Protecting Cooling Water Systems – Part 1

Brad Buecker, Buecker & Associates, LLC
Richard Aull, Richard Aull Cooling Tower Consulting, LLC

Posted 6/25/2024

Introduction

In a relatively recent article on flare tower safety, the author pointed out that “Operating personnel often do not consider flare systems to be as important as other plant equipment.  This issue is compounded by the fact that flares are usually located far from the operating plant, leaving them out of sight and, often, out of mind.” (1)  If something like a flare tower, which is very visible, especially at night, can become out of mind to plant personnel, what does that say about cooling towers and other cooling system components?  Numerous preventive measures are available during cooling system design and operation to establish and maintain reliability and efficiency.  In Part 1 of this series, we will examine some of the most important components of cooling towers, and from that will build upon issues that can affect cooling system performance and operation.  One of the later installments will address closed cooling water systems.

Important Cooling Tower Design Fundamentals

A large industrial plant such as a refinery, petrochemical facility, steel mill, etc., will have dozens if not hundreds of water-cooled heat exchangers to control various processes.  While some large heat exchangers such as steam surface condensers may be on once-through cooling, many cooling systems are open-recirculating design that have a cooling tower as the core heat discharge process.  Figure 1 outlines the basic schematic of a cooling system with a cooling tower at its heart.

Schematic of a cooling system with an induced-draft cooling tower.
Figure 1. Schematic of a cooling system with an induced-draft cooling tower. (2)  

The bulk of heat transfer (typically 60% to 90% depending on ambient conditions and other factors) in a cooling tower comes from evaporation of a small portion of the recirculating water.  Virtually all cooling towers contain some type of “fill” to enhance air/water contact and optimize the liquid-to-gas (L/G) ratio in the tower.  This reduces the size and material/operational costs of the tower and of auxiliary equipment such as recirculating pumps and fans.  Proper fill selection (and correctly designed water treatment programs, as we will explore in subsequent installments) are critical to maintain fill efficiency and reliability.  

Early cooling towers had wooden splash fill; a series of staggered slats below the water spray or distribution nozzles. 

Schematic of a cooling system with an induced-draft cooling tower.
Figure 2. General schematic of early wooden splash fill in a crossflow air pattern. (2)

Water impinging on the slats breaks into small droplets that increase the surface area.  Splash fill is common for crossflow towers, and the technology has been considerably improved, with a modern design shown below.

Schematic of a cooling system with an induced-draft cooling tower.
Figure 3. A modern splash fill arrangement.  

Splash fill may be the only choice in cooling towers where the water has a high fouling tendency, but in most towers film fill is the preferred material, as it enhances air-water contact.  Typical film fills are made of PVC per low cost, durability, good wetting characteristics, and inherently low flame spread rate. (3)  Numerous designs are available. The choice of flow configuration and the spacing between the fill sheets (flute size) must be evaluated carefully and are dependent upon the projected quality of the recirculating water.  Figures 4-7 below (courtesy of Brentwood Industries) show a progression of various film fill configurations moving from lower efficiency (and corresponding low fouling potential) to high efficiency and higher fouling potential. 

Vertical Flutes (VF).  
Figure 4. Vertical Flutes (VF).  
XF Stand-off. 
Figure 5. XF Stand-off. 

Offset Flutes (OF). 
Figure 6. Offset Flutes (OF). 
Cross-Flutes (CF).  
Figure 7.  Cross-Flutes (CF).  

Selection of the correct fill offers a classic example of preventive design.  The following abridged table, with data taken from Reference 4, outlines general guidelines for several of the configurations shown above. 

Table 1: Fill Section Based on Water Quality (4)
Fill Section Based on Water Quality

  1. “Good” biological control represents oxidizing biocide feed, with total aerobic bacteria (TAB) maximum plate counts not exceeding 100,000 cfu/ml (colony forming units) with minimal slime formation on heat transfer surfaces.
  2. “Poor” microbiological control implies minimal or no microbiological control, or control subject to significant disruption, with average TAB plate counts consistently over 100,000 cfu/ml.
  3. Values listed represent concentrations of light, soluble oils.  Heavy oils and greases would require a specific recommendation on a job-to-job basis.
  4. Values listed require limits on the concentrations of light, soluble oils to 5 ppm or less and 0 ppm on heavy oils & greases.

