Water Treatment for Cooling Towers and Evaporative Condensers Q&A
Monitoring water treatment performance is an essential part of many industrial equipment maintenance plans. Though the water treatment chemicals are introduced via the water, it is important to remember that the goal is not to treat the water. Instead, the water is use to carry the treatments to the areas of the equipment we wish to protect.
During a one-hour webinar hosted by Process Cooling in November (http://bit.ly/WaterTreatment2015), water treatment expert Paul Puckorius explained:
- How best to evaluate if your treatment program is cost effectively protecting all of the equipment in contact with the cooling water.
- How some water treatment products can supplement a standard treatment program — or even how they can interfere with protecting your system.
- How to prepare for the new standards from ASHRAE and the Cooling Technology Institute that require facility owners to manage the risk of Legionella colonization in their cooling towers.
As a part of his presentation, Paul also fielded questions from the audience about industrial water treatment. Because there were too many questions to answer during the time allowed, Process Cooling brings you this Q&A with Paul. If you’d like to view the complete webinar, visit http://bit.ly/WaterTreatment2015. Register and login, and you can watch the recorded presentation immediately.
Q. Can you review again briefly the major difference between a cooling tower and an evaporative condenser?
Paul Puckorius Responds: Glad to provide, and you may want to look at the webinar again for more details since it is available for continued viewing for a year.
The main differences are that the cooling water in an evaporative condenser simply goes from the basin to the top of the evaporative condenser since the coils/tubes are in the evaporative condenser. The cooling tower sends the cooling water to distant heat transfer equipment. There are no coils in the tower.
This means that there is less water in the evaporative condenser vs. the cooling tower; thus, water quality changes more quickly in the evaporative condenser. So, major water quality changes occur very quickly, and more attention is needed to prevent loss of inhibitors and other treatments.
Also the evaporative condenser is constructed only with either galvanized steel or stainless steel and use copper or galvanized steel tubes/coils. The cooling tower system can be constructed of wood, fiberglass, plastic and concrete as well as the materials used in the evaporative condenser.
Another change is that the cooling tower water system heat transfer surfaces maybe mild steel, copper alloys and stainless — and never galvanized steel.
Q. What is the best type of filter to use on an open cooling tower? Does the recommendation differ for evaporative condensers?
Paul Puckorius Responds: For both cooling towers systems and evaporative condensers, it depends on the type of material you need to remove from the cooling water.
If the material is sand or is of heavy particles that weigh more than water, a cyclone filter is the best. It does not need backwashing and little attention. If the material is lightweight vs. water and is suspended such as with fine particles and biological mass, a sand filter is best for large systems. For smaller systems, a cartridge filter — either one or a number of cartridges — would be best.
If there is any oil or oily material, there are special filters for this application.
Q. With scaled and organically fouled cooling tower fill, if the bulk cooling water has Legionella at high levels, to what extent do you judge the organism’s potential for residing within fouled tower fill at that time? And, does this potentially increase the chance for aerosolization of the bacteria outside the tower?
Paul Puckorius Responds: The quick answer is absolutely yes. Anytime there is fouling — mainly bio-fouling but also any fouling — this is often where the Legionella are living and entering the cooling water. So, high levels of Legionella could be present in the cooling water.
Also yes to the second half of the question: it does increase the potential for Legionella to escape the cooling tower via drift. If the cooling tower has high efficiency drift eliminators, this will reduce the amount of drift, but they do not stop all of the drift. So, please disinfect your cooling tower — if one exists with these conditions — using the Wisconsin Protocol for disinfection, which is to carry 10 ppm of free chlorine for eight hours or longer after applying 50 ppm of chlorine with the tower fans shut down to prevent any drift. Also keep pH bellow 7.5, or if it is higher, use a bromine-release biocide at same dosages.
Repeat if bio-mass is still present and retest for Legionella to be sure that none exist, or continue until tests show a clean tower. You might increase the corrosion inhibitors while dosing with high levels of chlorine or bromine to provide corrosion protection to your equipment.
