The potential to reduce the degree of outlet superheat is limited by the capability of control system. Almost all desuperheaters are used to reduce the steam (or gas) temperature as close to saturation as possible.
Most desuperheater models (types) are actually capable of achieving saturation or close to the saturation. The caveatis that these units must be controlled precisely to prevent flooding of the whole system. The limitation of the control system is its sensitivity range. Consider a system with a controlled outlet temperature set at 1 Deg.C above the saturation, where the controls are designed to maintain +/- 1 Deg.C.
The cooling water injection into the steam will continue till the temperature falls to saturation.. Then only the controls will reduce the cooling water injection. Due to the delays inherent in the control system (specifically introduction of derivative band) which is not in scope of desuperheater manufacturer, cooling water will continue to flow at the design rate for a short time. In that small amount of time, the saturated steam will be allowed to condense. Condensing vapor creates a vacuum, which sucks in more vapor, which too continues to condense. This results in loss of control, downstream equipment may be damaged & the downstream processes may be severely affected. Due to these reasons, we have opted to not guarantee desuperheating lower than 4 – 5 Deg.C above the saturation. The downstream processes do not suffer much from such small amount of superheat.