Even this abridged data clearly shows the tighter impurity guidelines for high efficiency fills (CF and XF designs) versus the lower efficiency VF types.  Additional notes in this technical bulletin mention the influence of other impurities including microbiological nutrients such as ammonia compounds and sugars.  We will examine microbiological fouling in greater detail in Part 2.

Additional Cooling Tower Maintenance Items

Other critical cooling tower equipment includes:

  • Water spray/distribution system
  • Circulating pumps
  • Fans and gear drives
  • Drift eliminators
Water Distribution System

Water distribution for a counterflow tower is typically provided from a header/lateral spray system that resides a few feet above the fill.

Fill Section Based on Water Quality
Figure 8. Schematic of an induced-draft, counterflow cooling tower. (2)

The headers, laterals, and nozzles are designed to provide an even distribution of water to the fill to minimize “channeling.” 

Photo of several laterals and spray nozzles in a counterflow tower. 
Figure 9. Photo of several laterals and spray nozzles in a counterflow tower. 

Obviously, failure or plugging of spray nozzles could cause flow maldistribution, and whenever opportunities permit, maintenance personnel should inspect nozzles for damage and fouling.   

In crossflow towers, the return water is typically discharged onto a distribution deck with uniformly spaced target nozzles within the deck that allows the water to feed by gravity into the air passing perpendicularly.

Schematic of an induced-draft crossflow cooling tower.
Figure 10.  Schematic of an induced-draft crossflow cooling tower. (2)  Return water is discharged onto distribution decks with a uniform pattern of nozzles, such that gravity provides the water pressure.

Figure 11 illustrates a small portion of a crossflow cooling tower deck with water draining through several penetrations.

Water flowing through several penetrations of the cooling tower deck.
Figure 11. Water flowing through several penetrations of the cooling tower deck.

Regardless of the cooling tower type, the fill requires uniform water distribution to minimize channeling.  Channeling by itself will reduce tower efficiency, but it can also establish low-flow locations that allow increased deposition and formation of microbiological colonies.  For crossflow towers, plugging of the nozzles by solid materials or algae can negatively influence water distribution.

Circulating Pumps

Recirculating systems are typically designed with multiple pumps so that the water flow rate can be adjusted per seasonal temperature changes, although with multi-cell cooling towers, cells can be placed in or removed from service to follow heat load.  As with other pumps in the plant, a preventive inspection and repair program is important to maintain pump reliability.  

Fans

Most large cooling towers are of the induced draft style where the multi-bladed axial fans pull air through the tower. 

An ID fan at the top of a cooling tower cell.
Figure 12. An ID fan at the top of a cooling tower cell.

Important aspects of fans beyond size and motor horsepower include fan speed and blade pitch.  Air flow can stall if the fan pitch settings are not correctly configured for the fan’s RPM.  Regular monitoring and maintenance are important. Fans may become unbalanced and misaligned from the accumulation of deposits that exit with the exhaust plume. Fan blade leading edges may become eroded due to constant impact with drift droplets in towers where drift is an issue. Gearboxes are another item that require regular inspection and lubricant monitoring.

A not uncommon arrangement is dual-speed fan control, or perhaps even variable frequency drive (VFD) control. Thus, rather than placing a cell or cells in or out of service to adjust for load or ambient air temperature changes, it may be possible to adjust the fan speed. 

Drift Eliminators

The interaction of air and water in the tower generates many fine droplets that can potentially exit the tower in the plume. The common term for this loss is “drift.”  Drift discharge is problematic for three reasons. First, solids within the droplets can deposit on induced-draft fan blades and gradually impact performance. Secondly, plant air emissions regulations often include cooling tower discharge. A facility may be in violation of discharge guidelines from the solids entrained in the droplets. Thirdly, drift droplets are considered a vector for the transmission of legionella bacteria, the cause of legionnaire’s disease, an illness that can be fatal to those with compromised immune systems. Accordingly, cellular-type drift eliminators are standard cooling tower items. They collect water by impingement and allow the water to drain back into the wet section of the tower.

A modern high efficiency drift eliminator design.
Figure 13. A modern high efficiency drift eliminator design.  Photo courtesy of Brentwood Industries.

Technology has advanced such that modern drift eliminators can reduce entrained moisture to less than 0.0005% of the recirculating water rate. To put that into perspective, the drift from a tower with a 100,000 gpm recirculation rate and 0.0005% drift would be 0.5 gpm.

