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 require a lower overall installation cost. However, there are some applications where water-cooled compressors could be advantageous. The best way to approach the air-cooled vs. water-cooled compressor decision is to consider your compressed air system installation. We'll take a quick look at the operation of air-cooled and water-cooled systems and then provide four questions to consider when deciding which way to go. These questions are not exhaustive, but will help establish which option might be best for the application at hand.
How Air-Cooled and Water-Cooled Compressors Manage Heat
Industrial air compressor cooling systems generate significant heat. In a rotary screw compressor, circulating oil removes most of the heat from the airend, but the oil and compressed air still need to be cooled before the air moves downstream. That's where the cooling circuit comes in, and the method you choose has real implications for your installation, your operating costs, and your compressor's long-term reliability.
An air-cooled air compressor uses a fan and radiator to move ambient air across the oil cooler and aftercooler, dissipating heat directly into the surrounding environment. These systems are self-contained and straightforward to install.
A water-cooled air compressor routes coolant through a heat exchanger, these can be plate-type heat exchangers or shell-and-tube heat exchangers, transferring heat from the compressed air and oil to a circulating water supply. These systems remove heat more precisely and can perform well in environments where ambient air temperatures make air cooling impractical.
Understanding how each industrial air compressor cooling system handles heat is the first step toward choosing the right configuration for your facility.
Question 1: How Much Room Do Compressors Need?
This question is a tad tricky. It seems straightforward, but there’s more than meets the eye with this one. It might seem counterintuitive, but it may be best to begin answering this question by first thinking about your plant’s overall demand. Is your compressed air demand steady, or does it vary throughout the day or week? If your plant demand isn’t constant, it may make more sense to have multiple smaller-horsepower compressors that can be coordinated to meet fluctuations in plant demand, rather than one large compressor sized to handle peak flow demand. The multiple smaller compressors option may also more effectively meet the price point for a backup compressor.
Smaller-horsepower rotary screw models(40 hp and below) are not typically available as water-cooled air compressors. When selecting larger air-cooled compressors, the required cooling airflow is proportionally greater; therefore, the limitations of the cooling air inlet and discharge in the room must be considered.
Take a look at the layout of your compressor room, play a little bit of mental Tetris, and see what will fit (with ample room for service around the equipment) and where the inlet air and discharge ducting would go.
Building on the idea of how much space you have leads us to the next consideration about compressor cooling methods.
Question 2: Is There Enough Ventilation in the Compressor Room?
Air-cooled compressors need a sufficient supply of cooling air for the inlet and enough space for the discharge. Improper ventilation is almost always the cause of temperature regulation issues and can lead to equipment failures and downtime. A compressor room without proper ventilation sets you up for costly shutdowns.
Additionally, if your compressor room is located near a hot boiler room or a process where fumes could be drawn into the inlet, this location may not be the best choice for your air-cooled compressor. Water-cooled compressors can be placed in confined spaces and are better suited for higher-temperature areas if there’s a sufficient supply (at a reasonable temperature and pressure) of cooling water.
Question 3: Do I Need To Purchase a Cooling Water Tower for a Water-Cooled Compressor, and What Are the Water Quality Requirements?
Water-cooled compressors require cooling water; obvious, right? It's not just about meeting the cooling water flow, but the quality of the water needs to be considered. The better the water quality, the longer the lifetime of the compressor heat exchangers. Therefore, you typically see closed-loop cooling systems that control water quality rather than city water. If there isn’t a closed-loop cooling system already on site, it may not make sense to purchase this additional equipment. Purchasing a closed-loop system with a cooling tower will significantly increase the capital cost of your new compressor purchase. The operational cost of the cooling system must then be added to the compressors' operational cost, further increasing the compressors' lifetime costs. That being said, many large plants already have cooling towers for other equipment, so then it becomes a question of whether the cooling tower can supply the required flow at the proper water quality.
Be sure to check with your compressed air manufacturer and ask for specifications for the required cooling water quality. If your water can’t meet the spec, go with an air-cooled compressor. You’ll save yourself a lot of headaches down the road.
Question 4: What Are the Energy Costs of an Air-Cooled vs. Water-Cooled Compressor?
Typically, a water-cooled compressor has a slightly lower specific power (kW/100 cfm) than an air-cooled compressor, meaning it’s a bit more efficient. You can compare specific performance values by looking at the compressors’ CAGI data sheets.
However, you also need to consider the electricity costs for the cooling system and the costs of water and water treatment. When you factor in those costs, an air-cooled compressor is more cost-effective in terms of energy savings. On the other hand, you can also consider the heat recovery potential by using the cooling water to preheat a plant process. This may offset the additional cost of cooling water and tilt the solution in favor of water-cooled compressors. Even with an air-cooled unit, you can make use of fluid heat exchangers. Keep in mind that there are also space-heating opportunities with air-cooled compressors, but these are typically only available seasonally.
To really understand how the savings compare between the two types and whether it would be better to go with one large compressor or multiple smaller ones, work with a compressed air specialist that can run a system simulation to show you multiple scenarios and the energy-savings potential for each. You’ll also have to consider the other three major items identified above: location, ventilation, and the need for additional support equipment.
Real-World Simulation: How One Plant Found $48,000 in Energy Savings
Here’s an example of a savings simulation we ran for a customer with three 200-hp water-cooled compressors in their existing system, running 24 hours per day, 5 days per week. The customer wasn’t sure whether to switch to air-cooled compressors. They already had cooling water towers, and the cooling water supply and quality were not issues. Also, the compressor room had enough ventilation and was not located near anything harmful that could be ingested into the compressed air supply. They had completed a compressed-air audit with a third-party provider, so we were able to use that information to build the simulation.
The audit data revealed that the system was oversized, so downsizing would offer immediate savings. Additionally, they had approximately 300 cfm during a non-productive time frame (which could be a combination of leaks and artificial demand). Eliminating ~ 300 cfm of demand would give an additional ~ $27,000 in savings per year.
As you can see here, the water-cooling tower costs account for nearly $48,000 in extra energy every year. Of course, your cooling water costs may vary widely from the rate used here. In this situation, switching to air-cooled compressors would offer considerable savings on top of the savings uncovered from fixing the leaks and properly sizing the compressors based on the demand.
After you’ve examined your compressor room, ventilation, and cooling water quality, take the time to look at the numbers. You might be surprised by how much you stand to save by choosing one compressor cooling option over the other; also, never underestimate the savings potential of heat recovery
Additional Resources:
You may find the following resources useful as you compare air-cooled vs. water-cooled compressors:
See How Much Your Cooling System Is Really Costing You
Most plants don't know how much their compressor cooling configuration adds to their annual energy bill until they run the numbers. Our engineers can build a no-cost, no-obligation system simulation for your facility, showing you exactly how air-cooled and water-cooled configurations compare under your operating conditions.