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The Benefits of vane technology

Product News Monday, September 27, 2010: Arthur Rushmer Engineering Ent cc

Though modern vane and screw compressors were both largely developed during the 1960s and 1970s, it is vane machines that are generally regarded as the older, perhaps more outdated, technology. However, according to Mattei, vane compressors offer significant advantages over screw compressors, and, with major advancements being made in recent years, the belief that vane technology is 'outdated' is unfounded. Here, Mattei's general manager Andy Jones discusses vane and screw compressors, with a particular focus on energy efficiency, operating speeds, volumetric efficiency, wear and maintenance and oil carryover.

Screw compressors currently account for the majority of industrial compressed air installations, and there are undoubtedly more manufacturers making screw compressors than vane compressors - perhaps because the technology has been easier to replicate, and patents have been simpler to overcome. This has led to the common misconception that vane technology is the older, more outdated of the two.

In Mattei's opinion, the widespread use of screw compressors has little to do with technological advantage - and we are now actively promoting the benefits of vane compressors, with the aim of redressing the balance. A leader in vane technology, Mattei has invested heavily in new product development, especially with regards to our rotor stator unit. Whereas some vane manufacturers have concentrated on engineering their products to reduce costs, we are committed to ensuring our machines can outperform their screw counterparts, and, through extensive research and development, we have overcome the negative issues previously associated with vane technology.

While vane and screw compressors share some similar components and are designed to produce the same end result, the two technologies differ quite considerably.

A vane compressor comprises a rotor with longitudinal slots, which house individual sliding vanes. The rotor is offset within a stator (a cylinder), in which it rotates. While the rotor turns on its axis, the vanes are pushed against the stator wall by centrifugal force, and the air is compressed.

Meanwhile, a screw compressor has two parallel rotors - a male and a female - which engage into each other. These two rotors intersect longitudinally with a critical minimal clearance and are in turn fitted into a stator. During rotation, the profiles mesh into each other, compressing the air.

We believe that vane machines offer advantages in several important areas - namely energy efficiency, operating speeds, volumetric efficiency, wear and maintenance and oil carryover.

For a compressor to work efficiently, it is essential for air leakage paths to be kept to an absolute minimum - because air lost during compression equals lost energy. Keeping internal leakages low increases the volumetric efficiency, and reduces the power required for a measured unit of delivered air.

In a vane compressor, internal air leakage is minimal. Because the vanes move freely in their slots, but are always in contact with the internal surface of the stator (albeit not directly, due to lubrication), the air seal is near-perfect. The clearances between the rotor, stator and end covers are also efficiently sealed by the oil that is injected into the stator to lubricate the moving parts and to cool the air during compression.

In comparison, screw compressors, by the very nature of their design, will have a 'blow hole', where the external profiles of the rotors meet. The air under high pressure returns to the area of lower pressure through this hole. Manufacturers of screw compressors have tried to reduce the 'blow hole' effect, but it is impossible to eliminate it completely.

To compensate for these air leakage paths, screw compressors need to run at high speeds, often relying on speed increasing gears or belt transmissions. Only recent optimisation of the screw profiles has allowed some manufacturers to engage their compressors through a direct drive, hence improving efficiency.

Because vane compressors do not have the same issues with air leakage paths, they are more volumetrically efficient. In addition, they have always operated at low speeds, from a direct drive coupling, which in turn leads to lower power consumption. Some of Mattei's fixed speed compressors operate at 1,000rpm, whereas the average screw compressor needs to work at 3,000rpm to achieve the same result.

The efficiency of the vane compressor has actually been enhanced in recent years - and, with the energy required to produce a given flow of air at a certain pressure being the most important feature of any air compressor, this is a crucial point. Traditionally, off-load energy consumption was considered too high with vane machines. Today, however, due to improvements in internal lubrication, Mattei has been able to reduce its off-load operating pressure - significantly improving the off-load energy efficiency.

As a result of Mattei's continuous development of the rotor stator unit, the on-load power consumption of our compressors has also been improved, and is amongst the lowest that can be found in any modern screw or vane machine.

Another important factor to address is compressor wear and maintenance. There is a common misconception that the sliding vanes used in a vane compressor will deteriorate, and will need to be changed regularly. However, in reality, there will only be negligible wear on the vanes, because the lubricating oil forms a film on which the vanes slide, meaning there is no direct metal-to-metal contact between the vanes and the internal stator wall.

Most significantly, a rotary vane compressor does not require roller bearings as used in a screw compressor. The vane compressor's rotor shaft is supported by simple white metal bearings, which ensure a long and inexpensive operating life.

So, providing it is looked after in accordance with the manufacturer's instructions, the operating life of a vane air compressor is virtually unlimited, and maintenance is minimal. An air end can easily run for 100,000 hours without wear - in fact, some Mattei machines have lasted over 230,000 hours in the most arduous operating conditions.

In comparison, screw compressors cannot offer the same levels of longevity. Because the rotors have to operate with high axial and longitudinal loads while maintaining minimum clearances, screw compressors use roller bearings by necessity. However, in order to prevent an expensive air end failure, screw manufacturers recommend that these bearings are replaced as frequently as every 24,000 hours. This re-bearing has its own cost implication, which is often overlooked at the time of purchase. Also, as there is no wear in the air end of a vane compressor, its efficiency remains constant throughout its operating life, whereas a screw compressor's efficiency will deteriorate over time.

A screw compressor also utilises additional parts, which aren't used in a vane compressor. Therefore, items such as gear trains and drive belts could potentially need replacing. Due to their design, vane compressors require fewer hydraulic hoses than screw compressors, therefore reducing the risk of leakage and failure.

All of this means that vane compressors offer lower maintenance costs, greater reliability and guaranteed lifetime performance.

Finally, there is oil carryover to consider. In the past, vane compressors have been criticised for their relatively high oil carryover. Twenty years ago this would have been a fair observation, however, in recent years this has been successfully addressed to tip the balance in favour of vane technology.

For example, in Mattei compressors the oil separation occurs in several stages, and our mechanical separation stages are particularly effective. We then use high-quality separator elements, which can last for up to 10,000 hours, to ensure that our machines have exceptionally low oil carryover throughout their operating lives. Our unique arrangement means that the oil quantity carried over from our compressors is less than three parts per million (ppm) as standard. Oil content can be further reduced to less than one ppm with filtration - so our machines can be used where an 'oil-free' compressed air supply is required.

Screw compressors may currently have large market share, but it shouldn't be assumed that they outperform vane compressors, or that they are a more modern solution. Today there is no technical argument against vane technology, in fact, vane compressors can be seen as superior, or at least equal, in many respects, particularly with regards to energy efficiency, operating speeds, volumetric efficiency, wear and maintenance and oil carryover - and we believe the tide is starting to turn. Mattei is the world's largest rotary vane manufacturer and a leader in developing vane technology.