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.
What is the purpose of a Cooling Tower?
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.
How Does a Cooling Tower work?
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.
Why is an Industrial Cooling Tower Needed?
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 Types
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).
Natural Draft Cooling Tower Systems
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.
Mechanical Draft Cooling Tower Systems
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.
Crossflow Cooling Tower Systems
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.
Counterflow Cooling Tower Systems
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.
Induced Draft Cooling Tower Systems
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.
Forced Draft Cooling Tower Systems
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.
What is the best material for a Cooling Tower?
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.
What are the advantages of using engineered plastic?
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:
- Life expectancy - A standard metal cooling tower has casings with thin sheets of galvanized steel. These sheets usually have welded seams that can deteriorate within a year and will require re-welding, patching, or coating to prevent leakage. Additionally, the treated water tends to attack the galvanized metal, essentially wearing it out in an extraordinarily short time. Environmental conditions such as sunlight, pollution, salt air, and harsh process chemicals also contribute to galvanized steel&#;s early demise. Even ambient air pollution can affect galvanized steel, leading to premature failure. Since metal expands and contracts, depending on the temperature, repeated cycling causes stress that can also accelerate corrosion, rust, and leakage. Even low-grade stainless shell options, series 300 stainless steel, are attacked and worn out against water treatment chemicals and environmental factors.
- Flexible modular design - In the past, a plastic cooling tower was too small for many industrial processes. For that reason, galvanized metal cooling towers were traditionally used for most applications above 250 tons but that situation has changed dramatically. Delta, for instance, has introduced its TM Series® of factory-assembled plastic towers that can be combined to provide up to 2,500 cooling tons in a single, modularized unit. Modular cooling towers also facilitate the use of an extra margin of cooling capacity that can be advantageous in adjusting to operational heat load or outflow changes, or in upgrading to meet future cooling requirements.
- Continuous, more economical operation - Engineered plastic can also reduce the expected and inconvenient consequences of operating a cooling tower, which include: electric power usage, water-treatment chemicals, labor and materials for maintenance, and unscheduled process downtime for cooling tower repairs. Maintenance and repairs usually mean process interruptions, the costliest of all problems related to cooling towers.
- Easier installation - The basic design advantages of the latest plastic cooling towers also include easier installation, especially on rooftops, because a lightweight plastic shell weighs as much as 40% less than a steel tower while being 5-10 times thicker. When modular cooling towers are combined in a cluster, installation is often faster and easier.
What is the connection between Cooling Tower systems and Legionnaires' Disease?
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.
Is a cooling tower environmentally friendly?
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.
How can Cooling Tower systems help businesses save money?
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.
Cooling Tower Basics: What are common cooling system terms?
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.
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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.
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Cooling towers are used to remove heat from water used in water-cooled applications. The heat is rejected to the atmospheric air that is passed through the system fill in a crossflow or counterflow method. A small percentage of the hot water from the system evaporates, cooling the remaining water which falls into the cold-water basin which is then pumped from the cold water basin to the system to absorb more heat in a continuous process.
It&#;s important to consider the efficiency of the cooling towers ability to reject heat for your project requirements. When you have the project requirements in the way of total tons, range and approach, then you can begin to analyze the cooling towers that would be appropriate for your water-cooled application.
Crossflow and Counterflow Cooling Tower
Induced draft crossflow and counterflow cooling towers both offer advantages and disadvantages based on specific project requirements and site conditions.
The obvious difference is in the method by which the air moves through the cooling tower in relationship to the cooling water, which is indicated by its description of a being crossflow or counterflow.
With a crossflow tower the air travels horizontally perpendicular) to the water traveling vertically down to the cold basin. With a counterflow cooling tower, air travels in the opposite direction vertically upward against the stream of falling water.
Cooling Tower Footprint &#; Physical Size
Air is required in sufficient amounts to accomplish the heat transfer requirements at the design conditions and at varying amounts for less than design conditions. Consideration must be made for this volume of air and the space required in the design of the tower to ensure the proper volume of air effectively interacts with the cooling water.
