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Wet cooling towers vs. dry coolers
Cooling towers are heat-transfer devices used to discharge heat loads into the atmosphere. Cooling towers cool circulating water in a wide range of applications, including process cooling and HVAC systems.
Cooling towers remove heat from the atmosphere by chilling a water stream to a lower temperature. Cooling towers are classified into two types:
Open-circuit cooling towers (also known as wet cooling towers), in which process water comes into direct contact with ambient air;
There is no direct contact between the ambient air and the fluid being cooled in closed-circuit cooling towers, also known as dry coolers.
Both serve the same function yet chill the water in distinct ways.
Wet cooling towers use the latent heat of water evaporation to exchange heat with the air stream passing through them and can lower the temperature of the process water to a certain degree above the dew point (approach temperature). In contrast, a dry cooler merely lowers the water temperature to the ambient temperature, although extra cooling can be performed by adiabatic pre-cooling of the incoming air. Wet cooling towers operate well in hot and dry areas, but when the surrounding air gets humid, their effectiveness suffers. Dry cooler performance, on the other hand, is unaffected by air humidity and is dependent on air temperature.
Dry Coolers in System Design
Dry coolers have a simple and sturdy design that incorporates a heat exchanger (usually finned tube or microchannel coils) and fans that drive the airstream through the heat exchanger to chill the water or glycol solution that passes through it. Dry coolers are often available with horizontal or vertical airflow configurations. Although large-capacity dry coolers are available on the market, heat rejection capacity normally varies up to 1MW.
Pre-cooling systems, such as adiabatic spray or wetted pads, are common in modern designs. Evaporative pre-cooling allows the process fluid to be cooled below the dry bulb temperature.
Cooling Towers in the Rain
A wet cooling tower's structure is far more complicated. It has nozzles that spray hot water uniformly onto the heat exchanging media (also known as the fill or wet deck), fans that direct ambient air throughout the cooling tower, drift eliminators that keep water droplets from discharging with the air, and a collection basin for the cooled water. The air flows from the bottom of the tower (counterflow tower) or perpendicular to the direction of water flow (perpendicular tower) (crossflow-type tower).
Film fill and splash fill are the two most frequent types of heat transferring material. Film fill is a collection of rough sheets with a large surface area on which water spreads and produces a thin film when it comes into touch with airflow. This ensures a rapid flow of heat between water and air. Splash fill is a multilayered construction that creates minute droplets to maximise air-water contact, resulting in a high heat transfer rate. The fill media should be chosen based on the water quality: film fill is best for clean water, while splash fill is best for unclean or dirty water with a high sediment content.
Specific treatments, such as anti-legionella fill media, are available to reduce the danger of bacterial growth. These solutions function by disturbing the metabolic process of microorganisms and interfering with bacteria's capacity to grow and reproduce.
Drift eliminators are essential for maintaining a cooling tower's performance and avoiding water loss. A drift eliminator's structure produces various deflections of the air stream, forcing water droplets carried away with the airflow to settle on the surface of the eliminator and drip down to the collection basin.
Advantages and disadvantages
Wet (open-circuit) cooling towers are better suited for large industrial operations that require significant amounts of water to be cooled:
Capability to cool process water below the dry bulb temperature; Reliable operation in high ambient conditions; High cooling capacity per unit.
High equipment and installation costs; high operational expenses, including periodic water treatment/refill, system component maintenance, and electricity costs;
Evaporation causes a relatively significant water usage.
Rust formation along the piping;
Bacterial growth, especially legionella.
Dry coolers are ideal for small-to-medium industrial and commercial applications, as well as free cooling systems:
Minimal startup, operating, and maintenance costs; ability to perform in any temperature, from hot and humid to very cold;
A wide range of unit designs allows it to be adapted to practically all settings and applications.
System fouling and process water contamination are kept to a minimum.
There was no water loss.
The process water cannot be cooled below the dry bulb temperature (without adiabatic pre-cooling); the capacity per unit is rather low.
Dry coolers, as closed-circuit systems, provide various advantages over wet cooling towers, including lower initial and ongoing costs, enhanced system dependability, and increased uptime. Dry coolers, which are fitted with evaporative pre-cooling, provide efficiency that is only restricted by wet bulb temperature, as opposed to wet-type cooling towers.
Given the foregoing, the most important consideration in deciding between an open-circuit cooling tower and a dry cooler is system capacity. Dry coolers can now be considered an effective aspect of modern environmentally friendly thermal systems because to the continuous introduction of new efficient technologies such as microchannel heat exchangers, EC fans, intelligent controls, and others.
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