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Introduction to Cooling Water System Fundamentals

Cooling of process fluids, reaction vessels, turbine exhaust steam, and other applications is a critical operation at thousands of industrial facilities around the globe, such as general manufacturing plants or mining and minerals plants. Cooling systems require protection from corrosion, scaling, and microbiological fouling to maximize performance, preserve equipment life and reliability, and most importantly, help ensure employee safety.

In this chapter, we examine fundamental cooling system design and heat transfer basics. This overview will provide the foundation for the cooling water treatment discussion in the next chapter.

Interested readers are also encouraged to explore the Cooling Technology Institute’s website. This long-standing organization provides excellent information on all facets of industrial and commercial cooling applications.

Table of Contents

Types of Cooling Systems
Cooling Tower Components
Overview of Natural Draft Cooling Towers
Cooling Tower Heat Transfer
Sidestream Filtration
Note About Tower Performance Monitoring
Cooling Tower Alternatives
Closed Loop Water Cooling
Heat Exchangers
Heat Transfer Fundamentals
Heat Exchanger Performance Monitoring
Conclusion

Types of Cooling Systems

The three major cooling system designs are once-through, open recirculating (cooling tower-based), and closed. The first two typically serve as primary cooling for the largest heat exchangers, with closed loops for auxiliary plant systems. The fundamentals of each are outlined below.

Once-Through Cooling Systems

As the name “once-through” implies, the cooling water comes from an external source such as a lake, river, or even the ocean. After serving the heat exchangers, the water is directly discharged back to the original source. A common example, especially in the last century, was turbine exhaust steam cooling at large power plants, as shown below.

Figure 6.1. Basic once-through cooling schematic of a power plant condenser.

Once-through intakes are normally equipped with bar screens and/or traveling screens to remove material such as tree branches, leaves, and other large items, including aquatic life, that would otherwise physically foul condenser and heat exchanger tubes. Years ago, it became evident that the screening process was fatal to many aquatic organisms, which either violently impinged on or became trapped against the screens. Increased concern about protecting aquatic life has brought about change to cooling system design and selection with a stronger focus on sustainable water solutions and advancements in sustainable water cooling. Some existing intakes have been retrofitted with modern screens that minimize harm to aquatic life, while for many modern plants once-through cooling is no longer permitted, rather cooling tower systems are required.

Note: While many nuclear plants have cooling towers, once-through backup systems are common for emergency cooling.

Also of concern with once-through systems is the discharge of warm cooling water to the supply source. Warm temperatures can be lethal to some organisms, while others such as fish will congregate at the discharge during cold weather months. Some plants were designed with discharge channels to allow the water to cool somewhat before entering the primary water body.

In a few once-through applications, a spray system assists with discharge cooling. Similar to the cooling tower process, which is examined in detail next, a spray system enhances cooling by evaporation of a small portion of the discharge.

Figure 6.2. Spray pond.
“Grey Water Pond at Palo Verde” by NRCgov is licensed under CC BY 2.0.

Chemical treatment of once-through systems is often straightforward but still very important to minimize micro- and macro-biological fouling and scale formation. These topics are covered in Chapter 7.

We will now examine alternatives to once-through cooling.

Recirculating Cooling Systems

In recirculating cooling systems, the water is recycled continuously. The simplest form of a recirculating cooling system is a cooling pond. Most cooling is by sensible heat transfer with minor evaporative heat loss that increases on windy and warm days. Cooling ponds require a large footprint, and thus open-recirculating systems are much more common.

Open Recirculating Cooling Systems

The ability to transfer large quantities of heat via a small amount of recirculating water evaporation is the basis behind cooling tower applications.

Figure 6.3. Photo of a counter-flow cooling tower.

The fundamental process is shown below:

Figure 6.4. Fundamental cooling tower flow diagram.

Millions of cooling towers are in service around the globe at facilities ranging in size from huge industrial plants to commercial facilities such as office buildings. 

Modern cooling towers are of two primary types, mechanical draft (fans move air through the tower) and natural draft (air flows naturally through the tower.) The latter is the huge hyperbolic towers at large coal or nuclear power plants, and are much less common than mechanical-draft towers, which are the primary focus of this section. 

