By: Neil Mehltretter
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.
How much room do the 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 counter-intuitive, 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 could be coordinated to meet the fluctuations in the plant demand instead of one large compressor sized to handle the peak flow demand. The multiple smaller compressors option may also more effectively meet a price point regarding the cost of a back-up compressor.
Smaller horsepower rotary screw compressors (40 hp and below) are not typically available water-cooled. When selecting larger air-cooled compressors, the amount of cooling air flow required is proportionally larger and therefore the limitations of cooling air inlet and cooling air discharge in the room will need to be considered.
Take a look at the layout of your compressor room and play a little bit of mental Tetris—and see what will fit and where inlet air and discharge ducting would go.
Building on the idea of how much space you have leads us to the next consideration—
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. If your compressor room isn’t properly ventilated, you are setting yourself up for downtime.
Additionally, if your compressor room is located near a hot boiler room or near a process where fumes could be ingested into the inlet, then 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 of cooling water.
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? Water quality is also an issue. 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 onsite, 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 the new compressor purchase. The operational cost of the system must then be added to the operational cost of the compressors, further increasing the compressor lifetime costs. That being said, many large plants already have cooling towers for other equipment as well, 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 on the cooling water quality required. 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.
What are the energy costs of an air-cooled versus water-cooled compressor?
Typically, a water-cooled compressor will have a slightly lower specific power (kW/100 cfm) than an air-cooled compressor—this means it’s a bit more efficient. You can compare specific performance values by looking at the compressors’ CAGI data sheets. If you’d like a line-by-line explanation on how to read these data sheets, download a free copy of our white paper here.
However, you also need to consider the cost of the electricity for the cooling system and cost for the water and water treatment. When you factor in those costs, an air-cooled compressor is more cost-effective from an energy savings perspective. 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 the cooling water and swing the solution in favor of the water-cooled compressors. Even with an air-cooled unit, you can make use of fluid heat exchangers. Keep in mind there are also space heating opportunities with air-cooled compressors as well, but this is typically only available seasonally. For more information, see our white paper on heat recovery.
To really understand how the savings compare between the two types and also if it would be better to go with one large compressor or multiple smaller ones, work with a compressed air specialist that can do a system simulation to show you multiple scenarios and the energy savings potential between them. You’ll also have to consider the other three major items identified above: location, ventilation, and whether additional support equipment is required.
Here’s an example of a savings simulations we did for a customer who had three 200 hp water-cooled compressors in their existing system. The customer wasn’t sure if they should switch to air-cooled compressors or not. They already had cooling water towers and cooling water supply and quality were not an issue. 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 and 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 200 cfm in artificial demand due to leaks. Fixing those would give an additional $27,000 in savings.
As you can see here, the water-cooling tower costs account for nearly $48,000 in extra energy every year. Plus your cooling water costs may vary widely from the $2.00/1000 gallons cost used here. 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 at much you stand to save by going one way or the other.
If you’d like our team of engineers to run a no cost, no obligation simulation for you, contact us. We’d be happy to help.