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What Is a Rotary Screw Air Compressor?
A rotary screw air compressor draws in air via the two interlocking screws that give the machine its name. The unit compresses the air by degrees before discharging it according to the machine’s psi rating. All air compressors are either oil-lubricated or oil-free.
- Oil lubrication is the more common. The air passes through a filter before it leaves the compressor to remove the oil, although traces may remain. Oil-lubricated compressors require regular maintenance to maximize the unit’s lifespan. This includes changing the filters, oil, and separator regularly.
- Oil-free lubrication employs a special coating to the pistons and rings, eliminating the need for oil. These units typically weigh less – and cost more – than their oil counterparts. In addition, they don’t have as long a lifespan due to increased damage from excessive heat. However, they are preferred in industries where cleanliness is imperative, such as dental offices.
Both types of rotary screw air compressors have more power and capacity than piston driven models.
Benefits of a Rotary Screw Air Compressor
There are a number of benefits to rotary screw air compressors compared to reciprocating models. These include:
- Lower maintenance costs: Thanks to fewer moving parts, rotary screw models require less maintenance. Of course, having fewer parts to service means there are fewer parts to replace.
- Cleaner air: The machine’s interlocking screws are highly efficient at sealing in the oil used for lubrication. Typically, oil carryover comes in at less than 3 parts per million, giving you much cleaner air with less moisture.
- Continuous duty : Air compressors come with numerous ratings: psi, CFM, hp, etc. The continuous duty rating tells you how many minutes per hour the unit can run safely. Many rotary screw compressors have a 100 percent continuous duty rating, meaning they can run for a full hour without rest. You get more work done in less time.
- Less noise: Reciprocating air compressors clock in as high as 100 decibels (db), which is equivalent to the noise made by a lawn mower or motorcycle engine. Rotary screw compressors are only around 70 db, about the volume of a dishwasher or vacuum cleaner.
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How Much Does a Rotary Screw Air Compressor Cost?
Tank size is probably the greatest price determinant for a rotary air compressor. Other cost factors include horsepower, CFM (cubic feet per minute), and accessories. The dealer you choose and your area also play a role in price.
- A tank-mounted Atlas Copco rotary screw compressor with 5 horsepower, electric, 250V, 16.6 CFM, and single-phase costs $4,500 on average.
- An Ingersoll Rand rotary screw air compressor with an 80-gallon, horizontal tank, 10 horsepower, 460V 3-phase and 38 CFM costs around $9,000 on average.
- A Kore rotary screw compressor with 120-gallon vertical tank, 20 horsepower, electric, oil lubrication, and 79 CFM costs around $10,500 on average.
- The Kaeser M27 is a towable, diesel-powered rotary screw compressor with 21 horsepower, Kubota motor, and 92 CFM at 100 psi for an average price around $14,500.
- The Sullair ShopTek ST3708 is a base-mounted rotary screw electric model with oil lubrication, 50 horsepower, 460V 3-phase and 219 CFM with an average price around $17,500.
If you need an oil free model, expect to pay around two to three times as much as for an oil-lubricated unit.
How to Choose the Right Compressor
Price is not the only factor to consider when buying a rotary screw air compressor. The following factors also play a role:
- Cubic feet per minute (CFM) measures the amount of air flow. To determine the CFM level you need for your air compressor, use the CFM of your highest-rated tool. Multiply that number by 1.5 to get the CFM rating your air compressor needs. (Sometimes labeled as standard cubic feet per minute, or SCFM.)
- Mobility may be via a self-contained trailer, wheels, or handles. If the air compressor is heavy but doesn’t have a trailer, wheels and handles make it much easier to move.
- Tank size determines more than price – it also dictates how long you can work. If you power multiple tools with one compressor, or use any continuously, you want a larger tank size.
You also need to know where you’ll place the compressor. These machines put out a lot of heat and noise.
Maintaining Your Rotary Air Compressor
Tracking running hours is a simple way to determine when the compressor needs maintenance. Many newer air compressors include indicators that let you know when it’s time to service your unit. The following five areas require regular maintenance:
- Airends and bearings: Excessive heat and contamination (from the air as well as the oil lubrication) damage these parts. Rebuild airends between 50,000 and 60,000 hours (or follow your manufacturer’s instructions).
- Drivetrain system: Maintain alignment, including lubricating gear drives regularly. Also, adjust v-belt tension every 500 hours (minimum) and replace worn or frayed belts immediately.
- Filters: Ensuring proper filtration protects your unit from early wear and is much cheaper than repairing or replacing the machine. Inspect and replace air inlet filters every 2,000 hours.
- Lubricants: Before refilling, drain the existing lubricant to maximize the machine’s lifespan. Follow the manufacturer’s recommendations for changing the oil. Always use lubricants designed for your air compressor.
- Motor bearings: Keep them lubricated and replace as required. Ensure proper ventilation and check amperage draw periodically to prolong the life of your motor.
Finally, keep your rotary screw air compressor clean. This helps maximize output and guards against the machine running hot, further increasing its lifespan