Slide Valve Control for Screw Compressors
This type of control incorporates a third bore in the compressor casing, at the cusp of the rotor bores. In this bore is positioned a slide valve at the discharge side of the rotors.
As the compressor is off-loaded, the valve moves progressively opening up ports which connect the compressed air back into the inlet. This form of control is only suitable when the compressor is driven by a constant speed device such as an electric motor. Speed control is usually preferable, as well as being simpler.