A cooling tower is a heat removal device that uses water to transfer process waste heat into the atmosphere. Likewise, an industrial cooling tower operates on the principle of removing heat from water by evaporating a small portion of water that is recirculated through the unit.
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The mixing of warm water and cooler air releases latent heat of vaporization, causing the water to cool. If you are ever looking down from a high-rise building, you might notice a square unit, with fans on top of it, on the buildings below. That's a water cooling tower.
No one wants to stay in a building with bad air conditioning—at least not for too long. On the other hand, buildings with excellent cooling make you want to return, even if it’s just to enjoy the air. That’s thanks, in large part, to the continued modernization and innovation of the commercial cooling tower system.
A water cooling tower is used to cool water and is a huge heat exchanger, expelling building heat into the atmosphere and returning colder water to the chiller. A water cooling tower receives warm water from a chiller.
This warm water is known as condenser water because it gets heat in the condenser of the chiller. The chiller is typically at a lower level, like in a basement. The cooling tower’s role is to cool down the water, so it can return to the chiller to pick up more heat.
Air conditioning equipment and industrial processes can generate heat in the form of tons of hot water that needs to be cooled down. That’s where an industrial cooling tower comes in. Overheated water flows through the cooling tower where it’s recirculated and exposed to cool, dry air. Heat leaves the recirculating cooling tower water through evaporation.
This is called evaporative cooling. The colder water then reenters the air conditioning equipment or process to cool that equipment down, and the cooling cycle repeats over and over again. When the warm condenser goes into the cooling tower, the water is passed through some nozzles which spray the water into small droplets across the fill, which increases the surface area of water and allows for better heat loss thru greater evaporation.
The purpose of the fan on top of the water cooling tower is to bring in air from the bottom of the tower and move it up and out in the opposite direction of the warm condenser water at the top of the unit. The air will carry the heat by evaporating water from the cooling tower into the atmosphere.
An industrial cooling tower is a critical component of many refrigeration systems and can be found in industries such as power plants, chemical processing, steel mills, and many manufacturing companies where process cooling is necessary. Also, a commercial cooling tower can be used to provide comfort-cooling for large commercial buildings like airports, schools, hospitals, or hotels.
An industrial cooling tower can be larger than an HVAC system and is used to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants, food processing plants, and other industrial facilities.
With the increased rate of the population all over the world, there has been a huge rise in the rate of needs and requirements by the world for manufactured products. This has forced the industrial sector to manufacture more and more products every day, which generates more manufacturing process heat.
The machines and processes of industries that generate tremendous amounts of heat must be continuously cooled so that those machines can continue to operate efficiently. The most efficient, effective, and least expensive solution to removing this heat is the installation of an industrial cooling tower.
Cooling tower systems are often vital to industrial processes. These tall, open-topped, cylindrical structures are responsible for cooling water generated from industrial or HVAC comfort cooling airflow. The different types of cooling towers are identified by the classification of the draft (natural or mechanical) and by the direction of airflow (counter or cross).
are usually used for large power plants and industries with infinite cooling water flow. The tower operates by removing waste heat by way of rising hot air that is then released into the atmosphere. These towers are tall and have a hyperbolic shape to induce proper airflow.
These towers have air forced through the structure by a fan that circulates air through the tower. Common fans used in these towers include propeller fans and centrifugal fans. While Mechanical draft towers are more effective than natural draft towers, they consume more power and cost more to operate as a result.
have a design that allows the air to flow horizontally through the fill and the tower’s structure into an open plenum area. Hot water flows downward from distribution basins. However, fans and motor drive require weather-proofing against moisture which can lead to freezing making it less efficient.
have a design where the air moves upwards and the counter-current, with hot water, falls downward to cool the air. This allows for maximum performance out of each plan area and helps minimize pump head requirements. Also, a counterflow cooling tower system is less likely to ice up in cold weather conditions and can save energy in the long run. All Delta Cooling Towers are counterflow and all include these advantages.
are typically mounted with a fan at the top of the cooling tower, which allows hot air out and pulls air throughout. The high exiting air velocities reduce the chance of re-circulation. To avoid the entrapment of water droplets in the leaving stream air, drift eliminators are used. Induced draft towers are more efficient as they use 30% to up to 75% less energy compared to forced draft designs.
This cooling tower system is similar to induced draft, but the basic difference is that the air-moving fan is placed at the base of the cooling tower, which allows the air to blow through from the bottom. Their use is limited due to water distribution challenges, high horsepower fans, and the possibility of re-circulation.
Water-cooled systems are primarily made from three materials: Metal, fiberglass, and plastic. As you know, metal can rust and corrode, and whatever’s inside of it can begin to leak over time.
To no surprise, a metal cooling tower only has an average shelf life of up to only 15 years and requires maintenance with epoxy paint, sealants, and more. That maintenance can lead to downtime for your business. This is why metal is now being replaced with better technology. Fiberglass cooling tower manufacturers, although providing a better alternative to metal, are still prone to cracks and wear which can lead to long-term higher maintenance costs.
An engineered plastic cooling tower is designed to stand up to wear and tear. It doesn’t rust or chip—and it can weather harsh environmental conditions. It also requires almost zero maintenance. High-density polyethylene (HDPE), a best-in-class engineered plastic used by Delta Cooling Systems, is seamless and resistant to environment-induced corrosion, unlike metal or fiberglass cooling towers. With a life expectancy of more than 20 years, you can install it once knowing you will not have to worry about it afterward.
