Chapter 32 - Closed Recirculating Cooling Systems

• Advantages of Closed Systems
• Scale Control
• Corrosion Control

CLOSED LOOP WATER SYSTEMS

A closed loop water system, also referred to as a closed recirculating water system, is a specialized setup used in various industrial applications to conserve water and minimize wastage. In a closed recirculating water system, water circulates within a closed loop, continuously moving through the system and undergoing cooling or heating processes before being reused. Closed recirculating cooling systems optimize industrial processes while reducing water consumption and environmental impacts, and play a key role in sustainable water management.

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The closed recirculating cooling water system evolved from methods used for the cooling of early engine designs. In a closed water system, water circulates in a closed cycle and is subjected to alternate cooling and heating without air contact. Heat, absorbed by the water in the closed system, is normally transferred by a water-to-water exchanger to the recirculating water of an open recirculating system, from which the heat is lost to the atmosphere (Figure 32-1).

Closed recirculating cooling water systems are well suited to the cooling of gas engines and compressors. Diesel engines in stationary and locomotive services normally use radiator systems similar to the familiar automobile cooling system. Other closed recirculating cooling applications include smelt spout cooling systems on Kraft recovery boilers and lubricating oil and sample coolers in power plants. Closed systems are also widely used in air conditioning chilled water systems to transfer the refrigerant cooling to air washers, in which the air is chilled. In cold seasons, the same system can supply heat to air washers. Closed water cooling systems also provide a reliable method of industrial process temperature control.

ADVANTAGES OF CLOSED WATER SYSTEMS

Closed recirculating systems have many advantages. They provide better control of temperatures in heat-producing equipment, and their small makeup water requirements greatly simplify control of potential waterside problems. Makeup water is needed only when leakage has occurred at pump packings or when water has been drained to allow system repair. Little, if any, evaporation occurs.

Therefore, high-quality water can usually be used for makeup, and as a result, scale deposits are not a problem. The use of high-quality water also minimizes the dangers of cracked cylinders, broken heads, fouled exchangers, and other mechanical failures. Closed systems are also less susceptible to biological fouling from slime and algae deposits than open systems.

Closed systems also reduce corrosion problems drastically, because the recirculating water is not continuously saturated with oxygen, as in an open system. The only points of possible oxygen entry are at the surface of the surge tank or the hot well, the circulating pump packings, and the makeup water. With the small amount of makeup water required, adequate treatment can virtually eliminate corrosion and the accumulation of corrosion products.

LIMITATIONS OF CLOSED LOOP WATER SYSTEMS

Closed loop water systems do many advantages, but they also come with certain disadvantages that need consideration:

1. Potential for contamination: Since the same water is continuously recirculated in a closed loop, there is a risk of accumulated contaminants or impurities affecting water quality over time.

2. Risk of scaling and corrosion: Closed systems can be prone to scaling and corrosion, leading to reduced efficiency and increased maintenance requirements.

3. Limited flexibility: The design of closed loop water systems can be less flexible compared to open systems, making it challenging to adapt to changing process requirements or accommodate varying water demands.

4. Higher initial costs: Closed loop water systems often require more upfront investment due to the need for specialized equipment and controls, making their implementation costlier than traditional open systems.

5. Energy consumption: These systems may require additional energy for pumping and cooling, which can lead to increased operational costs compared to open systems that rely on natural circulation.

SCALE CONTROL

Some closed systems, such as chilled water systems, operate at relatively low temperatures and require very little makeup water. Because no concentration of dissolved solids occurs, fairly hard makeup water may be used with little danger of scale formation. However, in diesel and gas engines, the high temperature of the jacket water significantly increases its tendency to deposit scale.

Over a long period, the addition of even small amounts of hard makeup water causes a gradual buildup of scale in cylinders and cylinder heads. Where condensate is available, it is preferred for closed-system cooling water makeup. Where condensate is unavailable, zeolite softening should be applied to the makeup water.

