Piston versus rotary screw compressors

There are many compressed air users (such as small automotive workshops and garages) where the requirement for compressed air is relatively low and infrequent, or the quality of the air produced is not of paramount importance. Here in particular it is very common to find these needs being sufficiently met with a piston compressor. However, if demand for compressed air grows or the applications that compressed air is required for expands to ones that require higher air quality, then a piston compressor may struggle to effectively, efficiently and reliably meet these needs.

A key difference between most piston and rotary screw compressors on the market, is that piston compressors cannot run continuously whereas rotary screw compressors can. Most piston compressors have a 60 - 70 % allowable duty cycle (which is the percentage of time they can operate before there is a risk of them overheating). In contrast, rotary screw compressors have a 100% allowable duty cycle, so if required they can run continuously. 

The duty cycle of a piston compressor tends to mean that they are often oversized for the application, in order to periodically allow the compressor to shut down and cool down. Having periods when the compressor shuts down can cause operational disruptions - even with sufficient air storage. And where a business expands and more compressed air is required, this can soon become a real issue.

If you use compressed air for body work - for example spray painting - then high air quality is essential if you want a quality finish to your paintwork. Moisture in an airline can for example cause fish eyes, leading to expensive re-work requirements. Equally, even if you just use compressed air to clean parts, such as to power a bead blaster, moisture in the airline will make the job more time consuming and less efficient. 

This is where you see a big difference in typical piston compressors versus modern rotary screw compressors. Whereas the internal operating temperature of piston compressors can be between 150 to 200^oC, for a rotary screw compressor this is significantly lower at around 75 to 95^oC. Why is this important to air quality? Because the hotter the air, the more moisture it will hold. 

Whereas modern rotary screw compressors are able to lower the temperature of the compressed air as it leaves the compressor thanks to built-in aftercoolers and powerful fans, even with an aftercooler and a special high temperature dryer, piston compressors struggle to reach the same dew point. Therefore, compressed air leaving a piston compressor will be very hot and hard to dry. 

In addition, more air volume is typically required to achieve high air quality. It's not unusual for auto workshops to be operating compressors as large as 22 kW. Here, rotary screw compressors offer a clear energy efficiency advantage, delivering more air per unit of input energy than piston compressors. And of course the cost advantage of operating an energy efficient compressor can for one, be seen in the electricity bill. 

Another consideration when it comes to producing high quality air is the oil carry-over. In a piston compressor as the moving parts wear (e.g. pistons, cylinders, rings etc), more lubricating oil is able to get past the rings and into the airline right through to the point of use. Like moisture, oil is another substance that you do not want in your compressed air - especially when you’re using that air for applications such as paint spraying. 

In contrast there is very little if any change in the performance of a rotary screw compressor over time. This is because the rotors in a screw compressor block do not wear because they do not touch each other or the rotor housing. The compressor lubricant acts as a non-wearing sealant, which is captured, filtered, cooled and recirculated. Not only does this extend the life of the compressor pump, but it means that very little lubricant gets downstream. So where a typical piston compressor can have anywhere from 10 ppm (or parts per million) upwards of oil carry-over, a rotary screw compressor has only 1 to 7 ppm. 

It’s also worth noting that even new piston compressors pass several times more oil than rotary screw compressors.

Piston compressors typically have a large footprint, create high vibrations and high noise levels. They cannot therefore be positioned at point of use. This creates additional requirements in terms of housing and piping.

Location also plays a key role in the overall performance and therefore useful life of the compressor. If the compressor is for example situated in its own room with poor air circulation, this can increase its operating temperatures and therefore make it harder to remove moisture and oil from the compressed air. 

In contrast modern rotary screw compressors emit only low noise emissions and are compactly designed, with only a small footprint. This makes them ideal at point of use but also provides more flexibility when it comes to deciding where they should be positioned. It may therefore be easier to choose a space that has better ventilation.

Routine maintenance on a piston compressor is generally simple and inexpensive whereas rotary screw compressors have more service points than need to be tended to. But, because of the high oil carry-over and loss of flow that you see in piston compressors, they eventually require major rebuild work which can be very expensive. 

The service frequency and therefore cost per annum of maintaining a rotary screw compressor, will differ from OEM to OEM. As an example, where one OEM might recommend that their rotary screw compressors require service after every 1,000 hours of operation, another might suggest after every 3,000 hours. For an auto shop that operates from let’s say 7 am to 4 pm, 6 days a week, 52 weeks a year, depending on the rotary screw compressor in question, that could be the difference between requiring service once a year as opposed to three times a year. If you are looking to move to a rotary screw compressor in your facility, it is therefore worth comparing the maintenance recommendations of a number of different makes of rotary screw compressors in making your final decision.

What the above hopefully demonstrates is that it is really important to look beyond the initial purchase price of a compressor and consider what the total cost of ownership will be. So, when comparing specifically piston and rotary screw compressors, it's worth specifically considering the following points: 

• Rotary screw compressors are more efficient than piston compressors and do not need to be oversized to compensate for limited duty cycles. By opting for a correctly sized rotary screw compressor that precisely meets your compressed air requirements, you will reduce your operating costs and the associated electricity costs.
• If you do require high quality compressed air for applications such as paint spraying, you will make savings by opting for a rotary screw compressor to meet these needs, in terms of materials (e.g. paint) and labour. You will also help to extend the life of your associated tools and equipment.
• With noise levels lower and sizing compact, you will not need to incur any additional costs associated with housing a piston compressor in a separate room by opting for a rotary screw compressor.
• If you require a consistent supply of compressed air throughout your working day, then 100% duty cycle means 100% productivity for your business.

For a more in-depth understanding on the total cost of compressor ownership, click here to read our blog post ‘Evaluating the true cost of compressor ownership’.

So which will be better for you? In the end it is not a one-size-fits-all approach. The best way to determine the ideal solution to meet your specific compressed air requirements, is to seek the advice of a compressed air expert, where they work closely with you to gather all the necessary production parameters and conditions, and then present a solution precisely tailored to meet your needs.

References / Further resources
Camber, M: Piston versus rotary screw compressors

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