Operation of a screw air compressor Gardner Denver

Operation of a screw air compressor
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Operation of a screw air compressor in a cycle of starting, loading, running, unloading, and stopping.

It is started manually or by remote command after safety checks are passed. Once running, it automatically loads and unloads based on pressure settings or remote signals. When unloading, it runs offload for a set time before stopping the main motor.

Timers prevent instant restarting or reloading after stops/unloads. The controller limits the number of starts per hour by adjusting the offload run time if the limit is reached. For a controlled stop sequence, the motor runs for an additional set time after stopping to safely de-pressurize the compressor. This cyclic operation with built-in delays and limits helps ensure safe and efficient compressor operation.

Pre-Start Safety Checks

  • Before starting the motor, the controller performs safety checks and ensures no inhibiting conditions are present.
  • If inhibiting conditions are detected, the compressor will not start, and a start inhibit message is displayed.

Starting Sequence

  • If no issues are found, the compressor will start, and the motor speed will stabilize (Load INH time).
  • After the load delay timer expires, the compressor will be loaded.

What is Offload pressure?

  • It is an adjustable setting that defines the maximum delivery pressure level.
  • At this pressure, the compressor will be offloaded and set to offload run mode.
Operation of a screw air compressor

What is Load pressure?

  • It is an adjustable setting that defines the minimum delivery pressure level.
  • The compressor will be loaded at or below this pressure level.
  • If in standby state, the motor is started before loading.

Main Motor Start Sequence (Y/D, DOL)

  • Controller energizes the star contactor first.
  • Then it energizes the main contactor.
  • After the Star/delta transition timer expires, it executes the star to delta contactor changeover.
  • If stop command received during sequence, it completes the sequence before stopping to limit break current.
  • For DOL compressors, main contactor acts as DOL contactor, and Y/D sequence runs in background.

What is Load Delay Timer?

  • It inhibits the compressor from loading after the star to delta transition.
  • Allows main motor speed to stabilize and pre-load functions to complete.
  • In VFD compressors, it starts after minimum air end speed is reached.

What is Reload Delay Timer?

  • Its purpose is to prevent immediate loading after unloading.

What is Venting Timer?

  • It starts immediately when the main motor is stopped.
  • Any start request is recognized but not initiated until this timer expires.
  • If internal pressure detection is enabled, restart also depends on pressure falling below inhibit level.

What is Offload Run Timer?

  • It starts when the compressor is unloaded.
  • If compressor remains unloaded and timer expires, main motor stops, and compressor enters standby.
  • Compressor automatically restarts and loads if required.
  • Improves efficiency during low demand and limits motor start events.

What is Stop Run Timer?

  • When compressor is stopped (stop button, remote input/command), compressor unloads first.
  • Main motor continues to run for this timer duration before stopping.
  • Allows venting of internal pressure and limits oil aeration before motor stop.

Safety functions of the controller

The following safety features will cause the compressor to stop

  • High motor temperature, thermistor or thermal relay.
  • High internal temperature, shut down at115° C.
  • High internal pressure (1,0 bar/10% above maximum working pressure).
  • High delivery temperature, shut-down at 70° C.
  • Analogue sensor fault.
  • High differential pressure in normal operation.
  • High ambient temperature (optional).

Operation modes in Air Compressor.

Loading Mode: When pressure reduce below 6.0 bar, then the control switch enables air inlet valve to open. Then air is compressed in compressor air end and after filtering and cooling it goes to system.

Unloading Mode: Here it waits for 200 seconds if the pressure reduces below 6.0 bar then again it goes to loading mode. In both loading/unloading mode compressor keeps running.

Stop Mode: If air pressure does not reduce in 200 seconds, it means there is no consumption. So, compressor stops.

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