CORROSION CONTROL

An increase in water temperature causes an increase in corrosion. In a vented system, this tendency is reduced by the decreased solubility of oxygen at higher temperatures. This is the basis of mechanical deaeration.

Corrosion rates at increasing water temperatures for two different sets of conditions.

Curve A plots data from a completely closed system with no provision for the venting of oxygen to atmosphere. Curve B shows data for a vented system. At up to 170°F (77°C), the curves are essentially parallel. Beyond 170°F (77°C), curve B drops off. This occurs because the lower solubility of oxygen with increasing temperatures in a freely vented system decreases the corrosion rate faster than the rise in temperature increases it. However, in many closed systems, the dissolved oxygen entering the system in the makeup water cannot be freely vented, resulting in the release of oxygen at points of high heat transfer, which may cause severe corrosion.

Untreated systems can suffer serious corrosion damage from oxygen pitting, galvanic action, and crevice attack. Closed cooling systems that are shut down periodically are subjected to water temperatures that may vary from ambient to 180°F (82°C) or higher. During shutdown, oxygen can enter the water until its saturation limit is reached. When the system is returned to high-temperature operation, oxygen solubility drops and the released oxygen attacks metal surfaces(Figure 32-1).

The metallurgy used in constructing modern engines, compressors, and cooling systems includes cast iron, steel, copper, copper alloys, and aluminum as well as solders. Nonmetallic components, such as natural or synthetic rubber, asbestos, and carbon, are also used. If bimetallic couples are present, galvanic corrosion may develop.

The three most reliable corrosion inhibitors for closed cooling water systems are chromate, molybdate, and nitrite materials. Generally, the chromate or molybdate types have proven to be superior treatments. For mixed metallurgy systems, the molybdate inhibitors provide the best corrosion protection.

Chromate closed loop water treatments

Chromate treatments in the range of 500- ppm as Cr4O2¯ are satisfactory unless bimetallic influences exist. When such bimetallic couples as steel and copper are present, chromate treatment levels should be increased to exceed ppm. Maximum inhibitor effectiveness can be achieved if the pH of these systems is kept between 7.5 and 9.5.

In a closed system, it can be quite difficult to prevent corrosion of aluminum and its alloys; the pH of the water must be maintained below 9.0. Aluminum is amphoteric-it dissolves in both acid and base, and its corrosion rate accelerates at pH levels higher than 9.0. The bimetallic couple that is most difficult to cope with is that of copper and aluminum, for which chromate concentrations even higher than ppm may not be adequate.

Where circulating pumps are equipped with certain mechanical seals, such as graphite, chromate concentrations may not exceed 250 ppm. This is due to the fact that water leaking past the seals evaporates and leaves a high concentration of abrasive salts that can damage the seal.

Another problem is encountered when chromate inhibitors are used in cooling systems serving compressors that handle sour gas. If sour gas leaks from the power cylinder into the water circuit, significant chromate reduction will occur, causing poor corrosion control and deposition of reduced chromate.

In very high heat transfer rate applications, such as continuous caster mold cooling systems, chromate levels should be maintained at 100-150 ppm maximum. Under these extreme conditions, chromate can accumulate at the grain boundaries on the mold, causing enough insulation to create equipment reliability problems.

The toxicity of high-chromate concentrations may restrict their use, particularly when a system must be drained frequently. Current legislation has significantly reduced the allowable discharge limits and the reportable quantity for the spill of chromate-based products. Depending on the type of closed system and the various factors of State/Federal laws limiting the use of chromate, a nonchromate alternative may be needed.

Molybdate closed loop water treatments

Molybdate treatments provide effective corrosion protection and an environmentally acceptable alternative to chromate inhibitors. Nitrite- molybdate-azole blends inhibit corrosion in steel, copper, aluminum, and mixed-metallurgy systems. Molybdates are thermally stable and can provide excellent corrosion protection in both soft and hard water. System pH is normally controlled between 7.0 and 9.0. Recommended treatment control limits are 200-300 ppm molybdate as MoO42¯. Molybdate inhibitors should not be used with calcium levels greater than 500 ppm.

