I believe you are referring to Variable Speed Screw Compressor.
In General, lot of compressor brands claim VSD compressor can give energy saving anything from 5% to 35% range. If the plant air usage pattern is very erratic, the energy saving will be higher as compared to a stable usage pattern.
However, please note that VSD will not give you any savings if the VSD compressor is run as at full load. In fact, it will be worst comparing to fixed speed compressor (of similar motor size) due to conversion losses.
One more thing, make sure VSD compressor is located at well-ventilated area, as pro-longed high heat will cause pre-mature failure to the VSD panel.
I bought a European brand VFD compressor about 2 years ago. Only 6 months running, the Converter board burnt. Supplier refused to give warranty and claim the compressor room was too hot. Bad experience.
My experience of Variable Speed compressor is good and bad.
Good is, my factory achieve about 7% energy saving on the Variable speed compressor, from replacing the old fixed speed unit.
But, bad is the supplier charged my factory very expensive to trouble shoot the inverter board issue when it happened 6 months ago. One morning, the Variable speed compressor just tripped and cannot be started. The supplier technicians came many times but he failed to find the root cause and blame the inverter board has burnt and he cannot do anything. Then, they sent the Inverter board supplier to investigate and replace a new board. The total cost of the repair (or replacing the board) is as much as 40% of the compressor price !
The compressor supplier didn’t highlight the issue of hot temperature when they promote the unit to us. Even their technician who commissioned the unit didn’t say anything about the room temperature.
In my experience, VSD compressor doesn’t guarantee saving when you get one for your plant. It all depend on what is your flow range of production. If your VSD is running at full load all the time, you are actually consume more power than a standard compressor.
The problem is many salesman promote VSD and give unrealistic promise. Be careful.