The key concept of cooling towers is, of course, to switch heat from heat heat exchanger discharge to air flowing via the tower such that the return water is properly cooled for reuse in energy plant condenser(s) and auxiliary coolers. Optimum tower design and operation requires an correct analysis of cooling tower course of variables for improvement of tower specifications or for evaluation of current tower efficiency. On the meeting’s Academic Seminar on Feb. 6, Rich Aull of Brentwood Industries outlined the basics of the CTI Toolkit software, which can calculate cooling tower demand curves. These calculations formerly required much effort and time, since they are based on the Merkel equation, which features a particular integral that have to be calculated numerically. Program inputs embrace fundamental info comparable to air temperature, humidity, tower design (counterflow or crossflow), cooling water circulate charge, air circulation charge and other commonplace gadgets to calculate how a tower will perform below any consumer-chosen set of conditions. The software program is a particularly powerful methodology for evaluating cooling tower design and efficiency points.
Any cooling tower that’s positioned in service will lose efficiency, and may even suffer severe damage, if it’s not correctly maintained, notably from a chemistry standpoint. I can not over-emphasize the issues that microorganisms can cause in cooling towers, together with biofilms that act as websites for growth of Legionella pneumophila, which gained notoriety because the organism that killed more than 30 American Legion members in Philadelphia in 1976, and which continues to trigger fatalities across the globe. A vital concept in prevention of microbiological fouling is to kill the organisms while they’re nonetheless in the planktonic state (suspended in the bulk water) and never enable them to turn into sessile, wherein they have formed colonies on cooling tower fill and in condensers and different heat exchangers. Once organisms type colonies, they secrete a polysaccharide coating that protects them from biocides. This movie also will acquire silt that enters the water by the makeup and air stream. In cooling towers, and particularly those with high-effectivity movie fill, even moderate growth will prohibit the passage of air and water via the tower. Severe microbiological fouling can add so much weight to the fill that tower collapse is feasible.
Nonetheless really useful is feed of an oxidizing biocide, with the commonest material being bleach (NaOCl) or bleach-activated bromine. Chlorine dioxide (ClO2) additionally could be an effective oxidizing microbiocide, however it have to be generated on-site. I’ve recently learned of two successful functions of monochloramine (NH4Cl) for sessile bacteria management. Although NH4Cl is a much less highly effective oxidizer than free chlorine or bromine, results indicate that it does a better job at penetrating biofilms to attack the organisms underneath.
Maintenance of a continual biocide residual within the cooling water system is vital, nevertheless it must be noted that discharge of residual biocides in the tower blowdown is often restricted. A technique to avoid this problem is continual feed of a lowering agent, corresponding to sodium bisulfite (NaHSO3), to the blowdown stream to remove oxidizing compounds.
Cooling water scale and corrosion treatment evolution
In recent decades, a typical technique for cooling tower treatment has been based around inorganic and organic phosphates, aka phosphonates, with operation in an alkaline pH range. Phosphates provide corrosion protection by forming precipitates that mask anodic corrosion websites, whereas organic phosphorous compounds additionally are environment friendly scale management brokers that inhibit formation of calcium carbonate and calcium sulfate deposits. Phosphonates attach to crystal nuclei as they are forming and disrupt crystal progress and crystal lattice strength. Very often they are used in combination with polymeric crystal modifiers to reinforce deposit control effectiveness, and to inhibit calcium phosphate deposition, which might be problematic in any other case.
A typical additive to phosphate/phosphonate packages has been a small focus of a zinc salt, since zinc will mix with hydroxide ions formed at cathodic websites to kind a precipitate of zinc hydroxide [Zn(OH)2], which is very effective as a pitting corrosion inhibitor for steel, especially in low hardness waters. Nonetheless, increasingly stringent restrictions on zinc in the cooling tower discharge are curtailing using this chemical.
Fast-forward to the present “Emerging [course of water discharge] laws embody a deal with reducing nutrient pollution, which will ultimately require states to develop and undertake numeric water quality requirements for phosphorous.”[three]
For many cooling techniques, these new standards will restrict using phosphorous compounds, particularly for larger programs that discharge on to public waterways. The event of phosphorous-free cooling water remedy applications is a multifaceted job requiring an method to regulate both corrosion and deposition.