Nitrite closed loop water treatment

Nitrite is another widely accepted nonchromate closed cooling water inhibitor. Nitrite concentra-tions in the range of 600- ppm as NO2- will suitably inhibit iron and steel corrosion when the pH is maintained above 7.0. Systems containing steel and copper couples require treatment levels in the - ppm range. If aluminum is also present, the corrosion problem is intensified, and a treatment level of 10,000 ppm may be required. In all cases, the pH of the circulating water should be maintained in the alkaline range, but below 9.0 when aluminum is present. When high nitrite levels are applied, acid feed may be required for pH control.

One drawback to nitrite treatments is the fact that nitrites are oxidized by microorganisms. This can lead to low inhibitor levels and biological fouling. The feed of nonoxidizing antimicrobial may be necessary to control nitrite reversion and biological fouling (Figure 32-3).

Product performance data developed in laboratory studies simulating a mixed-metallurgy closed cooling system identified steel and Admiralty corrosion rates for three closed system inhibitors at increasing treatment levels. As shown, the molybdate-based treatment provides the best overall steel and Admiralty protection. To achieve similar inhibition with chromate, higher treatment concentrations are required.

Nitrite-based treatment also provides effective steel protection, with results comparable to those obtained with molybdate; however, acceptable Admiralty corrosion inhibition is not achieved.

Closed systems often require the addition of a suitable antifreeze. Nonchromate inhibitors are compatible with typical antifreeze compounds. Chromates may be used with alcohol antifreeze, but the pH of the circulating water should be maintained above 7.0 to prevent chromate reduction. Because glycol antifreezes are not compatible with chromate-based treatments, nonchromate inhibitors should be used. Molybdate treatments should not be used with brine-type antifreezes.

In closed systems that continuously run at temperatures below 32°F (0°C), a closed brine system is often employed. The American Society of Refrigeration Engineers has established chromate limits in brine treatments. Calcium brines are limited to ppm chromate, and sodium brines are limited to ppm chromate. The pH should be 7.0-8.5 with caustic adjustment only. Some success has also been recorded with nitrite-based treatment of closed brine systems at treatment levels of about ppm as

NO2¯

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 6.3. Photo of a counter-flow cooling tower.

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.

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.

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 CF 21 mm OF 19 mm VF 25 mm M/S 38 mm VF 19 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  <5 Fibers  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.

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 man lift, or by suspending a section of fill beneath the main fill pack to allow it to be easily inspected and weighed. These monitoring techniques may be somewhat effective, but none have proven to be totally satisfactory. 

Several methods exist to remove biological/silt deposits from cooling tower fill. Hyper-halogenation is one method, but effectiveness may be limited. Furthermore, the high chlorine concentration can cause corrosion to system components, and, when the cleaning is complete, the waste stream may require treatment before discharge. Microbiological colonies have high water content and will shrink and detach from surfaces when thoroughly dried. US Patent 5,558,157 outlines this method for biofilm removal in shell and tube heat exchangers. However, effectively drying out cooling tower fill can prove problematic even with the help of fans. Chlorine dioxide has also served as a cleaner for cooling tower biofilms with some success.

The most widely practiced and effective cleaning chemical for microbiological deposits is hydrogen peroxide (H2O2). Peroxide is effective due to its oxidizing strength and from the physical action of oxygen micro-bubbles produced as it reacts with organic deposits. The decomposition products of peroxide are water and oxygen, and thus the compound has a very positive environmental profile. Typical dosages are in the 500-3,000 ppm range of active chemical. As with most cleaning processes of this type, addition of low levels of surfactants will help loosen deposits. Polymeric dispersants are often also included to keep dislodged solids in suspension until they can be discharged.

