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Different design, yet same result

25 January 2013

There are two main types of industrial rotary compressor available - vane and screw. Here,Martin Chitty, business development manager at Mattei, explains some of the key differences between the technologies Vane and

There are two main types of industrial rotary compressor available - vane and screw. Here,Martin Chitty, business development manager at Mattei, explains some of the key differences between the technologies

Vane and screw compressors were both developed in the 1950s, have continually evolved since then and essentially produce the same end result. But this is where the similarities end, with the two technologies differing quite considerably.

A rotary 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 in turn are fitted into a stator. During rotation, the profiles mesh into each other, compressing the air.

Longevity and maintenance These contrasting designs result in quite stark differences - particularly where longevity, maintenance and energy efficiency are concerned. Vane technology is particularly hardwearing. The use of a quality lubricating oil means there is no direct metal-to-metal contact between the vanes and the internal stator wall, so there will only ever be negligible wear on the vanes. In addition, a vane compressor's rotor shaft is supported by simple white metal bearings, which ensure a long and inexpensive operating life. In fact, providing a vane compressor is looked after in accordance with the manufacturer's instructions, its operating life 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 200,000 hours, in arduous operating conditions.

In comparison, I believe screw compressors cannot offer the same levels of longevity. Since the rotors on a screw compressor need to operate with high thrust and radial loads while maintaining minimum clearances it is necessary for manufacturers to use roller bearings. The recommendation is for these bearings to be replaced as part of the maintenance schedule after as little as 20,000 running hours, but this hefty cost is often overlooked at the time of purchase.

Energy efficiency For a compressor to operate efficiently, it is essential for internal air leakage paths to be kept to an absolute minimum; air lost during compression equals lost energy.

In vane compressors, internal air leakage is minimal, so they are volumetrically efficient.

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), so the air seal is near-perfect.

The clearances between the rotor, stator and end covers are also efficiently sealed by oil that is injected into the stator to lubricate the moving parts and to cool the air during compression.

In comparison, screw compressors have an inherent 'blow hole', created at the point where the external profiles of the rotors meet. The air under high pressure returns to the area of lower pressure through this pathway. Manufacturers of screw compressors have tried to reduce the 'blow hole', but it is impossible to eliminate it completely.

To compensate for these air leakage paths, screw compressors need to run at high speeds. In contrast, vane machines 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 1000rpm, whereas the average screw compressor needs to work at 3000rpm and higher to achieve the same result.

Advancements in vane technology There have been some major advancements in vane technology in recent years. For example, through developments in internal lubrication and the rotor stator unit,Mattei has been able to significantly improve offload energy efficiency and on-load power consumption respectively. The relatively high oil carry-over of traditional vane compressors has also been successfully addressed. In Mattei compressors, the oil quantity carried over is less than 3ppm as standard, and with added filtration this can be reduced to less than 1ppm.

Screw compressors currently have a large market share, but it shouldn't be assumed they are superior to vane compressors. In fact, we'd argue that the widespread use of screw compressors has more to do with the machinery being easier to replicate than technological advantage.
 
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