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What is a multi-stage air compressor? As the name implies, multi-stage compressors use multiple stages of compression to delivery higher airflow (CFM) or achieve higher pressures than are possible with a single-stage compressor. These workhorses in the air compressor industry may be two-stage compressors or even three-stage compressors. When are multi-stage compressors worth it, and when will a single-stage do the job?
Multi-stage compressors may be two-stage or three-stage, and they may be either piston-style or rotary screw. The working principles for multi-stage compressors are roughly the same for both styles.
With each stage of compression, the air is squeezed into a smaller and smaller space, creating higher and higher pressures (measured in pounds per square inch, or PSI). A multiple-stage compressor can reach a higher PSI than a single-stage compressor. A dual-stage air compressor may reach pressures of up to ~175 PSI, while specialty high-pressure air compressors (3-stage or 4-stage) may reach pressures of 2,000-6,000 PSI for breathing air and other applications. More commonly, a multi-stage industrial air compressor is used to deliver more air (in Cubic Feet per Minute, or CFM) at standard working pressures.
Both reciprocating air compressors (piston-style) and rotary-screw air compressors are available as multi-stage compressors.
A multiple-stage piston compressor has multiple sets of pistons with progressively smaller circumferences. Air is pulled into a piston chamber (cylinder); when the piston goes down, it is compressed into a smaller volume, which results in a higher PSI. In a dual-stage piston compressor, air is usually compressed to ~120 PSI during this first stage of compression. Air goes from the piston chamber to an intercooler system which brings the temperature back down using water or blown air. The cooled air then goes into the next cylinder for additional compression. Pressure may reach ~175 PSI during the second stage of compression. Additional compression or higher CFM may be reached with additional cylinders.
Rotary screw compressors are typically available as single-stage or dual-stage (two-stage). In a rotary screw air compressor, air is pushed through a pair of interlocking helical screws, which force the air into a smaller and smaller space as the screws turn. In a two-stage rotary screw air compressor, there are two sets of synchronized rotors. They may be housed in a shared rotor housing with one set underneath the other. Alternatively, they may be configured end-to-end with each set in its own housing. The rotors are turned with either a direct drive or a geared drive. The two rotor systems share the work of compression. Two-stage rotary screw compressors are usually used for high-CFM applications with pressures from 100 PSI to 175 PSI. Their primary advantage is not that they deliver higher pressures than a typical single-stage rotary screw model but that they can deliver more CFM with greater energy efficiency.
Specialty high-pressure air compressors are usually 3-stage or 4-stage piston compressors. They are capable of compressing air to very high pressures, up to 6,000 PSI or more. These air compressors are used for specialty applications requiring very high PSI, such as SCUBA diving, medical breathing air, paintball, firefighting and certain industrial applications.
Outside of the realm of specialty high-pressure air compressors, the chief advantage of a multi-stage compressor is its ability to deliver more air with greater efficiency and reliability. A multi-stage model can deliver more air (CFM) than a single-stage compressor of the same horsepower (at the same PSI). Advantages of 2-stage and 3-stage compressors include:
Disadvantages of a multiple-stage compressor come down to cost, size and maintenance complexity.
Multi-stage compressors may be a good choice if you:
Multiple-stage compressors are more commonly available in higher horsepower models of 100 HP and up. They are not always used to deliver high pressures; more commonly, they are used to deliver more CFM at standard working pressures. Multi-stage compressors are often found in industrial applications using large amounts of air at typical plant pressures of 100 – 120 PSI, such as automotive manufacturing, general manufacturing, chemical production and the energy sector.
How do you choose between a multiple-stage compressor and a single-stage compressor? There are several considerations in making the choice, including your PSI and CFM requirements, temperature control, size, maintenance and (of course) cost.
What are the PSI requirements for the machinery you are running with compressed air? Most industrial applications require around ~100 PSI. These pressures can easily be achieved with a single-stage air compressor. If you need higher pressure—especially pressure higher than 120 PSI—you are likely to need a 2-stage or even 3-stage compressor to achieve it.
Aire Tip: Many plants are running at higher pressures than are needed for their applications. Learn how you can reduce plant pressure to save money and energy.
What are your CFM requirements? A multi-stage compressor can pressurize air more quickly, resulting in a higher CFM than a single-stage compressor of similar size. Air production is also more efficient, which will save energy costs per CFM. Plants with high CFM requirements and continuous air use may want to consider a multi-stage compressor. Not sure how much CFM you need? Check out our compressed air CFM calculator.
How heat-sensitive are your applications? The intercoolers on a multi-stage compressor bring temperatures down, so discharge air is cooler. Cooler temperatures and dryer air (due to water dropping out in the intercooler) can be less stressful for air dryers or other equipment hooked up directly to the compressor. (However, temperature control alone is probably not a good reason to invest in the cost of a multi-stage compressor; there are other ways to handle temperature and moisture control for a single-stage system).
If you need a portable air compressor or a compact model, a small single-stage compressor may fit the bill—so long as it meets your CFM requirements. However, if you are purchasing a large industrial air compressor, remember that a 2-stage compressor will have a smaller footprint than a single-stage compressor of comparable CFM output.
A multi-stage compressor will have a significantly higher capital cost than a single-stage compressor of similar HP. However, for high-CFM applications, you will see cost savings through lower energy bills per CFM for the life of the compressor. You may also experience lower maintenance costs. Look at the total lifetime costs of the compressor when choosing between a single-stage vs. multi-stage compressor.
Is a multi-stage compressor right for you? Fluid-Aire Dynamics has a huge inventory of single-stage reciprocating and rotary screw compressors - which are a great choice for most basic compressed air applications. We also have large dual-stage rotary screw compressors available from PneuTech for high-CFM industrial applications. We can help you calculate the costs and benefits of moving to a multi-stage rotary screw compressor.
Contact us to learn more about single-stage and multi-stage compressors.
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.
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.
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 https://kaesertalksshop.com/2019/03/12/calculating-the-value-of-avoided-unplanned-downtime/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.