Mist 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.”  Moisture discharge is problematic for two reasons. First, solids within the droplets can deposit on induced-draft fan blades and gradually impact performance. Secondly, plant air emissions regulations usually also include cooling tower discharge. A facility may be in violation of discharge guidelines from the solids entrained in the droplets. Accordingly, chevron-vane mist eliminators are standard cooling tower items. The demisters collect water by impingement and allow the water to drain back into the tower.

Figure 6.16. A modern mist eliminator design.
Photo courtesy of Brentwood Industries and Rich Aull Consulting.

Technology has advanced such that modern demisters can reduce entrained moisture to less than 0.% of the recirculating water rate. To put that into perspective, the drift from a tower with a 100,000 gpm recirculation rate and 0.% drift would be 0.5 gpm. Very slight indeed!

Cooling Tower Heat Transfer

As air passes through a cooling tower, it induces evaporation. The water that evaporates consumes a large amount of energy during the change in state from a liquid to a gas. This is known as the latent heat of vaporization, which at sea level is typically around 1,000 Btu/lb. So, cooling towers remove much heat from the recirculating water by a small amount of evaporation.

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An important concept for understanding cooling tower heat transfer is that of “wet bulb” temperature. Consider being outdoors, but in the shade, on a 90°F day at 40% relative humidity. A standard thermometer would read 90°, which is the “dry bulb” temperature. Now, imagine if we placed another thermometer alongside the dry bulb thermometer, but in this case have wrapped a soaked piece of cloth around the bulb of the second thermometer, and have put both on a swivel such that the thermometers can be swirled very rapidly through the air. This instrument, a simple and common device, is known as a sling psychrometer. 

Figure 6.20. Illustration of a vintage sling psychrometer.
Photo courtesy of Rich Aull Consulting.

After a short time, the dry bulb thermometer will still read 90°F but the other thermometer will read 71.2°F. This latter reading is the wet bulb temperature, and is the lowest temperature that can be achieved by evaporative cooling. Modern psychrometers are mechanically aspirated (fans move air across the wetted wick) and are even more precise.

No matter how efficient, a cooling tower can never chill the recirculating water to the wet bulb temperature, and at some point, costs and space requirements limit cooling tower size. The separation in temperature between the chilled water and wet-bulb value is known as the approach. 

A graphical representation of range and approach, reproduced from Reference 1, is illustrated below. Obviously, these values will be variable over the wide range of conditions in which cooling towers operate.

Figure 6.21. Graphical outline of range and approach temperatures. (Reference 1)

The closest approach to wet bulb temperature that can economically be reached with a modern tower is about 4°F, with a typical value being 10°F.

Figure 6.22. Chart of relative cooling tower size vs approach temperature for general applications.
Illustration courtesy of Rich Aull Consulting.

The data needed to calculate heat transfer by air cooling and evaporation has been compiled in a graph known as a psychrometric chart. One version is shown below.

Psychrometric charts contain a large amount of data, and can sometimes be difficult to interpret. Appendix 6-1 outlines how to evaluate this data. 

Reference 8 provides a direct example of how to calculate cooling tower evaporation from psychrometric data, but a simpler equation is available that provides good approximations.

E = (ƒ * R * ΔT)/ | Eq. 6-1

E = Evaporation in gpm

R = Recirculation rate in gpm

ΔT = Temperature difference (range) between the warm and cooled circulating water (oF)

ƒ = A correction factor that accounts for evaporative and sensible heat transfer, where ƒ (average) is often considered to be 0.75 to 0.80, but will rise in summer and decline in winter.

The factor of 1,000 is the approximate latent heat of vaporization (Btu/lb) of water at ambient conditions. 

As an illustration of this calculation, consider a cooling tower at the following conditions:

• R = 150,000 gpm
• ΔT = 15oF 
• ƒ = 0.80

For these parameters, E = 1,800 gpm. Thus, the required cooling is achieved by just 1.2% evaporation of the recirculating water, with 20% sensible heat transfer.

