1、Helen Cerra is aTechnical Staff Consultant with Chemtreat, Inc., Richmond, Virginia. The Great Debate Between Non-Chemical Devices and Chemicals What Program Can Meet Water Treatment Performance Standards? Chemical Treatment, of Course! Helen R. Cerra Member ASHRAE ABSTRACT The debate continues betw
2、een the use of non-chemical water treatment devices and chemical water treatment for treating cooling water systems. The goals of any cooling water treatment program are to protect against corrosion, deposition and microbiological fouling. This paper will present the chemical water treatment method
3、to accomplish the best performance standards. In doing so, we shall critique both and highlight the benefits of chemical water treatment. We shall present results of trials where both technologies have been used to treat cooling water and will discuss the methodologies behind each technology. Many p
4、roviders promote “green”, “safe” and “economical” as the reason to use a certain technology. Many users select their appropriate method of treatment based on these same factors. What factors should determine the type of treatment? When we define success, we can determine the best method to achieve i
5、t. INTRODUCTION Debate continues on the better method to treat open and closed recirculating cooling systems. Is it chemical water treatment or a non-chemical device (NCD)? This paper will present three scenarios where chemical water treatment is the better method. Scenario one will discuss the need
6、 to meet performance standards as well as changing discharge limitations. The second discussion will focus on a facility site where an NCD was being used and its performance. The final discussion will highlight prior research using NCDs to control microbial growth related to human pathogens. There a
7、re many types of NCDs. This paper does not compare each type against chemical treatment, but presents the three cases with different points of focus on the great debate. WATER TREATMENT FACTORSThe main purpose of water treatment is to ensure the best heat transfer efficiency as well as protect capit
8、al equipment investments. In addition, a water treatment program permits maximizing cycles of concentration for water conservation and can ensure control of human pathogens. The goal of water treatment is to provide the best corrosion, scale/deposition and biological control. All these factors can i
9、nfluence each other when any scale or corrosion products drop out of solution, buildup on equipment surfaces and establish an environment for microbiological growth. If microbiological organisms are not controlled, they can lead to corrosion. Some cooling tower systems require pretreatment based on
10、the metallurgy of the tower. Some steel towers are manufactured with a coating to provide a protective layer against corrosion of the metal surface. Proper treatment at the startup of a new galvanized tower is essential to form the protective layer. It is important to have the correct pH, calcium, a
11、lkalinity, sulfate and chloride concentrations. Otherwise, the zinc carbonate, protective layer will not form and the tower will experience white rust. Can an NCD accomplish the subtleties of this procedure? If not, the OEM warranty will be voided in many instances. To determine the type of treatmen
12、t for scale or corrosion, one first has to characterize the water. This can best be done by utilizing different indices that have been developed to define the water quality and predict the solubility limit of the least soluble substance. Langelier Saturation Index (LSI) predicts the degree of satura
13、tion of calcium carbonate (CaCO3) with a positive result indicating scale is possible (precipitating CaCO3) and a negative index indicating scale is not possible (CaCO3 dissolution). LSI is a function of pH, calcium, alkalinity, total dissolved solids and temperature. LSI = pH pHs pHs = saturation p
14、H The Ryzner Stability Index (RSI) can also be used to predict corrosion/scale tendencies. An index value of 6.0 or less means scaling increases and corrosion decreases. As the index value gets larger than 7.5, the corrosion potential increases. RSI = 2(pH) - pHs The Larson-Skold Index is yet anothe
15、r index that predicts corrosion. It was developed for the Great Lakes watershed taking into account hardness, sulfate, chloride and alkalinity. An index value less than 0.8 indicates less corrosive water while an index value in the range of 0.8 to 1.2 indicates higher corrosion. Index values greater
16、 than 1.2 indicate that very high corrosion rates can be expected. Table 1 compares the values of these indices. Other proprietary computer models have been developed to characterize scale and corrosion potential. Larson-Skold = (epm Cl- + epm SO42-)/epm HCO3- + epm CO32-) Table 1. Comparison of Sta
17、bility Indices Langelier (LSI) Ryzner (RSI) Larson-Skold* Extreme Scale formation 2.5 1.2 Extremely Corrosive 10 3 *Larson-Skold is best used as a predictor of corrosion and not scale.Traditionally, phosphatebased chemistry is the water treatment of choice for controlling both scale and corrosion. W
18、ith the addition of polymeric dispersants, systems can keep higher concentrations of orthophosphate in solution. Due to the limitations of phosphate, azoles provide corrosion protection for yellow metals. The effectiveness of these treatments is contingent upon proper application and control. While
19、some may argue that chemical water treatment inherently has safety concerns, industry has developed applicable container, delivery and feed options that provide hands free and no contact with chemical. This paper will not discuss the energy demands of any NCDs, but will acknowledge that the “green”
20、claim may be misleading when energy may be needed to run the device and failure to meet performance standards may cost a facility much more than a chemical program.PERFORMANCE STANDARDSAs stated above, one of the goals of water treatment is to protect the system metallurgy by controlling corrosion.
