Recently there have been many comparisons between rotary screw and vane compressor technologies, looking at principles and performance. However, not all of these present all the salient data and insights to enable a potential user to make an informed choice. While both types of compressor rely fundamentally on displacement volumetric concepts, the simple vane principle has the longer track record, having been introduced almost 100 years ago. However, design longevity and simplicity do not necessarily equate to energy efficiency or allow for a full range of capabilities in comparison to those offered by rotary screw compressors. In terms of turndown rates, speed range limitations and energy demand, the innovation embodied in advanced rotary screw compressor technology provides a clear advantage in performance and compatibility in the era of the smart factory.
Wear, leakage and lubrication
Some say that the vane compressor offers a 100 per cent duty rating and becomes generally more efficient over time as its rotor blades wear in, actually improving its performance with use. This can be attributed to the vane blades settling into their stator and rotor slots, thereby producing a near-perfect seal, reducing leakage and increasing power efficiency at a level of 5 to 10 per cent higher than when new.
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 both the vane and slot are subject to excessive wear, eventually leading to failure of the vane and all that this entails. Large-vane models rated from 110kW upwards 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. Oil changes are required after every 2,000 hours of operation. In comparison, the oil-change interval for an oil-injected rotary screw compressor is 4,000 running hours.
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 1000 to 1100 rpm, and a third of that for screw compressor air ends. It is at maximum speed when vanes are most likely to flex, 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.
Modulating control strategies
Over time, compressor manufacturers have developed a number of different types of control strategy. Controls, such as start/stop and load/unload, respond to reductions in air demand, increasing compressor discharge pressure by turning the compressor off or unloading it so that it does not deliver air for periods of time.
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.
Fixed-speed vane compressors rely on 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.
Point of use
In the main, rotary vane compressors are constructed on a horizontal plane, which imposes some limitations on point-of-use applications. In contrast, the latest types of rotary screw compressor have been introduced in a vertical build concept, occupying extremely small footprints – under 2m2 in standard versions, and less than 3m2 for full-feature models with an integrated refrigerant dryer. These compact dimensions allow for installation close to a wall or even in a corner, and with no coupling or gears between the motor and screw element the screw compressors are very quiet in operation, in the order of 73dB (A).
Operational lifetime v. new technology
An advantage that is emphasised by vane compressor manufacturers is the 100,000-hour lifetime for vane equipment. That is equivalent to 12 years of operation. It is unrealistic to assume that an industry user would accommodate the progressive inefficiency, maintenance and running costs that a compressor would incur over such an extensive period − or ignore the benefits of the latest developments in design and the improved efficiency of today’s offerings in rotary screw technology.
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