Is a rotary screw air compressor right for you? Screw compressors are the workhorses of industry and manufacturing. These tough industrial air compressors are widely used for applications requiring high efficiency and continuous airflow, including conveyor systems, manufacturing equipment, process manufacturing, and more.
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Our complete guide to rotary screw compressors will help you understand how these compressors work and decide whether a screw compressor is the right choice for your application. Plus, how to install and maintain your compressor system for optimal performance.
A rotary screw air compressor (sometimes called a “twin-screw compressor”) is a type of air compressor that uses a pair of enmeshed helical screws to compress air. As the screws turn, the air is continually compressed. Screw air compressors are widely used for applications that require high CFM (Cubic Feet of air per Minute) and continuous use. This is the most commonly used air compression technology for heavy industrial applications such as powering pneumatic production lines and conveyor systems.
They can be contrasted with a reciprocating air compressor, which uses pistons driven by a crankshaft to compress air.
Read more: Reciprocating vs. Rotary Screw Air Compressors: What’s the Difference?
A rotary screw air compressor works by forcing air through a pair of rotating helical
screws (or rotors). The two rotors interlock as they turn, creating a series of chambers. As air is forced through the rotors, it is squeezed into a smaller volume. This reduction in volume compresses air as it moves the chambers. A rotary screw air compressor is a form of positive displacement compressor. The compression process is continuous and controlled by a drive motor and a system of inlet and discharge valves.
There are two basic kinds of rotary screw air compressors: oil-injected and oil-free. Oil-injected screw compressors are the most common type of industrial air compressor and work well for most applications. Within these categories, there are also some subtypes to consider.
Read more: Why Choose or Upgrade to a Variable Speed Drive Air Compressor?
An oil-injected (or oil-flooded) rotary screw air compressor uses oil to lubricate the screw unit and ensure its smooth operation. The oil also helps to cool the compressed air and prevent overheating of the unit. After the air is compressed, the oil is removed using an oil separator. A small amount of oil may be left in the compressed air (oil carryover), which can be further reduced using inline filtration.
An oil-free (or oil-less) rotary screw air compressor uses a two-stage compression process. Air passes through an intercooler between compression stages to bring temperatures down and prevent overheating. Oil-free rotary screw air compressors eliminate any oil carryover, producing very clean air. However, they tend to be more expensive, more complicated to maintain, and noisier than their oil-injected cousins. Oil-free screw compressors are most commonly used for specialty applications requiring very high-purity air. However, in most cases, it is possible to obtain the required purity levels using dryers and air filtration.
Read more: Do You Need an Oil-Free Compressor? Probably Not.
These are the basic components of a rotary screw air compressor.
The air-end is where the work of air compression happens inside the rotary screw air compressor. The air-end consists of several component parts.
A rotary screw air compressor must have several other components in addition to the air-end.
Read more: How to Adjust the Air Compressor Regulator.
Filters keep contaminants out of the air supply. Filters for a rotary screw air compressor may include:
Read more: What Type of Filtration Does Your Air Compressor Need?
Rotary screw air compressors generate a lot of heat during compression. This heat must be dissipated by a cooling system to prevent overheating. An aftercooler acts as a heat exchanger, absorbing heat from the compressed air and carrying it away where it can be dissipated. The aftercooler cools air and removes the majority of the moisture before it goes into the drying system and filters. There are two basic types of air compressor after-coolers: air-cooled and water-cooled.
Most screw compressors are air-cooled. Air-cooled rotary screw compressors use a series of coils that are cooled by a fan. Cooling fins provide additional surface area for air to move across, increasing the heat transfer capacity.
Water-cooled rotary screw compressors use water (or, sometimes, another liquid) to carry away excess heat. Water-cooled aftercooler system options include:
An oil-injected (or oil-flooded) rotary screw air compressor will have additional components as part of the oil injection system.
A complete compressed air system will include a number of other components in addition to the air compressor itself.
Hot, compressed air coming out of the air compressor will contain excess moisture. For most applications, it is desirable to remove moisture from compressed air to ensure a supply of clean, dry air for industrial processes. There are two basic types of compressed air dryers.
Read more: Refrigerated vs. Desiccant Air Dryers — How to Choose a Compressed Air Dryer.
The air receiver tank (or compressed air storage tank) stores compressed air after it leaves the air compressor. The air receiver tank may be placed either before or after the air drying system.
Read more: Air Receiver Tank Full Guidelines
Drain valves are used to remove excess liquid water from the compressed air system. When compressed air comes out of the rotary screw air compressor, it is very hot — and hot air can hold more moisture than cold air. As the air cools back down to atmospheric temperatures, excess liquid will fall out of the air as condensation. Drain valves are used to drain excess liquid out of the air compressor, air receiver tanks, air dryers, compressed air piping, and other parts of the system where liquid may accumulate.