21、Monitoring corrosion rates allows for the assessment of the performance of the water treatment program by placing coupons matching the system metallurgy in the path of the flowing water for a specific time period. To evaluate system performance, this discussion will use the following classification
22、of corrosion rates in mils per year (mpy) for open cooling water systems developed by Mr. Bennett P. Boffardi as shown in Table 2. Table 2. Corrosion Rate Classification Description Carbon Steel mpy Copper Alloys mpy Negligible or Excellent To 1 To 0.1 Mild or Very Good 1 to 3 0.1 to 0.25 Good 3 to
23、5 0.25-0.35 Moderate to Fair 5 - 8 0.35 to 0.5 Poor 8-10 0.5 to 1 Very Poor to Severe 10 1 For biological control, the performance standard that will be used is from the CTI Guideline 148, shown in Table 3. Table 3. CTI Target Values Parameter Result Planktonic Counts 10,000 CFU/ml Sessile Counts 10
24、0,000 CFU/ml Deposits No higher life forms DISCUSSIONCase I The subject facility of in this case is a Midwest university utilizing water within the Great Lakes watershed with the water characteristics shown in Table 4. The cooling tower discharged to the municipal sewer that charged the facility bas
25、ed on flow with additional charges for impurities, in this case, total phosphate. Total phosphate charges were expected to increase ten times in a short period as a result of new limits being imposed by the state upon the sewer authority. The makeup water, Table 4, is Great Lakes area water containi
26、ng high hardness and alkalinity with a Larson-Skold Index of 7.5. A review of Table 1, shows that at that Larson-Skold index, the system water is extremely corrosive. The chemical treatment at the tower consisted of an organic phosphate and polymer with acid feed controllers set at 8.4. The total ir
27、on in the system was about 4.0 ppm. Planktonic counts were 1,000 CFU/ml, well within performance standards. Table 4. Lake Water Analysis Analysis Lake Water pH 8.05 Conductivity, mho 587 “P“Alkalinity, as CaCO3, mg/ - “M“Alkalinity, as CaCO3, mg/L 232 Calcium Hardness, as CaCO3, mg/L 97 Magnesium Ha
28、rdness, as CaCO3, mg/L 149 Iron, as Fe, mg/L 0.05 Copper, as Cu, mg/L 0.05 Zinc, as Zn, mg/L 0.05 Sodium, as Na, mg/L 28 Potassium, as K, mg/L 3.3 Chloride, as Cl, mg/L 65 Sulfate, as SO4, mg/L 21 Nitrate, as NO3, mg/L 2.2 OrthoPhosphate, as PO4, mg/L 0.4 Silica, as SiO2, mg/L 0.25 Figure 1 shows a
29、picture of the tower sump, note the visible red hue. Figure 2 is a carbon steel corrosion coupon in the system for 30 days. The Corrosion rate was about 7 mpy. Figure 1 Case I Bulk Water on Phosphate Program Figure 2 Carbon Steel Coupon on Phosphate Program Comparing the corrosion rate with that lis
30、ted in Table 2 for mild steel, one might feel comfortable with a mild to fair corrosion rate. This brings up an interesting point. What one facility, or perhaps a better way to phrase it, what one water system can sustain may not be what another facility can expect. Thus, there exists the graduation
31、 of descriptions from excellent to severe corrosion potentials. Water quality and restraints of the solubility of compounds in the water will dictate corrosion rates. Additionally, the process or contamination of a process at a facility can influence the performance standard that can be attained. Hi
32、gher rates may be tolerable due to system limitations. Program costs are another factor. A program may provide good performance results at one price, while another program at a higher price provides excellent results. The choice of the program may be dictated by the process need where a facilitys ke
33、y performance indicators are achieved utilizing the more expensive program. In this case, the facility had been tolerating these corrosion rates. One can say that the chemical treatment was working with a fair corrosion rate. However, with a looming different type or “standard”, in this case, a regu
34、latory limit, there was a need to have a different water treatment program that did not contain phosphorous. At this point, one might ask the question, would you use an NCD to meet their needs? It could be said that probably no NCD would add phosphorous to the water, however, could a device provide
35、the corrosion control with this quality of the water? A decision was made to try a phosphorous free inhibitor program which resulted in much better looking bulk water with lower iron levels. See Figure 3. Corrosion rates fell to 2 mpy, into the mild to very good classification in Table 1. See Figure
