Dear Lance,

I hope the information is useful.

Positive displacement machines (pumps or compressors) do not follow the affinity laws as centrifugal pumps or fans do. The power savings are based on the capability of the compressor, based on tip speed of the rotors and how the slide valve responds to changing the effective length of the rotors.

Think of unloading a screw compressor as changing the length of the stroke of a reciprocating compressor. The shorter stroke would decrease the dispacement. When the slide valve unloads the rotors, the effective length of the rotor used for gas compression is shortened.

When all of this is taken into consideration with variable speed operation, the part load performance starts to become linear. At least down to about 50% speed with the slide valve fully loaded.

To get the exact data, you would need the manufacturers performance program. The various data points need to be ran for the suction and discharge pressure. Then vary the speed. Once the data is plotted as a percentage of full load capability, the curve starts to make sense.

The oil pressure is a major issue. If you have a full-time oil pump driven externally by a separate motor this becomes less of an issue.

It sounds like you are using some heat recovery devices off of the compressor discharge. Depending on the application, hot water generation will be reduced during part-load operation of the screw. Your operators may not want to decrease the discharge pressure if they think it affects the water heating.

On the other side of the discussion though, reducing the discharge pressure will reduce the motor amp draw, which also saves energy. You, or they may have to find a good balance between operation and heat recovery methods.

If your system uses water-cooled condensers, the water supply will determine how low the discharge pressure could potentially be reduced. If you have evaporative condensers, any lower wet bulb temperature will allow the discharge pressure to reduce. Assuming all fans and pumps are operating at full speed.

If you have VFD's on the condenser fans, it may be better to run the fans at full speed and reduce the compressor power input. The best course of action is whatever uses the least amount of energy.

If I can be of help, let me know.

Best of luck down under,
US Iceman