Air Compressor Control

This probably won't have a PLC in it, but I wanted to see if anyone could offer some enlightenment or experience, on how to implement this control scheme my customer wants for their air compressors.

They currently have a 40HP compressor, that is running near 100% Capacity. There is also two smaller 10HP spare compressors, they want to add to the system to handle surges along with a 1000 gal accum. tank. The question comes in how to turn on and off the two smaller compressors. He had suggested using current on the larger system to turn on and off the backup compressors. I was thinking probably using pressure to turn on the two small compressors, and then a current sensing relay, to turn them off, when the larger compressor unloads. Either that or a time based, if the main compressor is fully loaded for x amount of time, then turn on the other units.

I don't have any experience with this type of system, and wanted to see what the normal way of doing this was.

Thanks,

Do all the compressors have their own integral controller or are the smaller compressors just a motor and compressor. I did a lot of research on this when we were redoing our air system about a year ago.

compressedairchallenge.org is a good source of information generated by a partnership of industry and the epa.

To answer your question...
I would put a pressure transducer on the main header of the air line just before the air receiver or potentially on the receiver itself. You need to have a dead band in there say on at 90psi off at 105psi. Each compressor will require different on and off setpoints to keep both small compressor from kicking on when the big one is just kicking on. It has been my experience that you need at least 5 psi difference to coordinate this effectively. They then need to run to their setpoint, unload, and then they should timeout, say 5 min, before shutting down completely.

You could do pressure switches but it would cheaper to just get 1 transducer than 4 or more adjustable pressure switches.

A note on your receiver. If you have the luxury of getting a vertical receiver plumb the header near the bottom and the plant supply near the top. This will help collect the condensate in the receiver you should the place a coalescing filter and drier AFTER the receiver.

Sullair makes a standalone controller to do exactly what you are talking about. I have not priced one but am sure its extremely high for the quality. This unfortunately is something I feel is standard among air compressor "manufacturers", read assemblers.

Thanks Califflash,

The smaller systems are just a pressure switch to a starter, while the 40Hp has a controller which loads and unloads the compressor. They were only running 80% load when I was there today, with a steady pressure of about 103 psi. They do need to maintain a minimum pressure fo 90 psi. I had mentioned having the auxillary units on a pressure switch set lower than the main unit. But he was affaid this would be too late, and started discussing controlling off motor current of the main unit. Which is where I told him I would have to check on that.

We use ours set up on a lead/lag system. Reach a certain setpoint with your main compressor, then kick on the lag. We don't use volume as a % capacity indication, we use demand pressure. When the main isn't able to keep up with the demand for pressure, kick on lag. Your accumulator should be invisible to the air supply system.

Your main compressor has a turn-off setpoint - if you want to maintain pressure, kick on your lag before you reach that point. Turn it, and the main compressor off when the turn-off setpoint is reached. You have to think of compressors as "supply capacity" rather than the "percentage of demand".

We set the primary compressor for the highest pressure and set the secondary compressor a little below the main. The smaller third compressor is set just a little bit lower than the secondary.

Lead/Lag between the primary and secondary is done by changing the pressure settings around, which happens at a semi-annual PM.

Nothing fancy.

Consider who is/will be doing the maintenance. Make it easy for them. I would make the two smaller compressors turn on and off based solely on pressure set just a little lower than the main compressor. Most pressure switches have enough hysteresis in them that it should work well.