B. BUECKER, SAMCO Technologies, Lawrence, Kansas, U.S.
Heat exchangers are, of course, an absolute necessity for a countless number of industrial processes. Keeping exchangers clean from scale and deposition maximizes efficiency and reduces the potential for under-deposit corrosion. In this series, we will examine some of the primary influences on waterside deposition, with added discussion about modern control methods. Part 1 focuses on microbiological fouling and control.
The influence of and mechanisms behind deposition. The two primary heat transfer mechanisms in non-fired heat exchangers are convection and conduction; convection via fluid flow and conduction as heat moves from one fluid through heat exchanger tubes or plates to the other fluid. This heat must flow through several resistances in series, and in clean heat exchangers the resistances to be overcome are the two fluid-film resistances and the solid-wall resistance.1 The fluid films (FIG. 1) are the laminar sublayers that form at the metal surface, one on the cooling water side and one on the process side.
The combined resistance (Rt) to heat transfer is the sum of the individual resistances, shown in Eq. 1:
Rt = rwf + rmetal + rpff (1)
Factors that influence heat exchanger design include the required heat duty, cooling and process flowrates and temperatures, and exchanger metallurgy, among others.
Fouling and/or scale formation on the waterside and process side can greatly inhibit heat transfer, as shown in FIG. 2.
The total resistance equation expands to (Eq. 2):2
Rt = rwf + rwd + rmetal + rpsd + rpff (2)
Waterside deposition can have a huge influence on heat transfer, as even a very thin deposit layer can greatly influence heat transfer. Microbes and the slime layers they produce are even more troublesome.
Microbiological fouling. In this author’s experience, and as has been well-documented in the literature, microbiological fouling is often the number one enemy. Cooling systems and heat exchangers provide the ideal environment, warm and wet, for microbes (bacteria, algae and fungi) to flourish. Bacteria can form colonies in many locations; fungi grow on and in cooling tower wood (for towers that have this structural material); and algae will grow in sunlit areas, particularly on cooling tower decks and other exposed areas.
Bacteria are the primary organisms that affect heat exchangers. They are ubiquitous in the environment, with many different species. The two primary methods of microbe introduction to cooling systems are via the makeup water and the air that is drawn in through cooling towers. Free-floating “planktonic” bacteria are basically harmless, however, the nature of many bacteria is to settle on surfaces and establish colonies. As the name “slime-formers” indicates, some organisms will produce a protective polysaccharide film, which typically evolves into a complex structure with aerobic microbes near the waterside, anaerobic bacteria at the metal surface and facultative organisms (which can survive with or without oxygen) in the middle layers of the deposit. The slime collects silt and other particulates (FIG. 3) in the water to form mud-like masses.
Microbiological fouling can be very troublesome in other ways. Deposits in general establish corrosion cells where the oxygen-depleted area beneath the deposit becomes anodic to bare metal and begins to corrode. The presence of small anodes in a large cathodic field can generate pitting and through-wall penetrations in a short time frame. Beyond this issue is that some bacteria, such as the anaerobic sulfate-reducers, release metabolic byproducts [e.g., hydrogen sulfide (H2S)], that are very destructive to many metals, including steels. The attack is known as microbiologically induced corrosion (MIC) (FIG. 4).
Yet another dramatic example of the potential effect of microbiological fouling is the collapse of cooling tower fill support structures (shown in FIG. 5) due to weight gain from microbial slime/silt accumulation.
Finally, as microbial colonies mature, they will generate higher life forms, including protozoa and amoeba. These organisms can harbor Legionella pneumophila bacteria that multiply within the cells. Legionella was first discovered in 1976 when it infected American Legionnaires attending a convention in Philadelphia. Nearly three dozen people died, and many more became ill. The bacteria were traced to fine water droplets in the exhaust plume of a cooling tower on the roof of the convention hotel. The droplets entered the intake of an air handling unit, spread through the hotel and were inhaled by the guests (including the author’s parents, who suffered no long-term effects). These organisms can appear in many water systems (hospitals, spas, produce watering systems in grocery stores, etc.) that are not properly treated to prevent microbiological fouling.