Before selecting a rotary screw air compressor, make sure it is the right choice for your application. Here are five important questions to ask when making your decision.
Rotary screw air compressors are especially durable and are able to handle large amounts of demand for extended periods of time. They are still more efficient and durable than piston compressors, even at smaller sizes, but the efficiency benefits are most noticeable when getting into larger demands (i.e., 15 HP and more). Rotary screw air compressors are designed to run at 100% duty cycle, day in and day out.
While a rotary screw compressor may have higher air compressor maintenance costs and/or initial capital costs, its life expectancy is much greater than a piston-type air compressor. Piston-type (reciprocating) compressors typically last about 10,000 - 20,000 hours before a rebuild is necessary. On the other hand, a rotary screw compressor will last between 80,000 - 100,000 hours. Rotary screw compressors are known for their reliability and longevity.
Rotary screw air compressors are known for their quiet operation. Since most rotary screw air compressors come standard with sound-dampening enclosures, they generally operate between 65 – 75 dBA, which is much lower in comparison to other types of compressors. For comparison, a reciprocating or piston-type air compressor can operate at around 80 – 85 dBA.
Rotary screw air compressors are designed to have no more than 3 PPM of oil carryover, while piston compressors can have 10 PPM or more as they age. Less oil carryover means that you have to change your filter elements less often, therefore decreasing compressor maintenance costs.
Rotary compressors have become more and more popular because of their efficiency and their ability to keep up with constant high demand. Many rotary screw compressors are used 24/7.
Even if you don’t have a constant demand, rotary screw reliability, and durability come in a variable-speed drive version. A VSD compressor will speed up and slow down in relation to your demand at any given moment. This option should definitely be considered if your demand varies, because yesterday’s controls — including online/offline, modulation, and even turn valves — have proven to be costly when compared to the benefits of a VSD.
Rotary screw air compressors are the most popular type of air compressor for industrial applications — and for good reason. These air compressors are the workhorses of the industry: reliable, quiet, energy-efficient, and powerful. They are an excellent choice for high-volume applications requiring a continuous supply of air.
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Rotary screw air compressors are used across many industries that require high CFM or continuous airflow (100% duty cycle), including manufacturing, food processing, pharmaceuticals, processing plants, paint lines, packaging lines, and much more. These industry staples provide a reliable source of compressed air for many applications in manufacturing and industry, from powering equipment to pneumatic conveying, to process air.
Read more: Top 10 Industrial Compressed Air Applications.
Many manufacturing facilities rely on screw compressors to power production equipment, conveyors, and packaging lines. Rotary screw compressors provide consistent airflow and pressure for robotic production applications in manufacturing sectors such as automotive, aerospace, and general fabrication. Manufacturing plants also use compressed air to lift and move heavy parts and power manual tools. Other applications in manufacturing include:
Process industries, including chemical manufacturing and pharmaceutical manufacturing, rely on rotary screw compressors for both process air and to power machinery and conveyors. Air for pharmaceutical and process industries may need to meet very strict standards for purity. Screw compressors can be used to power applications such as:
Food and beverage manufacturers may use screw compressors in a variety of ways, from product blow-off and cleaning to packaging lines. A rotary screw compressor provides consistent and reliable air for food and beverage applications, including:
While small family farms may be more likely to use a reciprocating or piston compressor, large-scale commercial farming can benefit from the high efficiency and high CFM of a rotary screw compressor. Applications for screw compressors in farming include:
Rotary screw compressors are widely used throughout the energy sector for a variety of purposes. In the oil & gas industry, screw compressors are used to power drilling equipment and pneumatic systems for transporting drilled oil. Compressed air may also be used to power equipment for compressing natural gas and refining petroleum. Other uses in the energy sector include:
What kind of maintenance is required for a rotary screw air compressor? Rotary screw air compressors require regular inspection and maintenance to operate at peak performance and protect the system from excessive wear and tear. Check your owner’s manual for specific guidelines for your make and model of rotary screw air compressor. Air compressor maintenance tasks may include:
Read more: Maintaining Your Air Compressor.
Checking and changing the oil is one of the most important maintenance tasks for an oil-injected rotary screw air compressor. Depending on your compressor model and the type of oil used, you can expect to change the oil every 4,000 – 8,000 hours. However, if your rotary screw air compressor is operating in a dirty environment, the oil may need to be changed more often. Check oil levels and inspect oil for contaminants at least weekly. At least once a year, oil should be completely drained and flushed. Always replace your oil filter after flushing.