An advantage of mechanical-draft towers is that they can be designed and assembled in cells that sit side-by-side within a common structure. Individual cells may be placed into or taken out of service to handle changing loads. The towers may be either forced-draft, in which the fans push air through the tower, or induced-draft where the fans pull the air through.

Figure 6.5. An induced-draft fan in the exhaust hood of a cooling tower cell. Photo courtesy of International Cooling Tower.

Most large industrial towers are induced-draft, but smaller units are often forced-draft to simplify operation. In forced-draft towers the air velocity decreases during air passage through the tower. The lower velocity can lead to recirculation of exhaust air to the tower inlet, reducing efficiency. 

Another primary differentiation is crossflow or counter-flow, in which the air flows perpendicularly or counter-currently, respectively, to the water flow path.

Figure 6.6a. Schematic of an induced-draft, counter-flow cooling tower. Air flow is opposite water flow.Figure 6.6b. Schematic of an induced-draft crossflow cooling tower. Airflow is perpendicular to water flow.

Note that the towers shown in both Figures 6.6 a and b are dual-entry types, in which the air enters from opposite sides. These are more efficient than single-entry towers, where wind direction has a greater impact on efficiency. Large towers are often placed to take advantage of prevailing wind patterns. Occasionally, one might see an octagonal or circular tower for maximum efficiency regardless of wind direction, but the design and construction costs of these towers are greater than for standard rectangular towers, and thus they are not that common.

Cooling Tower Components

The tower components shown in the previous illustrations are critical for successful structural stability and operation. The next sections review the most important of these.

Structural Materials

Depending on size, age, and other factors, the structural supports and internal components of cooling towers may be of several materials. Knowledge of the various materials for any application is important for optimizing water treatment, as is discussed in Chapter 7.

In past years, large cooling towers had wooden support structures. Pressure-treated Douglas Fir and Redwood were the two most common choices. The primary advantages of these materials are reasonable cost, decent strength, and that the products can be easily cut to precise specifications in the field. Disadvantages include:

• Susceptible to fungal decay in the cooling tower plenum
• Susceptible to chemical attack in wetted zones
  • Chlorine
  • Low or high pH
• Combustible (destruction by fire of cooling towers is very well known)
• Variable degrees of wood quality, either from the source or, more commonly, the steps taken in the pressure treatment process.

The replacement for wood in many modern, large industrial towers is fiberglass-reinforced plastic (FRP).

Figure 6.7. FRP support structure for a counter-flow cooling tower. Photo courtesy of International Cooling Tower.

Benefits of fiberglass include:

• Decay resistant, especially compared to wood
• No chemical leaching
• Reduced flammability
• Chemical resistance is generally outstanding. FRP can tolerate high concentrations of chlorides and sulfates, which promote corrosion in cooling towers constructed of metals.
• Soft water is not aggressive to FRP

Limitations of fiberglass include:

• High temperatures can be problematic.
• Field repairs are virtually impossible. Once a section is damaged mechanically, it usually must be replaced.
• General lack of stiffness, relative to steel and wood, which results in limitations for use where mechanical loading might be large.

Typical for large cooling towers, and as is shown in Figure 6.8, is a concrete water basin. This can at times present corrosion challenges, as will be discussed in Chapter 7. In such cases, advanced methods can be used to protect cooling water systems as well as cooling water chemistry programs.

For smaller cooling towers such as those at commercial buildings, galvanized steel is a common structural material. Small towers can often be fabricated on one skid in the supplier’s shop and shipped to the site directly.

Figure 6.8. A package cooling tower.

Galvanized towers may have a carbon steel basin. Other small towers may be fabricated from stainless steel, sometimes in the mistaken belief that stainless is resistant to all forms of corrosion.

Tower Fill

The primary method of heat transfer in a cooling tower is evaporation of a small portion of the recirculating water. Key to maximum heat transfer (within various water quality restraints as we shall see) is correct fill selection. Proper selection lowers the liquid-to-gas (L/G) ratio for the tower, and correspondingly reduces the size and material/operational costs of the tower and of auxiliary equipment such as recirculating pumps and fans. 

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

Figure 6.9. General schematic of early wooden splash fill in a crossflow cooling tower.

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.

Figure 6.10. A modern splash fill arrangement. Source: Brentwood Industries and Rich Aull Consulting.