A critical aspect of cooling tower operation and cooling water treatment is that evaporation causes an increase in dissolved and suspended solids concentrations. The common vernacular in the industry for the concentration factor is cycles of concentration (COC). COC can be monitored by comparing the levels of a very soluble ion, such as chloride, in the recirculating (R) water and makeup (MU). However, this procedure requires laboratory analyses. A typical substitute is online-specific conductivity monitoring of the two streams, which can be programmed to automatically bleed off some recirculating water when it becomes too concentrated. A common COC range is 4 to 6. The water savings by increasing blowdown beyond this range become minimal, as the graph below clearly shows. 

Figure 6.24. Cycles of concentration vs. blowdown rate for the heat transfer example outlined earlier.

COC, or perhaps more accurately, allowable COC, varies from tower to tower depending upon many factors including makeup water chemistry, heat load, effectiveness of chemical treatment programs, and possible restrictions on water discharge quality or quantity. In arid locations, COC may need to be high, but chemistry control becomes more difficult. 

A straightforward set of equations is available to reasonably calculate the blowdown (BD) and makeup requirements from a tower when the evaporation is known and allowable COC has been determined.

BD = E/(COC – 1) | Eq. 6-2

MU = E + BD + D + L | Eq. 6-3

Regarding Equation 6-3, it has already been noted that some water escapes the tower as drift (D), but in towers with state-of-the-art drift eliminators, drift is quite small. Leaks in the cooling system are referred to as losses (L), which also contribute to blowdown. In older systems, leaks from corrosion of piping and other equipment may be significant. 

Chapter 7 discusses water treatment technologies to control scaling and fouling in cooling towers and the systems they supply, but the following section outlines a physical method for suspended solids control.

Closed Loop Water Cooling

Many plants have numerous heat exchangers that are embedded within one or more closed cooling water systems. These auxiliary “closed” heat exchangers reject heat to the primary open recirculation cooling system.

Figure 6.33. General schematic of a primary open-recirculating and secondary closed cooling system arrangement.

The term closed cooling water system is somewhat of a misnomer, as virtually all systems have leaks or small losses somewhere that require makeup. (If serious corrosion has occurred, these losses may be significant.)  A closed system is basically defined as a loop which has little or no evaporation, and where makeup requirements do not exceed
5–15% of volume each year.

Systems are often designed with a head tank for water makeup and to absorb volume changes per temperature and load fluctuations. This arrangement can allow oxygen to enter the cooling water, which, of course, influences the corrosion potential. Some closed systems may have a pressurized expansion or bladder tank to maintain constant water pressure. Makeup vessels or tanks are often located at the highest point in a closed loop to serve as an air release outlet for non-condensable gases that might otherwise accumulate in the system and can cause corrosion and pump cavitation.

Examples of industrial and commercial closed cooling applications include:

• Pump bearings
• Lubricating oil
• Automated welders
• Metal casting coolers
• Chilled water systems

The figure below shows the outline of a basic chilled water system.

Figure 6.34. Basic flow diagram of a chilled water system. A common chilled water temperature range is 40 to 45o F.

A variation of this design is shown below, with a closed cooling tower serving as the primary cooling circuit. 

Most closed systems are equipped with a small side-stream pot feeder having inlet and outlet isolation valves for batch chemical feed. An enhancement is a combination of feeder and side-stream filter to remove those metal corrosion products that inevitably form during operation. A filter can be particularly valuable for some applications. An example is the automated elders at automobile assembly plants, which have small-bore, serpentine cooling lines in the welder heads. Particulate accumulation (and fouling or scaling) can be very problematic. 

As will be covered in greater detail in Chapter 7, some closed water loops require high-purity makeup water, i.e., condensate. An example is a steel mill continuous casting cooler where the heat transfer rate is extremely large (106 Btu/ft2/hr). Corrosion or fouling that restricts heat transfer can be extremely detrimental and dangerous if “breakout” of molten steel occurs during the casting process.

Heat Exchanger Equipment

Perhaps the most common heat exchanger design is the shell-and-tube configuration. Shown below is a U-bend, two-pass exchanger.

