Rotary vane vs. rotary screw compressors: what's the difference?

But it is not all good news. Centrifugal force throws the vanes outwards against the stator casing, which seals each compression chamber. The vanes are therefore continuously sliding in and out of the rotor vane slot, and as a result this could lead to wear to both the stator slot and the vanes themselves. Some large vane models feature a twin compressor design configuration, which is likely to double the possibility of vane wear problems.

Internal air leakage, or the volume of air return from the compressor’s high-pressure side to the inlet, is an issue which can affect both types of compressor technology.

While it is true that early types of ‘dry’ screw compressor with symmetrical profile elements did display this tendency, the issue was largely remedied by the introduction of innovative oil-injected rotary screw compressor designs. A further step was the development of asymmetrical, helical element profiles manufactured with great precision to facilitate oil-free solutions for sensitive applications.

A characteristic of the vane compressor operation is the requirement for abundant lubrication within the rotor/stator chamber. The quality of the specific type of lubricant involved is critical in order to prevent sludge build-up and varnishing of stator bores. Therefore there is a risk that oil changes are required more frequently to prevent this from happening..

Speed control

There are significant disadvantages in terms of vane compressor minimum and maximum speed, which affects output. Because vane compressors rely on centrifugal force to operate, their maximum speed is limited to approximately a third of that for screw compressor air ends. It is at maximum speed when vanes are most likely to wear and ultimately fail.

At low speeds the vanes will not fully seal on to the stator casing, which enables leakage to occur between the compression spaces and, as a result, efficiency decreases. At speeds of 500 to 600 rpm and below there is insufficient force for the vane to seal against the stator casing, and so the compressor will run unloaded and eventually stop.

This is where the speed control advantage of the screw compressor is evident. VSD screw compressors can operate from 15 to 100 per cent load, and are able to match fluctuations in demand without energy penalty.

Similar limitations for the vane principle are evident when it comes to turndown and partial-load operation. In the main, fixed-speed vane compressors offer limited turndown opportunity. Even with VSD drive control to a close-coupled motor, the maximum turndown ratio is 50 per cent, compared to a VSD rotary screw compressor’s 80 to 85 per cent capability.

Inlet-valve modulation, as used on lubricant-injected rotary air compressors, allows compressor capacity to be adjusted to match demand. A regulating valve senses system or discharge pressure over a prescribed range, and sends a proportional pressure to operate the inlet valve. Closing the inlet valve causes a pressure drop across it, reducing the inlet pressure at the compressor and, hence, the mass flow of air. Since the pressure at the compressor inlet is reduced while discharge pressure is rising slightly, the compression ratios are increased so that energy savings are limited. 

Some fixed-speed vane compressors use modulating control units for partial-load operation, which has been proved to be energy inefficient in comparison to the load/unload method widely practised throughout industry. Modulating control has been described as like driving a car with the engine at full throttle and using the brakes to control the speed. On the other hand, the latest VSD vane units claim a maximum 35 per cent in energy savings, but still operate most efficiently in low-pressure applications up to 5 bar.

Although in some operations there may be a 20 per cent saving compared to the standard full load when taking on the modulating control option, that does not come close to the savings achieved by a rotary screw compressor’s variable-speed drive. If only 50 per cent of the vane's capability is used with modulating control, the machine will still be consuming 80 per cent of its total energy rating.

In contrast, the latest ultra-compact VSD rotary screw compressors incorporate high-efficiency permanent magnet motors and close-coupled drive trains. They have been proved to achieve up to 50 per cent energy savings as well as reductions in the compressors’ total lifecycle costs in the order of 37 per cent. Furthermore, the drive train in these screw compressor designs is a closed circuit in which both the motor and the compressor element are oil-cooled, thus contributing additional energy savings.