Compressor Technologies Comparision

The mechanical arrangement to deliver desired discharge pressure involves altering the effective volume of compression chamber/casing. Though it is anadditional complisity to a plain compressor, still it could be preferable compared to Variable Frequency Drive (VFD). In VFD, there is whole lot of an added system and just not few componets.

Sullair have a VSD model that combine Variable displacement and VSD.

They claim that it is more efficient. the machanical Variable displacement is simple and not so complicated.

the VSD is also simple. It requires a frequency inverter which is properly adjusted to suit the operation.

The Cost is the only difficult aspect.

In any case, a VSD Only unit will be more efficient in controlling the output and reducing the electricity bill if properly setup, than a variable capacity unit. This is my opinion: the motor will still be running at full speed while the bypass is being modulated --> losses due to friction in the bypass and to the electric motor running less efficiently. Even though you will consume less power when modulating the bypass, your consumption will be more than if it was a VSD only drive:(Motor speed changes and is controlled by the VSD to keep the motor efficient) Also, no friction losses in the bypass...

A combination of both methods seems to be more efficient in adjusting pressure and Volume... according to Sullair. I think we can trust them!