Continuous run air compressors

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DOT Sparky

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Good day all, while sizing a generator for a building at one of our facilities I installed our Amprobe DMII pro to log loads. Later when analyzing the data I noticed spikes that were traced back to this compressor that ran upwards of 340 amps @ 208. Running amps about 36. My cure for greatly reducing the size of the generator would be convert the compressor to constant run and running it with the unloader like they do in industrial situations. This facility uses a lot of air and the machine cycles on 2 minutes off 2 minutes with a 40 psi differential. Opinions on compressor life versus cost of constant run would be greatly appreciated.. Long time lurker, first time poster Thanks, Scott
 
this is actually a good question and I am sure there are going to be more responses..I actually have couple of questions is your compressor rated for continuous run..you need to weight the extra cost of the electricity it costs you now verus the annual or semi annual rebuild of the commpressor..Is the compessor going to be shut down at the end of the day..how many hours a day does it run..what type of compressor are we talking..these are just a few questions I have..
 
Carl, Good point that I Really spaced out on. cschmid, It runs 24-7, as for the rating, I will have to check, It's about 100 gallon horizontal with 2 seperate compressors (2 stage) that alternate between runs. I guess I'll get the bigger genset, but, I'm still curious about the benefits of continuous run...
 
coulter said:
Don't forget, continuous run or not, you still have to start it on the gen.

carl
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I was thinking the same thing and wondering if a soft start ( solid state reduced voltage starter ) wouldn't help. And a bigger holding tank for the compressed air so that thing wounldn't cycle on and off quite as often.
 
the bigger continuous run compressors do start unloaded and then after running for 15 seconds then the load begins..the benefit is not having to change your motor as often the down side is the compressor need regular maintenance..most of the operations where you run compressor 24/7 have have two compressors so you can take one down and service it and give it rest period then bring it up and do the same with other..
 
This installation is basically 2 compressors on one tank and they do get shut down alternately for service, and as far as I can tell they are starting unloaded, softstart is a thought, although I have not seen that done on a compessor before. There is still alot of stuff I haven't seen though.
 
run run run

run run run

let it run continuously if it makes you feel better

you may want to consider a different method of reducing load on motor besides blowing off the unloader

pneumatic belt tensioner
electric clutch
variable speed motor or drive
 
put them on and let them run continuously..you can run them on a clutch..it will need you to modify the control system..which would be my next question how are these controlled? like a computerized control or a relay logic type system..those are just examples..
 
How big is the generator? (what is your total load?) This load may be insignificant compared to the total load.

The generator manufacturers have software that can help you size them.

How many ATS's?... Multiple ATS's can have their transfer delays staggered...
 
let me add a idea here with the cont run compressor if you going to be on generator power you should add on delay timer when the compressor sense you are on generator power it will delay for 15 second to 1 min until the generator can get stablized with the load other wise you will have some pretty serious issue with voltage dip during start up otherwise some generator manufacter do have oversize generator head but used to handle extra surge during start up.

Merci, Marc
 
Thanks Marc, I like the delay option for this and the air handlers. To all other responders Thanks for all the helpful input, I will forward this info to the boss and see where it goes. engy, It looks like the genset will be around 115KW but I still don't want to be banging it every 2 minutes with this inrush so I still lean toward cont. run. 1 ATS will be used.Thanks again ALL. Scott Did not mean to bump to the top of the thread....
 
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I may have missed the size of your compressor. But at one time where I worked we had 9 compressors, from 1500hp to 100hp. You definitely do not want to start a compressor loaded - at least if it still has almost full pressure in the system. That is a good way to test the AIC of your breaker/fuses.
To restart every few minutes sounds like you are really reducing the life of those motors - can you even lay your hand on them? Or are they too hot? I'd look into running one of them continuously, run the other when you do maintenance on the other. I have seen some very serious explosions that resulted from starting a hi-volt "loaded" compressor. As a matter of fact, I wouldn't just look into it, I'd do it. What is happening with the rest of your system with that big current draw every few minutes?
 
Reciprocal compressors are generally NOT rated to be run continuously, nor are the motors they put on them. Centrifugal compressors ARE typically designed to run continuously. Starting current in either case can definitely be mitigated by soft starters, it's done all the time on large compressors. In fact it is one of the biggest applications of soft starters out there.

But at 36FLA I would hardly call that "big". I know that 340A might seem like a lot, but that is probably a peak reading of the magnetization inrush current, because even locked rotor current would be only 216A or so. I seriously doubt it is that high for more than half a second. Sometimes the newer digital DMMs with fast sample times will bite you for things like that. Magnetization inrush like that is not considered significant to the generator as it has virtually no power factor.

The cheap-and-dirty rule of thumb for generator sizing is to have the kW be at least 2.5x the HP rating that will be started. So at 36FLA 208V you must have a 10HP motor, maybe 15HP worst case. So 2.5x 15HP = a 37.5kW genset. Even if you wanted to start BOTH motors simultaneously, that's still 75kW and you say you have a 115kW generator; not going to be a problem. I wouldn't waste a lot of money on solving a non-problem.

On the other hand if your generator is bogging down trying to start these compressors, that's another issue. First check to make sure your unload valve is working properly. Decouple the motor and rotate the compressor by hand. If you can move it, the valve is open. If you can't, it's malfunctioning.
 
What sort of pressures and volumes are you running? It is quite likely that a VSD would pay for itself in energy savings as well as solving the starting current issue.

-Jon
 
I have three recip air compressors, I'm thinking they are 30-50hp each. All are rated continuous duty, although typical cycle is 6 mins on, 10 mins off (the "on" time includes a 2 minute unloaded cooldown).

We do use most of the methods as suggested by others:

-- startup while unloaded (20 secs) - gives motor time to come to rated speed, gives oil pump time to build oil pressure

-- startup is delayed for 2 minutes after power is lost and returns, whether by utility or generator - (A) prevents motor start while coasting down if power blinks instead of going out for a while, (B) gives generator time to stabilize before adding this load, (C) timer restarts if power goes out again during the timer's timing duration, (D) if 2-mins have passed, most blinking power interruptions are finished

System design hints:
-build a bigger "bottle" to store your compressed air if you want less cycles
-increase the spread of your on/off setpoints if possible
-alternate your compressors - either after each run, or on a weekly basis, etc.
-if able to load/unload your compressor, give thought to a 50% loading step. Use 100% loading up to a point just under your shutoff/cooldown setpoint then go to 50% load to finish the run. Finish with an unloaded cooldown period.

Is your load somewhat steady, or does it fluctuate greatly?
 
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