WHAT IS A COOLING TOWER?
Cooling is the process of thermal energy transfer via thermal radiation, heat conduction or convection. In industrial processes, cooling is achieved through various mechanisms requiring different equipment. The most commonly accepted industrial cooling mechanism is the use of a cooling tower.
Water is used as the heat transfer medium in cooling towers. Water has several key properties that make it a good choice as a heat transfer medium:
- High specific heat
- High heat of vaporization
- High boiling point
- Low cost
TYPES OF COOLING TOWER:
There are many variables that can affect cooling tower systems, including water make-up quality, operating schedule, and stressors. While there are only three basic types of cooling towers, design and operational differences make it difficult to generalize. For the purposes of this discussion, we group cooling systems according to what happens with the cooling water. The three types of cooling systems we focus on here are:
- Once-through cooling system
- Dry Cooling Tower (Closed recirculating system)
- Wet Cooling Tower (Open recirculating system)
Once-Through Cooling System:
In once-through cooling, water is pumped from a nearby source, and passes only once through the system to absorb process heat. It is then discharged back into the original source. This source may be a river, lake, ocean, or well.
This design is common where large volumes of low-cost water are available. Additionally, these systems are typical when cooling demand is low to moderate, processes are not critical, and there is room to accommodate large equipment and high volumes of water. One drawback to once-through cooling is susceptibility to disturbance by stochastic water events, such as flooding. Moreover, these systems are being phased out due to concerns about water quality and conservation.
Average Temperature Change: 5-10° F (3-6° C)
Amount of Water Used: High
Examples: Potable Water Systems, Process Water, General Service
Dry Cooling Tower:
In closed recirculating systems or dry cooling towers, heat absorbed by the cooling water is either transferred to a second coolant, or released into the atmosphere. The word dry is used because the water is never exposed to the air, and as a result, very little water is lost. An automobile engine is a good example of a closed cooling system.
Evaporation is not used in closed recirculating cooling towers. Instead, cool air rushes over a series of small tubes containing circulating coolant. Heat is transferred from the hot liquid inside the tubes to the cool air, resulting in cooling. The coolant is then returned back into the engine. Industrial applications for closed recirculated cooling include:
- Chilled water loops
- Computer room cooling
- Food temperature controllers
- Other processes where a small change in the temperature is important to the product or environment
Average Temperature Change: 10-15° F (6-8° C)
Amount of Water Used: Negligible
Examples: Automobile Radiator, Chilled Water Systems
Wet Cooling Tower / Evaporative Cooling Tower:
Open recirculating cooling systems or wet cooling towers are the most widely used designs in industry. Just as in closed recirculating systems, the open system uses the same water over and over again. Its most visible feature is the large, outdoor cooling tower that uses evaporation to release heat from the cooling water. Due to the mechanism, this type of cooling tower is also called an evaporative cooling tower. This system consists of three main pieces of equipment: the recirculating water pump(s), the heat exchanger(s), and the cooling tower.
Average Temperature Change: 10-30° F (6-17° C)
Amount of Water Used: Moderate
Examples: Cooling Towers, Spray Ponds
How Wet Cooling Towers Work:
Open recirculation cooling systems feature “wet towers,” where cooling water comes in direct contact with upward airflow. Water from the heat exchanger is pumped evenly over the top deck of the cooling tower. It cascades down and is broken into tiny droplets as it passes through a series of splash plates, called cooling tower fill. This fill can be corrugated plastic sheets, wooden slats, or other devices that increase surface area, thereby enhancing evaporation. As the water droplets bounce off the cooling tower fill, the hottest molecules break away from the water and are carried up and out of the tower as “drift”. The remaining cooled water collects in a tank at the bottom of the tower, called the basin. This cooled water can now be pumped back into the heat exchanger.
COOLING TOWER SCALING AND CORROSION
Advantages of Recirculating Cooling
A recirculating cooling tower design is preferred for promoting large decreases in coolant temperature and where feed water is an expensive resource.
