Rotary-screw compressor Working Principle

MODULATION

Instead of starting and stopping the compressor, a slide valve as described above modulates capacity to the demand. While this yields a consistent discharge pressure over a wide range of demand, overall power consumption may be higher than with a load/unload scheme, resulting in approximately 70% of full-load power consumption when the compressor is at a zero-load condition.

Due to the limited adjustment in compressor power consumption relative to compressed-air output capacity, modulation is a generally inefficient method of control when compared to variable-speed drives. However, for applications where it is not readily possible to frequently cease and resume operation of the compressor (such as when a compressor is driven by an internal-combustion engine and operated without the presence of a compressed-air receiver), modulation is suitable.

Variable displacement

Utilized by compressor companies Quincy Compressor, Kobelco, Gardner Denver, and Sullair, variable displacement alters the percentage of the screw compressor rotors working to compress air by allowing air flow to bypass portions of the screws. While this does reduce power consumption when compared to a modulation control scheme, a load/unload system can be more effective with large amounts of storage (10 gallons per CFM). If a large amount of storage is not practical, a variable-displacement system can be very effective, especially at greater than 70% of full load.^[4]

One way that variable displacement may be accomplished is by using multiple lifting valves on the suction side of the compressor, each plumbed to a corresponding location on the discharge. In automotive superchargers, this is analogous to the operation of a bypass valve.

Variable speed

While an air compressor powered by a variable-speed drive can offer the lowest operating-energy cost without any appreciable reduction in service life over a properly maintained load/unload compressor, the variable-frequency power inverter of a variable-speed drive typically adds significant cost to the design of such a compressor, negating its economic benefits if there are limited variations in demand. However, a variable-speed drive provides for a linear relationship between compressor power consumption and free air delivery. In harsh environments (hot, humid or dusty), variable-speed drives may not be suitable due to the sensitivity of the equipment.