Current phosphate/phosphonate applications function at a pH of eight.0 or above, however restrictions on using phosphate and zinc will favor still higher pH and alkalinity conditions that stretch the limits of scale control expertise. Thus, extensive work is underway to develop phosphorous-free management programs. The core chemicals being investigated embrace polyacrylates, enhanced acrylates, maleates, enhanced maleates, polyaspartate and enhanced aspartates, copolymers, terpolymers and quadpolymers. [Three, 4]
Environmental and water conservation issues
Water conservation and discharge, and plume discharge issues have gotten more and more vital at many power plants and other industrial amenities. Key points embody:
As we shall see, sometimes an effort to adjust to one concern leads to a conflict relating to another.
As a fundamental start line, allow us to look at the effect of cycles of focus (COC) on the blowdown rate from a cooling tower.
As is obvious, this curve is asymptotic, and the reduction in blowdown is vastly diminished with elevated cycles of concentration. So, in high COC instances the make-up water requirement approaches the evaporation fee, which is constant for any explicit set of conditions. The key good thing about operation at larger cycles is a much smaller blowdown stream for disposal. While this operating mode will be helpful from a discharge standpoint, it presents problems in some areas of the country. California is a notable instance, the place regulations are tightening on both the quality and amount of discharge. An increase in COC of the cooling tower might take the quantity of discharge under the plant’s permit limit, but the standard may then leap out of vary.
An possibility in these instances is zero liquid discharge (ZLD), but ZLD has its own set of points. It’s often needed to reduce the blowdown volume by some methodology previous to last ZLD course of, however blowdown chemistry can cause fouling and scaling in filters and different ZLD pre-therapy tools. These discharge-reduction purposes should be evaluated very carefully, as should the disposal technique for the ultimate small stream that continues to be. Some ZLD choices, relying upon the direction of the regulatory agency involved, include deep-properly injection, remedy in an evaporation pond, or the last word ZLD state of affairs, thermal evaporation to provide a stable material for landfilling.
Another factor that influences allowable COC is the makeup water high quality, since important makeup pretreatment may be required to extend COC. Sulfuric acid injection to scale back bicarbonate alkalinity is a standard methodology to attenuate calcium carbonate scaling, however other constituents comparable to silica can’t be eliminated in this manner. High non-carbonate hardness and silica might require lime softening and magnesium remedy in a makeup clarifier or softener to produce water suitable for cooling tower use. Many plants are being faced with rules that limit and even prohibit using contemporary water for make-up, with substitute by handled wastewater effluent. A common vernacular for this is “Title 22,based mostly on the regulations in California. It virtually goes with out saying that dealing with gray water can typically be a lot completely different than with contemporary water. The suspended solids focus could be larger and topic to fluctuation, as could be constituents reminiscent of ammonia and phosphate. The latter can really supplement phosphate feed to cooling towers, but as we have seen, phosphate discharge from some cooling programs is now prohibited.
Yet another subject of rising concern involves rules for cooling tower plume high quality. Modern towers have very efficient mist eliminators, however even so, a small quantity of moisture, known as drift, will escape with the exhaust water vapor. Drift is solely water droplets of the recirculating water, and thus has the identical chemistry. Because the droplets evaporate in the ambiance, the dissolved solids form particulates. Like particulates emitted by fossil fuel combustion and other sources, cooling tower particulates are underneath the scrutiny of the EPA and regional regulators per their influence on Nationwide Ambient Air Quality Standards (NAAQS). Cooling tower plume testing might be mandatory to make sure that the tower atmospheric discharge won’t violate future limits.
Brad Buecker is a process specialist with Kiewit Energy Engineers in Lenexa, Kan. He has more than 32 years of expertise in, or affiliated with, the facility industry, much of it in steam generation chemistry, water treatment, air high quality control, and outcomes engineering positions with City Water, Light & Energy in Springfield, Ill., and Kansas City Power & Light Company’s La Cygne, Kan., station. He has an associate’s degree in pre-engineering from Springfield School in Illinois and a bachelor’s degree in chemistry from Iowa State University. He’s a member of the ACS, AIChE, ASME, CTI, and NACE. He also is a member of the ASME Analysis Committee on Power Plant & Environmental Chemistry, the Electric Utility Chemistry Workshop planning committee and the Coal-Gen planning committee. It’s possible you’ll contact him by e-mailing firstname.lastname@example.org.