Risks of Open Recirculating Cooling
One drawback to wet cooling towers is that repetitive evaporation concentrates dissolved and suspended solids. These solids eventually reach a saturation point and begin to precipitate and deposit, leading to scaling or corrosion.
When insoluble materials, like calcium carbonate, calcium phosphate, magnesium salts and silica, precipitate out of cooling water, they form solid deposits called scale. A layer of scale as thin as 1/64 inch can reduce heat exchanger efficiency by 15%. Reduction in heat exchanger efficiency leads to wasted energy and increased operating costs. Scaling may be promoted by variations in pH, temperature, alkalinity, and concentration of scale-forming constituents in water.
Most metals used in cooling tower systems are very susceptible to corrosion. Surfaces must be kept clean to prevent under-deposit corrosion, which can lead to system failures. Corrosion can be uniformly distributed throughout the system, or it can be localized.
COOLING TOWER WATER TREATMENT
Water may be treated with chemical inhibitors to extend the saturation point of insoluble materials and prevent scaling and corrosion. Water treatment with cooling tower chemicals is preferred to supplying fresh water due to environmental and cost concerns.
Cooling Tower Blowdown
Cooling tower chemicals are useful to increase saturation point in cooling water. However, there is a limit to the solids concentration or cycle of concentration that can managed by a chemical inhibitor. For this reason, dissolved solids in cooling water are reduced by removing or “blowing down” a percentage of water from the system and replacing it with fresh water.
Cooling Tower Blowdown Control
Measuring and controlling conductivity allows accurate calculation of blowdown quantities and timing. Measuring and maintaining conductivity is critical, because poor blowdown control may lead to:
- Scaling and corrosion (when conductivity is too high) or
- Wasted resources, including water and cooling tower chemicals (when the system could tolerate higher conductivity feed water)
Risks of Closed Recirculation Cooling
Unlike wet cooling towers, dry cooling towers have a relatively low temperature profile and do not rely on evaporation. This reduces the risk of scaling in these systems. However, the risk of corrosion is still significant in dry cooling towers, due to the high solubility of dissolved oxygen at low temperatures. Additionally, low temperatures may promote microbial fouling due to the ability of bacterial populations to survive in the cooling tower environment.
Makeup water and wind can carry microorganisms into a cooling water system. Under certain conditions, microbial communities multiply in the system, causing problems. Microbiological stress can affect any component in a cooling system. In fact, microbiological stress is often the most significant stress on cooling systems, as it can promote other types of stresses. For example:
- Corrosion occurs under the bacterial slime layer
- Inorganic molecules become trapped in the slime layer, leading to scaling
Preventing Microbiological Deposits in Cooling Towers
An effective microbial control program focuses on the specific microbial communities damaging a given cooling system. Appropriate biocides may be selected, perhaps in conjunction with dispersants to penetrate and remove deposits. Chlorine oxidizing biocide is commonly used in biocontrol along with some non-oxidizing biocides. Bromine or chlorine dioxide is used instead of chlorine for heavy stress systems. Microbiological treatments may require control of pH and other water quality parameters to ensure maximum effectiveness.
Other Types of Cooling Tower Fouling
Solid material other than scale, like airborne debris, corrosion products, process in-leakage and suspended solids, accumulates in the system and contributes to loss in efficiency and equipment deterioration. A variety of factors influence the level of fouling. You may have to understand the type of fouling and the source in order to determine the best way to control it. This may involve recommendations for mechanical improvements, clarification of the makeup water or the addition of dispersants specific to the fouling problem. Side stream filtration can also be a solution for severe fouling stress systems.
SENSORS & INSTRUMENTATION FOR COOLING TOWER WATER TREATMENT:
The most important instrumentation control parameters in cooling tower water treatment are Conductivity and pH. Additional water quality parameters that should be measured online or sampled frequently include turbidity, total suspended solids (TSS), free chlorine, and oxidation reduction potential (ORP).