Heat rejection generally involves either air or water cooling. When adding a new exchanger, the practicality and economics of the choice vary with service requirements, climate conditions and site-specific factors. The cost of equipment certainly plays an important role. However, economics also significantly depend upon site factors, including differences between wet-bulb and dry-bulb temperatures, local climate (temperatures), availability of water, whether the exchanger will be stand-alone or added to a current system, and existing plant practices.
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Air coolers are self-contained units. An air cooler requires electrical supply for the fans but little else for installation. Water coolers include two major components, an exchanger that cools the process and a cooling tower that rejects the heat, as well as a water distribution system linking them.
The two components required in using cooling water can make even a straightforward economic comparison of air cooling versus water cooling difficult. However, as a rule of thumb, if the necessary process temperature is 50°F (28°C) higher than the dry-bulb temperature, the economics generally will favor air cooling. If the needed process temperature is closer than 50°F to the dry-bulb temperature, then water cooling likely becomes a strong contender.
You must consider both summer and winter operating conditions to understand the capital tradeoffs for air cooling versus water cooling. Air cooling effectiveness changes with dry-bulb temperatures while water cooling effectiveness changes with wet-bulb temperatures.
For air cooling, summer operation is most difficult. The dry-bulb temperature is high, which gives low temperature differences in the cooler, increasing needed surface area. The lower possible air-temperature rise also boosts total air required. Winter operation tends to be easier; the air temperature is lower, temperature differences increase, and necessary air decreases. This points up the value of including some method to vary air supply. Otherwise, the denser winter air will lead to greater power consumption then.
For water cooling, summer most limits the process exchanger. Wet-bulb temperatures are high, reducing temperature differences and increasing water demand. In the winter, cooling water temperatures drop, temperature differences rise and water demand falls. The process cooling exchanger has more duty capability in the winter.
However, roughly one percent of the water passing through the cooling tower evaporates for every 9°F (5°C) drop in water temperature. As air temperatures drop, the amount of water vaporized to go from the dry-bulb to the wet-bulb temperatures decreases. So, colder weather requires more air in the cooling tower to remove the same amount of duty. Operation of some cooling towers is more limited in the winter than the summer; this can be an unpleasant surprise.
You should include two other factors when evaluating air versus water cooling: variability within a day, and maximum and minimum temperatures.
In most areas, the dry-bulb temperature varies much more through the day than the wet-bulb temperature. This means air cooler performance tends to change more than water cooler performance during the day. Additionally, rain can dramatically alter air cooler performance. Rain storms rapidly convert air coolers into wet-surface coolers, substantially boosting their cooling capability. In contrast, upsetting a water cooler system requires a prodigious amount of astonishingly cold rain.
Essentially no upper limit exists to air cooler operating temperatures, so long as you ensure personnel safety and use correct mechanical arrangements. Indeed, air coolers have handled process inlet temperatures over 1,000°F (538°C). However, processes can suffer low temperature problems with air coolers; very cold air can solidify many process streams.
In contrast, you should not operate cooling water exchangers with excessive process temperatures. High film temperatures on the water can rapidly foul heat exchangers. You should keep bulk return water temperatures in water coolers below 125°F (52°C). Many plants that expanded without upgrading their utility systems now exhibit higher average return temperatures — often reaching 130–135°F (54–57°C) — particularly in summer operation. These plants tend to have high rates of carbonate fouling.
On the low temperature side, cooling water systems moderate temperature extremes. Outside of Alaska, Northern Canada or Northern Russia, few cooling water systems have problems with below-freezing temperatures — as long as they are running with some process heat input.
The selection between air and water cooling requires balancing many competing performance and cost tradeoffs. Neither method suits all applications.
