Session Code ICBSFM 2003 PS5-1220

Transcription

1 Session Code ICBSFM 2003 PS International Conference on Building Systems and Facilities Management (ICBSFM), 8th - 10th October 2003, Marina Mandarin Hotel, Singapore OZONATION The Answer to High Energy and Water Savings in Air-conditioning Systems P M Menon and Dr. A Appan Global Engineering & Management Services Pte Ltd, No. 61 Kaki Bukit Ave 1 #02-09 Shun Li Industrial Park, Singapore Abstract The use of ozone in treating cooling tower water in water-cooled condensers has received mixed reviews. Systems installed by engineers having a good understanding of the ozone chemistry in water applications have end users saving in make up water consumption in the cooling towers and high energy savings on account of reduced fouling factor in the condenser heat exchanger tubes. Systems installed with poor ozone engineering know-how and its applications have reported adverse results resulting in lack of confidence in its use in cooling systems. The conditions encountered in comfort cooling systems are clean with short turn over times and low heat exchanger skin temperatures than industrial/ process cooling systems. This makes ozonation an attractive stand-alone water treatment system for cooling tower water treatment. The authors review the fundamental bio fouling mechanism in cooling water and the effect of ozone in cooling water, as a biocide, scale and corrosion inhibitor. Case studies using ozonation on installations in the US, Singapore and recommendations from Federal Authorities in the US as a means of energy and water conservation are discussed. Keywords Air- Conditioning systems, ozonation, disinfection, water & energy savings Introduction Chlorine and other chemicals have been the choice of oxidation and disinfection for cooling water treatment for over three decades in this region. Very little attempts were made in considering alternative and cost effective technologies which are not only economical but are much cleaner and meet all the stringent environmental regulations. This was primarily due to the fact that Chlorination was traditionally a British practice, which seemed to

2 meet the requirements at that point of time. Ozone has been used extensively in drinking water disinfection for the last 100 years and in the cooling towers in the U.S.A. for the last two decades. Extensive water and energy savings has been the highlight of the introduction of ozone in cooling towers. The main objectives of this study are: to explain the mechanism involved and efficiency of the application of ozone in terms of its bio chemical and disinfection properties; to illustrate the method of application of ozone in cooling towers and explain the associated bio-film mechanisms; to appraise the energy and water savings with emphasis on case studies and to draw conclusion on ozonation system and make recommendation Advantages of ozone application When properly applied, ozone is an attractive stand alone alternative to multi chemical water treatment system for air-conditioning cooling towers. It is a strong biocidal agent against bacteria, viruses, protozoa and with simultaneous oxidation of organic matters results in improved water quality. The strong biocidal characteristics are due to the combination of its high oxidizing potential and its ability to diffuse through biological membranes. Ozone s killing mechanism is by direct lysing of cellular walls of the microorganism. Ozone readily oxidizes organic matter in bacterial membranes, which weakens the cell wall and leads to cell rupture. The internal cellular material/plasma is released in to the external environment, which causes immediate death of the cell. In contrast, chlorine and other oxidizing and non-oxidizing biocides must be transported across the cellular membrane where they act on the nuclear reproductive mechanism or enzymes life giving reactions in the cell. This mechanism of disinfecting is not as effective or efficient as ozone since these biocides need to be used in higher concentrations or much longer contact times. The use of ozone in cooling towers can check cooling tower associated diseases such as Legionnaires disease caused by the bacteria Legionella pneumophilia by virtually killing planktonic (free swimming) bacteria and indirectly by eliminating conditions that favor Legionella amplification, i.e., the elimination of biofilms and amoeba and other protozoa that feed on biofilms and which serve as Legionella host. It is a documented fact that, some protozoa serve as hosts for Legionella pneumophilia, which can enable rapid proliferation of Legionella. Ozone s biocidal action on protozoa eliminates any possibility of protozoan acting as host for Legionella pneumophilia. Micro organisms particularly bacteria and viruses develop an immunity to chemicals when used over a prolonged period of time, because of which 2

3 biocides needs to be changed/rotated for effective control of microorganisms. There is no microorganism that is known to have developed a resistance to ozone till date; thereby eliminating the need to rotate biocides. Application of Ozone to Cooling Towers Ozone is applied to cooling tower water as per the schematic diagram shown in Figure 1. The ozone rich water is distributed in the cooling tower sump to get the desired residuals in the entire cooling system. Figure 1: Typical cooling tower ozonation system The cooling tower systems become very clean because of the oxidizing and biocidal properties of ozone and the water clarity approaches 1 NTU. There will be no visible biogrowth on any wetted parts and the system will be very clean. Biofouling mechanism in cooling water The water used in cooling system contains small quantities of organics. These organics are also present in the potable water supply in low concentrations. Rice R.G and Wilkes J.F (1992) discuss the factors these organics have on the cooling system. The organics in the cooling water adsorb on to the heat exchanger pipe material. Thermodynamically there is a release of free energy, favoring this reaction to take place. The adsorbed organics are a source of food to the microorganisms. Colonization of microbes is always to these adsorbed surfaces where they thrive and multiply. The mechanism is simple - microorganisms typically found in cooling water circuits both eat and secrete on the pipe walls. This mass of organics with microbes with their metabolic waste etc is termed the biofilms. 3