Cooling Tower Footprint &#; Crossflow vs. Counterflow
Cooling tower manufactures will have different footprint thresholds where the vertically configured counterflow goes from the smaller footprint to the larger footprint. Because of the nature of the counterflow tower where the air and water interact vertically, this allows for a smaller footprint most of the time in towers around 800 tons or less. As cooling towers get larger than 800 tons, the advantage could switch to counterflow.
Site conditions will determine whether the physical size of the cooling towers footprint is more important, or any height restrictions imposed by site factors.
Maintenance
In order to keep the cooling tower operating for efficiently and for its useful life it&#;s important to provide periodic maintenance. This will require that you consider tower access as a factor in the design and location of the tower.
Crossflow Cooling Tower Maintenance and Access
Access to the internal working parts various between the two designs. With the crossflow tower most of the internal parts are easily accessible as the fill material surrounds the exterior portions of the insides, allowing for access to the fan, motor, drift eliminators and cold-water basin. With larger crossflow cooling towers, the manufactures have an option to have ladders and platforms preinstalled for easy access to the motor and drive.
Counterflow Cooling Tower Maintenance and Access
Access to the components of a counterflow tower are less friendly to the maintenance personnel because the overhead horizontal fill material provides a barrier between various components. The fan, motor, eliminators and spray system have limited access because they are located above the horizontal fill making then inaccessible from the cold-water basin.
Cooling Tower Operating Weight
Because of the crossflow towers larger footprint, it&#;s shipping and operating weight will often be greater than that of the counterflow tower. The bigger footprint of the counterflow tower can require additional structural support legs.
Water Distribution &#; Pressurized or Gravity
With the crossflow tower there is a hot-water basin above the fill that distributes the water that is pumped to the top of the tower. The water in the hot-water basin than makes its way through the distribution nozzles by gravity without the force of the pump. This allow for easier maintenance of the nozzles since the water is gravity fed and out of the air stream.
The design of the counterflow tower and the method by which the water descends in direct opposition to the vertically induced air, necessitates the need to pressurize the water distribution. Since the water is under pressure through the nozzles this requires that you shutoff the pump in order to clean or service the distribution system.
Another design and maintenance consideration are the method by which the water is distributed over the fill material.
Cooling Tower Capacity and Energy Efficiency
With many jurisdiction and states increasing energy efficiency requirements, the use of variable flow increases energy savings. As the tonnage requirement drops it&#;s wise to have the tower vary is flow in order to save energy.
Capacity can be increased basically three different ways. One way is by increasing the footprint of the tower, the second method would be to increase its height, and the third way would be to increase the HP of the fan motor to get more CFM out of the same tower size before having to go to the next size tower. These are some of the decisions that will need to be made based on project conditions.
- Increase Footprint (additional heat transfer surface area) #1 in image below
- Increase Height (additional heat transfer surface area) # 2 in image below
- Increase Fan HP (more CFM) #3 in image below
Cooling Tower Capacity vs Footprint, Height and HP (CFM)
Crossflow cooling towers are better at turndown than counterflow because of the inherent features of their water distribution methods.
Cooling towers are rated based on standard conditions of 95ºF (35.0ºC) entering water temperature to an 85ºF (29.4ºC) leaving water temperature at a 78ºF (25.6ºC) entering wet-bulb temperature. This correlates to 3 GPM of water per nominal ton. For non-standard conditions seek the assistance of your local cooling tower sales representative and their access to tower sizing software.
Cooling Tower Fill &#; Heat Transfer Surface
Cooling tower manufactures offer options on the type of fill based on factors such as the condition or chemistry of the water and the potential for biological growth. It&#;s important to prevent scaling which will reduce the capacity and ability of the tower to efficiently reject heat.
The use of PVC for fill material is common due to its low cost and effectiveness in clean water applications. If the water quality is not satisfactory than the use of splash fill might be required which will also increase the cost due to it be less efficient at heat transfer, which will also increase the size of the tower to meet the same project requirements. The use of splash fill is less prone to clogging due to dirty water as opposed to PVC Film which can clog more easily.
Cooling Tower Fans
There are anywhere from one or more fans that provide for the air movement through the towers. The most energy efficient being the axial fan which sits at the top of the tower. The use of centrifugal fans are more commonly used in forced draft fluid coolers.
Summary
Your project requirements and site conditions will determine the best option of either a crossflow or counterflow cooling tower including any consideration for maintenance.
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