Figure 6.37. Shell-and-tube heat exchanger design.

Such exchangers are common for liquid-to-liquid heat transfer when only one or neither of the fluids is water. This handbook, with its focus on water treatment, considers exchangers with water as the primary coolant, and where the water flow is through the tubes with the process fluid on the external tube surfaces. The design in Figure 6.37 is co-current with the coolant and process fluid flowing in the same direction. Note the baffle plates in the exchanger to enhance flow mixing and heat transfer. Two-pass heat exchangers are popular because they can provide greater cooling in a smaller amount of space than single-pass coolers.

Shown below is a counter-flow heat exchanger.

Figure 6.38. Counter-flow heat exchanger.

This design is often preferred because of lower thermal stress on the equipment, as the cooling water warms considerably before entering the highest-heat zone.

An interesting variation on this design is the steam surface condenser, which were so prominent at large coal and nuclear power plants, and are still prevalent at many combined cycle power plants.

Figure 6.39. Two-pass steam surface condenser.

The incoming turbine exhaust steam usually is (and should be) of 90% or greater quality. The heat exchanger, with thousands of tubes, converts the steam to liquid for return to the boiler. Condensation improves the thermodynamic efficiency of the power generation process by nearly a third. However, condensation generates a very strong vacuum when the steam collapses to water. The strong vacuum pulls in air at even the tiniest openings in the condenser shell or other spots. If the air is allowed to accumulate, it will coat the tubes and greatly reduce heat transfer. Thus, surface condensers are typically equipped with an air removal compartment that is continuously exhausted by vacuum pumps.

Older coal-fired plants often had vertically aligned shell-and-tube exchangers for feedwater heating. This orientation is practical where horizontal space is limited.

Figure 6.40. Vertical heat exchanger.

Suspended solids in the water may accumulate at the bottom of a vertical heat exchanger if flow is insufficient to keep solids in suspension. Periodic removal of the deposits may be necessary to prevent tubes from becoming blocked with material.

Other Heat Exchanger Designs

Another common design is the plate-and-frame heat exchanger.

Figure 6.41. Plate heat exchanger.

These units offer a smaller footprint and lower cost than shell-and-tube exchangers. The figure below illustrates a basic flow path.

Figure 6.42. Basic plate exchanger flow path.

A drawback is that the tightly-spaced plates provide locations for low fluid velocities that allow suspended solids to settle. Some exchangers may have corrugated plates to enhance fluid mixing, which can present cleaning challenges that require disassembly of the exchanger.

Figure 6.43. A corrugated plate from a plate and frame exchanger. Lower part of the plate already cleaned by water jet washing, upper part uncleaned with fouling clearly visible.

Stainless steel is a common material for plate exchangers, but more exotic materials may be required in high-stress or corrosive applications.

Other heat exchangers may be spiral or helical. A diagram of each is shown below.

Figure 6.44. A helical heat exchanger.

Figure 6.45. A spiral heat exchanger.

These heat exchangers are employed for specialty applications not covered in this handbook.

The above figures illustrate heat exchangers that provide a physical boundary between two fluids. In some applications, no boundary exists. A primary example is direct steam injection. The steam is then recovered as condensate further along in the process. However, the condensate may contain any number of impurities that require removal before return to the boiler. 

Heat Transfer Fundamentals

The three general modes of heat transfer are convection, conduction, and radiation. These are discussed in Chapter 4. For most of the heat exchangers outlined above, conduction and convection are the primary heat transfer methods. The mathematics of heat transfer can be quite complicated, especially when designing systems. However, a great deal of understanding is possible from straightforward calculations, “When heat flows from one fluid to another through a solid retaining wall, the total amount of heat transferred may be expressed as follows:

