caudal de agua en proporción directa a la temperatura de salida, debe estar situado tan cerca del enfriador como sea posible.
Cuando el agua de refrigeración está frío, la válvula reduce el caudal a un ligero sangrado. A medida que la temperatura de salida se eleva, la válvula se abre y se regula el flujo para mantener una temperatura de descarga constante. La válvula CW debe estar diseñado para mantener un flujo de purga constante. Sin algo de flujo, el elemento sensor de la válvula no puede decir lo que está pasando.
El uso de válvulas de control CW asegura equilibrado automático del sistema de agua de refrigeración, debido a que la válvula sólo utiliza tanta agua como el enfriador requiere. uso reducido de agua asegura se proporciona un suministro adecuado de agua de refrigeración, incluso a zonas alejadas de la nevera o en elevaciones más altas.
El mantenimiento de una temperatura de proceso en un valor preciso requiere un esquema de control diferente. Un sensor de temperatura (termopar), el controlador y la válvula de control accionada neumática o eléctricamente puede ser utilizado. Otra opción es una válvula de control de auto Cualquier disposición controla temperaturas de la corriente de proceso con diferentes grados de precisión. En muchos casos, la válvula de acción automática ofrece una precisión razonable a un menor coste de instalación.
pautas de aplicación
En aplicaciones que tienen una descarga abierta a un desagüe, la línea de descarga de la válvula CW debe estar siempre lleno. Esta condición puede garantizarse con un sello de bucle en la tubería de salida a una altura por encima de la válvula que luego va a grado (Fig. 3). Sin un sello líquido, líneas pueden vaciar cuando el equipo está apagado. El líquido o elementos de sellado termostáticas lleno de cera se pueden secar y fallar prematuramente.
Un colador puede ser instalado aguas arriba de la válvula de CW si las condiciones de calidad del agua requieren. Suciedad y los residuos afectan cierre adecuado y caricias de la válvula. Si existe este problema, una anulación de neumático (Fig. 4) puede ser utilizado para purgar la válvula de suciedad.
La mejor manera de controlar el crecimiento de algas y la acumulación es mantener la alta calidad del agua. Otros factores que contribuyen al crecimiento de algas incluyen la temperatura y la velocidad. Manteniendo la temperatura por debajo de 120 F es deseable. Dimensionamiento de la velocidad de flujo para lograr una mayor velocidad también tiende a obstaculizar el crecimiento de algas. Las algas limo ha sido conocido para formar en líneas con velocidades que pueden alcanzar tan alto como 10 pies / seg.
El lado del agua de refrigeración de un proceso a menudo se pasa por alto como incontrolable. Sin embargo, el enfriamiento válvulas de control de agua puede promover el ahorro al reducir el uso de agua, bomba de las necesidades de energía, y los costos de tratamiento de agua. En la nueva construcción, pequeños tamaños de tuberías y bombas pueden reducir los costes de equipamiento. válvulas CW también proporcionan mejor control del proceso mediante el mantenimiento de una diferencia de temperatura fijo a través de la entrada de agua refrigerante y la salida. En la mayoría de los casos, los análisis justificar la instalación de dichos controles.
- Editado por Jeanine Katzel, Editor Senior, 847-390-2701, j.katzel@cahners.com
Más información
El autor responderá a las preguntas técnicas sobre este artículo. Se le puede contactar por teléfono al 201
Conceptos clave
funcionamiento óptimo de los sistemas de agua de enfriamiento ayuda a limitar el crecimiento de algas y equipo fresco adecuadamente mientras se mantiene la temperatura adecuada.
Enfriamiento válvulas de control de agua reducen el uso del agua, las necesidades de energía de la bomba, y los costes de tratamiento de agua.
ahorro de agua y energía suelen proporcionar un rápido retorno de la inversión sistema de válvulas.
La justificación de los costes
Los ahorros en agua y la energía de refrigeración típicamente proporcionan una recuperación de la inversión rápida de la inversión en el sistema de la válvula. Enfriamiento válvulas de control de agua también reducen los costos de capital de una nueva instalación al permitir el uso de bombas y filtros más pequeños, y, en algunos casos, la reducción de tamaño de las tuberías.
Un ejemplo de ahorros obtenidos en un sistema retrofit se muestra a continuación.