Air compressor filters (including air intake filters and inline filters) should be changed at a minimum every 2,000 hours for a rotary screw air compressor. If your environment is very dirty, you may need to change filters more often.
Water should be drained for a rotary screw air compressor at least daily, if not more often. To reduce maintenance time, look for automated drain valves. Electric drain valves can be set to open on a timer to drain condensate regularly. A zero-loss drain valve uses a float mechanism to activate the drain, so it only opens when needed. This can reduce compressed air loss.
Proper compressor installation is very important for a screw compressor. There are several factors to consider for installation of a rotary screw compressor.
The experts at Fluid-Aire Dynamics can help you select the best rotary screw air compressor for your application. Choosing a rotary screw air compressor depends on your airflow requirements, production processes, and budget. Contact us for help in choosing the best compressor for your needs.
All in all, rotary screw compressors today have become popular because of their efficiency, noise level, and the fact that the cost for this type of compressor has dropped significantly in the last ten years. They have low oil carryover and are durable and reliable. They come in VSD for applications with varying demands throughout the day. Rotary screw compressors are a worthwhile investment for any manufacturer looking for a good quality, overall cost-effective unit.
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Use of a VSD air compressor may allow you to cut energy use and save money.
Deciding to install a VSD air compressor is a major step for any facility. On the positive side, VSD compressors can save energy (and significant amounts of money), earn rebates, allow soft starts and offer better control of your entire compressed air system.
But there are many concerns. VSD air compressors cost more than their fixed-speed counter parts. And, like any electronic device, they can be sensitive to harsh, demanding environments (see the “VSD: The Cons” section below).
Let’s start with the basics.
First, a little background on fixed and variable-speed drive.
A fixed-speed compressor operates at the same speed continuously, cranking along at the same RPM, no matter what the system’s demand. And, unless you change the pressure, the compressor’s output of CFM will also remain the same.
A VSD air compressor, on the other hand, is equipped with a variable frequency drive or inverter that allows it to adjust its motor speed or RPM (and thus its CFM) to match the real-time demand for compressed air. When the demand drops, the inverter dials back the motor operation to a lower speed (and lower CFM). It thus consumes proportionately less energy (and money), creating a relatively linear output-to-input efficiency curve.
VSDs are an extremely energy-efficient way to address rapid cycling, which occurs when your air compressor switches back into the loaded state before the transition to the no-load state is complete. If that happens too frequently, it can cause increased wear and reduce compressor reliability and lifespan.
VSDs are recommended when the compressor operates at a load between 30% and 80% of its total capacity most of the time. They are not recommended when your compressor is running at 100% load most of the time as the VSD drive itself has some losses (that can be overcome by optimizing tip speeds). But they are very valuable as a trim compressor complementing a base load compressor (see “System Reconfiguration” section below).
Variable-speed drives complement rotary screw air compressor technology exceptionally well.
VSDs allow the compressor to ramp up to full speed gradually, preventing a high in-rush current (up to six times as high as operating current) that fixed-speed compressors draw during startup. They automatically control acceleration and deceleration to reduce peak amps, thus avoiding utility company penalties for increased power demand. Utilities levy these costs to discourage spikes in usage that can overload the power grid.
This soft start capability saves energy and reduces the startup stress on compressor components. It lengthens the life of the motor by decreasing inrush current, which causes the coils to heat up and reduces the life of the insulation. It thus extends the lifespan of your equipment.
By allowing more finite control of your compressor, VSDs will enable you to calibrate the output to match compressed air demand, enhancing efficiency and performance.
In addition, you can control the pressure more tightly, keeping the pressure band—the gap you set between load pressure and unload pressure—to +/- 1.5 PSIG, according to CAGI. With a fixed-speed unit, that pressure differential will have to be much higher.
That’s a lot of wasted energy, especially if your plant needs 100 PSIG and you're compressing up to 130. The rule of thumb is that for every 2 PSIG increase in pressure, the energy consumption goes up by 1%. You can conserve energy by using a VSD air compressor and setting the header pressure as low as possible.
To combat rapid cycling, we often propose modulation that adjusts the inlet valve to a partially open position. As mentioned previously, variable-speed drives are a more energy-efficient way to address this issue but cost more up front.
While modulation has traditionally been viewed as a waste of money and energy, it may be more cost-effective than VSDs for some users. The key is to balance that potential cost with the negatives of rapid cycling.
Neither modulation nor VSDs, however, take the place of an appropriately sized compressor and storage tank. It’s a best practice to have both wet and dry storage to remove moisture and maintain stable header pressure. For more on tank storage, see our post, A Quick Guide to Accurately Size Air Compressor Tanks.
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