Advancements in the manufacturing and design of today’s engineered plastic towers changed the use of cooling towers from a valuable support tool, to one of productivity and savings in the cost of a cooling tower. Factory-assembled commercial cooling towers became popular at the same time when engineered molded plastics became more desired over the galvanized metal models that once dominated the industry. There are many reasons why you might want to consider an engineered plastic cooling tower to reduce costs and better meet your process needs:
According to the Centers for Disease Control and Prevention (CDC), water cooling towers can be a breeding ground for Legionella bacteria, the microbes that cause Legionnaires’ disease. Here’s why: Bacteria thrive in warm, wet conditions, making watercooling towers an ideal environment.
As a result, people can get Legionnaire’s disease, which can cause pneumonia, when they breathe in water droplets dispensed from HVAC systems that contain Legionella bacteria. In fact, a study by the CDC found six Legionnaires’ outbreaks in New York City that resulted in 213 cases and 18 deaths. Three of those outbreaks were linked to cooling towers.
To address this public health concern and liability, companies are treating the water inside of their industrial cooling tower with antimicrobial substances that kill the bacteria. For other water treatment applications, an air stripper is often used. As another precaution, plastic cooling tower systems can be manufactured with antimicrobial resins built into the unit materials and components to provide an additional layer of defense against Legionella. Find out more about the anti-microbial product technology at Delta Cooling Towers.
With the increasing concerns about meeting green standards and improving ROI on capital equipment expenditures, there are some standards to consider.
A systematic approach to cooling tower greenness will improve sustainability, increase energy efficiency, add water conservation, and create a smaller carbon footprint; all while improving some cost ramifications involved in achieving such green goals. Businesses can save up to 40 percent on energy costs. While conventional cooling towers, often constructed with sheet metal cladding, are environmentally challenging and maintenance intensive, the alternative of using cooling towers with molded seamless plastic is immediately beneficial to both the environment and your bottom line.
Traditional metal towers, which last only a few years in many applications, encounter environmental and economic issues including increased chemical use, higher maintenance costs, replacement costs, and disposal requirements. Engineered HDPE plastic design cooling towers allow the most aggressive water treatment options available. This can allow users to run at higher cycles of concentration, thereby saving make-up water. This can save tens of thousands of gallons of water per year.
These water and chemical savings can be significant and help solve water issues as well as save on operating costs. Cooling towers of this counterflow design also keep water totally enclosed and free from sunlight, thereby lessening the occasion for biological growth, which requires less harsh water treatment chemicals. Get more details here about Delta’s sustainable technology and products.
Think of it this way: Cooling tower systems are essential to many businesses, which means looking for efficiencies in operations and products can help impact the bottom line. Water consumption can be a major operating expense, and cooling towers can recycle about 98% of the water used in process cooling or air conditioning. If the unit is made from plastic and uses water instead of air as a cooling method, business owners can anticipate reduced energy costs, little-to-no maintenance, and extended product longevity, compared to older, metal systems.
That’s a highly desirable scenario for any business to cut down on costs. Furthermore, many customers appreciate knowing that the businesses and industries that support communities are environmentally conscious and working toward sustainable practices. That may not be a money-saving factor, but it could boost consumer confidence. And that’s good for business, too.
As you can see, there’s a lot to learn about cooling tower systems. Not only do they perform a function many of us couldn’t live without (that’s air conditioning, of course), they’re highly technological and, yes, cool. Perhaps knowing more about cooling towers will give you a greater appreciation for cool air.
The approach is the difference between the temperature of the cold water leaving the tower and the air's wet-bulb temperature. The establishment of the approach fixes the operating temperature of the tower and is the most important parameter in determining both tower size and cost.
Bleed Off: is the circulating water in the tower which is discharged to waste to help keep the dissolved solids concentration of the water below a maximum allowable limit. As a result of evaporation, dissolved solids concentration will continually increase unless reduced by bleed-off.
Biocide: a chemical that is designed to control the population of troublesome microbes by killing them.
Blowdown: is the water purposely discharged from the system to control concentrations of salts or other impurities in the circulating water. Units % of circulating water rate or GPM.
British Thermal Unit (BTU): is the heat energy required to raise the temperature of one pound of water one degree Fahrenheit in the range from 32° F to 212° F
Cooling Range: is the difference in temperature between the hot water entering the tower and the cold water leaving the tower.
Cycles of Concentration: compares dissolved solids in makeup water with solids concentrated through evaporation in the circulating water. For example, chlorides are soluble in water so the cycles of concentration are equal to the ratio of chlorides in circulating water to chlorides in makeup water.
Dissolved Solids: total solids that have been dissolved into a liquid. They may be ionic and/or polar in nature.
Drift: is the water entrained in the airflow and discharged into the atmosphere. Drift loss does not include water lost by evaporation. Proper tower design can minimize drift loss.
Heat Exchanger: is a device for transferring heat from one substance to another. Heat transfer can be by direct contact, as in a cooling tower, or indirect, as in a shell and tube condenser. Can also be the tube or fin tubed bundles in a wet/dry tower.
Heat Load: The amount of heat to be removed from the circulating water within the tower. Heat load is equal to water circulation rate (GPM) times the cooling range times 500 and is expressed in BTU/hr. Heat load is also an important parameter in determining tower size and cost.
Makeup: is the amount of water required to replace normal losses caused by bleed-off, drift, and evaporation.
Pumping Head: The pressure required to pump the water from the tower basin through the entire system and return to the top of the tower.
Ton: An evaporative cooling ton is 15,000 BTU’s per hour.
Wet Bulb: is the lowest temperature that water theoretically can reach by evaporation. Wet-Bulb temperature is an extremely important parameter in tower selection and design and should be measured by a psychrometer.
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.
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.
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.
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.
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.
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 NoneFigure 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.
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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!
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.
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:
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.
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:
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.
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.
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.
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
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
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) 12.7The 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 0.36Even 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.
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:
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:
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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