4 The organisms through their secretions and detritus produce a substance called musilage, which acts as a glue to trap precipitated solids. Without this glue the solids are carried by the flow velocities to the most quiescent portion of the cooling system - the tower basin. Since both aerobic and anaerobic microbes colonize the biofilms, pockets of oxygen depletion are created by their metabolic activity. A galvanic cell is thus created causing metal to go in to solution at the anodic regions; Figure 2. This is the start of under slime corrosion, which is the most common form of pitting corrosion encountered in cooling systems. Figure 2: Biofouling mechanism The precipitated solids can also be the seed crystals of calcium carbonate where they get to attach and grow to form calcium scales. The formation of scales and bioslime on heat exchanger surface reduce heat transfer efficiency of the cooling system. Biofilms, by itself, is a better insulator than typical calcium scale; the presence of biofilms can reduce heat transfer greater than an equivalent scale thickness. Elimination of the biofilms is an important step in elimination of scale formation. In ozonated system, the organics in the cooling water are oxidized by ozone by its high oxidation potential. Ozone being the most powerful biocide inactivates bacteria, viruses and the protozoa. Once the cooling surface is devoid of organics and the microbes, biofilms does not form. In ozonated waters, biofilms is non-existent, which also ensures there is no chance for seed crystals of calcium to form and adhere to the heat transfer surfaces. The heat exchanger surface is devoid of any fouling enabling the system to operate at the rated efficiency. Figure 3 shows the relationship between fouling factor, chiller capacity and condensing temperature (ASHRAE 1992 Hand Book). Clean condensers operate at fouling factor As the fouling factor increases, condensing temperature increases with increased compressor power consumption. Corresponding there is a fall in chiller capacity. 4

5 Figure: 3 Fouling Factor Vs. Chiller capacity Energy and water saving by using ozonation The twin saving involve can be quite substantial. Energy Savings: In ozone system, there is no Biofouling and scaling in the condenser tubes, enabling the system to operate at the rated fouling factor. The data given below, Table 1 (PEP Filtration, 2003) shows the increased power consumption with fouling factor. Table 1. Fouling Factor Vs. Energy increase. Fouling Thickness and Resulting Increase in Energy Use Scale Thickness (in) Fouling Factor (hrft 2 /BTU) Energy Increase % Increased Energy use means Decreased Efficiency and Higher Costs Immediately after ozonation, a dramatic improvement in chiller/ condenser efficiency was documented by Pryor A.E & Bukay M (1990). Within 8 days after ozonation, the compressor discharge pressure dropped from 275 psi to less than 190 psi; Figure 4. 5

6 Figure 4: Compressor discharge pressure after ozonation The U.S. Department of Energy (Federal Technology Alerts 1995) endorses the use of ozone in cooling tower waters as an Energy and water saving Mechanisms (Federal Technology Alert- Ozone Treatment for Cooling Towers). Average efficiency gain of 10% in chiller system performance can be obtained; case studies range from no improvement in efficiency to a 20% improvement in chiller performance is reported. Water Savings: Ozone treated cooling waters operate at higher cycles of concentrations due to the very short half life of ozone and in the process chemicals does not build up in the system. The system TDS does not increase on account of the treatment chemicals added. Pryor A.E and Buffum T.E (1993) has come up with a new useful index POSI (Positive Ozone Scaling Index) for predicting the safe maximum operating cycles of concentration in ozonated cooling systems. Conventional indices such as Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI) predict the solubility of calcium carbonate but do not predict the scale formation. Using POSI the safe maximum operating cycles of concentration based on the conductivity of the make up water can be accurately predicted using easily obtainable make up water parameters. The Table 2 shows the maximum operating cycles of concentration using PUB Singapore as the make up water source. Table 3. shows the blow down water in percentage of the make up water versus cycles of concentration. It may be noted that blow down can be reduced to 5% of the make up water in most cooling towers in Singapore. In the United States, there are references of large cooling systems operating at zero blow down (Holt T. 1993) and several references (Pryor A.E & Bukay M; 1990) of adopting ozone to conserve water by cooling tower ozonation. 6

7 Table 2: Analysis of make up water. Test Parameters Unit Make-Up Water Ca+ As CaCO3 ppm 40 Tower Water Cycles Mg+ As CaCO3 ppm 20 Na+ As Na+ 10 Cl- As Cl- 20 Alkalinity As CaCO3 ppm 30 Conductivity micromhos POSI Reference (POSI): Data Softened water 0 to 0.25 Up to 30 + High quality 0.26 to to 20 water Moderate 1.01 to 2 5 to 10 quality Poor Quality to to to 1.5 No scaling tendencies Thin glassy scale on dist. Pans 1.51 to Significant scale 2.0 accumulation Heavy Accumulation Table 3: Cycles Vs. Blow down water Cycles of % blow down of make Concentration up water Case Studies A case study done on water savings for a large Singapore based cooling tower is shown in Table 4. The chemical treatment system was designed to operate at a maximum cycles of 6, using PUB water as the make up water source. The table shows the estimated savings in make up water in m3 and 7