Microbiological fouling control. As logic would suggest, if microbes are allowed to form sessile colonies, with the attendant sticky slime deposits, the material can be quite difficult to remove. Accordingly, the best control method is to install a properly designed and reliable chemical feed system to kill planktonic microbes and to keep any survivors in a dormant state where they do not establish sessile colonies.
Oxidizing biocides are the backbone of microbial control programs, as this chemistry usually represents the most cost-effective method for maintaining system cleanliness. For a time, chlorine gas became the standard for drinking water and then cooling water applications. One-ton cylinders were a common method of storage and supply. When chlorine is added to water, the following reaction occurs (Eq. 3):
Cl2 + H2O ⇌ HOCl + HCl (3)
Hypochlorous acid (HOCI) is the killing agent, and it functions by penetrating cell walls and oxidizing internal cell components. Safety is obviously a major issue when handling chlorine cylinders and feeding chlorine gas, and years ago, many industrial facilities switched to liquid sodium hypochlorite (NaOCl, aka bleach), with a common active chlorine concentration of 12.5%. The change to bleach allowed for use of metering pumps to feed the chemical (FIG. 6); however, a problem that many operators encountered was pump binding due to bleach’s tendency to vaporize in piping locations around the pump. Pump designs have evolved to prevent or relieve vapor lock.
An alternative to bleach is onsite hypochlorite (CIO-) generation (FIG. 7).
The generatora produces a mixed oxidant from three common consumables: water, salt and electricity. Saltwater electrolysis generates chlorine and some peroxide, which also has biocidal properties. The process requires no storage of hazardous chemicals.
Chlorine/bleach alternatives. Prior to the 1980s, the common scale/corrosion inhibitor treatment for open recirculating systems consisted of sodium dichromate/acid feed, with pH typically maintained within or close to a range of 6.5–7. Concerns over the toxicity of acid/chromate programs [especially in regard to the formation of hexavalent chromium (Cr6+)] led to a changeover to inorganic/organic phosphate programs that operate near or slightly above a pH of 8. Unfortunately, the efficacy and killing power of chlorine significantly decline with rising pH, per the equilibrium nature of HOCl (FIG. 8) in water, as shown in Eq. 4:
HOCl ⇌ H+ + OCl- (4)
OCl- is a much weaker biocide than HOCl, probably because the charge on the OCl- ion does not allow it to effectively penetrate cell walls.
As is evident, the dissociation of HOCl is much more pronounced at a pH of 8 than in the 6.5–7 range of the former acid/chromate programs. Chlorine/bleach may not be the best oxidizer choice when combined with modern scale/corrosion control chemistry. Several alternatives are outlined below.
Bromine chemistry. A popular answer ha s been bromine chemistry, where a chlorine-based oxidizer (bleach is the common choice) and sodium bromide (NaBr) are blended in a water slipstream and injected into the cooling system. The reaction produces hypobromous acid (HOBr), which has similar killing powers to HOCl but is still 80% associated at a pH of 8.
Chlorine dioxide. Chlorine dioxide (ClO2) is a gas at room temperature that is stable and soluble in water to a maximum concentration of approximately 3,000 parts per million (ppm). It must be prepared onsite via the reaction of either sodium chlorite (NaClO2) or sodium chlorate (NaClO3) with an additional oxidizing agent under acidic conditions. ClO2 is more expensive than the halogens, but modern production techniques have lowered the cost.
Because ClO2 exists as a gas in solution, residuals that survive to the cooling tower can be stripped by air-water contact in the tower fill. ClO2 should be introduced below the tower basin water level near the circulating pump suction. Handling of the chemical precursors for ClO2, which may include sulfuric acid, requires attention to safety, although modern ClO2 generators are typically designed with safety in mind. Strict adherence to operational guidelines is important.
Monochloramine. Chloramines have served for microbial control in potable water systems for more than a century. In water containing ammonia, continued chlorine feed will produce a series of chloramines, starting from monochloramine (NH2Cl), then dichloramine (NHCl2) and finally nitrogen trichloride (NCl3). The solution reaches “breakpoint” once all ammonia has been consumed, upon which free chlorine appears. Monochloramine is the compound of interest for modern biofouling control, and technologies are now available to produce a pristine stream of NH2Cl for this purpose. Monochloramine is less reactive than the halogens, but the reduced reactivity allows it to penetrate biofilms and attack underlying organisms.