Condiciones existentes:
Q = calor tasa de eliminación del enfriador, 700.000 Btu / hr
T (sub i) = entrada de temperatura del agua refrigerante, 50 F
T (sub o) = temperatura de salida del agua de refrigeración sin control, 59 F
C (sub p) = calor específico, 1 Btu / lb / ° F
m = tasa de flujo de masa, lb / hr
v = velocidad de flujo volumétrico, gpm
m = Q / C (sub p) (T (sub o) - T (sub i)) = 700.000 / 1 (59 - 50) = 77.777 lb / hr
v = 155.5 gpm
(Para v, para convertir lb / hr en gpm divide por el factor de conversión de 500, que se obtiene multiplicando 8,33 lb / gal. De agua por 60 min / hr.)
Después de que se instala una válvula de CW, la temperatura de descarga se puede ajustar a 82 F. Inserción de la nueva T (sub o) en los rendimientos ecuación:
m = Q / C (sub p) (T (sub o) - T (sub i)) = 700.000 / 1 (82 - 50) = 21.875 lb / hr
v (nueva tasa de flujo volumétrico) = 43,7 gpm
En un sistema en el que no se recircula agua, el uso gotas de agua 72%. En un sistema de circuito cerrado, una cierta cantidad de agua se pierde por evaporación en la torre de enfriamiento y durante la purga. los costes de tratamiento del agua también deben tenerse en cuenta en el análisis.
Además de un ahorro de agua, la energía se conserva porque se necesita menos energía para bombear menos agua. La figura ahorro de energía de la bomba muestra tres gráficos de la cabeza de descarga. Eficiencia y consumo de energía de una bomba típica centrífuga se representan frente a desplazamiento de volumen. Tenga en cuenta que incluso con una reducción en la eficiencia, el consumo de energía es de 6,5 kW antes de instalar la válvula de control y 3,5 kW después, una reducción de energía del 46%.
A un costo de agua tratada de $ 0.50 / 1000 gal. y el costo de energía de $ 0.05 / kWh, el ahorro total anual de $ 7190. La figura supone una pérdida de agua 10% de la evaporación de purga. El ahorro anual para un sistema abierto de descarga son más de $ 60.000.
Using Control Valves To Optimize Cooling Water System Efficiency
The expense of treating water for use in cooling towers, boilers, and other plant applications is rapidly increasing. In addition to high costs, plants often face the need to comply with regulations governing effluents, including cooling water, sent through treatment facilities before being discharged.
Optimum operation of cooling water systems means minimum use of water while maintaining proper temperatures to limit algae growth and cool all equipment properly. One way to help achieve these goals while significantly reducing energy consumption is to install cooling water (CW) control valves.
A well-balanced system
Operating a cooling water system efficiently requires balance. A well-balanced system is one from which short-circuiting is eliminated. Short-circuiting occurs when excessive cooling water flows through one cooler causing insufficient flow through the others. This starving often occurs at the end of a system or in units at higher elevations.
Achieving a balanced system is a detailed and complicated process. Pressure drops must be figured for each piece of equipment and its associated piping, and for each branch of the cooling water circuit. Even if these calculations are done when a plant is new, conditions change over time. Deposits build up on surfaces altering the heat transfer coefficient and resistance to flow (pressure drop). Adding or removing equipment from the system also changes the balance and can lead to short circuiting.
Manually balancing a cooling water system using orifice plates is difficult and time consuming. Safety concerns often dictate that an orifice be sized for maximum demand. As a result, the cooling water pump must be sized for, and often must operate at, excessively high flow rates.
Sometimes attempts are made to balance a system by using a globe valve and manually throttling the flow. Unfortunately, this approach often leads to an operator opening the valve fully when maximum flow is needed, then never readjusting it. Again the result is high flow when the system requires average or minimum flow.
Too much flow is indicated by a cooling water outlet temperature only a few degrees above the inlet. This unbalanced flow condition leads to higher pump energy consumption and distant or elevated areas that are often starved for water.
Higher cooling water return (outlet) temperatures result in lower cooling water consumption. Under these conditions, cooling water temperatures should be increased to the maximum permitted by the process. This increase is accomplished by minimizing the flow. But before this action is taken, other conditions must be evaluated.
Higher temperatures (above 120 F) can cause calcium to precipitate out of water at a high rate, resulting in scaling and leading to increased pressure drop and reduced heat transfer. Increased temperatures also promote algae growth. The rate varies with quality of water and type of treatment.
Distributing cooling water throughout a system requires proper controllers that maintain outlet temperatures within a specified range, even during partial cooling. If outlet temperatures cannot be increased, controllers can still reduce the flow when water requirements drop.
Control valve configurations
Control valves are successfully applied in a variety of cooling water systems. In most systems, a proportional CW control valve can be installed in the return line (Fig. 1). The valve, which controls the water flow rate in direct proportion to the outlet temperature, should be located as close to the cooler as possible.