Additional Operational/Maintenance Concerns

Cooling towers are superb air scrubbers, and many air-borne materials enter cooling systems to cause potential problems in tower fill, cooling system heat exchangers, and other equipment.  (Cottonwood seeds are particularly notorius.)  Common cooling tower design guidelines recommend a sidestream filter to remove suspended solids.  Figure 14 illustrates two common filter locations. 

“kidney-loop” sidestream filter arrangement on a cooling tower basin
Figure 14. At left is a common “kidney-loop” sidestream filter arrangement on a cooling tower basin. An alternate location is the recirculating pump discharge shown at the lower right. (2)

The table below outlines four of the most common side stream filter technologies.

Table 2. Side Stream Technologies
Side Stream Filtration for Cooling Towers (energy.gov)
(Source: Side Stream Filtration for Cooling Towers (energy.gov)

Another maintenance issue is the potential for ice formation in northern climates.  Icing can damage equipment and restrict air flow through towers.  Plant personnel should work with the tower manufacturer to develop an operational program to control ice formation during the winter months.  Rotating cells in and out of service and changes in fan speed may help in this regard.

Conclusion

This initial series installment outlined many of the most important components of a cooling tower.  Well-designed operating procedures and regular maintenance are important for trouble-free operation.  The next installments will examine in greater detail the methods needed to maintain reliability of cooling towers and other cooling system components.  For those interested in plunging into greater detail, Reference 2 provides an expanded discussion of many of these issues.  Also, the Cooling Technology Institute (www.cti.org) offers a wealth of material about cooling systems.


References

  1. B. Karthikeyan, “Manage Changes to Flare Systems”; Chemical Engineering Progress, Vol. 116, No. 1, January 2020.
  2. Water Essentials Handbook (Tech. Ed.: B. Buecker). ChemTreat, Inc., Glen Allen, VA, 2023.  Currently being released in digital format at https://www.chemtreat.com/.
  3. Buecker, B., and R. Aull, “Cooling Tower Heat Transfer Basics – Part 2”; Hydrocarbon Processing, August 2020.
  4. “ACCU-PAC General Fill Selection Guidelines”; Brentwood Industries, March 2024.

richard aull

Richard Aull is president and principal engineer of Richard Aull Cooling Tower Consulting, LLC. He has been working in the cooling tower industry for over 40 years, with a specific interest in tower thermal design, performance rating, and analysis. Prior to starting his consulting business in 2017, he held technical positions within Brentwood Industries’ cooling tower business unit, first as R&D manager and lastly as director of application engineering.

Earlier in his career he held a variety of engineering positions within Research Cottrell’s Hamon Cooling Tower Division and the Ecodyne Cooling Tower Services Group. Rich is active in the Cooling Technology Institute (CTI) and has published technical papers and conducted seminars on a variety of cooling tower topics for the Cooling Technology Institute, Electric Power Research Institute, International Water Conference, IAHR Conference on Cooling Towers and Air-Cooled Heat Exchangers, India’s NTPC’s India Power Station O&M Conference, NTPC’s Global Energy Summit, and the American Society of Mechanical Engineers. Rich has B.S. and M.S. degrees in mechanical engineering from the New Jersey Institute of Technology focusing on thermodynamics, heat transfer, and fluid dynamics and is a registered Professional Engineer in the states of Pennsylvania, New Jersey, and Missouri.


avt-img

Brad Buecker

Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as a senior technical publicist with ChemTreat, Inc. He 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, IL, USA) and Kansas City Power & Light Company's (now Evergy) La Cygne, KS, USA, station. His work has also included eleven years with two engineering firms, Burns & McDonnell and Kiewit, and he spent two years as acting water/wastewater supervisor at a chemical plant.

Buecker has a B.S. 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 over 250 articles for various technical trade magazines, and has written three books on power plant chemistry and air pollution control. He is a member of the ACS, AIChE, AIST, ASME, AWT, the Electric Utility Chemistry Workshop planning committee, and he is active with the International Water Conference and Power-Gen International.

Picture of Brawley

Brawley

EXPLORE BY TOPIC

Join the discussion

Click here to join the Maintenance and Reliability Information Exchange, where readers and authors share articles, opinions, and more.

"*" indicates required fields

Get Weekly Maintenance Tips

delivered straight to your inbox

"*" indicates required fields