By Ron Marshall
One of the biggest wastes of energy in a compressed air system is allowing your air compressor to use energy when it is not producing air. This can be a big problem with screw compressors, due to the way these compressors typically operate. Well-maintained modern day screw compressors running in load/unload control mode will consume between 20-40% of full load power even in the unloaded state, consuming power but putting out zero air.
A sign you have a problem is if you have a low “loaded versus running hour” ratio. You can check this by looking at your compressor control panel and recording the loaded and running hours. It’s best to do this twice, once now and once in exactly one week. Subtracting the readings gives you the compressor duty cycle between observations. If your compressor loaded hours are about 40% or less of your compressor run hours, it is a sign that improvements could be made, possibly by putting the compressor to sleep in auto start when possible.
There may be times during your production cycle where the compressor is running unloaded most of the time. This could be, perhaps, during lightly loaded conditions during breaks or at night. It is also possible that the compressor is running fully unloaded for long periods of time with no load. A compressor not set to time out and shut off when lightly loaded will waste power (not in auto state).
A setting called auto/dual is built into almost every modern compressor control. This feature will monitor the compressor and turn off the motor if the control concludes the conditions are right to do so. Some controls work on a simple timer, after 10 minutes or so in the unloaded state the compressor will turn off (this is usually adjustable). Other controls have some “smarts” and will keep track of the compressor motor starts per hour, allowing the compressor to turn off immediately between cycles if the conditions are right, but not exceeding the number of starts that would damage the motor. The control stays armed, ready to start when the compressor is again required.
Some tips in working with the Auto feature:
- The auto feature must be manually turned on in some controls, read about how to do it in your compressor’s instruction manual, often plant personnel will forget to activate this feature, train your people to use it.
- The more storage receiver capacity the compressor has to work with, the better auto will work in saving energy
- The wider the compressor pressure band setting, the better the compressor will use the auto feature (within limits)
- Some controls have timer functions that could turn the units completely off between certain hours, say at night and on weekends. If you can use this, do so!
- Manually turning off compressors during nights, lunch breaks and weekends can save energy. If it is difficult to get to your compressors, wire in a remote stop/start button to save you steps
- Some VSD controlled compressors have an auto feature that will save energy when activated, VSD compressors can start and stop more often than fixed speed compressors, so they can often completely eliminate unloaded run time
- Like any feature, Auto should be used wisely, if you are not sure about it, ask your service provider to help you with the necessary adjustments.