When the cooling water is cold, the valve reduces the flow rate to a slight bleed. As the outlet temperature rises, the valve opens and regulates the flow to maintain a constant discharge temperature. The CW valve should be designed to maintain a constant bleed flow. Without some flow, the valve sensing element cannot tell what is going on.
Use of CW control valves ensures automatic balancing of the cooling water system, because the valve uses only as much water as the cooler requires. Reduced water use ensures an adequate supply of cooling water is provided, even to areas far from the cooler or at higher elevations.
Maintaining a process temperature at a precise value requires a different control scheme. A temperature sensor (thermocouple), controller, and pneumatically or electrically actuated control valve can be used. Another option is a self-acting control valve with a capillary tube (Fig. 2) inserted in the process stream. Either arrangement controls process stream temperatures with varying degrees of accuracy. In many cases, the self-acting valve offers reasonable accuracy at a lower installed cost.
Application guidelines
In applications that have an open discharge to a drain, the CW valve discharge line should always be full. This condition can be ensured with a loop seal at the outlet piping at an elevation above the valve that then goes to grade (Fig. 3). Without a liquid seal, lines may empty when equipment is shut down. Liquid or wax-filled thermostatic seal elements can dry out and fail prematurely.
A strainer may be installed upstream of the CW valve if water quality conditions require. Dirt and debris affect proper closing and stroking of the valve. If this problem exists, a pneumatic override (Fig. 4) can be used to purge the valve of dirt.
The best way to control algae growth and buildup is to maintain high water quality. Other factors contributing to algae growth include temperature and velocity. Keeping the temperature below 120 F is desirable. Sizing the flow rate to achieve a higher velocity also tends to hinder algae growth. Algae slime has been known to form in lines with velocities that can reach as high as 10 ft/sec.
The cooling water side of a process is often overlooked as uncontrollable. However, cooling water control valves can promote savings by reducing the use of water, pump energy requirements, and water treatment costs. In new construction, smaller pipe and pump sizes can lower capital equipment costs. CW valves also provide better process control by maintaining a fixed temperature difference across the cooling water inlet and outlet. In most cases, analyses justify the installation of such controls.
-- Edited by Jeanine Katzel, Senior Editor, 847-390-2701, j.katzel@cahners.com
More info
The author will answer technical questions about this article. He may be reached by phone at 201-403-1556 or by mail in care of his company, 10 York Ave., West Caldwell, NJ 07006.
Key concepts
Optimal operation of cooling water systems helps limit algae growth and cool equipment properly while maintaining proper temperatures.
Cooling water control valves reduce water use, pump energy requirements, and water treatment costs.
Water and energy savings typically provide rapid payback on the valve system investment.
Justifying the costs
Savings in cooling water and energy typically provide a rapid payback on the valve system investment. Cooling water control valves also reduce capital costs of a new installation by allowing use of smaller pumps and filters, and, in some cases, reduced pipe sizes.
An example of savings achieved in a retrofit system is shown below.
Existing conditions:
Q = heat removal rate of cooler, 700,000 Btu/hr
T(sub i) = inlet cooling water temperature, 50 F
T(sub o) = outlet cooling water temperature without control, 59 F
C(sub p) = specific heat, 1 Btu/lb/deg F
m = mass flow rate, lb/hr
v = volumetric flow rate, gpm
m = Q/C(sub p) (T(sub o) - T(sub i) ) = 700,000/1(59 - 50) = 77,777 lb/hr
v = 155.5 gpm
(For v, to convert lb/hr to gpm divide by the conversion factor of 500, which is arrived at by multiplying 8.33 lb/gal. of water by 60 min/hr.)
After a CW valve is installed, the discharge temperature can be set to 82 F. Inserting the new T(sub o) into the equation yields:
m = Q/C(sub p) (T(sub o) - T(sub i) ) = 700,000/1(82 - 50) = 21,875 lb/hr
v (new volumetric flow rate) = 43.7 gpm
In a system in which water is not recirculated, water use drops 72%. In a closed loop system, a certain amount of water is lost to evaporation in the cooling tower and during blowdown. Water treatment costs also must be taken into account in the analysis.
In addition to water savings, energy is conserved because less power is needed to pump less water. The pump energy savings figure shows three discharge head charts. Efficiency and power consumption of a typical centrifugal pump are plotted against volume displacement. Note that even with a reduction in efficiency, power consumption is 6.5 kW before the control valve is installed and 3.5 kW afterward, a 46% energy reduction.