36、 4. Figure 3 Case I Bulk Water on Non Phosphate Program Figure 4 Carbon Steel Coupon on Non Phosphate Program The point of this discussion was not to promote one program over another, rather to emphasize the options chemical water treatment offers. Often times there may be a clear choice when compar
37、ing chemistries where one program can provide better corrosion/scale control, higher cycles of concentration, and in this case, regulatory compliance. It is difficult to understand how an NCD can provide the flexibility of various chemical programs when makeup water sources change, for example from
38、surface to well water. Chemical treatment allows for options when the system parameters change. Case II As stated above, sometimes there is a necessary and clear choice of treatment, especially when the incumbent program is failing. Figure 5 is a picture of tube bundles from an ammonia condenser sho
39、wing gross scaling of calcium carbonate. This case is where a retailer was using a mechanical NCD for its water treatment. The water has high alkalinity and high hardness. Obviously this treatment was not working as is evidenced by the buildup of scale. Within weeks of applying a program of organic
40、phosphate and polymer, the system started to clean up. Chemical treatment allows varying concentrations to meet specific system needs, because a one size fits all approach just doesnt always work. Figure 5 Condenser Tubes with NCD treatment Figure 6 Condenser Tubes after Chemical Treatment Case III
41、This last example will be dedicated to biological control and the discussion of an independent study that evaluated five devices for effective reduction of microbial populations. All cooling towers need protection against microbiological organisms, consisting of bacteria, fungi, algae and higher lif
42、e forms, such as protozoa and amoeba. The larger industries, such as power plants that receive their makeup from surface waters, may need to control macro organisms; zebra mussels, Asiatic clams and fish. Accumulation of biological organisms has an enormous impact on system efficiency, maintenance c
43、osts, corrosion and human health. Biofilm has the lowest conductivity of any type of deposit that can inhibit heat transfer, including calcium carbonate. Microbiological fouling can provide an environment for the growth of Legionella bacteria which can cause Legionnaires disease. Industrial faciliti
44、es will use quaternary ammonium compounds to control macro biological organisms. We are not aware of any NCD that can provide that control. NCDs claim hydrodynamic cavitation or ultrasonics provide a physical effect against microbial growth. But are these sustainable with a residual effect throughou
45、t a system? In an ASHRAE funded study, Project Number 1361-RP Biological Control in Cooling Water Systems Using Non- Chemical Treatment Devices, five NCDs, including hydrodynamic cavitation, ultrasonics, pulsed electric field, electrostatic and magnet were studied in two untreated model cooling towe
46、rs. The cooling towers were operated under realistic process conditions. The study concluded that none of the NCDs tested provided significant biological control. It recommended facilities utilizing NCDs test biological counts more frequently. Evaluations showed combining chemical and physical treat
47、ment achieved significant kill of microorganisms and warranted further study. This last finding lends credibility to the advantages of chemical treatment for microbial control. CONCLUSION In the examples presented, chemical water treatment proved to be the best alternative when considering NCDs or c
48、hemicals. When faced with changing environmental regulations, it provides options of proven technology. There are cases when an NCD is not the correct treatment based on the system water. The ability to adjust chemical concentrations offers better options for meeting performance standards. The discu
49、ssion leads to questions on the viability of NCDs for passivation as well as microbiological control. Peer reviewed research has shown that NCDs are not an effective control for microbial growth. ACKNOWLEDGEMENTS The author would like to thank Jon Cohen, Ed Czaja, Ray Post and Richard Tribble for their assistance in developing this paper. REFERENCES Bennett P. Boffardi, Ph.D., FNACE. “Standards for Corrosion Rates”, AWT analyst, spring 2000. IWC 04-22, Non Chemical Devices: Thirty Years of Myth Busting, Timothy Keister, ProChemTech International, Inc., IWC Conference, 20
