Choosing the right compressor for the job
24 October 2019
A common question in the compressed air industry is choosing between a rotary vane and reciprocating (piston) compressor. Steve Downes, Hydrovane Sales and Product Manager at Gardner Denver, looks to settle this debate, outlining why rotary vane technology is often more suitable for those seeking a long-term compressed air solution.
Deciding between a rotary vane and a piston compressor will often depend on what it is a business is looking to achieve: a lower upfront purchase price in order to make a quick saving, or a machine that can deliver lower whole life costs over the lifetime of the compressor? Does the application in question require high air quality, or is the operator more concerned with a reliable and consistent supply of compressed air? Does the site have only a very occasional need for compressed air and electricity consumption is not a major concern, or will the system need to be used around-the-clock, meaning an energy-efficient model is an absolute must?
There are many factors to consider, and the compressor’s design should be the first one. A piston compressor uses the principle of displacement to increase the pressure of the air inside the cylinders. As the piston moves down, a vacuum is created above it. This allows outside air at atmospheric pressure to push open the inlet valve and fill the area above the piston.
In contrast, a sliding, rotary vane compressor features a simple design with minimal moving parts. As the rotor rotates, blade-shaped vanes slide in and out to form a compression cell and the atmospheric air is compressed and discharged. Typically oil-lubricated, oil is then injected continually to lubricate, cool and seal the compression cell. Any remaining traces of lubricant are then removed in a final separator element, ensuring high-quality air at all times.
Dry and oil-free air is often a pre-requisite for high quality manufacturing procedures. On the other hand, some applications will not require such stringent quality levels and a piston compressor – while it may have oil carryover of up to 25 ppm – can often provide the appropriate level of quality.
But rotary vane technology features an integrated filtration system, which removes lubricant internally before entering a final separator element. The final stage of separation ensures high air quality with very low levels of oil particulate. The system air is then passed through an optional aftercooler, reducing the outlet air temperature, which removes most of the condensate. It is also possible for clean and dry air to be achieved by installing additional downstream products.
As previously mentioned, the longevity of equipment is another factor to consider. If a long lifespan is not a key concern, then a piston compressor can often be purchased for a lower upfront cost and will typically operate reliably for a good two to three years, so can still be a sound investment.
In contrast, a rotary vane compressor might cost more initially, but can have a far greater lifetime than alternative technologies, which will be increasingly more cost effective. Rotary vane compressors run slower, with minimal friction. This means there are fewer moving parts and, therefore, the time between service intervals is increased significantly.
Another point to note is noise. Compressors can be fairly noisy during operation, which can become an expensive problem when there are staff working close to the unit. At best, operators and staff may need ear protection; at worst, the compressor may require a special noise enclosure with sound attenuation to reduce noise levels to an acceptable level.
Rotary vane compressors, however, are known for being quiet. Gardner Denver’s smallest Hydrovane unit, for instance, operates at just 62dB(A). This is quieter than having a conversation with someone stood next to you, and so quiet enough to be sited at the point of use in a factory or workshop, without needing any additional sound proofing.
For those looking for a long-term compressor investment, then, there are a range of reasons why it may be wiser to opt for rotary vane technology over a piston compressor.