At a treated water cost of $0.50/1000 gal. and energy cost of $0.05/kWh, annual savings total $7190. The figure assumes a 10% water loss from blowdown evaporation. Annual savings for an open discharge system are more than $60,000.
Historia de reciclaje de agua de purge de torre de enfriamiento
sby1 (Marino / Océano)
(OP)
19 de Ago 06 15:02
Estoy tratando de tratar a 50.000 galones por día de la torre de enfriamiento purga de aguas residuales para su uso como agua para irrigación. Mi pH es 8,6, sodio 270 ppm, chloride190ppm y TDS 1568. Las plantas no van a tolerar altos niveles de pH y de sodio. Alguna sugerencia
cuarc (Mechanical) 20 Aug 06 04:10
RO puede ser una buena opción en su caso. Ni sodio ni pH es un problema. De hecho, la alimentación de agua pH de 8 a 9 es bueno. Sólo echa de sílice.
Re: Torre de enfriamiento Purga del agua
cuarc (Mechanical) 20 Aug 06 04:13
PS: No estoy bien informado sobre los efectos perjudiciales de sodio en cualquier tipo de sistema de tratamiento de agua. ¿Puede usted explicar por favor? ¿Cuál es la composición de TDS 1568?
Re: Torre de enfriamiento Purga del agua
BigInch (Petróleo) 20 Aug 06 05:54
Las sales de sodio no son toleradas por la mayoría de los cultivos. TDS = sólidos disueltos totales
Going the Big Inch! http://virtualpipeline.spaces.msn.com
Re: Torre de enfriamiento Purga del agua
cuarc (Mecánico) 20 Ago 06 6:21
Gracias, lo confundí y por las plantas presumí plantas de tratamiento de agua. Pedí composición constituyente de TDS y no la forma completa.
Re: Torre de enfriamiento Purga del agua
LHA (Civil / Ambiental) de 21 Ago 06 08:23
De acuerdo con la preocupación de quark situada en la composición de TDS. Podría ser nada, que podría ser sólo las algas de las bobinas, que en realidad sería bueno para las plantas, aunque podría ensuciar las mangueras de distribución, boquillas, etc. Pero, mi conjetura es que usted ha utilizado un alguicida para evitar el crecimiento de algas para aumentar enfriamiento eficiencia bobina. Si esto es cierto, algún componente de la TDS es muy probable que las sales en el alguicida. Y si son sales de Cu - muy común en algicidas - esas son tóxicos para las plantas. Creo RO eliminaría la mayoría de los metales, en cualquier caso, sin embargo. Pero no habrá que ser muy caro?
La ingeniería es la práctica del arte de la ciencia
Re: Torre de enfriamiento Purga del agua
bimr (Civil / Ambiental) de 21 Ago 06 09:58
Esto ha sido discutido en otros mensajes, es posible que desee buscar en el sitio. El TDS de purga es demasiado alto para el riego de cultivos como otros han publicado, por lo que tendrá la desalación como tratamiento si desea volver a utilizar la purga. Un enfoque más práctico es convertir su sistema de torre de enfriamiento de una torre de refrigeración de descarga cero. Haciendo esto reducirá sus necesidades de suministro de agua (el agua adicional se puede utilizar para el riego en su lugar), y eliminar por completo la purga. No ha revelado qué tipo de planta de fabricación que se está trabajando, pero este enfoque ha sido utilizado en muchas plantas de energía, especialmente en las zonas con escasez de agua. Evaporadores se utilizan comúnmente para la desalinización en estos sistemas. Cero sistemas de refrigeración de purga cuestan más que los arreglos de torre de refrigeración convencionales. Sin embargo, se dará cuenta de que tratar para desalinizar la purga torre de refrigeración (con especial RO) es una propuesta problemático y difícil.
Re: Torre de enfriamiento Purga del agua
busby1 (Marino / Océano)
(OP)
21 de Ago 06 10:02
Gracias por las respuestas. Mi Cu es sólo 3ppm y zinc son .37ppm.Sulphates 705ppm.The RO es obviamente el camino a seguir, pero estamos tratando de ver si hay una manera de precipitar las sales de sodio, etc a través de filtros químicos o profundidad. He oído hablar de una "píldora" sólido puesto en el flujo de agua que reacciona con los nitratos de sodio dando, pero no soy un eng química así que no sé las composiciones de esta reacción y lo que la tableta podría ser hecha.
