What is a Rotary Screw Air Compressor?

Rotary screw air compressors or twin-screw compressors as they are also known are some of the most popular mobile compressors available. They are commonly used as a replacement for piston compressors and make up a large portion of the compressor market.

They are ideal for both mobile and standalone applications and can be found on vehicles and trailers as well as in production, industrial, and medical facilities where there is a need for very high-quality equipment.

How Does a Rotary Screw Air Compressor Work?

The rotary screw compressor process is fairly different from the reciprocating piston compressor. It works by generating the compression continuously at the rotary screw drive air end of the compressor. Air enters the unit at the inlet port of one of the two large screws rotating against one another, also known as the rotors or rotary screws. This air then moves down the length of the screws and compresses the air towards the outlet due to the air gaps in the rotors getting smaller and smaller. These rotary screws are machined with low tolerances to very highly accurate finished products through which the vanes on the rotary screws can be as low as a thousandth of an inch apart.

These pressurized rotary screw air systems consist of many components that interconnect and work together to compress air. Atmospheric air is used to provide the end goal of compressed air however, oil can be a very important characteristic of a rotary compressor. Oil lubricates the system and allows for the rotary screws to be far more efficient with the bonus of saving the user significant costs. Though oil brings these benefits, it does also require additional mechanisms resulting in a far more complex system. These mechanisms must be able to circulate, filter out and recycle the oil that enters the rotary-screw compressor.

The oil-fed rotary compressor provides a hydraulic seal between the rotary screw vanes and this allows for mechanical energy to be transported between the two rotary screws. Air enters the system in the same way as that of the oil-less system and moves down the length of the screws towards the outlet with the oil. A separation process is then required to ensure the air is routed back to the receiver and filter then re-route the oil back into the air end after cooling both. 

Advantages vs. Disadvantages of Rotary Air Compressors

Advantages

Delivery

Rotary screw air compressors hold some very important advantages over reciprocating air compressors. Generally, they can deliver more compressed air in comparison to reciprocating ones of the same size allowing for jobs to be completed faster and more on a day-by-day basis. Reciprocating compressors create air pulses in the compression process and require reservoir tanks to try to reduce these and also to be able to withhold greater quantities of air, which rotary screw compressors do not require for either reason.

Duty Cycle

Another key advantage that Rotary screw compressors hold is their ability to operate at 100% duty cycle. This allows for the continuous production of compressed air meaning you can work for longer and get jobs done without needing to wait around. The duty cycle in air compressors can be described as the amount of time that an air compressor can successfully operate without having to stop for any reason like overheating. For example, if you were to have a 10-hour working day period where the compressor runs for the full 10 hours then it is a 100% duty cycle. If it was only able to operate for 7.5 hours then it would be 75% duty cycle and so on. The key importance here is that in comparison to the rotary compressors, the majority of reciprocating compressors are only 50% duty cycle meaning that they can only operate for half the time which can cause significant reductions in completed work. 

Reduced Wear

Reciprocating compressors also wear significantly over time due to their engine-like parts coming in contact with each other. This wear then causes a detrimental reduction in the performance of the compressor, increased oil carryover, and an even greater amount of heat generation. Rotary screw compressors as previously described used the oil as a seal between the rotor screws, this allows the rotary compressor to maintain its high level of performance throughout its long life cycle worry-free. On the contrary, the reciprocating compressor declines and loses performance as it ages.

Lightweight

Rotary screw compressors allow you to work with so many more products due to their lighter weight and advantageous capacity when small in comparison to reciprocating air compressors. They can provide enough air to power and operate numerous very common pneumatic hand-held tools and allow vehicles to haul more equipment, tools, and materials. These all combine to an overall result of getting jobs completed a lot faster and more of them.

Disadvantages

Cost

On the other hand, they do have a couple of disadvantages.. first of all, they can have a higher upfront cost than reciprocating compressors. They can generally be upwards of twice the cost, but as you’ve learned can operate at twice the duty cycle so you essentially get what you pay for. This makes them a solid investment for long-term business operations that require air compression. Another disadvantage is that due to their complex nature, they require a far higher level of expertise to conduct maintenance on them in comparison to the reciprocating compressors.

Conclusion

It is clear that the advantages outweigh the disadvantages and that people tend to appreciate the efficiency, longevity, and reliability of the continuously run rotary screw air compressors.

Other Reading

Axial Flow Compressors Explained

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