(Q/t)total = U*A*ΔTtotal | Eq. 6-4

• Q/t is heat transfer per time, with the common English units of Btu/hr.
• U is the heat transfer coefficient (Btu/(hr) (ft2) (o F)
• A is the surface area (ft2) of the tubes or plates across which the heat is transferred.
• ΔT is the temperature difference between the two fluids. This is usually calculated as the “log mean temperature difference” (LMTD), which more accurately accounts for the change in temperature of the two fluids along the length of the heat exchanger.
  • LMTD = (Δt2-Δt1)/ln(Δt2/Δt1)

The key variable in Equation 6-4 is U. When fluid flows through a tube, pipe or along a plate, even if the bulk flow is turbulent, a thin, laminar sublayer forms at the material surface. This film influences heat transfer. Accordingly, for a clean surface, the equation for U becomes:

1/U = 1/h′ + 1/h″ + xw/k | Eq. 6-5

• h′ is the film coefficient for the first fluid (Btu/(hr) (ft2) (o F)
• h″ is the film coefficient for the second fluid (Btu/(hr) (ft2) (o F)
• xw is the tube wall or plate thickness
• k is the unit heat transfer coefficient of the material (Btu/(hr) (ft2) (o F/ft)

An excellent example of heat transfer from steam condensing on a 2”, Schedule 40 carbon steel pipe is outlined in Reference 11. All details are not repeated here, but worth noting is that h′ (water film) is given as 500 Btu/(hr) (ft2) (o F), h″ (steam side) is given as 2,000 Btu/(hr) (ft2) (o F), and xw/k is 0. (hr) (ft2) (o F)/Btu, where k for carbon steel is listed as 26 Btu/(hr) (ft2) (o F/ft). Taking the inverse of the first two and adding these numbers to the third value (and adjusting slightly for internal and external pipe surface area differences) gives a U value of 346 Btu/(hr) (ft2) (o F). For this particular example, with a single pipe 10 feet long and LMTD of 91o F, per Equation 6-4 the heat transfer to the cooling water is 170,000 Btu/hr.

A key observation from this example is that the heat transfer through the steam-side and the pipe wall are roughly equivalent, but that heat transfer through the water film is considerably lower. Thus, heat exchangers are often designed to maximize turbulent flow (within constraints on pumping requirements and metal resistance to erosion-corrosion) to reduce the laminar sub-layer thickness. For exchangers with liquid on both sides of the tubes or plates, the film heat transfer resistance increases. These factors are very important when designing the unit. If the process fluid is something other than water or steam, the calculations become more complex.

A very important criterion for selecting heat exchanger materials is the thermal conductivity. The data below offers some selected values from common heat exchanger materials.

Table 6-3. Thermal conductivities of some common heat exchanger materials.

Information provided by Dan Janikowski, Plymouth Tube Company per Heat Exchange Institute (HEI) data.

Metal Thermal Conductivity at 68o F (or near)(Btu/(hr) (ft2) (o F/ft) Carbon Steel  27.5 Admiralty Brass (70 Cu, 29 Zn) 64 90-10 Cu-Ni 26 70-30 Cu-NI 17 304 and 304L Stainless Steel 8.6 316 and 316L Stainless Steel 8.2 Titanium (Grade 2) 12.7

The table illustrates the large variety in thermal conductivities, and it clearly illustrates the much higher conductivity of admiralty brass as compared to carbon steel and especially stainless steel. (A caveat exists in this regard, as will shortly be outlined.)  For this reason, in the middle of the last century, admiralty brass was a common selection for condenser and feedwater heater tubes in coal-fired power units. Thick tube walls were possible; designed to provide long service life. However, it became increasingly apparent that gradual copper corrosion allowed transport of the corrosion products to steam turbines, which deposited on the turbine blades and reduced efficiency. Many plant staffs replaced the admiralty tubes with stainless steel to eliminate this problem. Unfortunately, when this modification was made in some condensers, the stainless steel tubes began to suffer from water-side microbiologically-induced corrosion (MIC). Copper ions that leached from the admiralty tubes were toxic to microbes, whereas the steel did not offer the same protection. This is another example showing the importance of matching metal corrosion characteristics with process conditions and will be covered in further detail in Chapter 7.

