Choosing a Rotary Screw Air Compressor: Air-Cooled vs. Water-Cooled

When selecting a rotary screw air compressor, you'll encounter two cooling configurations: air-cooled or water-cooled. These compressors generate significant heat during operation, mainly from the motor, air end, and cooling circuit. Customers must decide between air-cooled or water-cooled compressors based on factors like size, location, and available utilities.

Compression of air from 0 to 125 psig generates heat through the "heat of compression" process. Cooling circuits remove this heat from the compressor using either a radiator and fan combination (air-cooled) or a shell and tube exchanger (water-cooled).

In the rotary screw market, it is estimated about 80% of compressors are air-cooled, while 20% are water-cooled. Most rotary screw air compressors are air-cooled – especially those below 100 hp. Smaller rotary screw compressors, such as those 40 hp and below, are typically only available in an air-cooled configuration. Water-cooled compressors become more common in larger machines, between 125-600 hp.

The reason a customer opts for air-cooled versus water-cooled is based on their environment, space considerations, costs of water cooling and if water is available. The following questions can help you begin to determine which product to choose.

What are the requirements for water-cooled compressors?

In environments prone to caustic conditions or heavy contamination, a water-cooled compressor may be the optimal solution. Various cooling tower designs are available, each with its own set of advantages and drawbacks. Customers must weigh the initial cost of the tower against ongoing energy consumption and maintenance requirements. Additionally, alternative water sources such as public supply water, self-supplied well water, or natural sources like rivers, oceans, or lakes can be used. However, it's crucial to address water cleanup post-compression, especially with natural water sources. While chilled water systems are an option, they are often avoided due to their high associated costs.

What are the requirements for air-cooled compressors?

Air-cooled air compressors necessitate ample airflow for inlet cooling and sufficient discharge space. Inadequate space can lead to temperature control issues, resulting in operational disruptions or equipment breakdowns. Ductwork facilitates the circulation of cool air into the compressor package while removing hot discharged air from the facility. During winter, this discharged heat can be redirected indoors to aid in heating the facility.

Therefore, if space constraints are a concern, water-cooled compressors may present a more suitable option.

How are energy costs affected for air-cooled vs. water-cooled?

For larger industrial plants, a common consideration when selecting a rotary screw compressor is whether to go with an air-cooled or a water-cooled version. Both designs certainly have their advantages and naturally their disadvantages. In general, the vast majority of rotary screw installations are air-cooled. Air-cooled rotary screw compressors are typically more readily available, and will require less overall installation costs. However, there are some applications where water-cooled compressors could be advantageous. The best way to approach this decision is to consider your compressed air system installation. Here are four questions to answer when deciding whether to go air-cooled or water-cooled.

While water-cooled units may consume less power than air-cooled compressors, customers should take into account additional expenses such as electrical costs for the cooling system and water treatment costs associated with water-cooled units. After factoring in these expenses, air-cooled compressors generally prove to be more cost-effective.

However, some customers leverage cooling water for preheating plant processes, such as boilers, to reduce gas and heating expenses, resulting in savings in that regard.

What's the case for air-cooled compressors?

Given the rising costs and regulatory constraints associated with water usage, many plants are exploring ways to minimize their cooling water needs. Compressed air systems traditionally consume significant amounts of water for cooling purposes. However, modern air-cooled rotary screw compressors have undergone design enhancements, enabling them to tolerate higher ambient temperatures than before. Consequently, customers stand to benefit from reduced water consumption, along with lower capital and installation costs associated with air-cooled compressor systems, which can be conveniently installed anywhere within a factory setting.

Ultimately, the choice between air-cooled and water-cooled rotary screw compressors hinges on the specific application and location of the customer. By carefully considering these factors and assessing the financial implications with the guidance of a compressed air expert, customers can determine which option best aligns with their facility's requirements.