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?
How Multi-Stage Compressors Work
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.
- Atmospheric air comes into the compressor air end and goes through one full cycle of compression via the pistons or rotors.
- The air then goes through a cooling cycle in an intercooler. This allows moisture to drop out and prevents overheating of the compressor.
- The cooled, compressed air then goes into a second set of pistons or rotors, where it undergoes another stage of compression.
- For a three-stage (or even four-stage!) air compressor, the process repeats again.
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.
Types of Multi-Stage Compressors: Reciprocating vs. Rotary Screw
Multi-Stage Piston 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.
Multi-Stage Rotary Screw Air Compressors
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.
High-Pressure Air Compressors
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.
What Are the Advantages of Multi-Stage Compressors?
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:
- Better energy efficiency: Multi-stage compressors are more energy efficient than single-stage models (improved volumetric efficiency). With two or more stages of compression, the compressor does not need to work as hard to generate the same pressure. That translates to lower energy bills.
- Less moisture: The intercooler acts as a built-in air dryer; as the air cools, moisture falls out and is removed via a drain. That means less work for your air dryers and filters.
- Less overheating: Those intercoolers also reduce the risk of air compressor overheating by removing some of the heat of compression between stages. That means the compressor runs cooler while making air.
- Greater reliability: Lower heat and greater efficiency mean less wear and tear on the compressor motor and seals. Over time, that translates to less unexpected downtime and fewer repairs. Recommended service intervals are usually longer, too.
- Smaller size per CFM: If space is a consideration, a multiple-stage compressor will deliver more air (CFM) and/or higher pressure (PSI) than a single-stage compressor of similar size.
Cons of Multi-Stage Compressors
Disadvantages of a multiple-stage compressor come down to cost, size and maintenance complexity.
- Cost: A multi-stage compressor is more expensive than a single-stage compressor of similar capacity. A 3-stage compressor will be more expensive than a dual-stage compressor.
- Maintenance: While multi-stage compressors are more reliable overall, additional compression chambers also mean more parts and more ways something can go wrong.
- Size availability: A multi-stage compressor is a large, heavy piece of machinery. Most multi-stage compressors are going to be in the 100 HP and up category. If you don’t need this much CFM, it’s probably not worth the cost of upgrading to a multi-stage compressor that is bigger than you need just to get the other benefits.
Applications with Multi-Stage Compressors
Multi-stage compressors may be a good choice if you:
- Use a lot of air on a consistent basis (high CFM), and/or
- Need pressures (PSI) higher than ~120 PSI.
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.
Considerations in Choosing a Multi-Stage Compressor vs. a Single-Stage Compressor
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.
Required Pressure (PSI)
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.
Required Airflow (CFM)
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).
Space and Size Considerations
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.
Cost (Initial vs. Lifetime)
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.
Conclusion: Do You Need a Multi-Stage Air 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.