8 the savings in dollars and cents when operated at 15 cycles by ozonating the cooling water. Table 4: Water and chemical savings using Ozone Cooling Tower Particulars: Capacity of Tower 1, Tons No. of Towers Total Circulation rate 17, m3/hr System Volume (estimated) 1, m3 Operating load factor : 0.90 Evaporation 1,388, m3/yr Drift 0.005% 7, m3/yr Evaporation + Drift 1,395, m3/yr Utilities Costs: Make up water Cost 2.12 $/m3 Energy Cost 0.11 $/kwh Chemical Treatment Cost: Concentration ratio 6.00 Make up water 1,665, m3/yr Make up water cost 3,531, $/year Chemical treatment cost 161, $/year Annual total cost 3,692, $/year Proposed Ozone treatment cost: New concentration ratio Make up water 1,487, m3/yr Make up water cost 3,153, $/year Ozone electricity operating cost 54, $/year Annual total cost 3,207, $/year Annual savings using ozone treatment 485, $/year 8

9 Conclusions and Recommendations Ozone is an attractive stand-alone alternative oxidant and disinfectant. Ozone has the lowest ct value and is acknowledged as the most powerful disinfectant (EPA, 1989). The application of ozone in existing cooling tower is technically feasible. Ozonation is highly effective in the removal of biofilms leading to much lower fouling factor. This result in better heat transfer and leads to lower chiller power consumption. In conventional chemical systems, the maximum cycle of concentration is about six (6), where as in ozonation system it is as high as 15 cycles. Consequently, there is extensive water saving to the extent of 15% to 20%. In conclusion, the use of ozonation will not only save water in water-scarce Singapore but there will also be considerable saving in energy. Beside, the extensive disinfection properties of ozone will ensure that the cooling water quality falls well within the NEA s requirements. In other words ozonation is a sustainable and cost effective alternative in cooling water management. References ASHRAE 1992, HVAC Systems and Equipment hand Book. Federal Technology Alert Ozone Treatment for Cooling Towers; The U.S. Department of Energy; Holt T (1993), Rocketdyne Division-Rockwell International, Case Study: Ozonation of a 10,000 Ton Cooling Tower with near Zero Blow down ; Proceedings of the American Power Conference, Chicago, Illinois. McCoy, W.F. (1987), Fouling Biofilm Formation, in Biological Fouling of Industrial Water Systems: A problem solving approach (San Diego, CA: Water Micro Associates), pp24-55 PEP Filtration (2003); Pryor A E and Bukay M (1990), Economics and Performance of Cooling Tower Ozonation: Six Case Histories, 51 st. Annual Meeting International Water Conference, Pittsburg, Pennsylvania, October 21-24, Pryor A E & Bukay M (1990), Industrial Water Water conservation through Cooling Tower ozonation, Ultrapure Water Journal, May

10 Pryor A E and Buffum T E (1993), A New Practical Index for Predicting Safe Maximum Operating Cycles of Concentration in Ozonated Towers, Paper No.482, The NACE Annual Conference and Corrosion Show, Corrosion 93. Rice R G and Wilkes J F (1992), Fundamental Aspects of Ozone Chemistry in Recirculating Cooling Water System Data Evaluation Needs, Ozone Science and Engineering Journal, Vol. 14, pp Tierney D. (1995), Case History: Performance Evaluation of Ozone Water Treatment at Kennedy Space Center, International Water Conference, October 1995; Paper No. IWC Resume: 1. Pattathil Madhav Menon Mr. Menon graduated from the Regional Engineering College, Calicut, India (now called National Institute of Technology) in Mechanical Engineering in He later went on to receive a Post Graduate Diploma in Environmental Engineering in 1999 from the National University of Singapore. He has comprehensive experience in the engineering and application of ozone and carbon in air, water and wastewater treatment for the past 20 years. Mr. Menon has also published an article "Towards Cleaner Technologies- Ozone for cooling Towers"(Asian Water, October 1999) and was a speaker at the Environmental Business Opportunities in Industrial Development Conference (1996 Pollution & Environment Technology Indonesia 96). He is currently CEO of OzoneCarbon Technologies and is also Director of GEMS, and a partner in Neumann Systems Singapore. 2. Dr. Adhityan Appan Dr Appan has been both a practising civil engineer and an academic. He has had extensive experience in water resources management, water & wastewater treatment and solid waste management. He holds a Master s degree (London) and PhD (NUS) and is a Fellow of IES, ICE, IWRA, CIWEM and RSH. He also has licenses to practice in Singapore (P Eng), UK (C Eng) and EU (Eur. Ing). Dr Appan was attached to the PUB and then was Professor in the Nanyang Technological University till He has authored more than 125 papers and reports and has been an invited keynote speaker in eight international conferences. 10

11 He is currently an independent consultant in Singapore and is also Technical Consultant to GEMS, a partner in LBW Consultants and a Technical Adviser to HYFLUX. 11