Q. Would using reclaimed water create more sediment in tower basins? Also, does this in any way pose any increased risks of Legionella growth?
Paul Puckorius Responds: Glad to answer your question regarding reclaimed reuse water. I assume that you mean treated effluent from a municipal wastewater treatment plant.
I have worked with many municipalities providing reclaim water to refineries (even HVAC systems), and they usually supply a water quality often identical to California Title 22, which means less than 5 ppm total suspended solids (TSS) and a chlorine residual of 2 ppm.
If it is tertiary treated, there would be less than 2 ppm ammonia; 3 to 5 ppm of ortho phosphate; and 50 to 70 ppm of nitrate as NO3.
Thus, the TSS is very low — often less than 5 ppm — so it will be like potable water. With the chlorine this high, there is less danger from Legionella bacteria surviving vs. potable water. I have never had a case nor ever heard of a case of Legionnaires’ disease when on reclaimed municipal wastewater as 100 percent cooling tower makeup. The high chlorine levels actually have saved the facility chorine or bromine use for good bio-control.
The addition of the nitrate is a mild steel and stainless steel corrosion inhibitor, and the phosphate can be your mild steel corrosion inhibitor.
There is certainly a possibility of Legionella bacteria entering any cooling tower via the air, so Legionella can be present. However, I have never heard of reclaimed water systems ever reporting that occurrence.
Hope this is helpful. Remember, I feel that every cooling tower system should have a filter.
Q. Do you recommend testing for Legionella in water systems? If positive, do you notify building occupants?
Paul Puckorius Responds: First, I do recommend testing even though the CDC [Center for Disease Control] does not recommend routine testing for Legionella.
I believe one needs to know if you have a good microbio program that is keeping Legionella and other microbes to low and acceptable levels.
The second part of your question depends upon several items, including the level of Legionella bacteria found and the “sero-group” of the Legionella bacteria.
If you find low levels of Legionella bacteria, say 100 cfu/ml (colony-forming units per milliliter), this is a very low level. I believe there is no need to tell building occupants unless you want to create a panic.
The lab doing the testing should also give you the “sero-group” found. If sero-group 1, this is what is normally associated with outbreaks. If it is any of the other 13 sero-groups, they are not associated with the disease. If you have high levels such as 1000 cfu/ml and sero-group 1, you need to disinfect your cooling water system and retest until there is none or low levels.
Telling the occupants is a decision that is made by the building owner, but if the water treater does the testing, he certainly must notify the owner or manager of his findings. Finding even high levels of Legionella in the cooling water does not mean that there has been sufficient air contamination by the cooling tower or evaporative condenser and that persons in the facility have been infected.
Notifying the occupants that you have found Legionella in the cooling system and that you have implemented a disinfection of the system is a good idea so that if anyone comes down with pneumonia, they can tell their doctor that they may have been exposed to Legionella bacteria.
I believe that you should not panic occupants and say you have Legionnaires’ disease in your water systems but notify them you found the bacterium, which does not mean you have the disease.
Low levels of Legionella are common in water systems. Remember, infection can only occur if one inhales water droplets containing the bacterium plus the person needs to be susceptible to get the disease.
I might add that some certified Legionella testing laboratories put out a guideline of the level of Legionella bacteria found in cooling water and the actions that should be taken. One such lab is Pathcon Labs outside of Atlanta.
Q. Can UV light treatment be used as biological control for evaporative systems? Does UV perform well for biological control in evaporative condensers?
Paul Puckorius Responds: Yes, UV can be acceptable; however, you must remember that UV must penetrate the water, so the water must be crystal clear for effective killing of all microbes that pass the UV beam.
However, UV does not carry in the cooling water to any areas outside the UV beam. Thus, bio-mass and algae can still grow in some areas of the system.
There is some information that UV will reduce bio-organisms in the entire water system, but each system is different and it may not do that in your system.
UV is certainly something I would look at but you may also need a biocide applied periodically.