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. Film fill is not generic in nature, and 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. The following illustrations outline several film fill styles ranging from a low-fouling design for waters with strong fouling potential to high-efficiency types.

Figure 6.11a. Vertical Flutes (VF). Courtesy of Brentwood Industries and Rich Aull Consulting.Figure 6.11b. XF Stand-off. Courtesy of Brentwood Industries and Rich Aull Consulting.Figure 6.11c. Offset Flutes (OF). Courtesy of Brentwood Industries and Rich Aull Consulting.Figure 6.11d. Cross-Flutes (CF). Courtesy of Brentwood Industries and Rich Aull Consulting.

Figures 6.11a–d show a progression of various film fill configurations moving from low efficiency and corresponding low fouling potential to high efficiency and high fouling potential.

Cooling tower manufacturers continue to improve efficiency, but this is a double-edged sword in that the complex flow path increases potential locations for solids deposition. The following table outlines general guidelines for some of the designs shown above.

Table 6-1. Fill Selection Based on Water Quality1

Source: Reference 2

 19 mm CF21 mm OF19 mm VF25 mm M/S38 mm VF19 mm XF- Standoff4 Allowed TSS (ppm) with Good Microbial Control2 <100  <200  <500  <1,000  No Limit  <500  Allowed TSS (ppm) With Poor Microbial Control3 <25 <50 <200 <500 <1,000 <200 Allowed Oil and Grease (ppm) None <1 <5 <50 <25 <5Fibers None None None None None None 

1. These are general guidelines and may need modification based on site-specific conditions.
2. ‘Good’ microbiological control means oxidizing biocide supplied continuously with free oxidant residuals maintained, 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.
3. ‘Poor’ microbiological control implies little or no microbiological control or control subject to severe disruption, with average TAB plate counts consistently over 100,000 cfu/ml. Other potential fouling risk factors must be considered also, such as water-borne cross-contamination with process fluids containing ammonia compounds, sugars or other nutrients. Other airborne contaminants should be considered also, such as fine dust, dirt, and debris.
4. For high water loading typically found in crossflow towers

Figure 6.12 illustrates the effect of water velocity on the depth of biofilms.

Figure 6.12. Biofilm thickness as a function of water velocity (Reference 3, 4)

A comparison of this illustration with the types of cooling tower fill shown above highlights the vulnerability of cellular plastic film packs to biofouling. The water film velocity in typical cross-fluted film packs has been reported to be only 0.48 ft/s, and for fouling-resistant film packs, only 0.89 ft/s – 0.95 ft/s for an 8 gpm/ft2 water loading rate.

Figure 6.13. Water film velocity for typical cellular plastic fill packs of cross-fluted and fouling-resistant designs. (Reference 3, 5)

Biofilms collect suspended solids that enter the tower via the makeup and air flow to produce mud-like deposits that can become very thick.

Figure 6.14. An extracted section of film fill with microbiological/silt deposits.

The deposits can close off fill passages, which, of course, reduces air-water contact and degrades heat transfer. Deposition can also add enormous weight to the fill. Both effects are clearly shown below.

Figure 6.15. Tower capability loss vs. fill weight gain for a standard offset flute cellular plastic fill pack. (Reference 3, 6)

In extreme cases, fouled fill has collapsed, resulting in an unscheduled outage and large replacement costs. Fortunately, there are modern techniques for corrosion and fouling protection.

ADVANCED OXIDIZING BIOCIDE SELECTION FOR DIFFICULT COOLING AND PROCESS WATERS

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Microbiological colonies tend to accumulate in the middle of the fill pack. Water velocities directly under the spray nozzles are generally high enough to discourage microbe adhesion. Additionally, fouling tends to be more intense in the middle of the fill than at the bottom because suspended solids are filtered out prior to reaching the lowest fill layer and because the last few inches of fill do not physically support a soft deposit mass. The absence of microbial colonies at either the top or bottom of the fill, combined with the difficulty of inspecting the middle layers, often allows fouling to progress undetected until it has reached an advanced stage. Personnel at power plants and industrial facilities have attempted to monitor fill fouling during tower operation using sections of fill suspended from load cells, or by cutting an access window into the end of the tower casing to allow a middle section to be removed periodically for inspection using a