Re: Torre de enfriamiento Purga del agua
bimr (Civil / Ambiental) de 21 Ago 06 11:08
Un sistema basado RO funcionará, pero será problemática para operar debido a que el sistema de RO no puede tolerar los sólidos en suspensión en la purga. Por supuesto, puede filtrar los sólidos. Sin embargo, es más fácil decirlo que hacerlo para tratar de producir un bajo nivel de agua SDI que es adecuado para RO. No hay formas sencillas de obtener sulfato de sodio de la solución. La solubilidad del sulfato de sodio es de aproximadamente 2,500 mg / l, por lo que la precipitación de sulfato de sodio no va a ser de mucha ayuda. Que busca, ya sea por evaporación o RO. No hay otras opciones para la desalinización. Usted debe empezar a buscar otras alternativas primero. Por ejemplo, 1. ¿Es posible aumentar la purga por lo que se reduce la concentración de sales? 2. ¿Es posible ir a una torre de refrigeración de descarga cero. etcétera
Cooling Tower Blow Down Water Recycle Story
busby1 (Marine/Ocean)
(OP)
19 Aug 06 15:02
I am trying to treat 50,000 gallons per day of cooling tower blow down waste water to use as water for irragation. My pH is 8.6,sodium 270 ppm,chloride190ppm and TDS 1568. Plants will not tolerate high pH and sodium levels. Any suggestions
quark (Mechanical) 20 Aug 06 04:10
RO can be a good option in your case. Neither sodium nor pH is a problem. Infact, the feed water pH of 8 to 9 is good. Just check silica.
RE: Cooling Tower Blow Down Water
quark (Mechanical) 20 Aug 06 04:13
PS: I am not knowledgeable about detrimental effects of sodium on any kind of water treatment system. Can you please explain? What is the composition of 1568 TDS?
RE: Cooling Tower Blow Down Water
BigInch (Petroleum) 20 Aug 06 05:54
Sodium salts aren't tolerated by most crops. TDS = Total dissolved solids
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RE: Cooling Tower Blow Down Water
quark (Mechanical) 20 Aug 06 06:21
Thanks, I mistook and by plants I presumed water treatment plants. I asked for constituent composition of TDS and not the full form.
RE: Cooling Tower Blow Down Water
LHA (Civil/Environmental) 21 Aug 06 08:23
Agree with quark's concern on TDS composition. It could be nothing, it could be just algae from the coils, which would actually be good for plants, although it might foul distribution hoses, nozzles, etc. But, my guess is you've used an algacide to prevent algae growth to increase cooling coil efficiency. If this is true, some component of the TDS is most likely the salts in the algacide. And if they are Cu salts - very common in algacides - those are toxic to plants. I think RO would remove most metals in any case though. But won't that be pretty expensive?
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RE: Cooling Tower Blow Down Water
bimr (Civil/Environmental) 21 Aug 06 09:58
This has been discussed on other posts, you might want to search the site. The blowdown TDS is too high for the crop irrigation as others have posted, so you will need desalination as treatment if you want to reuse the blowdown. A more practical approach is to convert your cooling tower system to a zero discharge cooling tower. Doing this will reduce your water supply needs (The extra water can be used for irrigation instead), and eliminate the blowdown altogether. You have not revealed what type of manufacturing plant that you are working on, but this approach has been used on many power plants, especially in water short areas. Evaporators are commonly used for desalination in these systems. Zero blowdown cooling systems cost more than conventional cooling tower arrangements. However, you will find that trying to desalinate the cooling tower blowdown (with RO especially) is a problematic and difficult proposition.
RE: Cooling Tower Blow Down Water
busby1 (Marine/Ocean)
(OP)
21 Aug 06 10:02
Thanks for replies. My Cu is only 3ppm and zinc .37ppm.Sulphates are 705ppm.The RO is obviously the way to go but we are trying to see if there is a way of precipitating the sodium etc. out through chemical or depth filters. I have heard of a solid "pill" put into the flow of water which reacts with the sodium giving nitrates but I am not a chemical eng so do not know the compositions of this reaction and what the pill would be made of.
RE: Cooling Tower Blow Down Water
bimr (Civil/Environmental) 21 Aug 06 11:08
An RO based system will work, but it will be problematic to operate because the RO system can not tolerate the suspended solids in the blowdown. Of course, you can filter out the solids. However, it is easier said than done to attempt to produce a low sdi water that is suitable for RO. There are no simple ways to get sodium sulfate out of solution. The solubility of sodium sulfate is approximately 2500 mg/l, so precipitation of sodium sulfate is not going to be of much help. You are looking at either evaporation or RO. There are no other options for desalination. You should start looking at other alternatives first. For example, 1. Is it possible to increase the blowdown so the concentration of salts is reduced? 2. Is it possible to go to a zero discharge cooling tower. etc.
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