Q. What are your recommendations on duration (length of time) for passivating a cooling tower prior to startup?
Paul Puckorius Responds: I assume you are referring to a galvanized steel cooling tower. Two manufacturers — SPX (Marley) and Evapco — have a recommendation to maintain a pH of the cooling water in the range of 7.0 to 8.2 and no less than 50 ppm of calcium hardness (as CaCO3) for four to six weeks or until the dull coating is formed. Evapco also says to maintain the total alkalinity of the cooling water at 300 ppm.
I have used a more rapid method with phosphate at 100 ppm (as PO4) for 24 to 48 hours with the pH 7.0 to 8.0 plus use of a good phosphate scale inhibitor. I also have had success using sodium silicate at 100 ppm (as SiO2) and pH at 7.5 to 8.5 for 48 hours. The cooling water chemistry is important to form the passive zinc protective film.
Q. Have you ever seen or located a cooling tower in a phosphoric acid production plant? They normally utilize ponds. One company claims it can evaporate one ton of water for an energy cost of 20 cents. What do you think?
Paul Puckorius Responds: Yes, I have seen a cooling tower located at a phosphoric acid production plant. It was in Florida, and yes, most use a pond — called a spray pond — due the amount of water and wastewater used in the production of phosphoric acid.
Relative to the cost to evaporate a ton of water for only 20 cents, have all costs been included in this calculation? These include the pumping costs, even the amortizing of building the pond, and certainly dredging the pond periodically. Also, you must factor in maintenance of the spray nozzles plus the cost of water and certainly the electric costs.
Each site is specific to all the various costs associated and can be determined and compared to a cooling tower system.
Q. For inhibited acid descaling of CaCO3 on Cu/Ni tubes or stainless, what are the best post acid descaling and passivating agents? Elaborate for other metals as well if you can.
Paul Puckorius Responds: Desalting using acids is always tricky, and you should use inhibited acids. They are common for mild steel. For copper alloy, the use of specific inhibitors is needed and they are different. Often sulfamic and citric acids are used due to being weaker acids, but they take more and longer vs. hydrochloric or sulfuric acids. For stainless steel and galvanized steel use sulfamic acid, again inhibited. Never use hydrochloric acid for stainless steel.
For passivation, phosphate is an excellent passivator for mild steel and galvanized steel. The azoles are good passivators for copper alloys. Stainless steel is often passivated with nitric acid, but it is hard to handle and has fumes. Hydrogen peroxide is very good but air (oxygen) also will work (or aerated water) but takes longer.
Q. How big of impact on energy efficiency can a poorly maintained water system have?
Paul Puckorius Responds: Well, it depends upon on how poorly it is maintained. If there is heavy scale or other deposits in the condenser tubes of the chiller served by the cooling tower, there is a huge requirement for more energy — not only pumping energy due to restrictions in the tubes but also great impact on energy to provide sufficient cooling. Even with heavy corrosion deposits in circulating pipe, there is a huge increase in the energy requirement for pumping over that needed for clean lines.
If the cooling tower fill has heavy deposits so it is not able to supply sufficiently cool water to the chiller, it is another huge amount of energy lost or used with little or no cooling water obtained.
So, it depends upon how much cooling deposits are present throughout the system to determine the energy used and even increased due to pipe and tube restrictions. I would estimate that at least two and even three times as much energy is needed and still would not provide sufficient cooling. Each site is specific and the increase in energy can be calculated with good measurements of the energy and water temperatures.
I might add, the chiller manufacturer or a good service person can help determine the impact of a poorly maintained cooling water chiller.
Q. What is your experience with nonchemical water treatment?
Paul Puckorius Responds: I have had considerable experience with nonchemical water treatments. First of all, a filter is a nonchemical water treatment that every cooling water system should utilize.
I also have had experience with six different nonchemical water treatments that claimed scale, corrosion and microbiological control. Only two have worked to some extent. The other four did nothing or very little. I evaluated all six systems as an independent consultant and very thoroughly for scale, corrosion and microbiological actions. I cannot identify those that failed due to an agreement but the other two did work quite well for scale, microbiological and fairly good for corrosion control with no additional chemicals. PC