Based on the heat transfer area of the heat exchanger, the investment cost of air coolers is more than 2 to 3 times that of water coolers (referring only to hardware costs). There are two main reasons for this. First, the thermal conductivity of air is much lower than that of water, which inevitably leads to a lower heat transfer coefficient. Second, the ambient temperature used in the design is always higher than that of water, so the logarithmic mean temperature difference of the air cooler is always lower, especially when the outlet temperature of the process medium is very low.
Due to these two reasons, the heat transfer area required for air coolers under the same thermal load is much larger than that of water coolers. Additionally, the complex support system required for the larger heat transfer area further increases the costs.
However, as engineers know, the investment (or fixed) cost of equipment is only part of the total cost; it is important to consider the total cost, which is the sum of fixed investment costs and operating costs. The operating costs of water coolers are much higher than those of air coolers because they include the costs of initial makeup water, supplemental cooling water, water treatment chemicals, and cooling towers. When water is scarce, the operating costs of water coolers will increase, thus economically favoring the use of air coolers.
Advantages and disadvantages of air coolers
Air coolers have several important advantages compared to water coolers:
One of them is that water is not used directly as a cooling medium, so the costs associated with water, such as makeup water, supplemental water, and water treatment chemicals, are eliminated. The installation of coolers does not require proximity to water sources (such as rivers or lakes), thus preventing thermal loss and chemical pollution from water sources. Maintenance costs are also reduced because there is no need to frequently clean the water side of the cooler from scale, microbial fouling, and sediment. Additionally, the corresponding piping is eliminated, making installation simpler.
Another advantage is that air coolers can operate continuously, even in the event of power failure, by utilizing natural wind under reduced heat exchange capacity conditions.
Finally, the control of the outlet temperature of the medium (and the thermal load in this regard) can be achieved through various methods, such as starting or stopping fans, using two-speed or variable-speed motors, and using self-regulating fans (where the blades can be adjusted even when the fan is running), etc.
Limitations: Of course, air coolers also have many limitations. As mentioned earlier, compared to water, the thermal conductivity and specific heat of air are much lower, which results in significantly higher initial costs for air coolers than for water coolers.
In cold climates, additional anti-freezing facilities must be added to ensure that the medium does not drop below freezing temperatures, which also increases the initial investment costs.
A more economical approach is to maintain the temperature difference between the outlet temperature of the medium and the ambient air within the range of 10 to 15°C; in water coolers, this temperature difference can be as low as 3 to 5°C. This disadvantage can be compensated for by using an air cooler followed by a water cooler.
Due to the large heat transfer area, air coolers occupy more space than water coolers. However, this disadvantage can be overcome by placing the air cooler on a pipe rack, thus not wasting valuable floor space.
The specific heat of air is very low, requiring a large volume of air to be forced through the tube bundle. This can be achieved by using large-diameter, high-speed rotating blades, but it generates a lot of noise.
Seasonal variations in air temperature can affect the performance of air coolers, so expensive control systems must be employed to ensure operational stability.
Air coolers cannot be placed near large obstacles, such as buildings, as this can cause air recirculation and reduce efficiency.
The design of air coolers is quite complex, so there are far fewer manufacturers of air coolers compared to those of water-cooled shell-and-tube heat exchangers.
For cooling high-viscosity liquids, the low heat transfer coefficient within the tube makes air coolers more expensive (this fluid generates a very high heat transfer coefficient due to strong turbulence when flowing outside the tubes in shell-and-tube heat exchangers). However, this situation can be compensated for by using internal inserts to significantly increase the heat transfer area. However, this technology has not yet been widely disclosed.
Optimal selection between air cooling and water cooling
In many applications where the outlet temperature of the medium is very low, it is not feasible to use air coolers alone. For example, in a scenario with an ambient temperature of 42°C and a designed cooling water temperature of 33°C, it is impossible to cool a light hydrocarbon liquid to 40-45°C. In such cases, an air cooler followed by water cooling can be employed.
For certain other applications, air coolers are also not economically viable. For instance, in the aforementioned scenario with an ambient temperature of 42°C and a cooling water temperature of 33°C, for a naphtha stabilizer condenser with inlet and outlet temperatures of 50°C and 40°C respectively, the temperature difference is too small to use an air cooler; here, a water cooler should be considered.
Therefore, some situations are suitable for using air coolers alone, while others can use a combination of air coolers followed by water cooling, and some should use water coolers alone.
The most suitable temperature transition point for air cooling and water cooling (referring to the temperature at which the medium fluid leaves the air cooler and enters the water cooler) is determined by the overall economic plan of the specific engineering project. It will depend on the equipment costs of the air cooler and water cooler, the total costs of using water, and the power costs. Generally, the most suitable temperature is about 15°C to 20°C higher than the ambient temperature.
It is worth mentioning that even for combined coolers (air cooler plus water cooler), the air cooler will handle the majority of the thermal load, accounting for 80% or more of the total thermal load, thus significantly reducing the amount of cooling water used.
When using a combined cooler, if there is no downstream cooler, the best design for the air cooler is to take the ambient temperature slightly lower. When designing with a downstream cooler, the temperature of the medium fluid should be considered at the maximum (or near maximum) ambient temperature for the air cooler. This is because, when processing the medium at higher ambient temperatures, the costs of the downstream cooler are much lower than those of the air cooler.
