Above plants in the chemical, coking and thermal power industries, plumes of white mist drifting from cooling towers are a familiar sight. This "white plume" is essentially a mass of tiny droplets formed when saturated humid air leaving the tower meets cold air and condenses. It causes visual pollution, and in winter it also brings practical problems such as icy roads nearby and corrosion of equipment. As requirements for "plume abatement" in industrial parks are progressively rolled out across the country, mist eliminating cooling towers and dry-wet combined cooling towers are becoming popular choices for circulating water system retrofits. Drawing on the product range of Shandong Shengbao Heat Transfer Technology Co., Ltd., this article explains the technical principles of both types and how to select between them.
How White Plume Forms in a Cooling Tower
Conventional wet cooling towers reject heat through the evaporation of water, so the air leaving them is close to saturation. Once this warm, humid air enters the atmosphere and meets colder air, the water vapor it carries quickly reaches supersaturation and separates out as tiny droplets, and countless droplets gathering together form the white plume visible to the naked eye. The lower the ambient temperature and the wider the humidity difference, the more pronounced the white plume becomes, which is why northern China has a particularly urgent need for mist elimination in winter.
The problems caused by white plume fall into three categories. The first is perceptual: residents nearby easily take it for "smoke pollution", which leads to misunderstanding and complaints; the second is that dissolved salts carried by the drifting droplets settle and corrode surrounding equipment and buildings; the third is that the moisture in the warm humid air is never recovered, amounting to a continuing loss of water resources. A large circulating water cooling tower loses a considerable volume of water every year to evaporation and drift, and in water-scarce regions that item stands out sharply in the accounts.
How a Mist Eliminating Cooling Tower Works
1. Dry section preheating and condensate recovery
The idea at the heart of a mist eliminating cooling tower is "dry first, then wet, with mixing to reduce humidity". Fresh ambient air first flows through a dry heat exchange section arranged at the top or the side of the tower, where it is heated, and is then mixed with the saturated humid air discharged from the wet section. The mixed air is far from saturation, so once it leaves the tower it does not readily form mist, or forms noticeably less, which suppresses the formation of white plume at the source.
2. Moisture recovery and water savings
In the process described above, part of the water vapor condenses when the humid air passes over the cold surfaces of the dry section, and the condensate can be recovered into the circulating water system. For enterprises in water-scarce regions or with restricted water intake quotas, a mist eliminating cooling tower delivers tangible water savings at the same time as resolving visual pollution, which is why it is called an integrated "water-saving and mist-eliminating" solution.
3. Drift control technology
Mist eliminating cooling towers are usually also fitted with a drift eliminator, keeping the amount of droplets in the discharged air at a low level and reducing the dissolved salts carried out with those droplets, thereby saving water and protecting surrounding equipment at the same time.
Features and Suitable Applications of Dry-Wet Combined Cooling Towers
1. The logic of combined dry and wet operation
A dry-wet combined cooling tower brings a dry heat exchange section and a wet heat exchange section together in the same casing. By adjusting how air flow and water flow are distributed between them, it can switch operating modes from season to season throughout the year: mainly wet cooling in summer to secure full heat rejection capacity, both dry and wet in spring and autumn, and mainly dry cooling in winter for mist elimination and freeze protection alike. This structure brings together two requirements that seem contradictory, "heat rejection capacity in summer" and "plume abatement and freeze protection in winter", within a single piece of equipment.
2. Typical applications
Thermal power and chemical enterprises in northern regions where winter fogging is pronounced and icy roads are a risk; facilities located on the outskirts of cities or in the core zone of an industrial park, where the visible environment of the site matters; projects with tight water quotas that need water savings and mist elimination at the same time; and situations where space for converting the existing wet tower is limited and the functional upgrade has to be completed on the same tower footprint.
Parameters to Consider in Selection
Meteorological data: mist elimination is directly related to local winter temperature, humidity and wind speed, so hourly meteorological data for a typical year at the plant site must be obtained before design and the outlet air condition calculated for representative operating conditions. Area ratio of the dry section: the ratio of heat exchange area between the dry and wet sections determines the balance between mist elimination and initial investment, so the configuration should be derived from the target grade of mist elimination to avoid overdesign. Water quality and materials: where the circulating water has a high salt content, corrosion-resistant design of the drift eliminator and of the condensate recovery water line needs to be considered in advance. Winter freeze protection: the freeze protection logic for the dry section in severe cold and the fan variable frequency strategy should be designed together with the control system. Retrofit assessment: adding a mist eliminating module to an existing tower requires checking the resulting changes in the structural load of the original tower and in air duct resistance, with specialist verification carried out where necessary.
Frequently Asked Questions
Q: Can a mist eliminating cooling tower remove white plume completely? A: Most projects take "significant reduction" or "virtual elimination under defined operating conditions" as their target. In extremely cold weather the capacity of air to hold moisture is very low, and complete plume abatement tends to carry disproportionate costs, so it is usually enough to set the design target according to the plume abatement standard required by the local environmental authority or the industrial park.
Q: Will a mist eliminating retrofit affect the heat rejection capacity of the cooling tower? A: A well-designed system recovers part of the air-side heat load through the dry section, so the overall heat exchange capacity can be held close to the original design level. The key is to calculate air duct resistance and fan operating conditions before the retrofit, so that added resistance does not cause the air volume to drop.
Q: Are dry-wet combined cooling towers more expensive to run than wet towers? A: In winter a combined tower evaporates less water and recovers part of the condensate, so water charges and water treatment chemical costs both fall. At the same time, because the dry section is there, the spray system can run less often or stay off altogether in winter. Overall operating economics have to be calculated against local water and electricity prices, and most projects in water-scarce regions achieve good overall returns.
About the manufacturer and selection support
Shandong Shengbao Heat Transfer Technology Co., Ltd. is located in the Energy Conservation and Environmental Protection Industrial Park in Anqiu, Weifang, Shandong Province. Founded in 2012 with a registered capital of RMB 120 million, it covers an area of more than 180000 m² and has an R&D team of 58 technical staff together with more than 40 sets of production equipment. Its products cover a full range of mist eliminating cooling towers, dry-wet combined cooling towers, closed cooling towers, fiberglass cooling towers and energy-saving retrofits for turbine cooling towers, and it has served industrial enterprises in the chemical, petroleum, coking, thermal power, steel, pharmaceutical, food and other industries. If you need a mist elimination calculation and configuration advice for your specific operating conditions, please feel free to talk them over with our technical team.
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