If you need compressed air, there are a number of ways to get it (see chart above) and they all have their pros & cons. In order to choose the right compressor (or compressors), there are also a number of factors which will influence that decision:
- Specific requirements of the facility with regard to how much airflow, and at what supply pressure(s) is needed for the compressed air-operated equipment.
- Location of the facility…and the air compressor. Environmental considerations will absolutely influence the selection of filtration, drying, and even the type of compressor.
- Power cost – energy consumption is, by far, the largest portion of the total lifetime cost of any compressed air system.
With that in mind, the purpose of today’s blog is to lay out the case for rotary scroll compressors. These are positive displacement machines, meaning that they draw in a fixed volume of air and push it into a smaller volume, increasing the pressure. The earliest air compressors were positive displacement reciprocating piston types, where a piston moves back & forth in a cylinder, drawing in atmospheric pressure air on the ‘up’ stroke, and discharging it into the compressed air system on the ‘down’ stroke. These are simple machines – by adding cylinders, you can add air capacity, and by adding stages, you can generate high pressures.
The rotary scroll air compressor is a fairly recent development. While the first patent for one was issued in 1905, the machining technology required to make it work wasn’t available until the 1950’s, and it was the 1970’s before they started to enter mainstream use. Here’s how it works:
The main benefits of choosing a rotary scroll compressor are:
- Quiet operation. They can be thought of as the polar opposite of reciprocating piston models in this regard. Instead of slamming solid pieces of metal back & forth in a confined space, the continuous motion of the rotating scroll never brings it into contact with the stationary scroll, so mechanical sound generation and vibration are all but eliminated.
- Energy efficiency. Two main factors come in to play here:
- Continuous compression – if the rotary scroll is in motion, it’s compressing air constantly, in proportion to its speed of rotation.
- Low friction – because the scrolls don’t contact each other, they don’t lose energy due to friction between moving parts.
- Pulsation free operation. This is another benefit of continuous compression, as opposed to the pulsating airflow from a reciprocating piston design. This means less wear & tear on pretty much everything immediately downstream of the compressor.
- Fewer moving parts. The only moving part, really, is the rotating element. Compared to the pistons, rings, connecting rods, cylinders and valves of a reciprocating compressor, the reliability & durability of the rotary scroll compressors can’t be beat.
- Oil free air. No metal to metal contact in the air end means no lubrication is required.
That said, they’re not without limitations and potential drawbacks:
- Higher purchase price. The precision machinery needed to manufacture their high tolerance components aren’t cheap, and neither is their operation.
- Limited capacity. Because of the centrifugal force the rotary element generates, it’s necessarily limited in mass and therefore, size. Rotary scroll compressors typically top out at about 100 SCFM worth of capacity.
- Higher repair costs. Because of the tight tolerances and the fact that the air end has to be hermetically sealed, repairs of these compressors are high-tech operations…and the highly trained technicians with the specialized tools & equipment to perform such repairs don’t come cheap.
At EXAIR, we want to help you get the most out of your compressed air system. If you’d like to talk about that, give me a call.
Russ Bowman, CCASS
Application Engineer
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