Compressor Load Shedding

[jobemay 0]

Has anyone have experience programming compressor load shedding? I have 3 compressors hooked-up to a AB SLC 1500. I have a pressure switch monitoring the line presure and want to have a lead compressor and cut off the other ones when they are not needed. My main issue is how and when to turn off the slave compressors with-out cycling them too much.

[Ken Moore 23]

I think you need a lot more pressure switches, ideal would be about four, or perhaps two with a settable deadband. Let's assume normal air pressure is 110-120 psig. When pressure drops to 105 (pressure switch #1) the 2nd compressor starts and runs until pressure reaches 115 (pressure switch #2). If pressure drops to 100 (pressure switch #3) 3rd compressor starts and runs until pressure reaches 110 (pressure switch #4). Depending on the size of the system, gaining or losing 10 psi in pressure usually takes some time, that would minimize the start/stops of the back up units. You can then rotate the unit functions on some periodic basis, once a day, once a week, once a month etc.... You could also use a pressure transducer with one analog input and several compares.

[PdL 71]

Don't know about your compressors but you might want to check if your final program in theory keeps the number of starts in an hour below the maximum what's prescribed by the manufacturer. E.G. for medium sized screw compressors this is about 6 times.

[PdL 71]

From PM: (Lets keep the topic public jobemay, that way everybody can throw their thoughts in) I would stop the slave compressor on the pressure switch with a delay, and not stop it after a fixed time and then check the switch. That way if it is necessary to run the slave for 20 minutes, it will run for 20 minutes so the overal startup times will stay as minimum as possible. If you use a fixed time of X minutes, then check the pressure, if not sufficient start it again, it will be started unnecessarily often. Edited 4 Aug 2005 by PdL

[Ken Moore 23]

Here's a little snippet to get you started. If the pressure is low then the slave will run for three minutes after it is no longer low, then if the pressure goes low and remains low for two minutes, the slave starts. You can change the timer values to fit your needs.

[PdL 71]

I think jobemay is now out having a beer Edited 4 Aug 2005 by PdL

[jobemay 0]

Thank you.

[jobemay 0]

Is there only 3 rungs to the program example?

[PdL 71]

Well, it will require some more to complete your program, but the 3 rungs will do what Ken describes in his post. Doesn't it cover the part of which you didn't knew how to program !?

[gravitar 3]

I would think another part of your logic would be to select which compressor is the "master" for the day. You would want to alternate, right? You would probably need three integer words, and three rungs to do the shifting. Each rung would have three EQU instructions and a MOV at the end to change who holds the "token". Plus also the condition that tells the rungs that it's "time to switch".

[Guest Guest_Joe]

I do have the program completed. Thank you for your help