Methods and apparatus for unloading a screw compressor

A screw compressor unloading system comprising:

• a motor;
• a screw compressor operably connected to the motor for being driven thereby whenever the motor is running, the screw compressor including an air inlet and an air outlet, the air inlet having an inlet valve for closing the air inlet;
• an air reservoir;
• a main air discharge passage connecting the air outlet of the screw compressor with the air reservoir and including a first non-return valve preventing backflow of compressed air to the air outlet of the screw compressor;
• a conduit for conducting lubricating oil from the air reservoir to the screw compressor;
• a vacuum device of substantially smaller maximum capacity than the screw compressor and having an air inlet and an air outlet;
• and
• a secondary air discharge passage communicating the air outlet of the vacuum device with the main air discharge passage at a location downstream of the first non-return valve, the secondary air discharge passage having a second non-return valve for preventing a backflow of compressed air to the air outlet of the vacuum device;
• the air inlet of the vacuum device communicating with the air outlet of the screw compressor to enable the vacuum device to unload the screw compressor by sucking air out of the screw compressor when the inlet valve is closed.

A drilling apparatus comprising:

• a motor;
• a mast for supporting a drill string;
• a hydraulic device operably connected to the motor for rotating the drill string;
• an air reservoir for storing compressed flushing air to be supplied to the drill string;
• a flushing valve for selectively opening and closing communication between the air reservoir and the drill string;
• a screw compressor connected to the motor for being driven thereby whenever the motor is running, the screw compressor including an air inlet and air outlet for supplying compressed air to the air reservoir;
• an inlet valve for closing the air inlet of the screw compressor;
• and
• a vacuum device of substantially smaller maximum capacity than the screw compressor and including an air inlet and an air outlet, the air inlet of the vacuum device communicating with the air outlet of the screw compressor to enable the vacuum device to unload the screw compressor by sucking air out of the screw compressor when the flushing valve and the inlet valve are closed.

A method of unloading a screw compressor to facilitate start-up of a drive motor therefor, comprising the steps of:

• A) closing an air inlet of the screw-compressor;
• B) driving a vacuum device having a substantially smaller maximum capacity than the screw compressor;
• C) communicating an air inlet of the vacuum device with an air outlet of the screw compressor, causing the vacuum device to unload the screw compressor by sucking air out of the screw compressor;
• and
• D) starting the drive motor to drive the screw compressor.