To quote a superb reference manual on cooling towers, “Evaporation is utilized to its fullest extent in cooling towers, that are designed to expose the maximum transient water surface to the maximum movement of air for the longest time period. If cooling was solely a results of sensible heat switch, then cooling towers can be enormously large attributable to large air stream necessities. Evaporation is the key to heat switch. As air passes by means of a cooling tower, it induces evaporation. For water to evaporate, it should eat a considerable amount of vitality to alter state from a liquid to a gas. This is called latent heat of vaporization, which at atmospheric situations is usually around 1,000 Btu/lb. Maybe only 2 percent or so of the incoming water evaporates in the tower, but that is kind of enough to lower the temperature of the return water to the condenser and different heat exchangers.
Critical to maximum efficiency in a cooling tower is intimate contact between the warm inlet water and the air flowing by means of the tower. Area limitations forestall an in depth discussion of tower internals, however one technological achievement that has drastically improved heat switch is movie fill, as shown in Figure 1.
As the identify movie fill implies, layers of this material, placed between the inlet water sprays/distributors and the air rising from beneath, trigger the water to form a film on the material. The filming water exposes a lot floor area to the air. Fill varieties vary from open splash fill to high-effectivity movie fills with very torturous pathways. Water quality performs the main role in the type of fill that is suitable, but in all circumstances water chemistry must be correctly monitored and maintained to stop upsets. Microbes, and bacteria particularly, love heat and wet spaces to kind colonies. Micro organism, as a protecting mechanism, secrete a sticky polysaccharide coating (slime) that traps silt and other debris in cooling water. Entire sections of fill might change into fully plugged, drastically decreasing heat transfer. Moreover, the deposits add a lot weight to the packing that structural failures also might end result.
Condensers are one other excellent location for propagation of bacterial colonies, and in areas of the cooling tower exposed to sunlight, algae growth may be very problematic. Throughout the tower itself, fungi also will be a major issue, notably in wood cooling towers.
Regardless of the potential fouling mechanisms, at any plant with a cooling tower, and particularly if further nutrients arrive with the make-up water, microbiological control is of utmost significance. Frequent for a lot of towers has been bleach or bleach-activated bromine feed, since bleach is safer, albeit more expensive, than gaseous chlorine. Nonetheless, ammonia and organics in incoming make-up will consume chlorine, partially or totally destroying the residual concentration that is required to keep cooling methods clear. In addition, the chemistry would possibly kind halogenated organics, which additionally are unwelcome. Alternative treatments are becoming extra well-liked. One such different is chlorine dioxide (ClO2), the place technology methods have been enormously improved from the previous sodium chlorite (NaClO2)-chlorine reaction, and in which massive quantities of hazardous sodium chlorite needed to be saved on-site. One in every of the brand new processes utilizes a compact generator that combines sodium chlorate (NaClO3) with a pre-mixed mix of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) to induce the reaction proven in Equation 1.
Chlorine dioxide does not react with ammonia, nor does it react with organics to type halogenated organic compounds. Additionally, and in contrast to hypochlorite, chlorine dioxide is just not affected by pH. This can be an important benefit in those towers (the majority) whose chemistry programs operate in an alkaline pH vary.
Some plant personnel have had good success with on-line hypochlorite-producing programs, such because the MIOX course of, which through brine electrolysis produces a hypochlorite answer that additionally incorporates residual hydrogen peroxide. An advantage of this know-how is that the oxidant is produced on an as-needed basis, reasonably than being stored in giant tanks where it could actually degrade and lose energy.
A precise feed of monochloramine (NH3Cl) also might be efficient. This product is far weaker than a free oxidant, but that very property allows it to penetrate slime layers and attack the microbes beneath. Free halogens are typically consumed by the slime layer. The one potential drawback to monochloramine is that it introduces ammonia-nitrogen to the cooling water. If ammonia is restricted in the plant’s wastewater discharge permit, this therapy technique won’t be allowed.
A mechanical remedy expertise that is extremely beneficial for cooling towers is sidestream filtration. I often see requests-for-proposals (RFP) that name for direct makeup water filtration. Builders and owner’s engineers usually do not acknowledge that cooling towers are very good air scrubbers, and that many particulates are launched to cooling water by the scrubbing action. Make-up water filtration does nothing to manage airborne particulates. A large number of applied sciences can be found for sidestream filtration, ranging from standard multi-media filters to automated backwash methods with metal screens.
This brings us to an extremely essential improvement relating to general chemistry control in cooling towers.