The caveat mentioned above is that an oxide layer develops on many metals during service. The layer may be more or less protective depending on the environment and the metal. Copper alloys form an oxide patina, which is considerably more insulating than the base metal. This is another factor that must be considered during heat exchanger design.

Expanding on Equation 6-5, the equation below illustrates the influence of deposition on either side of the metal surface.

1/U = 1/h′ + 1/h″ + xw/k + 1/h′d + 1/h″d | Eq. 6-6

The last two terms account for deposition or film formation on each side of the tube wall or plate.

Mineral and microbiological deposits have low heat transfer coefficients as shown in the table below.

Table 6-4. Thermal conductivity of three common water-side deposits.

Information provided by Dan Janikowski, Plymouth Tube Company.

Foulant or Scale Thermal Conductivity (Btu/(hr) (ft2) (o F/ft) Calcium carbonate 1.25 Silica 0.8 Biofilm or stagnant water 0.36

Even a thin deposit layer will significantly reduce heat transfer. Note the confirmation of the insulating effect of a stagnant water layer. Porous biofilms can induce under-deposit corrosion, which may lead to premature failures and unit outages.

Heat Exchanger (HX) Performance Monitoring

Specification sheets are typical for new heat exchangers, and an example from the Tubular Exchange Manufacturers Association (TEMA) is shown below.

Figure 6.46. Specification sheet for a shell-and-tube heat exchanger.

When units are placed into service, it is important to collect all relevant operating data, as this is when the exchanger is at its most pristine with maximum heat transfer efficiency. Typically, the data will not exactly match the specification sheet, but it serves as the baseline for the future. Important data for inlet and outlet streams is outlined in Figure 6.46 and includes the following:

• Temperatures and pressures 
• Flow rates  
• Densities and viscosities 
• Specific heats

Also highly important are accurate process flow diagrams (PFDs) and process and instrument diagrams (P&IDs) that clearly outline the equipment and piping within a unit operation, including flow rates, pressures, and temperatures at full capacity. For plants such as refineries, chemical manufacturing, and other similar facilities, a huge number of diagrams is necessary. A frequent mistake at many plants has been lack of documentation for piping and equipment modifications. This becomes even more problematic when changes are made to underground piping without proper documentation. 

Common issues that affect heat exchangers materially and/or performance-wise include:

• Reduced cooling water flow due to pump impellor degradation or other problems. Flow losses are often gradual and may be overlooked. Reduced flow can increase process temperatures that in turn cause corrosion or scaling problems on the process side, and influence deposition and corrosion on the cooling side.
• Ill-considered manual adjustments to HX inlet and outlet valve positions. A common complaint heard at plants when equipment performance suddenly becomes erratic is that a “gremlin” must have tampered with it. The gremlin, of course, is someone from the plant who changed a valve setting, modified a pump feed rate, or made some other change without informing anyone.
• Incorrect materials selection for the application
  • Stainless steel is often selected in knee-jerk fashion during project design, only for rapid failures to be induced by a particular corrosive agent in the cooling water or process fluid. Chlorides can be quite corrosive to the austenitic stainless steels, especially if they concentrate under deposits such as those from microbiological fouling.
  • Sometimes the material selected for the application may be suitable, but the fabrication methods are faulty. A classic example was installation of new 90-10 copper-nickel tubes in a steam surface condenser for a once-through application with raw water from a recreational lake. The tubes should have lasted for decades but failed within 18 months from through-wall penetrations because the fabricator utilized a lubricating fluid that contained sulfides.
• Poor or ineffective water treatment program selection or operation (covered in Chapter 7)
• Inadequate initial cleaning and passivation (covered in Chapter 7)

Another common error, and especially at plants with many heat exchangers, is to monitor all exchangers rather superficially and overlook data from individual units that suggest a serious problem. Allocating resources for personnel to better focus on heat exchanger operation can be beneficial in finding and correcting problems before they become major issues.

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