METHOD AND APPARATUS IN CONNECTION WITH A SCREW COMPRESSOR

1 . A method of determining operation characteristics of a screw compressor drive comprising a screw compressor driven with a frequency converter, wherein the method comprises

pressurising a pressure vessel of the screw compressor with a variable rotational speed of the screw compressor, the rotational speed of the screw compressor having a speed profile in which the rotational speed is changed stepwise such that between stepwise changes the rotational speed of the screw compressor is kept substantially constant for a time period,

repeating the speed profile until the pressure of the pressure vessel reaches a set pressure value,

determining pressure of the pressure vessel, power consumption of the screw compressor drive and mass flow rate during the pressurising when the rotational speed of the screw compressor is kept substantially constant,

calculating energy efficiency of the screw compressor drive as a function of pressure of the pressure vessel and rotational speed of the screw compressor on the on the basis of the determined pressure of the pressure vessel and power consumption of the screw compressor drive.

2 . Method according to claim 1 , wherein the method comprises

selecting optimal rotational speed for the frequency converter as a function of pressure of the pressure vessel.

3 . Method according to claim 1 , wherein the calculating energy efficiency comprises

selecting multiple of rotational speeds,

calculating at least a second order polynomial fitting curves for pressure ratio and energy efficiency for selected rotational speeds,

selecting multiple of pressure ratios,

calculating at least a second order polynomial fitting curves for rotational speed and energy efficiency for selected pressure ratios, and

for each of the selected pressure ratios, determining a rotational speed having the best energy efficiency from the calculated polynomial fitting curves for rotational speed and energy efficiency.

4 . Method according to claim 3 , wherein after determining the rotational speed having the best energy efficiency, the rotational speed used in control of the frequency converter is selected based on the pressure ratio.

5 . Method according to claim 1 , wherein the power consumption of the screw compressor drive is estimated based on output power of the frequency converter and internal losses of the frequency converter.

6 . Method according to claim 1 , wherein the internal losses of the frequency converter are calculated on the basis of the torque of the motor, the nominal torque of the motor, output frequency of the frequency converter, nominal output frequency of the frequency converter and the losses of the frequency converter in a nominal operating point.

7 . A screw compressor drive comprising a screw compressor driven with a frequency converter, wherein the system comprises

means for pressurising a pressure vessel of the screw compressor with a variable rotational speed of the screw compressor, the rotational speed of the screw compressor having a speed profile in which the rotational speed is changed stepwise such that between stepwise changes the rotational speed of the screw compressor is kept substantially constant for a time period,

means for repeating the speed profile until the pressure of the pressure vessel reaches a set pressure value,

means for determining pressure of the pressure vessel, power consumption of the screw compressor drive and mass flow rate during the pressurising when the rotational speed of the screw compressor is kept substantially constant,

means for calculating energy efficiency of the screw compressor drive as a function of pressure of the pressure vessel and rotational speed of the screw compressor on the on the basis of the determined pressure of the pressure vessel and power consumption of the screw compressor drive.

8 . (canceled)

9 . Method according to claim 2 , wherein the calculating energy efficiency comprises

selecting multiple of rotational speeds,

calculating at least a second order polynomial fitting curves for pressure ratio and energy efficiency for selected rotational speeds,

selecting multiple of pressure ratios,

calculating at least a second order polynomial fitting curves for rotational speed and energy efficiency for selected pressure ratios, and

for each of the selected pressure ratios, determining a rotational speed having the best energy efficiency from the calculated polynomial fitting curves for rotational speed and energy efficiency.

10 . Method according to claim 9 , wherein after determining the rotational speed having the best energy efficiency, the rotational speed used in control of the frequency converter is selected based on the pressure ratio.

11 . Method according to claim 2 , wherein the power consumption of the screw compressor drive is estimated based on output power of the frequency converter and internal losses of the frequency converter.

12 . Method according to claim 3 , wherein the power consumption of the screw compressor drive is estimated based on output power of the frequency converter and internal losses of the frequency converter.

13 . Method according to claim 4 , wherein the power consumption of the screw compressor drive is estimated based on output power of the frequency converter and internal losses of the frequency converter.

14 . Method according to claim 2 , wherein the internal losses of the frequency converter are calculated on the basis of the torque of the motor, the nominal torque of the motor, output frequency of the frequency converter, nominal output frequency of the frequency converter and the losses of the frequency converter in a nominal operating point.

15 . Method according to claim 3 , wherein the internal losses of the frequency converter are calculated on the basis of the torque of the motor, the nominal torque of the motor, output frequency of the frequency converter, nominal output frequency of the frequency converter and the losses of the frequency converter in a nominal operating point.

16 . Method according to claim 4 , wherein the internal losses of the frequency converter are calculated on the basis of the torque of the motor, the nominal torque of the motor, output frequency of the frequency converter, nominal output frequency of the frequency converter and the losses of the frequency converter in a nominal operating point.

17 . Method according to claim 5 , wherein the internal losses of the frequency converter are calculated on the basis of the torque of the motor, the nominal torque of the motor, output frequency of the frequency converter, nominal output frequency of the frequency converter and the losses of the frequency converter in a nominal operating point.

18 . A tangible, non-transitory, computer readable medium configured to store instructions executable by a processor operably coupled with a screw compressor driven with a frequency converter, wherein the instructions comprise:

pressurising a pressure vessel of the screw compressor with a variable rotational speed of the screw compressor, the rotational speed of the screw compressor having a speed profile in which the rotational speed is changed stepwise such that between stepwise changes the rotational speed of the screw compressor is kept substantially constant for a time period,

repeating the speed profile until the pressure of the pressure vessel reaches a set pressure value,

determining pressure of the pressure vessel, power consumption of the screw compressor drive and mass flow rate during the pressurising when the rotational speed of the screw compressor is kept substantially constant,

calculating energy efficiency of the screw compressor drive as a function of pressure of the pressure vessel and rotational speed of the screw compressor on the on the basis of the determined pressure of the pressure vessel and power consumption of the screw compressor drive.