The phosphorus conundrum
For more than three decades, the most common cooling tower methodology has utilized a mix of natural and inorganic phosphates for major scale and corrosion control, along with a polymer for calcium phosphate scale prevention, and perhaps a small dosage of zinc for added corrosion protection. This chemistry will be efficient, but it surely requires very close monitoring and management to stop upsets. Accordingly, the key water remedy chemical companies developed subtle chemical feed/control programs for phosphate/phosphonate packages.
However, one other situation has arisen that’s changing into critically vital in lots of areas. With this chemistry, cooling tower blowdown obviously discharges phosphorus in the form of phosphate (PO4). Phosphorus is a major nutrient for aquatic organisms, together with toxic algae and cyanobacteria.  Many receiving our bodies of water are actually thought-about to be “phosphorus-impaired,which implies that discharges from new plants is perhaps severely restricted on the subject of phosphorus concentration. So primarily two choices are available, treat the plant wastewater, perhaps even to zero liquid discharge, to remove impurities, or make use of an alternate cooling tower treatment program. With regard to the latter, researchers are working diligently to good non-phosphorus (non-P) programs, and a few products have been very successful in full-scale purposes. The brand new programs are based on polymer chemistry, the place the polymer chains may include a number of of the energetic teams, as proven in Determine three.
The polymers serve as crystal modifiers and sequestering agents to inhibit scale formation. There also is proof that the polymers kind a thin coating on steel surfaces to inhibit corrosion. A typical dosage focus is 2-10 ppm active in the cooling water. In some cases, an all-P program is likely to be inexpensive than an equal phosphate/phosphonate program, although every potential application should be carefully evaluated. [Three]
Even a change to the sort of program still does not prevent problems that can occur due to introduction of impurities from reclaimed water sources. Phosphorus, ammonia and organics are three of probably the most potentially troublesome contaminants, and it is likely to be necessary to remove these impurities upstream of the cooling tower.
Cooling tower makeup water treatment strategies
The extremely well-established expertise of clarification might be employed to remove phosphorous and suspended solids from makeup.
Frequent iron or aluminum-based mostly coagulants will take away nearly all phosphorus as a phosphate precipitate. If the uncooked water incorporates high hardness and alkalinity, a softening clarifier could be applicable to cut back these constituents and reduce scale-formation potential earlier than the water reaches the tower. Lime [(Ca(OH2)] and soda ash (Na2CO3) are the common chemicals for this process.
If the raw water has a major ammonia focus, then extra therapy is likely to be required. A expertise that’s becoming more and more fashionable for reclaimed water therapy is biological processing of the plant intake. Bioreactors and membrane bioreactors (MBR) are two applied sciences in this regard.
Through microbes which are seeded and allowed to develop on internal units within the reactors, the incoming organics and nutrients are consumed.
The MBR outlined in the basic schematic above employs an anoxic zone (oxygen focus is limited) and an aerobic zone to process the incoming wastewater stream. The ultimate step, which is perhaps exterior or internal, is filtration to attenuate TSS discharge. With membrane bioreactors, where the membranes are of the micro- or extremely-filter variety, effluent turbidities is likely to be under zero.1 NTU (nephelometric turbidity units).
This is sort of satisfactory for common plant make-up, and even is suitable for feed to reverse osmosis models that produce excessive-purity make-up for the steam generator. The waste activated sludge might be very beneficial and from my own experience with an activated sludge plant this materials could be unfold on farm fields as a fertilizer.
The objects outlined above symbolize just some of the developments for bettering cooling tower operation and efficiency. For extra information, I invite you to analyze the Cooling Know-how Institute web site, www.cti.org. One other excellent forum for power plant chemistry studying is the annual Electric Utility Chemistry Workshop, www.conferences.illinois.edu/eucw.
Brad Buecker is a course of specialist in the process Engineering and Permitting group with Kiewit Engineering and Design Firm, Lenexa, Kan. The group supplies water/wastewater and air pollution management engineering and consulting companies. He has greater than 33 years of experience in, or affiliated with, the facility trade, a lot of it in steam technology chemistry, water treatment, air quality management and outcomes engineering positions. He has a bachelor’s diploma in chemistry from Iowa State College, with extra course work in fluid mechanics, materials and energy balances, and advanced inorganic chemistry. He is a member of the ACS, AIChE, ASME, CTI and NACE, the ASME Research Committee on Energy Plant & Environmental Chemistry and the program planning committee for the Electric Utility Chemistry Workshop. It’s possible you’ll contact him by emailing firstname.lastname@example.org.