What is a Cooling Tower?

A cooling tower is heat removal device that uses water to transfer process waste heat into the atmosphere. All cooling towers operate on the principle of removing heat from water be evaporating a small portion of water that is recirculated through the unit.  The mixing of warm water and cooler air releases latent heat of vaporization, causing a cooling effect to the water.  They are a key 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, cooling towers can be used to provide comfort cooling for large commercial buildings like airports, schools, hospitals, or hotels.

Types of Cooling Towers

what is a cooling tower?

Cooling towers might be one of the most vital part of any industrial process. These tall, open-topped, cylindrical structures are responsible for cooling water generated

from industrial or HVAC comfort cooling process. They are classified by the type of draft (natural or mechanical) and by the direction of air flow (counter or cross).

Natural Draft Cooling Towers are usually used for large power plants and industries with infinite cooling water flow. The tower operates by hot air in the tower rising removing waste heat and then releasing it into the atmosphere. These towers are tall and have a hyperbolic shape to induce proper air flow.

Mechanical Draft Cooling 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 Towers has 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. There is less recirculation because the speed of exit air is higher than entering air. However, fans and motor drive require weather-proofing against moisture which can lead to freezing making it less efficient.

Counterflow Towers has a design where the air is vertically upwards and the counter current with hot water is falling downwards to cool the air. This allows for maximum performance out of each plan area and helps minimize pump head requirements. Also, they are less likely to ice up in cold weather conditions and can save energy in the long run. All Delta Cooling Towers are counterflow with these advantages.

Induced Draft towers 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 reduces 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 towers are similar to induced draft but the basic difference is that the exhaust fan is placed at the base of the cooling tower, which allows the air to blow from the bottom. Their use is limited due to water distribution challenges, high horsepower fans and the possibility of re-circulation.

Engineered Plastic Advantages

types of cooling towersAdvancements in the manufacture and design of today’s engineered-plastic towers have initiated a change in the idea of the cooling tower from a valuable support tool to one of productivity and increases cost saving. Factory-assembled cooling towers formed with engineered molded plastics continue to gain popularity over galvanized sheet metal models that once dominated the cooling tower industry. There are many reasons why you might want to considered an engineered plastic cooling tower to reduce costs and better meet your process needs:

  • Life expectancy - Standard metal cooling towers have 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 temperature, repeated cycling causes stress that can also accelerate corrosion, rust and leakage.  Even low-grade stainless shell options, series 300 stainless steel, is attacked and wears out against water treatment chemicals and environmental factors.
  • Flexible modular design - In the past, plastic cooling towers were 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 means 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.

Green Cooling Towers

green cooling towersWith 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. 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 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 very large 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.

Cooling Tower Terminology

Approach: is the difference between the temperature of the cold water leaving the tower and the wet-bulb temperature of the air is known as the approach. Establishment of the approach fixes the operating temperature of the tower and is a 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 is the cooling range.

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 air flow and discharged to 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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