Halogen stabilizers. Several organic compounds are available that can stabilize chlorine and bromine and then release the oxidizers gradually, and where they are most needed. Stabilized halogens typically exhibit a lower oxidizing power than the parent halogen, but this reduced oxidizing power offers benefits with respect to microbial control in that it minimizes undesirable reactions with the protective slime. Three classes of stabilizers dominate the market: sulfamate, dimethylhydantoin and isocyanurates.
A supplemental treatment technique is the feed of a bio-surfactant prior to the oxidizer. These molecules typically consist of a non-polar, hydrophobic carbon chain with a charged molecule at one end. The hydrophobic portion penetrates the organic slime matrix, with the active group being attracted to water. Surfactants/penetrants can loosen the slime layer, allowing better penetration by the chemical agent. Most effective is to feed the surfactant perhaps a half-hour or so before the biocide to allow better penetration of the killing agent.
Non-oxidizing biocides. While oxidizing chemicals normally serve as the foundation of cooling water biocide programs, microorganisms can develop partial immunity. Feeding a non-oxidizing biocide on a periodic basis (e.g., once or twice per week for ~1 hr) can help control microbial growth. Non-oxidizers typically penetrate cell walls to then react with compounds within the cell that are necessary for life. Some of the most common non-oxidizers are 2,2-dibromo-3-nitrilopropionamide (DBNPA), glutaraldehyde, isothiazolones and quaternary amines. The compounds are typically not a “one size fits all” product, so programs must be tailored to address the most troublesome organisms in a cooling system.
Note: U.S. federal and state regulations may influence the choice of antimicrobial products. Industrial facilities are subject to regulatory approval processes before using or even testing a new chemical, with a strong focus on discharge limits. In the U.S., these regulations fall within the auspices of the National Pollution Discharge Elimination System (NPDES). The regulations require approved permits before a water stream with treatment chemicals, including biocides, can be discharged to the environment.
Individual states must enforce NPDES regulations but are free to impose more stringent guidelines, if desired. An issue that has become more pronounced is that the straight halogens, chlorine and bromine, can react with organics in water to form halogenated organic compounds. A plant permit may include restrictions on the concentrations of these compounds in the discharge.
Takeaway. Microbiological fouling is usually the most important concern in cooling water systems. Without proper control, deposits can cause incredible difficulties. Much additional information is available from the Cooling Technology Institute. Part 2 of this series (to be published in a future issue of HP) will examine the primary mechanisms behind, and control methods for, scale formation. HP
DISCLAIMER
This article offers general information and should not serve as a design specification. Every project has unique aspects that must be individually evaluated by experts from reputable water treatment and engineering firms before final treatment methods are selected. HP
NOTE
Morrow Water Technologies’ MIOX® onsite generator
LITERATURE CITED
Peters, M. S., Elementary chemical engineering, McGraw-Hill, 1954.
Betz Laboratories, Betz handbook of industrial water conditioning, 9th Ed., 1991.
Post, R., B. Buecker and S. Shulder, “Power plant cooling water fundamentals,” 37th Annual Electric Utility Chemistry Workshop, Champaign, Illinois (U.S.), June 6–8, 2017.
Brad Buecker serves as Senior Technical Consultant with SAMCO Technologies. He is also the owner of Buecker & Associates LLC, which provides independent technical writing/marketing services. Buecker has many years of experience in or supporting the power industry, much of it in steam generation chemistry, water treatment, air quality control and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company's (now Evergy) La Cygne, Kansas, station.
Additionally, his background includes 11 yrs with two engineering firms, Burns & McDonnell and Kiewit, and he spent 2 yrs as acting water/wastewater supervisor at a chemical plant. Buecker earned a BS degree in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored more than 300 articles for various technical trade magazines, and he has written three books on power plant chemistry and air pollution control. He is a member of the ACS, AIChE, ASME, AWT, CTI, and he is active with Power-Gen International, the Electric Utility & Cogeneration Chemistry Workshop, and the International Water Conference. The author can be reached at bueckerb@samcotech.com and beakertoo@aol.com.